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Herbert Rees OrmgeviEle, Ontario
Introduction is intended to acquaint the aspiring product designer with some of the tices with which he or she should be familiar before any product design. etaly new designs are rare, although they may be necessary for truly tions (patented or not) which have no precedent, not even in similar defmition, there can be no precedent for "new;" therefore, there can way to design and build a new product. g on a possible, practical, and acceptable target design, there p&hs open to the designer. The old way is to first experiment, with and test prototypes, and then proceed with the necessary strength on and graphicdelineationofthe new design so that it can be producedsafely. The modern way, after a suitable design has been s to use the preliminary drawings for stress and flow analyses; hich then appear necessary are made before the final drawings are
,m ~ sso-called t "new" designs are really "improvements" of existing C~m P based on similar, not necessarily related, products in other fields.
@ are worded ". . . this invention describes an improvement cofnmon practice and highly recommended that, prior to any attempt'tdmlesign or to simply copy an existing product, the designer should
gather as many s i d h r and related products as possible that are already on the market and stud @.em carefully to determine which "good" features to retain and which "bad' @Wmph&voidinthe "new"pr0duct. This phibs~phyapplies to any product, whether a k d e or a complicated rneehmbm. Most importantly, ' mu& be aware af and mnsider possible alternatives when designi ct, and then mswer all m1evmt questions before proceeding. The designer is rwnsib1e: fog &at t k ~;xr&uct fulfills all expectations, both in p & m a and in #Wit .cart
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be produced economically.
In the following chapters, the author highlighted many of the que tions and dwisions a designer will face while d e s i g ~ n any g product. Unfortuna kly, there
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Plastic Product Design
m y design a product made of (any) plilstic? Usually, the reason to design ets made of plastic is either to create new products, never made before, or omething similar to existing d d products but which is better, more to the user, or more economical to produce. i Typical examples of new products include compact disks (CDs) and enclo. mires far CDs, etc. There are many examples of old products which could be, or f~ be, redesigned for the following reasons:
Different appearance (shape) and more sales appeal (decorations, colors). Typical examples include kitchenware, containers, cutlery (some previously made from plastics or other materials such as glass, wood, metal, paper, etc.). Improved performance so that products last longer, are easier to use, and are easier to handle. For certain applications, some physical pr~pestjesof plastics may be more suitable, such as corrosion and scratch resistance, proof against shattering, and ligfiter weight. Typical examples are eyeglass and contact lenses, and protective eye wear. Weight (and cost) reduction of products previously made from plst6ifi.c~.Typically, weight reduction is most important in products used in the packaging industry and in the automotive and aircraft industris, wbre-lowerweights can result in significantfuel economy and albw greater payloads. Cost reduction, always important, is achieved by both reducing material costs and shortening molding c y d e ~while , improving the product quality. Chmgrss, in materials, for any of the above reasons. Typical exWnple~are vials and bottles [from glass); hinges (from metal); f h p p ~computer disk enclosure^ (from paper); hardware, such as h ~ and b tool handles (from wood); toys (from met& paper, or mhmotive, aircraft, m d naval parts (from cast zinc or ShmtsMl,w w a s ) ; and enclosures for electwic f f d ar
Making a New Product ~ ~ ~ 'does k wa designer ' hagk to mdLt the coofsmpltlted new or redisigded po&xt?Pint, the designer must decide which type of plastic to use for the product. Then, he or she must select the best method for processing create the finished product.
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Choosing the Proper Plastic
There are many thousands of plastics, each with specific chemical and physical characteristics which will affect the performance of the product. There can be vast differences in processing these-plastics. First, the designer must choose between two basic types of plastic, thermosets and thermoplastics. With thermosets, the raw material (resin) is "cured under high pressure inside a heated mold. The chemistry of the resulting product is different from that of the original resin, and it cannot be recycled. A cured thermoset product (or any s&ap) cannot be ground up to be reused, except perhaps as an inertfiller for some other products. However, thermoset products inherently resist higher tempera*& ;than most other plastics, and they have certain better physical, electrical, chemical properties, as well. . Most thermosets are molded with the compression or transfer molding pmiXSS (tires, dinnerware, electrical components, etc.). Some products, usually g ~ d l e ones, r can be made using the injection molding process in specially @&pt~d or modified injection molding machines. Some thermosets are used as W d s , applied to sheets or mats of glass and other fibers, and then cured under 4 ~ ~ v e low) l y pressure and heat. (Examples are plywood, particle board, @bmg1assstructures, etc.) With thermoplastics, the raw material must be heated before it is injected h a Q c ~ o l e dmold. The resin does not change (or only minimally changes) its ~ h ~ 8 tduring r y t h e m n o , molditlg, and cooling processes. Prod& (and any (.,I
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2.2 Selecting the Best Processing Method
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excellent surface definition, good accuracy, good repetitiveness, high productivity, and product finishing usually is not required after molding.
Disadvantage:
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This book will be concerned mostly with designing products for injection molding, but, before continuing, .we should consider s a n e of the other plastic sing m e t h a ,used.
Selecting the Best Processing Method
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High mold and machine cost.
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hY, &ere are a number of different plastic processing methods, and new ds are developed frequently:
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Molding (compression, transfer, injection, injection-compression), extruding (structural shapes, tubing, sheet), werU%ionblowing ("blow molding" or "bottle-blow molding"), Ljwtion blowing, stretch blowing (one- or two-step),
be perEomed wifh some
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f ~ i c o r molding e (an area of injection molding). '8
plastics may be suitable for only one or for just a few of the
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ated will play a large role in determining the lowing discussion will describe some of the he various methods.
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&is ~ a t ~ g includes ~ r y compression, transfer, or thezln~setsor thermoplastics. .,
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Suitable for practically any size product, but limited by available e%&mmt sizeb b fair accuracy, gad *'titivenms, high productivity, and P
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Limited application as a final product.
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which is su&ahle only for some t h e q l m t i c s , cut lengths
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Maki?~ga New Product
2.2 Selecting the Best Processing Method
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Suitability for all size bottles, toys, technical products (automotive b l tanks, air ducts, etc.), and for containers, up to large drums and t d , fair accuracy and repetitiveness, high productivity, and low-cost tooling.
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Disadvantages:
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Limited quality of surface definition and product must be finished (deburred, etc.) after blowing.
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1-Stage Method
Reforms are molded in a special machine and immediately, while still hot and without losing orientation, transferred first to a heat conditioning station and @en to a blowing station where the preforms are stretched (optional) and blown. The main difference between this method and injection blow molding is that the ~ f . o r m s a r eheat-conditioned between the molding and blowing steps, in the h e machine. Advantages:
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Injection Blow Molding
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Only one machine is required to do both malding and blowing, easier quality control, since each molding cavity corresponds to a specific blow cavity, and lower installation cost of total setup.
Disadvantages: 4
Suitable f@rxiw&y s o w thmoplastics, #injectionblow molding involves hot, injwtion m o l ~ p r @ mthat are transferred automaticw into cooled blow molds where fib1pfixms we blown into the final shape. This is only possible in special (injection blow) machines, or in special molds for standard injection sholding macbes. Advantages:
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Suitability for very small to medium-size bottles and jars, drinking cups, sac., good surface definition, high accuracy and repetitiveness, and high productivity.
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Special machine required, number and spacing of injection cavities limited by the number and spacing of blow cavities possible for the size of machine, lower productivity, since the molding cycle controls the overall cycle (molding of the preform usually 3 to 6 times slower than the blowing cycle required to shape the bottle), and complicated machine with poor accessibility.
2-Stage or Reheat and Blow (R & 0) Method
we molded in a standard injection molding machine. The cold s are then shipped randomly to a special stretch blow machine, where
Disadvantaga:
oriented, mleated, stretched, and blown.
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Special m W n e q u i r e & with(rehiively low mol&cr>~~~ or high mold cast f a we in standard injection molding machines.
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Stretch Blow Molding
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Most commonly used for molding PET bottles and jars, there are two methods of s t r a blow molding: the 1-stagemethod and the Zstage, or reheat and blow (R& B),method. The plastic is stretched to improve its physical properties.
Standard injection molding machine can be used, m U n g cycles and omperatme sethgs do not depend on blow @s,-eti., and can be &timized for &hest productivity, bcm be shipped to the mlatively s ~ ~ mh8t I B and bbw e q i p n t , k a & d at oa bottling plat, alidn&g the co&y ship-
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2.2 Selecting the Best Processing Method
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Tbermoforming
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M y possible for some plastics, thermoforming includes the molding of large,
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timt sdy!@ peformgare received8titthe bknwing
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dgid panels, such as refrigerator lining, but also small, lightweight disposable cts such as drinking cups, food packaging, etc. In this process, an extruded plastic sheet or strip is first heated, then placed into a forming station where it faces one or a number of cavities. Vacuum sucks the hot sheet into cooled cavities:to give itthe desired product shape. Occasionally, pressure plugs opposite the cavities assist the vacuum forming process. The sheet with the formed products is.then moved to a trimming station where the grohzcts are cut from thesheet and removed for additional forming (rim beading, where required, a d for stacking features. The web left over after trimming reprocessed. Thermoforming will not be further discussed.
Expandable Bead ~ o l d i n g 1
m o I I a qf,y q q ligbtwe* low ;&epdty ~ f i Z 7 ; @ ~ h t t ; ~ ~ s W~ r 1 ~ 8 ~ g ~ of large products such as storage tanks and containers, hollow automo~ & l e . f d , c a m ~ e r s , @wts siuch as cPrnpIex toys, mannequins (display figures), etc., is achieved using puts, ~ o @ t i & " d e ~ i c ~ ~ ,This , ~ c t cis. an molding. Lmly, there are more and more applications for this a tis limited c a few pi+.ti$ca. very ~dw'pressures are (Readag~. .ladand theno& are re:+ly.@ple and iny$nsiv,e: rotational molding, plastic powder or liquid is placed inside a hollow wbkh is then clorsd; placcdin.an oveh, A d rotated biaxially while the mlt~ and adhems to the inside contour of the mold wall. After a period t.S.7 Reaction Injection Molding h o l @m&g c h n b r , the mold is opened and theproduct is removed. The &&& simple, and there L no external (clamping) pressure involved; 'h%&d foam molding, reaction injection molding traditionally & o b g of f a r e r automotive and furniture components, but it also br beea wed lately for smaller products and strong, lightweight panels. @B
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twctural Foam Molding
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W M b injected with the plastic, to produce strong products with a &ind a faam interior. *;
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Lost-Core Molding
!@&% molding process is used for products wbch require complicated cores cannot be removed using any conventional injection molding technique. ly, automotiveproducts such as intake manifolds, which were previously t fkom cast iron, can be made using this process. aftenquite cumplicakd core is (injection) molded into one piece (or into E M ~ M : piems) ~ &am a special metal alloy which has, a very low $hisaretd J.W@(orcme awerably)is &en placed bto h e n t i a l l y $ ~ $ q iEOM. m After injection amd am& fhe peobuct,
1core, is put into a special aven-\;crberetbme&d is finished product. The metal can then be reused.
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Considerations for New Injection Molding Designs
e product designer should be familiar with the selection and properties of 1; thermoplas~csused for injection molding. This does not imply that the must know all the available plastics in this (very large) group, or be cal characteristics, but he or she should on necessary for making a preliminary ght offer suitableproperties for the product being considered. other) demands to which the product designer can select, basing this choice on past experience products, the plastic believed to be most suitable for the product denttion. In case there is m such precedent, the designer will have a "promising" plastic by comparing properties shown in handbooks such as the EncycZmpedia
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c e~onomically,as well. If it appears at this time that there is no plastic 1;2re s&ified requirements and which can be injection molded, the
rs faced with three possibilities:
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Approach the plastics material manufacturer to find out if there is a wne m@m@ ~5@om-intheSkistings, und such a plastic for the application on tdhmes w h s i d m d will it be while for the resin supplier to create a ction molding, and select another processing it possible, using the selected plastic, to use of plastic for the product, at this time.
Assuming One or more plastics are determined suitable for the demands of the contemplated product, the designer should approach one, or preferably several, mmuf&turers of the selected material to request their latest data sheets B in the selected group. These data sheets will not only cofl~rm for any (or re$ orta taken from the properties charts shown in handbooks or other data bases but d s o may state some examples in which such plastic was used. The data she## &o m y offer some advice about product and mold dosign when using tlkb1astic.h same cases, however, the information on moldability and mold design is n o t k q t up to date and must be used with cautioll. For example, the manufactum may describe results obtained using older types of molding a%,beaware of some more advanced molding techniques. udly in ~oopemtionwith the machine builder ahd the h9ve deuiaed fheir own rfierhocls of pro~ssing,whicb L' 'fiom'becoming widely known, Unless the new product is &a highly c o ~ t i anature, l approach a @der who is well experienced in molding the tyw ofpmduct under copsidwacitp for advice regarding materials sel~ction.This is r m i ~ t r a natn d b i g h l y r e e o m m ~ d step: eventually, the (pmdoct &awings) will e v e to b used to make molds, and unless tLe ghhqit deaigder ii well vsrsed in maid desip, be ,L or she should cooperate with an experienced hdlder and niold dekigner (in tlie appropriate field) before finalizing the product design and any detail drawings. This cooperation can save much time (and frustration) by ensuAng tbat ,h product is not only properly designed to accomplish what is expected but also that it can be molded, and further processed (assembled, decorated, after molding, without unexpected difficulties. In some cases, however, the 1.8
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k; &signer may not want to cooperate with somebody else, and may wish to do all $be experimentation in-house, regardless of the cost, to eventually take the @.$vantageof being ahead of others by creating something entirely new. This can 'fm a good policy if the investment in time and expenses for experimentation
Designing a Product apex and printed as "b1ueprjn~"( w h b lines o n l b b W m u n d ) (blue or gxey limes on white papeg), or as "hard copy," yhich made on a "plotter" that reads directly from illustrati~nss h w n on the
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Graphic Delineation
This is the method used to express the design graphically (with drawings) so that others can understand and use them without additional verbal explanations. Written explanations (notes) on the drawings are usually acceptable, and often even desirable, if it is more practical to explain in a few words that which otherwise would require extensive or easily misunderstood graphic illustrations. But such notes must be short and concise, and written in an unequivocal language. Excessive use of notes can detract from the understanding of a design; the use of notes should be held to a minimum.
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Drawing Board or Computer?
There is little difference in the method used to express a thought or concept for basic design, vvhether the old-fashioned drawing board or a modern computeraided desigh (CAD) program is used. The designer's purpose is to describe the required s h ~ of p the product when committing it to paper or to the screen. However, there is the question of speed and efficiency. When working with paper and pencil, using the most modern drafting machine, the actual drawing time islaboutthe same as with a computer; however, it is faster to chang& erase, and redraw lines with CAD.
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4.2 Starting the Design Process
Starting the Design Process *111ct%*3~
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s i m m l r a vcry time-consuming drafting)'ibbres, such as cross-hatch& of d numbering), and applying dimension lines and If there are drawing and design standards in tex @pgmat@basic desigq teners, it;.; this can take
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starting, the designer must have a complete list of questions about all the ments which must be filled by the product. Typical questions are:
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enerate additional views of the product, to add Might b d l s , if requited a b&&Uu$trate the pmduet. always keep'th&aumb.8~Icg"vkwSa ~ SC&& d to ., views may be re.quM for a &mpl'%hesive
, the operation, and the fuwtioning df my 11scale " (1:1);however, this is not always possible fore, one great advantage of CAD is that it areas of the product that a& difficult to see tter) and paper sizes available for making can easily work to fug scale on any arge them if n&ssiq, and then ,'total illustration ofthe product. smit theprodLtct design data to vided the program and' aomesigner will save much t h e delineation (line we@@, h better.&an*thathone
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Why do we need the new design? What must the product do (purpose or function)? What does the product replace? Are there competitive products? Are samples available? Are there any patents that would be infringed by a new design? Hpw will the product be used? Where will the product be used (surroundings, climate, etc.)? Who will use the product (educationaland/or cultural level of user)? Could the product be misused, creating danger or causing injury to $be user or bystanders? 4s @wproduct disposable or "single-use" (throw away after one
,Is@e product seasonal? L, B the product a promotional item (one time only)? $ t&e product expected to have a long time, frequent, repeated use? BQW m y times will the product be reused (100, 1,000, 100,000, millions, etc.)? e (fqish, colors, color match, How i m p o w t i s pr 2,- \s ""II! %I $@prqduct utilitarian or artistic? mlqtis the apected product life (time to product obsolescence)? stmoldi mold in$ operations are required (decorating,machining, ak.? factors will affect product life (wear, corrosion, erosion, hot or ~old'tem~eratures, chemical action, electric arcing,
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$e&l.qhacacteristics are required, such as electricresistance. optical & t ~ (hapsparewy), etc.? a h b emade (molding, blowing, etc.)? ba wde ecan~mically? &se&ly, pchagitlg, stacking, nesting, etc., will .
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wpik &emtion (geB3f - p q d d y Yqyinting)? How will the product be shipped?&a%& ~ p & m n - & h u k ? What is the expected cost range?
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When is it &i;e&iito be in produc?ion? Can the prcsduct be recycled? Does it require identification?
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has proven again and again that this method is fmter in the long run and better results (better designs) than any other method, bwauseit does catch ma1 emrs early in the project and t&es advantage of the fact that several e usually better than one when it comes to creative ideas and their
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*$&e design work, the easier will be the actual designing process. Conscienwvgly using the answers to these questions will greatly reduce the design work.
' It should also be made clear to the persons (the client or the boss) who can
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%ly the answers that these questions are not asked out of idle curiosity but @Getuse the relevant answers can have an important, even decisive, influence on aesign. There is nothing as embarrassing as finding out halfway through a roject that some of the given requirements or answers to these questions ng or incomplete, or that some key or even lesser requirements were d or their importance was not fully appreciated. r the need for a new (or revised) product has been justified and all ~ d m e n t have s been established by answering such questions as shown there are usually a number ofdesign possibilities which can be immeexcluded for not fitting some of the above requirements. But, there are rge number of solutions possible which all could be acceptable. 'l i%& is where the digerence between apoor, a mediocre or a good designer clearly evident. A poor or mediocre designer will, as arule, contemplate em, then jot down the first possible or practical solution that comes to tinue to elaborate on it. This designer may be right in choosing the ,but, historically, this is seldom the case. esigner will contemplate several or sometimes a large number of ible design solutions for any problem, and scrutinize each of them, ne whether they all satisfy the answers to the questions asked. A good ill also consider whether some portion of certain ideas in one layout could be joined with portions of some other sketches or ideas, to em, before settling on any one specific design. so the mark of a good designer that he or she will unhesitatingly submit f a &etches, and drawings to peers, a supervisor, or others who might have ng to contribute to the design and who could challenge the proposed Tt is a common experience amongst designers that it is very difficult to % own mistakes, while it is surprisingly easy for somebody else to pick epts or in the delineation, and/or to come forward with new else's drawing has been viewed. It is a well-known fact that ad gFLOt2* (by others) is always easier. Comments should never be c~Widere;da personal slight or a "nasty criticism." Wthod of cooperative design using "concept" and "design" reviews is ~rab&1$the best way to arrive at a good product. These concept and design r w h ~ ms y at first appear cumbersome md time consming. However, impI
s does not mean that the original designer should do a hasty or sloppyjob
". . . it will all -be taken apart by others anyway. . . ." After stadyi~~g the
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ing rquifments, a good designee wi11 initially came up a number of d can then defend the rationale behind each fdw pesenkd mdargue the of sQggeshns and criticisms; by o%hers,Hawever, the &signer must receptive mind and riot be "defemive" while his Q"S her designs even c o n d e m ~ d . &signers have often been disappointed or discoaraged aftex m from others, while fighting for their own ideas. In the Emg run, they will became much better designers who will have learn& what ~reatinga good design. hdatries where a number of designers are availabh t t ~ do similar prwtice to altmnate new projects between~~@de-signers b m dl rtctivsly p&cipm in &sign reviews, even if they are not " rn pne~mtingdesigner: Again, this will be to the h e f i t af all
g plastics, the designer must understand that plastics often behave m nonplastic materials, particularly from metals. Plastics are often E to replace steel, other metals, glass, paper, etc. Frequently, one type of k,will ba mplacetd by another type which may be more suitable for the k t , less expensive, or easier to convert (mold) into a product. will not di~cusshere the whole subject of characteristics of plastics but &a the deSip5;~to differences in the behavior between plastics and other &kdrs1used -udy for a similw product. If a similar product was &adc from a mrtain type of plastic, for instance, there is usually no @Wl&i . u&qg th@ or a rd&d material agtxin.
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of manufacturing- in this case, injection blow moldingthese obstacles disappeared. During the injection molding step, the rim and the portion not to be blown has an easily moldable thickness. The rest of the cup shape, which will be blown in the second step, is much thicker. This facilitates the first step by filling the thin rim through the heavier body walls. (Before, the heavy rim was filled through a very thin side wall, a condition which should generally be avoided in molding any plastic product.) After being transferred to the blow station, the heavy side walls and the bottom are blown to the final shape, resulting in extremely thin cross sections. While blowing, the plastic is biaxially oriented and thereby loses the brittleness formerly experienced. Even though it is now much thinner, the plastic is not brittle.
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and mre soj when it comes to dispmable items. As long as the p
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This example is given to highlight the necessary cooperation between the P d u c t and the mold designer, the materials supplier, the mold @er, the molding machine builder, and the molder to design a product which is more than r simple run-of-the-mill article. Typically, such cooperation applies whenever
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deiigner m h t consider s&eial facts. First, b9&$pr . ,y ;mitht:be ak&eoftthe 1imits:set by:~leinjkctiotipqi~vb~si
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the aregof injection capability. T,h&thiang$the I c !7sy 4 pmdubiwallb, tlw hi&er the ~ j e c t i opi r e s s u s ' ~ o n id ~~ds8';3equired, to fill the product withqaCshitable,$&j&at ;d iwill r e ~ a i ~ ~ ~ ~ ~ & ~ pixpose.~ , n o &&isie;'4&ing u ~ h ! [ ~ ~ ~ h ~ materials wi11 improve t$e'holding &a~abi$tyi-~Mti-'&&~e is h , ' -t , a danger 'of losing ~e~ui&?~rodugt ' :Jifi@er&psin . i costisalsdafactdr. k' .i Yn,V ' i A similar container could, hoderu; b i prodtke$u,&ng , an entirely different prcxek~rbimn~f~smingFin #$ich . ' ?\'b sn extrudedthin, wid6 strip df PP i% rehep&d bforeentq~ng tbe fo* station and sh& in a multimvity f ~ d n g I mold intothe desired shape. Afterfomhg, the stripadk&hPs ' b a trimming station; frornthere the productsare convkyed , &tHaelcing;ete. The unused Rortionof the strip (the Geb) ,- ib &en up and can be tecycled. , I
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drawing used will be slightly different for injection molding than for @ermoforming, even though the main dimensions will be the same for either
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e also very important: is more or less uniform molding speed, and the vihgs in the cost of transporting the raw ds, the cost of recycling, land fill case, ete. costs represent savings in energy (electricity or oil) and raft increases when their s is reduced. The lighter the structure,the more efficient it is, and
@eater are the possible payloads.
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,i%2 in&fy c51sc%i as good as injection m~lding,and is co d ma^, Bowever, be aware that there are certain
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nedmigping to replace a product previously made from other materials, P-1, brass, aluminum, &c or magnesium die cast metals, or wood, etc., W1:muat understand that the characteristicsof plastics in general cannot . s b p l y by comparing and extrapolating common b e n c w k s $ u ~ h
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d80, in general, not dqendent on the length oftime during which the p stressed. ~ l bthe, tensile strength of materials sukh as steel is defined very narrow range of values, and is quite repetitive. For most (conv inaterids, the yield point, the elastic Limit, and the proportional limit are very c l ~ toe each other and are often used interchangeably. For most thermoplastics, the tensile strength will vary greatly with chungeg kt ktemperature (see Fig. 4.2).Also, even for very similar plastics, the vaQ$ far &mile sueng€h can vary considerably, as can be seen in all charts and @& she* that show these values. -4, Pa~g;Eastics,t k length qftimfheprduct is stressBQ is very important. If the plastic is stressed (within the elasticlimit), but d y f o r a short time, it will retun ko i@original &ape. If it is stressed far a long time, even well below the elas&&limit, the moIecular m g e m e n t within ttre product will gradually c&& ar ''~mep," so that w b n the stress is eventually removed, the product will no2 ,return to its original shape. I
I
A & b p m t & e c t of this behavior (creep) is that a thermoplastic must m t be u& ;&se~ p ~ h ifgit requires even a small amount of preload, that is, if the plast prbg in its mt position remains stressed. However, it i8 quite p f c a 3 . to m e m ot'the elasticity of any plastic if the spring portion af the product is
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Prooortional limit
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together by the screw), the plastic female thread will be under stedy load an@ will gradually creep, eventually losing the force holding the two contact area6 together (see also Section 5.2, Screw Assembly, p. 88).
,
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I
Designing a Product
The relationships showing the increase in strain over time (or loss of elasticity) for various plastics are available in Engineering Properties of Thermoplastics [2].
The effect of humidify also can be significant with certain plastics when the mbanical strength, and in particular the impact strength, depends on the mount of water absorbed (i.e., a typical example is nylon). I
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I
4.3.2.2
' I
.
Compressive Strength
Nexural Strmgth ' -
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As with most metals and other nonplastics engineering materials, the compressive strength of plastics is about the same as the tensile strength.
a x @ m ~ n g t h refers to the resistme of a plastic against bending. In fact, it -1~ted b & e tensile and compressive strength of €he test specimen.The figures in ~ ~ m m o ~useful t l inycomparingthe relative stiffnessof materials.
4.3.2.3
4~'):
.
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I
MbdUb'ot Elasticrty (Tensile or Compressive)
6 As with most other materials, the shear strength of most plastics is about 4040% of the tensile strength.
4.32.4
I
Shear Strength
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Tke r n ; o d ~ @ ; $ (~E ~) i~& expressed ~ as the slope (tan a) of the s&dstr& mme u p & $ ~ & ~ c ] i p 6 ~limit o ~ a(see 1 Fig. 4.1, p, 27). The stiffw the mateAd, &d w a k ~ d b m .FOP steel, .on average, E = 29,000,000 p i , pra&ally IM'lm+ind tempemre range. t$"f&'& m g pla~tL,such m unfilled polpat.bona@I&@%&@ psmppo* dm values &$rE &mile)d 345,QQO psi: E ('t:aw@ive)=338jO@ g$& and E (iI~wztl) = 340,000 psi at room temperature decrease (or change) to back to c m CI. ' t . .#4$000 at 120 'C (250 OF). The designer must always refer
Impact Strength
The impact gtmngth of plastics varies over a wide range, depending on the type of pkwtic and on its molecular structure. Every material data sheet shows the irnpmt sltrength valws, together with the method that is used for the tesb. Typic& ~~g methods are Izod and Gkfupy geis& (ASTM D-256),tensile imp&,&st~ (ASTN D-18222, drop tests, ftdljng dmt twtsl, etc. ,When~ompming vaiou&.glastics, it is i m p m t &at thelbesigtzer c o q m ooly those values whiefi have been arrived at using the same metbds of tst&~g, , 3 % same ~ &sign r u b apply fis for m % d s or o& @?q$msringmatdals. Noses md hales, ourface finish, my cbmges in watl &.ic:lsne;ss,etc., must be carefullyoonsi&rd to minimize the damage they eat ewse to tbe strength (life and pe-rfomm) ols the product, Some:pltufics are W m n t l y name rmismt to impa~tthh others; s m e uwy mistant, while othersicwJxvery brittle. Fillers, such as glass a carbon fibrs, can add considerably1to the impact stragtil. " The &mperMve ,dur&g, ihk operation of the pmduct will have a much @eater M w e on the perfohanax of the product than what is nomsst.y g m t e d with a f b r en&ineeri~gmateridg. A plastic that may be suitable fa .t@p&cr8,~3rwrnrbmpera~r@ (252 OC ar 70 OQn~siybe uiieless at st &mperature
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s, scratch and abrasion resistance, and wear (friction) properties are ristics which can be important for the selection of the most suitable purpose of comparison, the designer must various charts have been produced using the
ummary, regarding mechanical properties, it is important that the scrutinize the material data sheets for all peculiarities of a plastic before a final specification; additional information on critical properties t] may be available from materials suppljers. Some of newly developed data, and it is up to thedesigner to find s whatis availablefor the appBeation. The
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It does not make much sense to include property tables or .charts of all in h i s book. There is ongoing demlopmnt of cmpletely new and difid resins, and engimring information which was aeonrate last y e a m y be superseded now. Usually, the best sources are up-to-date data sheets issued by the materials suppliers. However, charts shown in annual publications such as the Encyclopdia of Plastics [3] are a good start for directing the designer who has no idea which plmti~to selwt w b there is no precedent. When cantacting material s p i a l ; Ccompting) suppliers, the d e s i p r can request the latest data d w obtain additional infomati~nabout newer, potentidy more witable, materials. The designer must always keep an open mind. The fact that a material has bwf~us& a long time for a certain application does no&fl;ecesady mean it is the . Wst choice, even if it was gtcceptable in the past. There naw m y be newer, better tesisvailable, m y b e at lower mst, or with easier . p t m B g advantages in the areias d calors, ~ m s p ~ m,year, y, ,;, Q n l e there ~ is a mmpelling euonomkql, sdes, or engineering maon, it is sary to revise the materid far an e;jris&g product. But a,new to replace an ol&r one (m to @ompetewith it) &ould certainly a o&idateffor any ww mterid available.
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The importan'ce of knowing how the product will be made was stressed earlier in Chapter 2. Typical methods tbday include:
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Molding, blowing, forming, casting, and d!".vrrc machining from the solid?among other&? ?
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Physical Properties
f i e same comiderations listed above under mechanical properties apply here. m i c a l physical properties are: Gsefficient of thermal expansion, Me+aion tempmature under flexural load, : t'&&al couducl$vity, -. , d p c wphoperties (conductivity, dielectric strength, arc ns&,
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Processing MeZhods
4.3.4
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4.3.3
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t@vabx (cald, hot, stem), :to s d f w b . such as hydrocarbons or other orgaoic pu
As discussed earlier, the cost of plastic can represent a very 1
.
usual11 plastic surfacc tolerar specifi moldir
:X molded piece should have uniform thickness throughout. Un cas the qui
414 ,%&ematie drawings showing product shape f ~ arround ~ontaimr,such as k d e (left) or a laundry basket with a: h g e aead opmings (right)
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3, A dmp container (e.g., a latgepttil with a fhs> .or a l a ~ & basket Y pig. 4.4), ttc. The outside surface consists of the bottom, the sides wall, and any flange extending at the rim. 4. i?ayen&bsure,such as a l Ycabinet, housing for electric hand tools, - a.The owBM@k3ym EmwdI>of ;in gemad, it m y have an
@tm' araredthe above, the designer must also be aware that, use af: numerically controlled (NC) machin for manufactuirring if the shap
iebess is possible to achieve but is rarely the case; in fact, in most w s s i b l e . However, the designer should make every effort to make % inmy area of any product as uniform as possible. The reasons are e (refer to Mold Engineering [l] by this author):
z &k.er the plastic, the slower it will cycle in the machine. The &@stporti~ncontrols the cycle time, since the product cannot be &&amthe molding machine before it is reasonably cwl and lr p.@ ,tl d e , the increase in cycle time is not just linear with the b1$ but$ 2greater. In other words, twice:the wall t h i c k n ~we 8 e more than twice the cooling time. (Hovt m c k more time gmiiy ern the mold &sign.) a d r heavier swtion is .greater than that of a: "du'tlaar have wkis (empty spaces Mi&) and sdW +ws wp&&].T i s , w d i t i a n , w ~ r s eqe*4yy& m 4
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Figure 4.5 (next page) ifl;~trates the ~&c?&d the meanlng o?a v o ~ dand a df:&. Note that while sinks and voids occur mostly in plastics with high b k a g e , voids can also be present in thick sections of moldings made from @istic with low shrinkage. The effect of varying shrinkage within oneshape can result in &wer m l d h g cycb times, warp, and loss of dimensional accuracy and stability of the pm&art:t &br molding. It is p s i & t@ mQld au&
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4.4 Product Shape
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Rule 2: Make sure that the mold can (and will) be gated so that the thicknessof the plastic from the gate toward the remote areas either remains constant or diminishes. t this requirement, the product designer may want to seek the assistance experienced mold designer.
Pigure 4.5 Shrinkage during production may occur as a void (Ieft) or as sink marks
h?gfiF"'4.6 Various tiansitions b&eeti thicknei6.kes in i~ product: A. "notch" effect, a sev,embwss riser; B .a small radius at the:c d e r , C i4 large radius at the cmih, a d D.
37
,
a $bpe with a large, unnwhed radius t'.
4 1
I
haovy sections if they are necessary, but it will req&e special c k @ the selection of melt temperatmes, cooling temperaqs, and I, injection d hold pressures, and will usually result in slower ,V mdding cycles. , Bvery transition from thinner to heavim thickness creates stress ,Fs;s~esand an inherent weakness in the plastic prodpa. This is I , a ~ p i d l serious y if the transition is r d i ~ a lsuch , a~ a. step.
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e extended accordingly. there are often reasons why it appears that large changes in we unavoidable, the designer should still consider whether they can be hioid'd. Consider the following examples (Fig.4.7):
igure 4.8 Excessive thickness in a product (A) can be eliminated by coring out from the &wity (B) or the core (C) side of the product
The shape shown in Fig. 4.7A may be required for the purpose of the product, but it is obviously not a shape most suitable for molding, nor for equal strength. Instead, the designer can choose between Figs. 4.7B and 4.7C, according to the needs of the product. In Fig. 4.7B, the thickness remains more or less constant, staying well within the 25% "permitted" before affecting the moldability and product strength. The silhouette of the product, however, is changed. In Fig. 4.7C, the silhouette of the product is maintained, either by addling flanges at the ends of the step or by adding ribs of the shape shown. Similarly, excessive thickness can often be avoided by coring the thick section from one side or from the other, whichever is more suitable for the product (and its appearance). In contemplating such coring out of any heady( section, it must be a basic consideration for the product designer to avoid a waste of plastic, to reduce the molding cycles, and to avoid excessive, uneven shrinkage, as well as sinks and voids, to save power and to increase productivity4 Figure 4.8 shows two typical examples of coring. Obviously, Fig. 4.8A is bad design, since the change in thickness is far more than 100%.In Fig. 4.8B and 4.8C, however, either design is good, and the choice depends entirely on which better suits the desired product and, to some extent, on the appearance. In most cases, coring from the core side of the mold (the inside of the product) is preferable. The effect of coring will also help to keep the product on that side of the mold from which it will be ejected-usually, but not. necessarily, the core side of the product.
h
&tare 4.9 Schematic drawings of a shallow, round dish show plastic flow with center
@ting (left) and with edge gating (right)
-@3bviou@ly,there is an infinite number of design possibilities where these pmb1Rm.t~ can occur.) w, round dish is shown in Fig. 4.9 with a uniform thickness at the e rim. This is an ideal condition and is often possible in round etc. gating (Fig. 4.9 left), the plastic will flow uniformly from the mia&er toward the rim. This is a preferred gating method, but it is not dways Weptable; &rmmple, if the flat bottom must be free of any gate vestige, center gating (Fig. 4.9 right), the plastic will flow approximately as &strated by the mows and will eventually converge at the edge oppmite the m.T h m h no pmblem with venting. inFa$J.lO, therim is Uliclrer than tbc bottom. In a good design)W ahadd, often-¬, be avoided. The maja two mullant problems k e ~IW@I&?~.
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4.4.1.3
Flow Path
Rule 3: Plastic flow in a mold always takes the path of least resistance. The following two sketches (Fig. 4.9) illustrate how important it is for the product designer to understand the flow of plastic within the cavity space,
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~ithcenteap~~.4.1~~eft),~eEi~~~~wissi mdthe plastic ariU B mu ~ ~ l r m mwwd yy &.dm Cw$i@
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Designing a Product
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41
4.4 Product Shape
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Figure 4.18 Schematic &awings of a shallow, round dish with a thick rim show plastic flow with center gating (left) and with edge gating (right)
Edge gating is the worst possible flow path: the plastic flow always takes the path of least resistance and circles the thinner center before starting to fill it. The plastic will finally converge toward the center and, unless there are provisions in the mold (spot vents) to allow escape of the air trapped by the converging plastic streams, there will be a pronounced mark (a hole, void, burn, or weld) at the point where the plastic finally joins. While the examples here just show a simple, round dish, the same principle of the injected plastic selecting the path of least resistance applies to practically every product. This problem is not always as apparent as in the examples above, and often will be noticed only after also being overlooked by the mold designer. Eventually, such oversight shows up as an unfilled spot in the product, or as an unacceptable surface blemish. By that time, it is often too late. Although it may be possible to add some vents to the mold, change the gate location, or add more gates, all these mold changes can be very expensive. Some typical examples of designs that avoid heavy rims or any other thickening at the end of the flow path are shown below (Fig. 4.11). The end of the side wall, at the rim of any container, whether a drinking cup or industrial container, should for practical reasons be stronger (stiffer) than the wall itself. From design practices for other materials, typically metals, the first thought of the designer would be to increase the thickness at this spot. But with plastic, uniformity of thickness should be the first guiding mle; ifsecessary, narrow ribs (see arrow in Fig. 4.11) can be added b mangtka the rim withut increasing the thicbess.
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4.4.1.4
Parting Line
Rule 4: The outside surface should always end at the parting line. The "parting line" (PIL), which is actually a parting "plane," is the surface (area) where the cavity and core meets, and where they are clamped together under the locking force of the clamping mechanism of the molding machine. Designers must understand that, whe%r the cavity is gated at the edge or near the center of the product, most or all of the plastic will flow toward the parting line. The air that was inside the cavity space before the mold was clamped up must now escape to 1) permit easy filling of the cavity, 2) prevent burning of the leading edge of the plastic, or 3) eliminate the possibility of unfilled spots forming in the product. Venting at the parting line is no problem; therefore, a surface ending there can always be well vented using simple methods well known to the mold designer. (Unfortunately, the same cannot be said about venting for ribs, hubs, etc.; this will be covered later.) The shape of the product, at the parting line, needs special considerations:
,
F i s r e 4.13 Parting line for a rounded edge: A. ideal, B. and C. mismatched
1. What shape is required for the product? A sharp edge? A rounded 2. Which shape is easier to produce when building the mold?
In Fig. 4.12A, the wall surface ends with a sharp comer (S)on the outside (;' the parting line) and a radius (R) on the inside. This is a "natural" shape. ~ l i
a natural sharp corner (S) is created between the core and the insert. Fig. 4.12C, the design requires a rounded edge at the end of the side wall,
such as air blow off or mold wipers may then be required to ensure that s have ejected. There is usually no problem with heavier molded pieces mighing maybe 20 grams or more. possible between the cavity and core which can result in a small but sharp Figure 4.12 Three different product shapes at the outside edges: A. sharp outer edge with round (radius) inner edge, B. sharp corners on both inner and outer edges, and C. Round edge8 (radius an both inn& and auter edgm)
10
core, and/or the stripper ring.
cult challenge is to foresee where mismatch can occur, and to e how to dimension the product.
4.4.1.5
Parting Line Selection
Rule 5: Wherever possible, design a product so that the parting line can be in one plane.
Xn most molds, the P/L is at right angles to the direction of the opening of the -Id, as was shown in the foregoing examples; this is the simplest and best condition. For a good mold, the parting line should be ground on both the patching faces of the cavity and the core to avoid any "flashing" (escaping plastic film or burr) of the mold during injection. Such flash must be removed $€er molding and can require very costly (manual or mechanical) operations; &refore, it is least expensive to provide a well-fitting parting line by grinding. ,; . With some products, it may be necessary to have a parting line which is at &nangle to the direction of the mold. If this is unavoidable, it is still not to too @%pensive as long as the P/L is in one plane. In the example shown in Fig. 4.16, +
the core insert can be removed for grinding. If this was not the case, it would b~ more difficult to grind this P/L because the core projecting above the P/L woulc interfere with the grinding wheel. In the earlier days ofmold often finished bv hand. usingu u matching faces until a good fit was achieved. This is slow, costly, and never a good as grinding, but it may have to be use$ even today if there is no other way In some instances, electric discharge machining (EDM) is used. After on1 face is finished, it is used as the electrode to finish the matching surface. This too, is slow, expensive, and not always possible. 18 Three corner shapes: A. restriction to flow at a at a comer with radius Ri,and C . rounding of both
4.4.1.5.1 Venting at the Parting Line The venting at the parting line is usual1 easy; to achieve a uniform vent gap large enough to let the air escape but sm enough to keep the plastic from flashing, vents are usually produced by care grinding after the P/L is finished. The d e ~ t hof a vent is 0.005 to 0.015 m (0.0002 to 0.0006 in.), and occasionally somewhat larger, depending on the location of the vent and the viscosity of the injected plastic. The product designer who wants more information on venting is ref 7ld Enaineerina " " Tll. also bv this author. If the parting line is offset, which may be necessary for certain products sucl as cutlery (e.g., the shape of the spoonor fork handles), the grinding of the partin] line and of the vents becomes much more difficult and expensive, especially i the core ~roiects line shown Fig. 4.17 seem . -. above . . - the . - P/L. . -. The - -- .offset - - - - - .~arting. ..-.-----.-.. -- in -.. nple, but it requires very accurate grinding of all planes indicated both the cavitv and the core. to ensure "~erfect"fit. In summary, while it is possible to make any reasonable shape of parting line the cost of any P/L not at right angles to the opening of the mold, or of any offset will be much higher. This is an important point that should not be overlookec during ~roductdesian.
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Figure 4.17 An o&et paning line has many planes
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r-------- . -u $&&d out by arrow "S?'Such changes in direction occur frequently and must , k aref fully considered before finalizing a design. A sharp corner should be always avoided, for several reasons. In Fig. 4.18, different comer shapes are shown. The sharp comer in Fig. 4.18A has no s, so the plastic will flow over the corner and create a restriction behind the , which creates a pressure drop, affecting the filling of the mold further stream. The void shown next to the restriction will disappear as soon as the is filled and the plastic pressure in the mold rises. Fig. 4.18B, there is a radius R, on the core; this radius should be large h to permit easy flow of the plastic. However, the result of such a design kening at the comer which can result in more shrinkage at this point, sink marks as indicated by heavy lines. Also, unless the product really s a comer shape such as that shown in Fig. 4.18B, the material there is .In addition, sharp inside comers in any piece made from steel (or any rial, including plastics) are bad for the life of the product (or mold part) notch effect created by such an inside comer. minimum inside radius R,is suggested to be between 0.5 and 1.0 times the thickness t, but this may not always be possible. See Fig. 4.19 and Table 4.1. -
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4.4 Product Shape
Recesses can be defined as depressions in the plastic layer. Their defining length and width dimensions are usually larger than their height, and they do not, as a rule, present moldmaking problems. However, recesses can cause a restriction in the plastic flow that may split the flow path and result in backflow, as
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51
T b inside of a hub is a special type of recess. It is important to consider how the plastic will flow to form the hub. Depending on the dimensions of the hub and t* & wall or rib, and on the location of the gate, the flow may split and c&verge again at a weld line (W). In Fig. 4.21, the weld line is in a poor location because it weakens the plastic there; if, for example, the recess is used to receive a s c r w and then is stressed, it may burst along the weld line.
marks, and possibly may even trap air and cause holes in the surface. In fa
4.4.3
Holes and Openings
holes are usually round openings in a bottom, side wall, or rib; they can be small d large. The effect of any hole on molding is that the plastic flow will split and fhw around the core (or core pin) creating the hole. Where the flowing plastic led the "weld line" nes can cause 1) a physical weakening, because the joint at the weld terial; and 2) a poor uch as increasing which will add to
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er, however, can also prevent weld lines or reduce their e flow of plastic by relying on the designer's own ow analyses for different shapes and conditions. The cation of one or several gates, and select the shape of ses, flow paths, radii, etc.) so that weld line(s) will use the fewest problems, both in strength and in
Figure 4.20 Sufficient radius at the base of the recess facilitates the plastic eld line could be
4.
Figme 4.21.Poor pWc. flow inside a hub results in a poody located weld line (W)
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Holes in the~ottorn(or Top) of me Pioduct '
There is usually no problem in creating a hole in my portion of the product that is at right angles to the direction of openilvg of the mold. Small holes ar openjngs are often made with core pins; larger holes and openings can be machined right out of the core (or the cavity). The advantage of core pins is that they can be w i l y replaced if damaged. The disadvantageie that they must pass through larger parts of the cavity or core where they are located, thereby hindering tbe cooling layout of these parts, Ariother disadvantage is that core pins ate not w ~ lcooled l unless provided with their own, internal cooling, which is more expensive. 'Where the cores for the holes areleft standing an the mold part, their cooling is almost a good as the moM part itself. Danag~; &Qsuch cores, however, requires either welding or replaceznmt of the damage9 pa$. Why stress this subject, wb;ioh is really a moldmaker's problem? When spcifying smdl opeqings, the designer thould think of how they are going t~ be made. As a crass example: Imagine a sieve (or afilter) which requires many holes (Fig.4.23A). 1f hese holes are specified as round, there are two possible methods for creating them: 1, Create the hdes using many small p r q j h g pins (Fig. 4.23B), passing though the core (or the cavity), which waLlld make it virtually .impossible to place any cooling lines there. This k not a , gotxi idea if high productivity is expected. (Note that this m e W is quite easy in metal stamping dies.) 2. The round core pin (studs) could be left &ding frbm the steel (Fig, 423C) of the core (or cavity). This would be good for high produc@vity,since the mold part could be w6ll coaled, but the gee4
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4.4 P r h e t Shape
m n d the small studs would have to be removed; such a core would be very diffiicult (but not impossible) to machine, and subsequently polish if the spacing between these studs is very close.
6 . t 1 3 e s a w example, the designer could decide that the product would work j u % s well for its intended purpose if the holes were square, rectangular, or triangular, shapes which could easiiy be generated by milling or grinding t l ~ top of tlm w m (or cavity). Rectangular slots as shown in Fig. 4.24 could be even smaller 'than would be practical with,mmd holes. This shape would then allow an easy method &f manufacture (miWtegrinding, and polishing) and result in pan w h i ~ hcan be well cooled.
~ ~ t l s aHoles t l n g(or Openings) in Product Walls
4.4.3.2
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The pro&ms w c ~ c i a t dwith creating holes or openings at right angles (amy angle) to the mald openin$ mation are quite'-different from those p v i w s l y described for o p e a w makb using only tho simple (opening) 1&an ~f b mold. The corw $vhi& ae@e tHe apmbga from side premt product ejection. There are di&m&hod~ wb.i&, iiaxmbgiy, @i affect ll the product cost either by increasing the mold cost, or increasing product cog by ! . slowing the mold cycles or requiring post-molding operations. The designer must not forget the main goal: the product must functiaaqrequired but also must be produced at the lowest cost. Where low productioa quantities are expected, therefore, the manufacture may be planned so that s o m side openings are not molded but instead are drilled or stamped after moldings The savings in mold cost can easily outweigh the added cost of machining. A decision to select this method can also affect the shape and size of the opening. The product drawing must indicate that the opening will be added afFr molding, and must describe how this should be done. For mass production, however; the designer should endeavor to mold all openings.
v6rtical (as shown in Fig. 4.25) or at some (small or considerable) angle to the vertical. Any size and shape opening can be produced. The resulting opening can have sharp, chamfered, or rounded edges on the outside of the wall (Fig. 4.25A-C). The inside, where the side core meets the main core, will always be sharp. This method is frequently used and is based on either cam-actuated or hydraulically operated slides which move the side core in and out of molding position. This motion can be at righi angles to the center of the vertical axis of the product, or at any (reasonable) required angle ,in which case the mol cost would increase even more.
I
F i p e 4 . 2 6 Schematic of a basic mold with a moving side wall section shows movement &f various mold sections
Figure4.25 Opening shapes created by side cores: A. sharp edges, B. chamfere and C. rounded edges
.
4.4.3.2.2 Moving Side Walls A moving side wall is a section of the cav which forms the side (or a portion of the side) of the product. The section ei moves straight sideways, similar to a side core, or it slides up (together with the withdrawing core and the product) and outward at the same time. This comp motion (up and out) frees the side wall from any projections that are form that section of the cavity and from any cores that have openings. Figure 4.26 depicts the basic principle of a mold with a side section, in this case creates an opening and two projections. (For clarity, mold details such as cooling, venting, and the method of guiding and actuating the mo the side section are not shown, nor are the ejectors or strippers that remove the product from the core after the mold has fully opened.) The main reason for showing this example is so the designer can see how openings and projections in a side wall can be created, and where draft angles, radii, and/or chamfers are required to be shown on the product drawing. If these
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,while a radio cabinet may require only one. The designer can underaw ~omplicatedand expensive such molds can be.
ithout&& m
a or moving side wtions. The only condition is that
a Angle of shutoff fromtbtle mthd 19 D M & @ t
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r;tt the mviqwall: A. simple ejeetidn.bdt mppeaIiwwL @d W s m b & q w e~tk.def~&~aof~p~as~c J ~ , I \ , I -lf'j..h,
Figure 4.27 A vertical sbutoff creates an opening in a t a p e d wall
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the @gb of the side wall with the we&id, @eeasier it is to ,ma~&b ~il,
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Ljected (in the dii&$w of* $harts the cores which gknerate'~eside openings. The shape of the o cdarly appealing,however, with the sharp edge on top and a p attom. This method will work with any plastic, soft or hard. .2$B is a variation of this methad. Radii rare shown on Both ends ,which is now more appealing. There is no problem with the radius will now interfere with created by the radi and fhe smaller is the resistance
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M cost,&,a~,eygtuallY9 the p ~ u cmst t Note t&yt&e, j i4~8tr:ationsiq Figs, 4-29 and nd4e3Q,are only schem,tic,i; es an4pIcjf~#1qeshave been omjned f@ clarity. The mgle af #?&-
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own aIsDLFztte& ~n angle o f ; ~ 3 % is usW#~,s * ~ + p t . 3.efid6 6 a vidica~RIXI A condition i d j - siniilar PO thit f ~ r in up~f&cie rtical ' w+ e w R b 9 +p ~ @ g gahok,.i-&esi& ofarib. to p a k & a&& is often not pracricd6: my be ia9wgBitle Fw
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$sJll tp m a @swh a hole could be placed inside ojp40xA&ominFig,4.53 B Q ~ @ F84.' ka&ldbP w sli& zxdd csrrnv a core pin i~crnk@&
h are quite common1
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Extension of waH
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Outer surface or "wall"
Y,32 The outer surface extends or "projects" beyond the flange of this container .; '
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k e main problem connected with any projections is that they are accom-
@I &t by a thiokening of the plastic, with the following, sometimes costly, k3 ,' , Greater material concentration,
;i' 9 longer cooling cycles, and , less homogeneity of the plastic; therefore, less strength.
Reinforcements (Ribs and Gussets) I
$r,gussets are used to reinforce or stiffin an otherwise weak or flexing
pn The method of calculation for the moment of inertia and section b a r e basically the sameas that for any other machine or structuraldesign, @ f-;ormuLe used for strength of materials (shown in any engineering !&fa%) can be applied, except that, for plastics in general, the values for the !&w of elasticity (E) and the values for tensile strength and other properties, b&b lower than those for engineeringmaterials such as steel, duminum, etc. E$rWuesem be found in data sheets for the various plastics. b4&wigger will dtarrt with an assumption of the plastic thickness suitabi2e %iBb&nQed shape. If precedents exist, this is u s d l y no problem. However, B S i % ~ ! h ga de,sign "from smatch," it may be diffkult to determine an W k m w for a ~hapwb3chhas never been moMd N o P
verv smoath..can
. , . . . ..' . concerns. Of course, this may result in a pMtict that is'not st;rong enough to satisfy its icr&mgtbrq&e.ments. In same&~tms~ a B m W mock-up.&&ned from the solid or an actual molding fim an experimental mold can ~ Y M Bfaidy - ~good idea. of the
[email protected] iiidicate where reir$f&c&nhts are r e . The other advmtage d a ~11~ is t that w the function of b product can be better evaluated expensive, but is often from a drawing only. This m& be prefer& to buiIding a produ@on mold bnly then finding out wher6 'the weak spm are. i ! The modern alternative b a mock-up is-@-&avea computerflnite elements ambysis done of either the whole product or the critical areas only. This, too, can ~ c d u c t for s wM& an 1 be quite expqmivg hut w.iQ& xrmwsiw exprimental mold Or htmdrmde r n h p m y he nat practical: There are professional services and compumr programs available for finite. elements analyses. An ex@nced&m 3 1 9 agyrmch ~ the pchlem of stmngth by cdculatin%~ d timay whkh!meheavily stressed, wing methods similar to those used fox rnwhim or s t n z c a ~design, l and otherwise going by "gut feel." This m e w is frq~entlyPS& but may lead to "erring an the safe side" and o-vdedgning, phich'ofk~ yields a product that is too heavy or too complicated. Another pasgibility is to solely depend on the experienceof the designer (or groups of design&@who may have had experience with similar products. This may work in many eases, but there is still the risk of overdesigning and ending up with a product which, on second thsught, could have beenmqde lighter or less complicated, and therefore less costly. The designer usually has two alterna$ives: either make the plastic thicker 'i(heavier] wti1 the recjuired strength ar stiffnessis obtained (which is expensive), or add ribs or gpsets in strategic locatim where they achieve the same result as adding plqtic aZJ. over but without adding much mass when compared to the . originally @lameidprduct, where the wall was too thin and the product not strong b&ugh. Qf cotqe, there may bbe; the ~ ~ ~ swhere i a nboth ribs or gussets and thickening of the:bottom andor sides will be unavoidable. But $he dw@m will then hderstand that the piece will mid slower and cast more in prodaption. L
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4.4.4.2
Venting
Befow going into mote detd9; iz fs hp.ortrtnt that the designer is aware of the problems associated with venting in a mold. We have already discussed venting
of the requirad sliding fib, acts as a "natural" and "self-
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Before m@iming, we will refer to R a e 1,stated earlier on p. 35, and p v i d e a n l '+ a related statement about wnifonn thickness:
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Ru@&U iapvirtua~impossibleto avdd.milt h i d ~ & g at projec, , . tiom aa ribs. 1
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Theref& fhe designermust expect sinks or voids; these &re o h acceptable for the prodtic&&Eowsyer,if such "cosmetic" flaw. ztfe not aeceptabL, the mbsiding b@ a d j u ~ to d prevent the devilvent of unsightly shll on the prod$@tda~e Q&w&& S w b thickenings,wfiiehcan be msmpfi&db3' using high@hh$sction ~ 1 hb1d 4 p s s w s , 1mgef hjmticm @nd mdw b~ and higherrim* mbldl era-E all 6f;f.@e@e q n : t m ~ @a ' , ,I, slower c y c b .
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entice the plastic to flow faster throug the gate before the plastic would arrive there if there was no rib. Thbmay creak an encirclement of the inner, unvented areas of the mold (see Fig. 4.10, righa, and cause similar ~roblernsas explained earlier.
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4.4 Product S h a ~ e
h i & ~ ga Product
65
Ventilag, shrinkage, and flow paths rt@&ct the design of m y projectioa. Cmsi&r@ fhe flow path of th& plastic, t.bemh, ia of utmost importance in avoiding mphsmt svprises when the maid is m p i e d .
4.4.4.b1 P e p .t k€@$cl8;iY F.owPatA ,Messthe designer oan rely on his or her own experience, or the help of an expehnced mold designer, &e dwigmx & x M d l i n e acomputm program tiptias% simulate the flow~paph when a suggtmed design is ready to be analyzed. These are several such d a r n s c o available, arkd ~ there are~consultants~who speci&ze in this field.
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4.4.4.4
Flat Surfaces
Another point which requires careful considerationl is the design of "flat" surfaces. Although already mentioned on p. 35, it is important enough to bear repeating: It is very difficult to produce a really flat surface, especially in high shrinkage plastics, unless molding cmditions are carefully controlled, which unavoidably results in slower molding cycles. The main obstacle to achieving-flatness k umven shrinkage. Uneven shrinkage can l x caused by:
. . .
Uneven t$icsoess at the base of any projection, such as a rib; bosses that usually cannot be properly cooled; or poorly designed mold cooling.
Most mold designers wild attempt to provide the best possible cooling in maids, especially if large poduction is expected. However, the design of cavity and core cooli~gin areas where projec$ions are located may be hampered if they are too close together, which maka it very difficult to provide adeqwteccraling at that location. Such poorly cooled meas of the mold will then control (slow down). the molding cycle.
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pigure 4.32 b o s s w&msof b o ribs;
discussed later, i
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4.4.45
Ribs
The left illustration in Fig. 4.33 shows a rib which has (at its base) the same thickness E as the base itself. While this design is stronger than the om OR the right, the thickening (symhlized by acircle) is greater than that of the s h k h rn the right. .Such a thickening can cause a sink mark; therdore, t bdesiga @ right is usually preferable. IdeaIly, the thickness of the rib at#- base~shauldbe only about 0.5 t.
w)large r&+,
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(right) small V a a 8 I
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e n , the designer must select a compromise: the larger the radius, the better for I
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FigurbA43 ficiss sections of two ribs: (Left) rib thickness at base equals base t h i c k m ; (right) rib thickness is less than base thiclolcss
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t h stress conditions, but, at the same time, the thickening will be larger, as
*wn by the circles in Fig. 4.34. This figure shows the same iibs as before; hwever, the radius R on the left is larger than that on the right, and therefore the , of plastic is larger. It is up to the designer, in consultation with the client, to decide whether to W6ce appearance (accept sinks) or to accept a higher product cost as a result df dower production and wasted plastic. A major problem with rib design is the proper filling (molding) of the ribs. %&xw ribs no higher than H = 2t (Fig. 4.35) do not present any p;d,lenn, as a
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Designing a Radttct
Since flowing plastic will always take the path of least resistance, the plastic will fill the base before entering the rib. As a result, the air in the rib space is trapped. As the plastic continues to fill th&space in the cavity or core where the rib is being formed, the trapped air is compressed, This can result in two possibilities (Fig. 4.36): if the plastic enters rapidly and the volume of air is large, the compressed air can heat up so much that the plastic will burn at the crest of the rib. If the plastic enters slowly, the compressed air will form a pocket, and the rib could be unfilled at its crest. In some cas'bs, either flaw may be acceptable, but as a rule neither is acceptable and the product will be rejected. The problem of trapped air in ribs increases with the height H of the rib (depth of the "groove" in the mold). For higher ribs (H > 2t ): the volume of entrapped air increases; this air must be r&moved,or "vented," to ensure proper formation of the rib. Although this is reaby a moId design problem, the product and mold designers, by working together, can often find a suitable and relatively inexpensive solution to keep the mold cost down and t~ avoid costly venting solutions. If the rib ends in an outside wall, the air can usually escape through the parting line vents (Fig. 437). However, if the rib does not end at the outside wall,
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Figure 4.35 A shallow rib cross section showing dimensions H anad t,
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lFTj@re%$d WaP*d ah dtrhe &ksta&rfb f&*h& or burned plastic at the crest
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or if the plastic flow coming from the (heavier) wall arrives at the end of the rib before the plastic flow through the rib, the air will be blocked. In such caws, vents must be provided at the point(s) where air entrapment is expected. For a narrow rib, this can present a major mold design problem, especially if the rib is less than 2 mm thick at the crest. An inexpensive solution is to add ejector pins at the crest of the rib. This not only permits a good, natural venting through the clearance gap between the ejector pin and the c m block but also provides the best possible point from which to eject the product fromthe mold, ifpins are planned for the ejection of the prodwf. A disadvantage is that such added ejector pins can have a detrimental influence on the mold cooling by Ijmiting the space available for cooling lines, which cannot be located where there are ejector (or any other) pins. The can often be select& so that they will provide sufficient (if location of the not perfect) venting without too much effect cSn the cooling line layout. F i g W 4 B t fleft) illustrates an instan- where a gusset or rib is required b stiffen a wall, but the rib is too thin to add a reasonably sized ejector pin.Sirn the rib does not extend to the parting line, the trapped air must be vented. To improve the design, a stud has been added to Fig. 4.38 (right), with a diameter selected to be slightly larger than a standard size ejector pin. The trapped air cad now escape through the clearance where the ejector pin passes through theJ&0ld. This arrangement has the added advantage that the rib can be ejected from the lowest point, and it should ensure that the rib will not remain stuck in the mold at ejection. On the down side, the addition of the stud will fractionally add to the molding cycle because of the increased mass and the possibility of a sink mark, but it is better to mold somewhat slower than to be uncertain whether the piece, as designed, can be molded at all. The example in Fig. 4.39 is given with the intent to show the effect of stiffening of a rib added to a flat surface. For simplicity of the example, a strip
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f&X&'I&&sv3 :&yfik;&ilt~':$; am&al axis to extreme fiber Cy) L: info-& e s e ' f m l the ~ values selected for the reMarc4 strip
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modulus of elasticity for the material ofthe b c k ( i n our im mads*No@ that the pmduct i5 W&&I t 3 pmduce ~
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4.4Product Shape Beam
lengtho of berun
W = Load applied to beam
= Length of b w
Mc. = uniform load (over whole length)
f mt maximum ddlection at center of load 1 . .
pisure 4$5,Cross swtion af o beam supported at each end and under a uniform Load Figure 4.40 Cmsssecti~fi of a solidlymounted beam s b w s dimensionsused to calculate d e f l d o n and stress
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lues I and y can be calculated using equations 4.4 and 4.5.
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We see that the greater I, the smaller the maximum stress, but the greater y, $xe greater thb stress. In any case, ,s, must be well below spermissible for a we& , , dkigned prohct. '' The illustfation in Fig. 4.41 shows another typical case. Here, a bem is 1 I . Pdpported on bMh ends and &add with an evenly distributed load. ' The forrnhIi$efor deflection and stress-for rhis case only-are as follows: I
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modulus Zis the same hs cdcul~tedin equation (4.61,iS,a$p , b\
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rslf the beam gxaw5 very fast as the height of ribs .B ;.s : ,;q . : ,
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By s u b ~ t h k i nthe ~ <&tieof itid&,,iqrbldan 4.7, we get '
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rarely just a beam but is, in stress analysis, broken up into many (finite) elements (beams) which each can then be calculated for adequate strength. There are a number of specialists and computer programs to perform such "finite elements analyses." We will not further go into this subject. From equation 4.6, it is seen that the greater I, the less the deflection$ Referring back to Table 4.2, this influence is shown in the column "Gain in stiffness over flat strip;" a flat strip has the least resistance against deflection. The increase in I is quite spectacular as the height of rib increases. However, there are the other factors to considef, such as molding (filling, venting) and difficulties in making the mold with deeper ribs; such factors will usually limit the height of ribs to manageable proportions. Another important limitation to the height of ribs is the stress level at the crest of the rib (the point farthest from the neutral axis). This (or any) point must never be stressed beyond its permissible limit, that is, the tensile strength of the plastic, but must instead be reduced by an adequate margin of safety, or "safety factor," suitable for the performance of the product. The maximum stress s,, in the beam can be calculated-for this casconlyfrom the following equation: r ' '. . ., . , ,-:, t T r ) , 1.
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few problems with venting and filling. However, if the ribs do n end at the wall, provisions for venting are imperative, and a complicated mold construction (special ejectors, vented inserts, or vent pins) may be required and therefore increase the mold cost. 5. It may be of overall advantage to have several shallower ribs, morc closelv s~aced.rather than one d e e rib. ~
4
This is all pointed out so that the product desimer will think twice before specifying deep ribs, and if necessary, make a detailed stress analysis of the product before designing ribs which are unnecessarily deep, with all their potential problems and cost implications in molding and moldmaking.
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be 4.42 Side view (left) of gusset to stiffen end wall and end view (right) of wall C
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4.4.4.5.1 Using Ejector Pins Under Ribs A serious problem w ~ t hejector plns is that they often present mold maintenance problems, especially if the pins are long (and more so if the product requires a long ejection stroke). Any pin size smaller than 3 mm in diameter (even though they are commercially available in sizes as small as 1.5 mm in diameter) should be avoided to prevent frequent pin breakage and other maintenance problems. If there are other methods of ejection planned, such as the use of strippers or air ejection, the ejector pins and their mechanism can add considerably to the mold cost. Also, the effect on cooling (as outlined before) will i n c r e a s ~ t h ~ molding cycle and thereby reduce the productivity of the mold.
4.4.4.6
f
the bend region
¬ have to be round; they can have any shape suifable for the design, they can be molded (i.e., withdrawn from the mold part (cavity, core, in which they are formed. Studs are usually intended to:
tiussets
Basically, gussets are short ribs used either to 1) stiffen a wall where it makes significant turns, or 2) to support long, slender hubs or studs. A few typical exapples of stiffening at changes in direction or at corner are shown below.
4.4.4.6.1 Gussets in a Box In a box as shown in Fig. 4.42, a gusset would usually not be-requiredexcept to stiffen the side walls if they are very long. The stiffening effect of the end walls (sides) will often be enough to ensure the required stiffness of the bottom and sides of the box.
4.4.4.6.2 Ffat Strip with Upturned End The stiffening effect of the end wall is *ping on a flat strip, and a gusset may be necessary to ensure that the bent-up portion of the strip will retain its attitude under the expected load condition (Fig. 4.43).
x-x
f. %upport other components,
1 bcak (and also support) other components, $i be wed as rivets, for later joining other pieces, or
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&tandingstuds in the proportion shown in Fig. 4.44, or evenlonger, are ?@ady they am an axye than twice the height of the diameter (or the kk%t d i m e d o n at the base), because they are very sensitive to side loads & direction. P1 small radius where the stud meets the wall is absolutely w&r, ma large a ~ a d i uwill s be $ittonger-&utwill i h m e the mass bf D1a~tid h~tMi spot.
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p a t any other function for which a stud is suitable.
The e m p l e s . i nFig. 4.46 show that there a r e ~ ~ways n tof rea~hinga similar result (the aapport or the location of a ear;nponent), and that it is up to the dmigner to p c i @ a ,&ape h t not only is needed far the r w purpose but also can be easily mdded, t
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Top vimpfstud
L , I ! . . !(,' , '7 A hub cm he defmed as a dollowsd-out &d (or b6.s). All roles givep fogsatis
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the corresponding hub; dimension d and the hole and stud diameters must %acCpr-e with the proper range of fits selected for this assembly. Se.e dso Fig. 4.21 (~ectiob4.4.2, p. 50). There is always the risk pf 4 @es, which may weaken the wall of the hub. Flow analysis may predict @leas of weakening. ;&en though the hole d e ~ t needs h to be only sli6tbtly more than the height of
aware thpt the bncfick effect wring-&tby the core pin (io &w the is n a a v W l e here, d ~~s under the smd w8.l be larger dm that
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&hedesigner should make 8urethat the core pin, in other wmcls, thc mold p n e n t that will prodnce the inside shapeYhcan be easily made, md, 8 at all =&%if that it can be well cooled. The outside of tbe hub (or stud, w any
B% && mass. I During molding, it is surrounded by hot plas'tic. Becauw of @I mass, the core pin will quickly reach the temperature of the plastic. @use of the small cross section of the pin where it is located in the mold ttik94 core, or side cme), this heat will dissipate slowly into the mold steel. 4 udem h,pink well-cooid, it can dramatically affect the molding
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B
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w e 4.50 Two cross sections of a pail rim: A. projection is within the stripper rin @ B. projection is formed by side cores (Note that mold features such as cooling, kg, etc., are not shown) Figure 4.49 A core pin extends deeply into tke h ~ to b reduce plastic matis m d ~ c d l e better cooling
is nothing wrong with such designs, but the product designer must understand &tt these projections may presesnt some complications in moldmaking, k c r a s e and often increase the cycle time. There is a large number of wing sections will diwuss a few common examples. s a portim of the same: product (e.g., the rim of a pail). In e wall ends at the P/L, and the projection is within the stripper use of the draft of the projection within the stripper, the product can ted from the core, since the cavity withdraws (opens) without product. (NOTE:Air pressure may be required to lift the product per if the extension is long and could hold the product in the
.4.50B, the product does not allow a taper in the stripper as shown in c>nsi&fstio'a 1%-&'be
stressed endu*
@ ~ df 4 the poduct It? and &at klfill
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hnpodank d the mold (and the mold moling). I
.4.9
Projections on the Outside of the Product
;the wall ends at the P/L at a locatidn different from example A. The projection in B is now in the location where the wall was in A, and must be h m e d by side cores. This is a considerably more complicated and expensive method of building a mold. The point made here is that a small change in the product design-the simple draft angle-will permit the product to be made in a much simpler etnd less expensive mold. There are many other occasions where simplification dmilar to that suggested above can be applied.
4.4.4.9.1 Outside Threads Outside threads are outside projections, and will dways need at least two side cores to free the molded piece. In some cases, the mold designer may want to use more than two side cores (three or four) to reduce f the side cores. In the thread section of the product, ther- are always sls many witness lines visible as there are side cores.
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Figure 4.5 1 Cross section (left) of a container wall with a handle that is designed along an offset parting line (end view, right)
4.4.44.2 Other Outside Projectim Products in this group ;include handles, etc., which would hinder the stmght withdraw4 of the product from the mold cavity. la some cases, side cores can Ln used, ratha than split cavities. When ~ thedesigner may fiid ways to design the reconsidering the shape of t h prodata, projection alaag the PL, even if it requires an offset PJL dropping to the level of the projection (Fig. 4.51). Such dfset P L costs more than a stmight PJL but still'@mbe puch less expensive than either side cares or split cavitieg. ba in Fig. 4.50, there must be adsafe angle to allow the product to pull straight out of the core. By seieczting this design and agreeing to the concession of a draft angle, the use of a very costly side care to create the handle has been avoided. Note that there will be a uritnws h e where the inwrt meets the vertical wall. However, theae would also be witness lines if the handle were produced with a side care or with a split cavity. ,When the designer is in doubt and nmds to visualize the area in qu&tion, a simple (and very low cost) method is to model this a r a with mdelling clay (or "Plas:ticine"), T k more sophisticated designer atxuse computer modelling, if accessible:
4.4.4.10 Undercuts on th.e Inside of the Product Undercuts on the inside of a product am usually: .rlJ
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tigl dec methods; t m are not part ol drawing. T the unscre\ from the cc The most freq otatin stripper ring; the product must be held so that it can be unscr product turns relative to the core, and retracts. This usually ratchets on the underside of the wall of the screw cap, In one system, the product requires ribs or other projections on the outside, where an external unscrewing device can engage to grab the closures to remove them from the cores. The design of these aids for unscrewing should be discussed with the mold designer for this project. From the foregoing discussion, it becomes clear that any unscrewing method requires either complicated molds or special machines. The molding cycles are also slower than comparable products that need not be unscrewed. In low
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Designing a Product
the diameter d; or f is proportional to h/d. In other words, the greater the diameter d, the easier the product will slip out & i@ groove in the core. A product made from a relatively stiff plastic could be strip@ if d is large, but it would break during ejection if d is small. The type of plasf$&,and its modulus of elasticity E and tensile strength are, therefore, importmt factors in determining the sizes and shapes of an undercut planned to function in some manner, such as to hold (snap onto) some other product or to form a screw thread. Hate that many screw threads, and especially threads for closures, have standardized shapes, which originally evolved from threads on glass bottles and jars. Such standards should be followed, and their large tolerances used to obtain maximum benefits for plastic threads. Also, note that the finish of the groove also has sowe influence on the ease of stripping, especially with materials requiring high p o b h for good ejection.
I Figure 4.52 Cross section of a product ejected using a stripper to push the plastic over the "hump" in the core; the greater angle a, the greater the difficulty in stripping
quantity applications, the core could be also removable from the mold unscrewed from the product outside of the mold, by hand or by using fixture
4.4.4.1 1,2.2 Number of Threads (Pitches)
4.4.4.11.2 Stripping The designer must consider whether the product ca bestripped from the threads. Stripping is the easiest (and often the lowest cos +
solution for ejecting molded products; however, the ease of stripping depends o many equally important factors. The theory of stripping is quite simple. As the mold opens, and after th cavity has moved away from the core side, the ejection starts, caused by th stripper moving forward. In doing so, the plastic product is pushed over the hu in the core (Fig.4.52); this causes the plastic to expand so that the portion tha inside the groove in the core can slip out of the groove.
4.4.4.1 12.1 Influences Affecting the Ability to Strip a Product The greater the angle a, the more difficult is the stripping action. This is quite obvious: a sharp angle (a = 90% see Fig. 4.52, right) makes ejection virtually impossible. The projection would act like a hook, and the plastic would shear off in the groove rather than pull out. Screw threads could have a 90% angle if absolutely required, but that would definitely call for unscrewing; stripping would not be possible. The greater the height h, the more difficult is the stripping action. This, too, seems obvious, since the greater h becomes, the greater a becomes. However, there is an important relationship between the height h, the diameter d, and the ease of ejection. As the plastic is dragged out of the groove, it stretches and is stressed in tension. The amount of stretching can be calculated by using the circumference C of the circle, with the diameter d, (C = dn). The percentage of
The easiest and best way to strip threads is to specify only one thread (one pitch) so that the stripped thread will not slideinto an adjoining groove but continue to slide uninterrupted off the core. By chidering the factors influencing stripping as described above, the designer will w c a e d in creating a screw cap with the proper shape of the cross seaion of the thread (projection) and a suitable h/d ratio so that the product made from even a s&ff plastic can be readily stripped, without the need for an unscrewing i ' mold. L
4.4.4.1 2 Other Projections
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Almost anything can be molded in plastic; it is just a question of how much a mold will cost and how efficiently it will run. It is virtually impossible to describe all of thp different shapes that can be molded, and also all of the problems that can be encountered. A few more common designs are shown in Fig 4.53. Figure &53A shows a typical, frequently found design (in many variations) which requ*lr&sa 6ubsmtiaf 'hook below the top surface of a product. Figwe 4.53B illustrates a praGtical method to produce this shape by molding; however, it mquises more mechanism and permits ejeotion, the 8lide ejector advances while italso the hook b~f0t-ethe product is f'dl ejwted, ~ h d u q design t WI be dtexed design r&buires$ chinge tt, the f~foduc*:"
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4.4 Product Shane
Designing a Product
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L 4.54 Cross section (left) shows a product with a circular projection requiring a
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small, usually rectangular opening must be left in the top surface of the product to pennit the cavity insert to pass throligh and form the inside of the hook. This method requires just an "ordinary, up and down7'mold, without moving ejectors, etc. If such an opening is not objectionable for the appearance or function of the product, this method is the preferred design because it is much less expensive than any alternative.
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The product designer should understand that there are many different methods used to free seemingly trapped undercuts so that the product can be ejected. The following two examples illustrate what is involved to mold such shapes, and that such solutions are possible but more expensive than designs without such features.
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Freeing Undercuts in an Overcap
The inntral, circular projection in.^$. 4.54 ends in an enlarged bead, whi& is trapped by the sundunding mold s&l. If vould be impossible $ &i$be product using conventional matihods. By using a two-s@ge ejection bead ban -be freed before ejection is completed. As shown in Fig. 4.54 (1+2) until the bwd i s ahw the top at& (I), and s@ps tfre pmd#a$ in& the space previ@y ma& on wmplektl of
groove h, the diameter d, and the molding material is as insown for stripping screw threads.
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Other "Difficult" Shapes
4.4.5
,as with theshape of screw threads, etc., the shape of the bead o that it will slide out of its groove. Also, the relation between
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Gollapsible Cores collapsiblecores are the same as the slide ejector method s h o w in they MC used not just for one specific area, as illustrated, but for uts, which may be along puch of the inside of the product. The very complicated in design, expensive to'build, and mc& to ,it is usually difficult to provide good cooling for the moving mold e fators result in a higher prodat cost.
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5.1
Designing for Assemblies Mismatch
i.Matching of the outside of two molded pieces, or of one molded piece with a
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from more than one cavity. It is good practice to prevent unsightly mismatch between top and bottom parts by deliberately making one of the two parts larger along the matching face so that, even in case of mismatch, a more pleasant appearance can be achieved. The illustrations (Fig. 5.1, exaggerated) show one method for avoiding poor appearance due to mismatch caused by manufacturing and shrinkage variations. Only one design style of meeting surfaces is shown, but the principle applies to any design. (In other illustrations in this chapter, the matching of assembled
88
Designing for Assemblies
pieces will not show such provisions to hide mismatch b.yt will assume that ideal match has been achieved.) The following sections cover some of the most common assembly methods for plastic pieces and their effects on product design.
Screw Assembly
5.2
This description applies equally to assemblies using threads tapped in the plastic or to self-tapping screws. Screws are generally used for larger assemblies, such as enclosures for electrical apparatus, etc., and for any size assembIy that must be taken apart for servicing or into which me fastened other components which may have to be removed for sewicing, etc. The number of screws depends on the design (the shape) of the pieces to be joined. S c m s must ensure that the parts are held together under operating conditions of the assembly. If there is a seal placed between the two assembled parts, the distance ofthe screws must be such that the deflection of the sealing surfaces, caused by compressing a (soft) seal (rubber, cork, 0-riags, etc.), will not be so great that the surfaces can separate and cause leaks, or let air (dust) enter.
5.2.1
Holding Power of Screws
In mechanical assemblies of any kind, the screw force depends on the amount the screw is preloaded (stretched)when torquing while compressingthe assembly. Sufficient preload also ensures &at the screw will not loosbn as ?on$ as the preload prevails. With plastics, this reliance on preload is not possible, for two
sewn& 1- Pbstics in general are nor strong enough to d a w a sa8w @ ,be torqued (and stressed) as 'high as when used to hold s t r o t . maBrida such as steel. When torquing too.much, the tbmalebd in-&iplasticwQI strip,b d fhe mnlt v d l be lws of , InglWtic ass&qbli6s,therefore, screws can only be that fhe plastic @readwill not deform (by pmS&&, pemissi$rlelimit$ & si b & mdor dektiom), 17
metal (stee be tightenc Eomponenl passing '"plastic, Thc (h I the inse will mainti . Thepr
ence will prevent the plastic from flashing over the metal, which would the] require cleaning after molding.
Number of Screws
5.2.2
The number of screws should be held to a minimum. With increasing number screws:
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Mold cost increases, cost of the molded product increases, product assembly requires increased labor, screw cost increases, and cost of required nuts or inserts increases.
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However, with decreasing number of screws:
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Holding power may be insufficient, and a loss of parallelism 'md, therefore, sealing capability may result.
The designer must get an indication of, or estimate, the forces that will affect the joint, and also any internal pressures within the assembly which may tend to separate the joint and cause it to leak or allow air (and dirt) to enter. Once such forces are known, the designer can calculate the holding forces per screw (and screws is strong enough to withstand these bending forces without creating a ga in the seating (or sealing) surfaces.
Size of Screws
5.2.3 w
From the forces estimated as described above, the designer will have some indi cation of the minimum size screw required to hold the assembly together However, there are some practical limitations:
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If the screws are very small (1.5-,2.0-, or 2.5-mm diameter, or in the USA #4, #5, or #6), they are more difficult to handle, both in assembly and in maintenance, than larger screws. The holes into which these small screws will enter must be even smaller. (Hole diameters and shapes are specified by the screw supplier and are also shown on screw charts.) Note that there is a 'difference in hole size and shape 1) fm tapped threads in the plastic
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quently, will eventually increase the product cost. Although it cannot be said that such small screws should not be used, a good designer should be aware that, even if such a small screw would do the required job, he or she should, for practical reasons and provided the space is available, select larger screws of at least a 3-mm diameter (#8). The cost of a screw is not necessarily based on its size but on its commercial demand. A rarely used, small screw may cost more than a frequently used, larger one. The designer should look into the cost of the screw and determine whether the required size is available at Standard screw charts from suppliers show all available standard sizes (diameter and length). Special sizes can be very expensive unless the quantities required are large enough for the supplier to custom make them at a reasonable cost. The designer should never specify that a standard screw be modified for use with an assembly. This could be very expensive, especially in a mass-produced item.
Screw size considerations will affect the design, since they affect the length of the hole through which the screw must pass. The screw length (under its head) L the length of the clearance hole plus the amount the screw enters the (tapped) The length of effective engagement of the screw thread is that length which ely and effectively engages the plastic. In most screws, particularly in selfscrews, there could be a lengthy point (lead-in taper) which does little ding but which must be considered. This point can be of different lengths p d shapes for various self-tapping screw designs. The designer can either lpccify the type of screw required for the job or make the hole for the worst case. The length of effective engagement for steel is about equal to the screw @meter. In plastic, it should be greater-about two to three times the screw @ameter. The designer should follow the recommendations provided by the %xew manufacturer and the plastics supplier for the poper, effective engagemerit length for the type of plastic and screw planned. rn
Designing for Assemblies
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Screw Holes in Plastic nunI
For reasons of appearance, and also because of the available screw length, screw head is often located below the surface of the plastic. Therefore, the holc in the plastic must be "counterbored" and will be specified as such, even thougl it is molded, not counterbored by machining. There are many considerations in specifying the shape of the counterbore In molding, every hole in a plastic piece is created by a pin (the core pin). Thc core pins are often subjected to unbalanced side forces, caused by high injectiol pressures and poor flow conditions in the mold as a result of the gate location which will deflect these pins. The length of the hole will also depend on the available standard scr length. The following examples present a few typical cases. Example 1 A relatively shallow top ("top" in this context means that part where the screw head is located).
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Section at screw head LS . - = Standard . . .screw length
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Standard screw length LS should be a standard screw length, regardless of whether the screw requires a tapped hole or is a selftapping screw. LE Length of effective engagement Screws in plastic should have an LE 2 to 3 times the screw diameter d. LB Length (depth) of the counterbore LB is dependent on the overall design parameters. Where appearance permits, there is no need for a counterbore, and the screw head will seat on the flat top surface. In many designs, however, the screw head is desired below the top surface;LB is then usually greater than the height of the screw head. LC Length of the clearance hole The clearance hole is usually produced by the same pin that makes the LB. The longer the hole (LB +LC), the longer is the pin that is subjected to side forces, and
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5.2 Screw Assemblv
95
A special, long screw may still be required, and insertion of the screws during assembly is "blind," that is, in 5 assembly it is difficult to find the hole where the screw has to enter L .for threading. A tapered lead-in as shown can be helpful.
w#.ph'bi5 ".'2. positioning of a ahoR s c m in a l ~ n gcouhtw"9re du?@ : .el 4 , ;.,.
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is design is sometimes used but does not take into account the "groans" &d user, who may be faced with reassembling the enclosure after igl and then experience trouble when repositioning the screw. b that the above considerations apply not only to the few example shown $'to any product requiring long holes.
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Ldng Holes in Plastics
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: ' ~ ~ & s i g n i c o u uask l dtheend usnif it would be wcep&de to havesr m iti the sideof@etop,as &ownin exam* 2 ~In.this L@o61emo f c e o l i a V; ,( long pin ddnkppears: me n e w is npv.part.of the cavity MQ& it&; and, bthl,viewof the I&& mzas afthe cavity Mock, there ig no cwhgp p ~ ~ e fie& m-
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&signs (not only in the few cases described above), the length of any be produced using two pins which meet approximately at the center of I h such a case, the potential for deflection of the core pin is greatly Remember that the deflection at the end of a ufliformly loaded beam is power of its free (unsupported)le:ngth. In other words, in of 20 rnm in length will defllect eight times mare . " " h a p i n that is only 10 mm long (i'=1). m e of machining tolerances, it may be diffsult to match the csrabrlines dm-one of them located in the core, the other in the cavity. mgh ~s long hole is what the 'design in Fig. 5.3A requires, the mold , long and easily bent. B would be & & rep can create the long hole using two pins meeting halfway, as I~ i53B. ~ This . design will greatly improve the 1engtWdiarneterratio A
Becomes a "@ecial"'i s b k h should 6e uwd only as a , h t resort., , 2. A core pin of e ~irl recpird tb create SW& a longhiolein the pl~tit br-.tt' must be ay.td!b $, .p $:me mblh all circwmtaaces. ~ h pidri c much tea th$for& bngth6nd will r*fl&t kven ma*! easily duriq :, iajeetiw &&Jin'exakp1@1 above{' ,: ; y . .';'?: 9 5 &a% a am ~ m wbc t c o ~ l using d my k n v e n t i o d m d
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5.3 0ther.Aastmbly Methods
ibr use if the product
Solvents and adhesives also may be
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5.3.1
Bonding
~ependingon whether the plastic is to be bonded to another plastic, to metal, or to any other material, a different surface finish than that commonly tlsed in molding may be required. A rough surface may be most desirable for an adhesive, but thedesigner mast know that rough surfaces could create difficulty in ejecting the product from'the mold, especially if the roughness is krcqed on side walls or ribs with little draft'wgle. (No* that some p14stk1, .sxh as determining ehe type of adhesive Quitablefog Qe expert &o cm describe the surfbe specfim~m8 For some plagics, the us&of c w t a i solvtrea ~~ pieces. They dissdlve the surface layer and wi used before the s&&na evaporates. Some.solvcntr qq!
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Figure 5.5 Two pieces of a nylon fuel filter are joined under pressure and spin welded
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(5.2)
be larger than VH so that a dome can be formed. However, much of the rolding force generated by the cylindrical match of stud and hole. @e manufacturing tolerances of the sizes DS, L, DH, and H, and the tolermc@governing their location in relation to each other, require that the stud be the hole. However, for proper staking, the volume of the stud, VS, )atleast as large as that of the hole, to fill it completely, and then some ernate a dome over the stud and to apply sufficient pressure on the sides provide the required holding force for the joint. preferred design (Pig. 5.7), the holding capability is improved by a reverse taper shape so that the stud will be wider at the staked bd thm &tits base. This method eliminates the'ineed for a dome. with a flat tool, the stud will end flush with tRk3top surface. If ly too high (too much VS), the stud Will compress mote and/
5.3.2.4
heat sealing
Some bonding methods use applied heat, usually from microwaves. One example is to seal a plastic food container with plastic film. The designer should approach an expert on heat sealing, and the supplier of the plastic contemplated for the job, to arrive at the proper design (area, finish, etc.) of the sealing surface.
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Figure 5.7 Staking by reverse taper: A. stud, B. hole with reverse taper shape, C. before staking, and D. after staking-the stud is wider at the base and has no dome
or the hole will be stretched by the force exerted by the staking tool. Plastic is compressible and can accept such forces as long as they are within the allowable stress limits. If the stresses are too large, the hole could burst. Note that it is very important that every stud is well rounded at its base to reduce the effects of stress concentration and to prevent breakage of the stud. Gee Section 5.4, Chamfers and Radii.) The hole in Fig. 5.7B must be made by two core pins meeting at the narrow section. The same rule%apply as on p. 94, cwe pins meeting core pins. , Using a specially shaped staking tool, some plastics can be staked cold; o b r s require elevated temperatures. With some brittle plastics, cold staking may not be possible, and the staking tool (and/or the plastic) must be heated. Evep though a staked joint can be used only once, and the pieces cannot be separated without destroying at least the part with the studs, the method is ineapnsive and frequently used in mass production. Much work is often inydved in developing the most suitable shapes for stud, hole, and staking tool before settling on a final design. , %re is not much, if any, difference in assembly time when staking only one cv ~s@ltaneously) any number of studs per assembly; however, the number of studs (and matching holes) will affect the mold cost and may even affect the pmductivil) of the mold. If ejectors are required under the studs, some of them may interfere with an efficient cooling layout for the mold, thereby causing the Ibndd to run slower.
Figure 5.8 The core pin radius (A) is the "negative" of the radius in the molded part (B)
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5.4
Chamfers and Radii-In
the Plastic and in the Mold
The designer must be always aware that the mold presents the "negative" of the product shape. This applies not only to core,pins and holes but also to cavities and cores, in general.
Figure 5.9 Chamfer on the end of the core pin (A) creates two stress risers in the plastic (or molded) part (B)
An inside radius in a steel part is easy to produce using any machining method (milling, turning, or grinding). This radius is also important for the life Expectancy of the steel part, as it avoids stress concentration points (stress risers) and consequent breakage. The radius on the core pin will then produce a radius hthe molded piece where, in normal design practice, a chamfer would be exmted(Fig.5.8). On the other hand, a corner at the end of the core pin will be easier to chamfer than to make a radius; such chamfer will reproduce a chamfer on the inside of the plastic where normally a radius would be expected and desired. The (inside). chamfer in the plastic (Fig. 5.9B) now has two corners (stress risers);fherefore, , k is recommended that the designer specify the core pin chamfer to have wellrounded comers.
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5.10 For plastic st&, thu:hole entrance should be well rounded, and the radius BJ , @pDaOadM,bhWQ ~ ~ ~ h .~ t .. h ,
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Lettering and Other Distinctive Markings ngs are quite commonly shown on plastic products. or all of the following may appear: Manufacturer's name (and address), country of origin, recycling information,
patent and trademark information, cavity number (very important for quality control) date of manufacture, and batch number. igner must ascertain which of these data are required and indicate on the to appear. Some data may be legally required. data could also.be printed later, but at additional cost to the product. practice to have as much data molded in as possible, provided it will :in future batches. Printing is easier to change and may be the better he same product may require different data in future. ky molded-in data is either raised from or depressed in the product surface, st be located on surfaces where it will not mevent eiection from the mold. the top surface of the core or the inside bottom ~f the cavity.
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Lettering and Other Distinctive Markings
Shape of Engraving Engraving is the most common method used to machine the information into the mold. As was mentioned earlier, the mold has the inverse (mirror) appearance
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If the designer wmts raised letters (ar logos, etrc.) in the plastic, the mold must be engraved; that is, metal, (usually steel)must bemaved where the information is to appear. This is the simplest and least e x m i y e method, since only a minimum amount of metal is removed. If the information is other than a standard style af lettering, art wwk must be supplied to the moldmaker so that rhe necessary mas&rs for U e engraving process can be produced. Art work is usually in the form of an enlargemeat of the required shape, such as a photo or a drawing. The ratio of enlargement depends on the job and c a be anywhere from 2:lIto 29:1, as requested by the moldmaker.
Depressed Lettering Although sometimes requebted by a cpstomer, the designer must be aware that depressed lettering is much more difficult to achieve (and costs much more), since now the metal must be removed around the information so that the lettering is left s@ding above thq mold surface. Raised lettering is also far mQrqeasily damaged than engraved lettering. There are other methods available for cre example, the lettering could bR engraved into an e However, this too is mom oxpeksive than engraving One advantage of depressed lettering is that the
arule, depressed lettering in a molded product should be discouraged. On er hand, if other information must be added after molding, depressed ng could be of advantage.
Checklist of Additional Product Requirements m e checklist for the designer, below, outlines additional product requirements w t yet discussed which can influence a designer's decisions: '
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What limits are there to overall product size, shape, and weight? Will the product be stacked for easier transportation and packing? How many units per stack? Does the product require stacking lugs for easy separation either in assembly or by the end user? Will the product be packed in standard quantities (100,1000,dozen, etc.)? Are there size limitations for stack height or the size of cartons and boxes for packaging? Are standard box sizes to be used? Are there any other packaging requirements which can affect size and shape? Are there size limitations created by requirements for shipping in freight containers? How can the product be handled after molding and during secondary operation? Does the product need provisions to orient it for assembling, filling, or 0 t h seconckry operations? Is the product sensitive to scratches or other damage in handling? (This cadd affect the method of manufacture.) Wilt. h x e be a range of similar products (varying sizes, colors,
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m~~hi;ng* awe injury to the ~0spec:tiveuser (and ewn the "misuser") and to others. The gner must also consider how the prottuct will directly or indirectly affect the that
Foreseeable Areas of Risk
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bseeable areas of risk, such as causing injuries or death, could be in the !$rials selection and in the shape or strength of the product. Consider the .. owing examples: ,k
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possibility of being swallowed by infan&&? binall children, or all of the product, harm caused by catastrophic failure of 1' * fire hazard, or p.;* failure to perform as advertised. h e of some reluctance by the general public to the use of plastic instead : ~onventional,older materials, the designer of plastic products must be $@lly careful. Consider the following typical example: :
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Nabody has ever complained a h u t theuse o f g l t ~which s ~ is burn easily cm impact, p;Uti&ly when umd far pressurized miff b8tfim. Bowever, when the plastk intb&qp r o p o d Eo bttlm EnaBll PET, vewy ,&zing@ req-Esi s w ~ intro&uced.Chet& stipglaLtia mm hat thefi&d md pmmwhd plastic bottle mwt w i t b m d a -dmpBE 6 Eii ( 2 ~ ~ r n dff 3 breaking. At this same time, b d Y d 2-Wr bottles occasionally shattered when just tipped over on a flat surface.
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912
Safety in Product Design
Many accidents, particularly in North America, are exploited in the courts if they can be attributed to the failure of a product which caused bodily harm, or even only anguish. This "practice" is slowly spreading to other countries. The parties held responsible are often not only the seller of the so-called "unsafe" product; the designers who created it and the people involved . in the making of such a product (the mol @ % m p @ q y hj?h q~ espons* and must defend them st such attacks as " . . . not to haveforesee'n the possibility of misuse of the product. . . ." In many cases, the designer or the manufacturer has been found guilty because of a " . . .faiture to warn . . ." of posgible ris'bw'h&wing the fxckiuct, m h as prachg c d o n *and~~g Wmdon' on kht!.product, k i n lllaiiuals supplied with the product. ~ ? & u n f&me61y, thek are d$ojmtified'dl.glrir$if, fa$ exmp1e, the design&& not a r e to select the proper m&&d, -elr made s m e gl&g desia errd5&&en calculating the strength of the product. I
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Reaponsibilily and Liiibiflty
& w i v e resistance 29 Awuracy, dimensional 25 Ps,ppawance 3 Agsetnblies 87, 96
Creep 26,88 Criticism 20 Cross sections 18 Curing 5 Cycle time 35
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BefMe Selection &a rnsteer%~l,'parrticnlar~y if there me safety or health aspects in the ~ ; e a t i o n , + hdesigner e shedd demand &$haveall im@rtants-~&Eons cerhped by the supplier, and if m s s a r y , by an i&pendt?rra l i % M w .This may help to protect the designer and the m m u f w ~ a ~ f taew h e pr&mt.against liability claims for nonperformance of the product or srcciht6i qwlthg from the use of st& &d h a new qplicaitian. , O~WUW, making and SeSiwphtype-s will ga sl lmg w q t a w d pr~ving the new design and its rndterial selection. Unfortunately, thew mta e m o t p v e beyond doubt how the product will behaw whr& kia& ,&idverse! wditions. Usually, the manufacturer wants to get the p d w ? out into the field as soon as posSsible;to take .advantage bf' an early surt in,' a 'competiti~ek k e t . Wnfortunhly, this will often nqt dlow much dme fm k%hg aml pao~Mthe material to be used is gde, arrd'fhu3 br'ings a J C : e 6k o u n t of risk f k fhe rnadaahrmmdthe Wgpm-#
Data sheets, plastics 14 Definition, surface 24 Design process, questionnaire 19 Design review 20 , Designer I8 Designing 17 Dials 32 Diaphragm gate 52 Die cast 25 , Dimensional accuracy 25 Dewels 33 Drag angb 79 D r & m 18 Drawings 17 aFindrieg @up23 DupSWhg 34
Efficiency in design 17 Ejector pin, sleeve 63, 67, 72 Elastic limit 26
Electric discharge machining 46 Blecui~dcantact 89
Environment 11I Escutcheons 32
Injection blow molding 8 Injection-compressionmolding 5 Inside threads 81 Izod test 28
Extrusion blowing 7 Filling speed, mold 41 Finite elements analysis 62, 70 Flashing 45,46
Knock-outs 32
Flaws, cosmetic 63 Flexural strength 29 Flow of plastic 38, 60, 63 Flow path 38, 64 analysis 51, 77 Foreseeable risk 111
Lettering 32, 105 depressed 106 raised 106 Liability I12 Lid 33 Lightweighting 21 Live hinge 4 Logos 32 Lost-core molding 11 Louvers 32
Gate location 37, 51 center 39 diaphragm 52
Gussets 61, 72 Handles 32 Hardness 29 Heat conditioning 9 Heat sedhg 98 H o l d i i force, scrsem 27,7,8
composite. 58 for screws 92
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Margarine container 24 Markings lq5 Materids sdection 21 Mechanid properties 25: Mismatch 43,87 Models 34 Modulus d elasticity 24$29,61
e%xpmdable &ad I Q injection blow 8,23 iqjection-compression S insert 33 lmt-core 11 rektion injection 10 rotational 11 stretch blow 8
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Rotational molding 11
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tensile 25, 26, 61 ultimate 26 Stress 26, 48 maximum 70 risers 36, 60, 101 Stripping 82 Structural foam molding 10 Studs 32, 73 Temperature influence 28 Thermoforming 11,24 Thermoplastics 5 Thermosets 5 Thickness, plastic 35 uniformity 35 Threads 60 inside 81 outside 79 pitch 83 Time, stressed 26 cycle 35 Tolerances 44, 87 Toys 34 Transfer molding 5 Two-stage ejection 84
Undercuts 60, 80, 84 Unscrewing threads 81 User-friendly 22
Venting 39, 42, 62, 66 at parting line 46 Verbal instructions 17 Vertical shutoff 55 Views, drawing 18 Voids 35, 63
Wall 33 Wall thickness 25 warp 35 ' Wear resistance 29 Weight rduction 3 Weld line 51 Welding 97 heat 98 sonic 97 White prints 17 Yield point 26
