Plastics Technology Practice

IC LEARNING SERIES

Plastics Technology Practice

The Hong Kong Polytechnic University Industrial Centre

IC LEARNING SERIES

Plastics Technology Practice Suitable for the following learning modules offered by the Industrial Centre: TM4001 Integrated Training I for ME DG Students TM4009 Integrated Training for ISE DG Student TM4012 Integrated Training II for PIT HD Student TM9003 Rapid Product Development Processes TM9009 Reverse Engineering

Last updated: March 2012 Copyright reserved by Industrial Centre, The Hong Kong Polytechnic University


Plastics Technology P ract ic e Objectives:

 

To learn the practical applications of plastic technologies. To know the latest development of the plastic technologies.

1.

Introduction

In general term, plastic refers to the suitability for manufacturing and moulding into different shapes. Technically, plastics are polymers of high molecular weight by linking together many small monomers and may contain other organic, semiorganic or inorganic chemical substances to improve performance and/or reduce costs. Plastics are widely used in packaging, building & construction, transportation, communication, health, entertainment and many other industries for applications such as glazing panel, plumbing fixtures, helicopter blades, airplane fuselages, car bumpers, artificial hearts, food and drink containers, CDs, DVDs, electrical and electronic products. Plastics are useful but littering is not. It is estimated that everyday more than 60 million plastic water bottles are thrown away and most end up in landfills or incinerators in US. Plastics are non-biodegradable substance that degrade physically very slowly and prompt to pollute earth, air and water. On the other side of the coin, plastic packaging offers a superior ability to protect products against contamination; plastic pipes safely transport water or waste for their superior corrosion resistance and high strength to weight ratio; plastic vehicle parts consume less fuel for it weight to fuel impact; in electrical and electronics enable plastics to make our living easier, safer, less expensive and more fun for their ease of fabrication into complex shapes, insulation and colourful or transparent aesthetic aspect. Furthermore most plastics are petroleum base product and the energy required to produce plastics is just half of the energy required in producing paper and 1/5 in producing steel. Plastics can be firstly reused, replaced, and reduced and ultimately recycled at the end of their useful life. Plastic parts are littered because they are unfashionable rather than because they are worn out. Our living style is harming the earth not the plastics.

Page 1 IC Professional Training


The European Union has enforced a series of directives to ensure the sustainable development of mankind in using our nature resources. •

•

•

From January 2006 manufactures & retailers will be responsible for recycling waste electrical and electronic equipment under new EU legislation called the WEEE Directive (2002/96 EC). From June 2007, chemical substances are controlled through Registration, Evaluation, Authorisation and Restriction for their safe use under the REACH directives (1907/2006 EC). The EuP Directive 2005/32/EC on the eco-design of Energy-using Products encourages manufacturers to design products with the environmental impacts in mind throughout the product entire life cycle.

The use of plastics in our society should be undergone a holistic investigating or valuation in assessing the social benefits against with the social & environmental impacts starting from their raw materials extraction to final disposal: “Cradle to grave”. 1.1 Plastic Industry in Hong Kong Most of Hong Kong plastic manufacturing establishments have been blown up and moved to China after the economic reform in 1978. The Hong Kong entrepreneurs have been taking the leading role in transforming the Pearl River Delta into the heartland of China manufacturing industries and radiating their influences to the other provinces. In the past thirty years, China has grown into a giant in the plastics industry, ranking first in the world in the production volume of plastics processing machines, second in the production of plastic products, and third in the consumption of plastic resins and the largest importer of nature rubber. In 2007 annual plastics consumption in China is over 40 million tons and rubber consumption will reach 3.8 million tons in 2008. China is developing into one of the largest markets for plastic and rubber products in the world.

2.

Plastic Material

Plastics can be classified into three main types, namely the thermoplastics, thermosets and the elastomers, accordingly to their physical or chemical hardening processes. 2.1

Thermoplastics

Thermoplastic materials soften while heating and solidify while cooling. Thermoplastic can be mainly classified into crystalline and amorphous (noncrystalline type), they are different in molecular chain structure, such as linear, branched, comb, star, cyclic, dendrimer or randomly branched as shown in the following two diagrams. These chains associate themselves together through



weak Van der Waals forces or stronger hydrogen bonding or stacking of aromatic rings, while a highly crystalline structure is well order, an amorphous structure is random. Most of the plastics are in form of semi-crystalline by a combination of these two structures with certain degree of intra-molecular forces into a semiordered structure.



2.2 Thermosets Thermoset materials are heat-sensitive synthetic materials which, when subject to heat and usually pressure, will undergo chemical change with their molecules crosslinked together to become permanently insoluble and infusible. Thermosets cannot be remelted and reformed after cured and the process is irreversible. This reaction is somewhat like cooking an egg: once cooked, it is set permanently. 2.3

Elastomers

Elastomers are natural or synthetic materials with rubbery properties that can be stretched to at least 200 percent of their original length repeatedly (at room temperature) and which will return with force to their approximate original length when the applying stress is released. Natural rubber is an agricultural products harvested mainly from Thailand, Indonesia and Malaysia in meeting the rapid demand of automobile tyre industry, latex gloves, high pressure hydraulic hoses, escalator handrails, rubber seals, rubber pad, elastic rubber thread and ribbed rubber sheets. Thermoplastic elastomers (TPSs) is a kind of injection mouldable plastics that are low modulus, flexible with both thermoplastic and elastomeric properties in replacing traditional rubbers. The TPE is a class of copolymers based on urethanes, polyesters, styranics and olefins. TPSs are found in products for the consumers, medical, sports and leisure, automotive, lawn and personal care market segments for their ease of processing and soft to touch texture. 2.4

Additives and Fillers

Additives and fillers are added to improve the performance or to reduce cost of polymer during processing, or their servicing capabilities. The followings are some common additives and fillers. •

•

•

Anti-microbial imparts protection against mould, mildew, fungi and bacterial growth to materials. Without anti-microbials, polymers can experience surface growths, causing allergic reactions, unpleasant odours, staining, embrittlement, and premature product failure. Antioxidants are used in a variety of resins to prevent oxidative degradation. Such degradation occurs by the initiation of free radicals, which possess unpaired electrons and are highly reactive. These radicals are created by heat, radiation, mechanical shear or metallic impurities. Free radicals may also form during polymerization, processing of fabrication. The function of antioxidants is to prevent the propagation steps of oxidation. Antistatic agents are additives used in plastics to prevent the buildup of excess electric charge. This electricity is formed during processing, Page 4 IC Professional Training


•

• •

•

• • •

2.5

transportation, handling and final use. Secondary benefits of antistats can include improved processability and mould release. In fact, antistatic agents are used as lubricants, slip agents and mould release agents in some processes. Plastics are inherently insulated and do not allow built-up static electricity to dissipate easily. In most plastics, excess charges can linger or discharge, causing such problems as dust attraction, fire and explosion hazards, poor mould release, and damage to electrical components. Flame retardants additives for plastics are essential safety materials. The transportation, building, appliance and electronics industries use flame retardants in plastics to prevent human injury or death, and to protect property from fire damage. Fundamentally, flame retardants reduce the ease of ignitability and rate of burn of plastics. UV stabilizers are used in a variety of resins to prevent degradation caused by UV radiation from sunlight. Glass or Carbon fibres up to 40% (by weight) chopped Long and short glass fibers (GF) reinforced thermoplastic are added to a polymer matrix with distinguished good mechanical properties and high thermal resistance. Both Glass or Carbon continuous fibres are wound, weaved or braided into clothes and mats for transportation usage with superior fuel saving and reduction in production cost. Calcium Carbonate the least expensive and the largest mineral filler used (upto 70%) in thermoplastic to reduce shrinkage and offers good surface finish Barium Sulfate: the densest mineral uses in a few end products such as sound barrier or dampening applications. Talc enhance the stiffness and raise the heat deflection temperature significantly and better dimensional stability. Kaolin: clay or natural alumni-silicate provides good impact modification for automotive applications improves dimensional stability like talc. Clay provides better sound dampening but not as well as barium sulfate. Selection of Plastic Material

In order to choose suitable plastic material for our application, we need to understand the properties of different plastic material. The followings are some common properties we need to consider before choosing the plastic material. 2.5.1

Physical properties considerations

Physical properties can be observed or measured without changing the composition of matter. Physical properties are used to observe and describe matter. The followings are some common physical properties. •

Density is equal to mass per volume. Density = mass (g) / volume (cm3), by knowing the volume of the material, the weight of the material can be Page 5 IC Professional Training


•

•

•

•

• •

2.5.2

calculated. Smaller density means lighter in weight if the volume of the materials is the same. ISO 1183 Water Resistant describes how well an plastic part resists to water passage. Especially in some outdoor electrical device waterproof should be considered. Dimensional Stability is the property of a polymeric part to retain its form when subjected to varying degrees of thermal, moisture, pressure, or other stress. Softening and Melting Temperature is the temperature when the material will soften and melt. Different environment and purpose may require different temperature range. ISO 75 Flammability of plastics is tested accordingly Underwriters Laboratories UL 94 to measure the resistance of plastics to a fame source. UL approval is given for a particular product at a measured thickness with ratings ranging from least flame retardant to most flame retardant as HB, V2, V1, V0, 5VB and 5VA. Electrical properties are the resistance, insulation, dielectric strength, dissipation factor of the plastics. ISO 1325, ISO3915, ISO1325, ISO1325. Optical properties are the gloss, transparency, haze, colour and refractive index of plastics. Plastics can be product with a wide of range of colours to meet the lifestyle demand of people. ISO 489 Mechanical properties considerations

Mechanical properties describe how a material responds to the application of a force or load. The followings are some common mechanical properties. •

Tensile Strength is the ability of a material to withstand forces pulling it apart. ISO527-1



•

Impact Strength is the ability of a material to resist shock loading. ISO 179, 180.

•

Flexural Strength is the measure of how much stress (load) can be applied to a material before it breaks. ISO 181, 871, 1210. Ductility describes the extent to which a material can be deformed without fracture. Hardness is the resistance to compression, indentation and scratch. Durometer hardness tester is used to measure the material resistance against the indentor spring load balance. The hardness is ranging from 1 to 100 with Shore A. B, C, D, DO, E, M, O, OO, OOO,OOO-Sand R standards. The general Shore A standard is for normal elastomer and Shore D is for hard plastics (ASTM D2240 A and D testing standards). ISO 868.

• •



2.5.3

Practical application considerations

In practical application different usage have different requirement, the followings are some considerations. •

• • • • • • • 2.5.4

Weather Resistance is the ability of a material to withstand the effects of wind, rain, or sun(UV) and to retain its appearance and integrity. ISO-4892 , ASTM D-2565. Wear Resistance is the ability to resist removal of material from a surface as a result of mechanical action. ISO 2556, ISO 62,585, 960. Microwave Resistance is the ability of a material to resist microwave. Fire Resistance is the ability of a material to resist fire. Cost Factor is the amount of the money can be spent on the project or production. Manufacturability is the factors need to be considered in manufacturing like the method of manufacturing, shrinkage, tolerance. Environmental Factors is the impact on the environment when the material is disposed, is the material biodegradable? Can it be recycled? FDA Compliance is a certification of plastic materials that are used in contact with food certified by Food and Drug Administration (FDA) of USA. Major Consumption of Plastics

Majority of plastics consumption (over 90%) are commodity thermoplastics such as High Density Polyethylene (HDPE), Low Density Polyethylene (LDPE), Polypropylene (PP), Polystyrene (PS), Polyvinylchloride (PVC), Polyethylene Terephthalate (PET). Because of their popularity, they are respectively identified as the recycling codes as shown on forms of plastic packaging in the following diagram.

The second category of plastics (around 8% of total consumption) is known as engineering plastics for their improved mechanical properties and load bearing characteristics. Examples of engineering thermoplastics are Polyamides (Nylon), Polycarbonates (PC), Polyoxymethylene (POM), Styrene acrylonitrile (SAN), Acrylonitrile-butadiene-styrene (ABS) Polymethyl methacrylate (PMMA), cellulose acetate (CA), Polyphenylene ether (PPE), Thermoplastic elastomers (TPS), Polyurethanes (PUR) and the others. Lastly, there is only less than 1% of plastics consumption that is classified as high technology plastics. These plastics are with superior high temperature with much improved mechanical properties such as liquid crystalline polymers (LCPs), polyetheretherketone (PEEK), Polysulfones (PSU), Polyphenylene sulfide (PPS, Polyarylates (PAR), Polyimides (PEI) and the others.



2.5.5 Common Plastics Thermoplastics Polyethylene Terephthalate (PET): is used in beverage, food and other liquid containers; thermoforming applications. For post consumer, PET is one the easier collected and sorted in Mixed Plastic Wastes (MPW) for recycling purpose for its dominated application and ease of identification as the bottles for drinks and alcoholic beverage. High Density Polyethylene (HDPE): is one of the most stable and inert polymers, exhibiting very high resistance to chemical attack including alkalis, aqueous solutions, non-oxidising acids and to a lesser extent, concentrated oxidising acids. HDPE is used in hollow toys, playground equipment, tanks, milk bottles and water pipe and very thin carry bags. Polyvinyl Chloride (PVC): is commonly used as for the insulation on electric wires and over 50% of PVC manufactured is used in construction.PVC is used as magnetic stripe cards, window profiles, pipe, plumbing and conduit fixtures. Low Density Polyethylene (LDPE): is used for plastic wrap, plastic bags, dispensing bottles, wash bottles and food storage containers for its flexibility and soft features. Polypropylene (PP): has a melting point of ~160°C and is rated as 120°C operating temperature and is suitable for food containers that need to be dishwasher safe. Polypropylene is also very easy to add dyes to, and is used as hinges, food packaging, textiles, laboratory equipment, automotive components, and polymer banknotes. Polystyrene (PS): Pure solid polystyrene is a colourless, hard plastic and brittle, can be transparent for plastic assembly kits, plastic cutlery, rigid, economical plastics. Expanded polystyrene for packaging is used as foam for protection. Acrylonitrile-Buadiene-Styrene (ABS): is considered superior for its hardness, gloss, toughness, and electrical insulation properties. The nitrile groups making ABS stronger than pure polystyrene. The styrene gives the plastic a shiny, impervious surface. The butadiene, a rubbery substance, provides resilience even at low temperatures. ABS can be used between −25 °C and 60 °C. Styrene-AcryloNitrile (SAN): exhibits outstanding transparency, good chemical resistance, rigidity, dimensional stability and thermal shock resistance and excellent resistance to outdoor exposure, aging and yellowing and is used as appliance bodies, mixer bowls, water reservoirs. Polycarbonate (PC): is used to create protective features, e.g. in banks bullet-proof windows, lighting, lenses, sunglass/eyeglass lenses, compact discs, DVDs, and automotive headlamp lenses for its impact resistance and good strength at



elevated temperatures, to overcome the shortcomings of poor chemical and physical weathering in an Ultraviolet light environment . Polyamide (PA): is commonly known as nylon for its trade name, it is High mechanical strength, rigidity and thermal stability, good impact resistance even at low temperatures, advantageous sliding friction properties. Polyacetal Copolymers (POM): is with a high degree of rigidity and mechanical strength, outstanding resilience, optimal dimensional stability and excellent resistance against a variety of chemicals and is used to make gears, bushings, fasteners and other mechanical parts. PolyMethyl Mehacrylate (PMMA): is commonly known as acrylic, Perspex or Plexiglas for it clarify and transparent properties and is usually used as an substitute or glass and cheaper but with inferior mechanical properties than Polycarbonates. Thermosets Phenol-formaldehyde (PF): is the most widely used of all the thermosets for its excellent dimensional stability under thermal cycling and high stress conditions, low water absorption, and high surface hardness, compressive strength and highly resistant to petrochemicals and hydrocarbons. An Modified injection moulding method is by preheating, metering and plunging the PF resins into a mould that is embedded with heaters to cure the resin after the injection. Melamine – formaldehyde (MF): is usually formed by compression moulding method for its lack of pourability, it is with wide range of colour and scratch resistance and is often used in tableware, bowls and plates. However, it is not microwave safe. Epoxies: is cured by addition of a hardener to achieve total cross linking. It is used as electrical connectors, encapsulating components of Integrated Circuits, electronic components and coatings for its electrical, mechanical, chemical properties at elevated temperatures and its very high moisture resistance. Sheet Moulding Compounds (SMC) or Bulk Moulding compound (BMC) Composite: is typically 20-30% lighter than equivalent steel parts resulting in fuel saving and improved performance in transportation industries (automobiles and airplanes). Composites are with different resin systems (Polyesters, polyimides, expoxies and polyureas) and reinforcement (chopped or woven or filament winding of organic, boron, glass or carbon fibres), combination designed to meet different applications. They can be moulded by convectional compression, transfer and injection moulding and other techniques such as lay-up for exceptional strength requirements.



Elastomers Silicones: is used as seals, gaskets, o-rings, terminal covers, lubricants in food industries for its thermal stability, good flammability rating 94V0 in 1/16 in section, non-stick, low chemical reactivity, low toxicity and good electrical insulation. Natural rubber: is harvested as liquid suspension by scarping the barks of a rubber tree and can be cured by vulcanization using sulphur, peroxide or bisphenol. It is used as tyres, hoses, belts, matting flooring and dampeners in industrial uses and gloves for its good elasticity. Synthetic rubber: is made from the polymerization of monomers for a wide range of physical, mechanical and chemical properties while maintaining the elasticity properties. Thermoplastics elastomers (TPE): is a physical mix of polymers as polyolefin blends, thermoplastic polyurethanes, thermoplastic copolyester and theromoplastic polyamides with the flexibility of rubber, silent aesthetic and pleasant to touch. The typical crosslinking processing vulcanization in the thermosetting elastomers is through covalent bonding. While in TPE, the crosslinking is a weaker dipole or hydrogen bond suitable for recycling and reuse. General properties of Plastics Density g/cm3

Tm Melting ºC

Tg glass ºC

Tensile strength MPa

Elastic limit %

1.37-1.455

260

75

55-75

50-150

LDPE

0.910 -0.940

98-115

-

8.0 -31

PVC

1.30-1.58

100260

57-82

50-80

20-40

HDPE

0.952-0.965

130-137

-

18.5-24.8

55

PP

0.855-0.946

160

-

31-41

15

PS

1.04-1.05

240

95

45-60

3-4

ABS

1.04-1.05

-

105-115

29.6

20

SAN

1.06-1.1

-

102-104

32-40

4

PC

1.2-1.22

267

150

55-75

80-150

1.15

254

-

59-90

50

1.4-1.5

165-178

-

18-97

40

1.19

130-140

-

48-76

5

PF

1.30-1.51

-

-

50-55

0.45-2.3

MF

1.41-1.49

345

-

45

-

Name PET

PA Nylon 6 POM PMMA



3.

Plastics Processing

Many different methods are employed to convert plastics from their raw state into finished products or to fabricate stock plastics materials into finished products. In the industry the mass production processes are moulding and thermoforming. Moulding includes injection moulding, extrusion, blow moulding, compression and transfer moulding. 3.1

Injection Moulding

Injection moulding is the most important process used to manufacture plastic products. Today, more than one third of all thermoplastic materials are injection moulded. 3.1.1

Process Description

Injection moulding is the best process to use for high-speed, low-cost moulding of intricate plastics parts required in high volume. In this process, thermoplastic is fed from the hopper through an opening at the rear of the heated injection barrel (charging). The resin is forced forward to the front of the heated barrel by the rotation of a reciprocating screw, where the material is heated in various stages until it reaches a molten state. The injection screw forces the measured amount of molten resin into the shaped cavity of a closed mould through the nozzle/sprue/runner/gate by a ram action. The molten resin cools and solidifies in the mould cavity. After cooling, the mould is opened and the moulding is ejected. Almost all thermoplastics can be injection moulded and even some thermosets are being injection moulded with modified equipment. PE, ABS, nylon PA, acrylic and polystyrene are amongst the leading thermoplastics used in injection moulding. Typical injection moulded products include appliance housings, camera cases, lenses, gears, fan blades, spoons, wastebaskets etc…





3.1.2

Injection moulding machine

An injection moulding machine consists of three units; they are the plasticating & injection unit, the clamping unit and the mould cavity.

Plasticating and Injection Unit Clamping Unit

•

Plasticating & Injection Unit : The major tasks of the plasticating & injection unit are to melt the polymer, to accumulate the melt in the screw chamber, to inject the melt into the cavity and to maintain the holding pressure during cooling.



The recent development of all electrical or hybrid injection moulding machines offer greater high speed performance, low power consumption and increased precision, microprocessor control and robust & versatile in machine configuration using the more accurate electric servomotors .

•

Clamping Unit

The major tasks of the clamping unit are opening and closing the mould, close the mould tightly during injection. There are three clamping types: mechanical, hydraulic and their combination.



•

Specification of Moulding Machines

Shot size and clamping force are usually used to describe a machine. We need to consider both shot size and clamp tonnage when choosing a machine. i)

ii)

3.1.3

Shot size is the maximum amount of material the machine will inject per cycle (single shot) and the unit is ounces (oz) or grams (g). The standard for shot size measurement is general purpose polystyrene moulding in single shot. Clamping Force is the maximum force a machine can apply to a mould. The unit of clamping force is tons. Part Design for injection moulding

Part design is a very important in injection moulding, good part design can reduce the manufacturing cost and reduce the defects during manufacturing. •

Uniform Wall Thickness

Uniform wall thickness should be the primary consideration in part design because different wall thickness causes different shrinkage which increases the difficulties dimension control and cause serious warpage in the injection moulded products.

•

Draft Angles

Draft Angles are added in the internal and external walls for the mould part to be ejected from the mould. Draft angle requirement are smaller in external walls than internal walls. •

Radii/Fillet

Internal sharp corners and notches are the leading cause of failure in injection moulded thermoplastic parts. To avoid the problem occurred, radii / fillet is commonly employed to all “sharp” feature.



Fillet radius is determined by the wall thickness and the carrying load. Fillet radius should be between 25 to 60% the nominal wall thickness. The larger fillet radius is suggested for load carrying features. •

Ribs

Ribs are used to strength the structure and reduce the weight of the product.

t

h

Rib thickness should be 50 to 60% of the nominal wall thickness t, the rib height h should not excess three times of the nominal wall thickness. Spacing between two parallel ribs should be more than two times of the nominal wall thickness. Draft angle for ribs is 1 to 1.5°. •

Bosses

Bosses are thermoplastic cylinders attached to a side wall or end corners. They can be used for assembly with self-tapping screws. A boss should not be attached directly to a side wall because it will cause sinks or voids.

The outer diameter of the boss should be two times the inner diameter of the boss. The height of the boss should be less than three times of the outer diameter of the boss. The distance between two bosses should be more than two times of the nominal wall thickness t, the wall thickness at the base of the Page 17 IC Professional Training


boss should less than 60% of the nominal wall thickness t. the minimum draft angle on the outer diameter of the boss is 1/2° and inner diameter is 1/4°. Snap-fit Design

•

Snap fits are commonly used as an assembly method for injection moulded parts. Snap fits are very useful because they eliminate screws, clips, adhesives, or other joining methods. The snaps are moulded into the product, so additional parts are not needed to join them together. There are three main types of snap fits: Annular, Cantilever, and Torsional. Cantilever snap fits are the most widely used type of snap fit. There is a considerable amount of calculation and engineering that goes into designing a good snap fit.

•

Annular snap fits are generally stronger, but need greater assembly force than their cantilevered counterparts. They are basically interference rings.

•

The torsional snap-fit relies for its spring effect on twisting rather than flexing like the other types. It is a good way of fastening a hinged lid on a box or container.

•



3.1.4

Mould Design

The mould is an important element in the moulding machine, apart from determine the shape of the product, the mould vents the entrapped gas, cools the product and ejects the product. The quality of the product and the manufacturing cost are largely determined by the mould. The mould is comprised of mould base, core and cavity that determine the feature of the product, sprue, runner and gate that deliver the melt to the cavity, cooling system and ejection system. •

Mould Base: As a matter of fact, nearly all moulds consist of the same basic components. There are some standard mould bases in the market that provide cheaper and more reliable than custom design mould base. The followings are some standard mould bases.

In very broad sense, moulds can be classified as cold runner and hot runner moulds. Two-plate mould and three-plate mould are most common in cold runner moulds.



i)

Two-Plate Mould This is the most basic and most common type of mould. Twoplate mould has a single parting line, there are two plates in the cavity plate, with the central sprue bushing assembled into the stationary half of the mould, the moving half of the mould contains the cores and ejector mechanism, and in most designs the runner system.

ii)

Three-Plate Mould Three-plate mould has two parting line, one more intermediate and movable plate is introduced which increase the flexibility on gating locations.

There are other types of cold runner moulds like external under-cut mould, internal under-cut mould, side core mould, unscrewing mould, stack mould. iii)

Hot Runner Moulds

A hot runner mould refers to a mould in which the runner stays molten and is not ejected during the moulding cycle. In hot runner mould, the runner is eliminated so that the shot size, plastification time, runner cooling time and the clamping force can be reduced. The hot runner system is comprised of two primary components that are the manifold and the drop.



•

Core and Cavity

A mould must consist of core and cavity. The core (male) is fixed on the moving half of the mould and the cavity (female) is fixed on the stationary half of the mould. Gate is always on the cavity.



•

Feed System

The feed system is the flow-way of plasticized material to the cavity; it consists of a sprue, runner and gate. i) ii)

iii)

•

Sprue: is the entrance of the feed system. It is a divergent taper and highly polished. Runners is the channels through which the plasticized material enters the gate areas of the mould cavities are called runner. Normally, runner is round or trapezoidal in cross section.When designing a runner layout, the runner length should be minimized and balanced. The large parts and small parts should not be combined. Gate provides the connection between the runner and the mould cavity. It must permit enough material to flow into the mould to fill out the cavity. The followings are common types of gates.

Cooling system

Cooling means to maintain the temperature of the mould/die evenly in the moulding process by cooling channel, poor cooling design will affect the functioning of the mould and the quality of the moulded part. •

Ejection system

Ejection is necessary for part to be removed from the mould. The hot materials injected into the cavity will shrink and stick tightly onto the mould core. The ejector plate will be driven by the injection machine to carry the whole Ejection system travels sufficiently to clear the moulding from the mould.



3.1.5 Moulding Defects Injection moulding is a complex technology so defect may happen if it is not careful or experience enough. The followings are some common defects. •

Burn Marks are caused by poor venting. It can be solved by adding venting at parting line.

•

Sink Marks are caused by insufficient injection pressure and holding time. It can be solved by increasing holding pressure.

•

Warpage is the shape deformation due to uneven shrinkage. It can be solved by increasing cycle time and using shrink jig. Weld line is due to insufficient air venting and injection speed too low. It can be solved by increasing injection speed and providing air venting.

•

•

Air Trap is caused by poor venting, not enough injection pressure. It can be solved by adding air venting.



3.2

Blow Moulding

3.2.1

Extrusion Blow Moulding

The extrusion-blow moulding process is extensively used for making bottles and other hollow plastics parts having relatively thin walls. To blow mould a part, the extruder first extrudes a hollow tube (parison) in a downward direction, where it is captured at the proper time between two halves of a shaped mould. After trimming the top and bottom of the parison, air is blown into the soft parison, expanding it until it uniformly contacts the inside contours of the cold mould and solidifies. Then the mould automatically opens, the part is ejected and a new cycle begins. PE, PVC, PP and PS are commonly used plastics for blow moulded articles. Typical products include bottles, watering cans, display fruit and other hollow parts.

3.2.2

Injection Blow Moulding

In the injection blow moulding process, the polymer is injection moulded onto a core pin; then the core pin is rotated to a blow moulding station to be inflated and cooled. The process is divided into three steps: injection, blowing and ejection. The injection blow moulding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mould. The preform mould forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body. The preform mould opens and the core rod is rotated and clamped into the hollow, chilled blow mould. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.



3.3

Extrusion

Extrusion is common process in plastic manufacturing, nearly 40% plastic product are made from extrusion. It is a process used for making indetermined length of thermoplastics with constant cross-section. Pellets (or powder) are drawn continuously from a hopper into a heated barrel by the action of a rotating screw, where they are heated and softened as they progress through the heated barrel. At the front end of the extruder, the melted plastics are forced through a shaped die that determines the final cross-section of the extrudate, after which it is uniformly cooled and carried away on a continuous basis. Length can be cut as desired. ABS, PE, PS and PVC are extensively used in extrusion. Typical product includes piping, drinking straw, window track, wire and cable coating, film and sheet.



3.4

Compression Moulding

Most thermosets must be moulded under heat and pressure to achieve a satisfactory end product. The most widely used methods for moulding thermosets are compression and transfer moulding.

In compression moulding a pre-weighed and preheated amount of thermoset powder is loaded into a heated mould, the mould is closed and pressure is applied to the powder. The powder melts under heat and pressure and flows into all parts of the mould cavity, after which an internal chemical reaction crosslinks the plastics chain, hardening the plastics into its final irreversible state. The cured thermoset part is removed from the mould while still hot and allowed to cool outside the mould. 3.5

Transfer Moulding

Transfer moulding is similar to compression moulding except that the heated plastics powder is placed in a separate chamber in the form of a cylinder & plunger, then transferred under heat and pressure into a closed mould where the shape of the part is determined and the final crosslinking reaction takes place. Phenolic PF, melamine formaldehyde MF, urea formaldehyde UF and epoxy EP are common materials used in these processes. Typical products include pot handle, electrical connector, button, dinnerware, knob and tool handles.



3.6

Thermoforming / Vacuum Forming

This is a process for forming moderately complex shapes of uniform wall thickness, particularly when walls are very thin and cannot be injection moulded, or when parts are very large and too expensive to be injection moulded. The thermoforming processes uses sheet plastic that is softened by heat until pliable, then forced by vacuum, negative air pressure, or mechanical drawing against a cold mould surface where the sheet cools and retains the shape of the mould. Almost all thermoplastics sheet can be used in this process. The commonly used plastics are HIPS, ABS, PVC, acrylic, cellulosic. Typical application includes blister pack, suitcase and disposable plate.



4.

Other Plastic Fabrication processes

In addition of the above mentioned mass production manufacturing processes, single piece or small quantity of plastic models can be produced by others processes, the following session will introduce three common techniques for making plastic parts. 4.1

Plastic board Fabrication

In general, the working of plastic materials by hand tool or by machine usually uses the same methods as those commonly employed for work on wood and metals such as filing, drilling turning, milling , Hot Wire Bending, Engraving, Sand blasting, Fastening, Laser Cutting, Mechanical fastening, Bonding or sawing. 4.1.1 Cutting Plastics - Plastics can be cut by methods commonly employed for wood, metals and paper. Among the various cutting methods sawing is the most effective one. •

Hand Saws - Many hand saws can be used to cut plastics. Hack saws work well for cutting rod, tube and sheet. Jig saws are useful for cutting intricate shapes and holes in plastic sheets.

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Circular Saws - Circular saws are suitable for straight cuts. A speed of about 1,500 m/min is a reasonable average for cutting plastics. Carbidetipped saw blades will hold up longer with less maintenance, but hollowground cross-cutting blades with zero rake and 2-3 mm pitch will do many jobs well. All blades must be kept clean and sharp.



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Band Saws - Band saws are generally used for cutting curbes, irregular cuts, and thick materials. The advantage of using a band saw instead of a circular saw is that the cut is cooler. It is, however, more difficult to obtain as straight and as smooth a cut as with a circular saw.

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Jig Saws - Power jig saws are more efficient than hand jig saws. They are suitable for cutting intricate curves and holes.

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Sanding - Belt and disc sanding machines are effective finishing equipment for plastics. For parts which are too large to be worked by these machines, portable sanders or hand sanding may be employed. Page 29 IC Professional Training


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Buffing - Buffing is a polishing operation using a cloth or felt that contains fine abrasive. The coarseness of the abrasive used depends upon the original roughness of the part and the degree of luster desired. Buffing will not ture a surface, but tends to round sharp edges and produce a lustrous appearance.

4.1.2 Cementing- There are two basic methods of cementing plastics, i.e. Cohesive and Adhesive-bonding. •

Cohesive Bonding - Cohesive bonding is also known as solvent cementing, in which the surfaces of the joint are dissolved by a suitable solvent and then pressed together. However, this method is only suitable for thermoplastics and the material of the joint must be the same, e.g. acrylic with acrylic or styrene with styrene.

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Adhesive Bonding - This method is suitable for joining any materials, similar or dissimilar. It is necessary to find an adhesive which will stick to the materials involved. Although adhesive-bonding can be used with any plastics, it is generally not used where solvent bonding is satisfactory. This means that the most common application of adhesive bonding is with thermosets or where dissimilar materials are to be joined.

4.1.3 Mechanical Fastening The use of screws, bolts and nuts for fastening two pieces of plastics is also a common practice in joining plastics. The decision to use mechanical fasteners is based on the strength of the plastic



4.2

Welding

Welding by heat is only suitable for thermoplastics. The process consists of heating joint areas to fusion and then presses the joint surfaces together. After joining, the areas are cooled to their rigid forms. 4.2.1

High Frequency Welding

High-frequency Welding can only be carried out by a high-frequency (27.12 MHz) welding machine with a suitable welding electrode and a suitable plastics material. The equipment consists of a high-frequency generator, a press which is either pneumatic or mechanical operated, a machine table which forms the negative electrode and a welding tool which forms the positive electrode. The positive electrode defines the shape of the weld. When a material with a large dielectric dissipation factor is subject to a high frequency direct current, the molecules are forced to rearrange their polarities on either side of the positive and negative electrodes in the electric field, and thus cause a fusion by molecular friction heat.



4.2.2

Hot Air Welding

Hot-air Welding involves the heating of the plastics joining area to fusion state by a jet of hot air from a hot air gun and then filling up the joint by a filler rod which is similar in properties with the plastics sheet being welded.

4.2.3

Ultrasonic Welding

Ultrasonic plastic welding is the joining or reforming of thermoplastics through the use of heat generated from high-frequency mechanical motion. It is accomplished by converting high-frequency electrical energy into highfrequency mechanical motion. That mechanical motion, along with applied force, creates frictional heat at the plastic components' mating surfaces (joint area) so the plastic material will melt and form a molecular bond between the parts. The following drawings illustrate the basic principle of ultrasonic welding.

4.3

Resin Casting

For Resin casting please refer to the rapid tooling section in the reading material of Rapid Prototyping & Manufacturing



References •

Manas Chanda, Salil K. Roy, (2009) “Plastics Fabrication and Recycling”, CRC Press.

• • • •

Erik Lokensgard, (2004) “Industrial Plastics Theory and Applications”, Thomson. http://www.matweb.com http://www.dukcorp.com/us/PPL_WhatIsUPA.htm Osswald, T., Hernandez-Ortiz, J. P., (2006) Polymer Processing-modeling and simulation, Hanser

• • •

Hans-Georg, E. (2003) An Introduction to Plastics, Wiley-Vch Friedrich Johnnaber, (2008) Injection Molding Machine- A Use’s Guide, Hanser Osswald, T.,Turng, L.S., Gramann, P. (2008), Injection Molding Handbook, Hanser


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