Resin Casting In The Home Workshop Resin castings are ideal for reproducing car or wagon body sides. They can also be used to reproduce stone, brick, board or other siding material for use in architectural models. They are often made in open (one-piece) moulds although to ensure a uniform thickness a sheet of acrylic or rubber should be placed over the open surface. Take care to exclude air bubbles. More complicated castings involving detail on both sides of the pattern or even whole wagon bodies can be made using two-piece moulds.
Overview Of The Process A full size master (pattern) is made up using suitable materials. A mould is made of the pattern using two-pot silicon rubber. This material doesn't stick to anything except itself. Two-pot resin is poured into the mould and allowed to cure. The casting is removed and the process is repeated.
Making The Pattern Techniques are the same as for model making in general. The level of your crafting ability will be the bottom line as far as your p atterns are concerned. Styrene or acrylic fabrication, soldering of metal, lathework, carving and rivet embossing can all be used. Care must be taken to select materials that will not inhibit the curing of the mould although with the current generation of silicon rubbers this is not usually a problem. Some overhangs are possible, particularly if a more flexible rubber is going to be used to make the mould.
Making A One-Piece Mould The pattern is mounted on a flat base and surrounded by a fence high enough to allow effiecient thickness of rubber above the thickest pa rt of the pattern (see Fig. 1).
Freshly mixed silicon rubber is poured over the pattern and allowed to cure (see Fig. 2).
The rubber is removed carefully form the pattern and inverted. If all has gone well you will have a perfect, bubble -free negative of your pattern (see Fig. 3).
Making A Two Piece Mould This could be done in the same way as the moulds for metal casting are made. The method outlined below is described in Adrian Gunzburg's article in the Australian Model Railway Magazine, Issue 145, August 1987. The pattern is mounted temporarily on a platform, slightly larger than in area than the pattern itself. The pattern needs to be securely mounted because if it is filled with air (e.g., a complete, hollow wagon wago n body) it could float off! The platform is then mounted on a flat base and surrounded by a fence high enough to allow sufficient thickness of rubber above the thickest part pa rt of the pattern (see Fig. 5).
Freshly mixed silicon rubber is poured over the pattern and allowed to cure (see Fig. 6).
The rubber is now removed from within the fence, probably best achieved by breaking away the fence itself. The pattern p attern may come with the rubber or it may stick to the base. If the latter is the case it must be removed (see fig. 7).
Coat the inside of the mould with a mould release agent and replace the pattern. Place a suitable piece of scrap in position po sition for a tube (sprue) through which the resin can be poured when making the casting (see Fig 8)
. Pour another batch of freshly mixed rubber into the hollow of the pattern and filling up the first section of the mould (see Fig. 9).
When the rubber has cured, remove the sprue and pattern (see Fig 10).
Cut some small channels to allow for the release of air as the resin is poured in.
Making Castings The interior of the mould is coated with mould release or spray-painted. For a one part mould have a cover ready and coat it with mould release also. The cover can be from any smooth material. The base of o f old or unused moulds is an excellent surface because it releases so well. The mould should be assembled and set up on a level surface. Freshly mixed resin is poured into the mould, taking care to exclude or remove air bubbles. A toothpick is good for working wo rking bubbles out of fine detail on the mould (see Fig. 4). If the mould is in just one piece, the lid should be over ove r the resin, making sure that no air csavities are trapped.
For a two-piece mould it may be convenient to make a rudimentary funnel from paper or stout foil to direct the liquid resin through the hole.
When cured the casting is removed from the mould and, after cleaning, is ready for use.
Notes About Resin Casting General Remarks •
Resins can be a problem to people with sensitive skin. It is a good idea to wear rubber gloves when working with any of o f the chemicals used in casting.
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Many chemicals used in model making are injurious to the eyes. Protective googles or safety spectacles should be worn when ever working with them.
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The vapour from polyurethane resin should not be inhaled.
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Spread old newspaper around when casting in resin. A degree of mess is inevitable!
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See Graeme Brown's article 'The Compleat Chemical Mode ller' in Australian Model Railway Magazine, Magazine, Issue 144, June 1987 for safety tips regarding the use of chemicals in the home workshop.
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See the article on page 50 of the February 2002 issue (Number 232) of Australian of Australian Model Railway Magazine! Magazine!
Materials Rubber
Dow Corning J-Type ('green') rubber is quite flexible and is suitable for use with casting resins. It does need de-airing under a vacuum and is sensitive to a number n umber of inhibitors; you need to be careful about the materials that you use to make your mould. Rhodorsil RTV 585 is very flexible, is strong and doesn't need de-airing. It is also less sensitive to inhibitors. The catalyst ratio is low (2%). Dow Corning make a similar product. Resin
Polyester resin is available from any fibre-glass merchant. It has noticeable shrinkage. It is OK for scenery work but you would have to build in adjustments to pattern size if you used it for more precise work. It does not cure properly and is not recommended. The inclusion of fillers such as talc or aluminium dust are reported to reduce both deficiencies mentioned above. Polyester resins are relatively cheap. Epoxy casting resin shows negligible shrinkage and takes about 24 hour to cure. You should obtain the technical specifications from your merchant. A drying oven can speed up the process in cold c old weather but take care! Proportions of the two components are specified by weight and should be adhered to. Some epoxy casting resins are fairly colourless and transparent and can be used where you want to simulate windows or water. Significantly more expensive than polyester resins. Araldite Resin-M is one brand and va rious merchants have similar lines. Polyurethane resins are used by most VMRS members who do their own casting.
Fast-Cast 810 and Ezi-Cast both begin to cure within a couple of minutes of mixing and set in a short time. They should not be removed from the mould to soon because they 'remember' their shape as they cure and an d any deformity during this process will become permanent. Super-Cast is more viscous than these an d requires a little more fiddling to remove bubbles. This is compensated for by it being a little slower setting. It is reported to be more stable once cured. Polyurethane resins are slightly more expensive again than epoxy casting resins. Mould Release
Commercial types in spray cans are available from fibre-glass merchants. You can make your own by dissolving petroleum jelly ('Vaseline') in a suitable solvent such as paint thinners.
Suppliers Of Materials The following firms in Australia supply materials for casting. The list is not exhaustive. VMRS has no connection with these firms other than that our members have been satisfied with their service.
Rubber, resins and general supplies Solid Solutions (One Stop Plastics) http://www.solidsolutions.com.au 19 Ardena Court, PO Box 142 East Bentleigh,3165 Victoria Australia (03) 9579 2044 Monday to Friday 9am - 5pm Barnes Moulding & Casting Supplies http://www.barnesproducts.com.au 53 King Street Newtown NSW 2042 Telephone: (02) 9557 9056 Facsimile: (02) 9557 9246 Both firms have a good mail order service
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