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Silicone Moulding and Resin Casting Made Easy

By Grant Evans 

This presentation will teach you how to make a one piece basic open faced RTV silicone mould and produce a complete polyurethane resin casting in one hour. I will teach you all the tricks of the trade so you will finish with castings like a professional.

The piece you wish to make a mould from is called the master. The master is an original piece you have scratch built from various materials or have sculpted from an oil based or epoxy based sculpting clay. “Warning” if you copy another person or companies piece you may be infringing their copyright.

Once you have the master you wish to make a silicone mould of you need a mould box and a flat base to work on, eg a tile or piece of laminex. The mould box can be clay, a professionally made adjustable mould box with clamps, lego blocks, sealed timber or even the lid of a container. The important part of selecting you mould box is the size of it; you want to finish up with approx 8 - 10 mm of silicone surrounding your master. If the walls of your silicone end up too thin they will buckle under the weight of the casting resin and handling of the mould.

Once a mould box is selected it is important to seal your master down to a flat base. Reason for this is the piece will float up or move when pouring you silicone into the mould box, causing all sorts of dramas. This can be achieved using a thin layer of clay and compressing you master into it and removing neatly all the excess clay where it meets the mould box. The second option is to use ‘zap a gap’, which is a gap filling modelling glue, using this option there is a good chance you will break the original piece when removing it from the tile when you have finished making you mould. Now place the mould box around your master and use Klean Klay, which is an oil based clay to seal the mould box onto the tile and any corners that the silicone may leak from when pouring into the mould box.

It is now time to spray the inside of the mould box and the master with a paintable mould release. If you wish to paint the finished casting don’t use a silicone based release agent as this will transfer onto the surface of the resin casting and the paint will not bond.

We are now ready to mix and pour our RTV silicone into the mould box. There are many different types of moulding silicone. Room temperature vulcanising silicone is available in a condensate Cure and Addition Cure, these silicones have a shore A hardness which relates to the flexibility of the silicone the lower the number eg Shore A20 the softer and more flexible the silicone. When there are undercuts in the master / original piece, the silicone mould will need to be flexible and soft to stretch over these undercuts. Hence a lower Shore A RTV silicone with good tear resistance would be required.

We are using an RTV silicone called Pinkysil it has a Shore A of 20, a pot life, (which is mixing a pouring time) of 5 mins and demould (cure time) of 20-30 minutes @ 23 C. Air temperature affects all moulding and castings materials the higher the temperature above 23C the faster the cure time and less pot life or working time. The important part of moulding making is to always observe and mix exactly as the manufacturer of the materials recommends.

Weigh out equal parts of A and B of the Pinkysil using digital scales. Then pour part B pink catalyst into the part A white silicone. Stir thoroughly ensuring the pink catalyst is evenly mixed, a fast stirring action using an ice cream stick or tongue depressor is required, then pour the mixed silicone into another clear plastic cup making sure to scrape the excess silicone off the sides and bottom of the mixing cup. The reason for this is the silicone and catalyst never evenly mixes when it is stuck to the sides and bottom of the first mixing cup.

Pour the silicone into the corner of your mould box and not directly onto the master. Pour slowly with a thin stream initially lifting your mixing cup to approx 150mm above the mould box as you slowly pour into the same spot watching the silicone slowly flow over your master. Once the master is covered with silicone you can pour rapidly to fill the mould box approx 10mm above your master. Don’t touch the silicone; allow it to flatten with gravity and then cure. If there are undercuts or hollows you are concerned with in your master / original the silicone may not flow into, use a small brush and paint the silicone in prior to pouring your mould. The longer you allow an RTV silicone to cure the more life you will get from it relating to more castings in conjunction with good mould release techniques. Once the cure time has passed carefully remove your mould box and silicone mould from the master so you don’t tear any delicate parts of silicone that may get caught in the undercuts of the master.

You now will be looking at your completed silicone mould from here spray on a paintable mould release such as Stone Rocket Release E302, Chemlube PMR or dust a fine layer of talcum powder over your silicone mould. Release agents help protect the silicone mould from the chemicals in the polyurethane or polyester resin leaching into it. The chemicals eventually break down the silicone mould making it brittle on fine edges and then leading to the mould splitting.
It is now time to cast a polyurethane resin into our silicone, Polyurethane’s are available as a rigid, flexible, foam and clear product, so there are many different types and applications. The polyurethane I am casting is called Easycast it is ideal for small rigid casting applications. It has a low viscosity meaning very fluid and hence minimal chance of trapping bubbles. Easycast is white in colour has a pot life and mixing time of 90 seconds and a demould time of 30 mins @ 23C. So you can rapidly turnover castings from your mould. As you can see there many different casting resins available for your particular application. The other big advantage of polyurethane’s is you can add coloured pigments to them with appropriate Polyurethane BJB Pigments, or metal fillers for that object that must look like metal of any description. Other fillers include light weight fillers so the polyurethane can float, fillers that thicken and bulk out the resin reducing the cost of a large pour.

Measure Easycast by volume, lOOmls of part A to part B and then pour one into the other and mix rapidly for 15 seconds. I prefer to pour into a second clean cup and mix again. Pour the casting resin slowly into one place in your silicone mould and slightly elevate the cup. Continue to pour until the mould is full i.e level with the top of your silicone mould. I then wipe the top of the casting resin level with my mix stick and spray the resin from a height of approximately 3oo mm lightly with a squirt of mould release. This releases the surface tension on the casting resin and allows the bubbles trapped to rise to the surface before the resin sets. I then use a thin piece of stainless steel wire with the end rounded over so it doesn’t scratch the silicone mould to burst any small bubbles trapped in the resin, this must be completed quickly keeping in mind the Easycast has a pot life of 90 seconds. To have a flat back on your resin casting, spray mould release on to a small piece of glass and lay it on top of your silicone mould. Do not be impatient; leave your resin casting untouched until it is well cured. See the manufactures instructions on cure times and at what air temperature. Once cured carefully remove it from the mould, scrub the casting with warm soapy water and it is now ready to paint.

Important facts to remember:
• Follow the manufactures instructions on measurement by volume or weight
• Mix A and B thoroughly
• Do not take shortcuts, do not be impatient waiting for materials to cure
Air Temperature affects the pot life of all materials. Moisture in the air affects polyurethanes, always spray a product called Dry Air Blanket (nitrogen in an aerosol) into opened bottles of urethane resin to remove the air moisture prior to closing the lid.

Glossary of Moulding and Casting Terms and Products used:
• Casting - to pour a material into a mould to produce a copy
• Easycast - Rigid polyurethane used for rapid reproduction of cast parts
• Master - is the original piece you are going to duplicate
• Mould Box - used to surround the original piece the mould is being made of, to act as a dam wall so it holds the rtv, silicone in place while it cures.
• Moulding - to make a negative reproduction of an original piece, using products such as silicone, fiberglass, latex, alginate, gypsum to name a few
• Modelling Glue - Zap a gap used to hold master down to base of mould box
• Metal Fillers - fine metal powders available in all metal types eg bronze, copper, aluminium
• Klean klay - oil based sculpting plasticine clay
• Mould release - a material used as a barrier so materials do not stick to each other, such as aerosol sprays , paintable liquids and powders
• Rocket Release E302 - paintable aerosol mould release
• Talc Powder - brushable powder mould release
• Pigments - a coloured material specifically manufactured to blend with a particular product
• PMR - Chemlube paintable aerosol mould release
• Pinkysil - Rtv moulding silicone, Shore A hardness of 20, and a 20- 30 minute demould time
• Potlife - the time a product can be mixed and poured before it starts to gel
• Procast - Rigid polyurethane used for casting parts with a longer pot life, making it suitable to be degassed in a vacuum chamber
• RTV Silicone - room temperature vulcanizing silicone used for making moulds
• Vacuum Chamber - An air tight chamber placed under vacuum by a pump, with the polyurethane inside the chamber it removes all trapped air at a measurement called thirty inches of mercury. Anything less than full vacuum will not remove all the air trapped in the casting resin.
• Zap a gap - a high quality modeling glue with gap filling capabilities

Adelaide Moulding and Casting Supplies have a large range of products in their Retail Outlet (08 82940451) and on their Railway Modeller friendly and secure Shop Online Website - www.amcsupplies.com.au
(Grant Evans is a consultant at Adelaide Moulding and Casting Supplies)
(previously published as Notes for the 2007 National N Scale Convention)



 

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