Moulding and Resin Casting Workshop
by Jason Miller
Text and photos by Dominic Newland, Edited by Jason Miller
First published in the August 2009 AMMS Brisbane newsletter
At the April AMMS meeting Jason gave an excellent demonstration of making moulds and resin casting, explaining to the eager crowd
about many of his hard learned techniques and the pitfalls to avoid.
At the demonstration Jason made a one piece mould with a pouring plug, and showed how to pour resin into this and other types of moulds
Safety: Polyurethane resin can be a dangerous product to use.
- Do not eat.
- Do not use this product without proper fitting eye protection.
- Use in a well ventilated area.
- Wet sanding only.
Note: All silicone referred to in this article is "Pinkysil". An addition cure silicone rubber from Barnes Pty Ltd.
It is a very fast cure rubber, which has a pot life of about 5-6 minutes after mixing.
Creating a One Piece Mould for "Drop Casting" or "Home Casting"
This is good for simple parts that don’t have large undercuts. It also involves creating a pouring plug that leaves a small
casting block that needs to be removed from the resin replica that is created. The example here is a 1/35 Panther road wheel.
- Attach a small pouring plug (casting block) to the part in a location where the eventual resin casting block will be easily
removed without too much damage to the resin part. This pouring plug needs to be just large enough to allow a little extra resin to
sit above the part after pouring in the resin.
- If making a pouring plug for vacuum casting then you would need a pouring plug four to five times the size of non-mould making
pouring plug. This reservoir will fill and empty with resin several times as the vacuum sucks out the bubbles from the liquid resin.
- Sometimes you won’t even need a pouring plug as the part you are moulding has a plain back/lower surface that won’t be seen on the
eventual model. Examples of this are stowage bins or ammunition crates that sit flat on the model’s surface. Its best if you can get
away without needing a pouring plug as this minimises the amount of resin you need to remove from the resulting resin part.
- Make sure your part is clean, with no finger prints, ink markings, paint, sanding marks, scratches or dust. The silicon will
perfectly replicate any imperfections you leave on the cast part.
- Lay down Stitchery Tape onto a gloss tile, and add the part to be cast onto the centre of the tape. Stitchery Tape is not your
average double sided tape and is used to hold medals in medal mounting. It is quite expensive as well. You could also glue it down
onto some plasticard, but it needs to be quite firmly attached as the last thing you want is the part lifting off and disappearing
into the rubber.
- Create a reservoir for the pink silicon that will be poured around the part to be moulded. An easy way to do this is to use Lego
bricks (find them cheaply on Ebay or the kid’s room). Leave at least 10 to 15mm for the wall thickness of the silicon mould as the
resin reacts exothermally (creates heat) and will bulge the silicon walls if they are too thin. Build the Lego wall up high enough so
that it is 10mm to 15mm above the part.
- This Lego reservoir is pressed onto the Stitchery Tape, enclosing the part and making sure the part is central to the wall of the
Lego reservoir. Press it down firmly as the pink silicon will leak out through any gaps, leaving bubbles in your mould that create
weaknesses in the mould, or even bubbles next to your part.
- When pouring silicon to make a big part, the silicon will start to set (or "go off") too quickly before you have poured it all
into the reservoir, so unless your part is very small you will need to pour the silicon in two or more parts. A tank hull might take
six pours of silicon. A 1/35 Panther wheel might take two pours.
- Measure your two part silicon mix using scales to get a consistent mixture and best silicon mould strength. Photo 1.
- Mix the silicon by stirring with a stick in a circular motion until the rubber is a constant colour. Be sure to scrape the stick
along the inside edge of the cup to thoroughly mix the rubber.
- Using an old short haired brush, paint the liquid silicon over the part to ensure there are no trapped bubbles in any undercuts,
and so the part has at least one thin coating of rubber. This should ensure that when the rest of the mould is poured, that any
bubbles left will not be on the part. Rub the silicon off the brush using a cloth, and when dry pick out any left overs with a
toothpick. Photo 2.
- Pour the silicon in a thin stream from up high (to help get rid of air bubbles) into one corner of the Lego reservoir, whilst
rattling the tile (and the reservoir and poured silicon) to help release bubbles and get the silicon into the part’s recesses. Photos 3 and 4
- Mix more silicon to pour on top of the silicon you have just poured. Its better to not mix enough rubber than to mix too much.
Using scales to measure your mixtures will ensure your mix is consistent from one pour to the next. Keep doing this until the top of
your mould is reached. Never mix two different manufacturer’s batches of silicon. As silicon you bought at different times may
differ chemically and the mix will not set correctly, not at all or the mould life will be drastically shortened.
- Now that the mould is full of silicon you can rattle away some more and blow any bubbles off the surface. Repeat this until no
more bubbles are rising. You might need to pick out the bubbles with a pin or toothpick. The more you rattle then the more bubbles
you will get rid of. Then leave the mould somewhere level to set.
- When the silicon has set properly (for longer mould life you should leave it set for 24hrs, otherwise the part can be de-moulded
in 1 hr), remove the Lego then gently lift one corner of the mould away from the tape until you can see the original pouring plug.
Using flat screwdriver or such, pry the casting block off the tape as well. Then the whole mould will come away from the tape. Trim
off any access rubber from the outside of the mould. You can then get the part out by pulling back the lips of the mould and gently
removing the cast plug from the part then gently remove the part. At times you may need to cut the mould to remove the original part.
See Photo 5. DO NOT force the mould apart. It will tear and then all of your work will be for nothing.
- After extracting the original part from the mould, you may need to create small vertical cuts inside the mould where undercuts
might trap air bubbles in the resin when it is poured in. These small vertical cuts will be used to drag out the air bubbles, as
- When using a mould for the first time, Jason simply pours in the resin, gives the mould a squeeze and sees where the air pockets
form, so then he knows where to create the small vertical cuts inside the mould, and where to pull bubbles out of the mould.
Resin Pouring into a One Piece Mould (with and without a pouring plug)
Always have eye protection when handling resin, as a slip when handing the mould or the resin can lead to resin being splashed
into your face and or eyes. Jason can attest to not being able to see from his left eye for 3 days.
- Talc the inner face of the mould so as to help break the surface tension between the resin and the mould, which helps reduce the
number of bubbles in the resin. But too much talc will result in bubbles.
- Hold the mould together with a couple of rubber bands. The rubber bands should be holding the mould together firmly and not over
tight. As this will distort the mould and the part will be misaligned and misshapen. Photo 6
- Shake your resin bottles well to mix them properly and stand aside for 5 mins to rest.
- Pour each part of the two part resin into its own container, like a disposable drinking cup. Only pour enough resin for the jobs
you are doing. The resin will be ok in these cups for about half an hour, but less if there is humidity. Polyurethane will soak up
any moisture in the air as soon as the container is opened. Once water gets into the resin, it will create bubbles when the resin
exotherms . Any resin left over from the pouring SHOULD NOT be poured back into the main containers. It will contaminate the whole
batch. Regardless of how long it has been out.
- Use pipettes to draw up the desired amount of resin and squirt these into a disposable mixing container. With small amounts of
resin try to be fairly accurate in how much you draw up into the pipette. Photo 7.
Here Jason used tea light candle holders to mix the resin, but you can also cut the bottom off disposable drinking cups and use these.
- Note: - Don’t mix up your pipettes as this will cause resin to start setting in the containers – leave them in their respective
disposable drinking cup. These will be disposed of later. They are about $37 for 500.
- When the resin is mixed, pinch the mould open a little and pour in a little resin to coat the inside of the mould. Photo 8.
Use a rounded fine tipped stick (so the mould will not tear) to get the resin into any holes in the mould where bubbles might be. Photo 9
With moulds that have a lot of undercuts you will really need to get in there with the fine tipped stick to get the air bubbles out.
- Fill the mould 3/4 full and squeeze it a little to get more bubbles out.
- Fill the rest of the mould with resin until there is a slight mound of liquid resin on top of the pouring hole. Pick out any more
bubbles by pushing the thin stick into the mould and drawing up any bubbles through the small vertical cuts you made at any of the
- To get any more bubbles out slightly squeeze the sides of the mould so that the resin pushes gently up and down. This will also
ensure that any resin film will be as thin as possible.
- Minimise or even get rid of the casting block by placing on top of the still liquid resin a piece of plastic cut from clear
Chinese food take away containers. Weight this slightly so that it doesn’t lift away from the surface and accidentally pull bubbles
into your resin. This will leave only a minor sanding necessary on parts that have no pouring plug, such as stowage bins or
- Leave any resin on the outside of the mould, as this is easily picked off when cured.
Resin Pouring into a Two Piece Press Mould
- Mix enough resin so that it will be more than fill when the two parts of the mould when they are squeezed together. There will be
wasted resin as the moulds are squeezed together. Never mix too little resin as this will result in an under-filled mould and a waste
of resin. E.g. For a large part you could mix 150g of resin to end up with an 80g part. There will be wastage in a press mould, but
on the flip side there will also not be any casting plug which has to be removed and sanded, and which is wasted resin anyway.
- Press moulds are best for parts such as tyres where you don’t want a casting block on any of the part, as would otherwise occur
with a one part mould with a pouring plug. Here Jason is casting a tyre for an ASLAV. Photo 10
- Select the deepest mould half and quickly coat all internal surfaces of the mould with resin by using a disposable brush. Pick
out any bubbles using a fine round pointed stick.
- Coat the other half of the mould with resin and pick out any bubbles.
- Fill the rest of the two mould halves with resin and pick off any bubbles with a pin or fine stick. Pick up the shallowest mould
half and "roll" it onto the other half, ensuring the two parts are meeting correctly. Rolling the moulds together should
minimise air bubbles. Press the mould halves together slightly to squeeze out the excess resin, and until the two mould halves have
- Hold the mould together with a couple of rubber bands, or put a small weight on the mould to prevent the two mould halves
separating and pulling air into the mould. Photo 11.
Easy Cast is the thinnest resin on the market from Barnes Sydney. Jason can order it for AMMS members at cost plus a small $5
fee. Resin can be bought in packs of 500g, 1kg and 2kg of resin (which is a lot of resin). Jason can also make the mould of a part
for AMMS members, who can then buy the mould off him to use at home.
Shake your resin bottles occasionally to avoid it settling too much. Any particles in the resin bottles that become mixed in your
moulds will gradually leak unmixed resin chemicals out of your cast resin part, ruining it.
You know if your resin has become too old if the mixed resin starts to foam, or if the cured resin starts stays expanded in the
mould and is full of bubbles like a honey comb. It should be disposed of when it gets to this point.