What material do I use for mold making? (seeking professional advice)

Hi there. I’m a new here, so I would appreciate any professional advice. I have four questions about mold making and casting, so I’ll split it up by numbers. 

1) I’m making a mold for a monster that is roughly 10.5”-11” tall, and I started my using ultra cal. However I found that some of the finer details in the impression have broken off. Additionally, Ultracal mold is really heavy and hard to move around. I'm considering switching to a PU resin like Smooth-On 326. I feel like this alternative would retain details better and be much lighter. Please tell me what you think about this alternative. 

2) If I add metal powder to the PU resin, will it make the mold stronger?

3) When casting the character in silicone, I know that people normally inject more silicone into the character mold to fill (like Ecoflex) My question is, can I save on silicone if I first pad my puppet with foam around the body, and then coat the body with silicone before placing it in the mold to cast?

4) Between DragonSkin, Plasti-gel ,and Ecoflex, what silicone do you prefer?

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I mostly use ultracal or Hydrostone moulds, but for a couple of silicone puppets I used polyester resin with fibreglass reinforcing.  It makes a smaller, much lighter mould.  The surface will be very strong so you shouldn't have any issues with details breaking off.  I have also used epoxy resin with the fibreglass woven cloth the same way.  I haven't actually used PolyUrethane resin for moulds, but I don't see why it wouldn't work just the same.  

2) I don't know if metal powder would make it less prone to breaking, it might. Or it might make the surface harder, but not improve the tensile strength at all - so if it was a fairly flat mould and you pried it from the end of the arm, that whole thin arm section would be under stress and could crack.  Sometimes I put metal bars across to strengthen the mould with that in mind. That is also where the fibreglass reinforcing comes in, it gives it tensile strength.  You don't even put fibreglass in the surface coat, because you want a smooth coat with no hairy bits.   But you shouldn't need metal powder.  You are pulling clay out of it, and then soft silicone, there should be no real stress on the surface detailing. 

I usually don't get surface details breaking with Ultracal, the only time they get damaged is when prying them apart, if I haven't made strong prying points around the edge of the mould.  That is, I put some clay on the first half of the mould so when the second half is made on it, there will be a gap there when the clay is removed.  That makes a place to slide the chisel in without chipping away at the plaster.  And I use a bit more reinforcing around those places so they stand up to the prying.  Hard resin moulds should also have prying points so they are easy to open.

3) I made a Sumo wrestler in silicone, so I packed plenty of foam around the armature to keep it light and reduce the amount of silicone needed, just like i do with foam latex puppets.  But the silicone soaked in to the foam, and the result was the puppet was just as heavy, and quite a lot stiffer, than if I had just used silicone.  The waist, being thick, would hardly bend at all before it sprang back, and one leg could not support the weight during a walk.  I had to strip it off and re-do it with a softer silicone, Ecoflex 00-30 inside. (And a heavier armature.)

There is a 2-part expanding silicone foam that bonds onto the silicone skin, Soma Foama, but it was not available in my country.  Others have used it to fill the silicone skin with success, but tell me it is pretty quick acting so you don't have much time to mix it and pour it in. But it does make it lighter and easier to bend.  If you were going to use the urethane upholstery foam, you would need to seal the entire surface so the liquid silicone can't get in.  I suspect that is hard to achieve, like it only takes a small hole for a bucket to leak.  So I haven't tried.  I still use a thin layer of urethane foam on the armature to give it a porous surface, that lets the silicone have something to grip onto, just not the big thick padding I use with foam latex puppets.

4)  If I have both, I use Platsil Gel-10 for painting on layers of skin.  It is a little thicker so less of it runs down and pools in the bottom.  But inside, I pour Ecoflex 00-30 which is softer.  It is also a bit runnier, so hard to build up layers of skin, but easier to pour down the neck hole.  Platsil also make softer silicones, which I bought a small container of, but haven't used yet.  Might be Gel-0?  Both Smooth-on and Platsil are good, you can even mix the two brands, it's the shore hardness that matters.  Mostly I only make silicone heads, and the Gel-10 is just a bit stiff for facial movement if it is solid all the way through.  (00-30 is not like a Shore A hardness of 30 which is quite firm, it is a different scale and softer than Shore 10, but I can't figure out from the maker's website what order the numbers go in once you get to the 00 scale.  Is 00-20 softer, or are they like minus numbers so the bigger the number, the softer?)  

Couldn't find this pic before - the Sumo body mould, in polyester resin & fibreglass chopped strand mating, with the Ecoflex 00-30 being poured in.  A plaster mould would obviously have to have much thicker walls.  

Hello Nick, thank you so much for responding to my questions. I really admire your work on Youtube, so I'm kind of surprised you responded to my post. I'll take this advice into consideration when I make my final puppets. I might have some more questions about set design a little later down the line, but again, thank you for your input. 

StopmoNick said:

Couldn't find this pic before - the Sumo body mould, in polyester resin & fibreglass chopped strand mating, with the Ecoflex 00-30 being poured in.  A plaster mould would obviously have to have much thicker walls.  


Hey Nick,

So I've been looking at some epoxy resins, and I was wondering what brand you could recommend? I want to make sure that the resin I buy is proper for mold making. Sometimes when researching you buy something for animation, but it ends up not being suitable for your needs, like how someone once bought the Plaster of Paris brand for their mold, thinking it was just as a good cheaper alternative to Altracal, and their mold ended up breaking. I just want to be safe. I'm trying to find a good balance between cost and quality. Below are a couple links to resins I think might be good for my needs. Your input would be much appreciated. I'm leaning more towards the Smooth-On EpoxAmite 101

https://www.dickblick.com/items/33520-1019/

https://www.dickblick.com/items/artresin-epoxy-kit-studio/

https://www.dickblick.com/items/02918-1064/

https://shop.sculpt.com/smooth-on-epoxamite-101-fast-gallon-kit.html


StopmoNick said:

I mostly use ultracal or Hydrostone moulds, but for a couple of silicone puppets I used polyester resin with fibreglass reinforcing.  It makes a smaller, much lighter mould.  The surface will be very strong so you shouldn't have any issues with details breaking off.  I have also used epoxy resin with the fibreglass woven cloth the same way.  I haven't actually used PolyUrethane resin for moulds, but I don't see why it wouldn't work just the same.  

2) I don't know if metal powder would make it less prone to breaking, it might. Or it might make the surface harder, but not improve the tensile strength at all - so if it was a fairly flat mould and you pried it from the end of the arm, that whole thin arm section would be under stress and could crack.  Sometimes I put metal bars across to strengthen the mould with that in mind. That is also where the fibreglass reinforcing comes in, it gives it tensile strength.  You don't even put fibreglass in the surface coat, because you want a smooth coat with no hairy bits.   But you shouldn't need metal powder.  You are pulling clay out of it, and then soft silicone, there should be no real stress on the surface detailing. 

I usually don't get surface details breaking with Ultracal, the only time they get damaged is when prying them apart, if I haven't made strong prying points around the edge of the mould.  That is, I put some clay on the first half of the mould so when the second half is made on it, there will be a gap there when the clay is removed.  That makes a place to slide the chisel in without chipping away at the plaster.  And I use a bit more reinforcing around those places so they stand up to the prying.  Hard resin moulds should also have prying points so they are easy to open.

3) I made a Sumo wrestler in silicone, so I packed plenty of foam around the armature to keep it light and reduce the amount of silicone needed, just like i do with foam latex puppets.  But the silicone soaked in to the foam, and the result was the puppet was just as heavy, and quite a lot stiffer, than if I had just used silicone.  The waist, being thick, would hardly bend at all before it sprang back, and one leg could not support the weight during a walk.  I had to strip it off and re-do it with a softer silicone, Ecoflex 00-30 inside. (And a heavier armature.)

There is a 2-part expanding silicone foam that bonds onto the silicone skin, Soma Foama, but it was not available in my country.  Others have used it to fill the silicone skin with success, but tell me it is pretty quick acting so you don't have much time to mix it and pour it in. But it does make it lighter and easier to bend.  If you were going to use the urethane upholstery foam, you would need to seal the entire surface so the liquid silicone can't get in.  I suspect that is hard to achieve, like it only takes a small hole for a bucket to leak.  So I haven't tried.  I still use a thin layer of urethane foam on the armature to give it a porous surface, that lets the silicone have something to grip onto, just not the big thick padding I use with foam latex puppets.

4)  If I have both, I use Platsil Gel-10 for painting on layers of skin.  It is a little thicker so less of it runs down and pools in the bottom.  But inside, I pour Ecoflex 00-30 which is softer.  It is also a bit runnier, so hard to build up layers of skin, but easier to pour down the neck hole.  Platsil also make softer silicones, which I bought a small container of, but haven't used yet.  Might be Gel-0?  Both Smooth-on and Platsil are good, you can even mix the two brands, it's the shore hardness that matters.  Mostly I only make silicone heads, and the Gel-10 is just a bit stiff for facial movement if it is solid all the way through.  (00-30 is not like a Shore A hardness of 30 which is quite firm, it is a different scale and softer than Shore 10, but I can't figure out from the maker's website what order the numbers go in once you get to the 00 scale.  Is 00-20 softer, or are they like minus numbers so the bigger the number, the softer?)  

The sculpting and mould making materials shops here in Australia seem to sell their own branded epoxy resins.  So far all of them - from Barnes Products and Dalchem - have been good and as far as I can tell, the same stuff.  They would be classified as Laminating Resins, made for building up layers with fibreglass.  See Smooth-On's website:   https://www.smooth-on.com/category/epoxy-resin/

This page from Compleat Sculpt ( Sculpt.com ) on Smooth-on epoxy resin has some info:    https://shop.sculpt.com/smooth-on-epoxacoat-grey-gallon-kit.html

I don't usually buy a gel-coat, I just mix a little micro balloon filler into my laminating resin so it comes up white and is thicker.  That grey Epoxacoat stuff is a gel coat, for that first coat of resin on your sculpt, so you wouldn't want a big tub of it.  The Epoxamite Laminating Resin they mention for the following layers is more like the stuff I use.

Oh, just saw that you had put links in, should have looked at those first.

The first Dick Block link is casting resin, for pouring into a mould or encapsulating something in a block of clear resin.  It generally has less catalyst, because it would generate too much heat and possibly crack if it was the same as the laminating resin, which is applied in thinner layers.

The second one, Art Resin, I am not sure of.  It talks about coverage, so clearly used as a coating, but can't tell if it is ideal for laminating or not.

The 3rd one, Envirotek, says it is used like a clear varnish, only goes on thicker.

The fourth link is to the Smooth-On Epoxamite stuff I found, so I see you were ahead of me -that would definitely be suitable.  Not so sure about any of the Dick Block stuff.

Okay, so just so I understand. For epoxy resin, you mix it up, pour it into the mold, fill in the chopped strand fibreglass matt, or woven fibreglass fabric, wait for it to settle, and then add some more resin layer by layer? Or alternatively, you mix in micro balloon filler into the resin?

Just to add a little from my own experience with epoxy resin. Most resins are pretty similar, but not interchangeable. They are usually laminating resins, so intended, as Nick says, to be laid up in thin coats with glassfibre reinforcement. There are a few that are specifically designed to be able to be cast in thicker volumes.

If you were to try to cast a laminating resin as a thick section (I am talking 15mm and up), the exothermic reaction will heat the resin to such a degree that it will crack, smoke and may even catch fire. So it is in no way a substitute for plaster! This can be mitigated to an extent by including powder additives, but is not a good way to use an expensive product.

The most important thing about epoxies is that the hardener ratio should not be varied. It is unlike polyester in this respect. With polyester you alter the setting time by varying the amount of catalyst. The hardener (NB not catalyst, as it is incorporated into the final resin) in epoxy should be added only in the proportions stipulated on the pack. You vary the setting time by using slow hardener, also some tricks, see below...

Although epoxy resin smells not unpleasant, it does still give off fumes. You should use it in a ventilated space, and always, repeat ALWAYS, wear nitrile gloves. This is because the nasties in the resin accumulate in the body and can create a permanent allergic reaction, as in severe dermatitis. So treat it with respect!

As for using laminating resin to make a mould, you can use the properties of the resin to help you get a good result. A very thin layer of unthickened resin can be gently brushed or poured, to get into the fine detail, followed by one thickened with colloidal silica and/or microballoons. I recommend West System powders. You can then leave the mould for a short while to allow the resin to stiffen up a little - we refer to it as 'going chewy'. Test it with the remainder in the pot, being aware that thicker sections go off quicker. Then you can add the back-up layers, reinforced with glass fibre. Because the first layer is chewy, pressing or dabbing the glass/epoxy back-up layers on will not displace it.

Only use glass fibre that is suitable for epoxy. The standard chopped strand mat has a binder in it that is only dissolved by styrene, which is absent in epoxy. Ideally something without binder at all. One tip - if the mould is a bit fiddly to access, wet out the glass on a piece of polythene and then place it into the mould. I like using biaxial cloth, which drapes well. About 1000 to 1500 gsm will give a very substantial laminate. (By this I mean,  4 to 6 layers of 450 gsm cloth, ideally with the strands orienting differently.)

Finally, there is a product called peel ply, which is a PTFE impregnated nylon cloth. It is designed to give a good surface on the back of a layup. While maybe not essential, it does help to keep down those little sharp ends of glass that often pop up and catch your skin.

Using different resins, I have found that the setting times can be wildly different for the fast and standard varieties. West System fast resin will set up within minutes at temperatures over about 22 deg C, and their slow hardener is not much slower. SP resin is a bit more sedate, but in warm climates will also set up pretty fast. I have been using a generic resin recently that seems quite a lot slower.

This can also be manipulated when ambient temperatures are lower. Epoxy really doesn't set off below 10 deg C, and pre-preg epoxy/carbon (for racing boats) gets stored in a freezer for months without activating. So if you had a large mould, you could lay it up on a cool day to get longer setting times, and then speed up the setting by adding a heater. I like working with epoxy in ambient temperatures around 15 deg C, as it gives me time to work slowly and the opportunity to ramp up the reaction by adding some heat.

The other interesting feature of epoxy is that it continues to get harder after it has set. So leaving your completed mould in a warm place for several days to post-cure will likely add 25% to its strength. This also applies to the stuff we are normally sticking with epoxy. Leaving an item on a radiator for a while (or in a hot car...) will make it noticeably harder and tougher.

Hope some of this helps. It comes from my experience constructing a 10m catamaran from scratch, using about 1.5 tonnes of epoxy! After 8 years neither I nor my partner had any reaction to epoxy, but we did get through several thousand pairs of nitrile gloves! (I also make small stuff!)

Just to answer your direct question. Item 1 is polyester resin, not epoxy. Item 2 and 3 look like they are for coating, e.g. wood. Item 4 from Smooth-on looks fine as a laminating resin, but I am sure you can go to a fibreglass supplier and get almost ther same stuff much cheaper. The price drops dramatically with quantity.

Have a look at the West System website for some technical leaflets on using epoxy. They are well written.

wow, thank you for all the info Simon. Would a fiberglass cloth like this work?


https://www.homedepot.com/p/Evercoat-6-oz-4-in-x-10-yds-Fiberglass-...

Yes, that is absolutely fine. It may be quite heavy cloth, so if it is too resistant to getting into the shape of your mould, just cut it up into small shapes and then work it in. You may be able to buy fibreglass tape on a roll, and this is usually quite thin, so ideal for your purposes. Usually anything woven is good for epoxy, as it doesn't need binder. The stuff to avoid is the random fibre mat.

Sometime using a (disposable) brush is good, especially if you cut the bristles down to about half-way, which makes the brush good for stippling in the cloth and wetting out the fibres. But bear in mind that you don't want to risk damaging the surface of your sculpt, so this has to be done gently if the first layer is thin or not very 'chewy'.

If you find that you have quite a lot of resin in the mould, more than enough for the glass, then just keep adding layers of glass to soak up the resin. When the glass is wetted out it goes clear, so any white areas need more resin. The optimum ratio of resin to glass is quite low, meaning that the strongest combination has only just enough resin to wet out the glass and hold the matrix together. But for mould-making it is not at all critical.

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