Clear Cast Resin mould and release questions

[blennus]

I recently purchase some clear cast resin (and the hardening agent). I have two 500ml containers and read on the instructions that it mentions using a certain sort of wax in the mould before pouring... Am I correct in thinking this is simply as a release agent and something else may work as well?

My current thought is to create a sculpture using plasticine (possibly building on a skull to create an endo skull) then making a plaster cast mould of that... and then resin casting from that mould. Can I just use something like cooking oil (sprayed) into the mould as a release agent or are the chemical incompatible? I would have bought the wax but it wasn't readily available where I bought the resin so I'm not quite sure where to get it.

Also the instructions mentioned basically applying layers of the resin rather than dumping it all on at once so as to avoid bubbles... I was wondering if it would be possible to go one step beyond that and layer it to a certain thickness to create a form of hollow cast... I don't know what drying times are but imagine it may be possible to coat the mould, then when the first coating gets "solid" enough apply another coat and so on until the desired thickness is achieved... is this what is referred to as "slush casting".

Any advice, tips etc would be appreciated... alternative things to use the clear cast resin for are welcome also. It's Diggers brand if that helps... I bought it from a hardware chain that is going out of business so it was roughly half price.

 

[ChickenHaunt]

You'll have better luck running the resin in a flexible (ex: silicone) mold.

Yes, the wax would be your release agent. You could use other releases, as well. I probably wouldn't do the spray oil just because it might disrupt the smoothness of the surface, and you need it polished smooth in order for the clear to actually be clear. Technically you could run it in a silicone mold without a release, too.

Yes, you should be able to produce a hollow cast in that manner.

 

[blennus]

You'll have better luck running the resin in a flexible (ex: silicone) mold.

Yes, the wax would be your release agent. You could use other releases, as well. I probably wouldn't do the spray oil just because it might disrupt the smoothness of the surface, and you need it polished smooth in order for the clear to actually be clear. Technically you could run it in a silicone mold without a release, too.

Yes, you should be able to produce a hollow cast in that manner.

Thanks for the response. I presume the reason for a more flexible mold is it will make release easier? I don't have any silicone (plus not sure my budget would stretch to it anyway)... will see what I can do. I know it would be destructive to do so but I'm not completely against the idea of breaking the mould to get to the resin... or trying a seamed mould (if I'm going to polish it anyway... won't matter too much if part of that polishing is removing seam lines.

Great food for thought... thanks again ChickenHaunt

 

[martinwilson]

Yes - it is technically possible to cast resin from a plaster mold - but its probably the worst mold material you could choose. I did it while I was still a student and couldn't afford silicone - but I can promise you that the cost of your time wasted getting it to work is more than the cost of silicone.

There are many reasons why plaster is not good for this application:

First it is so porous that things like resin are able to penetrate the inner surface and grab onto each and every microscopic pore. Using oily release agents also don't work so well because they soak into the plaster mold instead of creating a barrier at the surface. Therefore to use a plaster mold you first have to seal it (you can use products for sealing concrete, or bricks etc, or I have had some success using many layers of soap), and then to use your waxy\oily release agent.

Secondly, unless you wait days for the plaster to dry out, it will be full of moisture. Moisture in usually a bad thing for resins - its can retard its curing, create unwanted surface marks, etc. Evaporation will also make your mold much colder than the ambient room temperature which can also be a problem.

Thirdly, as a very rigid and brittle material, even if you do everything else perfectly you will notice the inner mold surface begin to crumble after only a few casts.

I completely understand the problem of silicone being out of budget, but there are ways to make it cheaper. You only need a very thin layer of silicone to get all its chemical benefits - so you could paint on a layer of silicone onto your finished sculpture and then back it up with plaster. This way the resin only ever touches silicone, but you only need a small amount of it. If you absolutely can't use silicone, that doesn't mean that plaster is the next best choice. Even making a multi-piece fiberglass mold would be more appropriate than plaster.

And as for making a hollow cast: you need to find the data-sheet for the particular resin you bought (the shop that sold it to you should be able to print you out a copy - or maybe you can find it online). You need to know the resin's viscosity and precisely how long it takes to cure. Some resins set in 4 minutes so you have to be really fast pouring it in and have to rotate the mold like crazy. Others take 24 hours to cure - in which case a rotational cast would become a Herculean task. The reason they suggested brushing it in layers might be because it is very thick (high viscosity) (like syrup) which makes "sloshing" it evenly around the inside the mold very difficult.

I hope this helps - when in doubt do a small experiment using the materials before doing it on your full sculpture.
Anyway - let us know what you try and how it comes out.