Questions about casting

[MarcoSkoll]

I'm completely ill educated on the subject of casting, and as my Guard and Eldar projects require such knowledge, I come seeking such wisdom.

In both cases, I need to cast at least ten good copies of smallish parts for which the masters will be a mix of Green Stuff, brass and styrene. It needs to be something I can do at home, make copies of reasonably fast (2 or 3 copies a week without resorting to multiple moulds), and at a not exorbitant cost.

So, I'm looking for some advice on materials (both mould and casting medium) and method.

One other thing. The Eldar project will involve a Guardian model (well, probably two - one male, one female) that I'll want to be able to repose so that when they're converted into all the different Aspects , they'll all look different.
Would this be best done by sculpting the model(s) in a neutral pose, then hacking apart and reposing the casts; or cutting apart the master and casting it already in several small units that can be pinned together into the desired pose?

[Inquisitor Dionzi]

Well, issue #1 is that styrene doesn't cast well at all. Many silicone rubbers (used for the mold-making process) have some form of alcohol in them, or produce it through the catalyzation process, and will dissolve the styrene.

I have found great success in Smooth-On and Alumilite products. My molds are currently made with a semi-clear Smooth-On Mold Max 27T platsil silicone rubber (platsil tends to have a higher tensile strength and heat resistance than tinsil catalyzers). From Smooth-On, this semi-clear costs the same as a coloured silicone rubber with a longer usable life-span.  I have really been enjoying mucking about with Smooth-On's Task-18 aluminum filled resin (it's grey). It too has a higher heat resistance. They both have a decent work time, but a longer demold time as well. Platsil-catalyzed products will also allow you to cast in low-temperature-melt pewter, should you choose to cast in pewter rather than resin.

However, if you're looking for something easy to work with (particularly since I assume you don't have either a pressure chamber nor a vacuum chamber), find products with a LOW viscosity rather than a high one, and try to make sure your casts can be drop-cast.

I've found that a 3/5 ratio of molding product to resin is a good fit: I ordered a 1.5:3 silicone rubber to resin supply and found that it wasn't a good fit, not with how much I've been producing.

Mold boxes can be cheap: I use corrugated plastic hot-glued together. Make sure you have about 1cm to 1/2" of silicone rubber around each piece, otherwise you'll be likely to break the mold on the first go. Remember too that silicone rubber bonds to itself, so it may be advisable (if not time efficient) to pour many layers (2-3) of silicone rubber when making your molds.

Resin: it's best to have a low-viscosity resin so you can fill in fine detail without bubbling. If you have a vent at one end of your mold, you'll find degassing/deairing your resin far easier.

Make sure you always have a sharp hobby blade!

Thicker pieces of resin cure faster than thin: this is because of the heat the catalyzation process creates: the more heat, the faster the cure. Warm your mold in an oven (150 Fahrenheit) for a few minutes before pouring the cast.

Degassing can be accomplished by rapidly banging the mold/cast on a desk and blowing on the bubbles as they rise to the surface. Be sure to work on paper plates. I've also found that clear plastic wine cups ($4 for a dozen) make mixing far easier.  I find it best to use more catalyzer than recommended, simply because that ensures that I won't have mucked up the mixture. You'll end up with spare product at the end, but you won't have any fubared molds or casts.

Re: the eldar project. My 67mm dummy is neutrally posed and will be hacked and repositioned after casting. Makes molding and casting easier, at least from my perspective.

Does this help any?

[MarcoSkoll]

Well, that's quite a lot of information to get started with. I'm not sure the product specific stuff is going to be all that relevant this side of the Atlantic, but the general information is very useful.

Many silicone rubbers have some form of alcohol in them, or produce it through the catalyzation process, and will dissolve the styrene.

Is there any solution to this short of using something other than sytrene?

Re: the eldar project. My 67mm dummy is neutrally posed and will be hacked and repositioned after casting. Makes molding and casting easier, at least from my perspective.

Fair enough.

[Inquisitor Dionzi]

What are you using the styrene for anyhow?

[MarcoSkoll]

Well, for example, anything white or light grey except the main body of the bolter (which is resin).
Obviously the light grey stuff is 28mm plastic pieces (still styrene though - well, polystyrene if we're being pedantic), but the white bits are all cut from plastic rod/tube/sheet.

It's good for studs, rivets, flat panels, square pieces (my home build lasguns are mostly plasticard), curved pieces (if they'll cut out of the tubes I've got), and a lot easier to work than brass if I don't need the extra strength.

[DapperAnarchist]

Could priming work, like priming packing styrene before spray painting? And I'm watching this carefully, I'd like to learn more about casting...

[Hugh Jarse]

I did some casting and mould making a couple of years ago (made some Guard models), I bought all my stuff from Hobbies Catalogue

Linky

http://hobby.uk.com/moulding-casting.html

[Inquisitor Dionzi]

K, that clears things up: "styrene," colloquially, is something similar to packing peanuts. Easily dissolved. Polystyrene ("plasticard") doesn't dissolve under the solvents produced by the mold-making process. See, it's important to be pedantic at times.

It's NOT recommended to even attempt to mold styrene, Dapper, unfortunately. Most sealants I've come across have similar solvents and will dissolve your subject. One way to do that, however is the following: get a very fine plaster-of-paris mix and "paint" it onto your styrene (I learned that in high school).  You can then use a paintable latex to paint the inside of your plaster of paris mold, flip it inside out, and *then* fill it up with your resin. Another option is to find a clay that is not a sulfur base. That doesn't react to most molding silicones. "Monster Clay" is what I - and most of Canadian and USA's production companies use for swift-sculpts (i.e. facial prosthesis for movies: like Worf's forehead ridges - they were sculpted onto a plaster cast of Dorn's head, painted in latex, and then cast in a very flexible rubber. Seven of Nine's prosthesis were made in a similar manner, as were Trance Gemini's...and so on.)

[MarcoSkoll]

See, it's important to be pedantic at times.

If we're already being pedantic, packing peanuts are still polystyrene, but they're expanded polystyrene. Unless you actually work in a chemical lab or in the industry, you're unlikely to ever see styrene in its raw unpolymerised form.

But that's enough pedantry for now, I think.

@Hugh: Ah... they do aluminium powder as well. That reminds me, I'll need some for the Jubilee, although I'll have to see if I can get it in a smaller quantity than that.

[DapperAnarchist]

You can buy smallish quantities of aluminium powder from sports shops - I think its used to improve grip?

[MarcoSkoll]

Hmm. You might be getting it confused with Gymnastic chalk.

Aluminium powder is quite dangerously flammable stuff, so it's not used or sold casually.

[Hugh Jarse]

I used their resin, its quite, how shall we say, 'viscous' 

It was an absalute arse to get the bubbles out at times, but I found the best method to over come this, and for some of the more detailed parts, was to paint mixed resin on to the mould halves before putting them together quite thickly, then pouring in the remainder of the resin very slowly, then giving the mould several good swift taps to try and chace out the remaining bubbles..... results were still abit hit and miss at times though, but that might of been down to human inability rather than anything to do with the resin 

[Kallidor]

I get my casting stuff from TOMPS. I use their rubber and fast cast resin.

My advice on making moulds for small parts is to make the mould itself small. I use lego to make a box and blutac to hold the part in place rather than plastacene as I find that the models always end up gummed with plasticene but blutac just comes off easily. a bit of vaseline on the blutac and a spray of mould release and that's it. Very easy to do. I cut the poor channels afterwards and with a bit of tweaking you shouldn't have any problem with air bubbles.

Huge Jarse has alrady mentioned somethin else I sometimes do witch is to pour a little resin into the two halves of the mould in any areas with constantly catch bubbles.