How do you Cast Silicone?

[TheNickFox]

Okay, so there is PLENTY of material out there on how to cast plastics, (And I'm well-versed in enough of it) but not so much on rubbers. I have a prop project that could benefit from being able to cast silicone, and I'd like to know more about how other people do it, before doing it myself.

I start thinking "well you could just use a silicone mold, a good coating of mold release, and then pour it in," but I wanted to see what the "official" way of doing something was, whether there might be a more rigid substrate that wouldn't inherently require a release (though I'm sure it would be recommended).

So to start with, what are the molds made out of?

And if anyone else has a question about the process, products, or techniques feel free to post. I'd also appreciate it if someone with experience could give a run down of the entire process from molding to finished cast, or just some general tips and tricks from experience, I'd greatly appreciate it.

Thanks!

-Nick

 

[fettster]

there are loads of differnet ways to cast in silicone and some are more suited to different things.

in order to be able to help, i'd really need to know what it is you are trying to make

 

[Wesman]

Like fettster says, it would be a little easier to advise you if we had some idea of what you are making. I have cast silicone into silicone molds before, and yes you must use a good mold release. I would recommend de-gassing before the pour. Also, silicone is rather thick, so you have to make sure that the mold does not have "high points" that will trap air bubbles. Take your time pouring and allow the si to settle in. A tall spru will allow an excess amount of si in case more air comes out of the cast while you aren't still monitoring it. You can cut air vents into any areas that might trap air using a small brass tube filed sharp. Here again, we don't know if you are doing open molds, one-piece or two or more piece molds, so there's only so much we could tell you.

 

[clayslinger]

I'm sorry to jump in here but I would like to know what mold material is preferred when casting a silicone display head cast in a platinum silicone.

Are most made with a core and poured or brushed into an open mold?

Thanks!!!
Chuck...

 

[TheNickFox]

Okay, It's difficult to describe what I'm looking at molding exactly, so I'll just use something it's similar to so you guys can get the idea.

It's essentially an open-bottomed (But with a small lip on the bottom), wide but short cup. So once cast it will be a hollow piece of silicone about 3 inches high, 6 inches wide, and 3 inches deep with 2-3mm thick walls.

There are no undercuts or complex shapes, so I'd most likely be looking at a well-vented 2 piece mold.

I'm looking at using one of the food-grade platinum-cure silicones out there in a 40A-50A shore hardness.

I hope my description of what I'm making made sense. Thanks for all your help so far. This is why I love this place!

-Nick

 

[clonesix]

you cast silicone in a non-silicone mold.

Basic rule: cast a soft part in a hard mold; a hard part in a soft mold.


I sounds to me like you're making a baking dish. So let me ask some questions:

what type of finish will the finished part have? Shiney, dull, rough?

Is this piece symetrical? Round? Square?



I know how I would do this, on the assumption that I am making a baking dish. I would cut (profile) templates of the inner and outer surface, and use those templates for plaster turnings. If you don't know what that is, it is a pottery-wheel for plaster. The template cuts the surface of the plaster as it stiffens up. Plaster starts out as a blob, and as it turns against the template, it takes that shape as it stiffens and dries. When the plaster is nearly dry, it is like soap in consistency and depending on how sharp the edge of the template is, you will get a very clean turning. If you have an inside and an outside, with 3mm space inbetween, that is what you use for th emold.

 

[fettster]

id make a matrix mould in th silicone you plan to cast in. when casting, id pour in liquid wax or melted vasaline and pour out excess and let set, then pour in the casting silicone.

with that shore firmness, the silicone will really need de-gassing to produce a decent cast.

hope that helps.

at clayslinger. i use fiberglass moulds for heads and have brushed and cored depending on the piece. prefer coring though

 

[TheNickFox]

That's a great help, the only thing I'm a bit unclear about is what you mean when you talk about pouring wax or vasaline. Would I be pouring that into the mold cavity/void, or using it as part of the mold itself.

Sorry for all the questions, but I want to make sure I know what's what before I attempt this.

And thanks Fettster, having someone with your knowledge and experience answering my questions really is awesome.

I promise I should [mostly] be done with my questions now.

-Nick

 

[clayslinger]

id make a matrix mould in th silicone you plan to cast in. when casting, id pour in liquid wax or melted vasaline and pour out excess and let set, then pour in the casting silicone.

with that shore firmness, the silicone will really need de-gassing to produce a decent cast.

hope that helps.

at clayslinger. i use fiberglass moulds for heads and have brushed and cored depending on the piece. prefer coring though

Thank you sir!!!!
Just plain old polyester resin, right?

 

[MattMunson]

I have had great luck in calling technical support at Smooth-On. The folks who answer the phone usually have years of experience with the products, and can point you immediately to something that will do what you need.

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