[huertaarth]

Hello all!

This question is more along the lines of a production logistics question - but what are some effective casting methods to  create a silicone part? In particular, what is a good mold material that would be appropriate for delicate work and allow the silicone part to easily pop out? Anyone with any experience on this? Feel free to ask any relevant follow ups.

Thanks!

[billnotgatez]

I am at a loss here, since the molds (the negative) I make are composed of Silicone like material sometimes backed with plaster. I then make the final positive from something else like clay or wax. You on the other hand seem to be making the final positive from Silicone.

Are you currently using a metal like aluminum (corrosion resistant) as your mold?
or
Are you currently using another composite like urethane as a mold?

At this point, I would think plaster would be to fragile for your needs.

I would then think a Silicone based spray would work as a releasing agent.

If I run into the master mold maker that I know, I will pose your question to him.

[Enthalpy]

I too would go to metals rather than ceramics to help silicone pop out, and use a usual liner spray. Or try to brush the mold with an alkane.

A hollow silicone part is easier to take out of the mold, if you can. Once the casting insert is removed, you can deform the part to take it out.

Usually, silicone molds let the parts come out easily. It is much more difficult with silicone parts?

[huertaarth]

I am at a loss here, since the molds (the negative) I make are composed of Silicone like material sometimes backed with plaster. I then make the final positive from something else like clay or wax. You on the other hand seem to be making the final positive from Silicone.

Are you currently using a metal like aluminum (corrosion resistant) as your mold?
or
Are you currently using another composite like urethane as a mold?

At this point, I would think plaster would be to fragile for your needs.

I would then think a Silicone based spray would work as a releasing agent.

If I run into the master mold maker that I know, I will pose your question to him.

Hah yea I know it's far more typical to make the negative out of silicone, but in this case the positive is going to be silicone. It's the first time I've made a part out of silicone so I really have no idea if it's difficult to remove etc. - which is why I was hoping to find someone who knew better than I!

I was thinking of using urethane to make the mold, with a normal releasing agent, but I really didn't know if silicone posed any extra weirdness, especially because I need the part to come out precisely.

Also, any advice on type of silicone? I really had no idea there were so many options. I'm looking for a kind that is fairly hard but not brittle. Heat/Cold resistance isn't an issue. Ease of handling is a big plus since I'm obviously new at this! Even if you just have advice as to where I could effectively research this myself, that would be great.

Thank you for your *delete me*

[Enthalpy]

Most often, silicone doesn't adhere to other materials, except to some ceramics like glass or wall tiles. Hence the suggestion of metal or plastic as a mold. Liners are seldom necessary but exist if needed.

Users sometimes want silicone for medical cleanness or biocompatibility. Such a need would impose the choice of one material.

Yes, silicones are extremely varied, not only in hardness. I've forgotten a bit, but there are several families (3?) depending on the way they cure: by evaporation of acetic acid (not good within a closed mold), addition of a cross-linker (may include tin salts), and one (?) more. That would be the first decision, because some manufacturers or distributors provide only one family. Then, each family provides a broad choice of hardness.

Molding is rather easy, the biggest worry being bubbles, because of viscosity. It's a good idea to obtain a vacuum pump and bell jar, transparent for observation, to degas the resin after mixing has brought air bubbles in. Casting under vacuum would help, so the mold is properly filled. For bigger series, people mix the resin directly under vacuum.

Shape accuracy isn't a worry. I had poured (tin-linked) resin in a Tupperware: every so tiny scratch in the mold was visible at the part.

[huertaarth]

Most often, silicone doesn't adhere to other materials, except to some ceramics like glass or wall tiles. Hence the suggestion of metal or plastic as a mold. Liners are seldom necessary but exist if needed.

Users sometimes want silicone for medical cleanness or biocompatibility. Such a need would impose the choice of one material.

Yes, silicones are extremely varied, not only in hardness. I've forgotten a bit, but there are several families (3?) depending on the way they cure: by evaporation of acetic acid (not good within a closed mold), addition of a cross-linker (may include tin salts), and one (?) more. That would be the first decision, because some manufacturers or distributors provide only one family. Then, each family provides a broad choice of hardness.

Molding is rather easy, the biggest worry being bubbles, because of viscosity. It's a good idea to obtain a vacuum pump and bell jar, transparent for observation, to degas the resin after mixing has brought air bubbles in. Casting under vacuum would help, so the mold is properly filled. For bigger series, people mix the resin directly under vacuum.

Shape accuracy isn't a worry. I had poured (tin-linked) resin in a Tupperware: every so tiny scratch in the mold was visible at the part.

Thank you, this is excellent information. What family is best for biocompatability? Or if not by family, what parameters are there to distinguish biocompatability vs. not? Possible applications of what I'm working on would include medical, so I'd be interested to see how compatible it would be with my other goals.

[Enthalpy]

Medical use is a very strong constraint. Not always as critical as for an implant, but often you need the parts to be sterilizable, resist alcohol, and so on. While silicones meet these two easily, a serious difficulty is the release of minor components by the silicone over time. Typically the hardener contains unhealthy compounds. This must impose the silicone family, with a narrow choice.

Both because it's a genuine need and because stiff regulations exist for medical parts, you'll probably need to take a silicone formulation that is appointed for medical use, so you path could be to first find a producer whose silicone is appointed, then choose in his product family the proper hardness.