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I want to use liquid silicone to make several simple moulds, which I will then use to make resin casts. My aim is to mass produce a custom miniature base, that I originally 3D printed, in two part epoxy as it's faster to cast 50 bases than to print them if I make several molds.

What is the simplest possible container that I can pour the silicon in to while it sets around my miniature base?

For example, would a plastic Tupperware box be sufficient, or a Pyrex dish?

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According to AeroMarine Products' page on moldmaking, (emphasis from source)

Generally, silicone RTV mold making rubber does not stick to anything, and nothing will stick to it. The exception is that it will stick to itself, other silicones, silica, and glass.

In the past, I have used cut-down and clean milk cartons, which make an effective base for a square mold. YouTube resin caster Rybonator has used single-use plastic cups, paper cups, plastic food containers, and custom-built constructs made of stiff cardboard to produce molds. I've additionally seen multiple cases of people building their mold box out of Lego blocks, an example of which can be seen here.

The key points you'll want to consider are:

  • Avoid any material to which resin will stick (no glass, silicone, or other silicates).
  • Avoid anything that will later be used for food. If you aren't using food-safe silicone, it's best to err on the side of caution and keep your casting materials separate from anything you'll use for preparing or serving food.
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    I'd completely forgotten that glass was silicate. Sep 9 at 16:01
  • Easy thing to forget!
    – Allison C
    Sep 9 at 16:14
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Since you are already skilled at 3d printing, have you considered 3d printing a custom shaped box slightly larger than each specific base requires. Doing this would minimize the amount of rubber you need and give you a great place to story your rubber molds between uses.

You could even print thin seams in these custom shaped boxes to allow them to be bent away from the rubber, easing its removal from the box at the end of the mold making process. Such deflection points could easily bend back into their original shape to hold and reinforce the mold during future casting sessions and storage.

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