Silicone molds

A printed box you fill with silicone: which silicone, how to pour it, how the mold opens, and casting resin, plaster, wax or soap in it.

In a silicone mold the set silicone is the mold: it stretches off shapes that plaster would lock on. You print a box (or boxes), stand the model in it, and pour the silicone round it. Open the silicone mold tool for figures, tiles and anything with undercuts.

Three kinds of silicone mold

An open box: one piece. You peel the mold out of the box and off the model.
A block you cut open: the ridge on two sides of the block marks the cut. Cut down to the model along it with a scalpel, in a zig-zag so the halves key together, and stop short of the far end to leave a hinge.
Parts with keys: each box makes one part. Trim any thin skin at the edges with scissors.

Which silicone

Tin-cure silicone is forgiving. Platinum-cure lasts longer, but against some filaments and resins it stays tacky: seal the print with a clear coat first, and make a small test pour.

Pour the mold

Brush or spray a thin coat of release agent inside every box and let it dry. Too much pools in the detail.
Stand the boxes level, open side up.
Weigh both parts of the silicone to the gram, by the ratio on its data sheet; when the data sheet gives the ratio by weight, the pour sheet works out the grams of each. Stir slowly for 2 to 3 minutes, scraping the wall and the bottom, then pour it into a second cup and stir again: silicone left unmixed on the cup wall never sets.
Pour a thin stream into the lowest corner and let the silicone rise over the model by itself. Fill to the brim, then tap the table to bring the bubbles up.
Let it cure level and undisturbed at room temperature, as long as its data sheet says, often 4 to 24 hours. Cold slows the cure.

Cast in it

Close the mold (band it snug, not tight: a squeezed mold bends the cast), or stand an open box mold level. Mix the casting material as its maker says and pour it in slowly, through the cup when there is one, tilting and tapping the mold to let trapped air out. Resin and concrete warm up as they set: do not move the mold while it is warm, and check their cure heat against the limit on the silicone's data sheet. Then open the mold away from the cast a little at a time, and cut the sprue and vent stubs off flush.

Wash the mold with soapy water now and then, and store it flat, away from sunlight.

If something goes wrong

The silicone stays sticky against the print: platinum-cure silicone is stopped by some filaments and resins, or the two parts were off ratio. Seal the print and weigh both parts to the gram.
Bubbles on the face of the mold: air stirred in, or poured too fast. Stir slowly and pour a thin stream from high up into one corner.
Silicone leaks from a box: a gap between layers of the print. Press clay or hot glue over it at once, and print the next box with one more wall line.
The mold tears when it opens: a deep undercut, or it was pulled too hard. Bend the silicone away from the cast, never the cast away from the silicone.