Rubber Molds From Silicone Caulking

This page outlines a procedure for making high-quality silicone rubber molds out of silicone caulking, the really tacky stuff you find in tubes at the local hardware store. Its a surprisingly simple technique for mold-making that probably has applicability for all manner of craft-type endeavors, not just for concrete. The nice thing about it is that you don't have to buy any type of "specialty" rubber compound; everything you need can be purchased locally right off the shelf.

There aren't many pics, but I was tired of waiting for enough time to complete this page the way I would like. If you have any ?'s, feel free to contact me....

The basic approach is pretty simple: ← Expand All Sections →

1 → Make your model out of plaster. [ + ]
2 → Mix up some silicone rubber and mineral spirits. [ + ]
3 → Use a brush and give the plaster model a thin coating. [ + ]
4 → Repeat steps #2 & #3 [ + ]
4½→ Reinforce any outside corners with fabric. [ + ]
5 → Give the entire piece a thick coating using a caulking gun. [ + ]
6 → Be sure to give the silicone rubber enough time to cure [ + ]
7 → Wet the plaster when peeling off the mold. [ + ]
• Use the "good" stuff. I've tried various grades of silicone and even though its more costly, the higher the quality the rubber, the better the mold. At least that's been my experience. You won't regret using the highest grade rubber you can afford. You ***WILL*** regret using cheap stuff if you have the money to buy the good stuff.
• Most of the molds pictured above employ fiberglass shells to provide rigid support to the rubber while the concrete is still plastic. This tutorial does not outline the use of fiberglass. You will need to climb that learning curve elsewhere (sorry). I no longer make many silicone rubber molds so I no longer maintain a supply of resin or glass cloth, but I will say this, "fiberglass is an extraordinarily versatile material. It has a million uses. But it is nasty stuff to work with, the resin has a relatively short "shelf-life" and its rather expensive. Everything has its pros & cons.
• One possible "work-around" for creating a rigid shell to maintain the shape of a piece while waiting for the concrete to attain its initial "set" is to use plaster shells. Its not nearly as useful as fiberglass, but its cheaper, considerably less toxic and has a really long shelf life (as long as you keep containers tightly sealed). If you pick up some cheap "white glue" (i.e. the canonical "back-to-school" glue every grade schooler uses), you can squirt a bunch of glue into your mixing water. Once the plaster dries, the white glue will harden and impart some tensile strength to the plaster, which increases its versatility and durability. I've used this approach some, but not enough to supply much information beyond what is outlined here...
Even though this may appear a little long, it still represents a somewhat concise summary of the steps required for making a silicone rubber mold...but it *should* get you started in the right direction.
If you try this approach, start your efforts by testing on very small projects. These take less time and use only small amounts of material, but offer a lot of insight...there is absolutely *no* substitute for trying something yourself.