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| A selection of molds made 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.
It's 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.
The basic approach is pretty simple:
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1 → Your model can be just about anything.
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All of my modeling has been in plaster or clay, but just about anything will work as long as you have a good release agent (see the following).
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Take a bar of soap (I've always used Ivory but I suspect any would do) and toss it into a bowl/tub/bucket of water. Let it become totally water-logged.
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As the soap becomes more & more jelly-like, squish it up with your hand(s) to help turn the jellied soap into something more like syrup.
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When you have a mixture that is rather "goopy" (for lack of a better term), strain the syrupy, jellied mixture through a couple layers
of pantyhose. You might need to squeeze it a little. I find straining the mixture through the fine openings of pantyhose seems to help homogenize
the resulting water/soap mixture.
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The resulting syrupy liquid is your mold release. Use a brush to coat the mold. Let it dry overnight. Applying a second coat is never a bad idea.
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NOTE that depending on the consistency of the mixture you produce, it may appear somewhat thixotropic. If this bother you, adding a little water
should thin the mix to a point where thixotropy is no longer a property of the release agent.
2 → Mix up some silicone rubber and paint thinner (or mineral spirits).
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| Add some silicone caulking to paint thinner. |
The volume of paint thinner you use will depend on the size of the model. If the model
is relatively small, just a few ounces ought to suffice. Squirt some caulking into the
solvent (see the page on silicone & fabric for additional tips)
and mix until "smooth".
3 → Use a brush and give the model a thin coating.
Resist the temptation to "glob" it on. Use a brush with enough stiffness so that
you can just give the model a very thin coating.
Keeping the coating thin minimizes air bubbles. You'll thank me later.
4 → Repeat steps #2 & #3
Continue to add thin layers of rubber once (or twice)
a day for about a week. Your goal at this stage is to build up a continuous layer of rubber
measuring about 1 to 2 millimeters thick.
4½→ Reinforce any outside corners with fabric.
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Embedded fabric can be used to
reinforce outside corners. |
NOTE: not all molds will require this step. When you peel a mold off a casting, its a little like pulling a sock off your
foot by turning it "inside-out". During this operation, this imposes a fair
amount of tensile stress on portions of the mold that represent outside
corners. To minimize the potential for tearing, which can completely
destroy a mold if you don't watch out for it, you can strategically add
pieces of fabric to exterior corners
(use spandex or equivalent ... it needs to be ←stretchable→ fabric).
After you've built up several layers of rubber on the model, cut a few pieces of spandex (or equivalent),
saturate them with a silicone rubber/mineral spirit mix and attach them to the outside
corners of the model (see the above image). Be sure to work out any air bubbles. When it comes time to peel the mold off, this
can absolutely prevent tearing.
5 → Give the entire piece a thick coating using a caulking gun.
Once you've built up a nice (reasonably thick) coating, say 1/32nd to 1/16th inch
(approx 1 to 2 millimeters), then load the silicone into a caulking gun and give
the entire piece a "thick" coat, by just giving the entire thing a continuous "bead".
This additional coating, which I call the "structural" coat, gives the mold enough
strength to resist the stresses that will be imposed when you start using the mold
in some type of production capacity.
6 → Be sure to give the silicone rubber enough time to cure
When in doubt,
waiting another day or two is always the prudent approach...even if you're
anxious to see the results. Trust me...I've learned the hard way on this.
When in doubt, give it another day. My experience is that the more expensive silicone caulks take a longer time to fully cure than the less expensive ones.
7 → Wet the plaster (or clay) when peeling off the mold.
When it comes time to peel the mold off the plaster, if it appears to be
very tightly adhered to the plaster, pour a little water on the the plaster
right where you're trying to get the mold loose...I've found that silicone rubber
detaches *much* more easily from damp/wet plaster than dry plaster. For clay models, this can also be an issue.
NOTES:
• Use the "good" stuff.
I've tried various grades of silicone and even though it's 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.
One possible exception to this might be if you aren't planning on
using the mold for serious production work. If you only want a to make a few
castings for personal use, the low-cost stuff is fine.
There is one caveat to keep in mind, though: cured silicone
rubber shrinks over time. And the cheaper stuff shrinks more than the
good stuff. At least that's been my experience. It isn't dramatic, but it is noticeable.
• 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 it's 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. It's not
nearly as useful as fiberglass, but it's 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.
