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Mold making on the cheap

Hello all,

In Murray Bovins casting book he describes being able to vulcanize
rubber molds in a home oven, is this a viable technique for a
craftsman on a budget and will it yield satisfactory results.

Also if one wanted to go the route of rtv molds would painting the
mixture directly on the model help attain good results if one did not
have a bell jar vacuum. I have seen vibrating tables in the tools
catalogs the small ones run about 80 dollars are these better than
nothing at all. thanks in advance.


I use RTV on one-part molds and that is how I prevent bubbles. I get
an old children’s paint brush (Crayola or the like) and paint a
layer over the surface, the pour the RTV over to fill the cavity. I
have heard the opinion that if you are using a two-part mold, there
might be an issue with bubbles in the balance of the mold causing
some sort of problem because of porosity. It seems to me if you paint
a piece with RTV, then have to set it in a two-part mold and pour the
rest, the problem would be the layer on the piece curing before you
get the rest in the mold and it not adhering and splitting away when
you cut the mold. I don’t know if that is an issure or not. But check
into that.


Yes to both, it is just more trouble, I did it at the start, your
wife may complain about the oven…

I don’t think the vibration would help much but painting on would,
you can get some good two part pastes that work well.

regards Tim.

this a viable technique for a craftsman on a budget and will it
yield satisfactory results. 

I tried it once, and it didn’t work, the problem was not enough
pressure. My friend, bigger and more burly, it works for him.

Hard to Find Tools for Metal Clay

This is a great way to vulcanize molds (I prefer silicone). I use a
toaster oven and couple of peices of aluminum plate with bolt holes
in them. They should be bigger than your mold frame and my bolts are
pressed into the bottom plate with the top holes bored out enough to
allow it to move freely.

I just bolt the whole thing down tight and pop it in the oven using
an oven thermometer to mind the temp. I allow longer vulcanizing
times as the plates are cold going in and I leave it bolted until
it’s cool enough to handle. It’s slow going so I try to make sure I
make one double mold every day. Have fun (try powder seperation!)
and save your cash for stuff you really need - like stones!

Chas Hofmeister

Dear Bill,

This is Michael Knight at Castaldo. We’ve been publishing
instructions on how to vulcanize molds in a toaster oven for a long,
long time (although not Murray Bovins’s version). A copy is packaged
in every box of Castaldo.

It’s also on our web site at:

There are lots of other how-to articles on our site at:

Have fun!

Hey thanks all for the advice, I will probably see if i can pick up a
toaster oven at a yard sale soon. I also have a question about no
shrink pink aside from the name doesn’t is actually shrink about 3
percent or less and if i wanted no shrinkage rtv is the way to go?


I made an oven vulcanizer. Two plates of 1/2 inch aluminum about two
inches larger in each direction than your mold frame. One had holes
in each corner that bolts were put through with nuts to hold them in
place, the other had holes that would allow it to slip easily all the
way down to the other plate. Use washers and wing nuts on each bolt
to apply the pressure, I used channel lock type pliers that could
grip the aluminum plate firmly after the ten minute heating of the
mold so I could use pliers to tighten the wing nut to get the right

Richard Hart

Hi William:

There are two ways to make a mold that does not shrink.

You can achieve zero shrink by using our Akron RTV-RP clear

You can also use our 4X ZeroShrink, which just won an MJSA
innovation award for 2007.

The 4X ZeroShrink is cured from 160F through 250F–your
choice–depending on the model.

Keep in mind that only the mold does not shrink. Your wax and other
steps do produce shrink.

Contact me off forum for details and samples


Bill Mull
Zero-D Products, Inc.
precision engineered materials solutions

Dear William,

This is Michael Knight at Castaldo.

Molds made with our No Shrink Pink rubber will indeed exhibit 0.0%
shrinkage if done properly – something that requires at bit of
learning and experimentation to compensate for the wide variations in
the generally poor quality of vulcanizer thermostats.

Castings, however, are a different matter entirely. And I’m sure
that the final shrinkage of the finished piece is what you are really
talking about and what you are really concerned about.

Making a mold with 0.0% shrinkage is an excellent first step but
will not produce a final casting with no shrinkage-- there are
1,000’s of other factors involved, all of them introduced by the

A short and partial list of factors includes wax type, wax
temperature, wax injection pressure, sprue length vs. width ratio,
injection time, sprue placement, sprue design, investment shrinkage,
metal shrinkage, rubber mold use temperature one can go on and on.

All these factors will exist even with an RTV rubber mold.

Michael Knight