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Rio cold mold and wax injection

I ordered the Rio Cold Mold kit (part # 701041) so I could make
molds to inject wax into for lost wax casting.

I found an inexpensive, used wax injector on Ebay that works great.

I’m having trouble making the molds to inject wax into and filling
them with wax.

I can’t find any info online on how to do this. Making a one-sided
mold is easy, but that’s not what I need.

Basically, the stuff is too flexible. When I inject wax into the
mold, the wax shoots out the joint between the two halves, so the
mold isn’t filling up.

I’m also thinking I need to add a passage for air to escape in a
more planned manner, much like sand casting?

Any tips or links would be very helpful.
Thanks in advance!

Hi David,

I use RTV moulds with a wax injector.

You will always get some wax squirting out of the parting line. I
usually find the best way to hold a particular mould to minimise
this. This is where the air escapes when the wax is injected. The
best you can do is to try to make your parting line in a position
that makes subsequent clean up easy. Using the clear RTV compound
makes this process easier. I start my cutting from the sprue end and
attach the sprue to the pattern in line with the parting line you

If the mould is not filling completely it may just be that the
pressure is a bit low or the wax is too cool. Maybe try an additional
5C. I usually work at 70C. I usually use around 50 Kpa pressure (0.5
bar) but some of my moulds require a little more. The other problem,
particularly if the problem is with the end opposite the sprue not
filling, is that you may need to extend your parting line cut further
back. I usually don’t cut all the way back so there is a hinge at the
back of the mould. Most times I leave a minimum extension and see how
the mould injects and extend as necessary. This makes aligning the
halves more or less automatic (mostly anyway!) When you make your
mould measure out enough RTV to allow enough room for this – say
15-20 mm above the top of the pattern.

You may need additional venting for more complex patterns but so far
my work hasn’t required this.

All the best and have fun.

put the mould between 2 sheets of steel 1/4 in thick attach wire to
the master to allow the air to escape when injecting the wax

I found that holding molds with my fingers while injecting wax can
cause problems.

Finger pressure on the center portion of the mold can squeeze the
mold cavity deforming the wax model.

Holding the mold with fingers cannot provide uniform pressure along
the edges of the mold. This will allow wax to squirt out of the mold
wherever the pressure on the edges is less.

I have cut pieces of plywood the size of the length and width of my
molds. I place a piece of wood on top and bottom of the mold as I
prepare to inject wax.

The wood slabs, when compressed against the mold, provide uniform
pressure along the edges. The use of the wood slabs is especially
useful when injecting wax into soft molds.

However no matter how tightly the mold is held wax can sometimes
leak out. The mold should be cut so that any leakage of wax will be
on a portion of the mold that is easily cleaned.

A mold clamp is a much better way of holding the mold while
injecting wax. They provide uniform pressure on the mold so that
injected wax models are of uniform weight.

Lee Epperson

I have for years used pieces of plate glass and hold them in position
with large rubber bands. Works like a charm.

cheers, Don in SOFL