I’m having a problem, and I’m hoping someone here might know of a
solution. My company manufactures a line of local-themed jewelry.
I am in charge of wax production (injection, manicuring…), and we
have one collection that’s giving me fits. There are three sizes of
pendants that are designed to be completely flat on both sides (with
a rectangular cross-section). The collection can be viewed at http://www.mardonjewelers.com/html/traditional.html. Ideally, they
should come out of the mold perfect so there aren’t any marks to
clean up once these pieces are polished. The problem is that the
models are dipping in the middle on one side (the largest dips the
most, but the smallest dips enough that they have to be sanded
down). Shrinkage seems to be causing the problem. I have tried
numerous waxes (Kerr tuffy green, Rio pink buckle most recently). I
have tried keeping pressure on the molds until the wax is set. I
have refrigerated, frozen, and heated the molds before injection.
Pressure and temperature on the wax affect the dipping (using Kerr
Injectomatic II w/hand pump), but no combination of the two has
completely removed it.
If you have any suggestions, I would truly appreciate the help! My
bench jeweler, our wax carver, Rio Grande, and Kerr have all given
suggestions, but nothing has worked so far.
Have you tried injecting your designs at the low end of the wax’s
melting range? The cooler the wax is when it goes in, the less it
will shrink upon solidification.
Can you post a picture of your master or masters so that we can see
where and how the gate (sprue) attaches to the master? It would also
be nice to see the relationship of gate cross section to model cross
section. Maybe the model shrinking on one side has to do with the
gate solidifying before the model.
Shrinkage seems to be causing the problem. I have tried numerous
waxes (Kerr tuffy green, Rio pink buckle most recently). I have
tried keeping pressure on the molds until the wax is set.
There are several solutions. One is to make the original model domed
larger and thicker in the problem area so that when it shrinks it
comes close to the desired dimensions. next would be a spherical
shrinkage reservoir (a 4mm or larger ball of wax) sprued on a short
3mm or so sprue to the center of the shrunken area (lots of mess to
clean up). Another way is to make plexiglass plates which clamp the
mold while injecting. Above the area that shrinks you carve a domed
depression into the plexiglass with a large ball burr. When you
inject the rubber mold inflates a bit into the depression, thus
ending up with a wax which is thicker than the original model at
that point, and which shrinks back to the desired position upon
casting…
If you have any suggestions, I would truly appreciate the help! My
bench jeweler, our wax carver, Rio Grande, and Kerr have all given
suggestions, but nothing has worked so far.
HI
There are lot of variables to work with.
The pieces are very nice but look rather thick. Some times I have
run into problems with the more wax to cool the more sinks in the
wax due to contraction during cooling. I turn down the wax
temerature as low as it will go and still flow into the mold. Also
check the temperature to see that it is accurate. Turn the peasure
down as far as you can and still get a complete injection. I try to
get all my molds to inject at 6 psi. Then look at the mold to see if
it has enough air lines. When you inject the mold hold it loosly,
but not so loose wax escapes. for two reasons, first so you don’t
distort the mold with too much compression, the second is a tightly
compressed mold when released can draw or suck inward some what like
an eyedropper bulb. this can cause a depression in the wax. (hold
the mold loosly until the wax has cool at the sprue area the wax
will cool from the outside inward).
We make an injection wax that is specifically designed to resist
such dipping, which is caused when large wax pieces cool off. Some
people call it “cavitation”.
Please give me your shipping address & I’d be happy to send you a
small free sample of our CASTALDO SuperCera green wax to experiment
with.
Jenny, your injected wax shrinkage problem may be due to injecting
the wax too hot. I have had this problem with my flat work, and
running the wax as cool as possible has always solved it. Turn the
heat down on the injector until it will not work, then turn it up
just past that temperature. Yes, this takes some fiddling around, but
it is less work in the long run compared to trying to correct each
injected wax piece. And a corrected wax piece never seems to look as
good as one that came out right to begin with, does it? If the wax
you are using will still not work, then you will have to look for one
listed for less shrinkage. I think Rio Grande gives this kind of info
on their injection waxes.
The problem is that the models are dipping in the middle on one
side (the largest dips the most, but the smallest dips enough that
they have to be sanded down).
Hi Jenny,
I have had a similar problem with large, flat models that I have had
cast by a third party. In one case, the model was essentially a
frame around a flat backing. What I got back from the casters were
open frames with no backing at all! I have also had a few problems
with pendants being concave where they should be convex.
In many of these cases, I wonder whether the problem is simply
caused by applying too much pressure to the mold, causing the sides
of the mold cavity to cave in or press together. Perhaps an even
pressure around just the edges of the mold might help?