Vulcanized molds refuse to fill

Hello all,

I have a Kerr wax injector that I use to make waxes for casting. My
problem is that several of my vulcanized molds refuse to fill

Often I get partial fills, where the wax seems to have not quite
filled in the entire void and thus makes an incomplete wax model. I
have cut “air channels” into the spaces around the model, but it
hasn’t seemed to help. I have also experimented with different wax
temperatures and air pressures all to no avail. Even "Golden River"
wax added to the pot doesn’t seem to help the waxes to come out
complete. I usually use Silcone Spray to free the waxes, but switched
to mold release powder after hearing it might be better at allowing
air to escape the mold void and allow a better fill (if this was the
problem - I’m not sure)

These are not complex molds either, just flat simple domes about
31/2 inches in diameter and 1/32 inches thick with detail on the top.

Does anyone out there want to share their secrets for getting molds
to fill correctly? Sometimes I spend all day wax injecting and don’t
get any wax models made. It is getting frustrating.

Jim Manners


First make sure you have your wax melted at the manufactures
recommended temp.

Flat or near flat items can often be a little tricky to shot waxes

You may be applying too much pressure or uneven pressure to the

Do an experiment. Try placing the mold between your two plates and
very lightly holding it all together to shoot a wax. Just enough
light even pressure to hold the mold together. It is OK if wax shoots
out the seam of the mold, you are only testing to see if the mold
will fill completely. Shoot the wax at a low pressure say around 3 to
5 pounds of pressure. If it does fill completely then shoot the next
wax with just a little more pressure applied to the mold and plates
to see if it fills completely.

Keep on testing the amount of even pressure you apply to the mold
until the wax stops shooting out the seam.

You may also wish to slowly raise the pressure of the wax pot after
the above experiment to see if this helps.

Once you have the right combination of applied pressure and air
pressure make a note on the mold for the next time you use it.

Good luck
Greg DeMark


When you use the parting powder make sure some gets in the
air"vents"–slits-- that you have cut. This will hold those vents
open just a tiny, tiny bit which will help in evacuating the air.

I would also adjust my air or hand pressure, depending on the type
of injector that you use. With larger fills, sometimes too much
pressure will yield non fills.

I’d also look at my wax temp. Too hot or too cold can really make a

Good luck,
Andy cooperman

I have several molds of a dome like design that I had problems with
complete injections. You might try these two things.

  1. Powder the seams of the mold and the air vents but not the cavity
    where the wax fills. (sometimes the air vents close back up when
    using the silicone rubber molds. by powdering just the seam you get
    better air venting when injecting.)

  2. If the texture of the interior of the dome does not matter you
    can cut grooves in the rubber that goes inside the dome allowing for
    a more even fill. Cut the groves from the outside diameter to the
    center of the core, following the contour of the dome, I assume this
    piece is sprued from the center of the dome. Lay the lines out in a
    symmetrical pattern using 5-8 grooves. I use a seperating disk, a
    sanding disk, a v shaped chisel, or a hot tool to cut the grooves.

my best solutions. Frank Goss


I think the trick is making the rubber mold function like a flask
that is ready to cast. That is, the wax must fill the cavity, driving
out the air before solidification, much like a casting. So your
obvious variables are wax type and temperature, pressure of
injection, pressure of clamping, sprue diameter and placement, and
air relief. You have covered most of these.

However, 3 1/2 " is a very long way for wax to travel through a
thickness of 1/32". Medallion shapes seem particularly difficult to
fill. And then you have to resolve the same problems with the
casting. Were I doing it, I would want the piece to be considerably
thicker. I might also add a sprue on either side of the original, so
wax inflow covered about 240 degrees. After all that, I doubt I
would be successful with this project without adding a lot of weight.
I do not know whether vacuum assist would do the trick. Does your
design lend itself to an alternate approach, such as etching,
repousee, or sweating layers?

Jon Abbott

I second all the suggestions I’ve read so far. The only other thing
that has helped me is letting molds for thinner, flatter models sit
on top of the wax pot and heat all the way through before injecting.
This may degrade the rubber faster, but is the only way I’ve had luck
with some models.

Good luck,
Jenny Sweaney

Mardon Jewelers
3640 Main Street
Riverside, CA 92501


You don’t mention how you’re holding the molds for injection. My
experience when injecting a fairly thin wax is to hand hold it
between plates and not use a spring loaded mold clamp. I always used
a spray mold release (spray silicone) and hold the mold lightly at
the edges. You might get some flash if you’re holding it too lightly
but that’s easier to deal with than an incomplete wax.

Try that along with a bit more air pressure.

Hal Holzer

Sometimes I spend all day wax injecting and don't get any wax
models made. 

Dear Jim: Ouch. I do a lot of thin medallions that sound similar to
what you are doing. I too had many partials when I first started. I
tried many of the suggestions that have already been made. Here are
the things that work best for me:

  1. Make a larger vent than you think necessary at the back of the
    voids. Make sure you have excavated evenly from both sides of the
    mold. The more persistent the void the more incursion I make into the
    wax area. It’s a trade off between complete injection and extra work
    on the model.

  2. Shoot the wax a little hotter and at a higher pressure than
    normal. With the Kerr hand pump type I’ve gone as high as 29 lbs,
    with the compressed air type around 14 lbs.

  3. I think this is the best tip: I don’t use plates or pliers with
    thin molds. I cut a larger than normal sprue opening in the mold
    end. I fold two or three paper towels into a long rectangle just
    smaller than the mold. I wrap this around the mold several times so
    it ends just back from the sprue end. Hold the mold at the edges then
    inject the heck out of it and immediately turn it upright so you have
    a well of wax in the sprue opening. Quickly squeeze the mold several
    times and you will see the wax in the well pump in and out of the
    mold. Relax the mold to what you think is the right pressure to
    achieve its original shape and cool it for a minute before you set it

It takes some practice to get the feel for handling the mold, and
you might want to wear rubber gloves the first few times because it
can be messy (and hot) - but between all of the above I almost never
have a failed injection after the first couple of experimental tries
and vent adjustments.

Good luck!

I shoot lots of very long buckles. I use Rio Grande’s Pink Buckle
wax. I find that too low of a pressure will not fill the mold. If
the wax or mold is too hot you get voids. Do not overheat the wax and
let the mold cool down before injecting the following wax.

Some times too much clamping pressure on the mold body will prevent
complete filling. Place the mold between two blocks of wood. Use
just enough pressure to keep the mold together. Wax exiting the mold
should not be a problem if the mold is cut correctly.

If I have a long buckle mold that does not fill place 16 gage wires
at the very end of the cut of the mold (the very end of the
pattern). The wires are placed so that one end is just at the edge of
the mold pattern and the other end exists the mold body. This really
opens an exhaust at the end of the mold.

Photos available if requested.

Lee Epperson
About to do a show in San Diego Old Town.

I read a couple of replies to this about temperature, wax types,
pressure - all good advice, I thought. However, if all else fails,
you might just have a bad mold, which is to say, you molded a bad
model. 1/32" is not very thick for a 3 1/2" circle, and then you had
shrinkage in the molding. You can fix a bad mold sometimes by burning
it (only Castaldo - you can’t burn silicone). Remember what I said -
“If all else fails”. You can crank up a wax pen, or use a dental tool
and a torch in a torch holder to heat it. (alcohol isn’t hot enough).
I burn out sprues all the time, widening them and smoothing them. I
have a sprue former, too, that melts the “button” and makes it uniform

  • that can be a problem, that the nozzle has a bad seal. You can also
    melt around the part on the back to thicken it up, which can leave a
    bumpy texture in the wax, but you can get the piece. With a 3 1/2"
    circle, the back will be rough no matter how careful you are. Yes, it
    will stink, and it’s messy. Acetone will clean it up a lot, and then
    use talcum powder liberally, and throw away the first wax, as it will
    have residue. Maybe you won’t want to do this, but at times it’s a
    lot easier than rebuilding the model and making a new mold. You can
    also cut new sprues around the part up to 1/2 way or so. If you do
    that, you will be better off melting them after, so they are smooth.
    I can tell you what the problem is, for sure - your model is too thin
    for the width of it. But sometimes you can still get waxes if you do
    some of the things suggested here, mine and others.


Every one of the suggestions before mine are spot on, illustrating
how complex it really can be just to “shoot wax”. Jenny especially,
with her guidance about warm molds, helps nail down a special
flow/chill issue. After you solve injection temperature and pressure,
sprues and vents, hold times, release chemistry, flow dynamics gasp

  • note there’s a company called Moldflow that exists just selling
    software to design plastics molds for best flow/fill and molded
    properties - you are left wondering why some of the very experienced
    respondents talk about squeezing the mold progressively as the wax
    cools. The reason is (mostly) that injection wax is a partially
    crystalline thermoplastic. You don’t just suffer thermal contraction
    when the wax solidifies, you also suffer up to 14% volume reduction
    through the phase change that occurs!

If you cool it fast through the melt point, wax stays rather
amorphous with properties that include a tad more flexibility, lower
density (higher volume fill for a given weight of wax). Trouble is,
it can only cool from the outside surfaces, there is no “microwave
refrigerator” to cool it uniformly through thick sections. So fast
cooling of thick wax sections can chill the outside wax skin into a
rigid shell, allowing internal voids (even tears and cracks) to form
when the inside wax solidifies and shrinks. If it’s a thin molding,
the outer skin can be pulled inwards by the cooling central wax,
causing sinks in the final surface, near the thickest parts.
Eleventh-hour squeezing a flattish mold distorts the thickness a
little, but can prevent those voids, cracks and local sinks. Fast
cooling generally means less need for follow-up injection, especially
if you squeezed!

If you cool it very slow through the melt point, a better ordered
crystal structure can form, which behaves more rigidly, packs the
molecules tighter to a slightly higher density (less volume fill for
a given weight of wax). Trouble is, you need follow-up injection to
add wax volume because of the higher achieved density with slow
transit of the melt point. That volume needs to reach where the wax
is chilling LAST. And it ties up you and your mold longer.

Why am I telling you this? Because you should not feel bad that you
sometimes have a troublesome design & process. It’s awfully complex,
it’s most often hand-operated adding even a bit more variability. If
you do all the right things, all the individual advice given before
mine pushes you in the right direction. Mine can give the problem
another nudge, but all this materials science won’t stop wax casting
being a slightly black art. Now where’s that vial of powdered toad’s


Does anyone out there want to share their secrets for getting
molds to fill correctly? Sometimes I spend all day wax injecting
and don't get any wax models made. It is getting frustrating.

I had a similar problem just yesterday. I had six very thin flat
pieces all molded in Silastic silicone molds, and couldn’t get any of
them to fill. I tried everything that others had written about out
here (VERY timely thread! thank you all!) but nothing worked. I
wasted nearly an entire injection pot of wax trying different
methods. I kept getting close, but not 100%. Finally I held the mold
closed with two other Silastic molds instead of wood or metal sheets.
That worked, because the other soft molds “gave” enough to allow the
middle one to expand for filling, but held it together without
pressing too tightly. But then the wax model wasn’t evenly flat–it
was wedge shaped because my hand pressure wasn’t evenly distributed.
So I put the three molds into one of those mold clamp tools and tried
that, along with powdering all the air vent cuts heavily every single
time I injected the mold. That worked perfectly, and I finally got
acceptable models from all six molds.

Kathy Johnson
Feathered Gems Jewelry

A big Thank You to everyone who took the time to respond to my
questions about getting wax into my rubber molds. I know I didn’t
give much yet many jumped at the chance to share their

This is what makes Ganoksin great.



That is the beauty of working with Jewelry. It is part Art and part

No matter how much experience you have there is often something that
comes up either in the design of a piece or the actual creation of it
that makes you have to experiment and learn new things.

Congratulations on learning a new way to approach a problem.

Greg DeMark

Dear Kathy,

This is Michael Knight at CASTALDO with a suggestion for you and all
the others who are having trouble getting their molds to fill.

As you can all see from this thread, just injecting a wax is not as
easy or simple as it seems.

The advice and technical has been wonderful! But
sometimes you just need to shoot a wax with the mold that you have.
Not the mold you should have, but the one you have in your hand right
now, today. We’ve all been there.

We have a product called "Golden River Wax Additive - The Wax
Injection Problem Solver " that is perfect for just those situations.

It’s a compound – a wax really – that when added to your wax pot
one or two handfuls at a time will dramatically drop the viscosity of
whatever you have in there at the moment. In other words, it will
make your favorite wax FLOW like water into the pattern in your
rubber mold.

No need to change waxes, clean your wax pot (although that’s always
a good idea, but right now you just need to fill this one mold…)
And no need to experiment with new or different waxes recommended by
your cousin.

After you’ve injected that mold successfully and the problem of the
moment is solved, you can return to the comfort of your favorite wax
merely by adding an handful or two of that to your wax pot and you
are back where you started. And the rest of the bag of Golden River
just sits on your shelf waiting for the next time you get in trouble.

Success, viola!

You can read more about it at:

It’s sold by Gesswein, Rio (they use it in their production shop and
love it ) and most other CASTALDO dealers.

If anyone out there would like a small free sample, please just ask.
But please give me your full and complete contact info (UPS insists
on having phone number) and please – no P. O. box addresses.

Michael Knight

Now where’s that vial of powdered toad’s liver?

Yeah, I’m fresh out, myself. This thread has had about as great of
advise on the subject as you’ll ever find - I’d like to start from
square one, though, for those who can use it.

There have been a couple of threads here about, “How to get rubber
molds to fill properly.” The answer to that is simple - put the mold
to the nozzle and push. If your model is well made, and the mold is
well made, you should get 99-100% waxes, at first glance. You might
reject a few for details or finish. So, the point I am making is that
it is the wrong question. The real question is, “How to make a
successful model?”. I get beginners coming in who want their pieces
molded, and I reject 50% of them as being unmoldable, and the other
50% need to be reworked, and it is always (ALWAYS) the same reason -
they are too thin. I certainly can’t delve into model making
seriously, here, but there is one essential thing to understand: When
you make a piece of jewelry, you make it, and it’s done. When you
make a model, you are dealing with plumbing. The wax and metal are
fluids, and they need pipes to flow through, and that’s your job - to
make pipes. Most often jewelry has a fat side and a skinny side, or
is uniformly shaped, and you can just sprue the fat side and be OK.
But you just can’t do things like an hourglass shape without thinking
about the consequences. You’ll get what I call the coke bottle
effect: Get a full coke bottle, open it and quickly turn it upside
down and you’ll see. This would apply to a big shank and a big top
connect by skinny little wires and the like. In that case you would
mold the shank, perhaps with the wires, mold the top separately (can
be in the same mold on a separate sprue) and assemble after casting.
You have an 8% shrinkage in the direction of pressure with Castaldo
gold, and then don’t forget that you need to finish it after. If you
are molding a 3 1/2" circle, as in the recent thread, you should
probably be starting with 16 gauge metal at least for a good fill.
That means that it becomes 18 gauge or so out of the mold, from
shrinkage, and then 20 gauge after filing, give or take. Well, I’m
rambling a little, but you just need to think about plumbing - If you
have a 14 gauge wire, it will fill for 2" (Not really, just for
proportion), if it’s 16 ga. - 1 1/2", and a 22 ga wire will only fill
for 1/2 inch or something - never forget shrinkage - that 22ga wire
is now 24 ga. You need to go up a gauge in sheet metal for every inch
bigger (again not really, just proportion). You can mold a 4" circle
out of 22 ga metal just fine, but you will never get it to fill
unless you sprue it every 1/2" or something around - plumbing. If it
were 14 ga. it might work, but 12 ga. is what I’d want for that.
Finally, a wise man once said that you can cast anything if you sprue
it properly. Very rarely there is such a thing as a bad mold - poorly
cut or executed, and we all call molds that don’t shoot well “Bad
Molds”. They almost always are not, though, they are usually bad
models. Put your minds eye in the sprue, and swim up the model. Do
you flow like the lines of a Ferrari? Or do you have to squeeze up
and down and go around corners and have the maker expect you to jump
gaps or (heaven forbid) go backwards? Or flatten yourself impossibly
thin? If you can’t see it going “Whoosh” and filling in 1 second,
then it might be a problem - and don’t be afraid to make 2 part models
to be assembled later - that can often solve many problems in a

Hi Michael,

I would like to try a sample of the golden river additivr please.

Also,thenk you for the sample of Rapidio rubber. I tried it, and I
had excellentresults.

Andrew Charvoz
Scottsdale Bead Supply
3625 north marshall way
Scottsdale Az. 85251

Michael, got your free samples in the mail and just wanted to say
thanks, can’t wait to try them. One question comes up though on the
golden river (interesting name that) additive. I use Plasti-wax as
my main injection wax. Will the golden river work with this very
plastic wax??? Thanks again

Frank Goss


Great advice although you have not taken into account the person
that comes to you with her deceased family members cherished ring
that was completely worn out.

For the sake of very special memories they would like to have it
recast into a new ring that they can wear for another 20 years or so.
In your example you would turn this person away instead of helping
them relieve their grief.

This type of request requires doing a mold on a piece that is not an
ideal model and in doing so requires learning how to deal with
problem molds. I think that many of the suggestions that have been
made in this thread has been good advice for approaching this type of

Greg DeMark

We have a product called "Golden River Wax Additive - The Wax
Injection Problem Solver " that is perfect for just those
situations. It's a compound -- a wax really -- that when added to
your wax pot one or two handfuls at a time will dramatically drop
the viscosity of whatever you have in there at the moment. In other
words, it will make your favorite wax FLOW like water into the
pattern in your rubber mold. 

This sounds wonderful, and might really help with one mold problem
I’m still having! One question though-- I’m currently using the Kerr
Accu Carve purple injection wax. Will the Golden River additive change
the carveability (is that a word?) of the wax I’m using? Even if I
get the mold to fill properly, I will still need to make some
modifications to the resulting wax models before casting them.

Kathy Johnson
Feathered Gems Jewelry