Vulcanizing when using double frames

Once again, I’m turning to this list for advice (HELP). I’ve been
getting terrible results with my vulcanizing when using double frames.
I use only Castaldo precuts making a mold of three pink with one
outer gold in each side. I heat this five piece mold for about 40
mins at 300. I try to keep like thickness of originals in the two
molds. I tighten the frame slowly at about 20 sec intervals till I
can’t screw it any tighter. I have two double frames, one vented one
not. It doesn’t seem to matter. OK, here’s the problem. I get a bad
impression inside, like the piece has moved around during the heating
cycle. It seems as though one of the two will be OK, and the other has
moved. I tried adjusting the heat and went high and low. The only
thing that happened was bubbles and typical heat related problems.
It seems as though, if I only fill one side of the same double and
vulcanize that, it’s clean and perfect. AND sometimes, both come out
perfect. I took one Saturday and tried to figure this out and wasted
about 50 pieces of rubber. HELP!!! Bud Cravener

Bud ,I have run across a few problems with the mold process, I
personally would not mix different types of rubber,also 40 minutes for
5 sheets seems too long ,I like 35-40 minutes for 8 sheets thick
double frame. some small frames I have work great at 20 minutes. If the
rubber is gummy after heating, it is been heated to long, I also have
had bad results when one side or heating plate burnt out, it needed to
be replaced which was not difficult, I hope that helps, Michael Devlin

Hi Bud. Organic rubbers such as you are using require a fair amount of
pressure and require about 1/8 " of overpacking in the frame (unless
the frame is designed to have built in pressure plates that fit into
the frame which mine have) if you cure one frame at a time, you will
not have a problem . If you try to cure a double frame , you may have
a problem since it is impossible to tell when you have the same
pressure on both sides.(one side may have more pressure than the
other due to the method of packing and the amount of residual air
trapped in the rubber). Castaldo carries a large number of heat cured
silicone rubbers in sheet form that work very well in double
frames…I actually stack 3 double frames together with a 1/8 "
aluminium plate between each frame allowing me to knock out 18 molds
in about 2 1/2 hours using 2 vulcanizers.Using the Silicone, it is
Rare to get a mold that does not cure or fill properly.The silicone
requires that the models be cleaned of organic rubber or the molds
may not cure properly.The shrinkage in heat cured silicone is about a
1/4 size more than organic rubbers.I started using Castaldo rubber
in 1968 while in thailand and spear headed the development of heat
cured silicone rubbers for lostwax casting for another company in
1980. The silicone rubbers are very easy to use with a little
practice and castaldo has some good silicone. hope this helps.

Daniel Grandi

Dear Bud,

When figuring out how much rubber to use, stack them next to the
frame. The amount of rubber you use is that amount plus one. You cut
the extra sheet in half and put it on each side. If your piece is
shifting on the inside, you don’t have enough rubber pressing it into
place. If the mold is not working from the same PLACE in your
vulcanizer plates, you may have a bad heating element. I do two at a
time also with no problem. Make sure you count the same amount of
sheets for both sides.

Here’s a little tip. On the outside pieces, leave the cloth side on
the rubber. This fiberglass / cloth backing will shield the plates
from the sulfer content of the rubber. This sulfer stain is hard to
get off. If you use the vulcanizer for anything else you want to keep
your heating plates like new.

One other thing is that when setting the item up your spru rod should
go through the spru former into a hole in the frame. This will keep
the object centered while vulcanizing. It will also give you your
cutting path to your model.

Your model doesn’t matter it’s only the thickness of the mold that

The time vulcanizing is on a ratio of one inch of mold frame per hour
with a 40 minute minimum. Dont’t vary that instruction. Don’t vary the
temperature either (307 degrees F if you can do it). I believe
Castaldo documents this well.

The holes in the frame are a good success guide. You should have
rubber worms pushing out of the holes after 10 minutes. If you don’t,
you don’t have enough rubber. You can stop and with some hot gloves
on, add a half piece to each side and go back to pressing. Look for
the worms of rubber pushing out. Both sides of the frame should match.
If you have worm or vent holes on one side make them on the other

Best Regards,
TR the Teacher & Student

Hi Bud, I use 3 to 4 hundred pounds of Castaldo rubber each year
(yes, it is the best) I use a lot of the No-Shrink-Pink with good
results. There is a learning curve with the pink. If you stick with
it, it will pay off. First, don’t bother with using two colors of
rubber in the same mold. It will only cause your mold to warp. I know
some folks are doing this to save some cost on the pink rubber but, it
sounds as if you are using a lot of rubber experimenting! Next, you
MUST get an acurate temp reading on both the upper and lower heat
plattens on your vulcanizer. If you have only one heat control knob
(as most vulcanizers do, except the newer ones) you can isolate the
top from the bottom plates with a block of wood. I use a long stemmed
thermometer that came with one of my wax injection pots. Record your
readings from both the top and bottom plattens. If your vulc has only
one knob, you will usually find that one of the plattens is
consistently hotter than the other. When you identify which one, you
will be doing your calibration on the hottest side from now on. Place
the sensing area of your thermometer in the middle of the platten and
the block of wood will hold it in place as well as isolate it from the
other platten. Look at your thermometer constantly after your vulc has
warmed up. You will notice that when the “OK” light has gone out on
your vulc, the reading on your thermometer will continue to climb for
a minute or so before falling back down to where the heat cycle will
kick back on. Watch carefully for the very top of the cycle. It should
reach no higher than 310 F. As for your tightening proceedure, may I
suggest that you put the molds in, hand tighten with one hand, wait
two minutes, tighten with two hands, wait two minutes, tighten with
two hands and that should do it. I have taken off the cross bar on my
vulc and double nutted the main screw shaft and use a sears torque
wrench to quantify the pressure on my molds. Shifting inside the
rubber during vulc is not uncommon. This can be brought to a minimum
by having a soldered sprue to the model that has a cone slipped on the
other end. The optimum length of sprue measured from where the sprue
contacts the model to the pointed end of the cone is 3/4". I hope some
of this helps. J.A., J.A. Henkel Co., Inc. Moldmaking, Casting, &