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Pewter as model


#1

I’ve got another quick question.

If Pewter can be cast into rubber molds and has a low melting
temperature could it not be used in the place of our wax models? The
bulk of the pewter could be reclaimed after the furnaced reached its
melting temperature and could not the rest be vapourized? Which leads
me to my other question? I have just begun making rubber molds and
injections and I am not happy with the loss of character. I work for
hours with a design in wax, cast it, like it, make a rubber mold and
am left with a wax that looks unlike the original. I carve reliefs of
faces and something is lost. It loses its character, crispness. Do I
have to live with this? Is the pressure too high and distorting the
features ever so slightly.(I use between 7and 12 psi on larger
medallions) should I change wax(I use Kerr turquoise, or aqua)? Is
the temperature too high? Oh yes, being a poor craftsmen I do the
molds in the furnace with a thermometer, and it seems to work fine but
perhaps I’m missing something.Thanks

Manuel


#2

Hi Manuel, It is perfectly fine to make a modelusing pewter,But it is
not a good idea to attempt to use pewter in place of wax for the burn
out proceedure. The reason for this is that there will be many
impurities left in the plaster mold that will end up in your cast
metal making your metal useless. When it comes to loss of detail when
injecting a wax into a rubber mold, many factors have to be considered
other than just the injection pressure and temperature. Many molds
require extra vents to get certain areas to fill properly and
sometimes the venting can get quite complex. Talc powder or various
other powders can also be used to bring up the detail put the talc in
a small canvas bag and then put another small canvas bag over it so
that only the finest talc will go through the bag onto the mold. This
will usually solve the problem of dispearing detail on your waxes.
Making rubber molds using an oven and a clamping method will work
fine, it just takes a little longer to get everything clamped
…ocasionally, the clamp may not hold as well as a professional
vulcanizer. I hope this helps answer some of your questions.

Daniel Grandi
http:www.racecajewelry.com
We do casting, finishing in gold,silver,brass/bronze and pewter for people in
the trade.


#3

Perhaps because of my low tech equipment and/or clumsy methods I
never expect perfect reproductions from my rubber molds. I treat a
wax as I would a raw casting, ie, each one requires some touching up
and polishing.

Allan Wilkinson


#4

Thankyou Daniel

I use silicon mold release for the rubber mold. Do you find its
performance to be inferior to the talc which you mentioned. Today I
will make a no shrink pink mold but I assume that my problems were not
caused by the gold label as many of you use it with success I’m sure.
As for venting, the object in question is a 7cmx7mm (roughly
2.5inx1/4in…yes I read the measurement wars:)medallion not much
venting problems. I can almost hear you saying, ah ha, the quantity
of wax in such an object would cause considerable shrinkage and
distortion upon cooling. Could this be the problem? if so have any of
you tried a core in in the mold while injection? I’m going to figure
a way to do it but any tips would be appreciated.

Manuel


#5

From what I can understand, you are using an organic rubber such as
castaldo gold label which is a good rubber. We have not used organic
rubbers in many years .We prefer vulcanized silicone rubbers for all
our work As the model will never become black from the vulcanization
process and will not suffer from black build up . Granted, you can
remove the black build up through various means, but this is simply
not neccessary with the silicone rubbers. Michael knight from
castaldo
also sells a variety of silicone rubber that are vulcanized and work
very well. So does the www.contenti. and www.gesswein as well as
Rio.
The chief advantage is that these rubbers are like siliputty in
sheets an d the model is pressed into the rubber by hand . The rubber
can be manipulated so that the parting line is exactly where you want
it to be and can be seperated using mica powder instead of hand
cutting the mold.This is far easier and less expensive. Silicone
rubbers are completely self releasing … In other words, the wa x will
never stick to the mold and release sprays are not needed. However,
in
both rubbers, you may find that talc/ powder is beneficial i n getting
the detail to fill properly . Silicone sprayis not designed to d o
that. It is merely a release agent. As far as the problem getting your
large piece to fill properly, my advice is: Bring your wax temperature
down so that it injects at a cooler temperatu re Then, remember that
heavy items will heat the mold up very quickly and cause all kinds of
problems such as sinkage, shrinkage, airpockets and all the ugly
things you don’t want to see in a wax. So you have to allow

adequate cooling of the mold between shots.You can use an ice pack
wrapped in a towel and place the mold on it to keep the mold cold if
you are trying to run multiples quicker. Finaly, because the item is
thick , you may want to hold it against the wax
nozzle for a long time…say 15 seconds or maybe even longer. What
this will do is keep the wax pressurized while it is cooling down i n
the mold which will help reduce the shrinkage problem you are
experiencing. Another thing to check is that your sprue is adequately
large enough to feed the design . If the sprue / gate is too small,
you will be asking for problems. We do use plastic or metal cores /
rod/ wire in molds when we want to make sure that the wax does not
shrink in an area,or where rubber is inadequate. An Example: If you
had a design that had a 1/8 inch hole running through the piece and
your the hole in the model was 1/2 inch long … if you let the rubber
fill this area, then when you went to inject it, you may find that the
rubber might move when the wax is injected and/ or your hole may be
blocked by flashed wax inside causing you to have to drill this out by
hand in the wax or in the cast metal object. An easier way is to take
a piece of 1/8 inch rod ( brass/ steel / aluminium) and insert it into
the model so the rod protrudes past the edges of the model by 1/2 inch
on both sides. Now, mold the model and take a new piece of rod, Spray
it with silicone release and insert it in the mold. After you have
injected the wax and allowed the wax to cool, pull the ro d out and you
will find that you have a perfect round 1/8 in diameter hole going
clear though the wax. This same concept can be applied in all kinds of
different ways. I hope you may find this useful. Best
wishes,

Daniel Grandi We do casting and finishing in gold, silver,
brass/bronze and pewter for
people in the trade.


#6

Daniel thankyou very much,

You info was right on. However I had another problem with the rubber
molds. As you know I oven bake them. Now temp is the big factor
right…so I put a thermometer in the oven which of course has its
own pyrometer. When they both read around 310F I put in the mold
frame. The rubber I was using was pink labe and I was pretty generous
with it using only one layer of gold label top and bottom. There were
six layers in all. I place a sixteenth of an inch plate on either
side reinforced by a 1/4 inch plate to withstand clamping pressure.
All aluminum. I put four c-clamps on each corner. So its in the
oven…seven minutes per layer I take into account the timeit might
take for the mass of frame and plates to heat up. Out it came in 45
minutes. not cured. bum… so I put it in for another half hour…
bum again. I said to myself well I’ll just leave it in for a while
more. Well it must have been about twoandahalf hours when I took it
out and great the mold looked fantastic. it cut nicely, it have good
colour,good flex, but lo and behold the surface that had contacted
the metal was seriously stained brown with black blotces and some
parts were deteriorated on the surface. I decided to inject some
waxes anyway and was glad in one way because it was fairly true to the
character of the relief face but the deterioration necessitated a lot
of wax touch up. Does rubber react with the metal? The release. the
discolouration was not deep at all. It seemed like a reaction. Here
are some factors.

1.the model was coin silver(I used silicon parting agent before
putting it all together, I thought I read that somewhere)Man could
that have been it? Was it the release?Tell me I’m not suppose to use
release. Talc better maybe eh.

2.the pink label though I bought it in december was pretty stiff and
the blue plastic was coming off on its own, does this stuff have a
shelf life.

3.the time was long but it was uncured for so long.

4.I was expecting better. I assume it does get better. Here I
thought how wonderful to be able to make my own molds. A bunch of
you are probably thanking God you have a vulcanizer. Well if you have
one to spare or that’s lying around broken and can be fixed… please I
beg you:):slight_smile: Oh well I haven’t given up yet. We should probably change
the subject heading of this post. I’ll bet it was the release. Well
you all know what I’m trying tomorrow. Maybe I’ll wait for saturday
so I can read your input.

Manuel


#7
but lo and behold the surface that had contacted the metal was
seriously stained brown with black blotces and some parts were
deteriorated on the surface. 

Manuel, Don’t spray your model with silicone spray. Always have your
model absolutely clean. You don’t want any traces of silicone on it.
Silicone and rubber don’t get along. They prevent each other from
curing properly, resulting in gummy, yucky surfaces. That’s why you
use separate mold frames and plates for rubber versus silicone mold
compounds. Also, if you ever mold in rubber, remember to thoroughly
clean the model with alcohol or lightly repolish and clean before
molding it in silicone. Likewise for the reverse.

Good luck,
Donna


#8

Hi Manuel, I believe your problem is that you are not getting the
rubber to copy the correctly and it is not related to using silicone
release. Silicone release will not cause a problem . When you pack the
mold,gold label rubber needs 1 rubber thickness more than what the
frame thickness is. Then when you use an oven to cure it, after you
have clamped the mold very tight, you should put it in the oven for
about 5 to 10 minutes which will soften the rubber . Now, take the
mold out and you should be able to clamp the mold even tighter than
before as the rubber is now very soft( do this quickly). now put it
back in the oven for the specified timeand your mold should copy all
the detail in your model. When we use a vulcanizer on gold label
rubber, we put it in the vulcanizer as tight as possible , then after
5 minutes, we release the pressure slightly incase their is any air
that needs to escape and then we immediately tighten the vulcanizer
as tight as possible. A vulcanizer is more controllable in this
aspect than an oven. Your oven or dial thermometer may be inacurate
which is why it is taking so long to cure… try increasing the
temperature slightly… do not exceed 350 oF. The model will become
discoloured in the molding process unless the model is Rhodium plated
which I don’t suggest incase you wish to modify the model later. I
would suggest that you try heat cured silicone rubberwhich is far
easier to use and will not require that you take it out of the oven
and retighten the clamps. The model and mold frame needs to be cleaned
of discoloration and organic rubber residue or the silicone rubber
will not cure properly. I hope this helps.

Daniel Grandi
http://www.racecarjewelry.com