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RTV mold compounds?


#1

Has anyone had any luck with the "room temperature vulcanizing"
rubber compounds? or would I be better off baking something in my
oven? Also any suggestions for improvising a wax injector? I have
some ideas and was wondering at what temperature the wax is
injected at? thanks dave


#2

Yes, RTV is great mor for molding wax for example, and is
injected exactly like vulcanized rubber. It is more time
consuming as it must be vacuumed and allowed to set overnight.
Mark Parmenter has about newer rubbers that I am
anxious to learn about…

Jeffrey everett


#3

Dave,

I have used rtv to make molds of delicate originals that would

not take the heat and or preasure of a vulcanized or baked mold.
I use it mostly for original waxes that I don’t want to have to
carve again. I have used a few of the different SILICONE rubber
molding compounds. Notice I said silicone, it will stand up to
the heat of the wax injection without degrading too quickly. I
have used the RTV type E that Rio Grande sells and it makes a
nice tough stiff mold. I prefer to use silform which I can get
from a local ceramics supplier. It has a very high tear strenght
and is not stiff, it also has very low shrinkage and does not
create a exotherm reaction when cureing. It also has better mold
release qualities then RTV type E.

Any of the silicone two part mold compounds you use will be

thick and will need to be vacuumed. There is a thinner you can
buy and add to either rtv compound. It is added at 1-3 parts per
100 by weight. The catalyst is the added at 10 parts per 100 by
weight. You need to vacuum the mix for 3-5 min. You should not
exceede 5 min. or you may cause problems with the cure. The pot
life after vacuuming is about 1 1/2 hours with out vacuuming it
is 45 min.

There are several types of injection waxes but most are made to

be injected at a temperature between 160 to 170 degrees F. A
large syringe with a capacity of 200ml to 500ml such as one used
for irrigation with a cap for the nozzle can be used. The
syringe is filled with the injection wax pellets and then placed
in a pot of heated water until liquid. Try to get as close to 165
degrees as possible using a thermometer. Use the syringe to
inject the wax into the rubber mold. You might find the syringe
at a tack shop or pet store. Hope this works for you.

Joe B.


#4

A few problems with RTV molds. First is cost, its best to use
this for molding waxes and heat sensitive or fagile materials.
Second when you vacume it rises usually just higher than it
container in your vacume chamber, leaving a sticky goo like that
ooblick in the Dr. Suess story all over your bell jar. Third you
must measure carefully, if the two parts are not exact you can
wait for weeks and it will still be sticky. We cast every day
and only need this 5-6 times a year. Better to invest in a good
vulcanizer and start struggling with no shrink pink, but thats
another story.


#5

Better to invest in a good

vulcanizer and start struggling with no shrink pink, but thats
another story.

OK Mark, what do you mean by struggling? Share and help us all
out:) Charles

Brain Press
Box 1624, Ste M, Calgary, Alberta, T2P 2L7, Canada
Tel: 403-263-3955 Fax: 403-283-9053 Email: @Charles_Lewton-Brain

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#6

Dear Dave Each wax has it’s own personality, and specific use.
Some are better suited for heavier molds and others are better
for detail. The manufactuer will most likely list the uses a
particular wax is best suited for. As a general rule however the
lower the temp you can shoot a wax the better it will function.
If a wax is to hot it can bubble and the process of cooling in
the rubber mold can cause shrinkage. The cooler it is the less
it shrinks. As for wax pots there is a hand pump model on the
market ( Gesswein, Rio, Swestm ) that is low cost and works well.
It only runs around $130.00, you will spend much more time trying
to reinvent the wheel if you build one yourself. I do not take
that attitude most of the time and have built my own vacuum
chamber casting unit myself. But in this case I suggest buying a
wax injector. RED


#7

No-shrink-pink,share and help us all out.

A little story. You make rubber molds every day, usually of
customers merchandise that they want duplicated for whatever
reason. The catch is the shrinkage, usually a noticable amount,
really noticable when the customer compares it to the original.
The job is then returned to you from the jeweler with a note,
its smaller please redo at no charge, Ugh. Your response, several
years ago, was the well the advertised no-shrink-pink rubber. Now
instead of carving 99.9% of our custom jobs maybe you would only
carve 80%, mold the rest, save time and money, retire young (or
at least retire someday). Wrong, the vulcanizer that you had
bought from a freind in 1985 for $100.00 had always served you
well, until the day no-shrink-pink entered your life. In the ad
for this super rubber they show the original ring and the wax of
the NSP molded ring each on a mandrel each exactly the same size,
demonstrating no shrinkage. The procedure is to make sure each
plate on your vulcanizer is exactly 310 degrees, you can do this
by putting a block of wood between the two plates and inserting a
metal thermometer between the wood and one of the plates. When
you hit 310 F. you check the other plate, it is very unlikely
that the other will be the correct temp… So you get it as close
as you can, cook your mold for exactly 7.5 min. per slice, and
guess what, it didn’t work! You try and try again and always have
shrinkage. That is until that day that you get that job, the
mother of all carving jobs, you have a month to duplicate it
because they know its too difficult for a 10 day turn around.
This time instead of waiting until 5 days before its due to start
it, you order a brand spankin new vulcanizer with separate temp.
controlls for each plate (Gesswein about $600.00). You still
check each side making sure they are each 310 F., you cook for
exactly 7.5 min per slice and guess what, it still shrinks, but
not very much. After several molds of the same ring and
adjusting up about 4 degrees, it works. Life is good.

Mark P.