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Casting Polymer


#1

Help!!!
Ive been doing some work and experimentation with my “Model
Master”,(photo polymer system for making models of 2 dimensional
art work that can be vulcanized and then cast.) I’m having
terrible sucess with the casting of the models once I get a mold
made. I think there is some trick to casting a piece of sheet
with a pattern on it that I don’t know about. The piece I’m
trying to cast is .75-1mm thick and about 1/2" X 1". I’ve tried
casting the polymer and vulcanizing it and casting with wax, and
I still get the same results. Terrible surface texture,
inadequate fill, and investment like holes.I’m using the same
temp. and burnout cycle that I use for all my other castings and
I have an extremely low failure rate(usually). I cast 3 flasks
today, the other 2 come out flawless and the polymer flask was
awful. Any advice or experience casting this shape of item or
actually casting with this polymer???

Thanks in advance

Ray


#2

Ray, Try setting your oven at 200F (below boiling) for 3hrs and
then proceed with your regular burnout. Plastics don’t melt out
as fast as wax does so it’s still there when you have some steam
action which “scrubs” the surface of your pattern. This burnout
works for hard carving wax as well. As for non fill, try raising
you flask temp to 1200 or 1250F.

J.A._________


#3

Ray: If your piece has a lot of relief then it sounds like the
investment is breaking off. If you use Satincast you might try
Prevest SJ-27, it is stronger than Kerr. Also look at your water
to powder ratio. 38/100 gives you a stronger mold than the usual
40/100. A higher flask temp. should cure any filling problems.
You may want to look at the sprueing as well to make sure that
the wax is not getting trapped in the corners and boiling
against the investment causing it to break down. Prevest’s
number is 216-731-2608 if you want to give that a try.

Brett


#4

Ray, Have you tried using the metal backed polymer? I molded the
image on the metal backed, then shot the wax and cast that and
never had a problem, what kind of metal are you using? Amber


#5

Hi Ray,

I’ve used the Model Maker for a few years and know how
frustrating the first few tries can be. One of the tricks to
casting a patterned sheet is spruing- use one sprue for every
half-inch of area. It sounds like a lot of cleanup, and it is,
but it will help to get the casting complete. Also, it sounds
as if your flask is getting some thermal shock; heating or
cooling too fast. You’ll probably find that thin wall castings
are more susceptible to thermal shock, so try extending your
burnout time to produce a smoother thermal curve.

Last, remember " Cups up", as I heard recently in this forum,
when investing. My production crew loved that term. The small
open areas in the pattern act just like large ones in a regular
casting, so be sure to leave a way for the gases in the
investment to escape when investing. Oh, I have never gotten a
decent casting from the polymer which can supposedly be burnt
out. It cracks the investment every time, aside from smelling
horrible during burnout. Good luck, hope this helps. Metalsmth


#6

When I molded and then cast the metal backed polymer, the weight
was too high, ie. the piece was too thick. I should have shaved
down the back of the piece, but I wanted a nice, smooth backing.
Ruth