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Plastic Burnout

has anyone sprued up a piece of plastic for burnout instead of wax.
the plastic is the kind that small model cars are made of. i have
burned out several flasks only to have the resulting cast brass item
look spotty and rough in most areas. it looks like there may still
be some residue in the flask. maybe there is some kind of reaction
with the investment or since plastic melts at a higher temp than
wax maybe it should be burned out for several hours at 500-800 deg
before turning up the temp for vaporization. smaller items seem to
turn out nice. would appreciate any help during my next 50 burnout

Hi Don,

Some of the plastics expand before they burn out. That expansion
damages the plaster. I’ve tried adding very thin coatings of wax to
compensate for the expansion since the low temperature wax burns out
before most of the plastic expansion takes place.

I haven’t done a lot of plastic burnout to prove the effectiveness.
Some of the wax coating success could be attributed to just good

Donna Shimazu

The polystyrene used for models is loaded with fumed silica to make
it opaque, and less dense, and less expensive. The plastic burns out
but the silica is left behind and will cause the spots you refer to.
If you are going to use plastics they should be transparent
varieties as they don’t have the silica filler. However plastic has
a much higher expansion rate than the wax we normally use and will
sometimes crack the mold or degrade the mold surface due to the high
expansion. Some people have had luck burning out plastic models with
platinum investment. Due to its higher strength it is less likely to
crack from the plastics expansion – Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160

Member of the Better Business Bureau

Hi Don

One of my favorite past times is casting plastic models in parts,
then assembling them with solder. The problem you’re running into may
be that extremely fine clay is added to the plastic as part of the
opaque coloring agent. This leaves a residual dust in the flask. This
dust is difficult to remove from the flask, but it has not caused me
any problems as I can usually just brush it off (try a wet rotating
brass brush). I typically use a very slow burnout, and bring it up as
high as 1500 degrees for large flat and thin models being cast into
silver (using my Thermotrol casting machine and a 5x7 flask). I get
complete castings that have all the detail of the original plastic
model, all the fine engraved lines etc. As an engraver of metal
molds, my appreciation of the skills of the mold engravers that make
plastic models is very high, these mold engravers are some of the

Jeffrey Everett