Re: lost styrene casting - How it's done.

Schuyler Larrabee

How it works:

You have to have masters. You can make a master out of brass and have a rubber
mold made from it. This rubber has to be very hard, and the master should,
maybe has to be, made with silver solder to be durable enough to stand up to the
mold making process, which involves heat and pressure. The maker can then shoot
styrene into the rubber mold (which is why the mold has to be hard), to get
styrene copies of the master. There is a shrinkage at this point, which IIRC
from doing this probably ~20 years ago, is about 2%. The styrene used here is a
low temperature melting point version, actually quite a lot like wax.

It can also be wax. It's from this that the term 'lost wax' derives. You will
see why in a second.

It can also be styrene castings from some manufacturer, say Branchline for
instance (since I'd like to do this with Bill's bracket grabs . . .) That
styrene may not be quite the same as the styrene the mold maker would use, but
unless it's got some other problem, as was mentioned earlier today, that isn't a
real problem. I know that Delrin doesn't work well, but it can work. Again,
IIRC, there's solids in Delrin and other slippery plastics that just turn to
dust or something, and don't get out of the mold.

Now, all these 'masters' are assembled into a 'tree.' There may be more terms,
but that's the term I've heard before. It is like a fir, with a central shaft
and angled branches, with the masters as the 'leaves.' I expect there are
better and worse ways to position the 'leaves' on the branches. This tree, in
the place I worked with, was about 7-8" high, and had maybe a hundred masters
arrayed on it.

The tree is placed in a cylindrical mold, about the size of a coffee can. The
base of the tree is at the bottom. The can is filled with plaster. There's
probably a vacuum step here, to eliminate air bubbles. The plaster cures, and is
baked to make sure all the water is driven off. If you don't dry it completely,
it can explode in the next step: it is then heated up a LOT so that the styrene
melts and runs out (that's why I think the position of the 'leaves' matters).
Obviously, the wax versions work the same way. If everything comes out you now
have voids shaped like your masters inside the plaster.

Now, the plaster is placed in a centrifuge, and spins around with the trunk of
the tree radial to the shaft of the centrifuge, and the base of the tree toward
the central shaft. Molten metal (brass is our desired metal) is poured into the
center of the centrifuge, and the centrifugal force forces the brass to go into
the voids REALFAST! It solidifies. There is shrinkage here too, as discussed
earlier today, about 1 to 1.5%. Real pattern makers compensate for both
shrinkages by making the pattern oversize.

Later, after everything's cooled down some, the plaster is broken apart and
picked off the parts. The tree is cut up and the 'leaves' cut off. Some are
perfect, some aren't. Parts you might get from some suppliers may well have
bits of plaster still stuck to them. Obviously, a commercial part probably has
the plaster cleaned off pretty thoroughly. The brass in the trunk and branches
is recycled. Again, IIRC, only about 10-15% of the molten metal ends up as

Mr Jackman can tell me how I'm wrong now.


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