Okay try this...

Use putty style silicone mold material (it mixes by squishing it together).
Put the mixed material into the wheel and let set (about 10 mins). Remove
model once set by simply pulling it out.

Now use this mold to create another mold using more of the putty. Simply put
it on a flat surface, mold the new lump over it and let set. (Oh side note,
powder the model or it might stick permanently to the mold body.)

You should now have a mold of the correct dimensions of the wheels inner
spaces ready to receive molten lead (or lead free white metal/pewter) that
can withstand the low heat and be reused a great many times.

"tillius" wrote in message
ups.com...
I need to make some custom shaped lead weights that must be balanced
and fit precisely inside a small plastic wheel, the kind you might find
on a pinebox derby car.

I've tried making a casting of the inside of the wheel to use as a
pattern so I could make molds of the part and cast them out of lead,
but I've tried using plaster of paris, wax, clay and lead to cast the
pattern.

Plaster of parris disintegrates when I try to remove it from the wheel,
as does the clay when allowed to dry.

The wax and wet clay deform horribly when I remove them from the wheel.

The best success I has was lead, but the hot lead did melt the plastic
wheel enough to deform it.

When I made the final lead castings, they were still deformed enough to
deform the wheel when inserted. This and the fact that the weights were
unbalanced lead to a speed destroying oscillation.

If I can cast a better patters, I should be able to case weights that
are a lot more accurate then use the lathe to finish them, but I've
still got to figure out the balancing part, and, how dangerous would
the lead chips be?

I also thought about just turning a lead cylinder of the correct size
and boring out the center, then slicing it up into the individual
weights.

Any suggestions on getting this done right?