In response to the Amazing Technology question, Roger Moment has sent
me the following response:
Richard,
Here are some comments you are welcome to post. I
Shrinking of metal in molds is significant . 5% is a typical amount
and since it is a percentage, the larger the part the more noticeable
the change in size.
When I do investment casting (wax master, parts cast in bronze) there
are three shrinkage stages if I use an original as a master for my
mold. First is the mold rubber. Second is the wax and third is the
final bronze casting.
Some silicone rubbers shrink less than others.
There are also different types of wax that you can use, some of which
shrink more than others. In fact, the wax has a coefficient of
thermal expansion such that it can change dimensions if it is at room
temperature as opposed to inside a refrigerator. When I made a
reproduction of the earliest 100 flash that was attached to Healey
grilles (the numerals are only about 3/4" tall and the lightning bolt
does not have a slight raised ridge down the center but is flat with a
very fine scribed groove) my rubber mold. The rubber mold ended up
only very slightly smaller than the original piece I cast it around.
But when I made the waxes, if I chilled them in the 'frig to speed up
their solidifying, they would shrink away from the mold and be
measurably smaller. When I treed them up at room temperature, they
would be closer to the correct size. By changing wax material, I was
able to significantly reduce this thermal dimension changing.
The final bronzes still were about 0.080" shorter than the original
piece. But for this application it was acceptable.
I also reproduced some pieces for a lock assembly on a Victorian
house. Here the original part was cast in iron. The side cover
plates -- about 4x5" in size -- were measurably smaller than the part
they mated to, but I was able to get them to work.
I made an aluminum casting cup holder for my BMW and to get a proper
size to perfectly fit I had to make the master (built up out of
masonite and body putty) about 7% large. I think it took two tries
before the final Al sand casting came out the correct size. I was
fortunate in that a friend owned the foundry and helped me with my
sizing issues.
Casting urethane parts in rubber or aluminum molds does not encounter
such sizing issues. The final bits I have made came out
dimensionally within measurement error of what I wanted.
Roger
I hope that helps with the quest for perfection!
Richard Gordon
Rocky Mountain Austin-Healey Club
1830 South Newport Street
Denver, Colorado 80224
Home 303-756-7427
Cell 303-913-1171
HealeyHundred@comcast.net
_______________________________________________
Support Team.Net http://www.team.net/donate.html
Healeys@autox.team.net
http://autox.team.net/mailman/listinfo/healeys
http://www.team.net/archive
|