tom jelly wrote:

Funny thing about forming tubes- I saw what nasa did- they used SILICONE
RUBBER of all things inside a 2 piece mold- 2 half round aluminum parts and
a silicone male cylinder core.  unlimited pressure available as we all know
now....pushing on the ends of the silicone pushed it out, compressing the
matrix in the mold.

Tom>

I first saw the silicone forms used when a friend borrowed a nose mould for his Chevron sports racer. We made 4 noses and sent the last nose back to Canada still in the mould so he didn't have to pay duty on it as payment for using the mould. The brake duct male moulds were cast in silicone and protruded into the front bottom edge of the mould. They were rectangular at the front and changed to 3" diameter round at the inner end in about 7". They just pressed into the mould from the outside. No method of alignment other than the tight fit was used. They took wax as well as the rest of the mould and after the work had hardened for a few hours, we would pull on an "I" bolt that was cast into these duct moulds for handles and they would pop right out. We would free the piece from the mould, trim it up and clean the mould and put the piece back in for a week or until we had time to make another nose. The longer, the better for styrene because it will keep changing shape for some time
after it hardens (not quite).

 The silicone shapes come out easily because as you pull on one end the shape gets a bit smaller. And out she comes. Another ploy I have used is styrene shapes and epoxy or on the cheap, styrene shapes and syrene resin. For the styrene resin you have to protect the shape because the resin will desolve it into a pile of muck. You paint the shape with epoxy and let it harden. Then build it up with chopped glass and some cloth for a smooth outer surface. When it is completely hardened. Pour lacquer thinner through the part to desolve the foam. There are some giant foam candy canes that look alot like intake manifold runners to me.

For smaller parts like intake runners on go kart engines, I used modeling clay. I drilled out the ports and shaped them with a die grinder and connected them with rods made of clay. I build up a dam of clay arould the ports and pour it full of Devcon Plastic Steel. After the epoxy hardens, put the cylinder in the cleaning tank and desolve the clay. You can make some very complex shapes this way.

There is rigid polyethylene foam, just like the seat coushion stuff but stiff. It has no strength at all but can be had in sheets in any thickness. You cut it to shape with a sharp knife. It is open cell foam and you can sand it to shape with your hand, just rubbing on it. You can fasten it together with tooth picks and white glue. When you have the shape you want, cover it with two layers of glass cloth and paint on a thin styrene or epoxy. When hardened, scrape out the foam.
Since there is no draft required to remove from a mould, you can make nearly closed, and closed shapes like this. Like gas tanks, engine cowlings, air scoops and the like. Just some more ideas.
Lynn E. Hanover.

 
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