Originally Posted by gbell212 I'm looking for a plastic resin or something that can be poured into an aluminum extrusion, say a 4" x 6" tube that is approx. 200" long. Once it hardens it needs to be removed, thus leaving a mold of the inside of the extrusion. These need to be fairly flexible and tough enough to be reinserted time and time again into more of the same extrusions for stretchforming. Right now we are using sheets of ldpe plastic cut to shape and size on tablesaw. There's gotta be a cheaper, easier way. |
Polyurethane might be your best choice. It is possible to get grades that set to different flexibilities all the way from real squishy to almost solid as a rock. The technical term is Durometer or something; I think a typical O-ring is 90 Durometer.
Polyurethane is very tough; to remove it from the extrusion after curing it might be possible to cast a length of nylon rope in the center to use for pulling it out. As the rope is pulled it stretches and stretches the polyurethane which gets thinner and pulls away from the walls of the extrusion and comes out. I have done a similar thing with RTV silicone. Getting the length of polyurethane into another extrusion would be done the same way; pass the rope through and pull from the fare end.
But based on my experience bending thin wall steel tube I think you are already using the best procedure with the ldpe plastic shims. The reason for this is that if you take a stack of your shims and bend them they bend easily. Each shim can slide past the other and you see this at the ends when the stack is bent; the shims on the inside of the bend stick out past the others on the outside because neither have been compressed or stretched. If you take a solid block of ldpe the same dimensions it is much harder to bend because being one piece the material on the inside of the bend has to be compressed while around the outside it is stretched. But even though the stack of shims is easy to bend it is difficult to compress the stack so it supports the aluminum extrusion during the bend.
Any plastic in a solid piece stiff enough to support the extrusion against crushing around the bend is also going to be so stiff it will be very difficult to bend.