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.

I would use an air bladder inside of a woven “sock” (they are available in carbon, kevlar, and even fiberglass). You insert the bladder (deflated) into the dry “sock”, then slide the entire assembly into the extrusion. Once in the extrusion, air is applied to the bladder inflating it (and the “sock”) to the interior dimensions of the extrusion. At which point, an epoxy is “infused” (or drawn through with vacuum) into the fabric. Once the epoxy is cured, the bladder is deflated, and removed. The part can then be removed.
Note: Prepping the extrusion with mold release prior to the start of this process should aid in removal the finished part.

hey gbell, what kind of extrusions are you bending? Window industry? I used to do that. I had some success with filling the extrusion with silicia sand, taping the ends with duct tape. Depends on how tight you are trying to bend them. Also, for some extrusions we had the mill room setup a moulder and run the interior profile in oak. Then the people bending the extrusions would just stick a piece of oak in the opening before bending. Oak stays in, and it also gives you something to screw into.

Hope this helps,

Joe

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.

Shore. Typical O-rings are 60,70 Shore (soft) 90 Shore (harder). Normally cost a fortune as replacement part, cheap at the local hardware store.

Carel

Shore. Typical O-rings are 60,70 Shore (soft) 90 Shore (harder). Normally cost a fortune as replacement part, cheap at the local hardware store.Carel

From: http://www.matweb.com/reference/shore-hardness.asp

The Shore hardness is measured with an apparatus known as a Durometer and consequently is also known as 'Durometer hardness'.

:D

I would get a raised eyebrow here if I would use the term "Durometer". I googled it, both exist, so this must be a difference in local folklore.

Carel

If you're going to pour urethane into something make sure you use some kind of silicone-based release agent or you will never get your part out, we have our own "blend", but I'm sure you can buy something commercially for this purpose. Also, urethane shrinks 1.5-2% as it cools. The harder it is, the more it shrinks, I made a mold tonight for a 60a part (which is like jello when cured!) and made it 1.5 % big. That being said, your 4" x 6" piece of plastic will end up being 3.92" x 5.88". If your 200" length is going to be cut up into smaller lengths you are better off to cast to length, most plastic shops will actually charge less for this depending on the hardness of material, 90a is fairly soft and an absolute pain in the butt to do anything with after it is cast. Urethane material is way more expensive than polyethylene, so keep that in mind. Hope this helps!