Coroplast Kammback: joining or bending?

[evolutionmovement]

Polypropylene can be welded.

[Frank Lee]

Build a mold and a huge oven? Seriously?

Chances are you don't want to make a straight 90 degree duct. Tapering puts you at odds with the "grain".

It can be bent easily without slitting by using a heat gun. Heat guns are also very easy to regulate; by only applying the minimum heat necessary to do your shaping you can keep warpage and rippling way down. Even somewhat compound curves can be formed... but convex only, they get rippley on the concave side.

Chances are good you'll be better off going multi-piece and using something to join them together. I say that because chances are your edges will be curves and not straight lines. Good luck making a one-piece bend with curvature.

Coroplast has butt-ugly edges anyway. This is your chance to cover them up. I've used plastic extrusion for covering edges. In the future I may use it for joining too.

I've used screws with washers, spring-loaded wire retainers, you name it for holding that stuff together.

[Piwoslaw]

Originally Posted by evolutionmovement

Polypropylene can be welded.

I recently heard about that - ultrasonic welding, I believe. I have no information about though.

[euromodder]

For glueing :
Glueing polyethylene - polypropylene / coroplast

[Dr. Jerryrigger]

Originally Posted by Piwoslaw

I recently heard about that - ultrasonic welding, I believe. I have no information about though.

Well ultrasonic aside, polypropylene melts at 130–171C.
I've made some wax positives for casting and welding wax is quite simple with a soldering iron, though that was at around 90C. I used a light dimmer switch to control the temp.
For the poly it would be more difficult as it is far more sticky. But welding cloro would be far more tricky as it's so thin. Though if someone really wanted to it could likely be pulled off. Some sort of mold to hold the joint would be very beneficial. If that mold was then lined with wet paper towels it would prevent the outside from melting, and make it possible to build up a beefy slag on the inside.
Or a different method would be to heat both connecting surfaces and bond them. Maybe set up the joint, and then run a soldering iron between them, so that the weld is made 2in or less behind the iron.

These ideas all sound honorable in comparison to basic glue and mechanical fasteners. You can get some rather complex shapes by building a simple frame out of wood (or what ever you like) and a little bending and cutting

[Bicycle Bob]

I've always been quite satisfied using real silicon seal (not paintable) but if you really want welding, one way might be to use a few nails to fasten an overlap area. Then separate the surfaces until the nails are almost free. Then, string magnesium ribbon along the middle of the gap. Then, flash it off with a jolt of electricity and slam the assembly back together.