Help with PP/PE warping

[Reinier1994]

Hello guys,

I'm working at a company that has a lot of PP and PE waste, so they like to start 3d printing with their leftover material. I know these plastics are not the best materials to print, but that's the challenge. I have to find a way to make it work, so their waste gets reused.
The current setup is:

ABB IRB welding robot (from 2003)
Leister Weldplast S2 hand extruder. The extruder is connected to the robot I/O-board, and only has an on/off filament feed control. It's impossible to adjust temperature/filament speed while the robot is doing it's routine.
Vacuum table.

Right now I'm experimenting with HDPE filament, but the problem we get is that there's so much plastic, the internal forces get so high that a lot of warping occurs. I have attached some photos of the current setup and the problems. In the long run, the extruder will be fed with either granulate, or extruded filament out of granulate.

higher res images:

Huidige opstelling.jpgWarping.jpg

As you can see, some products turn out nicely, but a lot of them suffer from too much warping, to make it a valuable addition to the production plant. The following parameters have been tested.

- Filament feed speed
- Layer height
- Layer width
- Filament temperature

A lot of warping has been solved by using a vacuum table, on which a PP plate is secured with clamps on the edges. I had some thoughts on solving this issue, and I'd like to hear some suggestions and ideas from you guys.

- Heated pad, or a heat circulation. It's impossible to build a heated room, since the robot head cannot withstand a constant temperature of above 70C.
- Fluid cooling (oil/water) inside the workpiece. Let's say you're building a case, and after a few layers you start filling it with enough cooling solvant, so the piece cools down, except for the top 1-3 layers (so the next layer is hot enough to attach).
- Warm fluid inside the workpiece. Since it's not possible to make a heat room, could it be possible to fill the piece gradually with fluid between 50-120C, and when it's finished, cycle fluid through the piece while gradually lowering the temperature?
- Cool air gun. Make a cold air flow behind the extruder, so it cools down faster (but not too fast so the molecular structure is affected).
- Smaller layer width. By adjusting the extruder nozzle (filament is either 3 or 4 mm) it's possible to lay a smaller line. This means there's less material and hopefully less internal temperature differential.

Thank you for your time and hopefully I can work something out and test your ideas!