How to overcome warp in HDPE molding?
If you haven’t already considered pack pressure, pack/hold time and cushion size, you might want to take a look at it. Most non-filled, semi-crystalline materials will have warp problems if they are over-packed. Of course, part geometry, wall thickness, etc. will play a part in warping of the part. But I would start with checking the fundamentals of the setup; the pack/hold dynamics would be a good place to start.
I have summarized the advice I got with a few remarks of my own based on the experience gathered:
- Perform a mold flow analysis on the part especially concerning residual stresses, gate size and location and cooling channel location. This process is still on going so I really can’t comment on the results.
- Heating the mold helped considerably.
- Differential heating also helped (20C temperature difference between mold sides).
- Low pack and hold pressures.
- Use of fillers actually made things worse.
- Cooling on fixtures helped.
- Use of MDPE helped.
- Foaming didn’t help much.
- Making sure mold temperature is constant; this was never an issue so I can’t comment.
- Making sure wall thickness is constant; this was never an issue so I can’t comment.
- Annealing helped.
- Increase temperatures in the central nozzles were never tried.
- Nucleating agents didn’t help much.