Warpage PP/HDPE

All,
I have been bumping into an issue lately, one of my project had a material change and we are looking into producing a part, which was originally intended in PP, in HDPE. The part looks like an oval shape lid, part thickness is 0.9mm and I have a skirt on the side that is 5mm tall. The PP material flows ok, been developing the tool for a while already and it comes kind of flat (acceptable by our in house quality, overall flatness is within 0.5mm). It is 16 cavities, all valve gated.
When switching to HDPE, using similar filling profile (one step, switchover when all volume is filled and slowly adding plastic to reach a target weight). Most of the cavities will show warpage. The warpage leads to a dome forming on the surface of the lid, which would indicate that the perimeter is shrinking more than the top surface. But surprisingly, not all of them is doing that, one is relatively flat, not as flat as the PP part but flat enough. Looking into part thickness, some are closer to 1.00 mm, some are thinner, but the amount of warpage does not seem to be linked to the thickness of the part.
100% of my production is in PP and would like to know if somebody can tell me what should I expect in terms of difference processing PP and HDPE?
Given that the dome effect is quite wild... I mean, parts are doming up 8 to 10mm up, I was thinking that something might be really wrong with what I am doing, but I did try to change a few things, temperature/packing/fill speed etc... nothing really does seem to be linked to this.
Last but not least, I did run a flow simulation with generic HDPE on my part, but the result are WAY different than with the machine while the model I use should be ok as I fine tune my tool in PP with the same model and results are similar.
Being stuck... :(

Maybe you should look at the actual tool steel temps on a cavity by cavity basis. Then map out the temps and warpage with the data and see if there's something else going on here. I also believe you'll need to lower the pack psi and time but it may take individual settings on a cavity by cavity basis.
Rick.

Thanks Rick, I'll try to get the magnet thermocouple to get an accurate reading.