Quote:
Originally Posted by
ChrisA Edwards
Yes, as an experiment, I've bought two cheap cookie baking sheets. I'm going to put this piece between them and put it on my outside BBQ and try to control the temp to around 250F and keep it there for 2 hours. I'll put about 10lbs of weight on the top tray to try and hold it flat and then let it cool down to ambient temps.
Mr Kessler is exactly correct about injection molded polymers and influences on stress - only need to add duration of packing* pressure. *- hold pressure on mold for a few seconds to force just a bit more in as the 'shell' cools and contracts, so keeping the partproperly sized and in tight contact to the cold surfaces of the mold for good heat transfer.
Careful at 250degF with HDPE... I think you are trying to stress relieve this, not liquify it - and that is getting close. I'd probably shoot for 200-210degF, and key will be to heat soak all of the material, not just the outer shell. (Your slots should help a LOT in this regard.)
In a previous life, I processed 350,000-400,000 lbs of HDPE per month thru 2 injection molding machines. It was a tiny facility as such goes, just big machines (700t). It has been a long time, but IIRC we ran the extruder barrels at ~265-275degF? The exact temperature was determined by the melt index of a given batch of raw material (pellets). We WERE trying to liquify it and keep it that way all the way into a 2-3ft wide mold at 40degF.
...just in case you want to go into the molding biz. :confused:
ETA - Consider using boiling water - if you have a tray big enough, fill it with water and float the HDPE in it as it boils. Water boils at a constant temperature; you know you'll have 212degF (or close, depending on your altitude?).