Well, I decided to go in and experiment today. Shredded up a bunch of polypropylene caps, bottles and vials that had paper/labels attached. Screened for a small flake size. Heated the t-shirt press to 415º Fand made some sheets. Seemed to work great, and I can't see any label bits in it. I don't think I would use this for injection molding, but it seems like it would be fine for sheetpress and probably for extrusion into lumber, etc.
For me, 415F/212C is too hot and will cause plastic to overheat and stick to my mould. You might have your timing just right so the plastic doesn't actually see that high of a temperature.
If you're having good results with the way you're doing things, then right on! Keep it up! It's really difficult to have NO contamination and NO imperfections. Sometimes the imperfections should be looked as a feature and not as a flaw. But mostly, whatever you make should be functional.
Just be careful when you start diving in to HDPE. I always struggle with HDPE blow (blow moulded bottles) as it is much thicker and doesn't play nice.
This particular item is a heat press for t-shirts, so it only heats one face at a time, and I have the plastic between sheets of parchment baking paper. I'll have to experiment with lower temps; it might reduce the shrinkage during cooling. I'm also not actually sure if the machine's setting is accurate. And I suspect you're right that the plastic doesn't reach that temp, especially with the need to flip it over to heat both sides.
If you're using steel sheets, you dont need to use parchment paper. but you do need to use some demoulding agent to keep it from sticking. I use silicone release, you can find different stuff out there.
If the plastic sticks to the steel, then it might actually crack. I think it's less of an issue on smaller sheets but i could be mistaken. I've made a PP sheet about 45 x 45cm and it cracked, only one time. A friend makes 1m x 1m sheets and says he gets cracking issues. So you do have to let the plastic shrink. I don't recall what the issue was exactly but my guess is that there was pressure on the machine that held the plastic in place as it was cooling and caused it to become so stressed it cracked.
11:52 AM
You might not have this issue with parchment paper, but hopefully you dont have parchment paper sticking to your plastic.
Dialing in machines is important. Almost everyone uses different temperatures. So long as you are consistent with your results, you're doing well. I recently changed out my heater elements and repositioned my thermocouple. I have to dial it back in....keep overheating things and causing the plastic to stick to my steel. huhuhu
Just be careful when you start diving in to HDPE. I always struggle with HDPE blow (blow moulded bottles) as it is much thicker and doesn't play nice.