You guys are on the right track with overheating or slight moisture problems. I have used PETG and Polycarbonate for years. The polycarbonate must be dried or you get stuff that looks like this.
I formed the wrong sheet (I thought it had been dried).
The moisture turns to steam in the sheet and you get a tons of bubbles.
Regarding temperatures, I agree with
D48thRonin that it is a bit of an art. Generally speaking though, they suggest about 5 - 10 degrees above the heat deformation temp of the material. I use the data sheets at my
material supplier for getting this info.
Normally, you don't get this with the PETG, but we normally dry it for a little bit just to make sure. I once spoke with a technical guy at Sheffield Plastics in PA about PETG, and he said that they have to dry the resin before they extruded it into sheets.
Still, the PETG is the easiest and best clear material to form. Never heard of the forming over HIS, but we will give that a try.
Thanks for some new ideas.