If your model has any space under it, the plastic will form under the model and make it impossible to release. One way to prevent this is to place your model on a bit of wood and use modeling clay to block the bottom edge of the model.
I have tried both water based, air drying clay (the kind you might use to make a pot) and an oil based, never hardens modeling clay.
The never harden clay melted just a bit with the vacuum forming heat and was a mess to clean up. I placed the clay more as a "rope" around the model and that left a lot of nooks and crannies in the formed plastic that clay was stuck in. On one cycle, for some reason, I smoothed out the clay with my finger and that made the clean up much easier. Still, it is a pain and I don't recommend it.
The water based clay seems like it's going to dry on everything and be a mess. In fact it is easier to clean up. The clay did not "melt a bit" in the heat of forming; I think it became a bit harder. Then the clean up is easy with just flooding the formed plastic with water. I wish I had never thought to try the oil based clay!
- 0.04 inch... I bought a 24x48 inch sheet of 0.04 inch material from http://usplastic.com for $18 plus $10 shipping. Out of that I cut two 20x20 sheets. So the cost was $15 per sheet.
- 0.02 inch... Bought a precut 24x24 sheet from amazon. $5 for the sheet, $6 for shipping. Total of $11 per sheet.
- Tap Plastics. They charge $2.15 per square foot; sheet cost $6.00.
0.03 inch material: I set the vacuum former heaters on 3. This material took 50 seconds to be ready to form. It formed very nicely with excellent detail. I found that this is substantially thinner material than the 1/32 inch sheets from Tap Plastic.
I wish I had ordered 0.04 inch material from US Plastics.
- US Plastics. I bought a 40x72 inch sheet of 0.03 inch styrene from http://usplastic.com (item 043331) It cost $10 and shipping was another $10. It came in a rolled up tube. I cut it into 6 20x20 inch sheets on the power shear. The cost per sheet was $3.25
For styrene and PETG I used CRC Food Grade Silicone Mold Release. I think it really helped the plaster form to come loose. I recommend it. I bought my can at Amazon; it was inexpensive.