My tests in class were to see what was the best technique to vacuum form 1/16th inch acrylic. I tried flipping the acrylic as soon as it started to melt. The next two techniques were time based. One where I would leave the heater on for 30 seconds then off for 10 and the other was on for 100 seconds and off for 60. I found that so far the best way was the 30 seconds on and 10 seconds off.