I've worked with acrylic in many different situations, but have never yet put a piece in the oven to thermoform it. I've used strip heaters and heat guns before, but these techniques are more suited to edge bends, and today, I had the task to curve a thick piece in a consistent arc. Here is the machined blank, ready to be formed.
This is 3/8" thick cast acrylic (about 2" by 4") with a few slots cut into it. I put the acrylic on a piece of cardboard, since I was afraid the oven rack or metal pan would stick to the acrylic during heating. As it turned out, the hot acrylic had no tendency to stick to anything, and the cardboard got pretty toasted. The next time that I try this, I will use thin MDF or peg board to support the acrylic.
I put the acrylic and cardboard into my toaster oven, which was preheated to 350*F. After 5 minutes, the piece was getting flexible, but not soft enough. After 7 minutes total, the piece was perfect, and I quickly placed it onto my mold.The mold is made of three 3/4" thick pieces of MDF, which I cut into an arc with bandsaw, then sanded and primed the edge. After the primer was dry, I used double-stick tape to attach a piece of 1/32" delrin. This provides a slick surface that can withstand heat and many chemicals.
I used another piece of 1/32" delrin and a paper towel to push the softened acrylic down onto the mold. It took less than minute for it to hold its new shape. I deburred and flame-polished the edges to finished the piece.