Intro: Bending Plastic. I Made It at TechShop-Detroit
I found a trick for bending plastic sheets. You get straight, crisp, uniform bends. Cut a trench about .02" deep on the inside of the bend. When you soften the plastic with a heat source (hot wire, flame or even a heat gun - for short pieces), the bend happens exactly on the trench.
For tests and small pieces, I used a milling machine to cut the trenches, but for large items, I used the ShopBot CNC router at TechShop. http://techshop.ws
One thing to remember: each bend uses up some of the material (bend allowance). If this is an important issue, run a test. Cut a trench, say 2" from an edge, bend it, then measure. With the .12" thick ABS plastic I used, I lost .05" to the bend.
The trench puts the bend right where you want it, every time.