All thermoforming projects require some way to hold the plastics during the forming process. For some time now, many people have been using window screen material. Those types of frames are okay because they are easy to make and it is possible to mix and match the sides in order to get various aspect ratios and sizes for a number of different projects.
On the other hand, the material that aluminum window screens are made out of is quite flimsy. Therefore, when the project you are working on features large pieces or especially thick plastic, it is best to have a clamping frame that is considerably more rigid.
The ideal choice for building your plastic clamping frames is aluminum C channel. It is readily available and comes in a wide variety of thicknesses and sizes. It works well if you want to make a small frame of about 12 by 18 inches. For this you would use a small piece of aluminum channel that is 3/8, 1/16 or 1/20 inches thick. The same technique works just as well for larger frames. Simply use thicker and larger C channel.
The Basic Technique
The basic technique for building a plastic clamping frame out of aluminum C channel is simple. All you have to do is cut slits into the horizontal parts of the C profile. You can do this easily with an average hacksaw. Then, just bend the vertical part to construct the corners. Be sure to take care not to score the vertical parts of the C channel with you are sawing through the horizontal parts because it will cause it to tear.
Where the aluminum C channel comes back around to meet itself, there will be a little extra with the horizontal part. You will need to cut that part of in order to make a tab. Bend the tab at 90 degrees and overlap the vertical at the opposite end. Drill both pieces and pop-rivet it together. If you prefer, you can use a small bolt instead.
Only one of the frames will need to be rigid. The other one will only need to spread a bit to clamp the force halfway from one of the clamps to the other. Generally, in order to save on time and cost, it is best to combine a rigid, thick frame with one that is thinner and more flexible.
