I am cutting 0.050" UHMW with dies that are roughly 12"x12" and made a DIY press out of steel and a 20ton jack (pneumatically operated). It cuts though the plastic just fine, but the resulting puzzle pieces don't fit together very well. I am thinking that it must be stretching the plastic since the air jack is so slow. Do most presses rely on velocity for their cutting? Any ideas on ways around this in a DIY setting or am I destined to purchase a $7000 clicker press? Thanks!

I use a 4 tone eclectic clam press to press plastics and magnetic materials. Although I'm not sure the density of the plastics you are pressing, we cut .050 mill plastic with not much effort however we are considering a "Hot" press where the die is heated to a couple hundred degrees. That should cut much thicker synthetic materials like butter.

Depends upon the dies and how the cut is beveled. If you dont have the right bevel for the blade it will cut a different size due to compression or stretching depending upon the shearing angle.
If you are using steel rule dies you might have the wrong chamfer on the rule. If the die is homemade, ??? A quick strike is better for a shear but forming usually is a slower action. Thin forms are easy to quick strike. You need the right tonage for the action.

mc_n_g

All plastics I've punched or blanked will not punch out too size. Plastics expand after they've been punched, for example, a .125 dia. punch will only produce a .120 dia. hole. Holes contract, blanks expand. Count on it. I'm a stamping god!