You have two totally different materials there.
The ABS has one direction of shrinkage, whereas the UHMWPE has biaxial shrinkage, meaning it shrinks in the extrusion direction as well as across the width of the sheet.
In order to combat this excessive shrinkage and twist, you need to understand HOW much the material shrinks as well as how to control this shrinkage.
First off, most polyolefins shrink about .020" per inch in BOTH directions, so your mold must be built to accomodate this.
Secondly, the mold must be cooled to reduce the shrinkage either by cast in water lines or by cooling the part off of the tool with a mist of water on both sides of the part. Clamping the part along all four sides to reduce the twist you are seeing can help as well.
what are you trying to make??????
john