Hello. I am designing a passively athermal assembly, in which I am using a thermal compensator component made of UHMWPE. This hollow cylinder has one flat face resting against a stationary/fixed component, while the other face is pressing against a moving component. This moving component is spring loaded, so that if the thermal compensator contracts (as a result of entering a low temperature environment) the face of the moving component remains coincident to the thermal compensator.

I have been having trouble getting this simulation to match my hand calculations for expected directional displacement of the moving components. Initially I found, due to using bonded local interactions on each side, the resulting expansion/contraction seems incorrect. The resultant expansion and contraction always slightly bigger than it should be. I believe this is because of the material not being able to distribute evenly due to the bonded contacts acting as if the compensator was welded to each side.

How would I be able to create a contact where each side of the athermal spacer is able to expand/contract freely whilst pushing on both walls?