How does vacuum forming affect Pelite mechanical properties?

Pelite is a polyethylene closed cell foam commonly used as an interface material in prosthetics. Both normal and vacuum-formed Pelite were tested under compression and under shear loading. For shear testing, the results were not significantly different for normal and vacuum-formed Pelite. For normal Pelite, the slope of the stress-strain curve was 1.17MPa (+/- 0.14) while for vacuum-formed Pelite it was 1.24MPa (+/- 0.22). Compressive results, however, were significantly different. Below 80kPa of applied compression, the slope of the stress-strain curve for normal Pelite was 0.99MPa (+/- 0.11) while for vacuum formed Pelite it was 0.72MPa (+/- 0.12). Between 80kPa and 200kPa of applied compression, the slope of the stress-strain curve for normal Pelite was 0.45MPa (+/- 0.03) while for vacuum formed Pelite it was 0.55MPa (+/- 0.05). Reasons for the differences and their significance to interface mechanics and computer-aided prosthesis design are discussed.