Relevant materials parameters in cushioning for EPS foams

Cushioning of packaging materials is normally evaluated in terms of the maximum deceleration versus the “static stress” for a specific drop height (h) and cushion thickness (d) (ISO 4651:1988 (E)). These standard methods provide a general guidance to the evaluation of cushioning performance but require extensive work to build the curves for all the desired height to cushion thickness ratio (h/d). Moreover, they offer no insight into the material’s structure–performance relationships so it is difficult to compare different materials. Several authors approached the problem with empirical formulas, finite element analysis or parametric fitting to describe cushioning performances. Here we focus on the cushioning behavior of EPS (expanded polystyrene) which we show can be modeled with two contributions: a recoverable nonlinear elastic component due to the air compression and a non-recoverable ideal cushion due to the plastic collapse of the foam. We show how these two components are enough to describe the cushioning curve of EPS.