Thickness variation in the thermoforming of poly(methyl methacrylate) and high‐impact polystyrene sheets

The influence of material flow properties on the variation of wall thickness in a thermoformed part was investigated by measuring the thickness reduction at the pole of free‐formed axisymmetric domes of poly(methyl methacrylate) and high‐impact polystyrene. It was found that at a given pole height, the thickness reduction in poly(methyl methacrylate) was less than in high‐impact polystyrene, i.e., the wall thickness in a part formed from poly(methyl methacrylate) will be more uniform than in a part formed from high‐impact polystyrene by the same technique. This difference in formability was ascribed to a difference in the dependence of the flow stress σ at the thermoforming temperatures on time. The flow stress of both materials was given by σ = Ktm′ϵn, but whereas n was approximately 1 for both materials, m′ was −0.052 and −0.33 for poly(methyl methacrylate) and high‐impact polystyrene, respectively. A physical argument and simple analysis led to the conclusion that a large (negative) value of the “stress relaxation index” in a material reduces the degree of uniformity of sheet thickness in a formed part.