The extensional flow of poly(methyl methacrylate) and high‐impact polystyrene at thermoforming temperatures

The work described in the present paper was performed to establish stress–strain–time relationships at plastic sheet thermoforming temperatures. The relationships are correlated with sheet‐forming “formability”. Specimens of poly(methyl methacrylate) at 165°C and high‐impact polystyrene at 122°C were extended to large strains at constant cross‐head velocities. Initial strain rates were between 4.2 × 10 −3 /sec and 1.6 × 10 −1 /sec. It was found that the flow stress σ was related to the true strain ε and the elapsed time t by a relation σ = K t m ′ ε n , where K is a constant and n and m ′ are indices. The value of n for both materials was approximately one. The value of m ′ was −0.052 and −0.33 for poly(methyl methacrylate) and high‐impact polystyrene, respectively. Tests were also performed in which the cross‐head velocity was increased in steps. It was found that the flow stress in these tests followed the same relationship as in the constant cross‐head velocity tests.