Abstract Poly(α‐methylstyrene‐styrene‐acrylonitrile) (AMSAN) is an additive used to improve the heat deflection temperature of PVC. The effect of its addition on the HDT of a typical injection molding formulation is studied. Other physical properties like impact strength, yield strength, modulus of elasticity, and viscosity are also reported. It is shown that HDT and impact strength are decoupleable based on the amount of AMSAN and impact modifier in the formulation. Calculation of the required percentage of heat distortion additive and impact modifier for a typical injection molding application are demonstrated.