High impact polystyrene was extruded repeatedly at 220 degrees C and thermoformed under different heat time (Ht), plug assist time (PAt), mold time (Mt), and mold temperature (MT). The optimum thermoforming process conditions were determined for a material processed in one cycle, using an experimental design of the four variables mentioned before at two levels. Mt has no influence on the coefficient of wall thickness variation (CV), while the rest of the variables present a noticeable influence on CV, and they can be modified to obtain lower values of CV (i.e. CV = 1.3, using Ht = 46 s, PAt = 2.5 s, and MT = 80 degrees C). These conditions were then employed for a material processed under six cycles, resulting in an increase in the CV value to 3.5. The normalized thickness (NT) of HIPS with one and six processing cycles was also evaluated. It depends on Ht, Mt, and PAt. With the values of the calculated parameters for CV, it was possible to obtain NT values close to 1, using a material processed one cycle. The NT value obtained for six cycles processed HIPS was far from unity. The changes on thermoforming variables' responses are a consequence of the changes occurred in HIPS inner structure.