High density polyethylene (HDPE) is predominately processed from the molten state. However, there is the potential to enhance the mechanical properties by forming below the melt temperature, in the semi-solid phase. To further investigate this enhancement, HDPE sheets were deformed under constant width, simultaneous equal-biaxial (EB), and sequential biaxial deformation. The samples were deformed at strain-rates, from 4 to 16 s−1, up to nominal strains of 3.0 at temperatures below 130°C. The strain-rate and high-strain level applied were chosen so that the data was comparable to the thermoforming process. It was discovered that in order for the HDPE sheet to deform at the high-rate and high strains the processing temperature must be between 126°C and 130°C. This resulted in a processing window that was particularly small, 5°C, and that within this window a temperature change of 1°C had a significant impact on the stress–strain response. The samples were analyzed post deformation, where the elastic modulus was found to increase by a factor of 2.08 in the machine direction, when deformed via EB at 129°C. The results showed that HDPE can be deformed to strain levels comparable to the thermoforming process. POLYM. ENG. SCI., 2017.