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 degrees 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 degrees C and 130 degrees C. This resulted in a processing window that was particularly small, 5 degrees C, and that within this window a temperature change of 1 degrees 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 degrees C. The results showed that HDPE can be deformed to strain levels comparable to the thermoforming process. POLYM. ENG. SCI., 58:1516-1522, 2018. (c) 2017 Society of Plastics Engineers