A process model for ultrasonic decrosslinking of peroxide crosslinked high-density polyethylene (XHDPE) is proposed. The model is developed by modifying the earlier model describing ultrasonic devulcanization of rubber vulcanizates under static (no flow) conditions. The bubble cavitation during ultrasonic wave propagation in a viscoelastic solid containing small concentration of voids is considered. A method to calculate the ultrasonic power consumption and acoustic pressure in a bubble filled polymer is proposed based on the linear acoustic analysis. The rupture of network is modeled by considering the breakage of main chains caused by the large deformation arising during the bubble oscillation. Unlike the earlier process model for ultrasonic devulcanization of rubbers, the current model does not require kinetic parameters assigned to describe the rupture of network. The nonisothermal flow of decrosslinked XHDPE in the ultrasonic treatment zone in the twin screw extruder (TSE) is modeled by considering a helical flow of a power-law fluid with its material parameters related to the gel fraction. The simulation algorithms describing ultrasonic decrosslinking of XHDPE under static (no flow) condition and in TSE are presented. (C) 2017 Society of Plastics Engineers