ABSTRACTThe effect of screw design on decrosslinking of the crosslinked high‐density polyethylene (XHDPE) by means of ultrasonic twin‐screw extruder with two screw configurations is investigated. Die pressure and ultrasonic power consumption during extrusion are recorded. Swelling characteristics, rheological properties, thermal analysis, scanning electron microscopy, and tensile properties are used to investigate the structure–property relationship of decrosslinked XHDPE. It is found that the screw configuration with conveying elements and reverse conveying elements (decrosslinking screws) is an effective means to reduce the gel fraction and crosslink density of decrosslinked XHDPE and significantly improve its processibility. Rheological properties of decrosslinked XHDPE are correlated with structural changes occurring during ultrasonic decrosslinking. The presence of the highly branched sol in decrosslinked XHDPE is revealed through measurements of the activation energy for flow. Comparison of morphologies of the lamellar structure of HDPE, XHDPE, and decrosslinked XHDPE reveals that the presence of the crosslink network inhibits the lamella growth. Significant improvements in the mechanical performance of decrosslinked XHDPE are obtained by using decrosslinking screws. The molecular structure and morphology of the lamellar structure of decrosslinked XHDPE are used to explain the processing–solid‐state property relationship. The measured results on the gel fraction and crosslink density are compared with those of numerical simulations. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40680.