The chemical modification of high density polyethylene (HDPE) in a counter‐rotating nonintermeshing twin screw extruder is modeled using a simultaneous random scission and cross‐linking integro‐differential equation. The roles of mixing and kinetics in the free‐radical reaction are investigated. The modification experiments use a masterbatch feed of HDPE with various concentrations of 2,5‐dimethyl‐2,5‐(
t‐butylperoxy)‐3‐hexyne. The extruder operating parameters studied were barrel temperature, screw speed, and the degree of screw flight stagger. The reaction model is fit using the molecular weight averages and the gel content measured experimentally. The reaction time for HDPE is determined by use of the residence time distribution of the extruder with a segregation (macromixing) assumption. The segregation model is compared against the plug flow assumption, where the mean residence time of the measured distribution is used for the length of reaction of all polymer species within the extruder. The calculated molecular weight distributions are insensitive to the mixing model used. © 1995 John Wiley & Sons, Inc.