The effects of multiple-pass extrusion on the structure and properties of dairy-grade high density polyethylene (HDPE) have been studied. The samples were found to be thermo-oxidatively degraded by repeated extrusion and this led to crosslinking and chain scission reactions. The crosslinking process was dominant during the first two cycles, but the two processes proceeded at approximately equal rates after cycle 2. The melt flow index of the samples was observed to steadily decrease in accord with the crosslinking of chains. The tensile modulus initially increased before attaining an approximately constant value, while the yield strength was essentially unchanged. Impact strength was observed to steadily decrease after the third extrusion pass. The main practical problem associated with the multiple extrusion of the HDPE was observed to be severe discolouration of the sample and contamination with extraneous matter.