Failure analysis of extruder screw bushings used to process powder paint

This article aims to present the failure analysis of extruder screw bushings used for the manufacture of powder paint. Two bushings, one from the transition zone and one from the metering zone were selected to carry out the study. Chemical composition and microstructural analysis, hardness testing and a detailed analysis of the failed surfaces by visual examination and SEM were performed. The results showed that both bushings were made of steel DIN 1.2316 with a globular annealing heat treatment. Also, a nitride coating 4565 mu m thick with hardness greater than 1400 HV was identified. The analysis of the failed surfaces indicated that the main wear mechanism was contact fatigue wear, with the presence of micropitting, macropitting and cracking. This type of wear is due to the contact between the bushings surface and the high hardness abrasive particles of the paint formulation, added to the high pressures developed within the barrel and the cutting cyclic stresses that the bushings have to withstand. The presence of other wear mechanism like adhesion, corrosion and abrasion was discarded.