Cracking of Acrylonitrile-Butadiene-Styrene Protective Covers

Numerous protective covers, used in conjunction with an electrical appliance, failed during assembly with the mating components. The failures were traced to a particular production lot of the covers and occurred during insertion of the screws into the corresponding bosses. The parts had been injection molded from an ABS resin to which regrind was routinely added. Inspection of both the failed covers and retained parts, which exhibited normal behavior during assembly, included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The FTIR results indicated the presence of contaminant material exclusively within the ABS resin used to mold the failed covers, and the thermograms suggested contamination with a PBT resin. Further TGA analysis showed the contamination was estimated to account for approximately 23% of the failed cover material. The conclusion was that the appliance covers failed via brittle fracture associated with stress overload. The failures, which occurred under normal assembly conditions, were attributed to embrittlement of the molded parts, due to contamination of the ABS resin with a high level of PBT. The source of the PBT resin was not positively identified, but a likely source appeared to be the use of improper regrind.