A Preliminary Investigation into Premature Cracking of an Injection Molded Glass-Reinforced PET Plastic Receptacle

Premature cracking in injection molded, readily available thermoplastic polyethylene terephthalate (PET) glass-reinforced polyester receptacles was investigated by analyzing cracked “defective” and non-cracked “functioning” receptacles to discover compositional or processing deficiencies. Mechanical properties between both were also compared. Material samples were cut from both defective and functioning receptacles and subjected to thermal/chemical analytical and mechanical materials tests; thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), liquid nuclear magnetic resonance (NMR), mechanical bend, drop ball impact, and microhardness. Fractographic images were taken of both defective samples at cracked locations and functioning sample bulk material. The test results reveal that the “defective” receptacles exhibit lower thermal stability, increased crystallinity and lower glass transition temperature compared to the “functioning” receptacle, though compositional differences were not detected. Test results point to discrepancies in the injection molding process, possible moisture contamination, and/or possible differences in starting resin quality/glass fiber loading.