Effects of hairline cracks on the fracture behavior of polyethylene terephthalate injection moldings

Hairline cracks are microscopic defects found in most injection-molded components. The effects of such hairline cracks on the fracture modes of injection moldings, especially polyethylene terephthalate (PET), are still not well understood. The slow crystallizing nature of PET results in a distinct skin-core structure when it is injection molded. By controlling the hairline crack depth, the crack tip can be situated within the skin region of the moldings. The fracture behavior of hairline-cracked PET specimens during tensile loading was found to be dependent not only on the crack depth but also on the region in which the crack tips were situated. In contrast to our previous studies on a standard v-notch, very shallow hairline cracks resulted in catastrophic fracture of PET components. Crystallinity distribution along the skin core was found to affect the notch sensitivity of the materials. Annealing the specimens in hot oil was expected to create a crystallized outermost skin (referred to as the pseudo-skin) in the samples and remove internal stresses. This was intended to alter the skin-core ratio to evaluate its effect on the toughness of hairline-cracked specimens. The results revealed that annealing created a pseudo-skin layer and improved the toughness of the materials. Polymer Journal (2010) 42, 887-895; doi: 10.1038/pj.2010.86; published online 15 September 2010