Flow and Thermal History Effects on Morphology and Tensile Behavior of Poly(oxymethylene) Micro Injection Molded Parts

The micro injection molding process is a rapidly growing area in plastics processing technology. In this process, the polymer is exposed to both high shear rates and large thermal gradients. In view of the versatility of the process, both commodity and engineering polymers have been used in micro injection molded products. In the present work, poly(oxymethylene) (POM), a partially crystalline engineering polymer, was employed to evaluate the relationships between processing conditions, on one hand, and the morphology and properties of the final part, on the other hand. An unsymmetrical mold cavity to make parts in the form of stepped plaques was used in the study. This resulted in substantial differences in morphology, crystallinity and shrinkage of the zones of different constant thicknesses in the micro parts. Depending on the molding conditions and the location on the micro-part, the microstructure can display up to five crystalline layers. Of particular interest, shish-kebab crystalline structures were observed within the skin of the step with the smallest thickness. Differential scanning calorimetry (DSC) tests are used to distinguish between the melting points of the shish and kebab components of this particular structure. The degree of crystallinity as determined by wide angle X-ray diffraction (WAXD) and shrinkage across the thickness were also found to be highest in the step with the smallest thickness.