Flow-induced morphology evolution of uniformly miniaturized high-density polyethylene parts prepared by micro-injection molding

The morphology evolution of thermoplastics made by micro-injection molding (mu IM) in a uniform miniaturization manner has not been documented yet. In the present study, the crystal morphologies within mu IM high-density polyethylene parts were investigated with the aid of rheology measurement, polarizing optical microscopy, scanning electron microscopy, and differential scanning calorimetry. It was discovered that unlike conventional injection molded parts with spherulites predominating, uniformly miniaturized products have highly oriented lamellae starting to appear in the skin layer. For 175-mu m-thick mu IM parts (M175), interlocking shish-kebab structures exist throughout the thickness. Furthermore, the origins of different crystal morphologies with various part thicknesses were discussed mainly from the viewpoints of rheology and thermodynamics. It was revealed that the flow-induced crystallization is one of the key factors responsible for the unique super structures. More quantitatively, there is a critical shear rate of 3.0 x 10(5) s(-1) existing for forming highly oriented molecular structures. Meanwhile, on the basis of the model from the literature, an elongational rate as high as 4.4 x 10(4) s(-1) was also identified at the channel center for M175.