Observation of Specific Polymer Morphologies in a Microinjection Moulded Part

Abstract

The thermomechanical conditions involved in injection moulding of polymers affect their morphology, especially in the case of semi-crystalline polymers. Literature has shown that shear and thermal gradients occurring during moulding involve an anisotropic morphology along sample thickness, known as “skin-core”. In microinjection moulding high mechanical forces and cooling rates are present. Their effects on morphology are investigated here using a semi-crystalline polymer (HDPE) processed in standard injection moulding (1.5mm thick plate) and in microinjection moulding (μpart with 150μm thickness). The skin-core morphology is identified in the conventional part, while in the μpart it is absent and a specific “core-free” morphology results. TEM studies on the crystalline lamellae scale show highly oriented lamellae in the μpart, related to row and/or shish kebab structures. The high flow strength and cooling rates characteristic for microinjection moulding promote the homogeneity of the morphology through the thickness, but also induce flow-induced crystallization. As a result, highly oriented structures are created within the μpart, conferring anisotropy in the final product. This could be a challenge to overcome, as this anisotropy affects both polymer shrinkage and the overall final part behaviour.