Morphology of microinjection moulded polyoxymethylene

Abstract

Morphology of microinjection moulded polyoxymethyleneA microinjection moulding machine was used to obtain micromouldings of polyoxymethylene, in order to study morphology development during the process. The method of design of experiments was used to investigate statistically the effects of processing variables on the microstructural features of the mouldings. The morphological features were identified by microtoming the samples in both transverse and longitudinal (flow) directions and observing the microtomed sections under a polarised light microscope. Morphology evolution along the flow direction was followed by microtoming the specimens along the centre plane longitudinally and sequentially. A five-layer skin core structure was identified for micromoulded polyoxymethylene. The development of the structure was explained, based on mechanisms which were similar to those proposed for conventional injection moulding. Injection speed was found to be the most significant factor affecting morphological features of the final moulding. Moreover, the average plunger velocity, which is directly related to the cavity filling flow rate, was found to have good correlation with skin layer thickness. The distributions of crystalline polymorphs were observed and explained, in light of the distributions of the flow and thermal patterns in the mould. Morphology evolution along the flow direction reflected the distribution of pressure, temperature and velocity of the polymer melt during the microinjection moulding process. The results provided some indications regarding micromoulding mould design.

Flow Induced Crystallization of Poly(ether‐block‐amide) from the Microinjection Molding Process and its Effect on Mechanical Properties

“…The skin layer of cases 3, 5, 6, and 7 have a similar medium thickness from 36 to 43 μm. Statistical analysis in Figure b implies that the skin layer is influenced by injection velocity and mold temperature, as reported for Polyoxymethylene (POM) …”

“…In Figure , it is important to notice that we observed special spherulite‐free core morphologies in cases 3, 4, 5, and 8, compared to the typical spherulitic core morphologies of case 1, 2, 6, and 7. Some core‐free morphologies were found in micromolded parts of several materials, such as POM and HDPE, where the core was either composed of oblate spherulite structures or an oriented row structure because of fast quench and high shear rates. For Pebax, microscopic observation reveals no perceptible structures in the central region.…”

Effects of gate design and cavity thickness on filling, morphology and mechanical properties of microinjection mouldings

“…Kamal et al [6] studied process-morphology relationships for 300μm POM parts and found that the skin layer decreased and the core layer increased with an increase of injection velocity and mold temperature. Thickness of skin, fine spherulites and twisted lamellae layers gradually decreased from the gate to the part end, due to the combined effects of melt velocity, pressure and temperature.…”

“…Because of the intrinsic low thermal conductivity of polymer materials, a considerably high thermal gradient exists across the part thickness [1,5,6]. In order to fill such small features, high temperatures and high injection speeds are applied to reduce melt viscosity by shear thinning and these materials experience correspondingly high stresses and shear rates.…”

“…Thus, epitaxial growth of folded lamellae from fibril-like structures (shishes) leads to the formation of kebab structures. Except for such extreme conditions, the main morphology difference between micromoldings and conventional moldings is understood to be the relative thicknesses of different morphology layers, which determine the final physical properties of a component [5,6,[9][10][11][12][13]. The orientated layers account for more than 90% of the entire thickness of a micro part (thickness < 0.2mm), which is 3~4 times higher than that of a macro part (thickness > 1.5mm) [5,12,13].…”