Processing of Ultra-high Molecular Weight Polyethylene (uhmwpe) with Regular and Special Injection Molding Technologies PDF Download

Author: Galip Yilmaz
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Languages : en
Pages : 0

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Book Description
In this work, the processing of a unique ultra-high molecular weight polyethylene (UHMWPE) polymer, in both powder and pellet form, using both regular and special injection molding techniques, was investigated in an effort to mass produce this high-grade specialty polymer. The goals of this study were to (i) improve the processability of UHMWPE, (ii) enhance the mechanical performance and overall quality of the molded parts, and (iii) develop an in-depth understanding of how the supercritical fluid (SCF), machine configuration, mold compression, mold insulating techniques, and process conditions affect the flow behavior and final quality of the injection molded UHMWPE. In the first study, two common atmospheric gases in their supercritical states-namely, nitrogen (scN2) and carbon dioxide (scCO2)-were used as processing aids in a special full-shot, high-pressure microcellular injection molding (MIM) process for processing UHMWPE pellets. The mechanical properties in terms of tensile strength, Young's modulus, and elongation-at break of the SCF-loaded samples were examined. The thermal and rheological properties of regular and SCF-loaded samples were also analyzed using differential scanning calorimetry (DSC) and parallel-plate rheometry, respectively. It was found that the processing of UHMWPE with both gases effectively reduced the thermal degradation of the material and the injection pressure, compared to regular injection molding, while still retaining the mechanical properties of the resin. In the second part, a follow-up study was conducted on conventional injection molding (IM), along with the special full-shot, high-pressure microcellular injection molding (MIM) using UHMWPE in pellet form. A relatively complicated and thin-walled mold design was used to produce box-shaped parts with varying wall thickness. Although different processing settings were tested in order to eliminate persistent short shot issues, only high-pressure MIM processing was able to fill parts completely. Furthermore, not only did high-pressure MIM processing effectively promote the processability of UHMWPE, it also reduced the very high injection pressure requirement and the high part shrinkage issues associated with the IM samples. In the third study, UHMWPE powder was processed using injection molding (IM) and injection-compression molding (ICM). The processing parameters of feeding the powders were optimized to ensure proper dosage and to avoid damaging UHMWPE's molecular structure. Dynamic mechanical analysis (DMA) and Fourier-transform infrared spectroscopy (FTIR) tests confirmed that the thermal and oxidative degradation of the material was minimized but crosslinking was induced during molding. Tensile tests and impact tests showed that the ICM samples were superior to the IM samples. A delamination skin layer was formed on the IM sample surfaces, while it was absent in the ICM samples, thus suggesting two different flow behaviors between IM and ICM during the packing phase. The delamination layer defect was the subject of the fourth study as one of the main challenges of UHMWPE molding. The delamination layer hampers UHMWPE's two key properties: wear resistance and impact strength. A mold insulation method was employed to eliminate the formation of the delamination layer. The working principle of the method was to reduce the cooling rate and the shear stress of the polymer while improving polymer chain "interdiffusion" across the entangled chain bundles during the injection filling stage via a low thermal conductivity mold coating (e.g., epoxy coating). This method yielded molded parts free of delamination by delaying skin cooling during filling and packing. Therefore, it produced parts with enhanced mechanical properties, excellent impact strength, and improved surface quality.