Injection molding of delamination‐free ultra‐high‐molecular‐weight polyethylene

When ultra‐high‐molecular‐weight polyethylene (UHMWPE) in powder form is injection molded, the so‐called delamination layering occurs near the skin of the parts. This layering defect hampers UHMWPE's superior wear resistance property and part surface quality. The delamination layer was caused by a combination of excessive shear stress near the part surface and high degree of molecular entanglement of UHMWPE. A mold insulation method that delays the rapid cooling of UHMWPE to reduce the shear stress and improve the polymer chain “interdiffusion” across the entangled chain bundles was used to eliminate the delamination layer. When the insulation layer thickness and mold temperature were optimized, the delamination layer was eliminated completely while still maintaining a reasonable cooling/cycle time. The delamination‐free parts were found to regain UHMWPE's superior impact resistance and tensile properties. POLYM. ENG. SCI., 59:2313–2322, 2019. © 2019 Society of Plastics Engineers     

Effect of alkyl group on the chain extension of phosphites in polylactide

Polylactide (PLA), which is synthesized from natural resources and can degrade easier, possesses high mechanical strength, so it is a reasonable substitute for petroleum‐based plastics. Phosphites can increase the stability of PLA through chain extension with the hydroxyl and carboxyl groups simultaneously. But there are few reports on the structural effects of phosphites on the chain extension of PLA. In this article, three kinds of phosphites with different amounts of aryl and alkyl groups were used as chain extenders in PLA and were compared in detail. The molecular weights, complex viscosities, and storage moduli of virgin PLA and PLA stabilized by three different phosphites were characterized by gel permeation chromatography and rheometry. The results show that the presence of alkyl groups is not beneficial for chain extension, as the more alkyl groups there are, the worse the chain extension is. Regarding the three phosphite chain extenders added to PLA—triphenylphosphite (TPP), diphenylisooctylphosphite (PTC), and phenyldiisooctylphosphite (PDOP)—the number of alkyl groups in them can be ranked as follows: PDOP > PTC > TPP. Since PDOP had the most alkyl groups, the chain extension of PDOP was the weakest. In addition, the product, which was formed due to the chain extension of PLA and TPP, had some plastication, thus enabling PLA to move more freely and making it easier to process. J. VINYL ADDIT. TECHNOL., 25:144–148, 2019. © 2018 Society of Plastics Engineers     

Study of injection molded microcellular polyamide‐6 nanocomposites

This study aims to explore the processing benefits and property improvements of combining nanocomposites with microcellular injection molding. The microcellular nanocomposite processing was performed on an injection‐molding machine equipped with a commercially available supercritical fluid (SCF) system. The molded samples produced based on the Design of Experiments (DOE) matrices were subjected to tensile testing, impact testing, Dynamic Mechanical Analysis (DMA), and Scanning Electron Microscope (SEM) analyses. Molding conditions and nano‐clays have been found to have profound effects on the cell structures and mechanical properties of polyamide‐6 (PA‐6) base resin and nanocomposite samples. The results show that microcellular nanocomposite samples exhibit smaller cell size and uniform cell distribution as well as higher tensile strength compared to the corresponding base PA‐6 microcellular samples. Among the molding parameters studied, shot size has the most significant effect on cell size, cell density, and tensile strength. Fractographic study reveals evidence of different modes of failure and different regions of fractured structure depending on the molding conditions. Polym. Eng. Sci. 44:673–686, 2004. © 2004 Society of Plastics Engineers.     

Quantitative study of shrinkage and warpage behavior for microcellular and conventional injection molding

This research investigated the effects of processing conditions on the shrinkage and warpage (S&W) behavior of a box‐shaped, polypropylene part using conventional and microcellular injection molding. Two sets of 2^6‐1 fractional factorial design of experiments (DOE) were employed to perform the experiments and proper statistical theory was used to analyze the data. After the injection molding process reached steady state, molded samples were collected and measured using an optical coordinate measuring machine (OCMM), which had been evaluated using a proper R&R (repeatability and reproducibility) measurement study. By analyzing the statistically significant main and two‐factor interaction effects, the results show that the supercritical fluid (SCF) content (nitrogen in this case, in terms of SCF dosage time) and the injection speed affect the S&W of microcellular injection molded parts the most, whereas pack/hold pressure and pack/hold time have the most significant effect on the S&W of conventional injection molded parts. Also, this study quantitatively showed that, within the processing range studied, a reduction in the S&W could be achieved with the microcellular injection molding process. POLYM. ENG. SCI., 45:1408–1418, 2005. © 2005 Society of Plastics Engineers     

Microcellular injection-molding of polylactide with chain-extender

The effects of adding an epoxy-based chain-extender (CE) on the properties of injection-molded solid and microcellular polylactide (PLA) were studied. PLA and PLA with 8 wt.% CE (PLA-CE) were melt-compounded using a twin-screw extruder. Solid and microcellular specimens were produced via a conventional and microcellular injection-molding process, respectively. Various characterization techniques including gel permeation chromatography, tensile testing and dynamic mechanical analysis, scanning electron microscopy and differential scanning calorimetry were applied to study the molecular weight, static and dynamic mechanical properties, cell morphology, and crystallization behavior, respectively. The addition of CE enhanced the molecular weight but decreased the crystallinity of PLA. The addition of CE also reduced the cell size and increased the cell density. Furthermore, the decomposition temperatures and several tensile properties, including specific strength, specific toughness, and strain-at-break of both solid and microcellular PLA specimens, increased with the addition of CE.     

Preparation of thermoplastic polyurethane/graphene oxide composite scaffolds by thermally induced phase separation

In this study, biomedical thermoplastic polyurethane/graphene oxide (TPU/GO) composite scaffolds were successfully prepared using the thermally induced phase separation (TIPS) technique. The microstructure, morphology, and thermal and mechanical properties of the scaffolds were characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and compression tests. Furthermore, NIH 3T3 fibroblast cell viability on the porous scaffolds was investigated via live/dead fluorescent staining and SEM observation. FTIR and Raman results verified the presence of GO in the composites. SEM images showed that the average pore diameter of the composite scaffolds decreased as the amount of GO increased. Additionally, the surface of the specimens became rougher due to the embedded GO. The compressive modulus of composite specimens was increased by nearly 200% and 300% with the addition of 5% and 10% GO, respectively, as compared with pristine TPU. 3T3 fibroblast culture results showed that GO had no apparent cytotoxicity. However, high loading levels of GO may delay cell proliferation on the specimens. POLYM. COMPOS., 35:1408–1417, 2014. © 2013 Society of Plastics Engineers     

Process optimization of injection molding using an adaptive surrogate model with Gaussian process approach

This article presents an integrated, simulation‐based optimization procedure that can determine the optimal process conditions for injection molding without user intervention. The idea is to use a nonlinear statistical regression technique and design of computer experiments to establish an adaptive surrogate model with short turn‐around time and adequate accuracy for substituting time‐consuming computer simulations during system‐level optimization. A special surrogate model based on the Gaussian process (GP) approach, which has not been employed previously for injection molding optimization, is introduced. GP is capable of giving both a prediction and an estimate of the confidence (variance) for the prediction simultaneously, thus providing direction as to where additional training samples could be added to improve the surrogate model. While the surrogate model is being established, a hybrid genetic algorithm is employed to evaluate the model to search for the global optimal solutions in a concurrent fashion. The examples presented in this article show that the proposed adaptive optimization procedure helps engineers determine the optimal process conditions more efficiently and effectively. POLYM. ENG. SCI., 47:684–694, 2007. © 2007 Society of Plastics Engineers.     

Study of polystyrene/titanium dioxide nanocomposites via melt compounding for optical applications

This article presents the study of melt compounding of polystyrene (PS) with various types of titanium dioxide (TiO₂) nanoparticles and surfactants, using a corotating twin screw extruder with multiple screw element configurations. It was found that a properly designed high shear screw configuration and the copolymer of silicone, ethylene oxide, and propylene oxide‐based surfactant produced the greatest degree of nanoparticle dispersion in PS/TiO₂ nanocomposites, whereas a silane‐based surfactant and silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃) coated TiO₂ nanoparticles yielded nanocomposites with the least photocatalytic degradation effects and the best retention of tensile and impact properties. POLYM. COMPOS., 28:241–250, 2007. © 2007 Society of Plastic Engineers     

Fabrication of shish–kebab structured poly(ε-caprolactone) electrospun nanofibers that mimic collagen fibrils: Effect of solvents and matrigel functionalization

Surface properties of tissue engineering scaffolds such as topography, hydrophilicity, and functional groups play a vital role in cell adhesion, migration, proliferation, and apoptosis. In this study, poly(ε-caprolactone) (PCL) shish–kebab scaffolds (PCL-SK), which feature a three-dimensional structure comprised of electrospun PCL nanofibers covered by periodic, self-induced PCL crystal lamellae on the surface, was created to mimic the nanotopography of native collagen fibrils in the extracellular matrix (ECM). Two different kinds of solvents used in creating kebabs on the shish were investigated by real-time observation. It was found that the solvent solubility directly affects the formation of kebabs: the lower the solubility of the solvent, the easier the formation of the kebabs on the surface of the electrospun nanofibers. In addition, matrigel was covalently immobilized on the surface of alkaline hydrolyzed PCL and PCL-SK scaffolds to enhance their hydrophilicity. This combined approach not only mimics the nanotopography of native collagen fibrils, but also simulates the surface features of collagen fibrils for cell growth. To investigate the viability of such scaffolds, HEF1 fibroblast cell assays were conducted and the results revealed that the nanotopography of the PCL-SK scaffolds facilitated cell adhesion and proliferation. The matrigel functionalization on PCL-SK scaffolds further enhanced cellular response, which suggested elevated biocompatibility and greater potential for tissue engineering applications.     

Injection and injection compression molding of ultra‐high‐molecular weight polyethylene powder

Ultra‐high‐molecular weight polyethylene (UHMWPE) powder was processed using injection molding (IM) with different cavity thicknesses and injection‐compression molding (ICM). The processing parameters of feeding the powders were optimized to ensure proper dosage and avoid jeopardizing the UHMWPE molecular structure. Dynamic mechanical analysis (DMA) and Fourier‐transform infrared spectroscopy tests confirmed that the thermal and oxidative degradations of the material were avoided but crosslinking was induced during melt processing. Tensile tests and impact tests showed that the ICM samples were superior to those of IM. Increased cavity thickness and ICM were helpful for reducing the injection pressure and improving the mechanical properties due to effective packing of the material. Short shot molding showed that the UHMWPE melt did not exhibit the typical progressive and smooth melt front advancements. Due to its highly entangled polymer chains structure, it entered the cavity as an irregular porous‐like structure, as shown by short shots and micro‐computed tomography scans. A delamination skin layer (around 300‐μm thick and independent of cavity thickness) was formed on all IM sample surfaces while it was absent in the ICM samples, suggesting two different flow behaviors between IM and ICM during the packing phase. POLYM. ENG. SCI., 59:E170–E179, 2019. © 2018 Society of Plastics Engineers