Effects of nano‐ and micro‐fillers and processing parameters on injection‐molded microcellular composites

The effects of submicron core‐shell rubber (CSR) particles, nanoclay fillers, and molding parameters on the mechanical properties and cell structure of injection‐molded microcellular polyamide‐6 (PA6) composites were studied. The experimental results of PA6 nanocomposites with 5.0 and 7.5 wt% nanoclay loadings and of CSR‐modified PA6 composites with 0.5 and 3.1 wt% CSR loadings were compared to their neat resin counterparts. This study found that nanoclay was more efficient in promoting a smaller cell size, larger cell density, and higher tensile strength for microcellular injection molding parts. A higher nanoclay loading led to more brittle behavior for microcellular parts. It was found that a proper amount of CSR particles could be added to the microcellular injection‐molded PA6 to reduce the cell size, increase the cell density, and enhance the toughness of the molded part. However, CSR particles were less effective cell nucleation agents as compared to nanoclay for producing desirable cell structures, and a higher CSR loading was found to have diminishing effects on the process and on the properties of the parts. POLYM. ENG. SCI., 45:773–788, 2005. © 2005 Society of Plastics Engineers     

Fabrication of super‐ductile PP/LDPE blended parts with a chemical blowing agent

The mechanical blending of polypropylene (PP) and low density polyethylene (LDPE) is an economical and simple method for producing new polymeric materials for specific applications. However, the reduction in strain‐at‐break of the blend is one of its main shortcomings. In this study, PP/LDPE foamed parts were fabricated by conventional injection molding (CIM) with azodicarbonamide as a chemical blowing agent (CBA) and tested for tensile properties at two test speeds. Also, the fracture surfaces of the parts were investigated by scanning electron microscopy (SEM). In addition, to investigate the underlying mechanism of the super‐ductility, the tested samples were carefully analyzed and compared, and further characterized by differential scanning calorimetry and SEM. The results suggest that fabricating PP/LDPE super‐ductile parts using CIM with a CBA is feasible. The results also indicate that there is a close relationship between the mechanical properties and morphological structures, which are deeply influenced by the dosage of CBA, the PP/LDPE ratio, and the packing parameters. Furthermore, compared to conventional injection molded solid parts, the ductility of the foamed parts can be dramatically improved by the formation of microfibrils in the PP phase, which come into being under certain processing conditions. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44101.     

Experimental study on the penetration interfaces of pipes with different cross‐sections in overflow water‐assisted coinjection molding

The residual wall thicknesses (RWT) of the skin and the inner layers are important quality indicators of water‐assisted co‐injection molding (WACIM) parts. The influences of the shape of the cavity cross section and the processing parameters, including the water pressure, water delay time, inner melt temperature, and inner melt flow rate, on the penetration of the inner melt and water were explored via experiments. The results showed that the shape of the penetration section of the inner melt was closer to the cavity section with round corners, while that of the water ended up being round. Both the penetration ratios of the inner melt and the water increased proportionally with increasing circle ratio. Both the minimum values of the total RWT and the inner melt RWT increased with increasing circle ratio. Both the maximum values of the total RWT and the inner melt RWT increased with increasing Max_D, which is the maximum distance between the inscribed circle center and the wall. Both the penetration ratios of the inner melt and the water increased with increasing water pressure, decreased with increasing water delay time, and increased with increasing inner melt flow rate and increasing inner melt temperature. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42866.     

Fabrication of shish-kebab-structured carbon nanotube/poly(ε-caprolactone) composite nanofibers for potential tissue engineering applications

The electrospinning process was applied to fabricate the nanofibers of biodegradable poly(ε-caprolactone)(PCL) in which different contents of multiwalled carbon nanotubes(MWCNTs) were embedded. Afterward,the electrospun nanofibers were successfully decorated with shish-kebab structure via a self-induced crystallization technique. The topographical features and the mechanical properties of the composite scaffolds were characterized,and the biocompatibility of the material was assessed by using human osteogenic sarcoma osteoblasts(MG-63 cells). The carbon nanotube(CNT) concentration is found to affect the fiber diameter and mechanical properties of electrospun nanofibers and the periodic distance of the shish-kebab architecture. Cellular attachment and proliferation assays reveal that 0.5 wt% CNT-embedded PCL scaffold shows enhanced biocompatibility with MG-63 cells than their counterparts made of neat PCL, and the collagen-like nanotopology provided by the shish-kebab structure further facilitates the cell adhesion and proliferation. The superior interactions between cells and scaffolds demonstrate that the shish-kebab-structured CNTs/PCL nanofibers may be promising candidate for tissue engineering scaffold application.     

A novel thermoplastic polyurethane scaffold fabrication method based on injection foaming with water and supercritical carbon dioxide as coblowing agents

Injection foaming is an method for mass producing lightweight, foamed plastic components with excellent dimensional stability while using less material and energy. In this study, a novel injection foaming method employing supercritical CO₂ (scCO₂) and water as coblowing agents was developed to produce thermoplastic polyurethane (TPU) components with a uniform porous structure and no solid skin. Various characterization techniques were employed to investigate the cell morphology, crystallization behavior, and static and dynamic mechanical properties of solid injection molded samples, foamed samples using CO₂ or water as a single blowing agent, and foamed samples using both CO₂ and water as coblowing agents. When compared with CO₂ foamed samples, samples produced by the coblowing method exhibited much more uniform cell morphologies without a noticeable reduction in mechanical properties. Moreover, these TPU samples had almost no skin layer, which permitted the free transport of nutrients and waste throughout the samples. Such a mass‐produced, skin‐free structure is desirable in tissue engineering. In this study, the biocompatibility of the scaffolds was confirmed and the effect of these blowing agents on the TPU foaming behavior was studied. POLYM. ENG. SCI., 54:2947–2957, 2014. © 2014 Society of Plastics Engineers     

Semitransparent poly(styrene‐r‐maleic anhydride)/alumina nanocomposites for optical applications

This article presents the development and characterization of transparent poly(styrene‐r‐maleic anhydride) (SMA)/alumina nanocomposites for potential use in optical applications. Chemically treated spherical alumina nanoparticles were dispersed in an SMA matrix polymer via the solution and melt‐compounding methods to produce 2 wt % nanocomposites. Field emission scanning electron microscopy was used to examine the nanoparticle dispersion. When the solution method was used, nanoparticle reagglomeration occurred, despite the fairly good polymer wetting. However, through the coating of the alumina nanoparticles with a thin layer (ca. 20 nm) of low‐molecular‐weight SMA, reagglomeration was absent in the melt‐compounded samples, and this resulted in excellent nanoparticle dispersion. The resultant nanocomposites were semitransparent to visible light at a 2‐mm thickness with improved UV‐barrier properties. Their impact strengths, tensile strengths, and strains at break were slightly reduced compared with those of their neat resin counterpart, whereas a small enhancement in their moduli was achieved. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Novel foaming method to fabricate microcellular injection molded polycarbonate parts using sodium chloride and active carbon as nucleating agents

Microcellular injection molding can fabricate lightweight, dimensionally stable plastic parts while using less material and energy. This article investigates a new process using water vapor as the physical blowing agent and comparing two kinds of nucleating agents, namely, cubic sodium chloride (NaCl) and non‐uniform active carbon (AC). The effects of different nucleating agents on the surface roughness, mechanical properties, and microstructure of solid and foamed parts were characterized. Compared with typical microcellular injection molded parts, water vapor‐foamed polycarbonate (PC)/NaCl had a smooth surface comparable to that of solid parts, whereas foamed PC/AC had desirable specific mechanical properties as well as an attractive average weight reduction of 16.4 wt%. Low density and non‐uniform AC particles, used as a nucleating agent and reinforcement, improved the microcellular structure. Based on PC molecular weight measurement, the melt processing and water vapor‐foaming processes did induce a slight amount of thermal degradation and hydrolytic degradation, respectively. POLYM. ENG. SCI., 55:1634–1642, 2015. © 2014 Society of Plastics Engineers     

Injection molding quality control by integrating weight feedback into a cascade closed‐loop control system

Quality control plays a crucial role in injection molding control to meet stringent tolerance requirements and to facilitate automation. Previous work has tackled this challenging subject mainly through consistent machine operations or process variable control. In this paper, a direct quality feedback control system has been proposed and developed. The system has a cascade structure and combines both feedback and feedforward controls. An important quality index, namely, part weight, is measured in each molding cycle. The difference between this measurement and a quality target is used to adjust the mold separation at the process level. The mold separation is controlled via both a cycle‐to‐cycle mass‐based switchover point and a within‐cycle holding pressure control. Molding experiments have been conducted using different mold geometries and resins. Compared with the cavity pressure based control system currently used in industry, the control system in this study results in a significant improvement of both long‐term and short‐term consistencies in part quality. In addition, this direct quality feedback control has other benefits, such as 100% quality inspection and automatic process tuning. POLYM. ENG. SCI., 47:852–862, 2007. © 2007 Society of Plastics Engineers     

Ultrasonication‐Induced Modification of Hydroxyapatite Nanoparticles onto a 3D Porous Poly(lactic acid) Scaffold with Improved Mechanical Properties and Biocompatibility

A 3D porous poly(lactic acid) (PLA) scaffold with high porosity and well‐connected pores is fabricated using a vacuum‐assisted solvent casting technique. Its surface is modified with hydroxyapatite (HA) nanoparticles using ultrasonication to prepare an HA‐modified PLA/HA scaffold. For reference, an HA‐blended (b‐PLA‐HA) scaffold is fabricated via the solution blending method. The morphology, porosity, hydrophilicity, swelling ratio, mechanical properties, and cell viability of the PLA, b‐PLA‐HA, and PLA/HA scaffolds are systematically studied. The results show that HA nanoparticles are successfully introduced onto the surface of the PLA/HA scaffold, and strong interactions occur between the HA nanoparticles and the PLA matrix. The PLA/HA scaffold still has a high porosity of more than 85% after ultrasonication. The hydrophilicity and mechanical properties of the PLA/HA scaffold are significantly higher than those of the PLA and b‐PLA‐HA scaffolds. Compared with the PLA and b‐PLA‐HA scaffolds, the attachment and growth of mouse embryonic osteoblasts cells (MC3T3‐E1) cultured on the PLA/HA scaffold significantly improve, due to most HA nanoparticles on the surface, resulting in a good and direct interaction between the cells and the scaffold. Therefore, the PLA/HA scaffold possesses great potential to be used as a tissue engineering scaffold.     

Mathematical modeling and numerical simulation of cell growth in injection molding of microcellular plastics

A mathematical formulation and numerical simulation for non‐isothermal cell growth during the post‐filling stage of microcellular injection molding have been developed. The numerical implementation solves the energy equation, the continuity equation, and a group of equations that describe the mass diffusion of dissolved gas and growth of micro‐cells in a microcellular injection molded part. The “unit‐cell” model employed in this study takes into account the effects of injection and packing pressures, melt and mold temperatures, and super‐critical fluid content on the material properties of the polymer‐gas solution and the cell growth. The material system studied is a microcellular injection molded polyamide 6 (PA‐6) resin. Two Arrhenius‐type equations are used to estimate the coefficients of mass diffusion and solubility for the polymer‐gas solution as functions of temperature. The dependence of the surface tension on the temperature is also included in this study. The numerical results in terms of cell size across the sprue diameter agree fairly well with the experimental observation. The predicted pressure profile at the sprue location has also been found to be in good agreement with the dynamics of the cell growth. Whereas for conventional injection molding the pressure of the system tends to decay monotonously, the pressure profile in microcellular injection molding exhibits an initial decay resulting from cooling and the absence of packing followed by an increase due to cell growth that expands the polymer‐gas solution and helps to pack out the mold uniformly. Polym. Eng. Sci. 44:2274–2287, 2004. © 2004 Society of Plastics Engineers.