Microcellular Injection Molding Research Articles

Polylactide/thermoplastic polyurethane/polytetrafluoroethylene nanocomposites with in situ fibrillated polytetrafluoroethylene and nanomechanical properties at the interface using atomic force microscopy

This paper presents the characterization of in situ tri-polymer nanofibrillar composites (istp-NFCs) of polylactide (PLA)/thermoplastic polyurethane (TPU)/polytetrafluoroethylene (PTFE), which were fabricated using the in-situ generation of PTFE fibrils via melt blending with a twin-screw extruder (TSE). Nanofibrillated PTFE and its influence on ductility, crystallization, viscoelasticity and microcellular injection molding (MIM) of PLA/TPU blends were investigated. The immiscibility between PLA, TPU and PTFE were confirmed by scanning electron microscopy (SEM). The fibrillated PTFE in the PLA domain enhanced the distribution of dispersed spherical TPU particles. Significant enhancements in the crystallization behavior of PLA/TPU/PTFE were observed by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarized optical microscopy (POM). Tensile and rheological results revealed that the ductility increased 20 times with a brittle–ductile transition, and the viscoelastic properties of PLA matrix were dramatically changed by TPU and fibrillated PTFE. The local surface nanomechanical property variation of the PLA/TPU blend was examined by peak force quantitative nanomechanics (PQNM) based on atomic force microscopy (AFM). The average elastic modulus and deformation at the PLA/TPU interface varied from 28.2 ± 1.1 GPa for PLA to 2.3 ± 0.2 GPa for TPU, and from 7.3 ± 0.5 nm for PLA to 61.3 ± 3.8 nm for TPU, respectively. Furthermore, the thickness of the interface between PLA and TPU was around 208.3 ± 14.8 nm. MIM experiments confirmed that the presence of PTFE fibrils dramatically improved the foamability behavior of PLA.

Applications of Core Retraction in Manufacturing Low-Density Polypropylene Foams with Microcellular Injection Molding

Without modifying existing part and mold designs, the conventional microcellular injection molding (MIM) process can typically save about 5–10% material without encountering problems such as incomplete filling, excessive shrinkage, or deteriorating microstructure and mechanical properties. In this study core retraction was used in combination with the MIM process to produce thick polypropylene (PP) parts (up to 7.6 mm thick) with high density reductions of 30% and 55%. The cavity volume was modified by changing the retraction distance, which enabled control of density reductions. The lowest densities were achieved with this core retraction-aided microcellular injection molding (CR-MIM) process, the results of which could not have been achieved by the conventional MIM process alone. The effects of delay time in core retraction and weight reduction on the microstructure of the core and skin layers were investigated. It was shown that the CR-MIM process yielded better microstructure and tensile properties than the conventional MIM process. Use of core retraction also yielded more consistent densities and tensile properties throughout the length of the foamed parts.

Supercritical Fluid Assisted Microcellular Injection Molding and the Melt Foaming Behaviors

Supercritical Fluid Assisted Microcellular Injection Molding and the Melt Foaming Behaviors

Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding.

The processability of injection molding ultra-high molecular weight polyethylene (UHMWPE) was improved by introducing supercritical nitrogen (scN2) or supercritical carbon dioxide (scCO2) into the polymer melt, which decreased its viscosity and injection pressure while reducing the risk of degradation. When using the special full-shot option of microcellular injection molding (MIM), it was found that the required injection pressure decreased by up to 30% and 35% when scCO2 and scN2 were used, respectively. The mechanical properties in terms of tensile strength, Young’s modulus, and elongation-at-break of the supercritical fluid (SCF)-loaded samples were examined. The thermal and rheological properties of regular and SCF-loaded samples were analyzed using differential scanning calorimetry (DSC) and parallel-plate rheometry, respectively. The results showed that the temperature dependence of UHMWPE was very low, suggesting that increasing the processing temperature is not a viable method for reducing injection pressure or improving processability. Moreover, the use of scN2 and scCO2 with UHMWPE and MIM retained the high molecular weight, and thus the mechanical properties, of the polymer, while regular injection molding led to signs of degradation.

A novel gas-assisted microcellular injection molding method for preparing lightweight foams with superior surface appearance and enhanced mechanical performance

This paper proposed a novel gas-assisted microcellular injection molding (GAMIM) method by combining the gas-assisted injection molding (GAIM) with the microcellular injection molding (MIM). Firstly, under the assistance of the high-pressure gas from the GAIM, the amount of the polymer melt required for fully filling the mold cavity is reduced in the GAMIM compared with that in the MIM, thus leading to a high weight reduction of the foamed part. Secondly, the high-pressure assisted gas from the GAIM can dissolve all the cells generated in the melt filling stage back into the polymer melt, thus improving the molded part's surface appearance by eliminating the surface sliver marks. Thirdly, the secondary foaming process in a steady state triggered by releasing the high-pressure assisted gas makes the foamed part have a fine cellular structure and a compact solid skin layer, which can help to enhance the part's mechanical properties. In order to verify the effectiveness of the GAMIM, comparison experiments of the MIM and the GAMIM were conducted. The results demonstrate that the GAMIM can not only significantly increase the weight reduction, but also greatly improve the surface appearance and mechanical properties of the foamed part.

Effects of nanoclays on the thermal stability and flame retardancy of microcellular thermoplastic polyurethane nanocomposites

Microcellular nanocomposite foams based on thermoplastic polyurethane (TPU) and organically modified layered clays were prepared via an efficient foaming method, microcellular injection molding (MIM). The morphology of the nanoclays was characterized by X‐ray diffraction (XRD) and transmission electron microscopy. The results revealed that the layered nanoclays in the nanocomposite foams achieved a well dispersed morphology within the cell walls due to the MIM foaming process. Postburn SEM images showed that TPU/NC10 foam (with 10 wt% nanoclay) was able to maintain its foam porosity, original shape, and microcellular structure due to the synergistic effects of its microcellular structure and the fully dispersed nanoclays. Thermal stability and flame resistance were assessed by thermogravimetric analysis (TGA) and microscale combustion calorimetry, respectively. TGA results indicated that the nanocomposite foams had a significant decline in the thermal degradation rate compared to the unfilled TPU/NR foam. The flame resistance tests showed that the heat release capacity (HRC) of TPU/NC10 foams was substantially lower (27% lower) than that of the unfilled TPU/NR foam. The char yield of the TPU/NC10 foam was 19.14%, which was almost 18 times higher than that of the unfilled TPU/NR foam. With the presence of the microcellular structure, the HRC values decreased from 266.8 J/g k for solid TPU/NC10 to 235.5 J/g k for TPU/NC10 foams. This work combines two flame retardant mechanisms—the thermal barrier effect of the nanoclay layers and the microcellular self‐intumescing system—in the nanocomposite foams. Our study also provides substantial motivation to produce novel, environmentally‐benign, and flame‐retardant TPU foam using microcellular injection molding. POLYM. COMPOS., 39:E1429–E1440, 2018. © 2017 Society of Plastics Engineers

Formation mechanism of porous structure in plastic parts injected by microcellular injection molding technology with variable mold temperature

Microcellular Injection Molding (MIM) process combined with rapid heat cycle molding (RHCM) technique can fabricate microcellular foamed plastic parts with excellent surface appearance. The porous structure influences the foamed parts’ performances significantly and its evolution process is very complex in RHCM/MIM process. The temperature field and flow field play a very crucial role in determining the porous structure of the foamed parts. In this study, we developed a non-isothermal mathematical model based on the two-phase model to calculate the temperature field and flow field in RHCM/MIM process. Particularly, the coupling heat transfer between the injection mold and the polymer melt is considered by using the implicit domain coupled algorithm. By comparing with the experimental results, it is found that the developed model can predict the temperature field very accurately. The thermal response characteristics in RHCM/MIM process were further analyzed. By combining the simulation results with the cellular morphology obtained by experiments, we deeply analyzed the formation mechanisms of the cellular morphology in RHCM/MIM process.

Microcellular Foam Injection Molding of Thermoplastics Using Green Physical Blowing Agent

The microcellular injection molding technology, commercially offered by Trexel Inc. and other manufacturers, is primarily a close cell foaming technique. This process is capable of offering light weight non-porous thermoplastics moldings. The foaming of thermoplastics with open cellular morphology has got various high end applications among others like tissue engineering and membrane separation. Some of the researchers were successful in synthesis of open cellular thermoplastics at laboratory scale via solid state batch process. The growing demand for microporous thermoplastics, especially the biodegradable plastics (e.g. Polylactic acid), motivated the researchers develop a specialized microcellular injection molding process for processing of open cell thermoplastics using physical blowing agents such as supercritical nitrogen or carbon dioxide gas. A brief of theoretical and conceptual treatment of microcellular injection molding is presented.

Investigation on bubble morphological evolution and plastic part surface quality of microcellular injection molding process based on a multiphase-solid coupled heat transfer model

The surface of plastic parts molded by Microcellular Injection Molding (MIM) process usually has silver marks, spiral lines, pits and other defects. Rapid Heat Cycle Molding (RHCM) technology has become an efficient technology to eliminate these defects by controlling the heat and mass transfer in the MIM mold. However, the influence of heat transfer on the evolution of multiphase composite (polymer, supercritical fluid, air) in the mold cavity is still not clarified. To understand it, a new non-isothermal transient multiphase model based on finite volume method was established, and the Implicit Domain Coupling Algorithm (IDCA) was used to synchronously calculate the temperature field of mold and cavity regions. A compound experiment of RHCM and MIM (RHCM/MIM) with mold temperature control realized by electrical heating and water cooling was performed. Based on the simulation and experiment results, the transient flow behavior of foam composite in mold cavity was analyzed. The effect of mold temperature on the deformation, bursting and collapsing process of bubbles in fountain flow field was revealed. The phenomena of bubble bursting delay in filling stage of RHCM/MIM process were discovered, and the formation mechanism of the surface defects of RHCM/MIM plastic part was also revealed.

Recent Patents on the Equipment of Microcellular Injection Molding Machine

Background: In recent years, as microcellular plastics have been used in different economic and life fields, microcellular injection molding machine has been paid more and more attention and been developed into various structures. Objective: The purpose of this article is to provide an overview of the supercritical fluid injecting system, mixing, and nucleating equipment of the microcellular injection molding, and explain how the technologies influence the product's quality and application. Methods: The paper reviews various patents and research developments about supercritical fluid injecting system, mixing, and nucleating equipment of microcellular injection molding machine, and analyses their effects on cell diameter, weight reduction, and product appearance. Results: Current and future developments of the injection apparatus, nozzle, and mold apparatus of microcellular injection molding are finally provided to improve the microcellular injection molded product's quality. Conclusion: The considerable attention has been paid on the microcellular injection technology. The recent patents and technologies have provided help to apply microcellular injection molding in a wider range, however how to inject physical agent into resin melt, mix them, and form fine cell structure is still difficult. For further enhancement, more devices should be invented in order to solve or simplify the above difficulties. Keywords: Injection molding machine, microcellular, mixing, patent, structure, supercritical fluid.

Development of PLA/cellulosic fiber composite foams using injection molding: Crystallization and foaming behaviors

Poly(lactic acid) (PLA) and northern bleached softwood kraft (NBSK) or black spruce medium density fiberboard (MDF) fibers were melt compounded using a co-rotating twin screw extruder and subsequently microcellular injection molded. Poly(ethlylene glycol) (PEG) was used as a lubricant. The microcellular structure, thermal properties, and crystallization behaviors were characterized using scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, and wide angle X-ray diffraction. Results show that cellulosic fibers, acting as crystal nucleating agents, increased the crystallization temperature and the crystallinity and decreased the crystallization half time. The dissolved N2, the shear stress, and biaxial stretching during foaming also enhanced the crystallinity of PLA. Compared to PLA/PEG, a finer and more uniform cell structure was achieved in the cellulosic fiber composite foams. The improved foam morphology was attributed to the cell nucleating effects of the fibers and the increased melt strength by the addition of cellulosic fibers and by the gas- and fiber- induced crystallization.

Fabrication of highly expanded thermoplastic polyurethane foams using microcellular injection molding and gas‐laden pellets

Thermoplastic polyurethane (TPU) is one of the most widely used and versatile thermoplastic materials. TPU foams have been extensively applied in various industries including the furniture, automotive, sportswear, and packaging industries. In this study, two methods of producing highly expanded TPU injection molded foam were investigated: (1) microcellular injection molding (MIM) with N2 as a blowing agent, and (2) a novel gas‐laden pellet/MIM combined process with N2 and CO2 as co‐blowing agents. Two designs of experiments (DOEs) were performed to learn the influences of key processing parameters and to optimize foam quality. By using N2 and CO2 as co‐blowing agents, a bulk density as low as 0.20 g/cm3 was successfully achieved with a hysteresis compression loss of 24.4%. POLYM. ENG. SCI., 55:2643–2652, 2015. © 2015 Society of Plastics Engineers

Enhancing Nanofiller Dispersion Through Prefoaming and Its Effect on the Microstructure of Microcellular Injection Molded Polylactic Acid/Clay Nanocomposites

Promoting better dispersion of nanofillers is crucial in producing high performance nanocomposite foams. For the effects of nanofillers on controlling foam structures and mechanical properties to be enhanced, prefoamed pellets were produced via supercritical fluid (SCF) extrusion foaming and subsequently microcellular injection molded. Structural, thermal, and mechanical properties of the prefoamed pellets and resultant polylactic acid (PLA)/clay nanocomposite foams were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy, and tensile testing. The diffraction peaks in the XRD plots of extruded prefoamed pellets were observed to shift to lower angles, indicating that prefoaming improved the intercalation of clay platelets. The driving forces behind the intercalation and dispersion of clay were (i) enhanced diffusion via the plasticizing effect of SCF and (ii) the phase change of SCF from the supercritical to th...

Symulacje komputerowe procesu wtryskiwania porujacego

Computer simulations of the MuCell — microcellular injection moulding process were carried out using Moldflow Plastics Insight 5.0 software. An analysis of the effect of plastic type and wall thickness of the moulded part on its porous structure, internal stress and strain as well as the value of volumetric shrinkage was performed. The results of these simulations were compared with the relationships for cellular injection moulding process using chemical blowing agent. It was found that the cavity height has a significant influence on the distribution of temperature and pressure of liquid plastic as well as size and distribution of pores in the moulded parts. Application of this injection moulding method can significantly reduce the holding time and pressure and, in effect, the time of the injection cycle.

Morphology and Mechanical Characterization of ABS Foamed by Microcellular Injection Molding

In the present work ABS was used to inject cylindrical test bars, obtaining solid and foamed specimens. By varying the gas content, two levels of weight reduction were achieved. Morphology analysis revealed the presence of solid skin-foamed core structure in foamed samples. SEM micrographs showed a nucleus zone having bigger cells and irregular cell distribution, surrounded by a microcellular area with finer cell structure. Foamed bars with 10% and 17% of weight reduction presented similar values of cell size, cell density and solid skin thickness. On the other hand, results provided by simulation software were consistent with the experimental analysis. Mechanical properties were determined through tensile tests. Tensile strength and elastic modulus gradually decreased with decreasing apparent density. Experimental results were related to relative density and morphology parameters, and prediction models were employed to compare the estimated values to the experimental data.

Influence of Mold Cavity Gas Pressure on the Surface Quality of a Microcellular Injection Molding Part

Influence of Mold Cavity Gas Pressure on the Surface Quality of a Microcellular Injection Molding Part

Methodology to Analyze the Influence of Microcellular Injection Molding on Mechanical Properties with Samples Obtained Directly of An Industrial Component

Microcellular injection molding is a process that offers numerous benefits thanks to the internal structure generated, thus many applications are being arising in different fields, especially home appliance. In spite of it, when changing the manufacturing process of a component from conventional injection molding to microcellular injection molding, it is necessary to ensure the mechanical properties of the component. In this paper a study of the mechanical behavior of samples obtained directly from a manufactured component both by conventional injection molding and microcellular injection molding is carried out. This kind of samples take into account the process conditions under the final component is processed and the influence of these conditions on the mechanical properties. For this purpose special devices have been developed taking into account the characteristics of the component and the samples obtained. An X-ray 3D computed tomography is also carried out to validate the internal structure of the microcellular injection molded component. Tensile properties are reduced between 20–22% regarding flow direction when using microcellular injection molding. Impact properties are reduced up to 27%. However, flexural properties reduction for samples processed by microcellular injection is only 6.8%, so microcellular injection molding arises as a very suitable process for components working under this kind of load. Molding and impact tests are carried out by using specially developed devices.

Influence of relative low gas counter pressure on melt foaming behavior and surface quality of molded parts in microcellular injection molding process

A complex medical instrument exterior shell was chosen as a studying object to investigate the influence of relative low (<10 MPa) gas counter pressure process on microcellular injection molding process. The gas counter pressure microcellular injection mould and related experiments were designed. The relative low gas counter pressure under which the melt can foam was mainly considered to improve the surface quality of molded parts without significantly prolonging production cycle. The effects of the gas counter pressure parameters on the surface quality, cell morphology, and cell density of microcellular parts were studied. A critical melt flow front pressure to effectively eliminate surface swirl marks of microcellular injection molded part was proposed. The mechanism of the influence of gas counter pressure process on foaming behavior of melt in filling process was analyzed. The reasonable gas counter pressure parameters to improve surface quality of products without significantly increasing molding cycle were obtained. By using the obtained reasonable gas counter pressure parameters, a sound microcellular injection molded product was injected finally.

Analysis of the Influence of Microcellular Injection Molding on the Environmental Impact of an Industrial Component

Microcellular injection molding is a process that offers numerous benefits due to the internal structure generated; thus, many applications are currently being developed in different fields, especially home appliances. In spite of the advantages, when changing the manufacturing process from conventional to microcellular injection molding, it is necessary to analyze its new mechanical properties and the environmental impact of the component. This paper presents a deep study of the environmental behavior of a manufactured component by both conventional and microcellular injection molding. Environmental impact will be evaluated performing a life cycle assessment. Functionality of the component will be also evaluated with samples obtained from manufactured components, to make sure that the mechanical requirements are fulfilled when using microcellular injection molding. For this purpose a special device has been developed to measure the flexural modulus. With a 16% weight reduction, the variation of flexural properties in the microcellular injected components is only 6.8%. Although the energy consumption of the microcellular injection process slightly increases, there is an overall reduction of the environmental burden of 14.9% in ReCiPe and 15% in carbon footprint. Therefore, MuCell technology can be considered as a green manufacturing technology for components working mainly under flexural load.

Influence of Processing Parameters on Molding Process in Microcellular Injection Molding

In order to reduce weight and improve the dimensional accuracy of plastic products, microcellular injection molding technology is applied more and more in automotive. There are several defects are paid special attention by vehicle manufacturers, such as warpage and cell radius. This research investigated the effects of processing parameters in microcellular injection molding. The study investigated the influence of two kinds of gas foamers (CO2 and N2) on microcellular injection molding process, and the following processing parameters such as temperature, time, pressure and gas controlling were considered to research. Design of experiments was employed to perform the experiments. According to experimental analysis, not only the significant processing parameters to warpage were found out, but also the interactions between each factor were investigated. The results of this study could provide microcellular injection molding process design with theoretical basis and practical guidance.