Microcellular Injection Research Articles

Impact of Runner Size, Gate Size, Polymer Viscosity, and Molding Process on Filling Imbalance in Geometrically Balanced Multi-Cavity Injection Molding.

The injection molding process is one of the most widely used methods for polymer processing in mass production. Three critical factors in this process include the type of polymer, injection molding machines, and processing molds. Polypropylene (PP) is a widely used semi-crystalline polymer due to its favorable flow characteristics, including a high melt flow index and the absence of a need for a mold temperature controller. Additionally, PP exhibits good elongation and toughness, making it suitable for applications such as box hinges. However, its tensile strength is a limitation; thus, glass fiber is added to enhance this property. It is important to note that the incorporation of glass fiber increases the viscosity of PP. Multi-cavity molds are commonly employed to achieve cost-effective and efficient mass production. The filling challenges associated with geometrically balanced layouts are well documented in the literature. These issues arise due to the varying shear rates of the melt in the runner. High shear rate melts lead to high melt temperatures, which decrease melt viscosity and facilitate easier flow. Consequently, this results in an imbalanced filling phenomenon. This study examines the impact of runner size, gate size, polymer viscosity, and molding process on the filling imbalanced problem in multi-cavity injection molds. Tensile bar injection molding was performed using conventional injection molding (CIM) and microcellular injection molding (MIM) techniques. The tensile properties of the imbalanced multi-cavity molds were analyzed. Flow length within the cavity served as an indicator of the filling imbalance. Additionally, computer simulations were conducted to assess the shear rate's effect on the runner's melt temperature. The results indicated that small runner and gate sizes exacerbate the filling imbalance. Conversely, glass fiber-filled polymer composites also contribute to increased filling imbalance. However, foamed polymers can mitigate the filling imbalance phenomenon.

High-Quality Foaming and Weight Reduction in Microcellular-Injection-Molded Polycarbonate Using Supercritical Fluid Carbon Dioxide under Gas Counter Pressure.

Microcellular injection molding (MuCell®) using supercritical fluid (SCF) as a foaming agent to achieve weight reduction has become popular in carbon emission reduction. In the typical MuCell® process, SCF N2 is commonly used. Although SCF CO2 exhibits high solubility and can achieve a high weight reduction, controlling the foaming is not easy, and its foaming cells are usually larger and less uniform, which limits its industrial application. Our previous studies have shown that gas counter pressure (GCP) can improve the foaming quality effectively. Here, we investigated whether or not the CO2 SCF foaming quality could be improved, and weight reduction was achieved for polycarbonate (PC) material. This is quite important for the electronics industry, in which most of the housing for devices is made of PC materials. MuCell® was subjected to molding experiments using the parameters of the SCF dosage, melt temperature, mold temperature, and injection speed. The results revealed that using CO2 gas for the PC material can reduce the size of microcellular cells to 40 µm and increase the cell densities to 3.97 × 106 cells/cm3. Using GCP significantly improved the microcellular injection-molded parts by reducing the cell size to 20.9 µm (a 45.41% improvement) and increasing the cell density to 8.04 × 106 cells/cm3 (a 102.48% improvement). However, implementing GCP may slightly decrease the target weight reduction. This study reveals that microcellular injection molding of PC parts using SCF CO2 can achieve high-quality foaming and reduce the weight by about 30%.

Multi-objective optimization of microcellular injection molding process parameters to reduce energy consumption and improve product quality

Multi-objective optimization of microcellular injection molding process parameters to reduce energy consumption and improve product quality

High-strength, impact-resistant PP/PTFE composite foam with enhanced surface appearance achieved through mold-opening microcellular injection molding

Poor surface appearance and decreased mechanical performance are critical factors limiting the broad application of Microcellular injection molding (MIM) in foamed polymer products. Herein, in-situ polytetrafluoroethylene (PTFE) nanofibrils reinforced polypropylene (PP) foams with high strength, impact resistance, and enhanced surface quality were prepared by mold opening microcellular injection molding (MOMIM). PTFE nanofibers with different diameters were introduced into the PP matrix via twin screw extrusion and significantly enhanced the crystallinity and viscoelasticity of the PP matrix with the enhancement being more pronounced for the finer PTFE fibers. High-density oriented cellular structures were formed in the MOMIM PP/PTFE foams owing to the heterogeneous nucleation of PTFE and the stretching effect of the mold opening process. The optimum MOMIM PP/PTFE foam fabricated possesses a high cell orientation angle close to 90° and a large cell aspect ratio of 5.3, which reached a 34.37 % increase in tensile strength and a 73.08 % increase in impact strength owing to the synergetic effects of the PTFE network and the highly oriented fine cell structure, which effectively dissipated the tensile stress and impact stress by cell wall twisting and folding deformation. Furthermore, the MOMIM PP/PTFE foam showed a significantly enhanced surface quality compared to the MIM foam due to the reduced dragging flow in MOMIM, which greatly hindered cell rupture on the foam surface. Therefore, this work provides insights into the MOMIM of polymer composites containing fibrous fillers and the enhancement of tensile properties, impact resistance, and surface quality of foam products.

Injection molded lightweight composites from tea-stem fiber and polypropylene: Effect of fiber loading on forming properties and cell structure

China boasts abundant tea resources and substantial production output. To facilitate the high-value utilization of by-products generated during tea production, such as tea stems, thereby reducing resource wastage, this study pioneered the preparation of tea-stem fiber-reinforced polypropylene (TF/PP) composites through microcellular injection and conventional molding processes. The effects of tea-stem fiber (TF) content on the formability of foamed and unfoamed composites were compared and analyzed. The findings reveal that the inclusion of TFs fostered the crystallization of polymer chains, decreased the flowability of the composites in their molten state, and enhanced the density, surface hydrophilicity, and thermal stability of the molded composites. In the unfoamed state, composites with 20 wt%, 40 wt%, and 30 wt% TF content exhibited optimal tensile (35.92 MPa), flexural (58.43 MPa), and impact (6.49 KJ/m2) strength, respectively. Upon foaming, composites with TF contents of 30 wt%, 40 wt%, and 30 wt% demonstrated superior tensile (32.84 MPa), flexural (51.31 MPa), and impact (6.94 KJ/m2) strength, respectively. A 30 % TF content in foamed composites yielded the best reinforcement effect, creating an ideal balance of internal interactions. This resulted in mechanical properties that were close to or even exceeded those of unfoamed PP while reducing the density by nearly 6 %. The results from this study contribute towards the high-value application of tea by-products, promoting the development of the global tea economy.

Fabrication of lightweight polyethersulfone foams with enhanced surface quality and high specific flexural properties using micro‐opening microcellular injection molding

AbstractPolyethersulfone (PES) is distinguished by its exceptional comprehensive properties, making it a highly valued engineering plastic employed in various cutting‐edge fields. However, the high cost of PES limits its wider application in industrial fields. Considering the cost‐reduction advantages of polymer foaming, PES foams were successfully prepared in this work using microcellular injection molding (MIM) and micro‐opening microcellular injection molding (MOMIM) foaming methods. Compared with the MIM foaming method, the PES foam samples prepared by the MOMIM foaming method have better microcellular structure and surface quality. As the micro‐opening distance increased, the cell density of the PES foam samples showed a gradual rise before plateauing, with the cell size remaining relatively constant. As a result, the density of the PES foam sample dropped to 1.07 g/cm3, which is 80.5% of the solid PES sample. Meanwhile, the mechanical properties of the PES foam samples remained robust, with both the specific flexural modulus and the specific flexural strength showing increases as the micro‐opening distance increased. The knowledge obtained from this study provides a method for preparing high‐performance PES foam materials, which will facilitate the application of PES in more civilian fields.

Crystallinity and mechanical properties of polypropylene products foamed by microcellular injection molding process

AbstractMicrocellular injection molding (MIM) is a pivotal technique for lightweight molding. Polypropylene (PP) is widely used in MIM due to light weight and easy processing. The crystallinity of foamed PP products influences the load‐bearing structures and consequently mechanical properties. To address the deterioration of mechanical properties caused by internal porous structure, a novel method is proposed to regulate the crystallinity by optimizing process parameters, thereby improving mechanical properties. Wide‐angle x‐ray diffraction, mechanical testing, and melt flow simulation are applied to investigate the influence of process parameters on crystallinity and mechanical properties. Parameters are further optimized by establishing a multivariate nonlinear regression model for process parameters and crystallinity. The results show that increasing melt temperature, injection rate, mold temperature, and cooling time stimulate crystallization, enhancing strength and decreasing elongation at break. Conversely, high injection ratio of supercritical fluid obstructs crystallization, reducing strength, and increasing elongation at break. The crystallinity increases by 20.81%; the corresponding tensile, flexural, and shear strength and corresponding modulus increases by 5.39% and 10.28%, 7.60% and 18.50%, 5.00% and 11.37%, and elongation at break decreased by 5.19% through the optimization of regression model. The proposed method is feasible to upgrade the mechanical properties of foamed PP products.

Morphology and properties of PC/ABS notebook computer shell prepared by a combined in-mold decoration and microcellular injection molding process

In this paper, the PC/ABS notebook computer shells with PET film were prepared using a combined in-mold decoration and microcellular injection molding (IMD/MIM) process. Then the surface quality, morphology, impact toughness and wear resistance were investigated. The results shown that the surface quality is improved by IMD/MIM technology. The three dimensional surface roughness value (Sa) was about 1/3 of that of MIM parts. The cross-section of IMD/MIM samples have three layers. The first layer is a PET film decoration layer with about 132 µm thickness, followed by skin layer (about 22 µm) and foam layer. The range of thickness of the foam layer for the different specimens is about 7803 µm–7820 µm.The cell diameter is between 0.532 µm and 0.638 µm, which is about 2 times than that of MIM parts. Compared with MIM parts, the shape of the bubbles of IMD/MIM parts is ellipsoid with thinner wall. Moreover, the impact toughness of IMD/MIM parts are higher. The best impact toughness is 21.02 KJ/m2 within the scope of this experiment, which is 74% higher than the corresponding MIM part under the same foaming process. This may due to the PET film and micropores could block the initiation of the tip crack. Meanwhile, the IMD/MIM parts have excellent wear resistance property. In most cases, the wear rate of IMD/MIM parts is about 36.6% of MIM part under the same condition.

Study of Foaming Morphology in Microcellular Injection Molded TPU/MWCNT Composites under Gas Counter Pressure

Due to the global environmental issues, microcellular injection molding (MuCell) process with the advantages of the energy saving, environmentally friendly and light weighting has become more and more popular in industry. In order to improve the common defects of uneven cell size and cell rupture in MuCell, this study aims to ameliorate the issues of excessively large open-cell structure and density reduction caused by cell rupture in MuCell with thermoplastic polyurethane (TPU). TPU 1185A supplied by BASF was selected as the material, to which multi-walled carbon nanotubes (MWCNT) were added and a gas counter pressure (GCP) control was applied, subsequently. The cell structure change was observed via scanning electron microscope (SEM). The results showed that adding nanotubes to TPU can increase the viscosity of the polymer melt and lower the average cell size, which will significantly reduce the number of cells with a diameter greater than 100 μm. With the GCP technique employed, the overall cell size was further reduced as well as the number of cells with a diameter greater than 100 μm, and the cell density was further increased.

Fabrication of lightweight polymer products with excellent mechanical properties using a novel double-sided in-mold decoration combined with fiber-reinforced microcellular injection molding process

This paper introduces a novel molding process known as the double-sided in-mold decoration combined with fiber-reinforced microcellular injection molding (DS-IMD/FR-MIM) process. This process utilizes supercritical fluids to grow a microcellular structure for a lightweight product and adds glass fibers into polymer particles to reinforce mechanical properties. Concurrently, double-sided in-mold decoration (DS-IMD) technique provides a symmetric temperature field for melts during filling stage and high surface quality for parts after cooling. Tensile splines are simulated and tested to examine the impacts of various in-mold decoration methods on the melt temperature, freezing layer fraction, fiber orientation, cellular morphology and distribution, and mechanical properties. Applied splines include undecorated and fiber-reinforced microcellular injection molding (FR-MIM) spline, single-sided in-mold decoration combined with fiber-reinforced microcellular injection molding (SS-IMD/FR-MIM) spline, and DS-IMD/FR-MIM spline. Additionally, a flexural spline is used to test its flexural strength and surface quality. The results show that DS-IMD/FR-MIM process provides slower freezing speed and better foaming effect than FR-MIM process. It also generates more symmetric cellular distribution and more orderly fiber distribution than SS-IMD/FR-MIM process. DS-IMD/FR-MIM splines have high surface quality on decorated surfaces and 10.2 % weight loss. By comparison with FR-MIM and SS-IMD/FR-MIM splines, DS-IMD/FR-MIM splines increase an elongation at break by 24.3 % and 14.8 %, respectively. This paper contributes a feasible approach and theoretical groundwork for the fabrication of lightweight products with excellent mechanical strength and appearance.

Effect of supercritical nitrogen content on the weld lines and mechanical properties of microcellular injection molded double gate PP/Al parts

AbstractInjection molding products made of aluminum flakes and polymer blends exhibit a distinctive esthetic effect. However, during the filling process, the melt flows in different directions converge and collide, resulting in the flop effect of the aluminum flake and consequent weld line formation. Herein, microcellular injection molding (MIM) was employed to fabricate polypropylene/aluminum flakes (PP/Al) composite foamed parts with distinct weld lines using supercritical nitrogen (scN2) as the physical blowing agent. The scN2 content has a significant effect on cell diameter and cell density. When the scN2 content was 0.6%, the weld line width of the foamed part was 13.03 μm, while it was 30.41 μm for the solid counterpart due to the expansion and rupture of cells in the flow front during filling. Moreover, the orientation of Al flakes was mostly along the flow direction for the foamed parts, while it was generally aligned perpendicular to the flow direction for solid parts in the weld line region. In addition, the flexural modulus of foamed parts was increased by 29% compared with the solid parts, although the tensile strength was reduced by 18% due to the alignment of Al flakes and the stress concentration on the cell walls. Therefore, this work provides insight into the improvement of flexural property and the mitigation of weld lines for injection molded composite parts using MIM.

The Cellular Structure and Mechanical Properties of Polypropylene/Nano-CaCO3/Ethylene-propylene-diene-monomer Composites Prepared by an In-Mold-Decoration/Microcellular-Injection-Molding Process.

Polypropylene (PP)-composite foams were prepared by a combination process of microcellular injection molding (MIM) and in-mold decoration (IMD). The effect of ethylene propylene diene monomer (EPDM) on the crystallization properties, rheological properties, microstructure, and mechanical properties of PP-composite foams was studied. The effect of the additives on the strength and toughness of PP-composite foam as determined by the multiscale simulation method is discussed. The results showed that an appropriate amount of EPDM was beneficial to the cell growth and toughening of the PP blends. When the content of EPDM was 15 wt%, the PP-composite foams obtained the minimum cellular size, the maximum cellular density, and the best impact toughness. At the same time, the mesoscopic simulation shows that the stress concentration is the smallest, which indicates that 15 wt% EPDM has the best toughening effect in these composite materials.

Rapid mold temperature rising method for PEEK microcellular injection molding based on induction heating

As a high-performance polymer, Polyether ether ketone (PEEK) and its injection-molded products are widely used as an ideal metal replacement. The production of ideal PEEK injection molded products, including microcellular injection molded, requires the support of high mold temperatures that can be maintained. Given this, a rapid mold temperature rising technology based on induction heating was designed to take advantage of the ultra-high heating rate of induction heating, which effectively met rapid increase and maintenance of high mold temperatures for PEEK injection molding. Both finite element analysis and experimental results showed, compared to the electric heating method, the induction heating method increased the efficiency to 5600% when the mold temperature rise from 150 to 180 °C compared to electric heating. All heating methods were used for the validation tests of injection molding PEEK, microporous injection molding PEEK, and micro-open microporous injection molding PEEK. The experimental results showed that the surface quality of PEEK samples molded by induction heating was 9.8%–61.2% higher than that of the oil heater heating, while the mechanical properties were more than 18%. Besides, the adaptability to longer holding times for micro-open microcellular injection molding suggested it can effectively broaden the possibilities of developing higher-performance PEEK foam products. The pioneering design and analysis of heating methods in this work had important guidance for promoting high-performance polymer injection molding and the lightweight of its foamed products.

Cellular distribution and warpage deformation in double-sided in-mold decoration combined with microcellular injection molding process

Cellular distribution and warpage deformation in double-sided in-mold decoration combined with microcellular injection molding process

Microcellular injection molding of foamed engineering plastic parts with high dimensional accuracy

AbstractPolyformaldehyde (POM) and Polyamide 66 (PA66) are engineering plastics with excellent mechanical properties and thermal stability. Producing microcellular injection molded POM and PA66 parts with high dimensional accuracy would be beneficial to reduce material cost and product quality. In this research, foamed POM and PA66 gear parts were fabricated by using microcellular injection molding with supercritical nitrogen as the blowing agent. Compared to conventional injection molded parts (parts that foaming is not involved), the foamed POM and PA66 gear parts achieved 5% and 10% average weight reduction, respectively. The foamed parts displayed a lower shrinkage ratio when compared to the solid counterparts, which was attributed to the cell expansion that offset part of the inward shrinkage stress. Moreover, POM gear parts with a higher crystallinity degree presented more serious shrinkage ratio compared to the PA66 gear parts, which contributed to the denser polymer molecular chains arrangement. The shrinkage ratio in both directions of PA66 foamed gear parts depended on the injection volume, and the lowest shrinkage ratio of 0.043‰ was obtained at the injection volume of 74 mm, when the polymer reached the maximum foaming ratio. The findings from this study could provide practical guidance for preparing microcellular injection molded products with high dimensional accuracy.

Comparison of Conventional and Microcellular Injection Using New Chemical Agents

ABSTRACT This study is mainly focused to obtain stable dimensions for plastic products used in manufacturing industries. A technique of microcellular foaming injection method was chosen with added chemical blowing agents to decrease the mass, clamping force, and cycle time. Mold flow analysis was run to compare the conventional plastic injection (KP) method with chemical foaming injection molding method using added TecoCell H1 (TH) and CS (TCS). The primary process conditions are considered as temperature of the mold, velocity of injection, pressure and time of holding, and cooling time. The experiments were carried under polymer material as (polypropylene) Moplen EP548P. The new chemical blowing agents TH and TCS, having different reactive material ratios (50% and 25% endothermic) and gas concentration ratios, were utilized at a rate of 1% by weight claiming to be a novel method for producing the parts with high efficiency. The results of both the flow analysis and the injection molding experiments demonstrate that, plastic samples produced by conventional plastic injection could be made approximately 7.5% lesser weight, with 33% lesser force of clamping, and a gain of cycle time nearly 15% by adding chemical blowing agents to the process. Topsis technique is carried for determining the best fabrication method. It was observed that, the injection molding method with Tecocell H1 (TH) chemical foaming agent exhibited better results in terms of less clamping force, reduced cycle time, and weight of the product. The experimental results are validated using the Topsis method and the results are satisfied. Hence, the chemical blowing agent Tecocell H1 (TH) can be applied as a foaming agent for manufacturing the plastic products and increase the efficiency of the production.

Direct In-Mold Impregnation of Glass Fiber Fabric by Polypropylene with Supercritical Nitrogen in Microcellular Injection Molding Process.

Combining microcellular injection molding and insert injection molding, an injection molding technique for glass fiber fabric (GFF) reinforced polypropylene (PP) composite foams was proposed. The GFF was directly set in the mold cavity, and then the PP with supercritical nitrogen (SCN) was injected into the cavity for in-mold impregnation. The impregnation effects of two types of GFFs (EWR300 and EWR600) by the PP/SCF solutions at different injection temperatures (230, 240, and 250 °C) were investigated. The results of the morphological and tensile properties of the samples showed that the interfacial bonding was not good, because of the heterogeneity between the GFF and PP. In comparison with solid PP, the unfoamed GFF/PP did not present a higher tensile strength and presented a lower specific tensile strength. However, the increased tensile strength of the GFF/PP composite foams indicated an improvement in the impregnation effect and interfacial bonding. The SCN decreased the viscosity, which benefited the direct in-mold impregnation of the GFF. Increasing the temperature can improve the interfacial bonding, but it also influenced the foaming and thus led to a decrease in the tensile strength. According to the temperature distribution, the samples from different positions in the mold cavity had different properties.

Investigation of film–substrate interfacial characteristics of polymer parts fabricated via in-mold decoration and microcellular injection molding process

The appearance quality of foamed polymer parts can be improved by introducing high-appearance quality decorative films. However, the interfacial bonding characteristics of film that penetrated the surface of the substrate are few studied. In this paper, the foamed polypropylene (PP) parts with decoration films penetrated on the surface were prepared by the in-mold decoration and microcellular injection molding (IMD/MIM) process. The interfacial characteristics of the IMD/MIM parts were investigated experimentally through peeling tests and interfacial morphology. Based on the finite volume method, the coupled heat transfer model was established to calculate the temperature field in IMD/MIM process by taking into account the coupled heat transfer between polymer melt, film, and mold. The thermal response in the IMD/MIM process was numerically analyzed. The results show that the higher temperature on the polymer melt–film interface corresponds to relatively higher crystallinity and larger crystallite size and also favors the forming of the β-form crystal, which is beneficial to higher adhesion strength. The IMD/MIM parts can obtain a firm film–substrate adhesion and a uniformly strong bond between the film and the substrate.

Lightweight and High Impact Toughness PP/PET/POE Composite Foams Fabricated by In Situ Nanofibrillation and Microcellular Injection Molding.

Polypropylene (PP) has become the most promising and candidate material for fabricating lightweight products. Microcellular injection molding (MIM) is a cost-effective technology for manufacturing porous plastic products. However, it is still challenging to fabricate high-performance PP microcellular components. Herein, we reported an efficient strategy to produce lightweight and high impact toughness foamed PP/polyethylene terephthalate (PET)/polyolefin-based elastomer (POE) components by combining in situ fibrillation (INF) and MIM technologies. First, the INF composite was prepared by integrating twin-screw compounding with melt spinning. SEM analysis showed PET nanofibrils with a diameter of 258 nm were achieved and distributed uniformly in the PP due to the POE's inducing elaboration effect. Rheological and DSC analysis demonstrated PET nanofibrils pronouncedly improved PP's viscoelasticity and crystal nucleation rate, respectively. Compared with PP foam, INF composite foam showed more stretched cells in the skin layer and refined spherical cells in the core layer. Due to the synergistic toughening effect of PET nanofibrils and POE elastic particles, the impact strength of INF composite foams was 295.3% higher than that of PP foam and 191.2% higher than that of melt-blended PP/PET foam. The results gathered in this study reveal potential applications for PP based INF composite foams in the manufacturing of lightweight automotive products with enhanced impact properties.

Microcellular injection molded lightweight, strong and thermally insulating PP/fibrillated-PTFE composite foams with enhanced surface appearance

Microcellular polymer foams are a kind of advanced polymeric foams that show light weight and enhanced thermally insulating performance compared with traditional polymer foams. Microcellular injection molding (MIM) has emerged as a pollution-free technique to efficiently prepare microcellular polymer components. However, MIM products surfer from the problems including poor surface appearance, limited weight loss, and deteriorated mechanical properties, which seriously limit the application of this technology. Herein, we reported an approach to fabricate ultra-lightweight, high strength, surface quality and thermal insulation in-situ polytetrafluoroethylene (PTFE) microfibrils reinforced polypropylene (PP) foams using mold opening microcellular injection molding (MOMIM). Firstly, different morphology PP/PTFE composites were prepared by melt blending, and subsequently their crystallization behavior and rheological properties were further investigated. Afterwards, both MIM and MOMIM were conducted to produce PP and PP/PTFE microcellular parts. The results showed that the in-situ fibrillated PTFE benefit to accelerate crystallization, refine crystals and enhance viscoelasticity of PP compared with spherical PTFE. Both fibrillated and spherical PTFE led to greatly refined cellular morphology although the former is more effective. Compared with pristine PP foams, the tensile and impact strength of PP/fibrillated-PTFE foams could be enhanced by 21% and 133%, respectively. Meanwhile, the thermal conductivity of PP/fibrillated-PTFE foams could be reduced by 32% compared with the pristine PP foams. Moreover, MOMIM-fabricated parts exhibited better surface quality compared with MIM-fabricated ones.