Microcellular Injection Molding Research Articles

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.

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.

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.

Mechanical Response of Fiber-Filled Automotive Body Panels Manufactured with the Ku-FizzTM Microcellular Injection Molding Process.

To maximize the driving range and minimize the associated energy needs and, thus, the number of batteries of electric vehicles, OEMs have adopted lightweight materials, such as long fiber-reinforced thermoplastics, and new processes, such as microcellular injection molding. These components must withstand specific loading conditions that occur during normal operation. Their mechanical response depends on the fiber and foam microstructures, which in turn are defined by the fabrication process. In this work, long fiber thermoplastic door panels were manufactured using the Ku-FizzTM microcellular injection molding process and were tested for their impact resistance, dynamic properties, and vibration response. Material constants were compared to the properties of unfoamed door panels. The changes in mechanical behavior were explained through the underlying differences in their respective microstructures. The specific storage modulus and specific elastic modulus of foamed components were within 10% of their unfoamed counterparts, while specific absorbed energy was 33% higher for the foamed panel by maintaining the panel's mass/weight.

Modelling and Simulation of MuCell®: The Effect of Key Processing Parameters on Cell Size and Weight Reduction.

Microcellular injection moulding is an important injection moulding technique to create foaming plastic parts. However, there are no consistent conclusions on the impact of processing parameters on the cell morphology of microcellular injection moulded parts. This paper investigates the influence of the main processing parameters, such as melt temperature, mould temperature, injection pressure, flow rate, shot volume and gas dosage amount, on the average cell size and weight reduction of a talc-reinforced polypropylene square part (165 mm × 165 mm × 3.2 mm), using the commercial software Moldex 3D. The effect of each parameter is investigated considering a range of values and the simulation results were compared with published experimental results. The differences between numerical and experimental trends are discussed.

Microcellular injection molded lightweight and tough poly (L-lactic acid)/in-situ polytetrafluoroethylene nanocomposite foams with enhanced surface quality and thermally-insulating performance

High-performance microcellular polymer foams have been widely used all over the world, while the excessive usage of petroleum-based polymers caused serious environmental problems. As the eco-friendly awareness is increasing significantly, poly (L-lactic acid) (PLLA), as a typical biomass polymer, has gradually attracted widespread attention. However, the slow crystallization and poor melt strength of PLLA lead to low foaming ability and thus limiting its industrial applications. Herein, a novel and scalable strategy by coupling in-situ fibrillation and mold-opening microcellular injection molding (MOMIM) was developed to fabricate lightweight and tough PLLA/polytetrafluoroethylene (PTFE) foams. Thanks to the reticulated in-situ PTFE nanofibrils with a diameter of 100–200 nm, the crystallization and viscoelasticity of PLLA were dramatically promoted, and further contributing to its foaming ability. The expansion ratio of the MOMIM PLLA/PTFE foam was increased by 86 % compared with the regular microcellular injection molded (RMIM) PLLA foam. Moreover, the lower foam density and the toughening effect of PTFE nanofibrils resulted in the outstanding ductility of the PLLA/PTFE foams, whose tensile elongation, flexural strength, and impact strength were maximally increased by 52 %, 28 %, and 48 %, compared with PLLA foams. More importantly, the thermally-insulating performance and surface quality of PLLA/PTFE foams were also greatly improved.

Microcellular injection molding of polyether-ether-ketone

Polyether-ether-ketone (PEEK) has been widely used in aerospace field for its excellent performance. Microcellular injection molding (MIM) is a promising method to reduce product weight and save energy consumption. In this study, microcellular PEEK was prepared by injection molding. Firstly, the thermal properties of raw PEEK, injection molded PEEK, and foamed PEEK were investigated and results show that the thermal performance of the three types of PEEK were similar. Secondly, the effects of process parameters, including melt temperature, injection speed, injection volume and gas concentration, on microcellular PEEK were studied. The results show that injection volume had the most significant influence on the weight-reduction ratio and tensile strength, while the change of melt temperature, injection speed, injection volume and gas concentration could alter the internal cell structure. After optimization, two kinds of microcellular PEEK with the largest weight-reduction ratio of 17.29% and the greatest tensile strength of 74.13 MPa were obtained respectively. Thirdly, the effect of carbon fiber on microcellular PEEK was preliminarily studied. It was found that carbon fiber could promote the formation of dense cell structure. This work laid the foundation for further fabrication of high strength and light weight microcellular PEEK. • Effects of parameters on foamed PEEK are studied by single factor and orthogonal test. • Weight reduction and tensile properties are mainly dependent on injection volume. • Melt temperature, injection speed and injection volume greatly affect cell structure. • The weight-reduction ratio increases to 17.29% through optimization. • The tensile strength and tensile modulus reach 74 MPa and 2784 MPa after optimization.

Effect of mold opening on microcellular polyether-ether-ketone fabricated by injection molding

Polyether-ether-ketone (PEEK) is an excellent engineering polymer with numerous advantages. Processing by microcellular injection molding (MIM) can reduce product weight and save energy consumption. However, conventional MIM is plagued with limited weight-reduction and poor surface quality, so there have been few reports on microcellular PEEK with high weight-reduction based on injection molding. In this study, microcellular PEEK with weight reduction of 49.6% was prepared by mold-opening microcellular injection molding (MOMIM). The effects of holding time, holding pressure and mold-opening distance on microcellular PEEK were studied. Results show that incremental holding time and holding pressure could increase PEEK's density, promote the formation of dense cells and improve mechanical properties. Meanwhile, larger mold-opening distance reduced product's density, raised cell size and cell density, and lowered tensile properties, while the flexural properties increased first and then decreased when the mold-opening distance became larger. In addition, the surface qualities of microcellular PEEK prepared by MIM and MOMIM were compared, and it was found that MOMIM could significantly improve the surface quality. These microcellular PEEK prepared by MOMIM with tunable properties can further promote the lightweight application of PEEK.

Using Gas Counter Pressure and Combined Technologies for Microcellular Injection Molding of Thermoplastic Polyurethane to Achieve High Foaming Qualities and Weight Reduction.

Microcellular injection molding technology (MuCell®) using supercritical fluid (SCF) as a foaming agent offers many advantages, such as material and energy savings, low cycle time, cost-effectiveness, and the dimensional stability of products. MuCell® has attracted great attention for applications in the automotive, packaging, sporting goods, and electrical parts industries. In view of the environmental issues, the shoe industry, particularly for midsole parts, is also seriously considering using physical foaming to replace the chemical foaming process. MuCell® is thus becoming one potential processing candidate. Thermoplastic polyurethane (TPU) is a common material for molding the outsole of shoes because of its outstanding properties such as hardness, abrasion resistance, and elasticity. Although many shoe manufacturers have tried applying Mucell® processes to TPU midsoles, the main problem remaining to be overcome is the non-uniformity of the foaming cell size in the molded midsole. In this study, the MuCell® process combined with gas counter pressure (GCP) technology and dynamic mold temperature control (DMTC) were carried out for TPU molding. The influence of various molding parameters including SCF dosage, injection speed, mold temperature, gas counter pressure, and gas holding time on the foaming cell size and the associated size distribution under a target weight reduction of 60% were investigated in detail. Compared with the conventional MuCell® process, the implementation of GCP technology or DMTC led to significant improvement in foaming cell size reduction and size uniformity. Further improvement could be achieved by the simultaneous combination of GCP with DMT, and the resulting cell density was about fifty times higher. The successful possibility for the microcellular injection molding of TPU shoe midsoles is greatly enhanced.

Conventional and Microcellular Injection Molding of a Highly Filled Polycarbonate Composite with Glass Fibers and Carbon Black.

Conventional solid injection molding (CIM) and microcellular injection molding (MIM) of a highly filled polycarbonate (PC) composite with glass fibers and carbon black were performed for molding ASTM tensile test bars and a box-shape part with variable wall thickness. A scanning electron microscope (SEM) was used to examine the microstructure at the fractured surface of the tensile test bar samples. The fine and uniform cellular structure suggests that the PC composite is a suitable material for foaming applications. Standard tensile tests showed that, while the ultimate strength and elongation at break were lower for the foamed test bars at 4.0–11.4% weight reduction, their specific Young’s modulus was comparable to that of their solid counterparts. A melt flow and transition model was proposed to explain the unique, irregular “tiger-stripes” exhibited on the surface of solid test bars. Increasing the supercritical fluid (SCF) dosage and weight reduction of foamed samples resulted in swirl marks on the part surface, making the tiger-stripes less noticeable. Finally, it was found that an injection pressure reduction of 25.8% could be achieved with MIM for molding a complex box-shaped part in a consistent and reliable fashion.

Cell morphologies, mechanical properties, and fiber orientation of glass fiber-reinforced polyamide composites: Influence of subcritical gas-laden pellet injection molding foaming technology

Advanced materials and new lightweighting technologies are essential for boosting the fuel economy of modern automobiles while maintaining performance and safety. A novel approach called subcritical gas-laden pellet injection molding foaming technology (SIFT) was performed to produce foamed polyamide/glass fiber (PA/GF) composite. Gas-laden pellets loaded with nitrogen (N2) were produced by introducing sub-critical N2 into PA/GF composite during compounding using a twin-screw extruder equipped with a simple gas injection unit. Compared to the commercial microcellular injection molding (MIM) technologies, gas-laden pellets enable the production of foamed parts with a standard injection molding machine, which is more cost-effective and easier to implement. To the best of our knowledge, this is the first attempt that the SIFT technology is being used for the PA/GF composites for making foamed parts. The tensile strength, fiber orientation, cell morphology, and densities of foamed PA/GF parts were investigated, and the shelf life of N2-laden PA/GF pellets was examined. Results showed that the N2-laden pellets still possessed good foaming ability after one week of storage under ambient atmospheric conditions. One week is a noticeable improvement compared to those N2-laden neat polymer pellets without glass fibers. With this approach, the weight reduction of foamed PA/GF parts was able to reach 12.0 wt. %. Additionally, a nondestructive analysis of the fiber orientation using micro-computed tomography suggested that the MIM and SIFT samples exhibited a less degree of fiber orientation along the flow direction when compared to the solid samples and that the tensile strength of both technologies was very close at a similar weight reduction. Cell size increased and cell density decreased as the shelf life increased. These findings showed that this processing method could act as an alternative to current commercial foam injection molding technology for producing lightweight parts with greater design freedom.

Investigation on Film-Substrate Interfacial Characteristics of Parts Fabricated Via a Novel Integrated In-Mold Decoration and Microcellular Injection Molding Process

Investigation on Film-Substrate Interfacial Characteristics of Parts Fabricated Via a Novel Integrated In-Mold Decoration and Microcellular Injection Molding Process

Fabrication of Polyether–Ether–Ketone Foams with Superior Properties and Mitigated Weld Lines by Microcellular Injection Molding

Polyether–ether–ketone (PEEK) as an engineering plastic with excellent mechanical properties requires strict processing conditions. Introducing microcells in PEEK parts is beneficial for reducing the material cost and improving their tenacity. Herein, foamed PEEK parts are fabricated using microcellular injection molding with supercritical nitrogen as the blowing agent. Using optimum processing conditions obtained from orthogonal array experiments based on finite‐element simulation, the foamed PEEK parts achieve a 10% average weight reduction and display a typical sandwich structure with an average cell size of 30 μm in the core region. The cell size is larger, whereas the cell density and skin layer thickness are smaller in the region near the gate than the region far from the gate. The foamed PEEK parts show a slight drop in the tensile modulus and tensile strength but a significant improvement in the elongation at break when compared with the solid parts. Moreover, the foamed PEEK parts with weld lines exhibit less reduction in tensile properties than the solid counterparts, attributed to their tighter interfacial adhesion at the weld line. This study successfully produces foamed PEEK parts with high weight reduction and ductility and provides insights into the weld line mitigation through microcellular injection molding.

Lightweight and strong glass fiber reinforced polypropylene composite foams achieved by mold-opening microcellular injection molding

Microcellular injection molding (MIM) is a flexible, efficient, and environmentally friendly foaming technology to prepare plastic foams with complex shapes. However, MIM product suffers from deteriorated mechanical properties, limited weight reduction, and poor surface appearance. In this study, mold-opening microcellular injection molding (MOMIM) was used to prepare high-performance polypropylene/glass fiber (PP/GF) composite foams. GF was used to improve the foaming behavior of PP and to enhance the mechanical properties of foamed samples. Firstly, the crystallization and rheological behavior of PP and PP/GF were analyzed. Afterwards, the foaming behavior of PP and PP/GF in microcellular injection molding process were investigated. Finally, the mechanical properties of PP and PP/GF foams were measured and discussed. It was found that PP/GF composite foams show a rather uniform cellular structure than the pure PP foams. Moreover, PP/GF composite foams exhibit much higher strength and stiffness than the pure PP foams. Furthermore, MOMIM can significantly enhance the flexural modulus of the molded samples. For the foams with an expansion ratio of 1.5-fold, by adding 20 wt% GF, the tensile strength and modulus can be increased by 60% and 50%, respectively. For the PP/GF composite with 20 wt% GF, the foamed sample prepared by MOMIM can have a flexural modulus nearly 200% higher than that of the unfoamed sample. Thus, MOMIM PP/GF composites show a promising perspective in providing lightweight and strong plastic products that can be widely used in automotive, electronics, and appliances.

A Review on Microcellular Injection Moulding.

Microcellular injection moulding (MuCell®) is a polymer processing technology that uses a supercritical fluid inert gas, CO2 or N2, to produce light-weight products. Due to environmental pressures and the requirement of light-weight parts with good mechanical properties, this technology recently gained significant attention. However, poor surface appearance and limited mechanical properties still prevent the wide applications of this technique. This paper reviews the microcellular injection moulding process, main characteristics of the process, bubble nucleation and growth, and major recent developments in the field. Strategies to improve both the surface quality and mechanical properties are discussed in detail as well as the relationships between processing parameters, morphology, and surface and mechanical properties. Modelling approaches to simulate microcellular injection moulding and the mathematical models behind Moldex 3D and Moldflow, the two most commonly used software tools by industry and academia, are reviewed, and the main limitations are highlighted. Finally, future research perspectives to further develop this technology are also discussed.

High-Performance of a Thick-Walled Polyamide Composite Produced by Microcellular Injection Molding.

Lightweight moldings obtained by microcellular injection molding (MIM) are of great significance for saving materials and reducing energy consumption. For thick-walled parts, the standard injection molding process brings some defects, including a sink mark, warpage, and high shrinkage. Polyamide 66 (PA66)/glass fiber (GF) thick-walled moldings were prepared by MuCell® technology. The influences of moldings thickness (6 and 8.4 mm) and applied nitrogen pressure (16 and 20 MPa) on the morphology and mechanical properties were studied. Finally, the microcellular structure with a small cell diameter of about 30 μm was confirmed. Despite a significant time reduction of the holding phase (to 0.3 s), high-performance PA66 GF30 foamed moldings without sink marks and warpage were obtained. The excellent strength properties and favorable impact resistance while reducing the weight of thick-walled moldings were achieved. The main reason for the good results of polyamide composite was the orientation of the fibers in the flow direction and the large number of small nitrogen cells in the core and transition zone. The structure gradient was analysed and confirmed with scanning electron microscopy (SEM) images, X-ray micro computed tomography (micro CT) and finite element method (FEM) simulation.

Using P(Pressure)-T(Temperature) Path to Control the Foaming Cell Sizes in Microcellular Injection Molding Process.

Microcellular injection molding technology (MuCell) using supercritical fluid (SCF) as a foaming agent is one of the important green molding solutions for reducing the part weight, saving cycle time, and molding energy, and improving dimensional stability. In view of the environmental issues, the successful application of MuCell is becoming increasingly important. However, the molding process encounters difficulties including the sliver flow marks on the surface and unstable mechanical properties that are caused by the uneven foaming cell sizes within the part. In our previous studies, gas counter-pressure combined with dynamic molding temperature control was observed to be an effective and promising way of improving product quality. In this study, we extend this concept by incorporating additional parameters, such as gas pressure holding time and release time, and taking the mold cooling speed into account to form a P(pressure)-T(temperature) path in the SCF PT diagram. This study demonstrates the successful control of foaming cell size and uniformity in size distribution in microcellular injection molding of polystyrene (PS). A preliminary study in the molding of elastomer thermoplastic polyurethanes (TPU) using the P-T path also shows promising results.

Lightweight and strong gelling agent-reinforced injection-molded polypropylene composite foams fabricated using low-pressure CO2 as the foaming agent

Lightweight and strong polymeric foams show high potential application in alleviating the global energy crisis due to their capability of reducing material and resource requirements as well as decreasing energy consumption. However, it is inevitably difficult to produce lightweight polymers with satisfactory mechanical properties. Herein, we report an innovative method to produce high-performance polypropylene (PP) foams by combining a sorbitol gelling agent with the newly developed low-pressure microcellular injection molding (MIM) technique. Carbon dioxide (CO2) at an ultralow pressure of 5 MPa is used as a foaming agent in the low-pressure MIM process. The addition of a 1,3:2,4-bis-O-(4-methyl benzylidene)-D-sorbitol gelling agent (MDBS), which generates an in situ network structure, notably enhances the crystallization, viscoelasticity and melt strength of PP, resulting in PP foams with well-defined cellular structures. Compared with neat PP foams, the added sorbitol gelling agent leads to a four orders of magnitude increase in the cell density of PP foams that have a cell size of approximately 8.4 μm. Remarkably, the tensile toughness and tensile strength of the PP composite foam are approximately 1000 % and 150 % higher than those of the neat PP foam, respectively. These results demonstrate that lightweight and strong PP foams with high ductility can be obtained via the scalable and novel FIM technique, which shows a promising future in many applications, such as automotive, construction and electrical components.