Microcellular Injection Research Articles

Research on cellular morphology and mechanical properties of microcellular injection–molded BCPP and its blends

Research on cellular morphology and mechanical properties of microcellular injection–molded BCPP and its blends

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.

Effect of gas pressure on the microstructure of parts foamed with the novel microcellular injection molding technology Ku‐Fizz™

AbstractThe increasing demand for lightweight and economical automotive components boosts investigation of advanced materials and new lightweighting technologies. This work employs the novel microcellular injection molding technology Ku‐Fizz™. The process introduces gas with granulates at moderate low pressures into the feed zone of the injection molding machine. Ku‐Fizz is controlled by gas pressure; thus, a simple plate geometry was molded and the effect of various gas contents on the microstructure was analyzed. The material used was a chemically coupled glass fiber‐reinforced polypropylene compound. Optical microscopy was employed to measure the foam microstructure. Microcomputed tomography was used to quantify the fiber volume fraction and the orientation tensors. Results of the fully characterized microstructure showed cell density increasing and cell size decreasing with gas pressure and melt flow direction. Fiber length increased with gas content. Cell growth displaced fibers from the center of the part towards the mold surface, changing the fiber concentration and global fiber orientation.

Effect of film types on thermal response, cellular structure, forming defects and mechanical properties of combined in-mold decoration and microcellular injection molding parts

Different types of polymer films were used in the combined in-mold decoration and microcellular injection molding (IMD/MIM) process. The multiphase fluid-solid coupled heat transfer model was established to study the thermal response at the melt filling stage in the IMD/MIM process. It was found that the temperature distributed asymmetrically along the thickness direction due to the changed heat transfer coefficient of the melt on the film side. When polyethylene terephthalate (PET) films were applied, the temperature of the melt-film interface increased faster and to be higher at the end of melt filling stage in comparison with the application of polycarbonate (PC) and thermoplastic polyurethane (TPU) films. And the effects of film types on the cellular structure, forming defects and mechanical properties of IMD/MIM parts were also studied experimentally. The results showed that the film types had no obvious effect on the cells size in the transition layer and the mechanical properties of the parts. Under certain film thickness, the offset distance of core layer was the largest with PET film used, while the offset distance was the smallest with TPU film used. And similar results were found for the warpage of the parts. However, an exactly opposite change occurred for the thickness of film-side transition layer and the bubble marks on the surface of the parts.

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.

Microcellular injection molded outstanding oleophilic and sound-insulating PP/PTFE nanocomposite foam

Lightweight and open-cell thermoplastic foams have proven superior to common counterparts in terms of functional performances, such as oil absorption, sound and thermal insulation, contributing to energy saving, environmental protection, and sustainable development of the economy and society. Herein, a novel strategy was employed to prepare lightweight, oleophilic, and sound-insulating polypropylene (PP)/polytetrafluoroethylene (PTFE) composite foams by combining the microcellular injection molding (MIM) and in-situ fibrillation technologies. Thanks to the pronouncedly promoted crystallization and melt strength of the PP matrix by the nano-fibrillar PTFE fibrils obtained by the in-situ fibrillation process, the foaming ability of PP in the MIM process was effectively improved, reaching an expansion ratio of up to 20.2 and a cell population density of 108 cells/cm3. Notably, the nano-scale structures on cell walls together with thin cell-wall thickness of the PP/PTFE foam, significantly increased the open-cell content to 98.3%. The high porosities and large expansion ratios, assisted by the capillary penetration action caused by the uniquely elongated cells, effectively increased the oil absorption capacity up to 22.5 g/g with a high recovery of 97.4%, particularly for raw crude oil. Moreover, a high porosity and low airflow resistivity dramatically improved the sound insulation by reducing impedance and enhancing viscous dissipation, presenting as a sound absorption coefficient of higher than 0.5, and a transmission loss of up to 50 dB. Hence, the flexible, scalable and cost-effective MIM technology shows a promising future in the manufacture of multifunctional microcellular polymer products.

Polypropylene/talc foams with high weight-reduction and improved surface quality fabricated by mold-opening microcellular injection molding

Microcellular injection-molded part can save materials usage and reduce energy consumption. However, the part molded by conventional microcellular injection molding often encounters problems of a limited weight-reduction and poor surface quality. Focusing on these problems, this paper adopted mold-opening microcellular injection molding and talc as nucleating agent to prepare foamed polypropylene (PP) part. Different PP/talc composites were obtained by twin-screw extruder compounding, and their crystallization and rheological behaviors were investigated. The results show that talc can significantly promote PP crystallization, and the crystallization temperature changes slightly with talc content. However, the viscoelasticity of PP melt cannot be enhanced by the added talc. Then, the effects of packing time, mold-opening distance and talc content on the foaming behavior of PP were studied. It is found that a high packing pressure for long time can effectively eliminate the cells generated in melt filling process, thus a transparent part is obtained. With increasing packing time, the unfoamed skin layer of the part increases and cell size decreases. With increasing mold-opening distance, cell density increases. At a same mold-opening distance, cell density remarkably increases because of the heterogeneous nucleation of talc, thus cell size decreases. When talc content is 10 wt.% and weight-reduction of the part is 62.1%, the average cell size is 56.5 μm and the cell density reaches 1.54 × 107 cells/cm3. Because the high packing pressure is employed in the mold-opening microcellular injection molding, the surface quality of the part is improved.

Effect of microstructure induced by microcellular injection molding on electromagnetic interference shielding properties

AbstractPreparing lightweight and versatile products is the unremitting goal of industry to save resources and energy. Lightweight carbon fiber reinforced polypropylene (CF/PP) composite foams with high‐performance electromagnetic interference (EMI) shielding materials were fabricated by microcellular injection molding (MIM) technology. The average length and distribution of CF in CF/PP composite foams were examined. Thanks to the introduction of foaming process, the average CF length of composite foams was 33.98% longer than that of solids, which effectively enhanced the electrical conductivity and EMI shielding properties. The effect of shot size, gas content, and injection rate on the electrical conductivity and EMI properties was investigated. With melt shot size of 2/3 of the cavity volume, gas content of 0.5 wt% N2 and injection rate of 100 mm/s, optimal cellular structure of the composite material was obtained. The EMI shielding effectiveness (SE) reaches 36.94 dB, which is the highest value achieved by using MIM technology to the best of the authors' knowledge. In addition, the mechanical properties of cellular structure can still maintain good values, with the tensile strength and impact strength improved by 15.3% and 14.03%, respectively.

The Microcellular Structure of Injection Molded Thick-Walled Parts as Observed by In-Line Monitoring.

The aim of the study was to detect the influence of nitrogen pressure on the rheological properties and structure of PA66 GF30 thick-walled parts, produced by means of microcellular injection molding (MIM), using the MuCell® technology. The process was monitored in-line with pressure and temperature sensors assembled in the original injection mold. The measured data was subsequently used to evaluate rheological properties inside an 8.4 mm depth mold cavity. The analysis of the microcellular structure was related to the monitored in-line pressure and temperature changes during the injection process cycle. A four-times reduction of the maximum filling pressure in the mold cavity for MIM was found. At the same time, the holding pressure was taken over by expanding cells. The gradient effect of the cells distribution and the fiber arrangement in the flow direction were observed. A slight influence of nitrogen pressure on the cells size was found. Cells with a diameter lower than 20 µm dominate in the analyzed cases. An effect of reduction of the average cells size in the function of distance to the gate was observed. The creation of structure gradient and changes of cells dimensions were evaluated by SEM images and confirmed with the micro CT analysis.

Highly Compressible Polymer Composite Foams with Thermal Heating-Boosted Electromagnetic Wave Absorption Abilities.

Polymer composite foams are desirable materials for electromagnetic (EM) energy attenuation. However, a number of challenges limit improvement in the EM energy attenuation properties of foams. In this study, a simple microcellular injection molding method was used to fabricate highly compressible thermoplastic urethane (TPU)/carbon nanotube (CNTs) composite foams, which also had increased conductivity with an increase in CNT content. Compared to unfoamed composites, foamed composites exhibited higher conductivity and EM attenuation properties because of the presence of a microcellular structure. Moreover, the TPU/CNT foam with 4 wt % CNTs (F(4)) demonstrated strong EM dissipation and an optimal reflection loss (RL) value of -30.4 dB. Furthermore, stimulated by thermal heating and cyclic compression, EM attenuation was observed to increase because of the higher conductivity. Note that F(4) foam having a small thickness of 1.3 mm when treated at 333 K had the highest EM dissipation and the lowest RL value of -51.8 dB. Enhanced polarization and ohmic losses and multiscattering were responsible for the increased EM absorption. This behavior is attributed to the movement of CNTs within the TPU elastomer walls via thermal or compression stimulation. For designing stimulation-dependent multifunctional materials, composite foams with response to thermal heating were proved to be an alternative approach.

Microstructure and Properties of Glass Fiber-Reinforced Polyamide/Nylon Microcellular Foamed Composites.

The automobile and aerospace industries require lightweight and high-strength structural parts. Nylon-based microcellular foamed composites have the characteristics of high strength and the advantages of being lightweight as well as having a low production cost and high product dimensional accuracy. In this work, the glass fiber-reinforced nylon foams were prepared through microcellular injection molding with supercritical fluid as the blowing agent. The tensile strength and weight loss ratio of microcellular foaming composites with various injection rates, temperatures, and volumes were investigated through orthogonal experiments. Moreover, the correlations between dielectric constant and injection volume were also studied. The results showed that the “slow–fast” injection rate, increased temperature, and injection volume were beneficial to improving the tensile strength and strength/weight ratios. Meanwhile, the dielectric constant can be decreased by building the microcellular structure in nylon, which is associated with the weight loss ratio extent closely.

Establishing a rapid cooling complex mold design for the quality improvement of microcellular injection molding

AbstractInjection molding is one of the most widely‐used processes for the production of plastic parts, due to the utilization of diverse materials, the complex product‐shape moldability, and the rapid mass production. In relation to the environmental issues, light‐weight technology and green molding solutions are becoming more important. Microcellular injection molding technology is one of the green molding solutions for saving materials, as well as reducing the weight of molded parts. However, the molding process brings about some defects, including a sliver flow mark on the surface and uneven mechanical properties that are caused by the uneven cell size and their distribution within the part. Dynamic molding temperature control technology seems to be an effective way of improving the product quality. Until recently, there has been very little discussion about high‐efficiency cooling methods. A new complex mold for a cooling system has been designed. The basis cooling ability of different materials was investigated. The complex mold design has a faster cooling rate, and it has a greater surface temperature uniformity. This cooling technology was used to improve the quality of microcellular injection molded parts, which improves the glossy finish by about 73%. The results show that the faster cooling rate brings about the more uniform cell size with higher cell density from 9.43E+11 to 1.92E+12 cells/cm3. Otherwise, the cell size reduced from 192.92 to 84.97 μm.

Effect of POE on mechanical properties and cellular structure of PP/Nano-CaCO3 composites in IMD/MIM process

Nano-CaCO3 can effectively improve the strength of foamed Polypropylene (PP), ethylene-octene copolymer (POE) can greatly enhance the toughness of PP, and PP-g-MAH can improve the compatibility of Nano-CaCO3 with PP. The supercritical fluid-nitrogen (N2) was used as physical foaming agent, and the PET film was selected to study the effect of POE content on the mechanical properties and cellular structure of PP/nano-CaCO3 blends during combined in-mold decoration and microcellular injection molding (IMD/MIM) process. The results showed that the addition of POE did improve the impact strength of PP/nano-CaCO3 composites, while the tensile strength and flexural strength decreased. On account of the characteristics of POE, the melt strength change of PP/nano-CaCO3 blend system after POE’s addition was studied. The melt strength of the blend system decreased at first and then increased. It can be found that POE distributed uniformly in PP, and POE particle size decreased with the increase of POE content. Besides, it was revealed that POE could improve the cellular structure of the vertical section and the parallel section. The mechanical properties and cellular structure were optimized when 10% POE was added.

Lightweight and tough PP/talc composite foam with bimodal nanoporous structure achieved by microcellular injection molding

Although polypropylene (PP) foams fabricated by microcellular injection molding (MIM) have been widely studied and globally applied, it is still a big challenge to achieve lightweight products with high mechanical performance. Herein, the MIM method assisted by a fast cooling design was applied to prepare lightweight bimodal nanoporous PP foams with high toughness. A specific mold with a thin cavity was designed to achieve rapid cooling rate during MIM process, and to further accelerate crystallization and refine crystals, which was verified by visualization results, DSC curves, and WAXD patterns. Thus, nanocellular PP foams were prepared, whose high toughness was presented in tensile testing, especially for bimodal cellular foams, more than 327% higher than that of microcellular foams, and up to 53% higher than that of solid. It is owing to the introduced matrix craze, shear yielding, and larger plastic zone by nanocells and the transformed crack propagation direction by microcells. Therefore, this novel toughening method paves a promising prospect for MIM technology to achieve lightweight and tough products for industrial applications.

Effects of injection molding parameters on cellular structure of roofing tiles composite

A hybrid composite consists of two or more types of reinforcements to make a composite depending on the application. Due to their high specific stiffness and strength, fibre-reinforced polymeric composites are widely applied in diverse applications. The beauty of this combination (fiber–matrix system) is that it provides high fracture toughness at high strength levels, a property impossible to obtain with a single material. In this paper, the processing of polystyrene-based matrix composite (roofing tiles) is presented. Tensile testing of the specimen is performed on an Instron Testing Machine. The fabricated composites having a microcellular structure were used for making roofing tiles and building structures. The prepared samples were checked for hardness. These cellular composites are characterized by average cell diameter and cell density. The Microcell Distribution Index (MDI) has been proposed for the characterization of cell morphology. Microcellular injection moulded specimens for roofing tiles were analyzed for the effects of moulding parameters over microcell distribution. Various injection moulding parameters were analyzed for their effect over cell morphology characterized by cell size, cell density and distribution pattern. Effects of injection rate, back pressure, melt temperature and barrel residence time are being studied in the present work.

Morphology and mechanical investigation of microcellular injection molded carbon fiber reinforced polypropylene composite foams

AbstractEmploying microcellular injection molding technology, carbon fiber (CF)/polypropylene (PP) composite foams have been prepared. The influences of injection molding conditions and CF amounts relating to the flexural and impact performances have also been studied. X‐ray computed tomography scanning has been used for morphological observation. For the flexural specimens, although the solid skin and foamed core layers can be confirmed significantly, the intermediate layer is indistinct. Moreover, the stretched cells can be confirmed dramatically for the Charpy impact specimens. The cell density increases to 12.0 × 103 cell/cm2 when the nitrogen content is 1%. By contrast, the cell densities decrease with the injection speed and CF content increasing accordingly. Further, the maximum specific flexural modulus and Charpy impact strength of the foams can achieve 14 GPa/(g/cm3) and 6.2 kJ/m2, respectively, at the CF content of 30 wt%. Finally, the microcellular structure with the highest cell density can be confirmed with the nitrogen content of 1 wt%, the injection speed of 50 mm/s and the CF content of 10 wt%. Obviously, the CF contents have shown strong influences on the mechanical behaviors of the CF/PP composite foams compared with nitrogen contents or injection speeds.

Advances in microcellular injection moulding

Injection moulding is a well-established replication process for the cost-effective manufacture of polymer-based components. The process has different applications in fields such as medical, automotive and aerospace. To expand the use of polymers to meet growing consumer demands for increased functionality, advanced injection moulding processes have been developed that modifies the polymer to create microcellular structures. Through the creation of microcellular materials, additional functionality can be gained through polymer component weight and processing energy reduction. Microcellular injection moulding shows high potential in creating innovation green manufacturing platforms. This review article aims to present the significant developments that have been achieved in different aspects of microcellular injection moulding. Aspects covered include core-back, gas counter pressure, variable thermal tool moulding and other advanced technologies. The resulting characteristics of creating microcellular injection moulding components through both plasticising agents and nucleating agents are presented. In addition, the article highlights potential areas for research exploitation. In particular, acoustic and thermal applications, nano-cellular injection moulding parts and developments of more accurate simulations.

Solid and microcellular polylactide nucleated with PLA stereocomplex and cellulose nanocrystals

Blending polylactide (PLA) with poly(d-lactide) (PDLA) and cellulose nanocrystals (CNC)-based stereocomplex (SC) nanohybrids presents practical approach to produce fully biobased blends and nanocomposites with enhanced properties. This paper investigated the nucleation effect of PLA/PDLA and PLA/PDLA-g-MAH/CNC nanohybrids at low loading levels on the non-isothermal crystallization, morphology, as well as thermal and thermo-mechanical properties of PLA-based samples. Neat PLA, blends and nanocomposites were produced by conventional and microcellular injection molding. Nitrogen (N2) in a supercritical state was used as the physical blowing agent for preparation of microcellular samples. CNC-based SC showed superior nucleating efficiency compared to SC nucleating agents. However, low mold temperature resulted in relatively low degree of crystallinity (~ 15%). In addition, decrease in cell size and increase in cell density of microcellular samples have been observed after introduction of both nucleating agents. Slight increase in mechanical properties of nucleated samples compared to neat PLA has been ascribed to the higher degree of crystallinity. Despite these, decrease in all mechanical properties of microcellular samples has been noticed when compared to solid counterparts. Furthermore, dynamic mechanical analysis (DMA) reveals subsequent foaming and dramatic decrease in dimensional stability of microcellular samples above glass transition temperature (Tg). The storage modulus in a glassy region has been improved in both solid blends and nanocomposites. In addition, shift of Tg due to restricted chain mobility of PLA due to retarded relaxation of amorphous regions due to SC interactions has been observed.

Strong and super thermally insulating in-situ nanofibrillar PLA/PET composite foam fabricated by high-pressure microcellular injection molding

Strong, thermally insulating and biological PLA foams derived from cost-effective and environmentally friendly process are needed for saving resources and improving energy efficiency. PLA’s brittle feature and poor foaming ability are currently the major challenges which strongly restrict developing high-performance PLA foams. Herein, a scalable, cost-effective and environmentally friendly route by combining in-situ nanofibril reinforcement with high-pressure microcellular injection molding (HPMIM) was developed for producing high-performance PLA foams. Nanofibrillar PLA/PET composite was prepared by coupling twin-screw compounding and melt spinning. The presence of PET nanofibrils with an average diameter of 114.8 nm greatly improve PLA’s crystallization, viscoelasticity and melt strength, thereby enhancing its foaming ability. Thus, sub-microcellular nanofibrillar PLA/PET composite foam with an average cell size of 322.8 nm was produced using HPMIM, which shows dramatically enhanced mechanical properties than the pure PLA foam. Remarkably, the Izod and Gardner impact strengths of the HPMIM-fabricated PLA/PET foam are respectively more than 7-fold and 6-fold higher than those of the PLA foam fabricated by regular microcellular injection molding. Moreover, nanofibrillar PLA/PET composite foams with expansion ratio varying from 3.8-fold to 26.2-fold were fabricated. Thanks to the unique nanostructured cell walls with numerous micro/nano fibrils and voids, these nanofibrillar PLA/PET composite foams with large expansion ratios show outstanding thermally insulating properties. The 26.2-fold PLA/PET composite foam exhibits a thermal conductivity of 26.8 mW/m·K, much better than that of current polymer foams. The scalable and environmentally friendly method combined with the unique foam structure opens up new perspectives for developing high-performance multifunctional porous materials.

Investigation on Foamed PP/Nano-CaCO3 Composites in a Combined in-Mold Decoration and Microcellular Injection Molding Process.

A combined in-mold decoration and microcellular injection molding (IMD/MIM) method has been used in this paper. The foamed PP/nano-CaCO3 composites were prepared to investigate their mechanical properties, cellular structure, and surface quality. The content of nano-CaCO3 varied from 0 to 10 wt %. The results showed that nano-CaCO3 acted as a reinforcing phase and nucleating agent, which help to improve the mechanical properties of foamed composites. The cellular structure and mechanical properties were optimum when the nano-CaCO3 content was 6 wt %. In the vertical section, the cell size and density of transition layer on the film side was bigger than that on the non-film side. In the parallel section, the cell ratio of length to diameter of transition layer on the film side was smaller than that on the non-film side, and the cell tile angle was larger than that on the non-film side. With nano-CaCO3 content increasing, the surface quality showed a trend of decreasing first and then increasing.