Microcellular Injection Molding Research Articles

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.

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.

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.

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.

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.

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.

Investigation on forming defects and crystallization of plastic parts in combined in-mold decoration and microcellular injection molding based on a multiphase flow-solid coupled heat transfer model

Microcellular injection molding (MIM) combined with in-mold decoration (IMD) method can produce foamed parts with improved surface appearance by changing the heat transfer in the MIM mold. The temperature field has a significant effect on the forming defects and crystallization of the foamed parts. Based on finite volume method, a multiphase flow-solid coupled heat transfer model was established to calculate the temperature field in IMD/MIM process, and the implicit domain coupling algorithm (IDCA) was used to calculate the temperature field by taking the coupled heat transfer between the mold and the polymer melt into account. Compared with the experimental results, it is found that the established model can accurately predict the temperature field. The thermal response in IMD/MIM process was further analyzed. The effect of film on the forming defects of parts molded by IMD/MIM process was discovered by combining the simulation results with the experimental surface topography and warpage results, and the effect of film on the crystallization of parts molded by IMD/MIM process was also revealed.

Effects of dynamic mold temperature control on melt pressure, cellular structure, and mechanical properties of microcellular injection-molded parts: An experimental study

In this work, the effects of dynamic mold temperature control (DMTC) on melt pressure, cellular structure, and mechanical properties of microcellular injection molding (MIM)-molded parts are investigated experimentally. It is found that with the increase of the mold temperature, the duration of foaming pressure in the cooling stage increases. Meanwhile, the average cell diameter and cell diameter dispersion increases as well as the cell density decreases in MIM molded parts. The turning point of mold temperature after which the foaming pressure in the cooling stage and the cellular structure in MIM molded parts generate a significant change is around the glass transition temperature of the used plastic material. Under DMTC conditions, with the increase of mold temperature, the tensile strength, flexural strength, and impact strength of MIM molded specimens of single gate without weld line change a little, while the tensile strength, flexural strength of MIM molded specimens of double gates with weld line increase obviously. When the mold temperature increases to 120°C and over, the tensile strength, flexural strength of MIM molded specimens of double gates with weld line reach an equivalent level of specimens of single gate without weld line.

A Combined In-Mold Decoration and Microcellular Injection Molding Method for Preparing Foamed Products with Improved Surface Appearance.

A combined in-mold decoration and microcellular injection molding (IMD/MIM) method by integrating in-mold decoration injection molding (IMD) with microcellular injection molding (MIM) was proposed in this paper. To verify the effectiveness of the IMD/MIM method, comparisons of in-mold decoration injection molding (IMD), conventional injection molding (CIM), IMD/MIM and microcellular injection molding (MIM) simulations and experiments were performed. The results show that compared with MIM, the film flattens the bubbles that have not been cooled and turned to the surface, thus improving the surface quality of the parts. The existence of the film results in an asymmetrical temperature distribution along the thickness of the sample, and the higher temperature on the film side leads the cell to move toward it, thus obtaining a cell-offset part. However, the mechanical properties of the IMD/MIM splines are degraded due to the presence of cells, while specific mechanical properties similar to their solid counterparts are maintained. Besides, the existence of the film reduces the heat transfer coefficient of the film side so that the sides of the part are cooled asymmetrically, causing warpage.

In-situ ultrasonic characterization of microcellular injection molding

Microcellular injection molding (MIM) has been extensively employed for manufacturing foamed polymeric products. It has the advantages of saving energy and material costs, along with attaining better dimensional stability. However, the in-situ characterization of MIM is still challenging. In this study, an ultrasonic method for the real-time detection of variations in the foam structure—i.e., cell size (D), surface roughness (Ra), and skin layer thickness (h)—during the MIM process is proposed. This is the first time that ultrasonic technology has been employed to characterize the MIM process. Experiments were carried out to confirm the feasibility of the proposed method. Experimental results showed that the ultrasonic signals exhibited great consistency, and the duration process times of the ultrasonic signals (tduration) and the change of the ultrasonic speed (vvariation) in the transducer path could be used to characterize the variations of cell size and skin layer thickness. The time delay of the first ultrasonic signal (tstart) and the largest ultrasonic amplitude of the ultrasonic signals (Amax) could be employed to characterize the variations of surface roughness. The developed ultrasonic characterization method has several advantages, such as being low-cost, on-line, and non-destructive, and it has promising applications in the characterization of the MIM process.

Applicability of Microcellular Injection Molding Process

Microcellular injection molding is relatively new and progressive technology for production of lightweight constructions. The weight reduction, elimination of sink marks, internal stresses and deformation of products are the main advantages of this unconventional technology. On the other hand, decrease in mechanical properties and poor surface quality are the application restrictions. There are several aspects affecting formation of microcellular structure and final properties of injection molded products. As a one of the most important aspects there is used material. Therefore, the main goal of this article was investigation the influence of different types of thermoplastic materials on the microcellular structure formation and mechanical properties of products. Further, the influence of holding pressure on products quality was also evaluated.

Microcellular injection moulding: A comparison between MuCell process and the novel micro-foaming technology IQ Foam

The present work aims to compare two different injection moulding foaming technologies, the already known MuCell® process and the new emerged technology IQ Foam®, as well as the cell structure and mechanical behavior of the obtained components. Glass fiber reinforced-polypropylene (>PP GF<) was employed to produce rectangular plates at solid and foamed conditions by using MuCell® and IQ Foam® processes combined with the complementary Core Back expansion molding technology, and the material structure as well as the tensile, flexural and impact properties were studied.A solid skin-foamed core structure was observed in the samples foamed by both techniques. The mechanical properties decreased gradually with the apparent density of the microcellular plates. By increasing the thickness of the part because of the expansion of the cavity with the Core Back technology, the apparent density decreased but the flexural stiffness was greatly enhanced. Foamed samples obtained by IQ Foam® technology exhibited thicker solid surface layers and lower cell density than that of the MuCell® ones, but consequently higher resistant area, and thus, slightly higher mechanical properties. The new IQ Foam® technology is able to produce foamed parts with properties comparable to that of the MuCell® process, offering additional benefits such as cost-effectiveness, easy to use and machine-independence.

Design and fabrication of expandable cavity injection mold for open cell microcellular foam

Microcellular injection molding (MIM) of thermoplastics is a challenging molding process compared to conventional injection molding (IM) process because of its narrow processing window. As far as microcellular molding of open-celled foam is concerned, it is much more difficult because of extremely narrow processing window for open-cell foam and transient nature of melt temperature and pressure inside cavity during the molding process. However, it is expected that molding of open-celled microcellular foam may open many new areas such as tissue engineering, drug delivery, reverse osmosis, dialysis membranes, and fuel cell. The production cost may also be reduced manifolds. In present work a novel expandable cavity mold (EC mold) was designed and fabricated. EC mold has some nonconventional features like pneumatically actuated gate seal, mechanism for cavity expansion, and arrangement for cyclic heating-cooling of core and cavity blocks. In present study polylactic acid (PLA), a biodegradable polymer, was used and expandable cavity molding (EC molding) variables, namely, cavity expansion rare (mm/s), cavity expansion distance (mm), and cavity expansion-compression-expansion (ECE) profile mode, were studied. The results confirm molding of PLA foams with very high cell density and impinged structure. The micrographs and open-cell ratio confirm the formation of PLA foam with ~40% open-cell content.

Effects of Nano-CaCO₃ Content on the Crystallization, Mechanical Properties, and Cell Structure of PP Nanocomposites in Microcellular Injection Molding.

Using supercritical nitrogen as the physical foaming agent, microcellular polypropylene (PP) nanocomposites were prepared in microcellular injection molding. The main purpose of this work is to study effects of content of nano-CaCO3 on the crystallization, mechanical properties, and cell structure of PP nanocomposites in microcellular injection molding. The results show that adding nano-CaCO3 to PP could improve its mechanical properties and cell structure. The thermal stability and crystallinity enhances with increase of nano-CaCO3. As a bubble nucleating agent, adding nano-CaCO3 to PP improves the cell structure in both the parallel sections and vertical sections. The mechanical properties increase first and then decrease with increase of nano-CaCO3. The mechanical properties are affected by the cell structure, as well. The mechanical properties and cell structure are optimum when the content of nano-CaCO3 is 6 wt %.

Simulation Technology of Microcellular Injection Molding by using True 3 D Plastic Flow Analysis “Moldex 3 D”

Simulation Technology of Microcellular Injection Molding by using True 3 D Plastic Flow Analysis “Moldex 3 D”

Computer Analysis of Polyolefins Produced with Injection Molding Process Using Physical Blowing Agents

The complexity of today's plastic parts as well as the costs, quality makes it necessary to recognize potential errors early, already in the development phase of parts and moulds. Therefore, computer analysis became requisite part of any preprocessing phases. Due to the continuous effort decrease product weight and price many new techniques were developed recently. One relatively new end very effective technique is production of light construction products with using foaming agents. The main aim of this paper is therefore focuses on verification of computer analysis of Microcellular injection moulding process using physical blowing agents for the most used group of polymers which are polyolefins.

Foaming Mechanism of Polypropylene in Gas-Assisted Microcellular Injection Molding

In our recent study, it is reported that gas-assisted microcellular injection molding (GAMIM) is a promising method to fabricate plastic foams with good surface quality, high weight reduction, and improved mechanical performance. To clarify the foaming mechanism, the crystallization behaviors of polypropylene in both conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were studied. It is found that GAIM can significantly promote melt crystallization and refine crystals. To explore the improvement mechanism of crystallization, a numerical model was developed to simulate the filling and cooling processes of CIM and GAIM. The simulation results show that the much stronger shear field and the faster cooling rate lead to the refinement of crystals in GAIM. Finally, a crystallization-driven cell nucleation model was proposed to explain the improved foaming behavior of polypropylene in GAMIM. This paper provides a deep understanding of the foaming behavior and hence benefits the foaming process design.