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%.

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-Fizz^TM 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.

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.

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.

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.

Rheological behavior, crystallization properties, and foaming performance of chain-extended poly (lactic acid) by functionalized epoxy

The branched/micro-crosslinked structure formed by chain extension reaction between EGMA and PLA improved the rheological behavior and crystallization properties of PLA, which ameliorated the foaming performance of various PLA samples.

A Design of Experiment Approach for Surface Roughness Comparisons of Foam Injection-Moulding Methods

The pursuit of polymer parts produced through foam injection moulding (FIM) that have a comparable surface roughness to conventionally processed components are of major relevance to expand the application of FIM. Within this study, 22% talc-filled copolymer polypropylene (PP) parts were produced through FIM using both a physical and chemical blowing agent. A design of experiments (DoE) was performed whereby the processing parameters of mould temperatures, injection speeds, back-pressure, melt temperature and holding time were varied to determine their effect on surface roughness, Young’s modulus and tensile strength. The results showed that mechanical performance can be improved when processing with higher mould temperatures and longer holding times. Also, it was observed that when utilising chemical foaming agents (CBA) at low-pressure, surface roughness comparable to that obtained from conventionally processed components can be achieved. This research demonstrates the potential of FIM to expand to applications whereby weight saving can be achieved without introducing surface defects, which has previously been witnessed within FIM.

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.

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.