Effect of Gas Counter Pressure on the Surface Roughness, Morphology, and Tensile Strength between Microcellular and Conventional Injection-Molded PP Parts

Microcellular injection-molded parts have surface defect problems. Gas counter pressure (GCP) is one of the methods to reduce surface defects. This study investigated the effect of GCP on the surface roughness, morphology, and tensile strength of foamed and conventional injection-molded polypropylene (PP) products. GCP is generated by filling up the mold cavity with nitrogen during the injection-molding (IM) process. It can delay foaming and affect flow characteristics of microcellular and conventional injection-molding, which cause changes in the tensile strength, flow length, cell morphology, and surface quality of molded parts. The mechanism was investigated through a series of experiments including tuning of GCP and pressure holding duration. Surface roughness of the molded parts decreased with the increase in GCP and pressure holding duration. Compared to microcellular IM, GCP-assisted foaming exhibited much better surface quality and controllable skin layer thickness.

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.

A Novel Hybrid Foaming Method for Low-Pressure Microcellular Foam Production of Unfilled and Talc-Filled Copolymer Polypropylenes

Unfilled and talc-filled Copolymer Polypropylene (PP) samples were produced through low-pressure foam-injection molding (FIM). The foaming stage of the process has been facilitated through a chemical blowing agent (C₆H₇NaO₇ and CaCO₃ mixture), a physical blowing agent (supercritical N₂) and a novel hybrid foaming (combination of said chemical and physical foaming agents). Three weight-saving levels were produced with the varying foaming methods and compared to conventional injection molding. The unfilled PP foams produced through chemical blowing agent exhibited the strongest mechanical characteristics due to larger skin wall thicknesses, while the weakest were that of the talc-filled PP through the hybrid foaming technique. However, the hybrid foaming produced superior microcellular foams for both PPs due to calcium carbonate (CaCO₃) enhancing the nucleation phase.

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.

Lightweight High-Performance Polymer Composite for Automotive Applications

The automotive industry needs to produce plastic products with high dimensional accuracy and reduced weight, and this need drives the research toward less conventional industrial processes. The material that was adopted in this work is a glass-fiber-reinforced polyamide 66 (PA66), a material of great interest for the automotive industry because of its excellent properties, although being limited in application because of its relatively high cost. In order to reduce the cost of the produced parts, still preserving the main properties of the material, the possibility of applying microcellular injection molding process was explored in this work. In particular, the influence of the main processing parameters on morphology and performance of PA66 + 30% glass-fiber foamed parts was investigated. An analysis of variance (ANOVA) was employed to identify the significant factors that influence the morphology of the molded parts. According to ANOVA results, in order to obtain homogeneous foamed parts with good mechanical properties, an injection temperature of 300 °C, a high gas injection pressure, and a large thickness of the parts should be adopted.

Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding

The processability of injection molding ultra-high molecular weight polyethylene (UHMWPE) was improved by introducing supercritical nitrogen (scN₂) or supercritical carbon dioxide (scCO₂) into the polymer melt, which decreased its viscosity and injection pressure while reducing the risk of degradation. When using the special full-shot option of microcellular injection molding (MIM), it was found that the required injection pressure decreased by up to 30% and 35% when scCO₂ and scN₂ were used, respectively. The mechanical properties in terms of tensile strength, Young's modulus, and elongation-at-break of the supercritical fluid (SCF)-loaded samples were examined. The thermal and rheological properties of regular and SCF-loaded samples were analyzed using differential scanning calorimetry (DSC) and parallel-plate rheometry, respectively. The results showed that the temperature dependence of UHMWPE was very low, suggesting that increasing the processing temperature is not a viable method for reducing injection pressure or improving processability. Moreover, the use of scN₂ and scCO₂ with UHMWPE and MIM retained the high molecular weight, and thus the mechanical properties, of the polymer, while regular injection molding led to signs of degradation.

Investigation of Thermal and Thermomechanical Properties of Biodegradable PLA/PBSA Composites Processed via Supercritical Fluid-Assisted Foam Injection Molding

Bio-based polymer foams have been gaining immense attention in recent years due to their positive contribution towards reducing the global carbon footprint, lightweighting, and enhancing sustainability. Currently, polylactic acid (PLA) remains the most abundant commercially consumed biopolymer, but suffers from major drawbacks such as slow crystallization rate and poor melt processability. However, blending of PLA with a secondary polymer would enhance the crystallization rate and the thermal properties based on their compatibility. This study investigates the physical and compatibilized blends of PLA/poly (butylene succinate-co-adipate) (PBSA) processed via supercritical fluid-assisted (ScF) injection molding technology using nitrogen (N₂) as a facile physical blowing agent. Furthermore, this study aims at understanding the effect of blending and ScF foaming of PLA/PBSA on crystallinity, melting, and viscoelastic behavior. Results show that compatibilization, upon addition of triphenyl phosphite (TPP), led to an increase in molecular weight and a shift in melting temperature. Additionally, the glass transition temperature (T_g) obtained from the tanδ curve was observed to be in agreement with the T_g value predicted by the Gordon–Taylor equation, further confirming the compatibility of PLA and PBSA. The compatibilization of ScF-foamed PLA–PBSA was found to have an increased crystallinity and storage modulus compared to their physically foamed counterparts.

Morphological, mechanical, and thermal properties of injection molded polylactic acid foams/composites based on wood flour

In this work, injection molding was used to produce polylactic acid foams using azodicarbonamide as a chemical foaming agent and to study the effect of wood flour concentration (15, 25, and 40% wt.) on morphology (scanning electron microscopy), density (gas pycnometry), as well as mechanical (tensile, flexural, and impact) and thermal (differential scanning calorimetry) properties. In particular, density reduction was controlled by the amount of material injected (shot size). The results showed that polylactic acid properties increased with wood content, but decreased with density reduction. Nevertheless, specific flexural modulus (per unit weight) always increased with foaming. Foaming was also shown to significantly increase polylactic acid crystallinity.

The Influence of Blowing Agent Addition, Glass Fiber Filler Content and Mold Temperature on Selected Properties, Surface State and Structure of Injection Molded Parts from Polyamide 6

The effects of blowing agent and glass fiber addition and also mold temperature on selected properties of PA6 molded parts was presented in this work. Blowing agent was dosed to plastic in amounts 0.5 – 1%, and glass fiber was added in amounts 15 – 30%. Furthermore, molded parts from unfilled and unfoamed PA6 was also investigated. The experimental plan was prepared using Design of Experiments (DoE) method. The results of selected part properties: molded parts weight, thickness, hardness, impact strength, tensile strength, elongation at maximum force, and also surface state of molded parts (gloss and color) was presented. The article also presents microscopic investigations (using SEM method) of molded parts with blowing agent and glass fiber. It was found, that the glass fiber content has a larger impact on mechanical properties of parts than addition of blowing agent. The use of the blowing agent in an amount of 1% wt. will allow the reduce injection cycle time by reducing the hold pressure and hold time, without significant worsening their properties. The mold temperature has an impact especially on the gloss of molded parts and the pore size.

Comparative study of chemical and physical foaming methods for injection-molded thermoplastic polyurethane

Thermoplastic polyurethane is one of the most versatile thermoplastic materials being used in a myriad of industrial and commercial applications. Thermoplastic polyurethane foams are finding new applications in various industries including the furniture, automotive, sportswear, and packaging industries because of their easy processability and desirable customizable properties. In this study, three methods of manufacturing injection molded low density foams were investigated and compared: (1) using chemical blowing agents, (2) using microcellular injection molding with N2 as the blowing agent, and (3) using a combination of supercritical gas-laden pellets injection molding foaming technology and microcellular injection molding processes using co-blowing agents CO2 and N2. Thermal, rheological, microscopic imaging, and mechanical testing were carried out on the molded samples with increasing amounts of blowing agents. The results showed that the use of physical blowing agents yielded softer foams, while the use of CO2 and N2 as co-blowing agents helped to manufacture foams with lower bulk densities, better microstructures, and lower hysteresis loss ratios. Chemical blowing agent-foamed thermoplastic polyurethane showed an earlier onset of degradation. The average cell size decreased and the cell density increased with the use of co-blowing agents. A further increase in gas saturation levels showed a degradation of microstructure by cell coalescence.

Microcellular injection molding of recycled poly(ethylene terephthalate) blends with chain extenders and nanoclay

Poly(ethylene terephthalate) (PET) resin is one of the most widely used thermoplastics, especially in packaging. Due to thermal and hydrolytic degradations, recycled PET (RPET) exhibits poor mechanical properties and lacks moldability. The effects of adding chain extender (CE) and nanoclay to RPET were investigated. Melt blending of RPET with CE was performed in a thermokinetic mixer (K-mixer). The blended materials were then prepared via solid and microcellular injection molding processes. The effects of CE loading levels and the simultaneous addition of nanoclay on the thermal and mechanical properties and cell morphology of the microcellular components were noted. The addition of 1.3% CE enhanced the tensile properties and viscosity of RPET. The higher amount of CE (at 3%) enhanced the viscosity, but the margin of improvement in mechanical properties diminished. While the solid RPET and CE blends were fairly ductile, the samples with nanoclay and all microcellular specimens showed brittle fractural behavior. Finally, nanoclay and the increase of CE content decreased the average cell size and enlarged the cell density of the microcellular samples.

Morphology, mechanical and dielectric properties, and rheological behavior of EAGMA toughened microcellular PEI–EAGMA foam

EAGMA plays an important role in the microcellular structure formation and had an excellent toughening effect on the PEI–EAGMA system.

The Impact of Mould Temperature and Blowing Agent Content on Structure and Properties of Injection Moulded Parts

The investigation of the influence of mould temperature and blowing agent percentage on structure, and selected properties (weight, mechanical properties, surface state - gloss and colour) of moulded parts from HDPE was the aim of this work. The structure of moulded parts in cross-section perpendicular and parallel to the direction of polymer flow, and quantitative assessment of porous structure using microscopic image analysis were carried out. It was shown that the content of blowing agent in polymer influences not only the amount of pores in the part core, but also the thickness of solid skin. Lower mould temperature favors the formation of a fine porous structure. The formation of the porous structure influenced slightly the weight and mechanical properties of moulded parts. With the increase in blowing agent content, the gloss of parts decreased and also caused the change in their colour. Parts with higher amount of blowing agent were brighter.

Solid and Microcellular Polylactide-Carbon Nanotube Nanocomposites

In this study, polylactide (PLA)-multi-walled carbon nanotube (MWCNT) nanocomposites were melt-compounded using a twin-screw extruder. Solid and microcellular tensile bar specimens were produced via conventional and microcellular injection molding, respectively. Various characterization techniques were applied to study the static and dynamic mechanical properties, degree of MWCNT dispersion, cell morphology, and crystallization behavior. The addition of a small amount of MWCNTs led to a decrease in the cell size and an increase in the cell density of the microcellular PLA specimens. A transmission electron microscopy analysis of the PLA-MWCNT specimens revealed a higher degree of MWCNT dispersion in the microcellular PLA-MWCNT composite compared with its solid counterpart, indicating that the microcellular injection molding process further dispersed the MWCNTs. For both solid and microcellular specimens, the addition of 1.5 wt% MWCNTs reduced the specific strength, specific toughness and strain-at-break while exerting less impact on the specific modulus. The storage modulus was not affected significantly with the addition of MWCNTs, but was found to be higher for the microcellular specimens compared with their solid counterparts. Finally, the crystallinity of PLA increased with the addition of MWCNTs.

Injection Molded Solid and Microcellular Polylactide Compounded with Recycled Paper Shopping Bag Fibers

Recycled paper shopping bag fibers were melt-compounded using a batch mixer with biobased/biodegradable polylactide (PLA) at 10 and 30 wt.% using silane as a coupling agent. These PLA/fiber composites were then injection molded to produce both solid and microcellular tensile bars. The mechanical properties (specific modulus, specific tensile strength, specific toughness, and strain at break) of the neat PLA and PLA composites were tested and the cell morphology of the microcellular samples was examined using scanning electron microscopy. It was observed that the addition of the recycled paper shopping bag fibers resulted in an increase in cell density and decrease in average cell size for the microcellular components when compared with the neat PLA. The addition of the fibers increased the specific modulus of both solid and microcellular components, and high fiber contents (30 wt.%) resulted in an increase in specific tensile strength, yet yielded a decrease in the strain at break and specific toughness. The storage modulus was also improved with the addition of 10 and 30 wt.% fibers for both solid and microcellular components.