Study of the bubble nucleation and growth mechanisms in high-pressure foam injection molding through in-situ visualization

Optimisation of 3D Printing for Microcellular Polymers

Polymers are extensively used in various industries due to their versatility, durability and cost-effectiveness. To ensure functionality and longevity, polymer parts must have sufficient strength to endure external forces without deformation or breakage. Traditional approaches to increasing part strength involve adding more material; however, balancing strength to weight relationships is challenging. This paper explorers the viability of manufacturing lightweight components using a microcellular foaming polymer. Microcellular foaming has emerged as a helpful tool to achieve an optimal strength-to-weight ratio; offering advantages such as lightweight, improved mechanical properties, reduced material usage, better insulation and improved cost-effectiveness. It can also contribute to improved fuel efficiency and reduced carbon emissions, making them environmentally favourable. The combination of additive manufacturing (AM) and microcellular foaming has opened new possibilities for design innovation. This text highlights the challenges and efforts in incorporating foaming techniques into 3D printing processes, specifically fused filament fabrication (FFF). This study reveals that microcellular polymers are a viable option when balancing part strength and weight. The experiments completed during the formulation of this paper demonstrated that lightweight LW-PLA parts were significantly lighter than standard PLA parts and that a design of experiments approach can be used to optimise strength properties and provide insights into optimising manufacturability. Microcellular polymers present an opportunity for lighter and stronger 3D printed parts, offering potential energy and material savings for sustainable manufacturing practices.

Super toughened blends of poly(lactic acid) and poly(butylene adipate-co-terephthalate) injection-molded foams via enhancing interfacial compatibility and cellular structure

Biodegradable poly(lactic acid) (PLA) foams have drawn increasing attention due to environmental challenges and petroleum crisis. However, it still remains a challenge to prepare PLA foams with fine cellular structures and high impact property, which significantly hinders its widespread application. Herein, phase interface-enhanced PLA/ poly(butylene adipate-co-terephthalate) (PBAT) blend foam, modified by a reactive compatibilizer through a simple reactive extrusion, was produced via a core-back foam injection molding technique. The obtained PLA blend foams displayed an impact strength as high as 49.1 kJ/m², which was 9.3 and 6.4 times that of the unmodified PLA/PBAT blend and its corresponding foam, respectively. It proved that the interfacial adhesion and cell size both strongly affected the impact strength of injection-molded PLA/PBAT foams, and two major conclusions were proposed. First, enhancing interfacial adhesion could cause a brittle-tough transition of PLA/PBAT foams. Additionally, for foams with high interfacial adhesion, small cell size (<12 μm) was more favorable for the stretching of cells and extension of the whitened region in comparison with big cell size (cell size >60 μm), leading to the drastic toughening of PLA blends. This study provides a feasible, industrially scalable and practical strategy to prepare super toughened and fully biodegradable PLA materials.

Fabrication, property characterization, and benefit analysis of mixing mechanism of nitrogen and melt, and its comparison of the porous-foam polypropylene injection molding parts

In this study, an injection molding machine with a mixing mechanism of nitrogen (N2) and melt was designed, and the melt-fill/porous-foaming behavior is observed under the novel barrel design (plasticizing stage) of the gas/melt mixing for the porous injection molded thermoplastic. The thermoplastic employed in this study was polypropylene (PP), and the gas for forming the porous structure is N2. In addition, a thickness of 5 mm and a width of 10 mm paper-clip shape and the mold were constructed for studying the melt-fill-length and fill-length ratio through an experiment. The experimental results showed that the use of an injection molding machine with a mixing mechanism of N2 and melt decreased the melt-fill-length when the N2-output pressure was increased. The reason is that when the gas output, the speed of the screw will be affected. Therefore, during the gas/melt mixing and the plasticization rate will also affect the volume of the foam and the melt. But during plasticizing setting back pressure, can improve its melt volume reduction. When passing through the mixing mechanism and the injected melt, the melt is filled into the mold cavity, and the pressure in the melt is released/porous-foaming grows. At the same time, when the output pressure increases, the amount of melt in the injection barrel will decrease, and its relative porous structure/density distribution will increase. In addition, the mixing/flow direction of the melt impacted the density distribution and dispersion of porous foaming, thus the sample weight/shrinkage of melt-fill-length test sample (Mfl-ts) was improved.

Study on the definition, mechanism and controllability of secondary bubbles based on the bubble nucleation model in injection foaming polypropylene

The process of nucleation and growth of polypropylene foam was observed by using visualizations of the mold-opening foam injection molding (MOFIM) and free foaming (FREEF). The fitting of the mathematical model formula was used to supplement the judgment conditions of the secondary bubbles to explore the generation process and formation conditions of the secondary bubbles. The results of changes in blowing agent content and melt temperature proved the rationality of the judgment basis and the appearance of secondary bubbles started from the late stage of balanced-foaming. Then, the combined action of several nucleation mechanisms led to the emergence of secondary bubbles, which was observed utilizing glass fibers as nucleating agents and tracers. The data for the two foaming modes indicated that the formation of secondary bubbles is closely related to temperature and pressure drop. The bubble nucleation model was amended and validated by regulating the temperature variation in the mold cavity to control the number of secondary bubbles, which enabled the nucleation process of secondary bubbles to be fitted to an "S-shaped" curve. Finally, a controllable number of secondary bubbles was obtained from the bimodal bubble structure. Herein, this study enriches our understanding of the formation process of secondary bubbles, and provides theoretical guidance for fabricating high density, small size foam materials.

Autoclave foaming and steam-chest molding of polypropylene/polybutene-1 blend bead foams and their crystallization and mechanical properties

Expanded polypropylene (EPP) foams have showed wide applications in our daily life, such as automotive and packaging. Usually, autoclave foaming combined with steam-chest molding is the main artwork to prepare the high-precision EPP foam products. However, the foaming behavior of EPP and the excessive pressure required for molding still need to be further improved, which is great significance for energy saving and cost saving, etc. Herein, this study finds that adding a certain amount of polybutene-1 (PB-1) into the PP can help to reduce the temperature and pressure required for foaming/molding, and to broaden the foaming temperature. For example, in order to make the foam beads bonding well and with the expansion ratio of 20, the molding pressure should be higher than 2.7 bar for Neat PP foams, but just 1.5 bar for PP/PB-1 mixtures. Moreover, the effects of PB-1 content on the crystallization properties and foaming/molding behaviors of the PP/PB-1 bead foams are illustrated, and then the mechanical properties are also studied. Furthermore, the low-pressure foaming strategy presented here is beneficial for reducing the barriers of energy consumption and promoting the development of new bead foam materials.

Effect of Cell Morphology on Flexural Behavior of Injection-Molded Microcellular Polycarbonate

The quantitative study of the structure and properties relationship in cellular materials is mostly limited to cell diameter, cell density, skin layer thickness, and cell size distribution. In addition, the investigation of the morphology is generally carried out in two dimensions. Therefore, the interrelation between morphological properties and mechanical characteristics of the foam structure has remained in an uncertain state. In this study, during the physical foaming process, a foam morphology is locally created by using a mold equipped with a core-back insert. The variation in morphology is obtained by modifying the mold temperature, injection flow rate, and blowing agent content in the polymer melt. X-ray microtomography (μCT) is used to acquire the 3D visualization of the cells structure. The Cell Distribution Index (CDI) is calculated to represent the polydispersity in cell size distribution. The relationship between the wide range of morphological qualities and relevant flexural properties is made explicit via a statistical model. According to the results, the morphology, particularly cell shape, characterizes the mechanism of the linear elastic deformation of the closed-cell foams. IR-thermography reveals the bending failure of cellular structures in the tensile region despite the differences in cell diameter.

Analysis of Bubble Growth in Supercritical CO2 Extrusion Foaming Polyethylene Terephthalate Process Based on Dynamic Flow Simulation

Bubble growth in the polymer extrusion foaming process occurs under a dynamic melt flow. For non-Newtonian fluids, this work successfully coupled the dynamic melt flow simulation with the bubble growth model to realize bubble growth predictions in an extrusion flow. The initial thermophysical properties and dynamic rheological property distribution at the cross section of the die exit were calculated based on the finite element method. It was found that dynamic rheological properties provided a necessary solution for predicting bubble growth during the supercritical CO₂ polyethylene terephthalate (PET) extrusion foaming process. The introduction of initial melt stress could effectively inhibit the rapid growth of bubbles and reduce the stable size of bubbles. However, the initial melt stress was ignored in previous work involving bubble growth predictions because it was not available. The simulation results based on the above theoretical model were consistent with the evolution trends of cell morphology and agreed well with the actual experimental results.

Modification of sodium bicarbonate and its effect on foaming behavior of polypropylene

As a potential physical blowing agent, sodium bicarbonate (SB) is environmentally friendly and low in cost, but its low decomposition temperature cannot meet the requirements of polyolefin foam materials. Herein, for enhancing the thermal properties of SB, a modified way was offered to fabricate various SB-based capsules via suspension polymerization. As the modified SB-based capsules, epoxy resin (EP) accompanied with several organic acids was successfully coated on the surface of SB, serving as heat-insulation layer of SB. Various physicochemical characterizations provided reliable evidences for the good coating effect, and the thermal performance of the modified SB was improved. Further, the composite SB capsules were applied for the foaming of polypropylene (PP), and the foaming behavior of the SB-based capsules in PP was significantly improved, with more uniform distribution, smaller cell diameter, and higher cell density. In all, this work fully proved that the coated shells enhanced the thermal properties of SB, and the modified SB capsules significantly improved the foaming quality of foamed PP.

In-Situ Visualization of the Cell Formation Process of Foamed Polypropylene under Different Foaming Environments

In this paper, the dynamic foaming process of micro-foaming polypropylene (PP) in different foaming environments in real time was obtained via a visualization device. The relationship curve between cell number (n) and foaming time (t) was plotted, and then the nucleation kinetics of foam cells was analyzed. Results showed that the formation rate of cells changed obviously with the variation of melt temperature and the content of the foaming agent. The n-t curves presented a typical "S" shape, which indicated that the appearance of the cell number increased slowly in the initial foaming period, then increased rapidly in a short time, and finally maintained a certain value. When a certain pressure was applied to the PP melt, the external force had a great influence on the n-t curve. With the increasing external force, the rate of cell formation increased rapidly, and the shape of the n-t curve changed from "S" to "semi-S" without an obvious slow increase. The investigation of the n-t relationship in the PP dynamic foaming process under different foaming environments could provide effective bases for improving the foaming quality of injection molding foaming materials.

Experimental investigation on the forming and evolution process of cell structure in gas counter pressure assisted chemical foaming injection molded parts

By using a standard stretch spline as the research object, the influence of gas counter pressure (GCP) technology on melt foaming behavior in chemical foaming injection molding (CFIM) process was investigated. Related experimental line for GCP assisted CFIM foam was designed, and the effect of GCP technology on melt flow front, spline surface quality and internal cell was studied. According to the results obtained from the experiment, two critical GCP pressures and one critical GCP holding time were innovation proposed. Two critical GCP pressures are the critical GCP pressure of melt flow front cell not cracking and the critical GCP pressure of melt not foaming, respectively. The critical GCP holding time is the secondary foaming behavior time. Based on the proposed critical GCP pressures and critical GCP holding time, the influence mechanism of GCP technology on melt foaming action during CFIM process was revealed.

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.

Effect of Mold Opening Process on Microporous Structure and Properties of Microcellular Polylactide⁻Polylactide Nanocomposites

Cell structure is a key factor that determines the final properties of microcellular polylactide (PLA) product. In the mold opening process, adjusting the rate of mold opening can effectively control cell structure. PLA and PLA composites with a void fraction as high as 50% were fabricated using the mold opening technique. The effects of mold opening rate and the addition of nanoclay on the cell structure, mechanical properties, and surface quality of microcellular PLA and PLA composites samples were investigated. The results showed that finer cell structure was received in the microcellular PLA samples and the surface quality was improved effectively when decreasing the rate of mold opening. The effect of mold opening rate on the foaming behavior of microcellular PLA⁻nanoclay was the same as that of microcellular PLA. The addition of 5 wt % nanoclay significantly improved the foaming properties, such as cell density, cell size, and structural uniformity, which consequently enhanced the mechanical properties of foams and the surface quality.