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.

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.

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.

Due to the increasing demands on automotive components, manufacturers are relying on injection moulding components from fibre-reinforced polymers in an attempt to increase strength to weight ratio. The use of reinforcing fibres in injection moulded components has led to component failures whereby the material strength is hampered through the formation of weldlines which are also a problem for unreinforced plastics. In this study, an industrial demonstrator component has the injection locations verified through a combination of fibre orientation tensor simulation and optical microscopy analysis of key locations on the component. Furthermore, the automotive component manufactured from 30% glass fibre–reinforced polyamide 6-6 is simulated and optimized through a Taguchi parametric study. A comparison is made between the component, as it is currently manufactured, and the optimum processing parameters determined by the study. It was found that the component can be manufactured with roughly 7.5% fewer weldlines and with a mould fill time 132 ms quicker than the current manufacturing process.

Micro-injection moulding (µ-IM) is a fabrication method that is used to produce miniature parts on a mass production scale. This work investigates how the process parameter settings result in adiabatic heating from gas trapped and rapidly compressed within the mould cavity. The heating of the resident air can result in the diesel effect within the cavity and this can degrade the polymer part in production and lead to damage of the mould. The study uses Autodesk Moldflow to simulate the process and identify accurate boundary conditions to be used in a gas law model to generate an informed prediction of temperatures within the moulding cavity. The results are then compared to physical experiments using the same processing parameters. Findings from the study show that without venting extreme temperature conditions can be present during the filling stage of the process and that venting solutions should be considered when using µ-IM.