Open-cell foams are very versatile and continue to proliferate in our daily lives. Despite the bright future prospect, the open-cell plastic foam industry is facing many challenges. The use and end-of-life disposal the common non-recyclable thermosetting open-cell foams have received increasingly stringent regulation and public opposition over years. Hence, there is a strong demand and interest in the industry to devise strategies to produce recyclable thermoplastic open-cell foams with conventional processing processes, particularly with injection molding due to its manufacturing flexibility and cost-effectiveness. However, it has been technologically challenging to achieve foams with high foam expansion or open-cell morphologies with this process. Foams manufactured by conventional injection molding technique have a void faction much lower than 50%. Owing to that, the cell wall thicknesses are relatively thick, and therefore, it is very difficult for the bubbles meet with each other and create opening. Some open-cell structures had been reported earlier with injection molding, however, most of them were the result from excessive elongation of premature cells or from improper processing control;​ therefore, the overall structures tended to be very poor and not acceptable.

In this context, the objective of this thesis is to investigate open-cell foaming of recyclable thermoplastic foams with injection molding in conjunction with the mold-opening method to attain higher foam expansion. Processing control of the process was achieved by controlling the temperature of the foam expansion step through the varying the material cooling/dwelling time in the mold cavity. A numerical model was also established to provide a reasonable estimation on this material temperature, which is critical to the investigation of open-cell foaming with the proposed process. A clarifying-type crystal nucleating agent, MDBS, was also used to refine the size and increase the number density of the crystals of PP, and thereby, increasing the cell density for achieving thinner cell walls for cell wall opening. Results from the crystallization kinetics analysis, as well as the estimated temperature profile from the numerical simulations, were used to explain the evolution in the cellular morphologies and to postulate a potential mechanism for achieving open-cell and highly-reticulated foam structures in this study.