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Bead foams have been widely used for more than 70 years, with expandable polystyrene and expanded polypropylene (EPP) being the main materials. There are different processes for producing bead foams, depending heavily on the material used (e.g., their thermal behavior). EPP is usually produced by a discontinuous stirring autoclave process, which is the main subject of this study. In this process, thermal treatment during the saturation phase leads to the formation of a second melt peak, which is considered an important prerequisite for the subsequent welding process, in which the individual foamed beads are welded into complex shaped parts by applying saturated steam to the beads in a cavity (so-called steam chest molding). To date, EPP is one of the main bead foams used industrially, but other materials such as polylactide and thermoplastic polyurethane can also be processed using the same technique. This review focuses on the important thermal and physical mechanisms during saturation and expansion and the parameters (material and process) that affect them. The process itself provides multiple possibilities to influence thermal behavior (i.e., crystallization) and expansion. Typical properties of resulting bead foams, which are found in many applications, are also considered.

Developing novel foams with tailored properties is a challenge. If properly addressed, efficient screening can potentially accelerate material discovery and reduce material waste, improving sustainability and efficiency in the development phase. In this work, we address this problem using a hybrid experimental and theoretical approach. Machine learning (ML) models were trained to predict the density of polylactide (PLA) foams based on their processing parameters. The final ML ensemble model was a linear combination of gradient boosting, random forest, kernel ridge, and support vector regression models. Comparison of the actual and predicted densities of PLA systems resulted in a mean absolute error of 30 kg·m−3 and a coefficient of determination (R 2) of 0.94. The final ensemble model was then used to explore the ranges of predicted density in the space of processing parameters (temperature, pressure, and time) and to suggest some parameter sets that could lead to low-density PLA foams. The new PLA foams were produced and showed experimental densities in the range of 36–48 kg·m−3, which agreed well with the corresponding predicted values, which ranged between 38 and 54 kg·m−3. The experimental–theoretical procedure described here could be applied to other materials and pave the way to more sustainable and efficient foam development processes.