Errichiello, Fabrizio (2025) Implementing Graded Foaming Technology for Industrial Upscaling. [Tesi di dottorato]

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Abstract

This doctoral research explores the development and industrial implementation of graded foaming technology for thermoplastic materials. The study aims to establish a robust scientific and technological framework for producing polymeric foams with spatially varying density and properties, enabling the design of lightweight, high-performance and single-material components that meet localized functional requirements. By controlling the distribution of material density within a single polymer part, graded foaming provides a powerful means to enhance energy absorption, stiffness tuning, and overall structural efficiency while minimizing material usage. The research begins with an investigation of the fundamental relationships between processing conditions, foam morphology, and mechanical behavior. The effects of saturation pressure, temperature, and blowing agent selection are systematically examined to identify the governing parameters of gas dissolution, nucleation, and cell growth in thermoplastic polymers. Particular attention is paid to the mechanisms that control the onset of gradients during expansion and solidification. To accurately quantify local density variations, innovative non-destructive characterization techniques are developed, including 3D scanning and image-based reconstruction methods. Building on this foundation, the graded foaming approach is conceived as a design-driven methodology that integrates processing science with computational optimization. Through the coupling of process control and topology optimization, the applied loads (mechanical, thermal, acoustic, or other) are translated into targeted density profiles within the part. Prototype components designed following this approach, including protective shells and lightweight structural panels, demonstrate that tailored grading can substantially enhance mechanical efficiency without increasing overall density. The results highlight the potential of graded foaming as a bridge between advanced material design and scalable manufacturing. The core part of the research is focused on discontinuous foaming strategies, where gradient technology is applied through a variety of process techniques. In the case of bead foaming, expanded polymer beads with controlled densities are produced and simultaneously sintered into macroscopic structures in a single process. By modulating bead morphology and intra-bead welding conditions, continuous gradients can be replicated at the component level, resulting in enhanced mechanical performance. In addition, structured discontinuous foaming involves the creation of localized gradients directly within compact molded parts. This is achieved through selective saturation and expansion, enabling the realization of complex internal architectures within functional products such as shoe soles, protective elements, and flexible components compatible with industrial molding technologies. The concept is further extended to continuous manufacturing processes, particularly extrusion, to demonstrate the scalability of graded foaming beyond batch operations. By controlling the evolution of gas concentration, temperature, and pressure along the extrusion line, stable density gradients can be achieved over continuous profiles or sheets. This process configuration enables the production of lightweight panels, cushioning layers, and acoustic elements with position-dependent stiffness or damping properties, offering new possibilities for large-scale industrial applications. Overall, the thesis establishes graded foaming as a versatile and scalable technology that combines process innovation, material science, and computational design. The integration of experimental insights, advanced characterization, and prototype validation demonstrates the feasibility of tailoring foam morphology at multiple length scales through controllable gradients. The outcomes pave the way for a new generation of polymer foams designed according to functional requirements, representing a significant step toward the industrial realization of sustainable and high-performance graded materials.

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