A technique for initiating the formation of pores in a polymeric material that contains a thermoplastic composition is provided. The thermoplastic composition contains microinclusion and nanoinclusion additives dispersed within a continuous phase that includes a matrix polymer. To initiate pore formation, the polymeric material is mechanically drawn (e.g., bending, stretching, twisting, etc.) to impart energy to the interface of the continuous phase and inclusion additives, which enables the inclusion additives to separate from the interface to create the porous network. The material is also drawn in a solid state in the sense that it is kept at a temperature below the melting temperature of the matrix polymer.

The invention disclosed herein relates to relates to foamed thermoplastic material objects and articles of manufacture having an internal layered cellular structure, as well as to methods of making the same. In one embodiment, the invention is directed to a multi-layer foamed polymeric article of manufacture, comprising: a non-laminated multi-layer thermoplastic material sheet, wherein the multi-layer thermoplastic material sheet has first and second discrete outer layers sandwiching a plurality of discrete inner foamed layers, and wherein the two outer layers and plurality discrete inner foamed layers are integral with one another. The thermoplastic material may be a semi-crystalline polymer such as, for example, PET (polyethylene terephthalate), PEEK (polyetheretherketone), PEN (polyethylene naphthalate), PBT (polybutylene terephthalate), PMMA (polymethyl methacrylate), PLA (polylactide), polyhydroxy acid (PHA), thermoplastic urethane (TPU), or blends thereof. The two outer layers may be unfoamed skin layers having smooth outer surfaces, and the discrete inner foamed layers may be microcellular.