Effect of mold-opening microcellular injection molding on the cell structure and properties of TPU foams

Abstract

To optimize the cellular structure and enhance the performance of thermoplastic polyurethane (TPU) foams, this work addresses the limitations of conventional microcellular injection molding (MIM) by employing a self-designed mold-opening mold and a mold-opening microcellular injection molding (MOMIM) process, which uses a self-designed expandable-cavity mold to produce finer and more uniform cells with higher cell density and lower foam density. It also simplifies the mold structure without complex linkage mechanisms, improves product surface quality, enhances process versatility, and reduces production costs. The effects of key MOMIM parameters—including mold-opening distance, holding pressure, holding time, mold-opening speed, and mold-opening delay time—on cell size and density, cell wall thickness, foam density (expansion ratio), as well as mechanical and thermal properties are comprehensively investigated. The results demonstrate that the optimal process is: mold-opening distance 3 mm, holding pressure 35 MPa, holding time 6 s, mold-opening speed 20 mm/s, and mold-opening delay time 8 s. Under these conditions, TPU foam exhibits an average cell diameter of 43.40 μm, which is reduced by 34.63% compared with MIM foams; a cell density of 1.6 × 10⁷ cells/cm³, 22.18 times higher than MIM foams; an average cell wall thickness of 4.03 μm, reduced by 72.36%; and a skin layer thickness of 0.400 mm, reduced by 22.77%. The compressive strength is 4.58 MPa and the compressive modulus is 6.95 MPa, corresponding to reductions of 22.23% and 22.88%, respectively. The thermal conductivity at room temperature is 0.071 W/(m·K), representing a 55.91% improvemen. By precisely tuning the MOMIM parameters, this study successfully achieves simultaneous optimization of cell structure and macroscopic performance, balancing mechanical robustness and thermal insulation. It expands the application scope of TPU foams toward lightweight packaging, thermal insulation in new-energy vehicles, and building energy-saving materials, and provide a solid theoretical basis for the fabrication of high-performance, multifunctional foams.