Effect of Film Thickness on Crystallization Behavior and Mechanical Properties of Polymer Products Molded by Fiber‐Reinforced Microcellular Injection Molding Combined With In‐Mold Decoration

ABSTRACT Fiber‐reinforced microcellular injection molding (FR‐MIM) is a key technique for producing high‐performance, lightweight polymer products. Controlling polymer melt crystallization is essential for achieving optimal mechanical properties. However, the fiber‐reinforced phase reduces polymer chain mobility, slowing crystallization and complicating the formation of a uniform, load‐bearing structure. Additionally, the process is prone to surface defects such as floating fibers and bubble marks, which result from fountain flow, high shear forces, and cold tool walls. These challenges make it difficult to improve both crystallinity and surface quality in FR‐MIM. To address these issues, we introduce an enhanced FR‐MIM process that incorporates in‐mold decoration (FR‐MIM/IMD). This process applies a decorative film that not only improves surface quality but also provides thermal retardation. The film conceals surface defects and slows the cooling rate of the polymer melt, allowing it to remain within the crystallization temperature range longer. This extended cooling time promotes polymer chain alignment, enhancing nucleation and crystallization. We investigate the effect of film thickness on polymer crystallization and mechanical properties through numerical simulations and experimental tests. Additionally, we evaluate the film's effect on surface defects. Our findings show that the FR‐MIM/IMD process increases melt temperature during filling and cooling, accelerating crystallization and improving crystallinity, grain size, and β‐phase content. However, thicker films may reduce effectiveness by inhibiting crystallization, lowering fiber orientation, increasing cell size, and weakening mechanical strength. Furthermore, the film's heat retention helps re‐encapsulate uncured bubbles and fibers in the polymer melt, reducing visible defects while concealing residual imperfections beneath the film.