Carbon dioxide (CO₂) in a supercritical condition is utilized to develop hierarchical cellular structures in injection molding. Polypropylene is used as sample semicrystalline polymer. First, a single-phase CO₂-charged melt was produced under pressure. Then, a gradient cell structure was induced onto the polymer melt by pressure reduction and crystal-induced shrinkage. A visualization mold was employed to monitor real-time nucleation of CO₂ cells within the mold cavity. Distinctive cell nucleation behaviour was visualized for the first time in which a myriad of ultra-fine cells nucleated rapidly inside the system in an abnormal manner. This type of cell nucleation was different from that occurring due to a pressure drop during foam processing. This mode of cell nucleation was likely driven by the crystallization of the polypropylene chains in the presence of the supercritical CO₂. The crystallization behaviour of polypropylene was then investigated within the mold cavity and in the presence of dissolved CO₂ by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The presence of supercritical CO₂ affected the size of α and β crystals. Furthermore, γ crystals were detected in the core regions of samples. The formed crystals served as effective nucleating sites which promoted the nucleation of large number of fine cells and produced structurally graded foams. The effects of CO₂ content, melt packing, and melt temperature on the cell structure development was studied. It was concluded that these parameters can be regulated to design cellular structures with desired cell size and cell morphologies.

超临界条件下的二氧化碳 (CO ₂ ) 用于在注塑成型中开发分层蜂窝结构。聚丙烯用作样品半结晶聚合物。首先，在压力下产生单相填充CO ₂的熔体。然后，通过压力降低和晶体诱导收缩在聚合物熔体上引入梯度单元结构。使用可视化模具实时监测 CO _{2 的}成核模腔内的细胞。首次观察到独特的细胞成核行为，其中无数超细细胞以异常方式在系统内部快速成核。这种类型的泡孔成核与由于泡沫加工过程中的压降而发生的泡孔成核不同。这种细胞成核模式可能是由超临界 CO ₂存在下聚丙烯链的结晶驱动的。然后通过广角 X 射线衍射 (WAXD) 和差示扫描量热法 (DSC)在模腔内和溶解的 CO ₂存在下研究聚丙烯的结晶行为。超临界 CO ₂的存在影响 α 和 β 晶体的大小。此外，在样品的核心区域检测到γ晶体。形成的晶体作为有效的成核位点，促进了大量细孔的成核并产生了结构渐变的泡沫。研究了CO ₂含量、熔体堆积和熔体温度对泡孔结构发展的影响。得出的结论是，可以调节这些参数以设计具有所需细胞大小和细胞形态的细胞结构。