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Based on some assumptions, this study derives an equation that estimates the theoretical molding thickness hc of knitted fabric reinforced thermoplastic composites (KFRTC). This is required for controlling the actual molding thickness of KFRTC that are produced by hot pressing knitted preforms made from reinforcing fiber/matrix fiber commingled yarns. The relationship between the optimum actual molding thickness h that can result in composites with higher tensile strength and lower void content and the theoretical molding thickness hc are also determined experimentally in this study. It is found that the composites produced can have higher tensile property and lower void content when h and hc satisfy h = khc (0.85 ≤ k ≤ 0.95). This estimation of h contributes to manufacturing KFRTC.

Four groups of knitted glass fabric reinforced polypropylene (PP) composites with different cooling conditions are prepared by changing the method used in the cooling period. The crystal structure of the matrix in composites arising from different cooling conditions is studied using wide angle X-ray diffraction. It has been found that the various cooling conditions have no effect on the crystal types of the PP matrix. However, the crystal degree and sizes of microcrystal and spherulite vary with cooling conditions. They increase with the decrease of the cooling speed and the prolongation of the cooling time.