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Concepts of post‐yield fracture mechanics were used to characterize the crack initiation and propagation resistances of mica‐reinforced polypropylene containing different mica concentrations. Although mica addition leads to an apparently brittle composite, the crack initiation resistance is slightly increased with mica concentration up to 10 percent by weight; and significant improvement in crack propagation performance was found for polypropylene reinforced by up to 20 percent of mica in comparison to that of virgin polypropylene. The debonding of the interface between mica flakes and the matrix leads to a micro‐ductility ahead of the crack tip in which the matrix is able to pull‐out from mica particles and to stretch. This micro‐ductility also prevents the brittle, unstable crack propagation, which is due to the coalescence of voids in pure polypropylene. Above 20 percent of mica, the reduction of the effective amount of the matrix material results in a substantial drop in the resistance to crack growth of mica‐reinforced polypropylene.

Injection molding shrinkage deals with dimensional differences between a molded part and the cavity. By adding an array of orthogonal marks into a mold, local shrinkage values may be obtained by comparing dimensions of this array with dimensions of the array replicated on the surface of the parts. A profilograph is employed to obtain dimensional measurements, in the parallel to flow direction and in the cross flow direction. A sensitivity analysis is conducted to determine aspects of shrinkage evaluation causing uncertainty on the results. Prominent sources of uncertainty found are mark straightness defect and part warpage. Uncertainty on shrinkage is evaluated to 0.00025 mm/mm for a distance between the marks of 6.350 mm. Shrinkages have been evaluated locally for molded plates. Different distribution forms were observed for parallel to flow and cross flow shrinkage. Important anisotropy is also observed. The effects of holding pressure and injection velocity on shrinkages have been evaluated using a 2 3 factorial design of experiment for three locations on the plates. Finally, shrinkages for three mold geometries have been compared: constant thickness plate, variable in thickness symmetrical plate, and variable in thickness asymmetrical plate. Variable in thickness plates showed the importance of solidification dynamics on final shrinkages. POLYM. ENG. SCI., 46:1275-1283, 2006.

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This paper examines, through holding pressure, packing time, melt temperature, mold temperature and distance from the gate, the effect of molding conditins on the shrinkage of polypropylene and 40% calcium carbonate (CaCO3) filled polypropylene. The shrinkage longitudinal and transvrse to the flow direction were determined using a 127 × 76 × 4 mm thick plaque with a film gate. Marks were made ont he mold cavity to measure shrinkage at various distances from the gate. The results show that holding pressure and packing time are the most significant parameters. Cooling runners, however, could significantly influence local shrinkage values. Calcium carbonate reduces the shrinkage anisotropy as well as the cycle time of the molded parts. Shrinkage models have been developed using dual kriging statistical interpolation techniques and show an excellent fit with experimetnal data.

In this study, the distributions of both molecular orientation and crystallinity along the flow direction as well as across the thickness direction of injection‐molded specimens of poly(ethylene terephthalate) (PET) molded at different mold temperatures were investigated. The degree of molecular orientation at the surface of the specimens was compared with that of other injected materials (polystyrene, high density polyethylene, liquid crystal polymer) showing different thermal, rheological, and crystallization characteristics. It was found that the molecular orientation at the skin layer of the molding increases with the polymer relaxation time, the rigidity of the polymer molecules, and the crystallization rate of the polymer. Moreover, in the case of PET, it was found that the crystallinity at the skin layer and in the core of the molding depends on the mold temperature. For low mold temperatures, near the gate, the maximum of crystallinity was observed at the subskin layer because of the “shear‐induced crystallization” generated during the filling stage. On increasing the mold temperature, the maximum of crystallinity was found to shift to the skin layer as a result of the decrease of the thickness of this layer. For low mold temperatures, the variation of the molecular orientation in the thickness direction was found to be much the same as for the crystallinity of the polymer. This result indicates that the shear‐induced crystallization process improves the degree of molecular orientation in the flow direction since it inhibits the relaxation process of the polymer molecules.

Weldlines represent potentially fatal sources of weakness in injection molded thermoplastic parts. This is particularly true for filled and reinforced composi tions where usual remedies consisting in a modification of processing conditions such as increase of melt temperature, injection speed, and pressure do not always work. This paper deals with weldline strength of injection molded polypropylene containing spherical and irregularly shaped fillers (glass spheres, calcium carbonate), or reinforcements such as glass flakes and fibers, talc, and mica. Two types of weldline will be considered: weldlines formed when two melt streams meet head-on and those formed as a result of flow around an insert. It will be shown that the loss of strength is greatest in compositions reinforced with flakes and fibers. This is due to flow induced filler orientation in the weldline zone. Approaches to reduce the loss of strength which make use of known flow behavior of suspensions of fibers and flakes will be illustrated.