Molding processes skin-core morphology

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

Injection molding is the most widely used polymeric fabrication method and can produce WPCs products with complex three-dimensional shapes. In this process, WPCs pellets are heated and then the molten composites are forced into mold, often with a plunger action. The mold is cool, and after the part cools the mold opens and ejects it. Injection molded WPCs panels usually appear in a skin-core morphology. In such composites, fibers in the core layer are oriented perpendicularly to flow, while those in the skin layer are oriented parallel to flow [35, 36]. Temperature, pressure and flow are the three main variables that can influence the properties of composites. For example, a low mold temperature leads to a large skin thickness, while smaller thickness can appear when using higher barrel temperature, higher screw and injection speed. 

The crystallinity and crystallite size of injection molded isotactic polypropylene was measured by wide angle X-ray diffraction(WAXD) and the distinct skin-core morphology was visible under a polarizing optical microscope. The results show that the crystal structures are dependent on the injection molding processing conditions. The crystallinity and crystallite size decreases with the distance from the gate. The skin layer thickness is thinner with the higher injection temperature. 

Syndiotactic polystyrene will strain-induce crystalhze, as well as quiescently crystallize as discussed previously. Furthermore, imder certain conditions, it may also be quenched to the amorphous state. In injection molded parts, these processes may lead to skin/core differences in morphology that can be observed (111,112). With higher temperature molds (>150°C) the parts are generally fully... 

Several mechanical tests were conducted on specimens with morphologies similar to that in Fig. 10.5, for several processing conditions. From the results of such tests, it has now been established that improvement in mechanical properties of TP/LCP blends can be correlated with the processing parameters. In particular, these studies have established that the relative thickness of the skin/core structure in the cross section of injection molded samples correlates with the magnitude of the mechanical properties. The latter, in turn correlates with the magnitude of three of the most critical process parameters in injection molding process, namely, injection speed, mold temperature and melt temperature. 

Fig. 3.4 Possible effects of processing (injection molding) on the microstructure of a semicrystalline polymer. In contact with the mold wall (which is assumed to be perpendicular to the plane of the scheme) a surface skin morphology is formed. An isotropic microstructure can be observed in the center core interior. Adopted with permission from [15]...

The heterogeneous morphology and microphase separation process of block PLCs are closely related to the processing history and the molecular structures. For PET/PHB PLCs, PET-rich and PHB-rich phases are detected by SEM observation of etched samples [41,42]. When the PHB content is lower than 50 mol%, the PET-rich phase is continuous, and vice versa when the PHB content is higher than 60 mol%. The size of the PET-rich phase was found to be 10-20 pm with 40mol% PHB and 3-6 pm with 80 mol% PHB. In addition, the phase dimension will be influenced by the thermal and mechanical history. Joseph et al. have reported [41,42] that in an injection molded plaque of PET/PHB PLCs, PHB was richer in the skin while PET was richer in the core. However, there is controversy concerning this observation [3]. 

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