Melting molding cycle

This article is an overview of the novel technology of self-reinforced LCPs with polyesters, poly(ethylene terephthalate) (PET) and poly(ethylene naphtha-late) (PEN) [10-13, 21, 23], LCP/polyester blends in a polyester matrix form in situ fibrils which improve the mechanical properties. LCPs have an inherently low melt viscosity, and provide LCP/polyester blends that effectively lower the melt viscosity during melt spinning [24], and fast injection-molding cycles. The miscibility between the LCP and polyesters can be controlled by the degree of transesterification [25] in the reactive extrusion step, and fibril formation in LCP-reinforced polyester fibers has been studied. 

Stepto (1997) focused on the injection molding of potato starch including the basis of the process. In addition, the rheological behavior of starch/water melts during the refill part of the injection molding cycle was analyzed quantitatively to give apparent melt viscosities. Finally, the mechanical properties of molded starch materials and the dmg-delivery behavior of starch capsules were also discussed. 

The injection molding cycle is depicting in Figure 7.74. We can begin the cycle at any point we wish, but let us start at the point the screw moves forward and fills the mold with polymer melt. The screw moves forward and fills the mold with melt and maintains the injected melt under pressure, during what is called the hold time. To ensure that polymer does not flow backward, a check valve is attached to the end... 

Figure 13.44 represents the various stages of the compression molding cycle from the point of view of the plunger force needed to close the mold at a constant rate. In the first region, t < the force increases rapidly as the preform is squeezed and heated. At tf, the polymer is presumably in the molten state and, as such, is forced to flow into the cavity and fill it. Filling terminates at tc, when compression of the polymer melt takes place, to compensate for the volume contraction that results from the polymerization reaction. The bulk of the chemical reaction occurs after tc. We now comment on each of the steps of the compression molding process. 

Izod impact strength increases with increasing injection speed and lower melt temperatures. Since the fracture plane is normal to the direction of orientation in the Izod sample, the increased toughness with slower injection speed can be explained by the reinforcing effect of the oriented polymer on the surface. This effect is less noticeable at very high melt temperatures, where the surface can actually be annealed by the temperature of the core and subsequent extended molding cycle time required by the hot melt temperature. 

Most practitioners deflne the flow behavior of polymers based on the melt flow index however, this property is not entirely relevant to the rotational molding process because it is essentially a shear-free and pressure-free process. The use of zero-shear viscosity has been proposed as a better way to assess the coalescence behavior of resins. Resins with lower zero-shear viscosity coalesce at a faster rate and can thus be processed using a shorter molding cycle.The coalescence of individual powder particles is initiated as the particles stick and melt onto the mold surface or melt front. As the melt deposition process continues, pockets of air remain trapped between partially fused particles and lead to the formation of bubbles. In the rotational molding process, the coalescence of particles occurs at a temperature range close to the melting point of the material thus, from a processing standpoint, low values of zero-shear viscosity at low temperatures (i.e., close to the temperature at which the particles adhere to the mold surface) are preferable. 

Rapid crystallization of acetals from the melt contributes to fast mold cycles. Crystallization also causes a significant amount of mold shrinkage. Thus, it is necessary to compensate in mold design for dimensional changes that occur during the transformation from a hot, low-density, amorphous melt to a more dense, crystalline solid. 

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