Cooling melt flow

As the temperature is decreased, free-volume is lost. If the molecular shape or cross-linking prevent crystallisation, then the liquid structure is retained, and free-volume is not all lost immediately (Fig. 22.8c). As with the melt, flow can still occur, though naturally it is more difficult, so the viscosity increases. As the polymer is cooled further, more free volume is lost. There comes a point at which the volume, though sufficient to contain the molecules, is too small to allow them to move and rearrange. All the free volume is gone, and the curve of specific volume flattens out (Fig. 22.8c). This is the glass transition temperature, T . Below this temperature the polymer is a glass. 

Figure 15.22. Batch cooling crystallisation of melts flow diagram for the Proabd refiner...

Polypropylene (PP) films were first produced by extrusion casting. Polymer is extruded through a slit or tubular die and quenched by cooling on chill rolls or in a water bath. Cast films can be sealed over a wide range of temperatures and do not shrink in a steam autoclave, Polymers with melt flow rates below 5 dg/min are usually used to maintain the stability of the extra date. Higher clarity films are produced using random copolymers. 

Heating and cooling often take place while the polymer melt flows, making viscous dissipation an influencing factor during the process. However, since most plastic parts are thin, the conduction often occurs only across the thickness and the viscous heating is a result of shear within the narrow gap of a die or mold cavity. For such cases, the equations reduce to,... 

However, molecular orientation takes place in any case in some degree in any process involving melt flow and is frozen into the objects when they cool and the material solidifies. Such orientation can cause difficulties (like local brittleness and a tendency for the objects to distort), and will be especially troublesome if in the course of production the pattern of orientation changes. Orientation and strains in transparent objects can be studied conveniently using a source of polarized light (when they can be seen in different colours), but is more difficult with opaque items. 

When polymer melts are accelerated during the manufacturing processes such as injection molding and afterwards cooled down quickly, the stretched molecular chains cannot relax, which leads to orientations in the direction of melt flow. These oriented polymer chains act differently toward ESC influences as compared to nonoriented molecules. Orientation perpendicular to the crack propagation direction may result in a higher ESCR, as will be shown in the following discussion. 

Second, in the fast-cooling solidification process, the m-c interface is concave to the crystal and the melt flows from the periphery to the center along the solidification front surface. Thus, the impurities in the melt and the SiC particles generated at the interface are transported to and accumulated in the... 

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