Melt flow injection molding, influence

The flow behavior of the polymer preferentially influences the uniform shell thickness and is related directly to the molecular structure of the modifying comonomer. Modifications of PET, particularly with CHDM, improve the flow behavior during injection molding and significantly reduce the melting point of the polymer. The decreased melting point of the copolymer allows reduced processing temperatures and therefore correlates with a reduced formation of unwanted A A at shortened cycle times. 

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. 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

Shrinkage changes can occur at different rates in different directions for RTFs, particularly UTPs. These directional shrinkages can vary significantly due to changes in processes such as during IM. Activity is influenced by factors such as injection pressure, melt heat, mold heat, and part thickness as well as shape. The amorphous type melt flow can be easier to balance. 

Keywords Additives, Compounding, Deformation of melts, Extrusion foaming. Flow properties, Influences of processing on properties. Injection molding. Plastic melts, Processing of elastomers and thermosets. Processing of fiber composites. Processing of thermoplastics... 

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