Polyoxymethylene copolymer melting temperature

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]. 

Polyoxymethylene (POM) is commonly used as a direct replacement for metals due to its stiffness, dimensional stability and corrosion resistance. Copolymers including ethylene oxide are quite common, primarily because they reduce the propensity for depolymerization at normal processing temperatures. Some typical properties are Glass Transition Temperature, Tg = -30 C Melting Temperature, Tm = 183 C Amorphous density at 25 C = 1.25 g/cc Crystalline density at 25 C = 1.54 g/cc Molecular weight of repeat unit = 30.03 g/mole. There are a number of grade variations of this material commercially available. 

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