Which polymers are ideal for moulding?
When considering chemical resistance, stress-cracks are a special case. They develop in moulded parts when they are exposed to a combination of mechanical tensile stresses and a surfactant medium; this term also includes humidity. They lead to premature failure of parts through brittle fracture and they are a treacherous form of damage, because stress-cracks often show up only a long time after moulding, although their cause is faulty part design or internal stresses after processing. A uniform quantification of the susceptibility of various plastics to stress-cracking is not possible at present. In general, detergents (wetting agents) generate stress-cracking. Within any one type of plastic resin, resistance to stress-cracking rises with molar mass, i.e with chain length and branching. This knowledge enables producers to develop types with better stress-crack resistance. For a qualitative determination of stress-crack formation, a moulded part is immersed in a stress-crack generating
