MOULDED IN STRESS模塑的应力.doc

MOULDED IN STRESS模塑的应力 According to RAPRA Technology, who carried out post-mortems on thousands of industrial plastic parts, found that environmental stress cracking was the major cause of plastic part failures. Well, Stress cracking in service is due to the moulded-in (residual) stress in the moulding. There are three possibilities of inducing moulded-in stress in a plastic part. There are: When melt is over packed (while moulding) it can cause moulded-in stress. This can be avoided by applying follow up pressure just enough to avoid sink marks. Even unbalanced melt flow in the mould can cause overpack in some regions. It is not possible to avoid overpacking with unbalanced melt flow while processing. This problem should have been considered while designing the part itself. Un-equal freezing of melt through out the part, can also cause moulded-in stress. Cooling system, which does not provide uniform mould surface temperature can be responsible for this cause. This is the responsibility of mould design. MOULDABILITY It is assumed that every shape can be mouldable in plastics. It is partly not correct. For perfect moulding you should have 95 to 100% balanced melt flow. It means that melt front in different direction should reach the boundary of the mould at the same instant. Otherwise, you may have some region filled earlier and while filling unfilled region the previously filled region gets over packed.. To solve this problem flow balancing technique should be used to modify the part design. By this technique, flow leader or flow deflectors are used to balance the flow to avoid moulded-in stress. Moulded-in stress also causes unequal shrinkage in the moulding resulting in dimensional problem. To avoid moulded-in stress due to un-uniform freezing, we must ensure that melt flow velocity in the mould is uniform or constant through out the part during filling phase. STRESS CRACKING Stress cracking can be classified in to Environment stresscracking and Solvent stress