Determining Hydrolytic Degradation Thresholds in Virgin Resin Feedstocks
Hydrolytic degradation thresholds require drying virgin polyesters below 0.02 percent and polyamides below 0.10 percent water before melt processing.
Splay
Meaning
Visual defects consisting of silver-like streaks or fan-shaped patterns on the surface of a moulded part occur when moisture or trapped gases are dispersed in the melt flow. The appearance of splay indicates that volatile substances have been stretched along the cavity wall during the injection phase. These cosmetic blemishes weaken the surface layer of the component and can lead to reject parts.
Defect Origin
Inadequate pre-drying of hygroscopic resins is the most common cause of these surface streaks during processing. When the moisture is heated to moulding temperatures, it turns to steam and forms bubbles that flatten against the tool surface as the melt front advances. This moisture-induced splay must be distinguished from thermal splay, which is caused by the resin overheating and releasing gaseous degradation products.
Processing Control
Adjusting the injection speed and melt temperature helps to reduce the occurrence of these surface defects. Lowering the screw speed and back pressure can also prevent air entrainment and excessive shear heating that would otherwise vaporize volatile additives or moisture. Clean vents in the mould tool are necessary to allow these gases to escape before they can become trapped.
Quality Consequence
Scrap rates increase when these surface cosmetic defects appear on high-visibility or medical-grade plastic parts. Testing the moisture content of the resin prior to moulding remains the most effective way to prevent this defect from arising.