Mold shrinkage of Ryton® PPS compounds and Xtel® PPS-based alloys is affected by many factors including the type of compound, part weight and thickness, glass fiber orientation, and coring.
Glass fiber reinforced Ryton® PPS and Xtel® compounds will shrink anisotropically due to glass fiber alignment. For most parts having a wall thickness of 0.20 inch (5.1 mm) or less, mold shrinkage in the flow direction will typically be about 0.003 in/in for 40% glass filled PPS compounds and about 0.002 in/in for 65% glass/mineral filled PPS compounds. But, mold shrinkage will typically be about twice as much transverse to the direction of flow. Consult individual techncial data sheets for mold shrinkage measurements determined on 4 inch by 4 inch by 0.125 inch (102 mm X 102 mm X 3.2 mm) plaques.
The shrinkage factors provided in our techncial data sheets should provide a good indication of the shrinkage behavior for most parts having a wall thickness of 0.20 inch (5.1 mm) or less. But larger, thicker walled parts will tend to shrink more, and differential shrinkage may vary depending on part geometry and coring. For any given part, less mold shrinkage is observed with increasing part weight (less shrinkage with better part packing). Thicker wall sections generally exhibit higher shrinkage (up to an additional 0.001 in/in) than thinner ones since they hold heat longer. Lengthy thin wall sections tend to generate more glass fiber alignment resulting in greater differential shrinkage. Parts restrained during molding by coring will exhibit lower shrinkage than unrestrained parts. Mold shrinkage also increases somewhat with increasing mold temperature due to increased crystallization.
Cold molded parts will exhibit slightly less shrinkage than hot molded parts due to the lower degree of crystallinity. When heated above 190°F (88°C) after molding, cold molded parts will undergo thermal induced crystallization and additional shrinkage will occur. Conversely, hot molded parts that already have a high degree of crystallinity will undergo very little additional shrinkage when exposed to temperatures above 400°F (204°C).
Due to the low mold shrinkage of Ryton® PPS compounds, part-to-part dimensional molding tolerances are very reproducible, once shrinkage behavior under typical processing conditions has been established. Molding tolerances as tight as 0.0001 in/in may be achieved in optimally gated small parts, but molding tolerances are typically about 0.001 in/in, and may be as high as 0.002 in/in in some large parts. New molds should be cut “steel safe” to allow for later adjustments for differential shrinkage.
