A variety of different gating methods are suitable for molding Ryton® PPS compounds. Common sprue, pin, and tab gates as well as fan, flash, spoke, diaphragm, and submarine gates have all been used in Ryton® PPS molds.
Gate Location
The placement of the gate requires careful consideration since it has substantial ramifications on the strength and dimensional stability of the part. The designer should strive to maintain uniform flow throughout the part filling process and minimize internal stresses and weld lines. Since Ryton® PPS compounds are reinforced with glass fiber, they shrink anisotropically, about half as much in the flow direction as perpendicular to the direction of flow. This differential shrinkage incurs internal stresses that may cause part distortion or contribute to part failure. Differences in part cross section and abrupt changes in flow patterns during mold filling can generate additional internal stresses and/or voids. Gate location should promote steady material flow starting in thicker wall sections and proceeding to thinner wall sections, must avoid back filling, and should ensure that the last places to fill can be vented, preferably at the mold parting line. Filling patterns should be planned to promote glass fiber orientation along axes that experience more stress in service. If the part can not be molded without weld lines, the weld lines should be placed in the thickest section of the part or in areas where there is minimal stress.
Gate Size
Sufficient gate size is essential to ensure easy filling and adequate packing of the part, but it is also important that the gate not be so large that the time required for it to freeze off adversely affects cycle time. It is generally best to plan for the gate to be of minimal size, and then enlarge it later, if necessary. Although the wide variety of possible part geometries makes it impractical to establish criteria for any circumstance, we can provide the following general guidelines for gate sizing. We recommend an absolute minimum gate diameter or thickness of 0.035 to 0.040 inch (0.9 to 1.0 mm) for any part, even very small parts. This is to ensure adequate pack and hold pressure for the part and to prevent excessive shear that may cause overheating of the material and/or degradation of glass fiber length. A good guideline for larger parts is for the gate diameter or thickness to be about 60% to 75% of the maximum wall thickness of the part, but the graph shown below depicts the need for increasing the gate diameter or thickness as overall part volume increases. The “Minimum Gate Diameter” for round gates in the chart shown below may also be considered the minimum gate thickness for typical rectangular gates having a width of two times the gate thickness.
