Tool Steels, Coatings, and Surface Treatments
Because of the abrasive nature of the glass and mineral fillers used in Ryton® PPS compounds and Ryton® PPS Alloy compounds, hard tool steels are required. For long run production molds, we advise using A-2, D-2, or D-7 tool steels hardened to Rockwell C-60 or higher. For low volume runs, P20, S7, and H13 are acceptable softer steels. Slow deposition dense chrome and electroless nickel coatings provide good mold release characteristics and fairly long tool life. Surface treatments including Borofuse®, LSR-1®, and Nitride® may also be used to reduce tool wear. Steels with a surface finish of 0.0001 mm (4 microinch) or better typically experience extended service life. Because especially high rates of wear are typically encountered at gates, removable (replaceable) gate blocks are often used. For more information, refer to the Abrasion Resistant Steels and Surface Treatments for Extending Tool Life technical bulletin.
Mold Temperature Control
Molds for processing Ryton® PPS compounds and Ryton® PPS Alloy compounds must be designed to provide a mold surface temperature of at least 135°C (275°F). Various methods such as hot oil, electric cartridges, or high pressure water may be used to accomplish this. We generally advise using hot oil because it allows for addition or removal of heat for better temperature control under relatively low pressure.
Sprues and Runners
Molds for processing Ryton® PPS compounds and Ryton® PPS Alloy compounds generally use standard sprue designs with nominal 2° of draft and reverse taper or Z-cut sprue puller systems. The shaft should be as short and highly polished as possible to ease part removal from the stationary half of the mold. Runners of many types can be used, however, full round and trapezoidal runners are preferred and runner length should be kept to a minimum.
Multi-cavity molds should have balanced runner systems designed so that the mold cavities fill uniformly and in a balanced fashion. On multi-cavity molds with primary and secondary runners, the primary runner should carry on beyond the intersection with the secondary runner in order to provide a cold slug well for the runner flow front. As with engineering plastics in general, it is good practice not to use family mold layouts because difficulty in controlling filling patterns may produce parts with varying physical and mechanical properties.
Proper venting is essential for processing Ryton® PPS compounds and Ryton® PPS Alloy compounds because inadequate venting results in hard to fill parts as well as burning of the part and accelerated mold wear in the areas where gas is trapped. Venting can be accomplished with 0.008 to 0.013 mm (0.0003 to 0.0005 inch) deep by 6.35 mm (0.250 inch) wide channels cut on the parting line. Venting can also be accomplished by flattening ejector pins 0.018 mm (0.0007 inch) on one to four sides. Polishing vents to an A-1 finish in the direction of flow will help prevent buildup of residues during long part runs.
Stationary vent pins are typically not used because they can become clogged over long part runs. Vacuum venting has been used successfully in areas where a blind pocket exists. The vacuum is turned on after the mold closes and prior to the start of the injection cycle. Runners should be vented with 0.025 mm (0.001 inch) deep channels cut on the parting line.