The Effect of Internal Lubricants on the Accuracy of Injection Molded Thermoplastic Gears

This paper will examine the effect that PTFE, silicone and other internal lubricants have on the mold shrink rate of thermoplastic gear materials and the resulting effect on the accuracy of the molded gear. “Standard” thermoplastic mold shrinkage data is generated on relatively large test samples which do not represent the flow, orientation and heat transfer seen in a typical molded gear. Additionally, different parts of the gear can be expected to shrink at different rates, leading to potential errors if a single “generic” mold shrink rate is used to design and machine the gear mold cavity. Using a single cavity gear mold, the shrink rate of different internally lubricated gear materials is measured and compared to the unlubricated base resin. The shrink rate of individual parts of the gear are compared, as well as the resultant total composite variation. A discussion on the techniques used to measure the gears, as well as the expected consequences of the resultant shrinkage on the gear train, will be included. This data can be used to increase the accuracy of molded thermoplastic gears.