Design of Injection Molded Plastic Gears


Plastic gears show several advantages compared to metallic gears such as lower weight, dry run capability or the potential for cost-effective mass production. However, disadvantages, such as inferior mechanical and thermal properties and less dimensional accuracy need to be compensated by appropriate design measures. Present design methods, however, have a very limited applicability since several plastic specific properties such as viscoelasticity or damage mechanisms are neglected. Within the ”Design of Injection Molded Plastic Gearwheels” project, cooperating with Institute of Machine Elements, Gears, and Transmissions (Technical University Kaiserslautern), a multi physical modelling approach is supposed to be worked out and validated experimentally which should be used as a basis for applicable design rules.

Besides a comprehensive tribological material characterization for friction and wear analysis, an extensive mechanical characterization is also necessary to model the material considering temperature, strain rate, humidity content and fiber orientation. However, the transferability between mechanical characteristics of lab specimens and gears is a major point that has to be considered. Tensile specimens taken out of molded plates may differ in some degree from gears that were manufactured with the same raw material. Due to different flow paths or processing parameters various fiber orientations or fiber length distributions may occur, causing locally divergent mechanical characteristics such as strength and stiffness. To be able to estimate these differences, micromechanical simulation models were created, fed with the corresponding input data such as fiber orientation determined by computer tomography (calibrated using the results of the tensile tests) and their calculated effective stiffnesses were compared.  Thus, not only the transferability between lab specimens and gears can be analyzed but also the homogeneity along the different gear segments as well.

The project “Design of Injection Molded Plastic Gearwheels” is funded by the Federal Ministry of Economic Affairs and Energy, based on a decision by the German Bundestag (funding reference IGF 20379N).

Stefan Schmidt, M.Eng.

Design of Composite Structures
Leibniz-Institut für Verbundwerkstoffe GmbH
Erwin-Schrödinger-Str. 58
67663 Kaiserslautern
Telefon: +49 (0) 631/2017-274

Exemplary mean curves with scatter band from the tensile tests of short glass fiber reinforced polyamide

Micromechanical simulation model of a gear wheel segment

Calculated angle-dependent stifnesses of a micromechanical simulation model of a gear wheel segment

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