Creep failure analysis and shelf life determination (Prevention) of injection molded parts with and with out gamma irradiation

Creep is the inelastic response of materials exposed to constant load at a particular temperature. Creep characteristics play an important role in the design consideration of injection molded plastic parts where they can provide enable one to measure or estimate the creep strain when there are mating parts. However this does not consist of the information necessary to determine or calculate the outcome of creep failure of the mating parts under a constant load up to the end of the shelf life of the product. If the designer can understand the limits of creep failure in the plastic engineering part design then it aids in the determination of shelf life of the product. Also, the plastic parts which are predominantly used in the medical device industry, are exposed to sterilization prior to use. A common radiation dose used for plastics is in the range 15-25 kGy [8]. Therefore the objective of this study was to understand the creep failure of parts with or without gamma sterilization and help enable the designer to determine the shelf-life of the plastic components when they are exposed to gamma sterilization. Finite Element Analysis (FEA) was utilized to determine the impact of creep on the two mating PP (polypropylene) injection molded parts. The inputs needed for the FEA model, which are the temperature dependent coefficients (A, m and n) were determined by curve fitting the creep test results for PP with a time hardening formulation of power law creep model for the strain vs time data at 23°C, 40°C and 60°C with and without gamma sterilization. It is found that the Creep strain at given total time showed a decreasing trend [2]. The FEA model contains two PP resins namely a bearing and sleeve having a mating interference fit. Sleeve is inserted into the bearing and this insertion force is termed as attachment force and later the sleeve is pushed out from bearing and this force is termed as detachment force. In FEA model, in order to find creep strain produced between both mating parts, the Sleeve is retained in bearing for intended self-life duration. The detachment force of sleeve before and after shelf life for unexposed parts and 25kGy gamma exposed PP resins parts were calculated. The results shows that the detachment force reduces after aging, regardless of gamma exposure. These results assist the product designer to estimate the reduction in detachment force due to creep strain between the mating parts. It is also found that material and geometry are important to consider, so that the failure due to the creep can be avoided early in the design process and it is very critical to consider creep in order to ensure product performance. Therefore the results of this study can help one determine the required shelf life of the product by considering the creep failure in the successful design of the plastic Injection molding parts.