CREEP FAILURE ANALYSIS AND SHELF LIFE DETERMINATION (PREVENTION) OF INJECTION MOLDED PARTS WITH AND WITH OUT GAMMA IRRADIATION

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1 CREEP FAILURE ANALYSIS AND SHELF LIFE DETERMINATION (PREVENTION) OF INJECTION MOLDED PARTS WITH AND WITH OUT GAMMA IRRADIATION Trivikrama Bhanoji Pala, Mechanical Engineering, New Jersey Institute of Technology, Newark, NJ, I. Joga Rao, Mechanical Engineering, New Jersey Institute of Technology, Newark. NJ. Abstract 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 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 23ºC, 4ºC and 6ºC with and without gamma sterilization. It is found that the Creep strain at given total time showed a decreasing [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. Introduction Plastic parts have the ability to be pressed or snap fit which is not possible with metals, but it is important to consider creep characteristics to ensure product performance. Creep is the inelastic response of materials loaded at high temperatures. Temperature has important effects on deformation phenomena. Microstructural defect rearrangement processes are often accelerated at high temperatures. Since these thermal processes tend to relax the material they counteract the strain hardening produced by plastic deformation. Elasticity and plasticity are mechanical responses to loading which are independent of time. As soon as the load is applied, the corresponding level of strain sets in. In contrast, during creep the mechanical response is time dependent. This is somewhat analogous to viscoelastic behavior except that during creep an often significant portion of the strain is permanent and remains after unloading. With appropriate modifications, the standard tension test can be applied to investigate the mechanical response of materials deforming at high temperatures. Easiest to perform are tension tests at constant load and temperature. The resulting strain is measured as a function of time. This is often called the creep test and the resulting strain-time curves are called creep curves. If the creep test extends over a sufficiently long period of time, a stage may be reached at which the sample weakens rapidly and eventually breaks. Tests aimed at investigating this final stage of the creep process are called creep rupture tests. By appropriate modifications of the creep testing equipment, other useful tests can be developed such as the constant stress tests and the constant strain rate test. Using the results of creep tests, another manifestation of creep response has been observed; namely, the relaxation of stress over time. If a creeping material is strained to a specified level, the stress obtained decays from its initial value. Cyclic loading is often associated with material fatigue processes which eventually may cause the failure of the loaded component. SPE ANTEC Anaheim 217 / 12

2 At high temperatures, creep phenomena appear and they have an important influence on the fatigue process. The resulting creep deformation and relaxation phases affect the fatigue response of the material depending on their relative duration according to the specific form of the cycling pattern. Typical strain-time curves obtained from creep tests at constant load over sufficiently long periods of time often exhibit three characteristic stages. Primary or Transient Creep. Following the setting in of the instantaneous elastic strain, the material deforms rapidly but at a decreasing rate. The duration of this stage is typically relatively short in relation to the total creep curve. Secondary or Steady-State Creep. Here, the creep strain rate reaches a minimum value and remains approximately constant over a relatively long period of time. Tertiary Creep. In this stage, the creep strain rate accelerates rapidly rupture. The material is unable to withstand the load anymore and breaks. Creep Sample The samples used for this study were injection molded ASTM Type 1 PP specimen as shown in Figure 2. Specimen is made up of copolymer PP supplied by LyondellBasell Industries with less than 5% of blue color pigment concentration. These specimens were molded according to the manufacturer recommended injection molding conditions. Five were used for each set of test. Some of the specimen were gamma sterilized with a dose of 25 kgy (kilo Gray). Creep testing was performed per ASTM D Standard Test Method for Creep Test of plastics. Type 1 tensile bars were conditioned to 4ºC and, 5% RH. Figure 2: ASTM Type1 PP test specimen. Creep -Experimental testing Creep strain was measured as a function of time at three stress levels (typically 15, 3 and 45 % of total yield stress) and a single temperature, and the results were fit with the time hardening law model, because creep measurements are typically recorded for 1 hours. Creep strain versus time curves and the creep model is established. The creep model was used to simulate the effect of load being applied to a plastic part over a long period of time. The point of tertiary creep was selected based on the highest stress level tested for each temperature. Figure 1: Creep characteristics Creep Strain analyses are used to simulate the shelf life of a product. Viscoelastic materials exhibit dynamic behavior over time, depending on the given load and temperature. Physical tests are conducted in order to plot and understand a material s reaction to such loading. Three basic regions of the Strain vs. Time plot can be identified: Primary, Secondary, & Tertiary Creep. For cracking and rupture analyses, the onset of tertiary creep is most important. Beyond which, the material is predicted to become unstable. In general, creep strains above this tertiary strain limit should be avoided to prevent problems with structural integrity or part functionality. The t- time and strain level at which the tertiary limit occurs is dependent on the amount loading. The plot shown in Figure 1 depicts each region for the creep response curve of a given load. Methods Mathematical Modeling of Creep While the creep response of materials is intimately related to microstructural processes that take place inside the material during deformation, a continuum description of the creep process has proven to be of great engineering usefulness. From the standpoint of the continuum representation, solution of the creep problem requires statement of the mechanical equilibrium equations, appropriate constitutive equations for creep behavior and suitable initial and boundary conditions. Since temperature is a key variable in creep the energy balance equation may have to be solved also. The simplest case to investigate is uniaxial loading. A generic formal mathematical representation of the creep curve is as follows [9]: (1) T is temperature and t is time. Note that time and temperature are explicitly included in the representation since these two variables play key roles in the creep response. The introduction of the functions f, g and h above SPE ANTEC Anaheim 217 / 121

3 implies a frequently made assumption that of the effects of stress, temperature and time are separate. The above expression is the basis of the equation of state formulation used to investigate creep phenomena under variable stress. In this formulation the response depends on present state explicitly and it is in contrast with memory formulations that are somewhat less well developed. Creep Rate Laws and the Hardening Formulations In the design for structural integrity one is often most interested in the primary and secondary stages of creep. A commonly used equation of state representation of these two creep stages is provided by the Bailey-Norton law, sometimes called power law creep law. This is given by A generic representation of all the above creep laws is follows: Where v (σ) is the functional form used. Another set of creep laws have been proposed not in terms of the creep strain rate but the creep strain as follows: (8) (2) The above is often expressed in rate form as follows: This is called the time hardening formulation of power law creep, where A, m, n are temperature dependent material constants. If the Bailey-Norton law is solved for t and the result substituted into the above, the result is as follows: (4) This is called the strain hardening formulation of power law creep. The time and strain hardening formulations are used in practice to predict the creep curve under a variable stress history using only data obtained from multiple creep tests, each at constant stress. Experience indicates that the strain hardening formulation often produces better agreement with the results of actual tests under variable stress. Several other creep rate laws have been proposed, including Andrade's law (3) (9) FEA model description FEA model contains two PP resins namely a bearing and sleeve having a mating interference fit, Sleeve which is modeled with 18 hexahedral shaped elements (Figure 3) and bearing which is modeled with 645 hexahedral shaped elements (Figure 4). 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. A, n, m values which are obtained from the curve fitting of the creep test data are used as an input to the FEA model to simulate the creep characteristics of the model and obtain the attachment detachment forces. In FEA model, in order to find creep strain produced between both mating parts, the Sleeve is retained in bearing for self-life period of parts. The detachment force of sleeve before and after shelf life for no gamma exposed parts and 25kGy gamma exposed PP resins parts were calculated by applying the equations 5 to 9. Where k are constants; the exponential law (5) The hyperbolic sine law (6) Figure 3: Showing the Sleeve mesh generation in Abacus Software (7) SPE ANTEC Anaheim 217 / 122

4 C With Gamma strain vs yield strain vs 45% yield yield strain vs 3% yield yield Figure 4: Showing the Bearing mesh generation in Abacus Software Figure 7: After gamma irradiation at 23 C: Coefficients units (psi, hr units) A= 4.49E-13, n= 2.91, m=-.598 The Figure 6 and Figure 7 shows the creep strain decrease between before and after gamma radiation at 23 C. The above curves are plotted at 15%, 3% and 45% of yield strength of PP. Figure 5: Showing the Assembly of Sleeve and Bearing Results and Discussion The creep strain vs time at different loads for 23C, 4C and 6C for PP test specimens before and after gamma irradiation are shown in Figures 6 to 11 respectively. The Coefficients that are derived from the curve fitting method are displayed under each figure for a particular temperature and sterilization condition C without Gamma strain vs yield strain vs yield strain vs yield yield yield model@3% yield Figure 8: Before gamma irradiation at 4 C: Coefficients units (psi, hr units): A= 4.5E-7, n=1.6, m=-.73 Figure 6: Before gamma irradiation at 23 C: Coefficients units (psi, hr units): A = 6.7E-1, n= 2.56, m= -.78 SPE ANTEC Anaheim 217 / 123

5 C with Gamma strin vs 15% YIELD strain vs tim@3% yield strain v 45% yield 15% yield model@ 3% yield.35 STRAN(mm/mm).35 6C with Gamma.4 strain s time@15% yield strain vs yield strain vs 45% yield yield 3% yeield ) 3 4 Figure 9: After gamma irradiation at 4 C: Coefficients units (psi, hr units): A= 2.33E-12, n= 2.725, m= -.63 Figure11: After gamma irradiation at 6 C: Coefficients units (psi, hr units) A=3.85E-8, n= , m= -.74 The Figure 8 and Figure 9 shows the creep strain decrease between before and after gamma radiation at 4 C. The above curves are plotted at 15%, 3% and 45% of yield strength of PP. Figure 1 and Figure 11 shows the creep strain decrease between before and after gamma radiation at 6 C. The above curves are plotted at 15%, 3% and 45% of yield strength of PP. Table 1: Showing the Comparison of Mechanical Properties before and after gamma irradiation of PP. 6c Without Gamma strain vs tiime@15% yield strain vs yield load strain vs 45% yield 15% yield 3% yield.6.5 Base Resin PP Property Creep Strain at given total time Comments.4.3 FEA Results: Figure 1: Before gamma irradiation at 6 C: Coefficients units (psi, hr units) A= 5.91E-7, n= 1.163, m= -.84 Figure 1: Showing the Sleeve attachment to Bearing SPE ANTEC Anaheim 217 / 124

6 14.5 N Figure 11: Showing the Sleeve detachment from Bearing. Figure 14: Detachment force before shelf-life aging of PP resin mating parts exposed to 25kGy gamma irradiation. 3N 17N Figure 15: Detachment force after shelf-life aging of PP resin mating parts exposed to 25kGy gamma irradiation. Figure 12: Detachment force before aging of PP resin mating parts that were not exposed to gamma irradiations. 14 N Table 2: Shows the results of computational analysis produced from ABAQUS software for the detachment force of Sleeve in Newton at No gamma and 25kGy gamma exposed condition. No Gamma 25 kgy Gamma Before Aging 17 N 14.5 N After Aging 14.5 N 3N before and after aging before and after aging Figure 13: Detachment force after aging of PP resin mating parts that were not exposed to gamma irradiations. Comments Comments between Gamma and No Gamma between Gamma and No Gamma SPE ANTEC Anaheim 217 / 125

7 Conclusions It was found that the creep strain over time showed decreasing. The results shows that unexposed PP resin parts exhibit decrease in detachment forces after aging. It also shows that for 25 KGy gamma exposed PP resin parts the detachment force also reduced after aging. The results assist the design to estimate reduction in detachment force due to the creep strain between the mating parts. It is also found that material and geometry are important to consider such that failure due to the creep can be avoided early in the design process. When using press or snap fit features, it is important to consider creep to ensure product performance. Use of this experiment can determine the required shelf life of the product by considering the creep failure in successful design of the plastic injection molding parts. properties of commercial multilayer coextruded flexible plastics packaging materials. 8. Madhu Raju Saghee Application of Sterilization by Gamma Radiation for Single-Use Disposable Technologies in the Biopharmaceutical Sector. Published on IVT Network MEF2/Notes/ch9.pdf Acknowledgements The authors would like to acknowledge the support of Mahesh Krishnamoorthy, Hemanth Amarchinta and Aditya Velagaleti for their support. References 1. Antonios E. Goulasa, Kyriakos A. Riganakosb, MichaelG. Kontominas, Effect of ionizing radiation on physicochemical and mechanical. 2. Trivikrama Pala and Dr. I.J. Rao, Effects of small range color (pigment) concentration levels on plastic injection molded parts, - ANTEC 216, The Plastic Technology Conference 216. Indianapolis, USA, May Trivikrama Pala and Dr. I.J. Rao, Effects of external process (sterilization) on the plastic injection molded parts, - Proceedings of the 216 International Conference on Polymer Science and Engineering, New Orleans, USA, August Andrew W. Salamon and R. Bruce Cassel, the Perkin Elmer Corporation, Thermogravimetry of Polymers, - SPE ANTEC Myer Ezrin, Gary Lavigne, Mark Dudley, Laura Pinatti and Fiona Leek Retired, Institute of Materials Science, The Role of Analytical and Physical Methods in Plastics Failure Analysis, - SPE ANTEC M. Pentimallia,, D. Capitanib, A. Ferrandoc, D. Ferric, P. Ragnia, A.L. Segreb Gamma irradiation of food packaging materials: an NMR study. 7. Antonios E. Goulasa, Kyriakos A. Riganakosb, MichaelG. Kontominas, Effect of ionizing radiation on physicochemical and mechanical SPE ANTEC Anaheim 217 / 126