When a manufacturer of a material that is used for an entire line of products informs you that they can no longer supply this material it can be difficult to find a replacement material to perform to the same specifications. Once you’ve determined a number of potential alternative suppliers / materials, what’s next? Depending on how the material is used in the products and the percent of parts in the product line that are made from this material will probably determine how stringent and extensive of a qualification process should follow.
In a recent case of this kind at my company, this was exactly the case. A plastics manufacturer was experiencing issues with meeting demand and informed us that the material would no longer be available in 6 months. The typical qualification process usually consists of several destructive and non-destructive tests that are meant to evaluate the integrity of the alternative material relative to the proposed replacement. The tests that are common to our methods include moldability tests, drop test, tensile testing, UV exposure for color shift effects, and modest weatherability (more at environmental testing) tests for plastic warping. The moldability tests included 30 piece sample measurements of critical and overall dimensions, mold-flow testing, and visual inspection of all features to ensure of no short shots or other molding issues. To elaborate on the ‘weatherability tests’, essentially the materials were subjected to temperature chambers and run through high-heat and thermal cycling routines, but the only data collected was dimensional measurements before and after this testing only to gauge dimensional drift.
All the testing up to this point includes a relatively comprehensive testing platform and was a good overall gauge of the performance of the plastic as it is today. However, we realized it was lacking one major element, the performance of the plastic in 5 years, or near the end of its warranty period. There was only 1 test designed to evaluate the “performance” of the plastic after 5 years, and this was the UV test. However, this test does not evaluate the mechanical properties or performance of the material, solely the physical appearance. Not to mention there was no evaluation of acceleration factor for this test, only a subjective period of time in which the plastics are put into a UV chamber. The only testing to evaluate material properties is the drop testing and tensile testing performed on newly molded plastic.
This is where reliability becomes useful. Reliability forces you to consider the affects of the elements on the specimen / material / device at hand not just at the present moment, but in several hours, duty cycles, or years down the line. Since there had never been any consideration of the mechanical integrity of the material near the end of the warranty period for this type of testing, it was up to us to determine how to do that.
We set out to determine what factors would accelerate the life of the material. After debating on whether or not high temperature soaking would act as a stressor and accelerate the material to 5 years, we realized it probably would not since it would most likely only bring the plastic back up to its heat deflection temperature and only remove the residual stresses, perhaps allowing it to perform better in certain tests.
The next potential ‘stressor’ was UV. It was easily agreed upon that UV exposure would affect the mechanical properties of the plastic, but we needed a system to determine its equivalent 5-year exposure. The method we decided to implement was a 3-point test with molded tensile bars. We’ve decided to expose the material to UV for 50 days, 75 days, and 100 days. Once the material is finished with the ‘aging’ process, we intend to subject them to tensile testing along with ‘virgin’ tensile bars. As of this date, units are still in the aging process in the UV chamber. With 20 pieces of each, we should be able to determine the relationship of UV exposure to tensile strength with relatively high confidence. Results pending.
See the Ops A La Carte Seminar on Fundamentals of Climatic Testing