Southern California Section of the Society of Plastics Engineers
Promoting the scientific and engineering knowledge pertaining to plastics.
Case Study: Determining Root Cause -- Failure of Polycarbonate Connector Plug
Failure Root Cause
Polycarbonate parts were experiencing intermittent cracking in the area where PVC tubing was solvent bonded to one end of the connector using Cyclohexanone.
In order to conduct a meaningful failure analysis of the connector to determine possible cause of fracture and undesirable stretching, the following key areas of concern were proposed to be investigated.
Material verification: Conduct FTIR (Infrared analysis) and Intrinsic Viscosity (IV) tests to confirm the identity of the material of the failed part and to determine if the material was degraded during processing.
Process verification: Verify process; machine settings, set up parameters, drying practices, water hook-up, differences in set-up between two molds, annealing procedure, mold (actual Steel) temperature etc.
Tooling: Study the differences between two molds, gate size uniformity, cooling, ejection, venting, draft etc.
Design: Evaluate the part design to determine if there are any sharp corners which tend to act as stress concentrators and check the degree of draft in the part.
Chemical exposure: Check for chemical exposure, chemical compatibility and degree of chemical attack due to molded-in stresses as well as stresses induced by stretching of the molded parts. Conduct Solvent Stress analysis to determine critical stress level of the parts molded from old and new mold, annealed and unannealed. Mold parts in clear Polycarbonate to study molded-in stresses using photo elastic method.
Annealing: Check annealing process to determine if the parts are properly annealed by chemical exposure tests and optical tests.
Assembly operation and Service conditions: if necessary, look into these to area to study any abnormalities.
After verifying all other parameters and failure modes, the focus was directed to Solvent Stress Analysis and Photoelasticity to study the level of residual stresses in the part.
Solvent Stress Analysis: Solvent Stress Analysis is a valuable technique for quantitative determination of molded-in (residual) or externally applied stress in the molded part. Typical residual stress level in a properly designed and well molded parts range from 900 psi to 1200 psi. The test results showed that the batch of parts that were susceptible to cracking had over 2000 psi residual stress.
Photoelastic Analysis: Properly annealed and inadequately annealed transparent parts were analyzed using photoelasticity. The results showed that when properly annealed the residual stress is greatly reduced and parts are not susceptible to stress cracking when solvent bonded.
Results and Conclusions
The intermittent cracking issue in the area where PVC tubing was solvent bonded to one end of the connector using Cyclohexanone is due to poor annealing practices. It is recommended that annealing procedure evaluated thoroughly to minimize the level of residual (molded-in) stresses in the part.
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