10 Quality Control Guide from GE Plastics Design Guide

Quality Control GuideThe global marketplace demands highspeed to market, low cost and world classquality. GE Plastics is committed toproviding customers with the highest qualityproducts and services. Our Six Sigmaquality initiative results in the virtualelimination of defects from every product,process and transaction. This effort willpresent opportunities to reduce cost,expand and improve customer serviceQuality Control GuideContentsFailure AnalysisDegradationDegradationDegradation TestingDegradation Test ResultsChemical AttackStressStressDesign StressProcessing StressAssembly StressEnd-Use StressStress Concentration FactorsTroubleshooting GuideTroubleshooting GuideHistoryHypothesis of FailureTestingSourceFormulate SolutionCommunicateFeedbackInjection Molding Troubleshooting ConsiderationsFailure AnalysisWhen trying to determine the cause or causes of a part’s failure, it is good to maintain an open mind, best expressed by the phrase, "The Most Obvious Cause Isn’t." This reminds you to not assume anything during the analysis – to look for secondary or tertiary factors which may also be significant contributors to the part’s failure. The failure of a part molded from a thermoplastic resin can be caused by a number of factors during the life history of the part. Errors in design, processing, secondary operations, assembly and the end use environment can lessen the expected performance. Often two, three, or more factors can be found which may contribute to less than satisfactory performance of the part. However, of all the possibilities, by far the three most common problems are polymer degradation during processing, chemical attack and unexpected stress concentrations or accumulations.DegradationPolymer degradation is almost always associated with the processing of a material. If the degradation is a sudden or new occurrence, it is probably a result of changes in drying, processing parameters or the material. If the degradation is ongoing, (i.e., it cannot be resolved by standard troubleshooting), then it is more likely caused by part design, tool design or the molding machine.Degradation TestingFor most unreinforced thermoplastics, the test used to determine the level of degradation is the melt flow, or melt viscosity test. If the material is glass reinforced, a more accurate measurement can often be obtained from an intrinsic viscosity test. For a few resins, neither of these tests are accurate, and an alternative test must be used, such as the thermal gravimetric analysis – TGA.Degradation Test ResultsThe results of any analytical degradation test must be compared to some base value and reported as a percent change. The most accurate results are obtained by testing parts and pellets from the same lot of resin, in the same test equipment. The next best control value would be QC data from the manufacturer for the same lot of resin.There is always a small amount of degradation incurred in the molding process, due to the heat and shear that the material is subjected to in the mold. What percentage change is the dividing line between acceptable and unacceptable processing? General guidelines exist for most resins and processes. However, physical properties react differently to a particular level of degradation, and the needs of a specific part vary as well. Thus, the amount of acceptable degradation should be considered application dependent, and is determined either by the performance or failure of the part, or by thorough end use testing.9-1Degradation Testing GuidelinesChemical AttackExposure of a thermoplastic part to an incompatible solvent, solution or chemical may result in a variety of effects, ranging from surface melting, cracking and crazing, to degradation of properties such as elongation with little apparent evidence.When searching for a potentially incompatible material, it should be remembered that the attack may come from a minor component in almost any carrier – lubricants, cleaners, gaskets, adhesives, even mating plastic components! When in doubt, check it!Chemical compatibility – or the resistance of a material to a specific agent – depends upon a number of factors. The contact time may be critical; a cleaning agent which evaporates quickly may be acceptable for a part in its intended use, but if a molded plastic is immersed for extended periods of time, the part may be severely attacked. Similarly, elevated temperatures or stress levels may initiate or accelerate an attack. Remember that stresses are cumulative – a part may pass a compatibility test as molded, but with externally applied stresses, induced by an assembly operation for example, the part may be more susceptible to attack.StressA common factor in most part failures is stress. Compensating for all possible situations and their effects is virtually impossible. Even if all stresses are accurately evaluated, failure can still occur from some other unanticipated factor(s).Design StressHand calculations are typically used as a first approximation in the basic design of a part - and sometimes as the final analysis. Thus, it is important to understand their limitations:Most conventional hand calculations are only valid for simple geometries such as beams and plates.Hand calculations assume a linear relationship between stress and strain. The nature of plastic materials is that most exhibit a non-linear relationship, beginning at relatively low stress levels.Most hand calculations assume that displacements are relatively small so that changes in geometry do not significantly affect the stiffness of the part. Due to their relatively low stiffness and high deflections, this assumption is rarely valid for plastic parts. Thus, larger factors of safety are needed to compensate for the inaccuracies of hand calculations.Finite Element Analysis (FEA) utilizes the capability of a computer and specialized software to reduce a design to elements, each of which can be studied alone or as part of a system (or part).The most difficult area to analyze for both hand and FEA calculations is Stress Concentrations. Often, the best approach is simply to test actual parts and then modify the design accordingly.Processing StressResidual or molded-in stresses are inherent in all processing methods, but are most significant in injection molding. As the material is injected, the layer in contact with the mold quickly freezes. As the material filling the area between these frozen layers cools, it shrinks – pulling against the outside layers. This results in a slight compressive stress on the outside surface of a molded part and a slight tensile stress internally. The magnitudes of these stresses can be significantly affected by processing parameters and the cooling rate of the part. Warping of the part could indicate significant molded-in stresses.The most critical areas for molded-in stresses are the gate area, the farthest end of the flow, and any areas of sudden change in the flow, such as sharp corners and wall thickness transitions. In these areas, the level of molded-in stress is particularly high, and they should be evaluated carefully before being subjected to additional stresses.Assembly StressAdditional stresses are encountered when the part is joined as an element into an assembly. Mechanical fasteners are an obvious source of stress concentrations, but welding processes also induce residual stresses. Variations in tolerances and accumulations of tolerances can result in mismatched parts and cause sporadic high assembly stresses. Dissimilar materials will also cause additional stresses, if reactions to the assembly environment result in unequal size changes.End-Use StressAll possible end-use conditions and loadings must be anticipated in part design. Environmental changes can be significant – for example, large temperature excursions in combination with differences in coefficient-of-thermal-expansion (CTE) between components can result in high induced stresses. Some end-use loadings, such as impact, can be difficult to analyze before parts are available for testing. The most impossible task of all is to envision abuses that the part was not meant to experience.Stress Concentration FactorsStress Concentration Factors are a standard tool for design engineers. Developed from empirical data, they are used to approximate stress levels in areas that hand calculations cannot evaluate. Stress Concentration Factors are dependent upon the design only – they are the same regardless of the material. Since thermoplastic resins in general have relatively low stress limits compared to metals, consideration of Stress Concentration Factors is an important step in the design of thermoplastic parts.9-2Stress Concentration FactorsTroubleshooting GuideFailure of molded plastic parts can occur in several ways: mechanical failure, fatigue, thermal troubleshoot, chemical attack, improper processing, design and tooling, material inconsistencies, etc. The following is an operational outline for troubleshooting and correcting failures of molded plastic parts.1. Identify and Define Problem2. Research Failure History3. Hypothesis of Failure4. Testing5. Determine Source of Failure6. Formulate Solution7. Communicate Findings8. FeedbackThe first step is to identify exactly what the problem is, and define what would constitute an acceptable solution. Then characterize as completely as possible the failure:TYPE - brittle, ductile, single or multiple locations, etc.HOW - minor, catastrophic, slow, fast, etc.MODE - flexure, tensile, vibration, impact, creep, heat, etc.LOCATION - crack origin, thin wall, corner, gate, hole, boss, etc.SYMPTOMS - surface appearance, fracture surface, strength or properties reduction, snap, tear, etc. HistoryThe next step of information gathering is the history of the application, as well as the specific failed parts: FUNCTION - – what exactly is the part expected to doPERFORMANCE - – specifications, requirements, etc.MATERIAL - – resin, grade, color, lot, QC specificationsHISTORY - – has it ever worked, where did it happen, failure at inspection, percentage of failures, repeatability, etc.PROCESSING - – set-up, regrind, drying, gate and sprue, mold release, machine size and type, etc. DESIGN - – prints, analyses, testing, changes, etc.SECONDARY OPERATIONS - – fastening, bonding, welding, coating, painting, printing, etc. ENVIRONMENT - chemicals, weathering, UV, loading, temperature, time, etc. Hypothesis of Failure Use the accumulated information to determine the most likely contributing factor(s) to the failure. Decide if sufficient information is present to determine corrective measures or if there is a need for further information.With justifiable hypotheses, testing may be instituted to provide verification of the cause(s) of failure. Such testing might include:DEGRADATION - – melt flow, melt viscosity, intrinsic viscosity, or TGASTRESS - – print review, finite element analysis, physical testingCOMPATIBILITY - – data survey, chemical testing, chemical wash, microscopy, etc.FTIR - – composition, contaminationASH - – level of reinforcement, possibility and condition of regrind SourceUsing the test data, visual inspections, design and processing information, and the general background information, a "supported" conclusion of the source(s) of the part failure may be drawn. In many cases, the "cause" of failure may include multiple sources.Formulate SolutionAfter having identified and rated the most likely cause(s) of failure, it is necessary to isolate the optimum solution. The solution should be feasible, adequate and economical. This is often difficult and may involve a series of compromises. If possible, alternative measures should always be provided so that improvements, cost and timeliness can be evaluated for the best solution.CommunicateProper communication of findings is an integral step in the solution process. The findings must be forwarded in a concise, clear and timely manner or they are of little use. Verbal communication is the quickest method, and is often used to speed results. However, formal reports are beneficial in documenting the activity for immediate and future reference.Feedback is critical in that it reflects the effectiveness of the analysis. If the corrective measures did not resolve the problem, an alternative approach may be necessary. Even invalid conclusions provide important information, and can be used as the basis of a more focused, in-depth evaluation.This chart lists some of the factors that are considered in the design, development, manufacture and use of a part. When the part’s performance does not meet expected levels, it may be helpful to recreate and question the thought process behind some of these decisions.Injection Molding Troubleshooting ConsiderationsFeedback is critical in that it reflects the effectiveness of the analysis. If the corrective measures did not resolve the problem, an alternative approach may be necessary. Even invalid conclusions provide important information, and can be used as the basis of a more focused, in-depth evaluation.Page 11 of 11。