12| PLASTICS ENGINEERING | OCTOBER 2011| www.4spe.orgstronger and more resistant toimpact, so specialized equipmentand expertise are needed for charac-terization," Geissler says. Failure AnalysisWhen a problem with a plastic partarises-whether experienced by theconsumer or noticed after a part ismolded-it is important to analyzewhy the failure occurred. Failureanalysis can also be used to antici-pate problems or to find out whyprototype parts may have failed.The Madison Group in Madison,Wisconsin, USA, provides failureanalysis for plastics. Engineeringmanager Jeffrey A. Jansen says thatcompanies sometimes think theydon't have the time or resources forfailure analysis, but a scientificapproach to addressing a problem ismore cost- and time-effective thanguessing at the problem and tryingsolutions that might not work.During an analysis, engineers fromthe Madison Group aim to find outboth how and why a failureoccurred. In general, five factors areinvolved in failure: material, pro-cessing, design, installation, andservice conditions. The company begins an analysisby gathering backgroundinformation on the failedcomponent, such as howthe part was made andits service history. Theythen perform a visualexamination, checkingto see if the failure ishappening in the sameplace on all the parts orwhether discoloration ispresent, for example.Microscopic analysis isused to examine thefracture surface, whichshows how and where a crack beganand where it spread.Analytical characterization of thepolymer's molecular weight, com-position, and mechanical propertiesis required to understand why thefailures occurred. "Ideally we cancompare the characteristics of afailed part to a control that didn'tfail to show if some type of degra-dation, contamination, or materialanomaly was involved," Jansensays.All this information is used topiece together the primary cause ofthe failure as well as other con-tributing factors. "It is usually notjust one factor; usually two or threethings overlap," Jansen says, "par-ticularly if only 0.1 percent of theparts made are failing." For exam-ple, consider a part with sharp cor-ners and notches that focus forces.During molding, molecular degrada-tion occurs that compromises thematerial. Then during use, it under-goes strong impact. The failurewouldn't have happened if any ofthose things weren't present-thesharp corners and notches that con-centrate the force, the compromisedmaterial, or the strong impact dur-ing use. Viscoelastic MeasurementsTimothy A. Cassell, national salesmanager for rheology and solid sur-face analysis at Anton Paar USA,says there is a wider range of addi-tives on the market today for use inplastics. "With each additive it isimportant to characterize the materi-al. The goal is to understand howthe product will behave under man-ufacturing conditions and underSometimes the site of a failure is obvioussuch as in the plastic pipe on top.However, a closer look with microscopy(above) can reveal where a crack began.The Madison Group.A processed real-size automotive com-ponent for material characterization.Intertek.
use." The Modular CompactRheometer (MCR) from Anton Paarcan be used to measure the proper-ties of unmodified as well as modi-fied samples to identify the changes.The MCR characterizes a poly-mer's viscoelastic properties, infor-mation that can be used to under-stand how the material behavesunder application and in storageconditions as well as the inherentstructure of the material. Therheometer also allows the additionof environmental accessories for avariety of application-specific meas-urements. "Versatility is important;people want one instrument that cando multiple types of testing andhandle multiple types of samples,"says Cassell.With the MCR, a user can meas-ure the extensional and torsionalproperties of solid samples fordynamic mechanical thermal analy-sis. The addition of light-scatteringand optical viewing modules canallow study of microstructures andthe molecular or structural changesthat result from additives. Cassellsays that there continues to begrowth in application of plastics aslower-cost and lighter-weight alter-natives to metals. When plastic isused for moving parts in assemblies,measuring the friction factor for thepart is important. The company'stribology module allows the user toanalyze how a plastic part wouldwear over time.Weathering AnalysisDeveloping a new formulation orchanging an existing resin, additive,or color for a plastic part used out-doors requires assessing how wellthe plastic will weather over time.In the case of a change of manufac-turing location or a change in raw-material source, a company maywish to revalidate the UV enduranceof the product. In other cases, com-panies may use weathering testingto troubleshoot product failures. Asone example, when a seal on a partfailed, the customer speculated thatsunlight had changed the plastic'smaterial properties. Weathering test-ing showed thatalthough the color didnot change, the surfacechanged from glossy tomatte, and that there wasa strong possibility thatthe change in surfacetexture also affected theseal.Solar Light Companyis a U.S. manufacturerof precision UV lightsources, solar simula-tors, radiometers, sen-sors, and detectors forseveral industries. It alsooffers testing servicesand has seen increasingrequests for these servic-es over the past few years.Filters in the company's solar simu-lator tune the light source to simu-late only the UV portion of thespectrum, which has the advantageof minimizing the temperatureincrease of the sample to only a fewdegrees above ambient temperature.Because Solar Light's instrumentsends light through a lens, it canamplify irradiance by reducing thearea of the sample that is exposed."Using a 57-mm-diameter circle oflight, we can produce six years ofexposure in one year of acceleratedtesting time," explains Drew Hmiel,a physicist at the company."However, by reducing the circle toa 20-mm diameter we can furtheraccelerate six years of exposure toonly one month of testing."Solar Light is currently correlat-ing accelerated laboratory results tooutdoor exposure for a wide rangeof polymers. The company meas-ures color and appearance changesof weathered samples using col-orimetry as well as reflectance andtransmission spectrophotometry. UVtransmittance can be measured aswell. The company does not offertensile testing, although customerscan do this testing elsewhere if theydesire. Hmiel notes that simulationThe MCR from Anton Paar USA character-izes a polymer's viscoelastic properties.A plastic building product underwent anexposure test for 10 years. The lower-rightphoto illustrates the exposure arrangement.Solar Light.www.4spe.org | OCTOBER 2011 | PLASTICS ENGINEERING |13