Thermoforming2.810 Fall 2003Professor Tim GutowskiThermoforming Process PlanHardware Requirementsn Tooln Trim FixtureSet-upn Mount Tool in Thermoformern Mount Trim FixtureRunn Fixture Sheet; Heat; Form; Cool; TrimThermoformingHeaterPlastics sheetClampingVacuum**** Source: R. Ogorkiewicz, “Engineering Properties of Thermoplastics.”; ** http://www.arrem.com/designguide/dgprocesscap.htmThin cornerThermoformingVariations on the processDrape FormingVacuum FormingVacuum Snap-Back FormingBillow Vacuum FormingVariations on the processPlug-assist Vacuum Forming Plug-assist Pressure FormingPressure Reverse Draw with Plug-assistedProduction EquipmentHeat Transfer in ThermoformingHeatingconvectionq&radiationq&radiationq&conductionq&Coolingconvectionradiationqq&&&conductionq&Temperature regimes for polymersLog E(t)TemperatureTg TmSemi-crystalline PolymerLog E(t)TemperatureTg Tg+60°CAmorphous PolymerViscoelastic Effects During processingUnloaded sampleCoiled polymer chainExtended polymer chainLoaded samplePolymer chains tend to exist in coiled configurations. Loading the sample can extend the chain and alter the mechanical behavior. Generally, abrupt, high rates of loading will extend the chain and lead to elastic effects. On the other hand, gradual slow rates of loading allow the chain to more or less retain its coiled configuration, with a resulting primarily viscous responseSimple Viscoelastic Systemε&ddssdsdsE εµσεσεεεσσσ&&&&===+==; :behavior veConstituti2 :itycompatibil Kinematic :mEquilibriu Forceεµσσµ&&2 gives This =+EModulus ElasticViscosity Newtonian"" :isconstant Time tic viscoelasThe==Eµλ)1(2 0at 0 I.C. with 2 : oSolution tλεµσσεµσσλtet−−====+&&&behavior ous visc2 i.e. / of valuesLargeεµσλλ&≈>>ttbehavior elastic 2 behavior thens,let 2 ))/1(1(2 i.e. / of valuesSmallεσεελεµλεµσλλEttttt≈=⋅=−−≈<<&&&“FAST” t <<λ Elastic“SLOW” t >>λ Viscous“INBETWEN” t ~ λ ViscoelasticTemp. Dependence of Time constant, λTEeERTE00∆≅=µµλArrheniusRubber elasticityApproximationRTEe∆≅0λλ(For better accuracy, use Time-Temp shift, WLF eqn.)Example: PMMA Temp λ40°C 114 yrs100°C Tg135°C 3.5
View Full Document
Unlocking...