Thermoforming 2.810 Professor Tim GutowskiThermoforming Heater Plastics sheet Clamping Vacuum * ** * Source: R. Ogorkiewicz, “Engineering Properties of Thermoplastics.”; ** http://www.arrem.com/designguide/dgprocesscap.htm Thin cornerHeat Transfer in Thermoforming Heating convectionqradiationqradiationqconductionqCooling convectionradiationqq&conductionqRadiation Heat Transfer between two parallel plates = 5.67 x 10-8 W/m2K4 ex. 1 2 = 1 (black bodies) T1 = 533oK (heater) T2 = 293oK (plastic at room temperature) q = 4.2kW/m2 at T2 = 180oC = 453oK (forming temperature) q = 2.3kW/m2 )(1111424121TTq + =See Lienhard Text Ch 10 on Radiation Heat TransferHeating Time est. (Kydex sheet) = 1.35 g/cm3 w = 1/16 in or 1.59 mm c = 1.21 J/goK T = 180 – 20 = 160oK t = 130 sec q =wcTtTemperature regimes for polymers Log E(t) Temperature Tg Tm Semi-crystalline Polymer Log E(t) Temperature Tg Tg+60°C Amorphous PolymerViscoelastic Effects During processing Unloaded sample Coiled polymer chain Extended polymer chain Loaded sample Polymer 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 responseSimplified Rubber Elasticity W = F L = G = H –TS ideal rubber H = 0; T, P constant F =-T (S/L) F note that upon extension the change in entropy is negative (No volume change) FlorySimple Viscoelastic System Force Equilibrium: s=d=Kinematic compatibility: s+d=Constitutive behavior: s= Es;d=μdThis gives μE+=μModulus ElasticViscosity Newtonian"" :isconstant Time tic viscoelasThe==EμSolution to : + =μ with I.C. = 0 at t = 0 =μ(1 et)Large values of t / i.e. t >> μ viscous behaviorSmall values of t / i.e. t << μ(1 (1 t / )) = μt let t =, this gives E elastic behavior“FAST” t << Elastic “SLOW” t >> Viscous “INBETWEN” t ~ ViscoelasticTemp. Dependence of Time constant, TEeERTE00 =μμArrhenius Rubber elasticity Approximation RTEe 0(For better accuracy, use Time-Temp shift, WLF eqn.) Example: PMMA Temp 40°C 114 yrs 100°C Tg 135°C 3.5 millisecViscous behavior of “silly putty”deformation patterns d(Ah) = A dh + h dA = 0 dA/A = - dh/hDeformation PatternsThermoformingVariations on the process Drape Forming Vacuum Forming Vacuum Snap-Back Forming Billow Vacuum FormingThermoforming PatternsVacuum holesVariations on the process Plug-assist Vacuum Forming Plug-assist Pressure Forming Pressure Reverse Draw with Plug-assistedProduction EquipmentShow VideoDouble Diaphragm FormingTemp, Time, Size & ShearIdeal Shear Vs ActualParts made in LabDouble diaphragm forming Forming tool Curing toolFormer MIT grad student Sam Truslow MIT Building 35 Prototype machine at BoeingDiaphragm forming of Composites Demo part for Boeing
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