MIT 2 810 - Thermoforming (26 pages)

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Thermoforming



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Thermoforming

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Pages:
26
School:
Massachusetts Institute of Technology
Course:
2 810 - Manufacturing Processes and Systems
Manufacturing Processes and Systems Documents

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Thermoforming 2 810 Professor Tim Gutowski Thermoforming Heater Clamping Plastics sheet Vacuum Thin corner Source R Ogorkiewicz Engineering Properties of Thermoplastics http www arrem com designguide dgprocesscap htm Heat Transfer in Thermoforming Heating q radiation q radiation q convection q conduction Cooling q radiation q convection q conduction Radiation Heat Transfer between two parallel plates 1 q T14 T24 1 1 1 1 2 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 See Lienhard Text Ch 10 on Radiation Heat Transfer Heating Time est Kydex sheet T q wc p t 1 35 g cm3 w 1 16 in or 1 59 mm Cp 1 21 J goK T 180 20 160oK t 130 sec Temperature regimes for polymers Log E t Semi crystalline Polymer Tg Log E t Tm Temperature Amorphous Polymer Tg Tg 60 C Temperature Viscoelastic Effects During processing Unloaded sample Coiled polymer chain Loaded sample Extended polymer chain 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 response Rubber Elasticity F W F L G H T S ideal rubber H 0 T P constant F T S L note that the change in entropy is negative Simple Viscoelastic System s d Kinematic compatibility s d 2 Constitutive behavior s E s d d Force Equilibrium This gives E 2 The viscoelastic Time constant is Newtonian Viscosity E Elastic Modulus Solution to 2 with I C 0 at t 0 2 1 e t Large values of t i e t 2 viscous behavior Small values of t i e t 2 1 1 t 2 t let t thens behavior 2 E elastic behavior FAST SLOW INBETWEN t t t Elastic Viscous Viscoelastic Temp Dependence of Time constant E 0e E RT E0T Approximation Arrhenius Rubber elasticity 0 e E RT For better



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