8-thermoforming

8/13/2019 8-thermoforming

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Thermoforming

2.810T. Gutowski


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Vacuum Thermoforming

Heater

Plastics sheetClamping

Vacuum

*

**

* Source: R. Ogorkiewicz, Engineering Properties of Thermoplastics.; ** http://www.arrem.com/designguide/dgprocesscap.htm

Thin corner
http://www.arrem.com/designguide/dgprocesscap.htmhttp://www.arrem.com/designguide/dgprocesscap.htm


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Thermoformed Parts


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Some Basics

1. Heating and cooling

2. Viscoelastic behavior

Rubber elasticity Time-Temperature behavior

3. Deformation patterns

4. Production equipment5. Double diaphram forming


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Heat Transfer in Thermoforming

Heating convectionqradiatioq

radiationq

conductionq

Coolingconvectionradiation qq &

conductionq


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Radiation Heat Transfer between two parallel plates

s= 5.67 x 10-8W/m2K4

ex. e1e2= 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

)(1

11

14241

21

TTq s

e

e

See Lienhard Text Ch 10 on Radiation Heat Transfer


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Heating Time est. (Kydex sheet)

r= 1.35 g/cm3

w = 1/16 in or 1.59 mm

c = 1.21 J/goK

DT = 18020 = 160oK

t = 130 sec

q rwcDT

Dt


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Temperature regimes for polymers

Log E(t)

TemperatureTg Tm

Semi-crystalline Polymer

Log E(t)

TemperatureTg Tg+60C

Amorphous Polymer


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Viscoelastic Effects During

processingUnloaded sample

Coiled polymer chain

Extended polymer chain

Loaded sample

Polymer chains tend to exist in coiled configurations. Loading thesample can extend the chain and alter the mechanical behavior.Generally, abrupt, high rates of loading will extend the chain and leadto elastic effects. On the other hand, gradual slow rates of loadingallow the chain to more or less retain its coiled configuration, with a

resulting primarily viscous response


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Simplified Rubber Elasticity

DW = F DL = DA = DETDS

(A = Helmholtz free energy)

Conventional materials

F = DE/DL

Rubber

F =-T (DS/DL)

F

note that upon extension the change in entropy is negative

(No volume change)

Flory

QuickTime and a decompressor

are needed to see this picture.


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Simple Viscoelastic System

e

Force Equilibrium: ss sd s

Kinematic compatibility: es ed e

Constitutive behavior: ss Ees;sd ed

This gives

Esse

ModulusElastic

ViscosityNewtonian""

:isconstantTimeticviscoelasThe

E


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Solution to : sse with I.C. s 0 at t 0

se(1 e

t

)

Large values of t / i.e. t

se viscous behaviorSmall values of t / i.e. t

se(1 (1 t / )) et

let e te, this gives

sEe elastic behavior

FAST t ViscousINBETWEN t ~ Viscoelastic


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Temp. Dependence of Time

constant,

TE

e

E

RTE

0

0

D

Arrhenius

Rubber elasticity

Approximation RTEeD 0(For better accuracy, use Time-Temp shift, WLF eqn.)

Example: PMMA Temp

40C 114 yrs100C Tg135C 3.5 millisec


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Viscous behavior of silly putty


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deformation patterns

d(Ah) = A dh + h dA = 0

dA/A = - dh/h


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Deformation Patterns


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Thermoforming


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Variations on the process

Drape Forming

Vacuum Forming

Vacuum Snap-Back Forming

Billow Vacuum Forming


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Thermoforming Patterns


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Vacuum holes


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Variations on the process

Plug-assist Vacuum Forming Plug-assist Pressure Forming

Pressure Reverse Draw with Plug-assisted


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Production Equipment


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Double Diaphragm Forming


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Temp, Time, Size & Shear


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Ideal Shear Vs Actual


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Parts made in Lab


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Double diaphragm forming

Forming

tool

Curing

tool


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Former MIT grad student

Sam Truslow

MIT Building 35 Prototype machine at Boeing


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More videos

http://www.youtube.com/watch?v=KPFAoLmJ5og

superplastic forming of aluminum at Kirkham University

http://www.youtube.com/watch?v=NPLWxxyIJcE&NR=1&feature=fvwpthermoforming blow and then vacuum
http://www.youtube.com/watch?v=KPFAoLmJ5oghttp://www.youtube.com/watch?v=NPLWxxyIJcE&NR=1&feature=fvwphttp://www.youtube.com/watch?v=NPLWxxyIJcE&NR=1&feature=fvwphttp://www.youtube.com/watch?v=KPFAoLmJ5og

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