Thermoforming (MIT 2.008x Lecture Slides)

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Thermoforming
MIT 2.008x
Prof. John Hart

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What thermoformed object(s)
have you used already today?

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What is thermoforming
(process definition)?
à Forming a sheet (typically a
thermoplastic) by applying heat
then pressure against a mold.
Figure13.37fromFundamentalsofModernManufacturing(4thEdition)"by
Groover.(c)JohnWiley&SonsInc.(2010).

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Vacuum thermoforming
(vacuum forming, vacuforming)
Excerpt from: https://www.youtube.com/watch?v=BqV_jsxD0UA
http://formech.com/product/508fs/

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How is thermoforming
similar to injection molding?
à Both use heat and pressure
to shape thermoplastics.
How are thermoformed parts
different from injection
molded parts?
à Thermoformed parts are
typically thinner, and have less
complex shapes then injection
molded parts.
à The dimensional quality
(corners, edges) and
tolerances of thermoformed
parts are lower than injection
molded parts.

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Agenda:
Thermoforming
§ Basic equipment and process
configurations
§ Polymer mechanics during
thermoforming
§ Rate-limiting steps of
thermoforming
§ The process window and
design rules
§ Conclusion
Extra: Other polymer forming
processes

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Thermoforming:
2. Process and
equipment basics

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Lego IM vs TF comparison

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Thermoforming in the MIT shop

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Thermoforming in the MIT shop
Heater (also one above)
Clamps
Die (custom)
Sheet

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What is different about this part?
(hint: look at the surface features)

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Figures 13.36, 13.39 from Fundamentals of Modern Manufacturing (4th Edition) by Groover. (c)
John Wiley & Sons Inc. (2010)
Mechanical
thermoforming
Pressure
thermoforming Clamps
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Thermoforming:
3. Polymer mechanics
during thermoforming

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How does the polymer stress-strain curve change
with temperature?
à Recall from IM: Glass transition and softening
Figure 9.5 from Understanding Thermoforming (Second Edition) by J.L Throne. (c) Hanser, 2008.
Increasing temperature
≈TgBreak
Yield

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Temperature-dependent modulus of
thermoplastic
à Recall from IM: Glass transition and softening
Figure 9.1 from Understanding Thermoforming (Second Edition) by J.L Throne. (c) Hanser, 2008.

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Implication: thermoforming
temperature range
Figure 9.6 from Understanding Thermoforming by J.L Throne. (c) Hanser, 2008.
“Not too hot, not too cold.
Just right.”Increasing temperature
Forming range

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Demo: Stretching a thermoplastic
Heat
Heat
Pull

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Where is the strain greatest?
Figure 13.37 Fundamentals of Modern Manufacturing (4th
Edition) by Groover. (c) John Wiley & Sons Inc. (2010).

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Where is the strain greatest?

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0.394 mm
0.423 mm
0.290 mm
R = 0.310 mm
R = 0.201 mm
0.199 mm
0.154 mm
0.164 mm

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Generally, areas that touch the mold last
are thinnest
0.394 mm
0.423 mm
0.290 mm
R = 0.310 mm
R = 0.201 mm
0.199 mm
0.154 mm
0.164 mm
THICK AREAS
THIN CORNERS
AND EDGES
The area that stretched
the most to reach the
bottom is the thinnest.

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Simulation of TF (ANSYS): predicts strain and
thickness distribution
“For thermoforming a medical device package”
from http://www.ansys.com/Industries/Materials+&+Chemical+Processing/Polymer+Processing/Thermoforming
(left) finite element mesh automatically refined to capture mold curvature details
(right) predicted thickness distribution

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Thermoforming:
4. Rate limits and
continuous
processing

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What limits the rate of thermoforming?
§ Heating (à radiative transfer)
§ Stretching (à viscoelasticity)
§ Cooling (à contact with cold mold; see IM analysis)
Video: https://www.youtube.com/watch?v=YQ-s1BILiag

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Radiative heating (infrared)
Images from: http://heraeus-thermal-solutions.com/media/en/webmedia_local/media/pdfs/ir_basics_and_technology2014.pdf

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0.2 mm thickness

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Radiative heating of a plastic sheet
Lamp
Substrate
(to be formed)
h = thickness [m]
r = density [kg/m3]
cp = specific heat [J/kg-K]
a = total absorption coefficient of substrate [unitless]
plamp = lamp power [W/m2]
DT = temperature rise [K]

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Radiative heating of a plastic sheet
h = thickness [m]
r = density [kg/m3]
cp = specific heat [J/kg-K]
a = total absorption coefficient
of substrate [unitless]
plamp = lamp power [W/m2]
DT = temperature rise [K]
theat =
ρhcp
aplamp
ΔT
Lamp
Substrate
(to be formed)
DT = 250 K
h = 1 mm
r = 1200 kg/m3
cp = 1200 J/kg-K

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A continuous TF + packaging system (Ulma)
Image from http://www.ulmapackaging.com/packaging-machines/thermoforming-and-blister/tfs-700
Video: https://www.youtube.com/watch?v=qC5KFpNnR_4

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Thermoforming:
5. Process window
and design guidelines

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The thermoforming process window (P, T)
Figure 9.8 from Understanding Thermoforming by J.L Throne. (c) Hanser, 2008.

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Pressure and temperature ranges (for
pressure-controlled forming)
Table 9.1 from Understanding Thermoforming (Second Edition) by J.L Throne. (c) Hanser, 2008.

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Thermoforming strains
λ1/ λ2 = 1.28
λ1 = 1.50
λ1/ λ2 = 1.87
λ1 = 2.33
λ1/ λ2 = 4.01
λ1 = 5.17
λ1/ λ2 = 4.73
λ1 = 4.33
x1
x2
Draw ratio
L1
L2
L0
Biaxial stretch ratio
5 cm
8 cm
10 cm
~ 2.03
Note, that DR = 1 for the sheet material
prior to forming

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Areal draw ratios
Figure 9.11 and Table 9.2 from Understanding Thermoforming (2nd Edition) by Throne. (c) Hanser, 2008.

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Additional TF design guidelines
§ Avoid sharp corners in mold (R
~2*thickness) or greater.
§ Use draft angle if possible.
§ No undercuts (unless multi-part tooling)!
§ When you want to simplify mold making,
sharp corners are OK but beware of tearing.
§ For thin plastic, areal draw ratios >2:1
require careful optimization and suffer non-
uniformity.
Poor Design
Good Design
R = 2*t or greater
t
R
Draft angle: ¼°min for female tooling
1°for male tooling

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Additional TF design guidelines
Higher temperature: still cannot draw
deep teeth; non-uniformity results
Even higher temperature:
tearing
§ Avoid sharp corners in mold (R
~2*thickness) or greater.
§ Use draft angle if possible.
§ No undercuts (unless multi-part tooling)!
§ When you want to simplify mold making,
sharp corners are OK but beware of tearing.
§ For thin plastic, areal draw ratios >2:1
require careful optimization and suffer non-
uniformity.
Poor Design
Good Design
R = 2*t or greater
t
R
Draft angle: ¼°min for female tooling
1°for male tooling

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Positive versus negative mold

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Comparison of surface profiles
Positive mold
Negative mold

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Pre-stretching to reduce thickness
variation
Figure 13.38 from Fundamentals of Modern Manufacturing (4th Edition) by Groover. (c) John Wiley & Sons Inc. (2010).
https://www.youtube.com/watch?v=WJlXdb2zA0k

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Large TF tooling: car door panels

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Thermoforming:
6. Conclusion

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What’s new (and coming soon)?
§ Bio-derived and biodegradable plastics
§ Formable fiber materials
§ Paper (complex product packaging)
§ Carbon fiber (dream of auto industry)
For examples see:
§ http://vegware.com
§ http://www.billerudkorsnas.com/fibreform
§ http://www.darpa.mil/program/tailorable-feedstock-and-forming

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Reflection: the big four
Injection Molding Thermoforming
Rate High Greater (parts/time)
Quality Good Less
Cost Low (at high volume) Less ($/part, especially at
lower volume)
Flexibility Low (tooling cost high) Less: fewer shapes
Greater: lower tooling cost

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Thermoforming:
7. Other polymer
Processes

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Figure 19.1 from Kalpakjian and Schmid, Manufacturing Engineering & Technology (7th Edition)
Polymer processing overall
TP = thermoplastic
TS = thermoset
E = elastomer
Plastic bottles
Plastic bags
à Same physics,
different machine
and product format

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Blow molding of plastic bottles
Images: http://designtekplastics.com/tips/injection-molding-vs-blow-molding/, http://dtresource.com/images/what-is-stretch-blow-
molding-300x210.jpg, http://dongkong.en.ec21.com/500ml_water_bottle_blow_mold--4844865_4844892.html
Figure 13.32 from Groover, Fundamentals of Modern Manufacturing (4th Edition)

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Melt/Extruder
(Like an IM machine)
Rotating molds
§ 0.08 - 0.5 L containers
(e.g., PP, HDPE)
§ Multimold wheel system
(18-60 cavities)
§ Production rates of 7,500 -
30,000 bottles per hour
(500kg/h)!
Video of the machine: http://www.youtube.com/watch?v=u-eW2lrxrq0
Diagram and data from http://www.wilmingtonmachinery.com/media/pdf/small_bottle_insert.pdf
Continuous
process!

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How are trash bags made?

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Blown film extrusion
Figure 13.16, Groover, Fundamentals of Modern Manufacturing (4th Edition)

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Picture: https://www.hosokawa-alpine.com/film-extrusion/blown-film-lines/

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https://www.hosokawa-alpine.com/film-extrusion/blown-film-lines/
https://images-na.ssl-images-amazon.com/images/I/61uTRra5KkL.jpg

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Rotational molding
(‘Rotomolding’)
Fig. 19.15, Kalpakjian and Schmid, Manufacturing Engineering and Technology
Videos: https://www.youtube.com/watch?v=M0_l269cPvQ, https://www.youtube.com/watch?v=b619f-0QhEs

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References
1 Introduction
Photo of Lunch Tray © St. Louis County, Minnesota.
Photo of ATV by Vesa Minkkinen on Pixabay. This work is in the public domain.
Photo of Fruit Container by Vedat Zorluer on Pixabay. This work is in the public domain.
Photo of Refrigerator by US Consumer Product Safety Commission. This work is in the public domain.
Positive Mold Vacuum Thermoforming: Figure 13.37 from "Fundamentals of Modern Manufacturing (4th Edition)"
by Groover. © Wiley (2010).
Image of Formech 508FS © Formech International Ltd. 2016. All Rights Reserved.
2 Process Equipment Basics
Positive Pressure Thermoforming: Figure 13.36 from "Fundamentals of Modern Manufacturing (4th Edition)" by
Groover. © Wiley (2010).
Mechanical Thermoforming: Figure 13.39 from "Fundamentals of Modern Manufacturing (4th Edition)" by

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References
3 Polymer Mechanics
Stress-Strain vs. Temperature: Figure 9.5 from "Understanding Thermoforming (2nd Edition)" by Throne. ©
Hanser, 2008.
Elastic Modulus vs. Temperature: Figure 9.1 from "Understanding Thermoforming (2nd Edition)" by Throne. ©
Hanser, 2008.
Hanser, 2008.
Book Cover: Denslow's "Three Bears" (1901) on read.gov: Library of Congress. This work is in the public
domain.
Positive Mold Vacuum Thermoforming: Figure 13.37 from "Fundamentals of Modern Manufacturing (4th Edition)"
by Groover. © Wiley (2010).
Image of ANSYS Simulation © 2016 ANSYS, Inc. All Rights Reserved.
4 Rate Limits
Videos of Thermoforming and Sealing Packaging © ULMA Packaging, S.Coop.
Images of Infrared Heating Process and Equipment © 2016 Heraeus Holding
Video of TFS 700 Thermoforming Machine © ULMA Packaging, S.Coop.

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References
5 Process Window
Hanser, 2008.
Draw Ratio Diagram: Figure 9.11 from "Understanding Thermoforming (2nd Edition)" by Throne. © Hanser,
2008.
Draw Ratios: Table 9.2 from "Understanding Thermoforming (2nd Edition)" by Throne. © Hanser, 2008.
Video of Presuction © 2011-2015 EPW LLC
Thermoforming with Prestretch: Figure 13.38 from "Fundamentals of Modern Manufacturing (4th Edition)" by
Image of Automotive Doors: © 2011-2015 EPW LLC
6 Conclusion
Image of Carbon Fiber Manufacturing © Hearst Communications, Inc.
Images of Paper Products © BillerudKorsnas AB

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References
7 Bonus
Photo of Drinking Straws by User: Alexas_Fotos (Alexandra) via Pixabay CC0. This work is in the public
domain.
Photo of Water Bottles by User: PublicDomainPictures via Pixabay CC0. This work is in the public domain.
Photo of Kayaks by User: vonpics via Pixabay CC0. This work is in the public domain.
Photo of Trash Bag by User: cocoparisienne (Anja) via Pixabay CC0. This work is in the public domain.
Photo of American Football by User: Hans (Hans Braxmeier) via Pixabay CC0. This work is in the public
domain.
Photo of Disposable Cup by User: rodrigolourenco (Rodrigo Lourenço) via Pixabay CC0. This work is in the
public domain.
Photo of Sprayer Tanks © Copyright 2016. Den Hartog Industries, Inc.
Polymer Processing Overview: Figure 19.1 from Title: Manufacturing Engineering & Technology (7th Edition);
Authors: Serope Kalpakjian, Steven Schmid; © Prentice Hall; (2013);
Blow Molding: Figure 13.32 from Title: Fundamentals of Modern Manufacturing; Author: Mikell P. Groover;
Publisher: Wiley; 4 edition (2010); ISBN: 978-0470-467002

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References
Image of Bottle Blow Mold: Copyright ©1997-2016 EC21 Inc. All Rights Reserved.
Photo of Injection Molded Parisons © Steven Daly. All Rights Reserved.
Photo of Blow Molded Bottles ©2015 Design-tek Tool and Plastics Inc.
Image of Blow Molding Machine © Wilmington Machinery.
Image of Blow Molding Machine in Operation © Wilmington Machinery.
Blow Film Extrusion: Figure 13.16 from Title: Fundamentals of Modern Manufacturing; Author: Mikell P. Groover;
Publisher: Wiley; 4 edition (2010); ISBN: 978-0470-467002
Photo of Blow Film Extrusion Process © HOSOKAWA ALPINE Aktiengesellschaft. All Rights Reserved.
Images of Hefty Garbage Bags © 1996-2016, Amazon.com, Inc. or its affiliates
Rotomolding: Figure 19.15 from Title: Manufacturing Engineering & Technology (6th Edition); Authors: Serope
Kalpakjian, Steven Schmid; Publisher: Prentice Hall; 6 edition (January, 2009); ISBN-13: 9780136081685
Video of Rotational Molding Machine © Reinhardt Rotomachines
Video of Unmolding Rotational Molded Tank © Reinhardt Rotomachines

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