Slides accompanying 2.008x* video module on Thermoforming, Prof. John Hart, MIT, 2016.
*Fundamentals of Manufacturing Processes on edX: https://www.edx.org/course/fundamentals-manufacturing-processes-mitx-2-008x
3. 2.008x
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).
5. 2.008x
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.
6. 2.008x
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
18. 2.008x
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
2.008x
20. 2.008x
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
24. 2.008x
Where is the strain greatest?
Figure 13.37 Fundamentals of Modern Manufacturing (4th
Edition) by Groover. (c) John Wiley & Sons Inc. (2010).
27. 2.008x
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.
28. 2.008x
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
30. 2.008x
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
32. 2.008x
0.2 mm thickness
Images from: http://heraeus-thermal-solutions.com/media/en/webmedia_local/media/pdfs/ir_basics_and_technology2014.pdf
33. 2.008x
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]
34. 2.008x
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
36. 2.008x
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
39. 2.008x
Pressure and temperature ranges (for
pressure-controlled forming)
Table 9.1 from Understanding Thermoforming (Second Edition) by J.L Throne. (c) Hanser, 2008.
40. 2.008x
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
41. 2.008x
Areal draw ratios
Figure 9.11 and Table 9.2 from Understanding Thermoforming (2nd Edition) by Throne. (c) Hanser, 2008.
42. 2.008x
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
43. 2.008x
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
46. 2.008x
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
49. 2.008x
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
50. 2.008x
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
53. 2.008x
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
54. 2.008x
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)
55. 2.008x
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)
56. 2.008x
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!