injection molding (mit 2.008x lecture slides)
TRANSCRIPT
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2.008x
Injection MoldingMIT 2.008x
Prof. John Hart
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2.008x
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2.008xPlastics: 1950-2014
Data from Statistahttp://www.statista.com/statistics/282732/global-production-of-plastics-since-1950/
0
50
100
150
200
250
300
350
1950 1960 1970 1980 1990 2000 2010 2020
Volu
me
[mill
ion
met
ric to
ns]
Year
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2.008x
Adapted from: Figure 1, "Ideas in Motion Control from Moog Industrial" Newsletter Issue 34 © Moog, 2014
In North America and Europe, injection molding is used to process >10 million tons (10 billion kg) of polymers per year
Packaging
Automotive parts
Technical parts
Electronics and telecommunications
Medical, pharmaceutical, and
optical products
White goods, construction
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2.008x
Routsis Associates: https://youtu.be/cANvFsvY0Aw
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2.008xAn IM machine in the MIT manufacturing shop
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2.008xAgenda:
Injection Molding§ Fundamentals of polymers§ Mold tooling§ Process parameters and
equipment§ Cycle time, cooling, and
shrinkage§ Defects and design guidelines§ Advanced topics
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2.008x
Injection Molding:
2. Fundamentals of polymers
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2.008xIM feedstock: polymer pellets
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2.008x
Alibaba, February 2016http://www.alibaba.com/product-detail/Extruded-PP-granules-Polypropylene-PP-
Crush_60255030501.html?spm=a2700.7724857.35.1.1Fnwad
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2.008x
Kalpakjian and Schmid, Manufacturing Engineering and TechnologyGroover, Fundamentals of Modern Manufacturing
Poly (many) + mer (structural unit)-[C2H4]n- = poly[ethylene]
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2.008xThe ‘families’ of materials: modulus vs. density
Ashby, Materials Selection in Mechanical Design.
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2.008x
Manufacturing Engineering & Technology (7th Edition) by Kalpakjian, Schmid. © Upper Saddle River; Pearson Publishing (2014).
“Giant Dishes.” Daryl Mitchell (CC BY-SA 2.0) via Flikr
“Hydraulic seal kit cylinder seals o ring.” Devendra Dave (CC BY-SA 2.0) via Flikr
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2.008xPolymer network architectures
Thermoset Thermoplastic
(semi-crystalline)
(amorphous, linear) (amorphous, crosslinked)
à In all cases, the polymer chain length, interactions, bonding influence the part mechanics
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2.008x
What does the polymer ‘feel’ during injection molding?
à heat and pressure
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2.008x
What does the polymer ‘feel’ during injection molding?
à heat and pressure
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2.008x
J.L. Throne, Technology of Thermoforming
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2.008x
J.L. Throne, Technology of Thermoforming
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2.008x
Kalpakjian and Schmid, Manufacturing Engineering and Technology
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2.008xViscosity: resistance to shear
*at typical injection shear rate and melt temperature
Material Dynamic viscosity
Water (room temp) 1×10-3 kg/m-s [Pa-s]
Honey 10
Liquid thermoplastic* 102-103
Molten aluminum (600 C) 3×10-3
yU¶¶
= µt
Ux(H) = Ux
Ux(y)
Ux(0) = 0
h
Ux
x
y
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2.008xViscosity of polypropylene versus shear rate
and temperature
From Solidworks Plastics
µ = k !γ (n−1)
100 C
250 C
Ux(H) = Ux
Ux(y)
Ux(0) = 0
h
Ux
x
y
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2.008x
Injection Molding:
3. Mold tooling and configurations
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2.008xInjection molding of LEGO bricks
Excerpt from: https://www.youtube.com/watch?v=y1Zhpdx-XtA
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2.008xThe injection molding machine
Groover, Fundamentals of Modern Manufacturing
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2.008xKey features of mold tooling
Protomold ‘demo mold’
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2.008xVideo: MIT 2.008 injection molds and machine
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2.008xAn injection molded cap§ What features do you notice?§ Compare quality to LEGO bricks; what is different?§ What do the molds look like (draw the molds)?
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2.008x
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2.008x
Gate
Parting?
Parting
Draft
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2.008xMulti-part / multi-cavity molds
Kalpakjian and Schmid, Manufacturing Engineering and Technology
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2.008x
Kalpakjian and Schmid, Manufacturing Engineering and Technology
Three-plate mold
Two-plate mold
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2.008x
Kalpakjian and Schmid, Manufacturing Engineering and Technology
Hot runner mold (three plates)
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2.008xLego bricks: three-plate mold
http://www.cnet.com/pictures/how-lego-makes-its-bricks-photos/
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2.008x
https://www.youtube.com/watch?v=JSkz5eBJrCI
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2.008x
Injection Molding:
4. Injection process parameters
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2.008xHow would you choose
an IM machine?(important specs?)
§ Clamping force: force available to hold plates together.
§ Injection pressure: maximum pressure that can be developed to force the plastic into the mold cavity.
§ Shot size: amount of material that can be transferred to the mold (i.e., the part volume plus runners, gates, etc).
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2.008xSpecs of the IM machine at MIT
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2.008xSpecs of the IM machine at MIT
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2.008x
à Let’s relate the machine specifications to a basic model of the mold filling process
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2.008x
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2.008x
L
hh/2
h/2
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2.008x
L
hh/2
h/2
dPdx
= µd 2Udy2
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2.008xSimple scaling of injection parameters for a
2D rectangular channel
÷÷ø
öççè
æµ
÷øö
çèæ=D
=D
2
3
2
3
12
12
hwLF
hLP
whQLP
fillclamp
fill
tµ
tµ
µ
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2.008xSimulating injection using Solidworks Plastics
A simple plate:§ L = W = 100 mm§ h (thickness) = 2 mm§ Polypropylene (PP)§ Tmelt = 250C§ Tmold = 70C
à Above, we predicted injection pressure DP = 3 MPa
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2.008xSimulation: injection flow
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2.008xViscosity of polypropylene versus shear rate
and temperature
From Solidworks Plastics
µ = k !γ (n−1)
100 C
250 C
Ux(H) = Ux
Ux(y)
Ux(0) = 0
h
Ux
x
y
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2.008xSimulation: injection pressure
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2.008x
Injection Molding:
5. Cycle time, cooling, and shrinkage
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2.008xThe injection molding
cycle
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2.008xTemperature vs time
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2.008xHow do we model cooling of the part?
2
2
2
2
yT
yT
ck
tT
p ¶¶
=¶¶
=¶¶ a
r
Mold
Mold
Part
x
y
L
hh/2
h/2
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2.008x
Exact solution for a plateTm = melt temperatureTw = wall temperatureTe = ejection temperature
Drawing from Leinhard, A Heat Transfer TextbookAlso see BASF ‘estimating cooling time’ http://www2.basf.us/PLASTICSWEB/displayanyfile?id=0901a5e1801499d3
a4
2htcool =
MoldMold Part
a = thermal diffusivity = k / rcp~0.1 mm2/s for thermoplastics
÷÷ø
öççè
æ--
=we
wmcool TT
TThtpap4ln2
2
à We define the cooling timeas the time until the temperature at the centerline of the part reaches the specified ejection temperature
‘Rule of thumb’
if (Tm-Tw) ≈ 10(Te-Tw)
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2.008xCooling time scaling for plate geometry
Tm = 200 ºC = 473 K Tw = 77 ºC = 350 K
tcool =h2
4αtcool =
h2
π2αln
!
4
π
Tm − Tw
Te − Tw
"
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2.008xHow do process parameters vary with
part size? ΔP = 12µ
τ fill
Lh"
#$
%
&'2
Fclamp ∝µτ fill
wL3
h2"
#$
%
&'
tcool ∝h2
4α
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2.008x
Pack
Clo
se
Fill
Eje
ct
Gat
e fre
ezes
Time
Pres
sure
[MPa
]5
10
15Cool
Cycle time
Pressure vs time
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2.008x
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2.008xResidual stress in LEGO® block
(polarized imaging) 2.008x
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2.008xPolymers change volume with pressure and
temperature
à imagine a sponge that tries to shrink but is glued to the inside walls of a rigid container: residual stress!
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2.008x
http://www.lati.com/pdf/technical_data/dimensional-molding-shrinkages.pdf
Practically, how much shrinkage?
Longitudinal shrinkage (parallel to flow)
Late
ral s
hrin
kage
(per
pend
icul
ar to
flow
)
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2.008xIn other words…§ Polymers shrink during cooling; that’s a fact.§ If the shrinkage is constrained by the mold, residual
stresses are ‘trapped’ because the part cannot relax as the polymer shrinks.
§ During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses or residual stresses that act on a part with effects similar to externally applied stresses.
§ These residual stress can cause the part to will warp upon ejection from the mold or crack when loaded during use.
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2.008x
Injection Molding:
6. Defects and the ‘process window’
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2.008xThe injection molding
‘process window’
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2.008xShort shot
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2.008xFlash
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2.008xBurning (thermal degradation)
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2.008x
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2.008xProtomold ‘design cube’
§ What is the molding orientation?
§ Where are the ejector pins?§ What defects do you notice?§ …
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2.008x
Ribs
Undercuts
‘Living’ hinges
Straight-pull transverse hole
Side-pull transverse hole
Surface finishes
Thick and cored out sections
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2.008xSimulation: filling
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2.008xSimulation: cooling time
309 s
1.4 s
63 s
124 s
186 s
248 s
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2.008xSimulation: shrinkage
0.45 mm
0.01 mm
0.10 mm
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2.008xSimulation: warp
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2.008xCorners, fillets and hinges (‘living hinges’)
R = 0.2 mm
2 mm
R = 1 mm
Fillets
Corner radius
2 mm
0.25 mm
Hinges
Note blistered edges
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2.008xDraft angles
2 mm
Fins on Protomold cube
à Draft angles enable easier part ejection.
à The required draft angle depends on thickness, and surface texture.
LEGO brick
100 µm
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2.008xSurface finishes
PM-F0 PM-F1
SP-B1 SP-A2
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2.008xThe process window always applies, but the
conditions are different everywhere in your part!
à Therefore it’s not good enough to be in the process window!
à Beware of common defects, design for maximum uniformity, and reduce risk by following DFM guidelines (see supplements)
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2.008x
Injection Molding:
7. Advanced topics
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2.008xMolding with ‘side action’
Animation from protomoldElbow fitting: http://www.plastic-injectionmoulds.com/sale-1133447-household-plastic-injection-
molded-parts-pvc-pb-pp-for-water-tank-fitting.html
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2.008x
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2.008xInsert molding
Plasticmolded item
Metal terminal
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2.008xOvermolding (two plastics)
How?§ Insert brush§ Mold rigid base (white)§ Mold elastomer (black) at
lower temperature
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2.008xMetal injection molding (MIM)à Perform injection molding using a metal powder mixed with polymer binder; then anneal the part to achieve higher density (with significant shrinkage)
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2.008x8. Conclusion: the big four
Injection Molding Machining
Rate High Low-Medium
Quality Good As good or better!
Cost Low (at high volume) Almost always greater
Flexibility Low (tooling cost high) High (within machine constraints)
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2.008xReferences1 Introduction
Photo of Electrical Plug by User: Taken on PIxabay.com. This work is in the public domain.
Image of Plastic Production Industry © John Hart. Adapted from http://www.statista.com/statistics/282732/global-production-of-plastics-since-1950/
Image of Plastic Production Industrial Branches by Burkhard Erne © MOOG Inc. 2013. All Rights Reserved.
Video of Process Overview © A. Routsis Associates Inc. 2015
2 Fundamentals
Photo of Pellet Costs © 1999-2016 Alibaba.com. All Rights Reserved.
Polymer Representation: Figure 8.2(3) from Title: Fundamentals of Modern Manufacturing; Author: Mikell P. Groover; Publisher: Wiley; 4 edition (2010); ISBN: 978-0470-467002
Polymerization Reaction: Figure 7.3b from Title: Manufacturing Engineering & Technology (6th Edition); Authors: Serope Kalpakjian, Steven Schmid; Publisher: Prentice Hall; 6 edition (January, 2009); ISBN-13: 9780136081685
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2.008xReferencesModulus vs. Density Plot: Figure 4.2, page 60 from Title: Material Selection in Material Design; Author: Michael Ashby; Publisher: Butterworth-Heinemann; 4 edition (2011); ISBN: 9780080952239
Stress-Strain Comparison: Figure 7.10 from Title: Manufacturing Engineering & Technology (7th Edition); Authors: Serope Kalpakjian, Steven Schmid; © Prentice Hall; (2013);
Photo of Giant Dishes by Daryl Mitchell on Flickr. (CC BY-SA) 2.0
Photo of Hydraulic Sealing by Devendra Dave on Flickr. (CC BY-SA) 2.0
Networked Polymer Structure: Figure 7.5d from Title: Manufacturing Engineering & Technology (6th Edition); Authors: Serope Kalpakjian, Steven Schmid; © Prentice Hall; (2009)
Image of Semicrystalline Polymer by Dr. Michael Eastman, P.I.; © Copyright UTEP 2010
Stress-Strain Comparison of Amorphous Thermoplastics: Figure 2.12 from Title: Technology of thermoforming; Author: James L. Throne; © Hanser/Gardner Publications; (1996);
Tensile Strength vs. Temperature: Figure 2.26 from "International Plastics Handbook" by Osswaldet al. © Hanser Publishers (2006).
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2.008xReferencesGlass Transition Temperature: Table 7.2 from Title: Manufacturing Engineering & Technology (7th Edition); Authors: Serope Kalpakjian, Steven Schmid; © Prentice Hall; (2013);
Image of Viscosity Shear Thinning ©Dassault Systemes; SolidWorks Corporation 2016
3 Mold Tooling + Conf
Video of LEGO © User: Mister Rolls on YouTube
Injection Molding Machine: Figure 13.21 from Title: Fundamentals of Modern Manufacturing; Author: Mikell P. Groover; © Wiley; (2010);
Multi Cavity Mold: Figure 19.10 from Title: Manufacturing Engineering & Technology (7th Edition); Authors: Serope Kalpakjian, Steven Schmid; © Prentice Hall; (2013);
Injection Molding Molds: Figure 19.11 from Title: Manufacturing Engineering & Technology (7th Edition); Authors: Serope Kalpakjian, Steven Schmid; © Prentice Hall; (2013);
Photo of LEGO Mold © Daniel Terdiman / CNET.
Video of Ball Point Pen Clips © ARBURG.
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2.008xReferences6 Cycle Time Shrinkage
Cooling of Slab: Figure 5.6 from Title: A Heat Transfer Textbook (4th Edition); © 2000-2015, John H. Lienhard IV and John H. Lienhard V. All Rights Reserved.
Specific Volume vs. Temperature and Pressure © John Hart. Image adapted from cnf-moldmaking.com, original image Copyright © 2011 CNF Molds & Plastic Co., Limited.
Image of Shrinkage © LATI S.p.A. 2008
8 Advanced
Image of Side-Action © Proto Labs 1999–2016
Image of Pipe Fitting © Westside Wholesale Inc. 2016. All Rights Reserved.
Photo of Side Action Tooling © 1999-2016 Alibaba.com. All Rights Reserved.