injection molding (mit 2.008x lecture slides)
TRANSCRIPT
2.008x
Injection MoldingMIT 2.008x
Prof. John Hart
2.008x
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
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
2.008x
Routsis Associates: https://youtu.be/cANvFsvY0Aw
2.008xAn IM machine in the MIT manufacturing shop
2.008xAgenda:
Injection Molding§ Fundamentals of polymers§ Mold tooling§ Process parameters and
equipment§ Cycle time, cooling, and
shrinkage§ Defects and design guidelines§ Advanced topics
2.008x
Injection Molding:
2. Fundamentals of polymers
2.008xIM feedstock: polymer pellets
2.008x
Alibaba, February 2016http://www.alibaba.com/product-detail/Extruded-PP-granules-Polypropylene-PP-
Crush_60255030501.html?spm=a2700.7724857.35.1.1Fnwad
2.008x
Kalpakjian and Schmid, Manufacturing Engineering and TechnologyGroover, Fundamentals of Modern Manufacturing
Poly (many) + mer (structural unit)-[C2H4]n- = poly[ethylene]
2.008xThe ‘families’ of materials: modulus vs. density
Ashby, Materials Selection in Mechanical Design.
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
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
2.008x
What does the polymer ‘feel’ during injection molding?
à heat and pressure
2.008x
What does the polymer ‘feel’ during injection molding?
à heat and pressure
2.008x
J.L. Throne, Technology of Thermoforming
2.008x
J.L. Throne, Technology of Thermoforming
2.008x
Kalpakjian and Schmid, Manufacturing Engineering and Technology
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
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
2.008x
Injection Molding:
3. Mold tooling and configurations
2.008xInjection molding of LEGO bricks
Excerpt from: https://www.youtube.com/watch?v=y1Zhpdx-XtA
2.008xThe injection molding machine
Groover, Fundamentals of Modern Manufacturing
2.008xKey features of mold tooling
Protomold ‘demo mold’
2.008xVideo: MIT 2.008 injection molds and machine
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)?
2.008x
2.008x
Gate
Parting?
Parting
Draft
2.008xMulti-part / multi-cavity molds
Kalpakjian and Schmid, Manufacturing Engineering and Technology
2.008x
Kalpakjian and Schmid, Manufacturing Engineering and Technology
Three-plate mold
Two-plate mold
2.008x
Kalpakjian and Schmid, Manufacturing Engineering and Technology
Hot runner mold (three plates)
2.008xLego bricks: three-plate mold
http://www.cnet.com/pictures/how-lego-makes-its-bricks-photos/
2.008x
https://www.youtube.com/watch?v=JSkz5eBJrCI
2.008x
Injection Molding:
4. Injection process parameters
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).
2.008xSpecs of the IM machine at MIT
2.008xSpecs of the IM machine at MIT
2.008x
à Let’s relate the machine specifications to a basic model of the mold filling process
2.008x
2.008x
L
hh/2
h/2
2.008x
L
hh/2
h/2
dPdx
= µd 2Udy2
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
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
2.008xSimulation: injection flow
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
2.008xSimulation: injection pressure
2.008x
Injection Molding:
5. Cycle time, cooling, and shrinkage
2.008xThe injection molding
cycle
2.008xTemperature vs time
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
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)
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
"
2.008xHow do process parameters vary with
part size? ΔP = 12µ
τ fill
Lh"
#$
%
&'2
Fclamp ∝µτ fill
wL3
h2"
#$
%
&'
tcool ∝h2
4α
2.008x
Pack
Clo
se
Fill
Eje
ct
Gat
e fre
ezes
Time
Pres
sure
[MPa
]5
10
15Cool
Cycle time
Pressure vs time
2.008x
2.008xResidual stress in LEGO® block
(polarized imaging) 2.008x
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!
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
)
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.
2.008x
Injection Molding:
6. Defects and the ‘process window’
2.008xThe injection molding
‘process window’
2.008xShort shot
2.008xFlash
2.008xBurning (thermal degradation)
2.008x
2.008xProtomold ‘design cube’
§ What is the molding orientation?
§ Where are the ejector pins?§ What defects do you notice?§ …
2.008x
Ribs
Undercuts
‘Living’ hinges
Straight-pull transverse hole
Side-pull transverse hole
Surface finishes
Thick and cored out sections
2.008xSimulation: filling
2.008xSimulation: cooling time
309 s
1.4 s
63 s
124 s
186 s
248 s
2.008xSimulation: shrinkage
0.45 mm
0.01 mm
0.10 mm
2.008xSimulation: warp
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
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
2.008xSurface finishes
PM-F0 PM-F1
SP-B1 SP-A2
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)
2.008x
Injection Molding:
7. Advanced topics
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
2.008x
2.008xInsert molding
Plasticmolded item
Metal terminal
2.008xOvermolding (two plastics)
How?§ Insert brush§ Mold rigid base (white)§ Mold elastomer (black) at
lower temperature
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)
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)
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
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).
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.
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.