injection molding (mit 2.008x lecture slides)

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

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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

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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.