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Injection Molding (MIT 2.008x Lecture Slides)

Slides accompanying 2.008x* video module on Injection Molding, Prof. John Hart, MIT, 2016.

*Fundamentals of Manufacturing Processes on edX: https://www.edx.org/course/fundamentals-manufacturing-processes-mitx-2-008x

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Injection Molding (MIT 2.008x Lecture Slides)

  1. 1. 2.008x Injection Molding MIT 2.008x Prof. John Hart
  2. 2. 2.008x
  3. 3. 2.008x Plastics: 1950-2014 Data from Statista http://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 Volume[millionmetrictons] Year
  4. 4. 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
  5. 5. 2.008x Routsis Associates: https://youtu.be/cANvFsvY0Aw
  6. 6. 2.008x An IM machine in the MIT manufacturing shop
  7. 7. 2.008x Agenda: Injection Molding § Fundamentals of polymers § Mold tooling § Process parameters and equipment § Cycle time, cooling, and shrinkage § Defects and design guidelines § Advanced topics
  8. 8. 2.008x Injection Molding: 2. Fundamentals of polymers
  9. 9. 2.008x IM feedstock: polymer pellets
  10. 10. 2.008x Alibaba, February 2016 http://www.alibaba.com/product-detail/Extruded-PP-granules-Polypropylene-PP- Crush_60255030501.html?spm=a2700.7724857.35.1.1Fnwad
  11. 11. 2.008x Kalpakjian and Schmid, Manufacturing Engineering and Technology Groover, Fundamentals of Modern Manufacturing Poly (many) + mer (structural unit) -[C2H4]n- = poly[ethylene]
  12. 12. 2.008x The ‘families’ of materials: modulus vs. density Ashby, Materials Selection in Mechanical Design.
  13. 13. 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
  14. 14. 2.008x Polymer network architectures ThermosetThermoplastic (semi-crystalline) (amorphous, linear) (amorphous, crosslinked) à In all cases, the polymer chain length, interactions, bonding influence the part mechanics
  15. 15. 2.008x What does the polymer ‘feel’ during injection molding? à heat and pressure
  16. 16. 2.008x What does the polymer ‘feel’ during injection molding? à heat and pressure
  17. 17. 2.008x J.L. Throne, Technology of Thermoforming
  18. 18. 2.008x J.L. Throne, Technology of Thermoforming
  19. 19. 2.008x Kalpakjian and Schmid, Manufacturing Engineering and Technology
  20. 20. 2.008x Viscosity: 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 y U ¶ ¶ = µt Ux(H) = Ux Ux(y) Ux(0) = 0 h Ux x y
  21. 21. 2.008x Viscosity 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
  22. 22. 2.008x Injection Molding: 3. Mold tooling and configurations
  23. 23. 2.008x Injection molding of LEGO bricks Excerpt from: https://www.youtube.com/watch?v=y1Zhpdx-XtA
  24. 24. 2.008x The injection molding machine Groover, Fundamentals of Modern Manufacturing
  25. 25. 2.008x Key features of mold tooling Protomold ‘demo mold’
  26. 26. 2.008x Video: MIT 2.008 injection molds and machine
  27. 27. 2.008x An 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)?
  28. 28. 2.008x
  29. 29. 2.008x Gate Parting? Parting Draft
  30. 30. 2.008x Multi-part / multi-cavity molds Kalpakjian and Schmid, Manufacturing Engineering and Technology
  31. 31. 2.008x Kalpakjian and Schmid, Manufacturing Engineering and Technology Three-plate mold Two-plate mold
  32. 32. 2.008x Kalpakjian and Schmid, Manufacturing Engineering and Technology Hot runner mold (three plates)
  33. 33. 2.008x Lego bricks: three-plate mold http://www.cnet.com/pictures/how-lego-makes-its-bricks-photos/
  34. 34. 2.008x https://www.youtube.com/watch?v=JSkz5eBJrCI
  35. 35. 2.008x Injection Molding: 4. Injection process parameters
  36. 36. 2.008x How 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).
  37. 37. 2.008x Specs of the IM machine at MIT
  38. 38. 2.008x Specs of the IM machine at MIT
  39. 39. 2.008x à Let’s relate the machine specifications to a basic model of the mold filling process
  40. 40. 2.008x
  41. 41. 2.008x L h h/2 h/2
  42. 42. 2.008x L h h/2 h/2 dP dx = µ d2 U dy2
  43. 43. 2.008x Simple scaling of injection parameters for a 2D rectangular channel ÷ ÷ ø ö ç ç è æ µ ÷ ø ö ç è æ =D =D 2 3 2 3 12 12 h wL F h L P wh QL P fill clamp fill t µ t µ µ
  44. 44. 2.008x Simulating 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
  45. 45. 2.008x Simulation: injection flow
  46. 46. 2.008x Viscosity 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
  47. 47. 2.008x Simulation: injection pressure
  48. 48. 2.008x Injection Molding: 5. Cycle time, cooling, and shrinkage
  49. 49. 2.008x The injection molding cycle
  50. 50. 2.008x Temperature vs time
  51. 51. 2.008x How do we model cooling of the part? 2 2 2 2 y T y T c k t T p ¶ ¶ = ¶ ¶ = ¶ ¶ a r Mold Mold Part x y L h h/2 h/2
  52. 52. 2.008x Exact solution for a plate Tm = melt temperature Tw = wall temperature Te = ejection temperature Drawing from Leinhard, A Heat Transfer Textbook Also see BASF ‘estimating cooling time’ http://www2.basf.us/PLASTICSWEB/displayanyfile?id=0901a5e1801499d3 a4 2 h tcool = MoldMold Part a = thermal diffusivity = k / rcp ~0.1 mm2/s for thermoplastics ÷ ÷ ø ö ç ç è æ - - = we wm cool TT TTh t pap 4 ln2 2 à We define the cooling time as 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)
  53. 53. 2.008x Cooling time scaling for plate geometry Tm = 200 ºC = 473 K Tw = 77 ºC = 350 K tcool = h2 4α tcool = h2 π2α ln 4 π Tm − Tw Te − Tw
  54. 54. 2.008x How do process parameters vary with part size? ΔP = 12µ τ fill L h " # $ % & ' 2 Fclamp ∝ µ τ fill wL3 h2 " # $ % & ' tcool ∝ h2 4α
  55. 55. 2.008x Pack Close Fill Eject Gatefreezes Time Pressure[MPa] 5 10 15 Cool Cycle time Pressure vs time
  56. 56. 2.008x
  57. 57. 2.008x Residual stress in LEGO® block (polarized imaging) 2.008x
  58. 58. 2.008x Polymers 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!
  59. 59. 2.008x http://www.lati.com/pdf/technical_data/dimensional-molding-shrinkages.pdf Practically, how much shrinkage? Longitudinal shrinkage (parallel to flow) Lateralshrinkage(perpendiculartoflow)
  60. 60. 2.008x In 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.
  61. 61. 2.008x Injection Molding: 6. Defects and the ‘process window’
  62. 62. 2.008x The injection molding ‘process window’
  63. 63. 2.008x Short shot
  64. 64. 2.008x Flash
  65. 65. 2.008x Burning (thermal degradation)
  66. 66. 2.008x
  67. 67. 2.008x Protomold ‘design cube’ § What is the molding orientation? § Where are the ejector pins? § What defects do you notice? § …
  68. 68. 2.008x Ribs Undercuts ‘Living’ hinges Straight-pull transverse hole Side-pull transverse hole Surface finishes Thick and cored out sections
  69. 69. 2.008x Simulation: filling
  70. 70. 2.008x Simulation: cooling time 309 s 1.4 s 63 s 124 s 186 s 248 s
  71. 71. 2.008x Simulation: shrinkage 0.45 mm 0.01 mm 0.10 mm
  72. 72. 2.008x Simulation: warp
  73. 73. 2.008x Corners, 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
  74. 74. 2.008x Draft 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
  75. 75. 2.008x Surface finishes PM-F0 PM-F1 SP-B1 SP-A2
  76. 76. 2.008x The 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)
  77. 77. 2.008x Injection Molding: 7. Advanced topics
  78. 78. 2.008x Molding with ‘side action’ Animation from protomold Elbow fitting: http://www.plastic-injectionmoulds.com/sale-1133447-household-plastic-injection- molded-parts-pvc-pb-pp-for-water-tank-fitting.html
  79. 79. 2.008x
  80. 80. 2.008x Insert molding Plastic molded item Metal terminal
  81. 81. 2.008x Overmolding (two plastics) How? § Insert brush § Mold rigid base (white) § Mold elastomer (black) at lower temperature
  82. 82. 2.008x Metal 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)
  83. 83. 2.008x 8. 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)
  84. 84. 2.008x References 1 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
  85. 85. 2.008x References Modulus 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 Osswald et al. © Hanser Publishers (2006).
  86. 86. 2.008x References Glass 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.
  87. 87. 2.008x References 6 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.

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