Injection Molding Fall 2008 Professor Tim Gutowski

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1 Injection Molding Fall 2008 Professor Tim Gutowski

2 Short history of plastics 1862 first synthetic plastic 1866 Celluloid 1891 Rayon 1907 Bakelite 1913 Cellophane 1926 PVC 1933 Polyethylene 1938 Teflon 1939 Nylon stockings 1957 velcro 1967 The Graduate

3 Outline Basic operation Cycle time and heat transfer Flow and solidification Part design Tooling New developments Environment

4 Readings Tadmore and Gogos Molding and Casting pp Boothroyd Dewhurst Design for Injection Molding pp Kalpakjian (5 th ed) see Ch 19 Injection molding case study;washing machine augers; see on web page

5 30 ton, 1.5 oz (45 cm3) Engel Injection Molding Machine for wheel fabrication

6 Process & machine schematics * * Schematic of thermoplastic Injection molding machine * Source:

7 Process Operation Temperature: barrel zones, tool, die zone Pressures: injection max, hold Times: injection, hold, tool opening Shot size: screw travel Processing window Temp. Thermal degradation Flash Shortshot Melt Pressure

8 Typical pressure/temperature cycle * * Time(sec) Time(sec) Cooling time generally dominates cycle time * Source: t cool = α = 10 ( half thickness) 3 cm 2 2 α sec for polymers

9 Calculate clamp force, & shot size F=P X A = 420 tons 3.8 lbs = 2245 cm 3 =75 oz Actual ; 2 cavity 800 ton

10 Clamp force and machine cost

11 Heat transfer Note; α Tool > α polymer 1-dimensional heat conduction equation : q x q x + Δq x Fourier s law Boundary Conditions: qx (ρ c p T ) ΔxΔy = t x T q x = k x 1st kind 2nd kind 3rd kind T ( x = T k x T k x ( x = ( x ΔxΔy 2 2 T T T T ρ c p = k or = α 2 2 t x t x x') = constant x') = constant = x') = h ( T T The boundary condition of 1 st kind applies to injection molding since the tool is often maintained at a constant temperature )

12 Heat transfer t T ii Let L ch = H/2 (half thickness) = L ; t ch = L 2 /α ; ΔT ch = T i T W (initial temp. wall temp.) -L x T W +L T θ = T T T Non-dimensionalize: O 2 i W W ; ξ = x + 1; L F α t = L Dimensionless equation: 2 θ θ = 2 ξ F O Initial condition = 0 θ = 1 Boundary condition F O ξ = 0 θ = 0 ξ = 2 θ = 0 Separation of variables ; matching B.C.; matching I.C. θ ( ξ, F ) = f ( F ) g( ξ ) O O

13 Temperature in a slab Centerline, θ = 0.1, F o = αt/l 2 = 1 Bi -1 =k/hl

14 Reynolds Number Reynolds Number: Re = V ρ L V μ 2 L 2 inertia viscous = ρvl μ For typical injection molding ρ = 1g cm Part length V Fill time 3 = 10 3 N 10 = 1s 1 m ; 4 s 2 ; L Z = 10 μ = m N s thickness m 2 Re = 10 4 For Die casting Re = 300 * Source:

15 Viscous Shearing of Fluids h F F A v h v F/A μ 1 v/h Newtonian Viscosity v τ = μ h Generalization: τ = μ & γ γ& : shear rate τ η(γ &) & γ = Injection molding Typical shear rate for Polymer processes (sec) -1 Shear Thinning ~ 1 sec -1 for PE γ& Extrusion 10 2 ~10 3 Calendering 10~10 2 Injection molding 10 3 ~10 4 Comp. Molding 1~10

16 Viscous Heating Rate of Heating = Rate of Viscous Work P Vol = F v Vol = F A v h = μ v h 2 Rate of Temperature rise 2 2 ρ c p dt dt = v μ h or dt dt = μ ρ c p v h Rate of Conduction out dt dt = k ρ c p 2 d T 2 dx ~ k ρ c p ΔT 2 h Viscous heating Conduction = 2 μv kδt Brinkman number For injection molding, order of magnitude ~ 0.1 to 10

17 Non-Isothermal Flow v Flow rate: 1/t ~V/L x Heat transfer rate: 1/t ~a/(l z /2) 2 Flow rate Heat xfer rate ~ 2 V Lz 1 = 4α L 4 x VLz α L L z x Small value => Short shot For injection molding Flow rate Heat xfer rate ~ cm / s 0.1cm 0.1cm 2 10 cm / s 10cm 3 = 2.5 For Die casting of aluminum Flow rate Heat xfer rate ~ cm / s 0.1cm 0.1cm cm / s 10cm * Very small, therefore it requires thick runners 2

18 Injection mold die cast mold

Fountain Flow * ** * Source: http://islnotes.cps.msu.

19 Fountain Flow * ** * Source: ; ** Z. Tadmore and C. Gogos, Principles of Polymer Processing

20 Shrinkage distributions sample Transverse direction V=3.5cm/s V=8cm/s * Source: G. Menges and W. Wubken, Influence of processing conditions on Molecular Orientation in Injection Molds

21 Gate Location and Warping Sprue Shrinkage Direction of flow in/in Perpendicular to flow Before shrinkage After shrinkage Air entrapment Gate Center gate: radial flow severe distortion Edge gate: warp free, air entrapment Diagonal gate: radial flow twisting End gates: linear flow minimum warping

22 Effects of mold temperature and pressure on shrinkage Shrinkage LDPE PP Acetal Nylon 6/6 Shrinkage LDPE PP with flow PP across flow Acetal Nylon 6/ PMMA PMMA Mold Temperature (F) Pressure on injection plunger (psi)

23 Where would you gate this part?

24 Weld line, Sink mark Gate Weld line Mold Filling Solidified part Sink mark * Source: Basic rules in designing ribs to minimize sink marks

25 Injection Molding * * * Source:

26 Where is injection molding?

27 Effects of mold pressure on shrinkage LDPE Acetal PP with flow Shrinkage PP across flow Nylon 6/ PMMA Pressure on injection plunger (psi)

28 Tooling Basics Sprue Nozzle Cavity Plate Core Plate Cavity Moulding Core Cavity Basic mould consisting of cavity and core plate Gate Runner Melt Delivery

29 Tooling for a plastic cup Nozzle Knob Runner Cavity Part Stripper plate Core

30 Tooling for a plastic cup Nozzle Nozzle Knob Runner Cavity Runner Part Cavity Cavity Part Part Stripper plate

31 Tooling * * * * * ** * * Source: ** (E-trainer by HZS Co.,Ltd.)

32 Part design rules Simple shapes to reduce tooling cost No undercuts, etc. Draft angle to remove part In some cases, small angles (1/4 ) will do Problem for gears Even wall thickness Minimum wall thickness ~ in Avoid sharp corners Hide weld lines Holes may be molded 2/3 of the way through the wall only, with final drilling to eliminate weld lines

33 New developments- Gas assisted injection molding

34 New developments ; injection molding with cores Injection Molded Housing Cores used in Injection Molding Cores and Part Molded in Clear Plastic

35 Environmental issues System boundaries Polymer production Compounding Machine types Out gassing & energy during processing

36 CRADLE Naphtha, Oil. Natural Gas Ancilliary Raw Materials Additives Compounder Thermoplastic Production (Boustead) Polymer Delivery Internal Transport Extrusion Drying Pelletizing Emissions to air, water & land Building (lights,heating, ect..) Emissions to air, water, & land Polymer Delivery Injection Molder Energy Production Industry Emissions to air, water, & land Note to Reader: = Focus of this Analysis = Also included in the Paper Anciliary Raw Materials Internal Transport Drying Injection Molding Scrap Building (lights,heating, ect..) Packaging 1 kg of Injection Molded Polymer Service Period Emissions to air, water & land FACTORY GATE Waste Management

37 What is a polymer: Polymer Production Largest Player in the Injection Molding LCI How much energy does it take to make 1 kg of polymer = a lot!!! Sources HDPE LLDPE LDPE PP PVC PS PC PET Boustead Ashby Patel Kindler/Nickles [Patel 1999] Worrell et al. [Patel 1999] E 3 Handbook [OIT 1997] Energieweb Values are in MJ per kg of polymer produced. Thiriez 06

38 Compounding - extrusion An extruder is used to mix additives with a polymer base, to bestow the polymer with the required characteristics. Similar to an injection molding machine, but without a mold and continuous production. Thus it has a similar energy consumption profile. Environmentally Unfriendly Additives: Fluorinated blowing agents (GHG s) Phalates (some toxic to human liver, kidney and testicles) Organotin stabilizers (toxic and damage marine wildlife)

Injection Molding Process Source: http://cache.husky.

39 Injection Molding Process Source: ochures/br-hylectric03a.pdf Machine types: Hydraulic, electric, hydro-electric

40 All-electrics have very low fixed energy costs (small idling power). SEC is constant as throughput increases. 9 8 SEC p v SEC (MJ/kg) All-Electric - 85 tons Hydraulic - 85 tons Material: PP Throughput (kg/hr) Source: [Thiriez]

41 For Hydraulics and Hybrids as throughput increases, SEC k. SEC (MJ/kg) Variable Pump Hydraulic Injection Molding Machines. HP 25 HP 50 HP 60 HP 75 HP 100 Low Enthalpy - Raise Resin to Inj. Temp - PVC High Enthalpy - Raise Resin to Inj. Temp - HDPE Throughput (kg/hr) Does not account for the electric grid. Source: [Thiriez] Enthalpy value to melt plastics is just 0.1 to 0.7 MJ/kg!!!

42 All-electric vs. hybrid Power Required (kw) Plasticize Cool Clamp open-close Ton Buildup t Inject high Inject low Time (seconds) MM 550 Hybrid NT 440 All-Electric Source: [Thiriez] The hydraulic plot would be even higher than the hybrid curve

43 Driers Used to dry internal moisture in hygroscopic polymers and external moisture in non-hygroscopic ones. It is done before extruding and injection molding. Specific Power Consumption (MJ/kg) W150 W600 W300 W400 W200 W800 W1000 W1600 W2400 W3200 R 2 = W Same as P m& = E m Throughput (kg/hr) = SEC = P 0 m& + k Power Trendline Source: [Thiriez]

44 LCI Summarized Results ENERGY CONSUMPTION BY STAGE in MJ/kg of shot Thermoplastic Production Generic by Amount Extras HDPE LLDPE LDPE PP PVC PS Consumed Inj. Molded PC PET avg low high Polymer Delivery avg low high avg low high Internal Transport Compounder Building (lights, Drying Extrusion Pelletizing heating, ect..) Subtotal avg low high Polymer Delivery avg low high

45 Injection Molder avg low high Internal Transport Drying Injection Molding (look below) Scrap (Granulating) Building (lights, heating, ect..) avg low high Injection Molding - Choose One Hydraulic Hybrid All-Electric Subtotal avg low high TOTAL w/ Generic Inj. Molded Polymer avg low high Hydraulic Hybrid All-Electric TOTAL w/o Polymer Prod avg low high Notes Drying - the values presented assume no knowledge of the materials' hygroscopia. In order words, they are averages between hygroscopic and non-hygroscopic values. For hygroscopic materials such as PC and PET additional drying energy is needed (0.65 MJ/kg in the case of PC and 0.52 MJ/kg in the case of PET) Pelletizing - in the case of pelletizing an extra 0.3 MJ/kg is needed for PP Granulating - a scarp rate of 10 % is assumed Source: [Thiriez]

46 Energy Production Industry United States Electricity Composition by Source Hydro Nuclear Other Coal Oil Gas Waste/ Renewable 7.1% 19.6% 0.0% 50.7% 3.1% 16.7% 2.2% The Grid is about 30% efficient For every MJ of electricity we also get: g of CO g of SO g of NO x 6.24 g of CH mg of Hg

47 Scale HDPE, LDPE, LLDPE, PP, PS, PVC Compounder and Injection Molder 6 Main Thermoplastics All Plastics U.S. GJ/year 9.34E E+08 Global GJ/year 4.01E E+08 The Injection Molding Industry in the U.S. consumes 6.19 x 10 7 GJ of electricity (or 2.06 x 10 8 GJ in total energy). This is larger than the entire electric production of some small countries. In such a scale imagine what a 0.1 % energy savings mean!!!

48 The printer goes in the hopper

49 And comes out.

50 Readings 1. Z. Tadmore et al., "Molding and Casting" p G. Boothroyd et al., "Design for Injection Molding p S. Shingo, "Single Minute Exchange of Dies 4. Thiriez et al, "An Environmental Analysis of Injection Molding 5. "Injection Molding Case Study 6. Kalpakjian Chapter 19 (Chapter 18)

Injection Molding T. Gutowski. D. Roylance 1

Injection Molding T. Gutowski. D. Roylance 1 Injection Molding 2.810 T. Gutowski D. Roylance 1 V-6 air intake manifold Water control valve Brass Vs injection mold www.mnrubber.com 2 Short history of plastics 1866 Celluloid 1891 Rayon 1907 Bakelite