Injection Molding T. Gutowski. D. Roylance

Transcription

1 Injection Molding T. Gutowski D. Roylance

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 Ref Kalpakjian and Schmid McCrum, Buckley, Buckknall

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

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: half thickness t cool cm sec for polymers 2

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 Boothroyd/Busch

11 Heat transfer Note; Tool > polymer 1-dimensional heat conduction equation : q x q x + q x Fourier s law Boundary Conditions: t ( c T )xy q x x xy T k x q x c T t k 2 T x or T 2 t 2 T x 2 1st kind 2nd kind 3rd kind T ( x T k x T k x x') constant ( x ( x 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 / ; T ch = T i T W (initial temp. wall temp.) -L x T W +L T T T T Non-dimensionalize: O 2 i W W ; x 1; L F t L Dimensionless equation: 2 2 F O Initial condition 0 1 Boundary condition F O Separation of variables ; matching B.C.; matching I.C. (, F ) f ( F ) g( ) O O

13 Temperature in a slab Centerline, = 0.1, F o = t/l 2 = 1 See Heat Transfer Text By Lienhard on line Bi -1 =k/hl

14 Reynolds Number Reynolds Number: Re V L V 2 L 2 inertia viscous VL For typical injection molding 1g cm N m 4 V Part length Fill time s 2 ; L Z 10 3 m thickness 101 1s ; 103 N s m 2 Re 10 4 For Die casting Re * Source:

15 Viscous Shearing of Fluids h F F A v h v F/A 1 v/h Newtonian Viscosity v h Generalization: : shear rate ( ) Injection molding Typical shear rate for Polymer processes (sec) -1 Shear Thinning ~ 1 sec -1 for PE 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 : v h 2 Rate of Temperature rise c dt dt v h 2 or dt dt c v h 2 Rate of Conduction out dt dt k c d 2 T dx 2 ~ k c T h 2 Viscous heating Conduction 2 v kt 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 Péclet No. Flow rate Heat xfer rate ~ 2 V Lz 1 4 L 4 x VLz L L z x Small value => Short shot For injection molding Flow rate Heat xfer rate ~ cm / s0.1cm 0.1cm 2 10 cm / s 10cm For Die casting of aluminum Flow rate Heat xfer rate ~ cm / s0.1cm 0.1cm cm / s 10cm * Very small, therefore it requires thick runners 2

18 Non-Isothermal Flow v Flow rate: 1/t ~V/L x Heat transfer rate: 1/t ~a/(l z /2) 2 Péclet No. Flow rate Heat xfer rate ~ 2 V Lz 1 4 L 4 x VLz L L z x Small value => Short shot For injection molding Flow rate Heat xfer rate ~ cm / s0.1cm 0.1cm 2 10 cm / s 10cm For Die casting of aluminum Flow rate Heat xfer rate ~ cm / s0.1cm 0.1cm cm / s 10cm Very small value for aluminum requires thicker runners 2

19 Injection mold die cast mold

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

21 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

22 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

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

24 Where would you gate this part?

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

26 Injection Molding * * * Source:

27 Where is injection molding? Controlled by shrinkage and warping. Hence, polymer, fillers, mold geometry and processing conditions can all influence the final tolerance. Shrinkage is of order /1000 for unfilled and 1-10/1000 for filled across the thickness

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 Tooling Alternatives Kalpakjian & Schmid

33 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

34 Novel development- Gas assisted injection molding

35 Novel development ; injection molding with cores Injection Molded Housing Cores used in Injection Molding Cores and Part Molded in Clear Plastic

36 Micro injection molding

37 Micro embossing Replacing serial processes with parallel processes at small scales B. Kim UMass

38 Conformal Cooling Channels Tooling built using Additive Manufacturing Innomis.cz

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

40 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

41 Polymer Production Largest Player in the Injection Molding LCI What is a polymer: 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

42 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)

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

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

45 SEC (MJ/kg) For Hydraulics and Hybrids as throughput increases, SEC k Variable Pump Hydraulic Injection Molding Machines. HP 52 HP 05 HP 06 HP 57 HP 001 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!!!

46 Power Required (kw) All-electric vs. hybrid Plasticize Cool Clamp open-close Ton Buildup Inject high 60 ton 40 Inject low Time (seconds) MM 550 Hybrid NT 440 All-Electric Source: [Thiriez] The hydraulic plot would be even higher than the hybrid curve

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

48 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

49 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]

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51 Do Polymers get recycled? Ref Ashby 2009

52 The printer goes in the hopper

53 And comes out.

54 Readings Tadmore and Gogos Molding and Casting pp Boothroyd Dewhurst Design for Injection Molding pp Kalpakjian Ch 7 & 19 Thiriez et al, "An Environmental Analysis of Injection Molding "Injection Molding Case Study (Gas Assist)