Injection Molding T. Gutowski. D. Roylance 1

Transcription

1 Injection Molding T. Gutowski D. Roylance 1

2 V-6 air intake manifold Water control valve Brass Vs injection mold 2

3 Short history of plastics 1866 Celluloid 1891 Rayon 1907 Bakelite 1913 Cellophane 1926 PVC 1933 Polyethylene 1938 Teflon 1939 Nylon stockings 1957 velcro 1967 The Graduate 1970 Earth Day recycling 3

4 Ref Kalpakjian and Schmid McCrum, Buckley, Bucknall 4

5 Outline Basic operation Cycle time and heat transfer Flow and solidification Part design Tooling New developments Environment 5

6 30 ton, 1.5 fl oz (45 cm3) Engel Injection Molding Machine for wheel fabrication 6

7 Process & machine schematics * * Schematic of thermoplastic Injection molding machine * Source: 7

8 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

9 Typical pressure/temperature cycle * * Time(sec) Time(sec) Cooling time generally dominates cycle time * Source: 9

10 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

11 Clamp force and machine cost Boothroyd/Busch 11

12 Heat transfer Note; a Tool > a polymer 1-dimensional heat conduction equation : q x q x + Dq x Fourier s law Boundary Conditions: The boundary condition of 1 st kind applies to injection molding since the tool is often maintained at a constant temperature 12

13 Heat transfer t T ii T W Let L ch = H/2 (half thickness) = L ; t ch = L 2 /a ; DT ch = T i T W (initial temp. wall temp.) -L x +L Non-dimensionalize: Dimensionless equation: Initial condition Boundary condition Separation of variables ; matching B.C.; matching I.C. 13

14 Temperature in a slab Centerline, q = 0.1, F o = at/l 2 = 1 See Heat Transfer Text By Lienhard on line Bi -1 =k/hl 14

15 Reynolds Number Reynolds Number: For typical injection molding For Die casting * Source: 15

16 Viscous Shearing of Fluids h F v F/A µ 1 v/h Newtonian Viscosity Generalization: Injection molding Typical shear rate for Polymer processes (sec) -1 Extrusion 10 2 ~10 3 Calendering 10~10 2 Shear Thinning Injection molding 10 3 ~10 4 Comp. Molding 1~10 ~ 1 sec -1 for PE 16

17 Viscous Heating Rate of Heating = Rate of Viscous Work Rate of Temperature rise Rate of Conduction out Brinkman number For injection molding, order of magnitude ~ 0.1 to 10 17

18 Non-Isothermal Flow v Flow rate: 1/t ~V/L x Heat transfer rate: 1/t ~a/(l z /2) 2 Péclet No. For injection molding Small value => Short shot For Die casting of aluminum * Very small, therefore it requires thick runners 18

19 Non-Isothermal Flow v Flow rate: 1/t ~V/L x Heat transfer rate: 1/t ~a/(l z /2) 2 Péclet No. For injection molding Small value => Short shot For Die casting of aluminum Very small value for aluminum requires thicker runners 19

20 Injection mold die cast mold 20

21 Fountain Flow * ** * Source: ; ** Z. Tadmore and C. Gogos, Principles of Polymer Processing 21

22 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

23 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

24 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) 24

25 Where would you gate this part? 25

26 Design Features * * * Source: 26

27 Weld line, Sink mark Gate Weld line Mold Filling Solidified part Sink mark * Source: Basic rules in designing ribs to minimize sink marks 27

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29 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 29

30 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 From Boothroyd et al 30

31 Tooling Basics Sprue Nozzle Cavity Plate Core Plate Cavity Moulding Core Cavity Basic mould consisting of cavity and core plate Gate Runner Melt Delivery 31

32 Tooling for a plastic cup Nozzle Knob Runner Cavity Part Stripper plate Core 32

33 Tooling for a plastic cup Nozzle Nozzle Knob Runner Cavity Runner Part Cavity Cavity Part Part Stripper plate 33

34 part Side pull Side pull in position Toy tooling example from Protomold 34

35 Tooling * * * * * ** * * Source: ** (E-trainer by HZS Co.,Ltd.) 35

36 Tooling Alternatives Kalpakjian & Schmid 36

37 Injection Molding Homework Undercut features to hold tape on Evidence of tooling feature 37

38 Part design rules Simple shapes to reduce tooling cost n No undercuts, etc. Draft angle to remove part n n 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 n Holes may be molded 2/3 of the way through the wall only, with final drilling to eliminate weld lines 38

39 Novel development- Gas assisted injection molding 39

40 Novel development ; injection molding with cores Injection Molded Housing Cores used in Injection Molding Cores and Part Molded in Clear Plastic 40

41 Micro injection molding 41

42 Conformal Cooling Channels Tooling built using Additive Manufacturing Innomis.cz 42

43 Environmental issues Energy n n n Polymer production Compounding Machine types Recycling 43

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45 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!!! Values are in MJ per kg of polymer produced. Thiriez 06 45

46 Gold ~ 250 GJ/kg Ashby

47 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) 47

48 Injection Molding Process Source: ochures/br-hylectric03a.pdf Machine types: Hydraulic, electric, hydro-electric 48

49 All-electric vs. hybrid Source: [Thiriez] The hydraulic plot would be even higher than the hybrid curve 49

50 For Hydraulics and Hybrids as throughput increases, SEC à k. Does not account for the electric grid. Source: [Thiriez] Enthalpy value to melt plastics is just 0.1 to 0.7 MJ/kg!!! 50

51 All-electrics have very low fixed energy costs (small idling power). SEC is constant as throughput increases. Source: [Thiriez] 51

52 Driers Used to dry internal moisture in hygroscopic polymers and external moisture in non-hygroscopic ones. It is done before extruding and injection molding. Same as Source: [Thiriez] 52

53 LCI Summarized Results 53

54 Source: [Thiriez] 54

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56 Do Polymers get recycled? Ref Ashby

57 The printer goes in the hopper 57

58 And comes out. 58

59 Readings (first 3) & Refs Tadmore and Gogos n Molding and Casting pp Boothroyd Dewhurst n Design for Injection Molding pp Kalpakjian Ch 7 & 19 Thiriez et al, "An Environmental Analysis of Injection Molding "Injection Molding Case Study (Gas Assist) 59