Kursus i Produktions- og materialeteknologi

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Maryann Richards
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1 Kursus i Produktions- og materialeteknologi Plastsprøjtestøbning / Injection Molding Basics

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 Basics 2

Cellophane 1926 PVC 1933 Polyethylene 1938 Teflon

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

4 Commonly used Polymers in Injection Molding Basics 4

5 Injection Mold Tool Basics 5

6 Mold Construction Consists of two parts a stationary half (cavity plate) and a moving half (core plate) Needs to have a central feed channel (sprue) Runners connect the sprue to the part Ejector pins in the cavity plate Fine vents to allow the air to escape during filling Basics 6

channel (sprue) Runners connect the sprue to the part Ejector pins in

7 Mold Types Two-plate molds Three-plate molds Side-action molds Unscrewing molds Basics 7

8 Two Two-plate mold Two active plates Part and runner system ejected as a single item Basics 8

9 Three Three-plate mold Three plates: runner, cavity, and core plate This design facilitates separation of the runner system and the part when the mold opens Basics 9

10 Side-action molds Used in molding components with external depressions or holes parallel to the parting plane. Referred to as undercuts Basics 10

11 Unscrewing Mold Basics 11

12 Basics 12

13 Basics 13

14 30 ton, 1.5 oz (45 cm3) Engel Injection Molding Machine for wheel fabrication Basics 14

15 Process & machine schematics * * Schematic of thermoplastic Injection molding machine Basics 15

16 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 Basics 16

hold, tool opening Shot size: screw travel Processing

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

Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.

18 Calculate clamp force, & shot size F=P X A = 420 tons 3.8 lbs = 2245 cm 3 =75 oz Actual ; 2 cavity 800 ton Basics 18

19 Clamp force and machine cost Basics 19

20 Heat transfer Note; α Tool > α polymer 1-dimensional heat conduction equation : q x q x + q x Fourier s law Boundary Conditions: (ρ c p T ) x y = t T q x = k x 1st kind 2nd kind 3rd kind T ( x T k x T k x q x ( x ( x x x y 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 Basics 20 )

y 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

21 Heat transfer t T ii Let L ch = H/2 (half thickness) = L ; t ch = L 2 /α ; T 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 Basics 21

22 Temperature in a slab Centerline, θ = 0.1, F o = αt/l 2 = 1 Bi -1 =k/hl Basics 22

23 Reynolds Number: Reynolds Number Re = V ρ L V µ 2 L 2 inertia ρvl = viscous 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: Basics 23

24 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& Injection molding 10 3 ~10 4 Extrusion 10 2 ~10 3 Calendering 10~10 2 Comp. Molding 1~10 Basics 24

25 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 T 2 h Viscous 2 heating v = µ Conduction k T Brinkman number For injection molding, order of magnitude ~ 0.1 to 10 Basics 25

26 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 2 10 cm / s 0.1cm 10cm = 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 runne 2 Basics 26

27 Injection mold die cast mold Basics 27

28 Fountain Flow * ** * Source: ; ** Z. Tadmore and C. Gogos, Principles of Polymer Processing Basics 28

29 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 Basics 29

30 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 Basics 30

31 Effects of mold temperature and pressure on shrinkage LDPE PP Acetal Nylon 6/6 Shrinkage LDPE PP with flow PP across flow Acetal Nylon 6/ Mold Temperature (F) PMMA PMMA Pressure on injection plunger (psi) Basics 31

32 Weld line, Sink mark Gate Weld line Mold Filling Solidified part Sink mark Source: Basic rules in designing ribs to minimize sink marks Basics 32

33 Injection Molding * * Source: Basics 33

34 Where is injection molding? L total = L mold + L shrinkage Basics 34

35 Effects of mold temperature and pressure on shrinkage LDPE PP with flow Acetal Shrinkage PP across flow Nylon 6/ PMMA Pressure on injection plunger (psi) Basics 35

36 Tooling Basics Sprue Nozzle Cavity Plate Core Plate Cavity Moulding Core Cavity Basic mould consisting of cavity and core plat Gate Runner Melt Delivery Basics 36

37 Tooling for a plastic cup Nozzle Knob Runner Cavity Part Stripper plate Core Basics 37

38 Stripper plate Tooling for a plastic cup Nozzle Nozzle Knob Runner Cavity Runner Part Cavity Cavity Part Part Basics 38

39 Tooling * * * * * ** * Source: ** (E-trainer by HZS Co.,Ltd.) Basics 39

40 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 Basics 40

41 New developments - Gas assisted injection molding Basics 41

42 New developments; injection molding with cores Injection Molded Housing shown in class Cores used in Injection Molding Cores and Part Molded in Clear Plastic Basics 42

43 Environmental issues Petroleum and refining Primary processing Out gassing & energy during processing End of life Basics 43

44 Environmental loads by manufacturing sector Carbon Dioxide and Toxic Materials per Value of Shipments Chemicals Petroleum and Coal Plastics and Rubber Primary Metal Fabricated Metal Machinery Electronic Transportation Manufacturing industries CO2 (metric ton/$10,000) Toxic Mat'ls (lb/$1000) Weight/Dollars EPA 2001, DOE 2001 Basics 44

45 Environmental loads The estimated environmental performance of various mfg processes (not including auxiliary requirements) nergy per wt. normalized the melt energy ** total raw material normalized by the part wt. Basics 45

46 The printer goes in the hopper Basics 46

47 And comes out. Basics 47

48 The problem with plastics is Basics 48

49 Or remanufacture. Basics 49

50 Summary Basic operation Cycle time and heat transfer Flow and solidification Part design Tooling New developments Environment Basics 50

Injection Molding. 2.810 T. Gutowski. D. Roylance

Injection Molding. 2.810 T. Gutowski. D. Roylance Injection Molding 2.810 T. Gutowski D. Roylance Short history of plastics 1862 first synthetic plastic 1866 Celluloid 1891 Rayon 1907 Bakelite 1913 Cellophane 1926 PVC 1933 Polyethylene 1938 Teflon 1939