Injection Molding Fall 2002 Professor Tim Gutowski

Transcription

1 Injection Molding Fall 2002 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 see Ch 18 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 3 2 α sec for polymers

9 Calculate clamp force, & shot size FP 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 y x x q y x T c t x p ) (ρ ) ( ) ' ( 3rd kind constant ) ' ( 2nd kind constant ') ( 1st kind T T h x x x T k x x x T k x x T Boundary Conditions: 1-dimensional heat conduction equation : The boundary condition of 1 st kind applies to injection molding since the tool is often maintained at a constant temperature x T k q x q x q x + q x or x T t T x T k t T c p ρ α Fourier s law

12 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 1 Boundary condition F O θ 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 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 * 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 T 2 h 2 Viscous heating µ v Conduction k T 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 ~ 1 10cm/ s 0.1cm 0.1cm cm / s 10cm For Die casting of aluminum Flow rate ~ Heat xfer rate cm/ s 0.1cm 0.1cm cm / s 10cm 2 * Very small, therefore it requires thick runners

18 Injection mold die cast mold

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

20 Shrinkage distributions sample Transverse direction V3.5cm/s V8 cm/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 L total L mold + L shrinkage Where is injection molding?

27 Effects of mold temperature and 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 shown in class Cores used in Injection Molding Cores and Part Molded in Clear Plastic

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

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

37 The estimated environmental performance of various mfg processes (not including auxiliary requirements) *Energy per wt. normalized by the melt energy ** total raw mat l normalized by the part wt.

38 The printer goes in the hopper

39 And comes out.

40 The problem with plastics is

41 Or remanufacture.

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