Injection Molding 2.810 Fall 2002 Professor Tim Gutowski
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
Outline Basic operation Cycle time and heat transfer Flow and solidification Part design Tooling New developments Environment
Readings Tadmore and Gogos Molding and Casting pp584-610 Boothroyd Dewhurst Design for Injection Molding pp 319-359 Kalpakjian see Ch 18 Injection molding case study;washing machine augers; see on web page
30 ton, 1.5 oz (45 cm3) Engel Injection Molding Machine for wheel fabrication
Process & machine schematics * * Schematic of thermoplastic Injection molding machine * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
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
Typical pressure/temperature cycle * * Time(sec) Time(sec) Cooling time generally dominates cycle time * Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html t cool α 10 ( half thickness) 3 cm 3 2 α sec for polymers
Calculate clamp force, & shot size FP X A 420 tons 3.8 lbs 2245 cm 3 75 oz Actual ; 2 cavity 800 ton
Clamp force and machine cost
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 2 2 2 2 or x T t T x T k t T c p ρ α Fourier s law
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
Temperature in a slab Centerline, θ 0.1, F o αt/l 2 1 Bi -1 k/hl
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 10 3 3 m N s thickness m 2 Re 10 4 For Die casting 3 3 10 10 Re 3 10 1 10 3 300 * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
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
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
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 2 4 10 cm / s 10cm 3 2.5 For Die casting of aluminum Flow rate ~ Heat xfer rate 1 4 10cm/ s 0.1cm 0.1cm 10 2 0.3cm / s 10cm 2 * Very small, therefore it requires thick runners
Injection mold die cast mold
Fountain Flow * ** * Source: http://islnotes.cps.msu.edu/trp/inj/flw_froz.html ; ** Z. Tadmore and C. Gogos, Principles of Polymer Processing
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
Gate Location and Warping Sprue 2.0 60 1.96 60.32 Shrinkage Direction of flow 0.020 in/in Perpendicular to flow 0.012 2.0 1.976 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
Effects of mold temperature and pressure on shrinkage Shrinkage 0.030 0.025 0.020 0.015 0.010 LDPE PP Acetal Nylon 6/6 Shrinkage 0.030 0.025 0.020 0.015 0.010 LDPE PP with flow PP across flow Acetal Nylon 6/6 0.005 PMMA 0.005 PMMA 0.000 100 120 140 160 180 200 220 240 Mold Temperature (F) 0.000 6000 10000 14000 18000 8000 12000 16000 Pressure on injection plunger (psi)
Where would you gate this part?
Weld line, Sink mark Gate Weld line Mold Filling Solidified part Sink mark * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htm Basic rules in designing ribs to minimize sink marks
Injection Molding * * * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_2.htm
L total L mold + L shrinkage Where is injection molding?
Effects of mold temperature and pressure on shrinkage 0.030 LDPE Acetal 0.025 PP with flow 0.020 Shrinkage 0.015 0.010 PP across flow Nylon 6/6 0.005 PMMA 0.000 6000 8000 10000 12000 14000 16000 18000 Pressure on injection plunger (psi)
Tooling Basics Sprue Nozzle Cavity Plate Core Plate Cavity Moulding Core Cavity Basic mould consisting of cavity and core plate Gate Runner Melt Delivery
Tooling for a plastic cup Nozzle Knob Runner Cavity Part Stripper plate Core
Tooling for a plastic cup Nozzle Nozzle Knob Runner Cavity Runner Part Cavity Cavity Part Part Stripper plate
Tooling * * * * * ** * * Source: http://www.idsa-mp.org/proc/plastic/injection/; ** http://www.hzs.co.jp/english/products/e_trainer/mold/basic/basic.htm (E-trainer by HZS Co.,Ltd.)
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 ~ 0.025 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
New developments- Gas assisted injection molding
New developments ; injection molding with cores Injection Molded Housing shown in class Cores used in Injection Molding Cores and Part Molded in Clear Plastic
Environmental issues Petroleum and refining Primary processing Out gassing & energy during processing End of life
Environmental loads by manufacturing sector Carbon Dioxide and Toxic Materials per Value of Shipments 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 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
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.
The printer goes in the hopper
And comes out.
The problem with plastics is
Or remanufacture.
Summary Basic operation Cycle time and heat transfer Flow and solidification Part design Tooling New developments Environment