Injection molding professor tim gutowski fall 2008

Injection Molding 2.810 Fall 2008 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 •BoothroydDewhurst – Design for Injection Molding pp 319 - 359 •Kalpakjian(5 th ed) see Ch 19 • 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 * * Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm * Schematic of thermoplastic Injection molding machine Process Operation • Temperature: barrel zones, tool, die zone • Pressures: injection max, hold • Times: injection, hold, tool opening • Shot size: screw travel Flash Melt Thermal degradation Short- shot Temp. Pressure Processing window Typical pressure/temperature cycle ( ) polymersfor sec10 thicknesshalf 23 2 cm t cool − = = α α Time(sec) Cooling time generally dominates cycle time Time(sec) * Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html * * Calculate clamp force, & shot size F=P X A = 420 tons 3.8 lbs = 2245 cm 3 =75 oz Actual ; 2 cavity 800 ton Clamp force and machine cost [...]... 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 Where is injection molding? ... 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 Cores used in Injection Molding Cores and Part Molded in Clear Plastic Environmental issues • System boundaries • Polymer production • Compounding • Machine types • Out... Reynolds Number Reynolds Number: V2 inertia ρ ρVL L = Re = V μ μ 2 viscous L For typical injection molding ρ = 1 g cm3 = 103 N m 4 s 2 ; LZ = 10 −3 m thickness V≈ −1 Part length 10 = ; Fill time 1s For Die casting μ = 103 N ⋅ s m 2 3 ⋅103 × 10 −1 × 10 −3 Re ≈ = 300 −3 10 * Source: http://www.idsa-mp.org/proc/plastic /injection/ injection_process.htm Re = 10 −4 Viscous Shearing of Fluids F v F/A μ h v τ =μ h... Shearing of Fluids F v F/A μ h v τ =μ h 1 F v ∝ A h v/h Newtonian Viscosity Generalization: τ = μγ& τ = η (γ& ) γ& γ& : shear rate Injection molding “Shear Thinning” ~ 1 sec-1 for PE γ& Typical shear rate for Polymer processes (sec)-1 Extrusion Calendering Injection molding Comp Molding 102~103 10~102 103~104 1~10 Viscous Heating P F ⋅v F v ⎛v⎞ = = ⋅ = μ⎜ ⎟ Vol Vol A h ⎝h⎠ Rate of Heating = Rate of Viscous... dt ρ ⋅ c p dx 2 ρ ⋅ c p h 2 Viscous heating μv 2 = Conduction kΔT Brinkman number For injection molding, order of magnitude ~ 0.1 to 10 Non-Isothermal Flow Flow rate: 1/t ~V/Lx v Heat transfer rate: 1/t ~a/(Lz/2)2 Flow rate V ⋅ L2 1 VLz Lz z ~ = ⋅ Heat xfer rate 4α ⋅ Lx 4 α Lx Small value => Short shot For injection molding Flow rate 1 10cm / s × 0.1cm 0.1cm ~ ⋅ = 2.5 −3 2 Heat xfer rate 4 10 cm / s... (lights,heating, ect ) Emissions to air, water, & land Polymer Delivery Injection Molder Energy Production Industry Internal Transport Drying Injection Molding Emissions to air, water, & land Scrap Anciliary Raw Materials Emissions to air, water & land Building (lights,heating, ect ) Packaging FACTORY GATE Note to Reader: = Focus of this Analysis 1 kg of Injection Molded Polymer = Also included in the Paper Service... 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 V=3.5cm/s V=8cm/s * Source: G Menges and W Wubken, “Influence of processing conditions on Molecular Orientation in Injection Molds” Gate Location and... ρ ⋅ cp = k 2 or =α 2 ∂t ∂x ∂t ∂x 1st kind 2nd kind 3rd kind T ( x = x' ) = constant ∂T ( x = x' ) = constant −k ∂x ∂T −k ( x = x' ) = h (T − T∞ ) ∂x The boundary condition of 1st kind applies to injection molding since the tool is often maintained at a constant temperature Heat transfer Tii t TW -L x +L Let Lch = H/2 (half thickness) = L ; tch = L2/α ; ΔTch = Ti – TW (initial temp – wall temp.) T −... molding? Effects of mold pressure on shrinkage LDPE 0.030 Acetal PP with flow 0.025 Shrinkage 0.020 0.015 PP across flow Nylon 6/6 0.010 0.005 PMMA 0.000 6000 10000 8000 14000 12000 18000 16000 Pressure on injection plunger (psi) Tooling Basics Sprue Nozzle Core Plate Cavity Plate Moulding Core Cavity Cavity Gate Runner Melt Delivery Basic mould consisting of cavity and core plate Tooling for a plastic cup... 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 . Injection Molding 2.810 Fall 2008 Professor Tim Gutowski Short history of plastics 1862 first synthetic plastic 1866 Celluloid 1891. Design for Injection Molding pp 319 - 359 •Kalpakjian(5 th ed) see Ch 19 • Injection molding case study;Washing machine augers; see on web page 30 ton, 1.5 oz (45 cm3) Engel Injection Molding Machine for. http://www.idsa-mp.org/proc/plastic /injection/ injection_process.htm * Schematic of thermoplastic Injection molding machine Process Operation • Temperature: barrel zones, tool, die zone • Pressures: injection