Injection molding

Injection Molding 2.810 Fall 2002 Professor Tim Gutowski Short history of plastics 1862 1866 1891 1907 1913 1926 1933 1938 1939 1957 1967 first synthetic plastic Celluloid Rayon Bakelite Cellophane PVC Polyethylene Teflon Nylon stockings velcro “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) Cooling time generally dominates cycle time tcool * Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html ( half thickness ) = α α = 10 −3 cm3 sec for polymers Time(sec) Calculate clamp force, & shot size F=P X A = 420 tons 3.8 lbs = 2245 cm3 =75 oz Actual ; cavity 800 ton Clamp force and machine cost 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 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 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 4.5 3.5 2.5 1.5 0.5 CO2 (metric ton/$10,000) Manufacturing industries Transportation Electronic Machinery Fabricated Metal Primary Metal Plastics and Rubber Petroleum and Coal Toxic Mat'ls (lb/$1000) Chemicals Weight/Dollars Carbon Dioxide and Toxic Materials per Value of Shipments 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 [...]... 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? ...  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... Shearing of Fluids F v F/A v τ =µ h 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... Reynolds Number Reynolds Number: V2 ρ inertia 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 v τ =µ h µ h 1... dt ρ ⋅ c p dx 2 ρ ⋅ c p h 2 Viscous heating µv 2 = Conduction k∆T 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 ⋅ L2z 1 VLz Lz ~ = ⋅ 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... 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 ∂x ∂T −k ( x = x' ) = h (T − T∞ ) ∂x −k 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 ; ∆Tch = Ti – TW (initial temp – wall temp.) T −... 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... 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... mold temperature and 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 ... 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... 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? ∆Ltotal = ∆Lmold + ∆Lshrinkage... 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