Injection Molding
Content
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
• Boothroyd Dewhurst
– Design for Injection Molding pp 319 - 359
• Kalpakjian (5th 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
*
*
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
tcool
* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html
2 ( half thickness ) =
α
α = 10 −3 cm 2 sec for polymers
Calculate clamp force, & shot size
F=P X A = 420 tons
3.8 lbs = 2245 cm3 =75 oz
Actual ; 2 cavity 800 ton
Clamp force and machine cost
Heat transfer Note; α
1-dimensional heat conduction equation :
qx qx + Δqx
Tool
> αpolymer
Fourier’s law
∂q ∂ ( ρ ⋅ c p ⋅ T )ΔxΔy = − x ΔxΔy ∂t ∂x ∂T q x = −k ∂x ∂T ∂ 2T ∂T ∂ 2T 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
Boundary Conditions:
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 x
Let Lch = H/2 (half thickness) = L ; tch = L2/α ; ΔTch = Ti – TW (initial temp. – wall temp.)
-L
+L
T − TW α ⋅t x ; ξ = + 1; FO = 2 Non-dimensionalize: θ = Ti − TW L L
Dimensionless equation: Initial condition Boundary condition
∂ 2θ ∂θ = 2 ∂FO ∂ξ
FO = 0
θ =1 θ =0 θ =0
ξ =0 ξ =2
Separation of variables ; matching B.C.; matching I.C.
θ (ξ , FO ) = ∑ f ( FO ) g (ξ )
Temperature in a slab Centerline, θ = 0.1, Fo = αt/L2 = 1
Bi-1 =k/hL
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
Part length 10 V≈ = ; Fill time 1s
For Die casting
−1
μ = 103 N ⋅ s m 2
Re = 10 −4
3 ⋅103 × 10 −1 × 10 −3 Re ≈ = 300 −3 10
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
Viscous Shearing of Fluids
F h 1 v
F/A
μ
F v ∝ A h
Generalization:
v τ =μ h
v/h
Newtonian Viscosity
τ = μγ&
γ& : shear rate
Injection molding
τ = η (γ& ) γ&
Typical shear rate for Polymer processes (sec)-1 Extrusion Calendering Injection molding Comp. Molding 102~103 10~102 103~104 1~10
“Shear Thinning” ~ 1 sec-1 for PE
γ&
Viscous Heating
Rate of Heating = Rate of Viscous Work Rate of Temperature rise
P F ⋅v F v ⎛v⎞ = = ⋅ = μ⎜ ⎟ Vol Vol A h ⎝h⎠
2
ρ ⋅ cp
dT ⎛v⎞ = μ⎜ ⎟ dt ⎝h⎠
2
or
dT μ ⎛v⎞ = ⎜ ⎟ dt ρ ⋅ c p ⎝ h ⎠
2
Rate of Conduction out
dT k d 2T k ΔT ~ = 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
v
Flow rate: 1/t ~V/Lx 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
For injection molding
Small value => Short shot
Flow rate 1 10cm / s × 0.1cm 0.1cm ~ ⋅ = 2.5 −3 2 Heat xfer rate 4 10 cm / s 10cm
For Die casting of aluminum
Flow rate 1 10cm / s × 0.1cm 0.1cm −2 ~ ⋅ ≅ 10 Heat xfer rate 4 0.3cm 2 / s 10cm
* 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
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 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
0.030 0.025
LDPE
PP Acetal Shrinkage Nylon 6/6
0.030 0.025 0.020
LDPE Acetal PP with flow PP across flow
Shrinkage
0.020
0.015
0.015 0.010 0.005
Nylon 6/6
0.010 0.005
PMMA
0.000 100 120 140 160 180 200 220 240 0.000 6000
PMMA
10000 8000 12000 14000 16000 18000
Mold Temperature (F)
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?
Effects of mold pressure on shrinkage
0.030
LDPE Acetal PP with flow
0.025
0.020
Shrinkage
0.015
PP across flow
Nylon 6/6
0.010
0.005
PMMA
6000 8000 10000 12000 14000 16000 18000
0.000
Pressure on injection plunger (psi)
Tooling Basics
Sprue Nozzle Cavity Plate Core Plate
Moulding Cavity
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 Runner Cavity Runner Part Cavity Cavity Knob
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
Cores used in Injection Molding
Cores and Part Molded in Clear Plastic
Environmental issues
• System boundaries • Polymer production • Compounding • Machine types • Out gassing & energy during processing
CRADLE Naphtha, Oil. Natural Gas Ancilliary Raw Materials
Additives
Compounder
Internal Transport Drying Pelletizing Emissions to air, water & land
Thermoplastic Production
(Boustead)
Polymer Delivery
Extrusion
Building (lights,heating, ect..) Emissions to air, water, & land
Polymer
Delivery
Injection Molder Energy Production Industry
Internal Transport Drying Emissions to air, water & land
Injection Molding
Emissions to air, water, & land Scrap Anciliary Raw Materials Building (lights,heating, ect..) Packaging Note to Reader: = Focus of this Analysis = Also included in the Paper
FACTORY GATE
1 kg of Injection Molded Polymer
Service Period Waste Management
Polymer Production
Largest Player in the Injection Molding LCI
What is a polymer:
How much energy does it take to make 1 kg of polymer = a lot !!!
Sources Boustead Ashby Patel Kindler/Nickles [Patel 1999] Worrell et al. [Patel 1999] 3 E Handbook [OIT 1997] Energieweb HDPE 76.56 111.50 ------------------131.65 80.00 LLDPE 77.79 ------------------------121.18 ------LDPE 73.55 92.00 64.60 71.00 67.80 136.07 68.00 PP 72.49 111.50 ------------------126.07 64.00 PVC 58.41 79.50 53.20 53.00 52.40 33.24 57.00 PS 86.46 118.00 70.80 81.00 82.70 ------84.00 PC 115.45 ------80.30 107.00 78.20 ------------------81.00 PET 77.14 ------59.40 96.00
Values are in MJ per kg of polymer produced. Thiriez ‘06
Compounding - extrusion
• • • An extruder is used to mix additives with a polymer base, to bestow the polymer with the required characteristics. Similar to an injection molding machine, but without a mold and continuous production. Thus it has a similar energy consumption profile.
Environmentally Unfriendly Additives: •Fluorinated blowing agents (GHG’s) •Phalates (some toxic to human liver, kidney and testicles) •Organotin stabilizers (toxic and damage marine wildlife)
Injection Molding Process
Source: http://cache.husky.ca/pdf/br ochures/br-hylectric03a.pdf
Machine types: Hydraulic, electric, hydro-electric
All-electrics have very low fixed energy costs (small idling power). SEC is constant as throughput increases.
SEC ≈ pv
9 8 7 All-Electric - 85 tons SEC (MJ/kg) 6 Hydraulic - 85 tons 5 4 3 2 1 0 0 5 10 Throughput (kg/hr) 15 20 Material: PP
Source: [Thiriez]
For Hydraulics and Hybrids as throughput increases, SEC Æ k.
8 7 6 SEC (MJ/kg) 5 4 3 2 1 0 0 50 100 Throughput (kg/hr) 150 200 Variable Pump Hydraulic Injection Molding Machines. HP 25 HP 50 HP 60 HP 75 HP 100 Low Enthalpy - Raise Resin to Inj. Temp - PVC High Enthalpy - Raise Resin to Inj. Temp - HDPE
Does not account for the electric grid.
Source: [Thiriez]
Enthalpy value to melt plastics is just 0.1 to 0.7 MJ/kg !!!
All-electric vs. hybrid
120
Cool Ton Buildup Clamp open-close Inject high
Power Required (kW)
100
Plasticize
80 60 40 20 0 0 1 2 3 4 6 7 8 9 10 11 Time (seconds) MM 550 Hybrid NT 440 All-Electric 5 12 13 14
t
Inject low
Source: [Thiriez]
The hydraulic plot would be even higher than the hybrid curve
Driers
• • Used to dry internal moisture in hygroscopic polymers and external moisture in non-hygroscopic ones. It is done before extruding and injection molding.
1.8 Specific Power Consumption (MJ/kg) 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 500 1000
W600 W800 W1000 W2400 W1600 W3200 W5000 W150 W300 W400 W200
Power Trendline R2 = 0.8225
1500
2000
2500
3000
3500
Throughput (kg/hr)
Source: [Thiriez]
Same as
P0 P E = = SEC = +k & m & m m
LCI Summarized Results
ENERGY CONSUMPTION BY STAGE in MJ/kg of shot
Thermoplastic Production
HDPE LLDPE LDPE 89.8 79.7 73.1 77.9 79.7 64.6 111.5 79.7 92.0 PP 83.0 64.0 111.5 PVC 59.2 52.4 79.5 PS 87.2 70.8 118.0 Extras Generic by Amount PET Consumed Inj. Molded PC 81.2 74.6 95.7 78.8 69.7 62.8 78.2 59.4 102.7 97.6 117.4 96.0 avg low high 0.19 0.12 0.24
avg low high
Polymer Delivery
Compounder
Internal Transport 0.09 --------avg low high 5.51 3.25 8.01 Polymer Delivery avg low high 0.19 0.12 0.24 Drying 0.70 0.30 1.62 Extrusion 3.57 1.82 5.00 Pelletizing 0.16 0.06 0.31 Building (lights, heating, ect..) 0.99 ---------
avg low high Subtotal
Injection Molder
Internal Transport 0.04 --------Drying 0.70 0.30 1.62 Injection Molding - Choose One Hydraulic Hybrid All-Electric 5.56 4.89 11.29 3.99 3.11 1.80 69.79 8.45 15.29 13.08 5.35 72.57 Hydraulic 93.60 71.65 178.68 18.97 8.84 81.04 7.35 4.47 11.22 Hybrid 87.87 70.77 117.34 13.24 7.96 19.70 6.68 3.17 18.06 All-Electric 87.20 69.46 124.18 12.57 6.66 26.54 Injection Molding (look below) Scrap (Granulating) 0.05 0.03 0.12 Building (lights, heating, ect..) 0.99 ---------
avg low high
avg low high Subtotal avg low high
TOTAL w/ Generic Inj. Molded Polymer TOTAL w/o Polymer Prod Notes
avg low high avg low high
Drying - the values presented assume no knowledge of the materials' hygroscopia. In order words, they are averages between hygroscopic and non-hygroscopic values. For hygroscopic materials such as PC and PET additional drying energy is needed (0.65 MJ/kg in the case of PC and 0.52 MJ/kg in the case of PET) Pelletizing - in the case of pelletizing an extra 0.3 MJ/kg is needed for PP Granulating - a scarp rate of 10 % is assumed
Source: [Thiriez]
Energy Production Industry
United States Electricity Composition by Source Hydro 7.1% Nuclear 19.6% Other 0.0% Coal 50.7% Oil 3.1% Gas 16.7% Waste/ Renewable 2.2%
The Grid is about 30% efficient
For every MJ of electricity we also get: Æ171.94 g of CO2 Æ0.76 g of SO2 Æ 0.31 g of NOx Æ 6.24 g of CH4 Æ 0.0032 mg of Hg
Scale
HDPE, LDPE, LLDPE, PP, PS, PVC
Compounder and Injection Molder 6 Main Thermoplastics All Plastics U.S. GJ/year 9.34E+07 2.06E+08 Global GJ/year 4.01E+08 6.68E+08
The Injection Molding Industry in the U.S. consumes 6.19 x 107 GJ of electricity (or 2.06 x 108 GJ in total energy). This is larger than the entire electric production of some small countries.
In such a scale imagine what a 0.1 % energy savings mean !!!
The printer goes in the hopper…
And comes out….
Readings
1. 2. 3. 4. 5. 6. Z. Tadmore et al., "Molding and Casting" p. 584 - 610 G. Boothroyd et al., "Design for Injection Molding“ p.319 - 360 S. Shingo, "Single Minute Exchange of Dies“ Thiriez et al, "An Environmental Analysis of Injection Molding“ "Injection Molding Case Study“ Kalpakjian Chapter 19 (Chapter 18)
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
• Boothroyd Dewhurst
– Design for Injection Molding pp 319 - 359
• Kalpakjian (5th 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
*
*
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
tcool
* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html
2 ( half thickness ) =
α
α = 10 −3 cm 2 sec for polymers
Calculate clamp force, & shot size
F=P X A = 420 tons
3.8 lbs = 2245 cm3 =75 oz
Actual ; 2 cavity 800 ton
Clamp force and machine cost
Heat transfer Note; α
1-dimensional heat conduction equation :
qx qx + Δqx
Tool
> αpolymer
Fourier’s law
∂q ∂ ( ρ ⋅ c p ⋅ T )ΔxΔy = − x ΔxΔy ∂t ∂x ∂T q x = −k ∂x ∂T ∂ 2T ∂T ∂ 2T 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
Boundary Conditions:
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 x
Let Lch = H/2 (half thickness) = L ; tch = L2/α ; ΔTch = Ti – TW (initial temp. – wall temp.)
-L
+L
T − TW α ⋅t x ; ξ = + 1; FO = 2 Non-dimensionalize: θ = Ti − TW L L
Dimensionless equation: Initial condition Boundary condition
∂ 2θ ∂θ = 2 ∂FO ∂ξ
FO = 0
θ =1 θ =0 θ =0
ξ =0 ξ =2
Separation of variables ; matching B.C.; matching I.C.
θ (ξ , FO ) = ∑ f ( FO ) g (ξ )
Temperature in a slab Centerline, θ = 0.1, Fo = αt/L2 = 1
Bi-1 =k/hL
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
Part length 10 V≈ = ; Fill time 1s
For Die casting
−1
μ = 103 N ⋅ s m 2
Re = 10 −4
3 ⋅103 × 10 −1 × 10 −3 Re ≈ = 300 −3 10
* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm
Viscous Shearing of Fluids
F h 1 v
F/A
μ
F v ∝ A h
Generalization:
v τ =μ h
v/h
Newtonian Viscosity
τ = μγ&
γ& : shear rate
Injection molding
τ = η (γ& ) γ&
Typical shear rate for Polymer processes (sec)-1 Extrusion Calendering Injection molding Comp. Molding 102~103 10~102 103~104 1~10
“Shear Thinning” ~ 1 sec-1 for PE
γ&
Viscous Heating
Rate of Heating = Rate of Viscous Work Rate of Temperature rise
P F ⋅v F v ⎛v⎞ = = ⋅ = μ⎜ ⎟ Vol Vol A h ⎝h⎠
2
ρ ⋅ cp
dT ⎛v⎞ = μ⎜ ⎟ dt ⎝h⎠
2
or
dT μ ⎛v⎞ = ⎜ ⎟ dt ρ ⋅ c p ⎝ h ⎠
2
Rate of Conduction out
dT k d 2T k ΔT ~ = 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
v
Flow rate: 1/t ~V/Lx 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
For injection molding
Small value => Short shot
Flow rate 1 10cm / s × 0.1cm 0.1cm ~ ⋅ = 2.5 −3 2 Heat xfer rate 4 10 cm / s 10cm
For Die casting of aluminum
Flow rate 1 10cm / s × 0.1cm 0.1cm −2 ~ ⋅ ≅ 10 Heat xfer rate 4 0.3cm 2 / s 10cm
* 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
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 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
0.030 0.025
LDPE
PP Acetal Shrinkage Nylon 6/6
0.030 0.025 0.020
LDPE Acetal PP with flow PP across flow
Shrinkage
0.020
0.015
0.015 0.010 0.005
Nylon 6/6
0.010 0.005
PMMA
0.000 100 120 140 160 180 200 220 240 0.000 6000
PMMA
10000 8000 12000 14000 16000 18000
Mold Temperature (F)
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?
Effects of mold pressure on shrinkage
0.030
LDPE Acetal PP with flow
0.025
0.020
Shrinkage
0.015
PP across flow
Nylon 6/6
0.010
0.005
PMMA
6000 8000 10000 12000 14000 16000 18000
0.000
Pressure on injection plunger (psi)
Tooling Basics
Sprue Nozzle Cavity Plate Core Plate
Moulding Cavity
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 Runner Cavity Runner Part Cavity Cavity Knob
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
Cores used in Injection Molding
Cores and Part Molded in Clear Plastic
Environmental issues
• System boundaries • Polymer production • Compounding • Machine types • Out gassing & energy during processing
CRADLE Naphtha, Oil. Natural Gas Ancilliary Raw Materials
Additives
Compounder
Internal Transport Drying Pelletizing Emissions to air, water & land
Thermoplastic Production
(Boustead)
Polymer Delivery
Extrusion
Building (lights,heating, ect..) Emissions to air, water, & land
Polymer
Delivery
Injection Molder Energy Production Industry
Internal Transport Drying Emissions to air, water & land
Injection Molding
Emissions to air, water, & land Scrap Anciliary Raw Materials Building (lights,heating, ect..) Packaging Note to Reader: = Focus of this Analysis = Also included in the Paper
FACTORY GATE
1 kg of Injection Molded Polymer
Service Period Waste Management
Polymer Production
Largest Player in the Injection Molding LCI
What is a polymer:
How much energy does it take to make 1 kg of polymer = a lot !!!
Sources Boustead Ashby Patel Kindler/Nickles [Patel 1999] Worrell et al. [Patel 1999] 3 E Handbook [OIT 1997] Energieweb HDPE 76.56 111.50 ------------------131.65 80.00 LLDPE 77.79 ------------------------121.18 ------LDPE 73.55 92.00 64.60 71.00 67.80 136.07 68.00 PP 72.49 111.50 ------------------126.07 64.00 PVC 58.41 79.50 53.20 53.00 52.40 33.24 57.00 PS 86.46 118.00 70.80 81.00 82.70 ------84.00 PC 115.45 ------80.30 107.00 78.20 ------------------81.00 PET 77.14 ------59.40 96.00
Values are in MJ per kg of polymer produced. Thiriez ‘06
Compounding - extrusion
• • • An extruder is used to mix additives with a polymer base, to bestow the polymer with the required characteristics. Similar to an injection molding machine, but without a mold and continuous production. Thus it has a similar energy consumption profile.
Environmentally Unfriendly Additives: •Fluorinated blowing agents (GHG’s) •Phalates (some toxic to human liver, kidney and testicles) •Organotin stabilizers (toxic and damage marine wildlife)
Injection Molding Process
Source: http://cache.husky.ca/pdf/br ochures/br-hylectric03a.pdf
Machine types: Hydraulic, electric, hydro-electric
All-electrics have very low fixed energy costs (small idling power). SEC is constant as throughput increases.
SEC ≈ pv
9 8 7 All-Electric - 85 tons SEC (MJ/kg) 6 Hydraulic - 85 tons 5 4 3 2 1 0 0 5 10 Throughput (kg/hr) 15 20 Material: PP
Source: [Thiriez]
For Hydraulics and Hybrids as throughput increases, SEC Æ k.
8 7 6 SEC (MJ/kg) 5 4 3 2 1 0 0 50 100 Throughput (kg/hr) 150 200 Variable Pump Hydraulic Injection Molding Machines. HP 25 HP 50 HP 60 HP 75 HP 100 Low Enthalpy - Raise Resin to Inj. Temp - PVC High Enthalpy - Raise Resin to Inj. Temp - HDPE
Does not account for the electric grid.
Source: [Thiriez]
Enthalpy value to melt plastics is just 0.1 to 0.7 MJ/kg !!!
All-electric vs. hybrid
120
Cool Ton Buildup Clamp open-close Inject high
Power Required (kW)
100
Plasticize
80 60 40 20 0 0 1 2 3 4 6 7 8 9 10 11 Time (seconds) MM 550 Hybrid NT 440 All-Electric 5 12 13 14
t
Inject low
Source: [Thiriez]
The hydraulic plot would be even higher than the hybrid curve
Driers
• • Used to dry internal moisture in hygroscopic polymers and external moisture in non-hygroscopic ones. It is done before extruding and injection molding.
1.8 Specific Power Consumption (MJ/kg) 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0 500 1000
W600 W800 W1000 W2400 W1600 W3200 W5000 W150 W300 W400 W200
Power Trendline R2 = 0.8225
1500
2000
2500
3000
3500
Throughput (kg/hr)
Source: [Thiriez]
Same as
P0 P E = = SEC = +k & m & m m
LCI Summarized Results
ENERGY CONSUMPTION BY STAGE in MJ/kg of shot
Thermoplastic Production
HDPE LLDPE LDPE 89.8 79.7 73.1 77.9 79.7 64.6 111.5 79.7 92.0 PP 83.0 64.0 111.5 PVC 59.2 52.4 79.5 PS 87.2 70.8 118.0 Extras Generic by Amount PET Consumed Inj. Molded PC 81.2 74.6 95.7 78.8 69.7 62.8 78.2 59.4 102.7 97.6 117.4 96.0 avg low high 0.19 0.12 0.24
avg low high
Polymer Delivery
Compounder
Internal Transport 0.09 --------avg low high 5.51 3.25 8.01 Polymer Delivery avg low high 0.19 0.12 0.24 Drying 0.70 0.30 1.62 Extrusion 3.57 1.82 5.00 Pelletizing 0.16 0.06 0.31 Building (lights, heating, ect..) 0.99 ---------
avg low high Subtotal
Injection Molder
Internal Transport 0.04 --------Drying 0.70 0.30 1.62 Injection Molding - Choose One Hydraulic Hybrid All-Electric 5.56 4.89 11.29 3.99 3.11 1.80 69.79 8.45 15.29 13.08 5.35 72.57 Hydraulic 93.60 71.65 178.68 18.97 8.84 81.04 7.35 4.47 11.22 Hybrid 87.87 70.77 117.34 13.24 7.96 19.70 6.68 3.17 18.06 All-Electric 87.20 69.46 124.18 12.57 6.66 26.54 Injection Molding (look below) Scrap (Granulating) 0.05 0.03 0.12 Building (lights, heating, ect..) 0.99 ---------
avg low high
avg low high Subtotal avg low high
TOTAL w/ Generic Inj. Molded Polymer TOTAL w/o Polymer Prod Notes
avg low high avg low high
Drying - the values presented assume no knowledge of the materials' hygroscopia. In order words, they are averages between hygroscopic and non-hygroscopic values. For hygroscopic materials such as PC and PET additional drying energy is needed (0.65 MJ/kg in the case of PC and 0.52 MJ/kg in the case of PET) Pelletizing - in the case of pelletizing an extra 0.3 MJ/kg is needed for PP Granulating - a scarp rate of 10 % is assumed
Source: [Thiriez]
Energy Production Industry
United States Electricity Composition by Source Hydro 7.1% Nuclear 19.6% Other 0.0% Coal 50.7% Oil 3.1% Gas 16.7% Waste/ Renewable 2.2%
The Grid is about 30% efficient
For every MJ of electricity we also get: Æ171.94 g of CO2 Æ0.76 g of SO2 Æ 0.31 g of NOx Æ 6.24 g of CH4 Æ 0.0032 mg of Hg
Scale
HDPE, LDPE, LLDPE, PP, PS, PVC
Compounder and Injection Molder 6 Main Thermoplastics All Plastics U.S. GJ/year 9.34E+07 2.06E+08 Global GJ/year 4.01E+08 6.68E+08
The Injection Molding Industry in the U.S. consumes 6.19 x 107 GJ of electricity (or 2.06 x 108 GJ in total energy). This is larger than the entire electric production of some small countries.
In such a scale imagine what a 0.1 % energy savings mean !!!
The printer goes in the hopper…
And comes out….
Readings
1. 2. 3. 4. 5. 6. Z. Tadmore et al., "Molding and Casting" p. 584 - 610 G. Boothroyd et al., "Design for Injection Molding“ p.319 - 360 S. Shingo, "Single Minute Exchange of Dies“ Thiriez et al, "An Environmental Analysis of Injection Molding“ "Injection Molding Case Study“ Kalpakjian Chapter 19 (Chapter 18)
