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2.008 Design & ManufacturingII
Spring 2004
Polymer Processing II Injection Molding
2.008-spring-2004 S. Kim 1
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What is a mold?From Webster: a cavity in which a substance is shaped: as (1) : a matrix for casting metal (2) : a form in which food is given a decorative shape
-Net shape manufacturing
-Volume vs. cost
-Life of a mold
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Casting
1. Assemble a mold.2. Pour molten metal
in.3. Cool down.4. Open the mold and remove the part.
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Die casting
1. Assemble a mold2. Inject molten metal into3. Open and remove
aluminum, magnesium Precision parts
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Injection Molding
2.008-spring-2004 S. Kim a slide from B. Kim, 1982, MIT 5
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Injection Molding Machine
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Steps of Injection Molding -Mold closing -Filling, packing, holding -Cooling -Opening, part removal.
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Injection Cycle Time–$$$
– Typical Cycle of Injection Molding
• Mold Close 1-2 sec • Injection 2-5 sec• Pack and Hold 8-10 sec• Part Cool 10-20 sec – Screw return 2-5 sec• Mold open 1 sec• Ejection 1 sec
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Design for Manufacturing
• Part design – Moldable – Draft angle – Shrinkage – Reinforcements (ribs and bosses) – Cycle time – Appearance (defects)
• Mold Design – Gate – balancing
• Process Control
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Injection molding process window
Poorquality
flash
Degradationburn
shortshot
Better weldPmax
Pmin
TmaxTmin
Good
Low cost
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Flashes
Flashes develop at the mold parting line or ejector pin installation point. It is a phenomenon where molten polymer smears out and sticks to the gap.
CausePoor quality of the mold. The molten polymer has too low viscosity. Injection pressure is too high, or clamping force is too weak.
SolutionAvoiding excessive difference in thickness is most effective. Slow down the injection speed.Apply well-balanced pressure to the mold to get consistent clamping force, or increase the clamping force.Enhance the surface quality of the parting lines, ejector pins and holes.
Gate Runner
SprueFlash developedat Parting Line
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Short shotThis is the phenomenon where molten plastics does not fill the mold cavity completely. and the portion of parts becomes incomplete shape.
CauseThe shot volume or injection pressure is not sufficient.
Injection speed is so slow that the molten plastics becomes solid before it flows to the end of the mold.
SolutionApply higher injection pressure. Install air vent or degassing device. Change the shape of the mold or gate position for better flow of the plastics.
Proper Shape of Mold Incomplete FillingGate Runner
Sprue
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Injection Molding Parameters • Temperature and Pressure: Function (x,y,z) • Melt Temperature Control – Through Cylinder(Barrel) • Frictional Heating • Heating bands for 3 zones – Rear zone – Center zone (10F-20F hotter) – Front Zone (10F-20F hotter) • Nozzle Solid Plastic Pellets
Barrel
Screw
Melted Plastic
Mold
Figure 2: Detail of Screw and Barrel
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Plasticating Extrusion
rough surface
smooth surface
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Suggested Melt Temp at nozzle Acetal (coploymer) 400 F Acrylic 425 F ABS 400 F Liquid Crystal Polymer 500 F Nylon 6 500 F Polyamide-imide 650 F Polyarylate 700 F Polycarbonate 550 F Polyetheretherketone 720 F Polyethylene LDPE 325 F Polyethylene HDPE 350 F Polypropylene 350 F Polystyrene 350 F Thermoplastic polyester (PBT) 425 F Urethane elastomer 425 F
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Temperature Cycle
TEMPERATURE HISTORY IN AN INJECTION MOLDED PART
Inside the PartGate
Cooling
Pack
Time (Sec)
Tem
per
atu
re (
0 C)
Fa
st
Clo
sin
g
Gat
e F
reez
e O
ff
Pa
rt E
lec
tio
n
Fil
l
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-Flow path ratio is the ratio between L (the distance between the gate and the farthest point in the molding dimension) and T (the thickness of the part) .-When molding large or thin parts, the flow path ratio is calculated to determine if molten plastics can fill the mold cavity.
Polyethylene (PE) L/T = 280-100 Polypropylene (PP) L/T = 280-150 Polyvinyl chloride (PVC) L/T = 280-70 Polystyrene (PS) L/T = 300-220 Polycarbonate (PC) L/T = 160-90 Acrylonitrile butadienstylene (ABS) L/T = 280-120 Polyamide (PA) L/T = 320-200
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Flow path ratio
Rule of thumb
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Mold Temperature Control • Mold Temperature Control
– Mold cooling with water, oil. – Hot mold for less residual stresses (orientation)
• Low thermal inertia – Uneven cooling
• warpage, twisting, shrinkage defects • Shrinkage can progress for up to 30 days.
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Mold coolingMolding cycle can be shortened by reducing time for cooling and solidification of molten plastics.
Solidification time, t thickness2/α, thermal diffusitivity
Warpage or stress in a part can be generated when mold shrinkage varies due to different thickness, leading internal residual stress difference.
Even cooling
Cooling Water
Part
Cavity
Part
Pipe
Core
CoolingWater
Cooling Channel
Cavity
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Warpage
This deformation appears when the part is removed from the mold and pressure is released.
CauseUneven shrinkage due to the mold temperature difference (surface temperature difference at cavity and core), and the thickness difference in the part. Injection pressure was too low and insufficient packing.
SolutionTake a longer cooling time and lower the ejection speed. Adjust the ejector pin position or enlarge the draft angle. Examine the part thickness or dimension. Balance cooling lines. Increase packing pressure.
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Pressure Control
• pressure distribution
• Injection unit
– Initial injection pressure
• Applied to the molten plastic and resulting from the main
hydraulic pressure pushing against the back end of the
injection screw (or plunger).
– Packing pressure
– Injection Pressure inside mold
• Usually 1,000 psi to 5,000 psi
• Lower than hold and pack pressure between 10,000psi and
• 20,000 psi
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Pressure Control PRESSURE HISTORY IN AN INJECTION MOLDED PART
Cycle Time
Mold Closed Time
Cooling
Pack
Time (Sec)
End of filling
Ca
vity
Pre
ssu
re (
MP
a)
F
ast
Cas
ting
Gat
e F
reez
e O
ff Par
t E
ject
ion
Fill
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Pressure Control – Hold pressure (packing)
• Compensate shrinkage • Rule of thumb: Hold pressure = 150% of injection pressure. • Applied at the end of the initial injection stroke, and is intended to complete the final filling of the mold and hold pressure till gate closure
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Clamp force• Pressure Required – Total force = projected area times injection pressure (A X P) – Rule of thumb 4 to 5 tons/in2 can be used for most plastics. – Example,
Part is 10 in by 10 in by 1 in Projected area = Surface area = 10 in x 10 in = 100 in2
Injection Pressure = 15,000 psi for PC Tonnage required to keep mold closed is
– 100 in2 x 15,000 psi= 1,500,000 lbs = 750 tons (note : 2000 lbs = 1 ton)
Parting line ?
10 in
10 in
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Mold Structure - Cavity and core
Integral Method Insert Method
Part
Cavity(Concave Side of
Cavity Plate)
Core(Convex Side of
Core Plate)
Part
Insert
Part
CoreInsert
Cavity Insert
Part
Cavity Insert
Part
Core Insert
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Mold Structure: Parting line A dividing line between a cavity plate and a core plate of a mold. - Make a parting line on a flat or simple-curved surface so that flash cannot be generated. - Venting gas or air.
Cavity Plate
Core Plate
Parting Line
Gate Runner
Flash developedat Parting Line
Sprue
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Mold Structure: Undercut, Slide core
Undercut
Undercut
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Delivery
Sprue A sprue is a channel through which to transfer molten plastics injected from the injector nozzle into the mold.
Runner A runner is a channel that guides moltenplastics into the cavity of a mold.
Gate A gate is an entrance through which molten plastics enters the cavity
Gate
Runner
Part
Sprue
RunnerSprue
Gate Part
Sprue
Sprue Bush
RunnerGate
Cavity
Injection Device
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Two plate mold
One parting line
Runner Sprue
Gate
Part
Runner Process
Part
FixedSide
Gate Sprue
Part
MovableSide
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Three plate mold
Two parting linesSprue
Runner Gate
Fixed Side
MovableSide
Part
RunnerStripper Plate
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Runner balancingCavity
Runner
Runner
Cavity Sprue Bush
Sprue Bush Example of MoldingFour Parts
Cavity RunnerRunner
Cavity
Example of MoldingEight Parts
Sprue BushSprue Bush
balanced
Not balanced
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Runner cross sectionParting line
Round Trapezoidal U-shape
Runner cross section that minimizes liquid resistance and temperature reduction when molten plastics flows into the cavity.
- Too big- Longer cooling time, more material, cost
- Too small- short shot, sink mark, bad quality
- Too long- pressure drop, waste, cooling
Hot runner, runnerless mold
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Gate-Restricts the flow and the direction of molten plastics.
-Quickly cools and solidifies to avoid backflow after molten plastics has filled up in the cavity.
-Simplifies cutting of a runner and moldings to simple finishing of parts.
Gate
Runner
Part
Side gate
Submarine gate
Part
Gate
Length
CrossSection
Thickness
Width
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Fan gate, Film gate, Direct gate Sprue
Runner
Gate Part
Sprue
Runner
Gate Part Sprue Gate
Part
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Gate PositioningPoint 1: Set a gate position where molten plastics finish filling up in each cavity simultaneously. Same as multiple points gate.
Point 2: Basically set a gate position to the thickest area of a part. This can avoid sink marks due to molding (part) shrinkage .
Point 3: Set a gate position to an unexposed area of part or where finishing process can be easily done.
Point 4: Consider degasing, weldline, molecular orientation.
Point5: Fill up molten plastics using the wall surface in order not togenerate jetting. Die swell > Thickness, t Gate
Runner
Jetting
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Molecular orientation
Gate 2Gate 1
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Design for Manufacturing
Moldable: flow path ratio, machine
size Draft angle Shrinkage Reinforcements (ribs and bosses) Cycle time Appearance, defects Balance, balance, balance!!
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Draft angle -for removing parts from the mold - 1-2o, material, dimension, texture dependent - Cavity side smaller, core side larger. - Crystalline material has more shrinkage. - Amorphous material has smaller shrinkage.
Shrinks andadheres tothe core
After Cooling Mold
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Shrinkage
Resin Name Molding Shrinkage (%) Polyethylene (PE) 1.5-6.0Polypropylene (PP) 1.0-3.0Polyvinyl chloride (PVC) 0.1-0.5Polystyrene (PS) 0.2-0.6Polycarbonate (PC) 0.5-0.8Acrylonitrile butadienstylene 0.3-0.8Polyamide (PA) 0.6-2.0
∆L = α L∆T = α Tα: shrinkage rate
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Ribs and Bosses
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Examples of DFM
Compensation for distortion
Strengthening rib Stiffen the bottom by doming
(Like beverage cans)
Poor Design Improved Design
Compensation for swell
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Strength Issues
Creak
Make base thick
The molecules align in themajor direction of flow, andhence there is greater strength
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DefectsMolding defects are caused by related and complicated
reasons as follows:
* Malfunctions of molding machine* Inappropriate molding conditions* Flaws in product and mold design* Improper Selection of molding material
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Sink marks
-Equal cooling from the surface-Secondary flow-Collapsed surface
→ Sink Mark
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Whitening
After the ejection by the ejector pin, the surface of the ejected part or surrounding part turns white. When some portion of the part is hard to remove from the mold, that portion also turns white.
CauseThe part was hard to remove from the mold. Poor quality of the mold surface
SolutionPolish the mold well to facilitate removal of the part. Lower the injection pressure to facilitate removal of the part Reduce the ejector pin speed, and increase the number of ejector pins.
Whitening
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Weldline
This is a phenomenon where a thin line is created when different flows of molten plastics in a mold cavity meet and remain undissolved. It is a boundary between flows caused by incomplete dissolution of molten plastics. It often develops around the far edge of the gate.
CauseLow temperature of the mold causes incomplete dissolution of the molten plastics.
SolutionIncrease injection speed and raise the mold temperature. Lower the molten plastics temperature and increase the injection pressure. Change the gate position and the flow of molten plastics. Change the gate position to prevent development of weldline.
Weld Line
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JettingThis is the phenomenon where the part has a wire-shape flow pattern on the surface.
CauseDue to inappropriate gate position, a flow of molten plastics into the cavity is cooled in a line shape and remains undissolved with other plastics flow coming later.
SolutionRaise the molten plastics and mold temperature, and increase injection speed to make the initial and later flows of molten plastics dissolve completely. Change the gate position to make the molten plastics touch the facing side before making a line shape.
Gate
Runner
JettingDie swell > Thickness, t
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Flow mark
This is a phenomenon where the initial flow of molten plastics which solidifies mixes with a later flow and remains undissolved. It develops distinctive patterns such as clouds, scales or tree rings.
CauseInjection speed is too fast.Mold or molten plastics temperature is too low.
SolutionEnlarge the gate area to decrease the speed of the molten plastics flowing through the gate.Increase the pressure retention time for better pressure quality.
Flow MarGate position