design principles
TRANSCRIPT
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Introduction toIntroduction to
Injection Molding
Flow Behavior
Design Principles
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Injection Molding MachineInjection Molding Machine
ClampingUnit Hydraulic
Unit
Hopper
Screw(Ram)
Barrel
Heaterbands
Mold cooling fedfrom external unit
Nozzle betweenbarrel and mold
(not shown)
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Cooling Time
Mold Open TimeFill Time
Hold Time
Injection Molding CycleInjection Molding Cycle
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Cycle Time
Fill Time
Hold Time
Cooling Time
Mold Open Time
22Sec.
1
9
10
2
Injection Molding CycleInjection Molding Cycle
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Filling– Mold closes, screw rapidly
moves forward, frozen polymer skin forms at mold walls
Packing Time (Holding)– Cavity filled, packing
begins, cooling occurring
Hopper
Barrel
Screw
Mold
Screw is applying a specifiedpressure to the polymer meltin order to pack more plasticinto the cavity. Also called“compensation stage”.
Injection Molding ProcessInjection Molding Process
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Cooling– Packing complete,
gate freezes off, cooling continues
– Screw moves back and begins plasticating resin for next shot
Mold Open– Cooling completes,
mold opens
Injection Molding ProcessInjection Molding Process
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The Injection MoldThe Injection Mold
a.k.a. Stationary Half
a.k.a. Moving Half
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Flow BehaviorFlow Behavior
What Does a Plastic Molecule Do in an Injection Mold?
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Filling PhasePressurization PhaseCompensation Phase
Filling, Volumetrically fill the cavity Pressurization, Build up pressure in the cavity Compensation, Add extra material to reduce
shrinkage
Phases of MoldingPhases of Molding
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Fountain FlowFountain Flow
Describes the phenomena of how plastics flows in a mold
Material that first enters shows up at the surface near the gate
Material that enters the cavity last, shows up in the center downstream
Has direct influence on molecular and fiber orientation at the part surface
Melt
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Molecular Orientation is caused by shear flow. The high amount of shear is inside the frozen layer, therefore the highest orientation
Cross-Sectional FlowMolecular Orientation
Cross-Sectional FlowMolecular Orientation
Shear rate min max
tensile force tensile forceLow orientation
High orientation
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There should be a balance between heat input from shear and heat loss to the tool.
Cold MoldHot Plastic Melt
Heat Loss
into the Tool
HighShearRateHeat
Input
PlasticFlow
Frozen Layer
FasterInjection Rate
SlowerInjection
Rate
VS.
Cross-Sectional Heat Transfer
Cross-Sectional Heat Transfer
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Faster injection times will produce a thinner frozen layer, and a thicker flow channel.
Injection Time /Frozen layer thickness
Injection Time /Frozen layer thickness
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Pressure-Volume-Temperature
Pressure-Volume-Temperature
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Gate along edge
Final Part Mold
ShrinkageShrinkage
Shrinkage in the direction of flow is usually much greater than across the flow for un-filled materials.
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Use Design Principles and Moldflow technology so you don’t have to do this:
Design PrinciplesDesign Principles
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Number of gates– The number of gates used is based on the pressure to fill the cavity. In general,
one selects the minimum number of gates to fill the cavity. Position of gates
– The position of the gate is determined by the flow balancing principle. Flow pattern
– The mold should fill with a straight fill pattern with no changes in direction during filling.
Runner Design– The runner system is designed to achieve the required filling pattern in the cavity.
Sequence of Analysis– The procedure of the mold design always starts with the cavity.
Moldflow Design Philosophy
Moldflow Design Philosophy
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Determine the design criteria for the project Use previous experience of analyst Discuss the project with all disciplines involved in the project Use Moldflow Design Principles Use Moldflow Design Rules with the software Interpret results and make changes where necessary Discuss changes with all disciplines involved in the project Repeat Moldflow analysis to ensure acceptable results
Project Design Procedure Using Moldflow
Project Design Procedure Using Moldflow
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Unidirectional and Controlled Flow Pattern
Flow Balancing Constant Pressure Gradient Maximum Shear Stress Uniform Cooling Positioning Weld lines and
Meld lines
Positioning Weld lines and Meld lines
Avoid Hesitation Effects Avoid Underflow Balancing with Flow
Leaders and Flow Deflectors Acceptable runner/cavity
ratio
Flow ConceptsFlow Concepts
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BAD!!, Orientation is different Directions, flow marks, high stresses, & warping.
MUCH BETTER!!, Orientation in one direction, Uniform, shrinkage, & stresses.
The uni-directional flow principle says that the plastic should flow in one direction with a straight flow front throughout filling. This gives a uni-directional orientation pattern.
Uni-Directions and Controlled Flow Pattern
Uni-Directions and Controlled Flow Pattern
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Naturally balanced runner system– Same distance and
conditions between the nozzle and all the cavities
– All cavities filling at the same time pressure and temperature
Artificially balanced runner system– Sizes of the runners are
different in order to deliver plastic melt to all cavities at the same pressure so that all the cavities fill at the same time
The flow balancing principle says that all flow paths within a mold should be balanced, i.e. fill in equal time with equal pressure.
Flow BalancingFlow Balancing
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Artificially balanced runners– Limitations
» Very small parts» Parts which contain
very thin sections» Parts where sink
marks are important» Where the ratio of
runner lengths to be balanced is too great
BeforeAfter
Flow BalancingFlow Balancing
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The constant pressure gradient principle says that the most efficient filling pattern is when the pressure gradient, i.e. pressure per unit length, is constant along the flow path.
Constant Pressure Gradient
Constant Pressure Gradient
Pressure Spiking at end of Fill
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The shear stress during filling should be less than a critical level. The value of this critical level depends on the material and application.
Stress plotted above the material limit
Material: ABSStress Limit: 0.3 MPa
Maximum Shear StressMaximum Shear Stress
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Uniform CoolingUniform Cooling
When plastic is in contact with the mold, and one side is cold and the other is hot, differential cooling takes place. This causes a bowing to the hot side, as the hot side has a longer time to cool and shrink.
Hot Side
Cold Side
Tensile StressFrozen and Shrunk
Hot Side
Cold Side
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Uniform CoolingUniform Cooling
Part cross-section should cool evenly, cavity to core. If it does not in a corner, the corner will pull in to less than 90 degrees producing the typical bowed box warpage.
Heat is concentrated in the corner of the core
CavityCold
CoreHot
HOT Corner(shrinks relative to frozen sections, causing warpage)
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Position weld and meld lines in the least sensitive areas, if they can’t be eliminated.
Weld Lines are formed when twoflow fronts meet head on
Meld Lines are formed when two flow frontsmeet and flow in the same direction
Positioning Weld and Meld Lines
Positioning Weld and Meld Lines
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Avoid Hesitation EffectsAvoid Hesitation Effects
Position gates as far away as possible from where the flow divides into thick and thin flow paths to avoid hesitation effects.
Gate
Gate
Rib did not fill
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GATES make poor flow control devices
Low pressure drop in runners
Middle cavity hesitating more than right cavity
HESITATION EFFECTMaterial freezes off in the gate closest to the sprue
TRADITIONAL APPROACHFirst gate opened 0.010” in thickness and width, from 0.030” to 0.040”
Now first cavity filling much faster than other cavities
Avoid Hesitation EffectsAvoid Hesitation Effects
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A change in flow direction between the time an area fills and the end of fill
Blue Velocity Angle arrows should beperpendicular to the multi color fill contour lines
Good
Not Good!
Avoid UnderflowAvoid Underflow
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FLOI4
Arrows show direction plastic moving at the instant of fill
Avoid UnderflowAvoid Underflow
Flow front
Weld Line moves inside
frozen layer
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Subtly increase (leader) or decrease (deflector) the wall thickness to influence the filling pattern to create a balanced fill within the part.
Uniform Thickness
Balanced Thickness Balanced Filling
Unbalanced Filling
Balancing with Flow Leaders and Flow
Deflectors
Balancing with Flow Leaders and Flow
Deflectors
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Design runner systems for high pressure drops, thus minimizing material in the runner, in order to give a low ratio of runner to cavity volume.
Volume of parts 192.0 ccVolume of feed system 13.4 ccFeed system 7.0% of part volume
Acceptable Runner/Cavity Ratio
Acceptable Runner/Cavity Ratio
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QUESTIONS?QUESTIONS?