Introduction to
Injection MoldingFlow Behavior
Design Principles
Hopper
Screw (Ram)
BarrelHydraulic Unit
Tool
Injection Molding MachineInjection Molding Machine
Cooling Time
Mold Open TimeFill Time
Hold Time
Injection Molding CycleInjection Molding Cycle
Cycle Time:
Fill Time:
Hold Time:
Cooling Time:
Mold Open Time:
22Sec.
1
9
10
2
Injection Molding CycleInjection Molding Cycle
Hopper
Barrel
ScrewMold
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
FillingMold closes, screw rapidly moves forward, frozen polymer skin forms at mold walls
Packing Time (Holding)Cavity filled, packing begins, cooling occurring
CoolingPacking complete, gate freezes off, cooling continuesScrew moves back and begins plasticating resin for next shot
Mold OpenCooling completes, mold opens
Injection Molding ProcessInjection Molding Process
The Injection MoldThe Injection Mold
a.k.a. Stationary Halfa.k.a.
Moving Half
(a) (b)
Nozzle
Flow Behavior
What Does a Plastic Molecule Do in an Injection Mold?
Phases of MoldingPhases of MoldingFilling
Volumetrically fill the cavity Pressurization
Build up pressure in the cavityCompensation
Add extra material to reduce shrinkage
Filling PhasePressurization PhaseCompensation Phase
Melt
Fountain FlowFountain Flow
Describes the phenomena of how plastic flows in a moldMaterial that first enters shows up at the surface near the gateMaterial that enters the cavity last, shows up in the center downstreamHas direct influence on molecular and fiber orientation at the part surface
Shear rate - min max
Low orientation
High orientation
tensile force tensile force
Cross-Sectional FlowMolecular OrientationCross-Sectional FlowMolecular Orientation
Molecular Orientation is caused by shear flow. The high amount of shear is inside the frozen layer, therefore the highest orientation
FasterInjection Rate
SlowerInjection
RateVS.
Cold MoldHot Plastic Melt
Heat Loss
into the Tool
Frozen Layer
HeatInput
HighShear
RatePlasticFlow
There should be a balance between heat input from shear and heat loss to the tool
Cross-Sectional Heat TransferCross-Sectional Heat Transfer
MOLDFLOW Scan Injection Time - Flow 1Pressure [MPa]
Time [sec]
Injection Time /Frozen layer thicknessInjection Time /Frozen layer thickness
Faster injection times will produce a thinner frozen layer, and a thicker flow channel
Pressure-Volume-Temperature (PVT)Pressure-Volume-Temperature (PVT)
Describes the temperature/pressure relationship for polymers over the entire processing range
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
Design PrinciplesDesign Principles
Use Design Principles and Moldflow technology so you don’t have to do this:
Moldflow Design PhilosophyMoldflow Design Philosophy
Number of gatesThe 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 gatesThe position of the gate is determined by the flow balancing principle.
Flow patternThe mold should fill with a straight fill pattern with no changes in direction during filling.
Moldflow Design PhilosophyMoldflow Design Philosophy
Runner DesignThe runner system is designed to achieve the required filling pattern in the cavity.
Sequence of AnalysisThe procedure of the mold design always starts with the cavity.
Project Design Procedure Using MoldflowProject Design Procedure Using Moldflow
Determine the design criteria for the projectUse previous experience of analystDiscuss the project with all disciplines involved in the projectUse Moldflow Design PrinciplesUse Moldflow Design Rules with the softwareInterpret results and make changes where necessaryDiscuss changes with all disciplines involved in the projectRepeat Moldflow analysis to ensure acceptable results
Flow ConceptsFlow Concepts
Unidirectional and controlled flow patternFlow balancingConstant pressure gradientMaximum shear stressUniform coolingPositioning weld and meld linesAvoid hesitation effectsAvoid underflowBalancing with flow leaders and flow deflectorsAcceptable runner/cavity ratio
Orientation is different Directions, flow marks, high stresses, & warping.
Orientation in one direction, Uniform, shrinkage, & stresses.
Uni-Directions and Controlled Flow PatternUni-Directions and Controlled Flow Pattern
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
Flow BalancingFlow Balancing
The flow balancing principle says all flow paths within a mold should be balanced, i.e. fill in equal time with equal pressureNaturally balanced runner system
Same distance and conditions between the nozzle and all cavitiesAll cavities filling at the same time ,pressure and temperature
Flow BalancingFlow Balancing
Artificially balanced runner systemSizes 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
Before After
Flow BalancingFlow Balancing
Artificially balanced runnersLimitations:
• 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
Pressure spiking at end of fill before switchover lowers
pressure
Constant Pressure GradientConstant Pressure Gradient
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
Maximum Shear StressMaximum Shear Stress
The shear stress during filling should be less than the critical level. The value of this critical level depends on the material and application
• Material: ABS• Stress Limit: 0.3 Mpa
– Stress plotted is above the material limit
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 takes longer to cool and shrink
Uniform CoolingUniform Cooling
Hot Side
Cold Side
Tensile Stress
Heat is concentrated in the corner of the coreCavity
Cold
CoreHot
Hot Corner(shrinks relative to frozen
sections, causingwarpage)
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
Position weld and meld lines in the least sensitive areas, if they can’t be eliminated
Weld Lines are formed when two flow fronts meet head on Meld Lines are formed when two flow fronts meet and flow in the same direction
Positioning Weld and Meld LinesPositioning Weld and Meld Lines
Gate
Rib did not fill
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
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
Good
NotGood!
Avoid UnderflowAvoid Underflow
A change in flow direction between the time an area fills and the end of fillThe blue velocity angle arrows should be perpendicular to themulti-color fill contour lines
FLOI4
Arrows show direction plastic moving at the instant of fill
Flow front
Weld Line moves inside
frozen layer
Avoid UnderflowAvoid Underflow
Uniform Thickness
Balanced Thickness Balanced Filling
Unbalanced Filling
Balancing with Flow Leaders and Flow DeflectorsBalancing with Flow Leaders and Flow Deflectors
Subtly increase (leader) or decrease (deflector) the wall thickness to influence the filling pattern to create a balanced fill within the part
Volume of parts: 192.0 ccVolume of feed system: 13.4 ccFeed system: 7.0% of part volume
Acceptable Runner/Cavity RatioAcceptable Runner/Cavity Ratio
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
QUESTIONS?QUESTIONS?