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?