Design Principles

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Introduction toIntroduction to

Injection Molding

Flow Behavior

Design Principles


Injection Molding MachineInjection Molding Machine

ClampingUnit Hydraulic

Unit

Hopper

Screw(Ram)

Barrel

Heaterbands

Mold cooling fedfrom external unit

Nozzle betweenbarrel and mold

(not shown)


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


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


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


The Injection MoldThe Injection Mold

a.k.a. Stationary Half

a.k.a. Moving Half


Flow BehaviorFlow Behavior

What Does a Plastic Molecule Do in an Injection Mold?


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


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


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


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


Faster injection times will produce a thinner frozen layer, and a thicker flow channel.

Injection Time /Frozen layer thickness

Injection Time /Frozen layer thickness


Pressure-Volume-Temperature

Pressure-Volume-Temperature


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.


Use Design Principles and Moldflow technology so you don’t have to do this:

Design PrinciplesDesign Principles


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


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


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


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


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


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


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


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


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


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)


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


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


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


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


FLOI4

Arrows show direction plastic moving at the instant of fill

Avoid UnderflowAvoid Underflow

Flow front

Weld Line moves inside

frozen layer


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


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


QUESTIONS?QUESTIONS?

Design Principles

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