3 03 Design Principles [Read-Only]

7/25/2019 3 03 Design Principles [Read-Only]

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Copyright 2000 Moldflow Corporation

All Rights Reserved

Introduction to

Introduction to

Injection Molding

Flow BehaviorDesign Principles


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Injection Molding Machine

Injection Molding Machine

Clamping

Unit HydraulicUnit

Hopper

Screw

(Ram)Barrel

Heater

bands

Mold cooling fed

from external unit

Nozzle between

barrel and mold

(not shown)


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Cycle Time

Fill Time

Hold Time

Cooling Time

Mold Open Time

22

Sec.1

9

10

2

Injection Molding Cycle

Injection 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 specified

pressure to the polymer melt

in order to pack more plastic

into the cavity. Also called

compensation stage.

Injection Molding Process

Injection Molding Process


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Cooling

Packing complete,gate freezes off,cooling continues

Screw moves backand beginsplasticating resin fornext 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 Phase

Pressurization Phase

Compensation 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 molecularand 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 force

Low orientation

High orientation


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There should be a balance between heat input from shearand heat loss to the tool.

Cold MoldHot Plastic Melt

Heat Loss

into the Tool

HighShear

RateHeat

Input

PlasticFlow

Frozen Layer

FasterInjectionRate

SlowerInjection

Rate

VS.

Cross-SectionalHeat Transfer

Cross-SectionalHeat 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-TemperaturePressure-Volume-Temperature


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Gatealongedge

Final Part Mold

ShrinkageShrinkage

Shrinkage in the direction of flow is usually muchgreater than across the flow for un-filled materials.


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Use Design Principles and Moldflowtechnology so you dont 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 indirection 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 PhilosophyMoldflow 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 andControlled Flow Pattern

Flow Balancing

Constant Pressure Gradient Maximum Shear Stress

Uniform Cooling

Positioning Weld lines andMeld lines

Positioning Weld lines andMeld lines

Avoid Hesitation Effects

Avoid Underflow Balancing with Flow

Leaders and Flow Deflectors

Acceptable runner/cavityratio

Flow ConceptsFlow Concepts


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BAD!!, Orientation is

different Directions, flowmarks, 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 PatternUni-Directions and Controlled Flow Pattern


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Naturally balanced runner

system

Same distance and conditions

between the nozzle and all thecavities

All cavities filling at the same

time pressure and temperature

Artificially balanced runner

system

Sizes of the runners are

different in order to deliverplastic 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 GradientConstant 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 plottedabove the

material limit

Material: ABS

Stress 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 takesplace. 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

Cavity

Cold

Core

Hot

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 cant

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


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Avoid Hesitation EffectsAvoid Hesitation Effects

Position gates as

far away aspossible from

where the flow

divides into thickand 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

APPROACH

First gate opened0.010 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 be

perpendicularto the multi

color fill

contour linesGood

Not Good!

Avoid UnderflowAvoid Underflow


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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 cc

Volume of feed system 13.4 cc

Feed system 7.0% of part volume

Acceptable Runner/Cavity RatioAcceptable Runner/Cavity Ratio

3 03 Design Principles [Read-Only]

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