Product Design For Plastic Technology

8/9/2019 Product Design For Plastic Technology

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PST66

Principles of Product Design

Teaching Methodology: Lecture

Course Work : 60%

Final examination 2-hour paper: 40%


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PST66


References

Morton-Jones, D.H.and Ellis, J.W., Polymer

Products , Chapman and Hall.

Lockett, F.J., Engineering Design Basis for

Plastics Products, HMSO

Malloy, Robert A.,Plastics Part Design forInjection Molding: An Introduction,,(1994)


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PST66


Product design consideration; from the concept of design

to the drawing of selected design in view of the product

application and functionality.

Selection of appropriate materials according to design,manufacturing and cost of production.

Analysis of the preliminary designs for uniformity of

section thickness, strength and assembly.

Mechanical behaviour for structural designs; Stress analysis for polymers, dynamic and cyclic loading,

static loading and stiffness.

Designing for quality,

Function, production, economics, rigidity and toughness


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PST66


Mould design :Sprue, runner and gate designs.

Design for manufacturability

Mould filling, shrinkage, warpage and tolerances

Design for assembly

Screw fittings, press fit, snap fit, metal inserts

Welding of plastic products

Hot gas, hot plate, friction, electric resistance, magnetic

induction, radio frequency (microwave) and ultrasonic

Decoration in plastics

Self colouring, marbling effects, painting, metallization, hot

and mould foiling and printing


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Concept of Design to the Drawing of Selected Design in

View of the Product Application and Functionality


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Selection of Appropriate Materials According to Design,

Manufacturing and Cost of ProductionMake Molding Simple


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Manufacturing and Cost of Productionheat vs density of melt


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Manufacturing and Cost of Productionheat vs density of melt


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Selection of Appropriate Materials According to

Design, Manufacturing and Cost of ProductionMelt at Rest vs During Flow

Polymer chains are forced tochange from their random coilstate to an elongated coil

The degree of elongation/alignment is dependent on thenature of polymer and shearrate

Shear rateis dependent on the

channel dimensionDecrease of channel dimensionwill increase the shear rate


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Shear rate - min max

Low orientation

High orientation

tensi le forc e tensi le forc e

Selection of Appropriate Materials According to

Design, Manufacturing and Cost of ProductionMolecular Orientation

Molecular orientation is caused by shear flow

The high amount of shear inside the frozen layer, produce

high orientation


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Manufacturing and Cost of Productionorientation vs nisotropy

Molecular Orientationcaused by

weak/strong anisotropy depending

on the direction of orientation

The difference in anisotropy caused

Shrinkage to occur.Shrinkage is greater in the flow

direction (orientation)than in the

cross direction (transverse)


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Summary

Flow Patterns

Orientation

Shrinkage


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Manufacturing and Cost of ProductionCOMPENST ION FOR SHRINK GE


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Orientation that are Frozen

Stressthat weaken product and causes

failure at lower applied stress levels. This

caused cracks to propagate in the direction

of floweasily. This can also leads todimensional instability

However, when heat is applied, it inducesmolecular relaxation which produces

warpingor distortions.


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Frozen orientation (cont)


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Frozen orientation (cont)


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Pseudo Plastic Flow behaviour

Orientation Extension Deformation Destruction

of Aggregates

Liquid not sheared

Liquid sheared


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(Dynamic)Viscosity h

Shear stress t

Shear/deformation gShear rate g

t = h * g

y

A

F

A

[ = Pa]Nm2

g =dxdy

[ = ]1s

m

s * mg =dvdy

=dg

dt

v,F

x

.

.

.

Area A Force F


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Dynamic viscosity h[Pas]

t

= shear stress [Pa]

g= shear rate [1/s]

1 Pas = 1000 mPas

1 mPas = 1 cP (centi Poise)

Kinematic viscosity n[mm2/s]

= density [kg/m]

1 mm/s = 1 cSt (centi Stokes)

h

=n

g

t=h

.


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Newtonian Flow Behaviour

0 5 10 15 20 25 30 35 40 45 50

[1/s]

0

50

100

150

200

250

300

350

400

450

500

[Pa]

1

10

100

[Pas]

Flow curve

Viscosity curve

.

Constant Proportionality between Shear Stress and Shear RateViscosity remains constant


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0 50 100 150 200 250 300 350 400 450 5000

20

40

60

80

100

120

0.1

1.0

Flow Curve

Viscosity Curve

[1/s].

[Pas]

[Pa]

At constant time, if viscosity decreaseswith shear rate

SHEAR THINNING

Non Newtonian Behaviour of Plastics

Shear Rate Dependent


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0 50 100 150 200 250 300 350 4000

500

1000

1500

2000

2500

3000

1

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Non Newtonian Behaviour of Plastics

Shear Rate Dependent

Flow Curve

Viscosity Curve

.

[1/s].

[Pas]

[Pa]

At constant time, if viscosity increaseswith shear rate

SHEAR THICKENING


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At constant shear rate,if viscosity

Decreases with time :THIXOTROPYIncreaseswith time : RHEOPEXY


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Heating and Cooling

Problems:

Dimensional Tolerance Internal Stresses

Dimensional Stability During Service

Plastics has a high thermal expansion and contraction

Coefficient of linear expansion,

= expansion / (original length x temperature rise)

Coefficient of cubical expansion,

= 3

The expansion of the material per degree rise in temperature

Typically a 1000C rise in temperature produces an increase

between 0.005 and 0.2 mm/mm depending on the material grade

and the molding conditions


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Shrinkage

Excessive decrease in dimension in a part after processing or cooling

Typically due to:

Temperature gradient

Rate of cooling Pressure during shaping

Anisotropy due to orientation

Amount of crystals

Semi crystalline Vs Amorphous

PS do not shrink in comparison to PE

Degree of coolingPET can produce up to 50% crystallinity

when cooled rapidly


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Shrinkage (cont)


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Shrinkage (Rectify)

Inside Mould

Adjust the mould temperature. A cold mould solidifies and forms aplastic skin sooner than a hot mould, resulting in a shrinking of plasticbefore full injection pressure is applied. However, a hot mould allowspolymer melt to continue to move and be compressed by injectionpressure before solidifying.

Typically, a 10%change in mould temperature can result in a 5%change in original shrinkage.

Inside Barrel

Adjust the barrel temperature while plastic resides in the barrel

In general, the higher the plastic temperature, the greater the amount

of shrinkage.This is because of the increase in activity (expansion ofmolecules) of the individual plastic molecules as the temperaturerises.

Typically, shrinkage rates can be changed 10 % by changing barreltemperatures 10 %


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Shrinkage (cont)

Minimize by:

Incorporation of fillers

Thermal expansion of the plastics lowered.

However, fillers may affect dimensional

stability and produce anisotropy

Optimise mould/melt temperature, reduce

variation in temperature Reduce variation in wall thickness.


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Warpage

Moulded part is twisted or bent from the intended shape

after ejection.Common in thin walled containers and large

flat moulded parts

Some possible cause:Differential shrinkage within component

Remedies, to check:

Mould temperatures for both halves of the mould

Injection rate- may be too slow (or too fast). Mould cooling - avoid differential cooling


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Warpage


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Warpage


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Part Shrinkage Versus Mould


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Part Shrinkage Versus Mould


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Temperature and Rheology

On application of heat, molecules vibratesand mobility increased

Viscosity, for at given polymer melt at different temperaturesaresuperposable by shift at constant stress


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Temperature and Rheology

Viscosity is lower at high temperatures

The elastic modulus of polymers is less sensitive to temperature change


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Table compares the Relative Fluidity Index value (RFI) at different temperatures for different polymers


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Due to Non-Newtonian character of the polymer melts, superposition offlow curves at produces temperatures shiftat constant stress.

The viscosity dependence on temperature decrease at high shear rates


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Importance of Temperature Monitoring in Rheology

In polymer processing, high temperature is expensive

More energy is required to raise the temperature.Time necessary to cool the material to form a stable

product.May lead to decomposition

Thus, in practice we usually seek to process in thelowest temperature in which the temperature willincrease with work output

Hence, if material can be softened by heat input early inthe process, then non-uniform heat generation can beavoided at the later stage in the processing


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Effects of Pressure Pressure reduces both free volume and molecular mobility so leading to an

increase in viscosity

The influence of pressure on viscosity is qualitatively similar but opposite insign to that of temperatures.

ressure may be considered as the negative of temperature

The application ofP (differential pressure) increases the viscosity;T is the differential temperature rise viscosity

( T / P) where = viscosity

However instantaneous temperature rise, T resulting from theinstantaneous pressure, P.( T / P) s,

where s is the entropy which is a measurement of disorder.


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The correlation suggests that if no direct measure of the influence of pressureon viscosity is available, then the thermodynamic function may be used as aguide.


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In polymer processing, the combination of high pressure and lowtemperaturewill tend to promote the crystallizationof some materials sothat in some cases, the harder one pushes, the less the material

will flow.


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Sinking

Depressionin a moulded product caused by

shrinking or collapsing of the resin during cooling.

Main cause: As the resin changes from a molten state to a

solid state, it occupies a smaller volume (this iscalled shrinkage). As more and more of themolten resin solidifies a vacuum is formed in thethicker sections and this tends to pull the surfaceof the moulding inwards and forms a depressioncalled a sink mark


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Sinking


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Sinking


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Ways to Reduce Sinking

Hold-on pressure - too low

Hold-on pressure time - too short

Mould temperature - too low.

Gateincrease in gate number

Part sectiontoo thick

Incorporate fillers


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Thick sections

When moulding thick sections, the surfacelayer becomes hotter than the interior layers

Due to poor conductionof the polymersexpansion on the surface is g reater than theinter iorhence developing differentialexpansion

Requires the removal of heat efficiently, thusmore energy needed for cooling


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Uniform Wall Thickness

Constant wall thickness or gradual transition between thickan thin wall section

Ratio of thick to thin is 3: 1.

All surfaces perpendicular to the parting lines are tapered to

assist ejection of the components from the mould

Any abrupt changes to flow leads to internal stresses anddifferential thermal gradient.

These stresses can lead to an area of tension and

compression that will result in warping and fracture. However,the thermal stresses can be reduced by annealing duringmoulding or post moulding.


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Corners

Two walls adjoin andinterrupt the smooth flow

of melt.

Set up internal stresses,

especially with fiber

reinforced materials

Act as notched which

encourages failure

Should be designed with

the radius rounded


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Part Thickness and Corners

Range 0.5 mm to 4 mm(0.02-0.16in), dependent

on the part design and

size.

A molded piece shouldhave uniform thickness

throughout


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Corners

Bosses


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BossesBosses are used for mounting/fasteningpoint purposes

or to serve as reinforcement around holes


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Boss


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Boss

To support moulded parts or studs for assembling

components.Metal inserts should not have sharp

corners and boss must sink into the inserts on

cooling because of higher coefficient of plastics

Can be incorporated via ribsat corners or along theside of the wall

Ejector pin must be incorporated at the base of

each boss at the cavity side cavity to facilitate

extraction. This also allows air to escape, thusavoid burn mark on the surface and incomplete

filling


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Boss


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Weld line

HEAD ONof 2 flow front

Multi gating system

Caused local weakness

Inconspicuous areas

PARALLEL FLOW of 2

flow front

Flow around pins

Caused local weakness

Inconspicuous areas

Melt line


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Weld lines


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Weld lines


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Designing for Stiffness

Resist deflection under bending load; deflection (Y)

is inversely proportional to the stiffness factor(1 / ( E I ))

Two ways of resisting Y

select material with high E

select a suitable cross section geometry & design, I

Important :

shear ( torsion) forces and deformation must be

considered as a cross section may have highresistance to bending but not necessarily totorsional shear

Reinforcing ribs


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Reinforcing ribs Effective way to improve the rigidity and strength

Save material and weight

Shorten molding cycles and eliminate heavy crosssection

Disadvantage, may produce warpage and stress

concentration


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Analyzing Defects


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Chart of Defects

Documents

Product Design For Plastic Technology