injection moulding technology

Injection Moulding Technology - Part 3

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Injection Moulding Technology

Part 3

Mould Design & Construction

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To gain an understanding of general mould design and construction techniques, and to become familiar with the purpose and function of the various component parts.

Session aim

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By the end of the session you will be able to:

Detail the requirement for venting the mould.

Identify the component parts of a two plate mould.

List two advantages and limitations for various gate design alternatives.

State the three elements of the feed system.

Identify applications for typical runner less mould designs.

Explain the importance of mould temperature control.

Session objectives

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Mouldmaterial

Costrestraints

Mouldmaking

techniques

Mouldingmachine

Componentmaterial

Quantity

Postmoulding

operations

Mouldlife

Componentdetail

Mould Design

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Core plate Split line

Cavity plate

Fixed back plate

Cooling channel

Guide bush

Air ventDowel or Guide pin

Ejector plate

Runner

ColdSlugWell

Sprue puller pin

Push back pin

Moving backplateSupport blocks and risers

Two Plate Mould Two Plate Mould ToolTool

Locating ring

Sprue bush

Ejector pin

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Question 1

List the advantages & disadvantages of a multi-plate mould?

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Multi-plate mould tool

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Higher cost of tooling.

More moving parts.

Critical opening stroke.

Runner wastage.

Centre gating of multi cavity. Side gating easily achieved. Multi-point gating easily

achieved. Automatic de-gating.

AdvantagesAdvantages

DisadvantagesDisadvantages

Multi-plate mould tool

Stripper plate

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Question 2

On a mould that incorporates a hot runner, what might soft start facility be used for?

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Hot runner

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Ideally the sprue bush, hot tipsand the manifold should haveseparate controllers and thermocouples.On the examplefour zones are required.

The hot runner

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Hot runner systems

Increases themould height.

Material delivered to the cavities in the same condition as the melt left the barrel.

Saved material waste from the sprue & runner.

The 2 plate tool

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Question 3

Please list 3 advantages and 3 limitations when using hot runner moulds?

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Limitations of a hot runner

Increased set-up times.

Warm up period required.

Increased tooling costs.

Manifold leaks.

Higher maintenance costs.

Degradation with certain polymers.

Heat expansion problems.

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When a hot runner leaks ….it leaks !

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Advantages of a hot runner

No feed system to be removed.

Reduced mould open stroke.

Reduced shot weight.

Injection time reduced.

Automatic de-gating.

Easier to balance gates.

Increased number of cavities possible.

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When using hot runner moulds

Avoid regrind to avoid blocked nozzles.

Avoid heat sensitive polymers.

Do not flick off cold slugs.

Keep systems fully maintained.

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Question 4

What is the primary function of a stack mould?

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Braun Ireland

Stack mould - reduced projected area

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Valve to Valve

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Spring opening

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Question 5

What are the different types of ejector pins and methods of ejection?

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Combination round/blade ejection

Blade ejectors

Round ejectors

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Sleeve ejection

Core pin

Sleeve

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Good even ejectionaround bosses but quite expensive.

Sleeve ejection

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Stripper plate ejection

Stripper block

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Air off - spring returns valve to its rear position

Air ejection

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Question 6

When would mechanical or hydraulic core movement be used?

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Mechanical core pin movement

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Core pulling

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Points to remember:

If in doubt ASK!If in doubt ASK!

Core movement by hydraulics

Machines will need to be programmed.

There are many different sequences.

Must know the sequence, before opening or closing the mould.

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Question 7

When designing a mould, utilising a cold runner system, what are the primary considerations to be addressed?

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The feed system

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Runner shape

Which shape will give the best flow?

10 deg

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Runner layout

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Runner layout

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General guidelines• Single Mould

• As the number of cavities increases, so must the machine tonnage

• 2,4,6,8,12,16,24,32,48,64 cavities are typical. No odd numbers!

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Question 8

Please list advantages & disadvantages for the selected gates:

Sprue gate

Pin gate

Diaphragm gate

Tab gate

Flash gate

Fan gate

Tunnel gate

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Tab gate

Diaphragm gateSprue gate Pin gate

Flash gate Fan gate

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Runner

Submarine/tunnel gate

Component

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Submarine/tunnel gate

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Gate considerations

Size in relation to section thickness of the moulding.

Always feed into the thickest section.

Position gate to give even filling.

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Question 9

Why is mould venting required?

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Exhaust grooves

Vent depth 0.025mm

Mould venting

To prevent burning/dieseling.

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Question 10

What are the general considerations for the correct moulding temperature?

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Mould temperature control

As uniform as possible, to ensure the mould is cooled equally in all areas.

Mould temperature has the greatest influence on part quality.

High enough to allow filling of the mould, without premature freezing of the polymer melt.

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Piping up considerations

1in 2in1out 2out

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Coolant flowUsed as an alternativeto cooling channels.

Cool core pin technology

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Beryllium copper insert to improve cooling

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Cooling baffles

Baffles may be offset to achieve turbulent flow.

Must be piped correctly.

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Spiral flow cooling

Materialentering

the mouldwill be at

itshottest

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Machine Practical 1

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Part 3

Mould Design & Construction

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Part 3

Plastics Materials

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To provide an understanding of plastics materials technology and processing behaviour when injection moulded.

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Describe, in simple terms, the molecular structure of plastics materials.

State three methods of classifying plastics, giving examples.

Describe the MFI test and it’s relevance.

Identify five common additives to modern plastics.

Objectives

By the end of the session you will be able to:

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What are plastics?

Synthetic - Man made

Organic - Based on carbon

Polymers -

Describes the MOLECULES of polymers

Many units

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Polymerisation example

ETHYLENEMonomer

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POLYETHYLENEpolymer

ETHYLENEmonomer

Polymerisation example

Catalyst - heat, pressure and time

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Polymer examples

Polypropylenepolymer

Propylenemonomer

Polystyrenepolymer

Styrenemonomer

Polyvinylchloridepolymer

Vinyl Chloridemonomer

Polycarbonatepolymer

Phosgene Bis-Phenol Acetonemonomer

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Summary

Polymer chains have a coiled spring like structure.

About 5,000 times longer than their diameter.

Chains are upwards of 1,000,000 repeat units long.

Processing of polymers reduces the chain length and hence the loss of properties when using regrind.

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Classifying plastics

ThermosetsThermosets

Example - Bakelite

Curing/solidification

Heat & PressureHeat & PressureCross-linking reaction

Flows due to pressure

HeatHeat

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Bakelite

Rubber

Melamine

Polyester

(Chemical change - Boiled egg)(Chemical change - Boiled egg)

ThermosettingThermosetting

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ThermoplasticsThermoplastics

Example - Polyethylene

Flows underpressure

Heat Cool/remove heat

Solidification

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Polyethylene

Polystyrene

Polycarbonate

Polyester

(Physical change(Physical change - Ice to water)- Ice to water)

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Commodity Engineering

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Commodity Commodity

£600 - £950

£500 - £900

£600 - £900

£600 - £1200

£900 - £1500

£1500 - £2500

Polypropylene

Polystyrene

Acrylic

Price/tonnePrice/tonnePlasticPlastic

Prices are only approximatePrices are only approximate

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EngineeringEngineering Plastic Price/tonne

Acetal (POM)

Nylon (PA)

£1000 - £1200

£1100 - £1900

£1600 - £2400

£1800 - £2700

£2500 - £3500

£35000 - £40000

£38000 - £69000

Prices are only approximate.Prices are only approximate.Manifold PA66 GF35

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Amorphous versus Semi-crystalline!

What are the differences?What are the differences?

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Structural changes of polymers

Semi-CrystallineOrdered structure

Molten state

AmorphousRandom structure

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AmorphousAmorphous Semi-crystallineSemi-crystalline

Polycarbonate

A.B.S.

Polystyrene

S.A.N.

Acrylic

Polyethylene

Polypropylene

Acetal

Polyester

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AmorphousAmorphous Semi-crystallineSemi-crystalline

Wide melting range Narrow melting range

Generally clear Generally opaque

Property differences

High mould Shrinkage 1.5%-2.5%

Low mould shrinkage 0.5%

Why is crystallinity important?

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Amorphous plasticsAmorphous plastics Melt and solidify over a WIDE temperature range.

SemiSemi-c-crystalline plasticsrystalline plasticsMelt and solidify over a NARROW temperature range. (i.e they have a crystalline melting point)

Flow behaviour

Amorphous

Ease of

+Temperature

Semi-crystalline(PA)

(PMMA)

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Homo-polymers

The homo-polymer chains comprise of the same repeat unit for their entire length.

Examples: HDPE, PA, POM, PVC, PS, PP.

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Co-polymers

Co-polymer chains comprise of two or more different repeat units, which are linked together.

Examples: ABS, SAN, POM, PP, HIPS.

(ABS – Acrylonitrile-Butadiene-Styrene)

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BlendsBlends

Blends & Alloys

Two or more different polymers in the same matrix but not linked.

Noryl PPO/PS

HIPSHigh Impact Polystyrene

AlloysAlloys

Two or more different polymers in the same matrix plus a chemically adhesive additive.

Bayblend

PC/PBT

PC/ABS

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Why use additives?Why use additives?

Additives in polymers

Modify service performance.

Modify processing performance.

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Assist processingAssist processing

Modify bulk mechanical propertiesModify bulk mechanical properties

Reduce costs???Reduce costs???

Heat stabilisers Lubricants

PlasticisersReinforcing fillersToughening agents

Powdered fillers e.g. talc or chalk

Types of additives

Essential additives

= Flexible PVC

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Modify surface propertiesModify surface properties

Modify optical propertiesModify optical properties

Prevent ageingPrevent ageing

OthersOthers

Anti-static agents

Pigments and dyes

U.V. stabiliser

Blowing agentsFlame retardants

Additional additives

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Melt Flow Index apparatus (MFI)

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Melt Flow Index apparatus

Barrel

Insulation

Polymer Melt

Thermometer inside an oil filled well

Weight

Avg. weight of extruded plastic in 10 mins.

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Chain lengths can be shortened by processing or regrinding.

Chain length

Effects of processing

First pass

Lower viscosity

Higher MFI

Lower strength material

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Melt Flow Index

The bigger the value - the easier the flow.

Values can reflect relative chain lengths.

Low shear (not representative of injection moulding)

Spiral flow test better.

SummarySummary

Used as a quality control for incoming material.

Specific test conditions must be used.

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Cycolac, Lustran, Terluran ABS

Delrin, Hostaform, Kematal Acetal (POM)

Plexiglas, Lucite (Perspex) Acrylic (PMMA)

Lupolen, Rigidex, Vestolen HDPE

Styron, Lacqrene HIPS

Ultramid A, Zytel 66, Maranyl A Nylon 66 (PA 66)

Common trade names

Trade namesTrade names Family namesFamily names

Santoprene, EPDM TPE’s

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Moisture absorption - HygroscopicMoisture absorption - Hygroscopic

Not affected - drying not usually required.

POM, PE, PP, PS, PVC

Water absorbed - may need drying.PMMA, ABS, SAN, PP + talc

Damaged by water - needs drying.

PC, PET, PA, PBT, PUR

Pre-treatment of plastics

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Methods for removing moisture:Methods for removing moisture:

Vacuum oven Dehumidifying dryer

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Type of material – ABS, PC, PA plus virgin or regrind.

Temperature and time required.

Volume held in dryer to give correct throughput.

Maintenance of dryer.

Key drying considerations:

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Part 3

Plastics Materials

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Part 3

Processing & Troubleshooting

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Processing

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MouldCloses

MouldOpens

CoolingPhase

HoldingPhase

InjectionInjectionPhasePhase

Mould 99% Full

Injection Speed & Pressure

Apply Locking TonnageStart Mould Safety

Open Period

Aims during the injection phase

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Mould fill analysis

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Melt flow behaviour

Mould cavity

Fountain flow

Frozen layers

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Example of skin formation

New colourPrevious colour

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Molecular orientation

Slow injection speedSlow injection speed

Low orientation/Stiff flowLow orientation/Stiff flow

Mould cavity

Thicker frozen layersThicker frozen layers

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Molecular orientation

Mould cavity

Fast Injection SpeedFast Injection Speed

High orientation/Easy flowHigh orientation/Easy flow

Thinner frozen layersThinner frozen layers

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Residual stress

Slower filling or colder mouldsSlower filling or colder moulds

Mould cavity

Thick frozen layersThick frozen layers

High residual stress !High residual stress !

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Residual stress

Faster filling or hotter mouldsFaster filling or hotter moulds

Thin frozen layersThin frozen layers

Low residual stress !Low residual stress !

Mould cavity

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Effect of residual stress

Fast fill rate - Lower internal stress

Slow fill rate - Higher internal stress

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Injection phase

Aim to fill the mould as quickly as possible.

Uniform melt front speed as part fills.

Mould cavities fill evenly.

Ensure mould is 99% full.

SummarySummary

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MouldCloses

MouldOpens

CoolingPhase

HoldingHoldingPhasePhase

InjectionPhase

Gate Freeze-off

HoldPressure& Time

Mould 99% Full

Apply Locking Tonnage

Start Mould Safety

Open Period

Aims during the holding phase

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Polymer compression

Pressure is applied to the melt.

Melt density increases.

Material shrinkage reduces.

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Holding time (secs)2 4 6 8 10 12 14 16

Gate freeze-off

Gate frozenusing 35oC mould & 230oC melt

Gate frozenusing 55oC mould & 230oC melt

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Ensure adequate compression of the melt.

Reduce sinks and voids by compensating for the shrinkage of the polymer chains

Obtain consistent component quality by establishing gate freeze-off time.

Obtain correct component weight/dimensions by the application of a suitable pressure.

Rule of thumb:Rule of thumb:

Amorphous plastics require a low holding pressure. Semi-crystalline plastics require a high holding

pressure.

Holding phase

SummarySummary

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MouldCloses

MouldOpens

CoolingCoolingPhasePhase

HoldingPhase

InjectionPhase

Screw BackCompleted

Start Screw Rotation

Gate Freeze-off

HoldPressure& Time

Mould 99% Full

Apply Locking TonnageStart Mould Safety

Open Period

Aims of the cooling phase

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Factors to consider

Polymer type – Semi-crystalline or Amorphous.

HDT – Heat Distortion Temperature.

Capacity of temperature control unit.

Cooling time versus refill time (open nozzle).

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HOT SIDE

COLD SIDE

Mould Tool

Cooling rate, shrinkage & distortion

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Cooling rate, shrinkage & distortion

Moreshrinkage

Cold side

Hot side

Normal shrinkage effect

Residual orientation effect (stress)

Hot side

Cold side

Low orientation

High orientation

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Cooling phase

To remove heat at an appropriate rate to promote maximum service properties.

To remove the heat from all cavities evenly.

To allow the part to be ejected without distortion.

To provide sufficient time to plasticise a homogeneous mass of molten polymer.

SummarySummary

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Troubleshooting

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Troubleshooting

ASSESSMENTASSESSMENT

CORRECTIONCORRECTION

Is the fault acceptable – check the first off.

If not, then check the basics first:

The mould (damage, vents).

The material (grade, batch, moisture).

The machine (wear, malfunction).

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Residual stress

Slower filling or colder mouldsSlower filling or colder moulds

Mould cavity

Thick frozen layersThick frozen layers

High residual stress !High residual stress !

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Residual stress

Faster filling or hotter mouldsFaster filling or hotter moulds

Thin frozen layersThin frozen layers

Low residual stress !Low residual stress !

Mould cavity

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Relationship between faults & cycle steps

OUT OF CYCLECOOLINGHOLDINGINJECTION

Note:Note: Some faults can be related to more than one specific part of the injection moulding cycle.

Weight

Sinking

Dimensions

Contamination

Mica marks

DistortionFlash

Gas trap

Jetting

Flow lines

Weld lines

Dieseling

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Troubleshooting

Examine the mouldings and determine the:

Fault Cause Remedy

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Example of Fault - Cause - Remedy

Burn mark (Dieseling)

REMEDYFAULT CAUSE

Trappedgas

Clean vents.

Reduce Locking tonnage.

Reduce injection speeds or profile.

Reduce melt temperatures.

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Short termShort term

Locking tonnageHolding pressure timeCooling timeInjection speedHolding pressureChange over positionShot weightDecompressionEjector speed

Long termLong term

Melt temperatureMould temperature(Oil temperature)

Determine actual temperatures usinga pyrometer for both mould and melt.

Back pressureScrew speed

Processing variables

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Systematic approach

Eliminate faults in order of priority:Eliminate faults in order of priority:

Those that potentially damage the mould

- Flash- Gas burns/traps

Optimise the filling phase

- 95 - 99 % full

Filling phase faults

- Jetting- Flow lines- Weld lines

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Systematic approach

Holding phase related

- Voids- Sinks- Undersized

Optimise the holding phase

- Holding pressure- Gate freeze-off time- Cushion Size

Optimise the cycle time

- Cooling time

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Troubleshooting SummaryTroubleshooting Summary

Adopt a logical and systematic approach

Change one condition at a time

Consider the interaction of conditions

Keep samples

Keep notes

Be patient

Don’t be a knob Twiddler!!Twiddler!!

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Injection Moulding TechnologyPart 3

Processing & Troubleshooting

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Blade ejection

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Round ejection

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Sleeve ejection

Return

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Return

Sprue bush

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