MANUF PROCESS_Troubleshooting App. Defects in Automotive Plastic Injection Molding DCC 2007

8/14/2019 MANUF PROCESS_Troubleshooting App. Defects in Automotive Plastic Injection Molding DCC 2007

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Troubleshooting AppearanceDefects in Automotive PlasticInjection Molding

Identifying, Analyzing, andCorrecting A-Surface Issues

Kurt Beyerchen

Nissan TechnicalCenter North


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What is Quality?Phillip Crosbys Definition

Quality means conformance torequirements, not goodness.Quality is achieved by prevention, notappraisal.Quality has a performance standard of

Zero Defects, not acceptable qualitylevels.Quality is measured by the Price of Nonconformance, not indexes.


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What is a Defect?If quality is a conformance torequirements, a defect is a deviationfrom those requirementsRequirements are created based oncustomer expectations

Therefore, a defect is a deviation fromcustomer expectationsA source of dissatisfactionA limiter of use


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Customer ExpectationsHigh quality materials

Planned execution

Seamless integration


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Customer ExpectationsHigh quality materials

Appropriate glossConsistent surface finish, visual and tactile

Good touch characteristicsCharacter of textureConsistent color

A sense of value; getting more than is paid

for


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Customer ExpectationsPlanned Execution

Parts have an intended appearance; noparts look like theyre from the reject bin

The right material for the jobNo stress whitening or signals of impending failure

No warpage


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Customer ExpectationsSeamless Integration

Solid surface no appearance of cracks or splitsThought given to compatibility and harmonyUniform appearance of surface, no visible parting

lines or non-intended changes


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Communicating Customer Expectations

D e s i g n T r e n d s

i n O t h e r P r o d u c t s

S a l e s,

M a r k e t,

D e a l

e r F e e

d b a c k

P r e s s

T G W

W a r r a n

t y, J D P o

w e r,

C o m p l

a i n t s

T G R / W a n t s

C u s t o m e r S u r v e y s

J D P o w e r

OEMSPEC


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Material SelectionCustomer Specs

Appearance RequirementsImpact ResistanceChemical and UV Resistance

WeightCost

Raw Material Per PoundFinished Part After Forming

IncumbencyExisting EquipmentFamiliarity


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Material SelectionSurface Appearance

Inherent GlossMatch to Master / Surrounding Parts

Color GlossGrain

Surface AspectSink / Read Through

Paintability


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Material PropertiesInherent glossChemical ResistanceDimensional StabilityImpact ResistanceFlame Resistance / FlammabilityUseful Temperature RangeHardness

Abrasion ResistanceCreep ResistanceTensile StrengthFatigue EnduranceElectrical Conduction / ResistanceCompression SetResilience

Dielectric StrengthAdhesive CompatibilityColorant Technology CompatibilityWeight / Specific GravityTransparencyWeldabilityAntistatic

UV / Sunload ResistanceChemical ResistanceGas ResistancePaintabilityLaser MarkabilityCrystallinityCost / PriceAvailabilityRecyclabilityRegrindPost-Processing MachinabilityMelt FlowProcess CompatibilityShrinkageViscosityMelt Point / Processing TemperatureFillersLubricative PropertiesHygroscopicReinforcement

Fume Release (ex. Formaldehyde)


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Material Properties Related toMoldability

Melt Flow ViscosityMelt Point / Processing TemperatureFillersMultiple-Shot Application Compatibility

RegrindColorant / Colorant Carrier LubricativeHygroscopicityCrystallinity

Use of fillers / reinforcements


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Problem Solving AnalysisMachine

Barrel / nozzle temp, injection speed / pressure, shot size,contamination, nozzle size, pack pressure, cushion

MoldCavity / core temp, venting, runners & gates, polishing, moldcleanliness, surface finish, part thickness / geometry

MaterialMoisture, contamination, regrind, processing temperature,colorant

Operator Cycle time, post-ejection handling, process consistency


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Barrel Temperature EffectsHigh barrel temps can cause degradation of resin and colorant, causing the part to appear discolored or have black streaks

Low barrel temps may increase shearing dueto poor mixing, knitlines, jetting, flow lines

Injection Speed EffectsHigh injection speeds can cause shearing,burning, blushLow injection speeds can cause short shots,knitlines


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Identification And Correction

Of Defects

Some Common Injection MoldingProblems


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Gate Blush

What it is: Discolored area of the part, radiatingfrom the gate, resulting from shearing of material at the gate. May also be indicated by

streaking.


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Gate Blush Causes

MachineInjection fill speed too fastMelt temperature too highor too lowInjection pressure too lowNozzle diameter too smallNozzle temp too low


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Burn Mark

What it is: Discoloration of material, usuallyyellow/brown, brown or black depending onseverity. Usually located at edge of part, away

from the gate. Frequently also seen as shortshot in this area.


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Burn Mark Causes

MachineInjection speed or pressure too highScrew speed too highImproper compression ratio of screwExcessive back pressureNozzle too hot or nozzle diameter toosmallFaulty temperature controller Barrel temperature too high

MoldInsufficient venting size and/ or locationVents plugged or shutoff Improper gating size and /

or locationMaterial

Excessive regrind useExcessive lubricant

Operator

Inconsistent cycles


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Discoloration

What it is: Deviation from the intended color.May be local or throughout entire part


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Discoloration Causes

MachineExcessive residence timeBarrel temp too highNozzle temp too highExcessive cycle timeExcessive shot size ratio

Mold

Improper mold tempInefficient coolingInadequate ventingGrease, contaminated moldcomponents

MaterialContaminated materialIncorrect regrind ratioIncorrect color blending

Operator Inconsistent cycles


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Contamination

What it is: Foreign substances mixed orembedded in the part


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Contamination Causes

MachineOil and grease leaks and drips

MoldExcessive lubricationGrease, contaminated moldcomponents

MaterialContaminated materialIncorrect regrind usageExcessive moisture

Operator

Poor housekeeping


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Short Shot

What it is: Missing plastic or features not fullyformed. Edges have smooth, roundedappearance. May be accompanied by distinct

knitlines, sink, or burns in surrounding area.Generally located away from gate at the last


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Short Shot Causes

MachineInjection speed or pressuretoo low

Insufficient material feedInsufficient press capacityInadequate back pressureNozzle too smallFaulty check valve

Excessive feed cushion

MoldInsufficient ventingMold temp too lowGates / runners too smallCross-section too thinSprue bushing too longSprue diameter too small

MaterialImproper flow rateExcessive regrind use

Moisture in resinNon-uniform particle size


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Part Sink

What it is: Localized depression on the surfaceof the part, usually adjacent or above ribs orthicker areas of the part


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Part Sink Causes

MachineBarrel temp too highInsufficient injection

pressure or timeInadequate cooling timeInsufficient cushion or hold timeFaulty check valve

MoldMold temperature too high

opposite ribsGates or runners too smallor improperly placedExcessive rib thickness(>50% of wall)

Excessive part thicknessGate land too longUnbalanced flow pattern

MaterialImproper flow rate

Excessive regrind use


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Knitline

What it is: Location in part where melt frontsmeet. Incomplete adhesion occurs between thefronts, causing poor appearance and structural

weakness


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Knitline Causes

MachineBarrel temp too lowBack pressure, injection pressure, or speed too low

MoldMold temperature too lowGates or runners too small or incorrectly placedGate land too long

Material

Improper flow rate


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Jetting

What it is: Serpentine discoloration in directionof the gate.


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Jetting Causes

MachineExcessive injection speed

Barrel temp too high or too lowNozzle opening too smallNozzle temp too low

Mold

Mold temperature toolow

Gates or runners toosmall or incorrectly placed

Gate land too long

Material

Improper flow rate


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Gloss Variation Causes

Machine

Inadequate injection pressureInadequate residence timeBarrel or nozzle temp too lowExcessive feed cushionNozzle too smallInadequate cycle time


Mold

Mold temperature too lowGates or runners too small or

incorrectly placed

Inadequate venting

Inadequate polishing of moldsurfaces

Contaminated mold surfacesfrom buildup of chemicalresidue from outgassing or buildup of mold release(plateout)

Material

Improper flow rate

Inadequate lubrication

Moisture in resin


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Bubble / Blister

What it is: Gas trapped within part
http://www.rotuba.com/plastics/solutions/blisters.htm


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Bubble / Blister Causes

Machine

Injection temp too high

Injection pressure too low

Insufficient material feed

Improper injection tempprofile

Excessive injection speed

Operator

Inconsistent cycles

Mold

Improper runners or gates

Improper ventingSection thickness too great

Mold temp too low

Material

Excessive moisture in resin


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Black Specks / Streaks

What it is: Black specks or streaks visible in thepart

l k S k S k


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Black Specks / StreaksCauses

MachineExcessive residence time in barrelHang up of material in barrel or runner systemContamination of injection barrelDegradation of material due tomalfunctioning heater bands or

thermocouplesDefective nozzle shutoff Cracked injection cylinder or pittedscrewOil leaking into injection unit


MoldSprue bushing rough or notseatedBurned material caused byimproper ventingContamination from grease

or lubricantsMold too small for machinesize

MaterialContamination of raw

materialWron material for mold


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Pin Push

What it is: Stress whitening on the a-surface of the part, often accompanied by a bulge in thesurface. Occurs above ejector pins.


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Flow Line

What it is: Light or dark streaks running parallelto the direction of flow


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Flow Line Causes

MachineInadequate injectionpressureInadequate residence timeBarrel temp too lowNozzle temp too low

MoldImproper gate locationMaterial flow over ribs

Operator Improper cycling


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Flash

What it is: Excess plastic squeezing outperpendicular to the parting line, often felt as

sharp


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Flash Causes

MachineExcessive injectionpressure

Excessive shot size ratioInsufficient press capacityBarrel temp too highExcessive cycle timeExcessive residence time

MoldInsufficient venting

Inadequate coolingInadequate mold supportsPoor match of shutoff area(poor spotting)Sprue bushing too long

Improper stackupdimensionMaterial

Improper flow rateExcessive mold lubricant

Operator

P t d T li P t


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Part and Tooling ParametersAffecting Grain and Gloss

Wall with greater than nominal thicknesswill pull away from the textured moldsurface, resulting in poor grain and high

glossWall with less than nominal thickness willtend to adhere to the textured surface,resulting in a flat, chalky appearanceWall-stock variation can result in glossand grain definition variation

R l ti hi f S f


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Relationship of SurfaceRoughness to Gloss

The texturing process and subsequent media blasting introduces surface roughness to the previouslysmooth mold surfaceGloss is closely related to surface roughnessThe distribution and scale of the roughness in the plane of the surface is criticalAnother issue to consider is the refractive index of the material, and crucially (and often ignored) the

_ distribution _ and scale of the roughness in the plane of the surface - there is typically lots of microstructure in a polymer at a similar scale to the wavelength of light.Fillers are often added to the material to roughen the surface that are huge compared to this crucial

wavelength, so it is the coupling between the large and small scale roughness that is the keyLeather seems to succeed by blending large- and small-scale roughness

M ld T t Eff t


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Mold Temperature Effects onGloss

High cavity temps can decrease gloss byallowing material to flow easier, thereforeenhancing transfer of tool surface texture

High cavity temps can increase gloss byincreasing material shrink, causing decreaseddwell time on tool surface and decreasingtransfer of surface texture

How does mold temp affect gloss?It depends!

M ldi g P t Aff ti g


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Molding Parameters AffectingColor and Gloss

Mold TemperatureBarrel Temperature

Injection SpeedPack PressureScrew Speed

Cycle TimeBack Pressure

The Big

Three

M ldi g P t Aff ti g


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Molding Parameters AffectingColor and Gloss

Mold temperature has the greatest effectCan affect L*, a*, b*, Gloss

Machine barrel temp has a large effectCan affect L*, a*, b*, GlossInjection speed has a lesser effect

Mainly affects L*, Gloss; slightly affects a*Pack pressure has a slight effect

Slightly affects L*, Gloss; minor affect to a*

Molding Parameters Affecting


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Molding Parameters AffectingColor and GlossGeneral rule *

a*, b*, Gloss are positively correlatedIf measured a*, b* or Gloss increase or decrease,

the other two will follow suitL* is negatively correlated with a*, b*, GlossIf L* increases or decreases, measured a*, b*,Gloss will react oppositely

If a*, b*, or Gloss increase or decrease, L* will reactoppositely

* Rules are made to be

Color Master


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Color Master Recommendations

Since color is linked to process, develop masters inmaterials that are the same as the materials thatwill be used for parts, and set process parameterswithin normal operating ranges

If a master must be processed within a smallprocessing window, change to a combination of machine+mold+material+operator that will yieldlarger process parameters

Develop a robust process early on in the masteringprocess

Color Matching


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Color MatchingRecommendations

Since color is linked to process, try someminor changes to the process first

Select materials that are less sensitive toprocess changes

Develop a robust process early on in thematching process


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MANUF PROCESS_Troubleshooting App. Defects in Automotive Plastic Injection Molding DCC 2007

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