HPDC Design Gate and Venting

7/22/2019 HPDC Design Gate and Venting

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CAEDSHighPressureDieCastingDesign

Usually foundry technicalpersonneldesign thegating system togetherwith thedie

makingcompany.Thecastingdesignershouldbeawareofthetechnicalrestrictionsin

gatingtobeabletoproducecastfriendlyshapes.Fromthegatingpointofview,most

important istoshapethecastingsothatthemetalentersthemouldcavityfromone

side,flowswithdirectandclearroutesthroughthedieandpushesthegasesoutfrom

the

opposite

side

of

the

cavity.

Shapes,

which

block

the

metal

flow

or

form

closed

cavitieswithoutventing,arenotdesirable.Insomecasestheyarenoteventolerable.

Thegatingdesignincludesthefollowingsteps:

1. Analysisofthemetalflow2. Selectionofthebestplaceforthegateononesideofthecastingandventson

theoppositesideandselectionofasuitablediecavityfilltime

3. Divisionofthecastingintogatingsegments4. Fill time and gate area calculationsby segment; gate velocity selectionby

segment

5. PQ2analysis6. Modificationsandanewtryout

1.Analysisofthemetalflow

GateandVenting 2

An idealcastingdesignallows themetal topass thediecavitywithdirectandclear

routes.Usuallythereisaneedtocompromise.Onlyseldomitispossibletodesignan

idealgateandrunnersystem.(Seeimages.)

Image3.Acupshapedcastingwithaflange.Metalflowstartsfromthepartinglineandfinishestothepartinglineontheoppositeside.Nolargebossesoutsideorinside.Clearflowpatternandenoughspaceforthegate.

Image4.Aflatcasting.Nohighbosses.Clearroute.Thereareblindspotsbehindtheholesintheendofthemetalflowpath.

Metalentersthelastpointfromtwodirectionsanditispossiblthattherewillbeanareawherethemechanicalpropertiesarenotasgoodasontheotherareasofthecasting.

e

Image5.Acastingwithcoolingribs.Thisisnotanoptimalsolution.Ribsformclosedcavitiesoutsidethemainrouteofthemoltenmetal.

Gatedesignsteps


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Image6.Bettertogatefromthisdirection,butthereisnotasmuchspaceforgatingasinthecaseabove.

GateandVenting 3

Image7.Ablindboss.Aclosedshapeabovethecavitysurfacewithoutventing.Notagoodsolution.

Image8.Betterthanabove.Metalflowsaroundtheprojectedshapesandpushesgasestowardventingintheoppositesideofthecavity.

Image9.Betterthantheexampleintheimage7.Metalflowsthroughtheribsandgasesouttowardstheventingintheoppositeside.Ribscancausesomemoredifficulties,forexampletherecanbeshrinkmarksonthebottomofthepart.Ribsarerelativelyexpensivetomachinetothemould.

Possiblecastingdefects:

If themetaldoesnot flowefficiently through thecavity it is likely that thecastings

willhavegasporosity,gasbubblesoreven largevoids intheproblemareas.Blind

projectionsdonotfillproperly.Forexamplethecoolingribsintheimage5donotfill

withease.Onesolutiontothefillingproblemistousevacuumvalveortoadjustthe

castingshotpressurescarefully.Castingswithshapes,whichfillwiththemetalflow

ing from twodirections, tend to formcold flows.Thecasting in the image5might

endinhavingcoldflowsbehindtheholesinthelastfillingpartsofthecasting.Below

areimages

of

these

casting

defects.

Image10. Porosityandalargebubbleinsideanaluminumcastingwall.Iftheventingdoesnotworkproperortherearesomeblindareaswithoutventing,theairinsidethecavitymaymixtothemetalandformvoids.Thesevoids

canbeverylargeevenifthecastingseemsfineoutside.

Castingdefects

resultingfrom

poorflowprofile

ly


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Image11. Left:Coldflowandflowlines.Thesewilloccurifthemetaltemperaturedropstoolowbeforethediecavityistotallyfilled.Right:Anonfillinacoolingrib.Possiblecausesapoorventingorbadshotparameters.

2.

Selection

of

the

best

place

for

the

gate

on

one

side

of

the

casting

andventsontheoppositesideandselectionofsuitablediecavityfill

time

Filltime

Thecastingshouldhaveenoughspaceonthepartinglineforthegateandvents.The

gate length isthegateareadividedbythegatethickness.Thegateareadependson

selecteddie cavity fill time and gatevelocity. Die cavity fill time is selectedon the

groundsof:

Thinnestcastingwallthickness:Thickwallallowslongerfilltimethanathinwall.Thinwallstendtosolidifyprematurelyifthefilltimeistooshort.Also

theflowlengthiscritical.Iftherearelargeareasofthinwallsorthethinwalls

areindistancefromthegate,thefilltimemustbeselectedshorter.

Thermalpropertiesofthecastingalloyanddiematerials:Liquidustemperature,widthofthesolidificationrangeandthermalconductivityofthemould

material.Theseinfluencethesolidificationtime.

Combinedvolumeofthecastingandoverflows:Thinwallcastings,castingswithlongflowdistancesthroughthecavityandcastingswithspecialsurface

qualityrequirementsneedlargeoverflows.Largevolumeofthemetalisable

tokeeptheheatlongerthanasmallervolume.

Percentagesolidifiedmetalallowedduringfilling:Thebetterthewishedsurfacequalitythelesssolidifiedmetalisallowedandtheshorterthediecav

ityfilltime.

GateandVenting 4


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GateandVenting 5

OneofthebestknownformulasfordeterminingdiecavityfilltimeistheNADCAfill

timeequationbyJ.F.WallaceandE.A.Herman1:theequationtakesslightlydifferent

forms in different literature. The following equation and parameters are modified

fromMikeWard:GatingManual,NADCA,USA,2006.

t=maximumfilltime,s

TTTSZTTKtdf

fi

+= K=empiricallyderivedconstantrelatedtothethermal

conductivityofthediesteelT=characteristicthinnestaveragewallthicknessofthe

casting,mmTf=liquidustemperature,CTi=metaltemperatureatthegate,CTd=diesurfacetemperaturejustbeforetheshot,CS=percentsolidsattheendoffill,%Z=solidsunitsconversionfactor,Cto%,relatedto

thewidthofthesolidificationrange

Thepartoftheequationbetweenthebracketssetsarelationbetweentheconsumable

heatduringthecavityfilltimeandthetemperaturedifferencebetweentheminimum

flowtemperatureanddiecavitysurfacetemperature.ConstantKrelatesthistothedie

material thermal conductivityandT to the thinnest wall thicknessesof the casting.

More information about this equation canbe found in the document Runner and

gatingsystems.

ConstantKis:

0,0312s/mmbetweenAISIP20(prehardenednitratingplasticmouldsteel)steel

and

zinc

alloys

0,0252s/mmbetweenAISIH13(hotworkingtoolsteelalloyedwithchromium)andAISIH21(hotworkingtoolsteelalloyedwithchromiumand

tungsten)steeland magnesiumalloys

0,0346s/mmbetweenAISIH13andAISIH21steelsandzinc,aluminumandbrassalloys

0,0124s/mmbetweentungstenandmagnesium,zinc,aluminumandbrassalloys

Solidifiedmaterialcanbeallowedaccordingtothefollowingtable.

Table1.Recommendedpercentageofsolidifiedmaterialasafunctionof theaverage thinnestwallthickness.Ifthereisaneedtohavegoodsurfacequalityinthecasting,uselowervalues.MikeWard:GatingManual,NADCA,USA,2006.

Alumi in i Magnesium Sinkki

< 0,8 5 10 5 - 15

0,8 - 1,25 5 - 25 5 - 15 10 - 20

1,25 - 2 15 - 35 10 - 25 15 - 30

2 - 3 20 - 50 20 - 35 20 - 35

Seinmn-

paksuus, mm

Suositeltu kiteytyneen aineen mr (S), %

1MikeWard:GatingManual,NADCA,USA,2006.


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ConstantZis:

4,8C/%foraluminumalloysASTM360,380ja384,allundereutectic,lessthan12%SicontainingAlSi(Cu/Mg)alloys

5,9C/%foraluminumalloyASTM390,overeutecticAlSi(Cu/Mg)alloy 3,7C/%formagnesiumalloys 3,2C/%forzincalloys12and27 2,5C/%forzincalloys3,5and7 4,7C/%forbrass

BrassHPDCdiefilltimecanbedeterminedbymultiplyingthewallthicknesswitha

constant2:

s


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GateandVenting 7

Theformulagivesalimittothelowestrecommendedgatevelocityasafunctionofthe

gate thickness. It is not a good practice to choose a low velocity with a thin gate.

Typicalgatethicknessis0,8 3mmforaluminumalloys,0,72,2mmformagnesium

alloys,0,351,2mmforzincalloysandfrom1,5mmupto4mmforbrassalloys.

Someexamplesofgateareasandgatelengthsareinthefollowingtables(Table1and

Table2).

These

tables

are

prepared

for

total

casting

and

overflow

volume

of

0,1

dm3.

For other volumes multiply the values with the actual volume. For example: if the

casting+overflowvolumeis0,283dm3,multiplythetablevaluesby2,83.

AfterthegatelengthandthicknessapproximationsthereisaneedtodecideWHERE

toplacethegate.Therearesomebasicruleseventhougheachcastingisdifferent:

Placethickersectionsnearthegateandthinnernearoverflows.Thisarrangementdoesnotblockthemetalflowduringthesolidificationshrinkage

compensation.

Avoidtwometalfrontstoencounterindistancefromthegate(Seeimage4).Thisisanunwantedsituationonthewhole,butsometimesnotavoidable.

Frameshapedcastingsshouldbegatedfrominside. Trynottoplacethegateinfrontofcores.Highgatevelocitybrakesorwears

thecorerapidly.Iftherearecoresandtheycannotberemoved,seeifthegate

couldbedividedintosectionsbetweenthecores.Consultthefoundryanddie

makingcompany.

Possiblecastingdefects:

Gasandshrinkageporosityarethemostcommonandmosttroublecausingdefectsin

HPDC. There are contradictory opinions about the causes of gas porosity and the

influence

of

gate

velocity

to

the

problem.

One

reference

convinces

that

the

atomized

flow phenomena or high gate velocity causes the gas porosity. Another reference

recommendsusingashighgatevelocityaspossible,evenover50m/s.Itseemsthatif

othervariablesarethesame,raisingthegatevelocityreducesporosity.Buthighgate

velocitycausesexcessivediewear.Ifthegatevelocityistoolow,thismaycausepoor

flowprofileandflowporosityinsidethecasting.Itisworthnotingthatthegatingis

designedtosomefilltimeandgatevelocitycombinationandthisisthecombination

thegateworksbest.Foundryshouldkeepthedesignedparameters.

Castingdefects

resultingfrom

wronglyselected

gatevelocity

Table2.Gatevelocity,minimumfilltimeandgatedimensionrangesfordifferentalloytypes.Indicativevaluesfor11,25mmminimumaveragewallthickness.CalculatedwiththepresentedWallaceandHermanfilltimeequation.

Al 17 - 40 m/s 0.016 - 0.038 s 0.8 - 3 mm 65 - 370 mm2 43 - 215 mm 16 - 93 mm2 82 - 460 mm

Zn 30 - 60 m/s 0.011 - 0.021 s 0.35 - 1.2 mm 80 - 305 mm2 70 - 780 mm 20 - 77 mm2 100 - 380 mm

Mg 25 - 50 m/s 0.023 - 0.036 s 0.7 - 2.2 mm 55 - 175 mm2 25 - 125 mm 14 - 44 mm2 70 - 220 mm

Cu (brass) 20 - 50 m/s 0.007 - 0.010 s* 1.5 - 4 mm 200 - 700 mm2 70 - 245 mm 50 - 175 mm2 250 - 875 mm

Gate area

Gate thickness

range for the

typical ingate

velocities

Minimum fill time

range for a casting

with 1 - 1.25 mm

smallest average wall

thickness

Typical ingate

velocityAll oy

Ingate lengthMinimum vent

areaVent length**

Gate and vent measures for a cast part with total cavity and

overflow volu me of 0.1 dm3


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Table3. Indicative valuesfor 2mmminimum averagewall thickness.Calculatedwith thepresentedWallace andHermanfilltimeequation.




Cu (brass) 20 - 50 m/s 0,02 s*** 1.5 - 4 mm 100 - 250 mm2 35 - 86 mm 25 - 63 mm2 125 - 313 mm

Gate and vent measures for a cast part wi th t otal cavity and

overflow vol ume of 0.1 dm3

Gate area Ingate lengthMinimum vent

areaVent length**

All oyTypical ingate

velocity

Minimum fill time

range for a casting

with 2 mm smallest

average wall

thickness

Gate thickness

range for the

typical ingate

velocities

Table4. Indicative valuesfor 3mmminimum averagewall thickness.Calculatedwith thepresentedWallace andHermanfilltimeequation.




Cu (brass) 20 - 50 m/s 0,03 s*** 1.5 - 4 mm 70 - 167 mm2 25 - 58 mm 18 - 42 mm2 88 - 210 mm

Gate and vent measures for a cast part with t otal cavity and

overflow vol ume of 0.1 dm3

Vent length**

All oyTypical ingate

velocity

Minimum fill time

range for a casting

with 3 mm smallest

average wall

thicknessGate area Ingate length

Minimum vent

area

Gate thickness

range for the

typical ingate

velocities

* Estimatedbycalculatingwiththeformulat=sx7.

** Ventthickness0,2mm.

*** Estimatedbycalculatingwiththeformulat=sx10.

AdditionalinformationunderthetitleRunnerandgatingsystems.Diemakingcompanyand/orfoundry calculate thegatearea,gatevelocityand cavityfill timemoreprecisely.Theyhavemanyyearsexperienceinthistopicanditisrecommendedtorelyontheiropin

ion.Thetablesandequationsareonlyforchecking.

GateandVenting 8


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3.Divisionofthecastingintogatingsegments

Gatingsegmentisaportionofthecastingwheremetalflowstoarelativelycoherent

direction.Avoidclosedends:Thereshouldalwaysbeaventon theoppositesideof

the

gating

segment.

Tangential

runner

gives

good

possibilities

to

direct

the

metal

flow.(Seeimages.)

Image12. Acupshapedcastingwithnarrowribtypeprojectionsinthemiddle.Themetalflowisdirectedthroughtheflatprojectionsinthesidesofthecastingbothinupwardandsidewarddirections.Theribtypeprojectionswillcauseproblems.

Image13. Overflowsforth casting.e

Image14. Segmentedcasting.Theribshapedprojectionsformclosedcavitiesinthemiddlesegmentandmakethemetalflowmorecomplicated.Itispossiblethatsomebackflowoccursinthemiddlesegment.Backflowmixesgastothemetalandcausesporosity.

Image15. Modifieddesign.Thelongribshapeprojectionsarenowsmaller.Metalflowsbettertotherightdirectionsandtherearenoclosedshapes.

Foundry and die making company may suggest some modifications to the casting

design.Sometimesitisnotpossibletomakethesechanges.Thereasonscanbetechni

calortheycanberelatedtothevisualappearanceofthecasting.Itisrecommendedto

consultthefoundrytechnicalpersonneltofindagoodcompromisingsolutioninthese

cases. Blind cavities are cast able and canbe produced in good surface and inner

qualitywithvacuumsystemorothertechnicalsolution.

NOTE:It is thedieandgatingsystemdesignerstasktodividethecasting intoseg

ments,but

the

casting

designer

should

also

be

aware

of

this

step

to

be

able

to

make

properdesignsandtakethegateandventpositioningintoaccount.

GateandVenting 9


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4.Filltimeandgateareacalculationsbysegment;gatevelocityselec

tionbysegment

3D CAD software gives an excellent possibility to measure casting total volume,

volumefor

each

gating

segment

and

the

projected

area.

Total

volume

and

segment

volumes are used in fill time calculations. Projected area determines the required

locking force in theHPDCmachine.Thebestoption is todeliver thenative format

castingmodeltothefoundryanddiemakingcompany,buttheneutralformats(IGES,

STEP and parasolid) are also feasible. If neutral format is chosen itbetter that the

castingdesignermeasuresthethinnestaveragewallthickness.

Example:Filltime,gateareaandlength

TotalcastingvolumeVgis0,0375dm3and

total overflow volume Vo 0.0147 dm3.

Casting width is 120 mm, length 80 mm

andheight45mm.Averageminimumwall

thickness is 1,8 mm. Overflow volume is

about40%ofeachsegment,whichshould

be enough to produce a good surface

qualityforacastingwith1,8mmminimum

wall thickness. There are totally 5 casting

segmentsand6overflows.(Seeimage.)

Image16. Segmentedcastingandoverflows.The casting material is aluminum, AlSi10Mg. The process is cold chamber HPDC.

Liquidus temperatureofAlSi10Mgalloy is600Candsolidus530C. If thecasting

temperatureis690C,temperatureattheingate(Ti)isaround660C.Minimumflow

temperature (Tf) forthisalloy is570Canddiecavity temperature (Td)afteragood

productionstart340C.Only15%solidsareallowedtoobtainagoodsurfacequality.

SZwillbethen72C.Diecavitymaterialsaresomecommontoolsteel.TheconstantK

is0.0346s/mm.Withthisinformationminimumfilltime(t)willbe:

smmCC

CCC

mms

TTT

SZTT

Kt df

fi0325,08,1

260570

725706600346,0 =

+=

+=

Gatethickness(h)isselectedto1,0mm,onethirdoftheflangethickness.Gatevelocity

is selected to32m/s,as lowaspossiblenot to cause excessivediewear.Minimum

velocityat1,0mmgateis32m/s.

Gatearea(A)is:

2

33

2,50/3200325,0

0147,00375,0/ mmsdm

s

dmdm

v

t

VV

vQAg

og

g =+

=

+

==

Gatelengthwillbethen:A/h=50,2.Thecalculatedvaluewasaminimumfilltime.Itisrecommendedtouselowervalues.Ifthefilltimeiscutto70%oftheoriginalvalue,0,7 x 0,0325 s = 0,0228 s, the gate length willbe 71,5 mm. This value is accepted.

GateandVenting 10


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GateandVenting 11

Thenexttablepresentsthefilltime,gateareaandgatelengthpercastingsegment.

Table5.GatelengthpercastingsegmentSegment

Segment + overflow

volume, dm3

Minimum fill

time, s

Gate area,

mm2

Gate length,

mm

1 0.0063 0.0228 8.7 8.7

2 0.0114 0.0228 15.6 15.6

3 0.0167 0.0228 22.9 22.9

4 0.0114 0.0228 15.6 15.6

5 0.0063 0.0228 8.7 8.7

Total 0.052 0.0228 71.5 71.5

5.PQ2analysis

PQ2analysismatches the selectedgatevelocity to theHPDCmachineplungerhy

draulic system. The plunger hydraulics consists of nitrogen bottles, accumulator,

computercontrolled

valve

system,

and

ahydraulic

cylinder

to

which

the

plunger

is

attached.Thepurposeoftheplungerhydraulicsistomovetheplungerandfillthedie

cavity.(Seeimage)

Image17. Coldchamberhighpressurediecastingmachineplungerhydraulics.

Plungermovementhasthreephases:

Slowphaseduringwhichtherunnerisfilleduptothegate. Fastphaseduringwhichthecavityandoverflowsarefilled.Fastphaseisad

justedtofillthemouldcavityinthecalculatedfilltime.

Intensificationphaseduringwhichacastingispressedwithaveryhighpressure.

Gatevelocitydependsonthemetalpressureduringthefastshotphaseaccordingto

thefollowingformula4:

Pm=metalpressurePa =metaldensitykg/m32

2

=

d

g

mC

V

gP

g=gravitationalconstantm/s2

Vg=gatevelocitym/sCd=coefficientofdischarge

HPDC machines have unique pressure and velocity profiles. The coefficient of dis

chargerepresentsthevariationbetweenmachines.Typicalvalueis0,45 0,5.

4MikeWard:GatingManual,NADCA,USA,2006


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HPDC foundries analyze their machines to find out the dependencebetween the

velocityandthepressureinsidetheplungerhydraulics.Pmistheoretical,actualvalue

canbedifferent.

TheHPDCmachinesareclassifiedby their locking force.Locking force is the force,

which resists themouldopening in the endof the shot.When themould is totally

filled,ahigh

pressure

forms

inside

the

mould

cavity.

The

pressure

is

still

increased

in

thethird,intensificationphaseoftheshot.Thesepressuresformaforcewhichisthe

proportional to the projected area of the casting. Projected area is the area of the

castinginthepartingsurfacedirection.(Seeimage.)

GateandVenting 12

Image18. Projectedareaofthecastingintheimage2.Projectedareaisthecasting,runner,biscuitandoverflowsprojectioninthepartingsurface.

Metalpressurecreatesmoldbreakingforcewhichisproportionaltotheprojectedarea

withtheequationF=PxA.ThiscalculationisusedinestimatingtherequiredHPDC

machinesize.Forexample ifthe intensificationpressure is550bar=550x105N/m2,

theprojectedareaof1,49dm2createsadiebreakingforceof820kN.Thisforcewould

requirea82kilotonneHPDCmachine,which isverysmall.PresentHPDCmachine

sizevaries

from

100

to

1000

kilotonnes.

Consequenceofthedependencesbetweenthemetalpressureandgatevelocityandon

theotherhandthegatevelocityandthepressureinsidethediecavityis,thatitisnot

always possible to produce wide casting with high gate velocity and/or high end

pressure.Thereisaneedtocompromise.

6.Modificationsandanewtryout

Gating design is compromising. It is done together with HPDC foundry, casting

designerandmoulddesigner.Itislikelythattherewillbesomethingtocorrectafter

firsttryout.

There

are

also

many

other

details

than

gating

to

discuss

and

develop.

Noticethatdesigningacastablepartmaybeatimeconsumingproject.


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GateandVenting 13

References

D.R.Gunasegaram,B.R.Finnin,F.B.Polivka:EffectofFlowVelocityontheProperties

ofHighPressureDieCastAlSiAlloy,MaterialsForum29,2005

H.H.

Pokorny

and

P.

Thukkaram>

Gating

Die

Casting

Dies,

Society

of

Die

Casting

Engineers,USA,1981

J.Orkas,edit.E.R.Keil:Painevalumuotinsuunnittelu,seminar20.22.10.1998,Espoo,

Finland

MikeWard:GatingManual,NADCA,USA,2006.

P. H. Andersson, P.Jrvel, P. Peltola,J. Mkel, V. Koskenniska, M. Heikkil,J.

Saarinen, P. Mikkola,J. Kokkonen, I. Nieminen: Muotin suunnitteluja valmistus,

TampereUniversityofTechnology,Finland,2004

W.G.Walkington>DieCastingDefects/Causesandsolutions,NADCA,USA,1997

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HPDC Design Gate and Venting

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