hpdc design gate and venting
TRANSCRIPT
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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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CAEDSHighPressureDieCastingDesign
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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CAEDSHighPressureDieCastingDesign
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.
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CAEDSHighPressureDieCastingDesign
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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CAEDSHighPressureDieCastingDesign
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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CAEDSHighPressureDieCastingDesign
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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CAEDSHighPressureDieCastingDesign
Table3. Indicative valuesfor 2mmminimum averagewall thickness.Calculatedwith thepresentedWallace andHermanfilltimeequation.
Al 17 - 40 m/s 0.031 - 0.060 s 0.8 - 3 mm 42 - 190 mm2 30 - 110 mm 11 - 48 mm2 53 - 238 mm
Zn 30 - 60 m/s 0.022 - 0.033 s 0.35 - 1.2 mm 51 - 152 mm2 45 - 390 mm 13 - 38 mm2 64 - 190 mm
Mg 25 - 50 m/s 0.047 - 0.058 s 0.7 - 2.2 mm 34 - 85 mm2 16 - 60 mm 9 - 22 mm2 43 - 106 mm
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.
Al 17 - 40 m/s 0.047 - 0.090 s 0.8 - 3 mm 30 - 125 mm2 20 - 75 mm 8 - 32 mm2 38 - 157 mm
Zn 30 - 60 m/s 0.033 - 0.050 s 0.35 - 1.2 mm 35 - 101 mm2 30 - 260 mm 9 - 26 mm2 45 - 127 mm
Mg 25 - 50 m/s 0.070 - 0.087 s 0.7 - 2.2 mm 23 - 57 mm2 15 - 41 mm 6 - 15 mm2 30 - 72 mm
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.
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CAEDSHighPressureDieCastingDesign
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.
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CAEDSHighPressureDieCastingDesign
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.
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CAEDSHighPressureDieCastingDesign
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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CAEDSHighPressureDieCastingDesign
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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CAEDSHighPressureDieCastingDesign
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