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CAEDSMouldDesign

NumberofCavitiesandPositions 1

NumberofCavitiesandPositionsSchoolofTechnologyandManagement,PolytechnicInstituteofLeiria

Carefulconsiderationshouldbegiventothenumberofcavitieswithinamould.The

costsofmakingthemouldmustbebalancedagainsttheproductionrequirementsfor

the injectedpart.Goodmarketingdata thatcanpredict thesalesvolume for the in

jectedpartoftenarethekeytosuccessfullydeterminingthepropernumberofcavities.

Alsotheinjectionmachinemodelisverydetermentindefiningthenumberofcavities.

Singlecavitymouldsarefrequentlyusedforlimitedproductionrunsorwhenthepart

isverylarge,sothatthesizerequirementoftheinjectionmouldingmachinedoesnot

becomeexcessive.

Whenmore than one injected part, all the same, ismade in the samemould, the

mould

is

called

a

multi

cavity

mould.

It

can

be

stated

that

the

more

parts

produced

to

specificationinasinglemouldingcycle,thehighertheprofit.Theobjectiveofamulti

cavitymouldistoproducemultipleidenticalpartswithineachmouldinjectioncycle.

This ismuch simpler said thandone, even ifweassume that steeldimensions and

coolingbetweencavitiesare identical.Successfulmultiplecavitymoulding requires

thatthemeltconditionsintroducedtoeachofthecavitiesbethesame.Forinstance,a

treeorfishbonerunnerlayoutwilluselessmaterialthanmostrunners,butwillresult

in the most imbalanced filling of cavities in multicavity moulds when precision

moulding isneeded.Asmelt enters this runner system, itwill progressively reach

eachofthegatesandcavitiesalongitslength.Atarelativelyfastfillrate,inamould

withnonrestrictivegatesorcavitywall thicknesses, thecavitiesclosest to thesprue

will fill first and then progressively fill the remaining cavities along its length. In

doingthis,

each

cavity

will

be

filled

at

adifferent

pressure,

flow

rate,

and

melt

tem

perature. A balanced flow to each of the cavities in a multicavity mould will

maximizethepotentialofproducingpartswithinspecificationandprovidethelargest

processwindow.Whenproducingamulticavitymould,amouldthatallowstheuse

ofmouldinsertstodefinethecavityandcore,mightprovevaluable.Therefore,when

apartneedstobechanged,theinsertsforonepartcanberemovedandtheinsertsfor

thesecondpartputintheirplace.

Somemouldsarebuiltso that thecavitiesaredifferent fromeachother, inorder to

injectdifferentpartswith thesamemould.Thesemoulds,called familymoulds,are

usuallyconstructedsothatallofthepartsofaparticularassemblycanbemouldedat

thesametime(likemodelairplanes,etc.).Theobviousadvantageoffamilymouldsis

thatonlyonemouldneedstobeusedtomakeallofthepartsintheassembly.How

ever,therearealsosomedisadvantages.Ifpartsofdifferentshapesaretobemadein

onemould,therunnersystemshouldbesizedsothattheflowdowneachleg is the

same,thusfillingallofthemouldssimultaneously.Thesizingofrunnersystemsfor

nonuniformpartsisverycomplicatedandresultsinaslightdecreaseinpartaccuracy

inmouldswherethecavitiesaredifferentifnotproperlydesigned.

Introduction

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CAEDSMouldDesign

Experiencehasshownthatfamilymouldsaremostusefulwhensmallquantitiesofan

assemblyaretobemadeor ifthemould isjust forprototypingandseparate,multi

cavitymouldsmaking identicalparts are tobe constructed after theparthasbeen

exactlydefinedandthemarketestablished.

Whendesigningamouldforinjectionmoulding,certaininformationmustbedefined

in

order

to

start

the

design

process,

such

as

process

variables

and

process

parameters.

Processvariablesare:partgeometry,mouldingmaterial,demandsonthepart,lotsize

and delivery date. The process parameters are: number of cavities, major mould

dimensions,injectionmachinemodel,mouldcostsandcostofinjectedparts.

These variables and parameters are all dependent of each other, for instance, the

numberofcavitiesalsodependson: theavailableproduction time,productquantity

required,machineshotsizeandplasticizingcapacities,shapeandsizeofthemould

ings, and mould costs. In spite of these dependencies, the number of cavities is

directly dependent of themajormould dimensions,which isdetermined from the

injection machine model. Once the injection machine is chosen, the major mould

dimensionsareautomaticallydefined,whichthendefinethe limitforthenumberof

cavitieswithin

the

mould.

Inordertoaidthedeterminationofthenumberofcavitiesinthemould,andbearing

inmind the limitnumber of cavities, three simple formulas arepresented.Use the

minimumvaluederivedfromthefollowingformulastodefinethenumberofcavities.

ProductquantityIf the dimensional tolerance of the part is not very critical and a large number of

mouldingsarerequired,multicavitymouldsarepreferred.Thenumberofcavitiesis

dependentonthetimeavailabletosupplyaspecific lotofparts (tm), thenumberof

partsinthelot(L),thecycletimetoproduceasinglesetofparts(tc),therejectfactor

(K),expressed

as

K

=1/(1

reject

rate).

Therelationis:

tm

tcKLNC

=

ShotcapacityThe injectionmachine shot capacity is also a factor in determining the number of

cavities.Take80%ofthemachinecapacityastheshotweight(S)anddivideitbythe

part

weight

(W)

to

get

the

number

of

cavities.

Therelationis:

WNCS

=

Determinationofthe

number

of

cavities

NumberofCavitiesandPositions 2

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CAEDSMouldDesign

PlasticizingcapacityThe injectionmachine plasticizing capacity is also a factor.Divide the plasticizing

capacity(P)ofthemachinebytheestimatednumberofshotsperminute(X)andpart

weight(W).

Therelationis:

WXNC

=P

Afterthenumberofcavitieshasbeenestablished,thecavitieshavetobeplacedinthe

mouldas ingeniouslyaspossible.In injectionmouldingmachines, thebarrel isusu

allypositioned in the centralaxisof themould.Thisestablishes thepositionof the

sprue.Thecavitiesthenhavetobearrangedrelativelytothecentralsprueinsucha

waythatthefollowingconditionsaremet:

Allcavitiesshouldbefilledatthesametimewithmeltofthesametemperature.

Theflowlengthshouldbeshorttokeepscraptoaminimum. Thedistancefromonecavitytoanotherhastobesufficientlylargeloprovide

spaceforcoolinglinesandejectorpinsandleaveanadequatecrosssectionto

withstandtheforcesfrominjectionpressure.

Thesumofallreactiveforcesshouldbeinthecentreofgravityoftheplaten,inotherwords,theremustbeequilibriumoftheforcesinthemouldduring

injection.

Bearinginmindtheconsiderationsforcavitylayout,threepossiblesolutionsmaybe

used.Inthetablebelow,isillustratedthepossiblesolutionswiththeadvantagesand

disadvantagesofeachone.

CavityLayouts

NumberofCavitiesandPositions 3

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CAEDSMouldDesign

Table1:Comparisonofcavitylayouts.

NumberofCavitiesandPositions 4

CavityLayout Advantages DisadvantagesCircular Equalflowlengthsto

allcavities,easy

demoulding(especiallyofparts

requiringunscrewing

device).

Onlyalimitednumber

ofcavitiescanbe

accommodated.

Inseries Spaceformorecavitiescompared

withacircularlayout.

Unequalflowlengths

toindividualcavities

(auniformfillingis

onlypossiblewith

correctedchannel

diameters).

Symmetrical Equalflowlengthstoallcavitieswithout

gatecorrection.

Largerunnervolume,

muchscrap,rapid

coolingofmelt.

Toavoidthis,ahot

manifoldoran

insulatedrunner

systemmustbeused.

BibliographyHowtoMakeInjectionMolds;3rdedition;G.Menges,W.MichaeliandP.Mohren;Han

serPublishers

SuccessfulInjection

Moulding

Process,

Design

and

Simulation;

J.P.

Beaumont,

R.

Nagel

andR,Sherman;HanserPublishers

PlasticsMaterialsandProcessing;3rdedition;A.BrentStrong;PearsonPrenticeHall