8/12/2019 Bag Molding

1/12

8/12/2019 Bag Molding

2/12

Lecture11

Manufacturingof

Thermoset

Composites

8/12/2019 Bag Molding

3/12

Ba Moldin

Ba moldin techni ueworkswellwithwetformin rocess andalso

withpremix/prepreg formingprocess.Itisaveryold,andyetversatile

composite

fabrication

process.

Inthisprocess,layersoffibers,impregnatedwithuncuredresin,arelaid

ontopofamold,layerbylayer.Oncethelayupiscomplete,theoverall

fiberstack

up

is

covered

with

aflexible

bag

or

diaphragm.

temperatureforpurposesofreliablycuringtheresininrelativelyshort

periodof

time.

Oncecuringiscomplete,constituentmaterialsbecomeoneintegrated

. , ,

followingwhich

they

can

be

used

in

actual

application.

8/12/2019 Bag Molding

4/12

Ba Moldin Layingupandproperbaggingofthematerialareverycriticalstepsinthe

entireprocess,astheyinfluencethequalityofpartproduced. Toensurethis,andelaboratesetupasshowninFig.11.1isrequiredforbagging.

, ,andcaul plate,whicharecoatedwithasprayofreleasecoatfilm,aswellasreleasefabric,whichensurethatthepartdoesnotgetstucktothemold late.Thecom osite artiscoveredwith eel lies which rotectitagainst

contaminating

agents

.

Further,the

laminate

is

also

covered

with

bleederplies,whichabsorbexcessresin,andbreatherplies,whichactasapathwayforairbubblesandvolatilematerialstoexitoutfromthecompositeduringcuring.Theentiresetupissealedinavacuumbagusingappropriatesealants.

Inthisprocess,whileapplicationoftemperatureisrequiredtofastenthecuringprocess,applicationofpressureisimportanttoensuregoodsurfacefinish dimensional accurac and also to eliminate resence of air and

porosity

in

the

composite

component.

8/12/2019 Bag Molding

5/12

Fi .11.1:T icalBa Moldin ProcessSetu

8/12/2019 Bag Molding

6/12

Ba Moldin

Ba moldin rocesscanbecate orized basedonthemethodof

applicationofexternalpressure,intothreegroups.

Pressure

bag

molding:

Here

pressures

exceeding

1

bar

are

applied

on

the

composite

materialbein rocessed.

Vacuumbagmolding:Inthisprocess,thecompositematerialsissubjectedtovacuumto

removeairbubblesfromthelaminate.Postthisstage,thematerialmaybesubjectedto

atmospheric

pressure

while

it

undergoes

curing

process

in

an

oven.

Autoclavemolding:Here,thecompositeissubjectedtovacuumpressures,andalso

elevatedpressuressimultaneously,whileitundergoescuringatelevatedtemperatures.

Inthisprocess,thepartisenclosedinabagconnectedtoavacuumpump.Further,the

exteriorof

the

bag

is

subjected

to

pressures

exceeding

1bar.

Finally,

curing

of

the

resin

s n a e oug a s ng e empe a u eo ma e a yp ac ng nanau oc ave

chamber.

,

othervolatiles,

increased

wetting,

and

improved

impregnation

of

fibers

with

resin.

8/12/2019 Bag Molding

7/12

Ba Moldin

Pressureba moldin isrelativel ex ensiveduetotoolin costs.Inthis

method,toolingisexpensivesinceitisintegratedwithcuringpressure

system.

Further,toolsforthistypeofbagmoldingarepartspecific.

Incontrast,

vacuum

bag

and

autoclave

molding

approaches

are

relatively

inexpensiveasthebasiccuringequipmentdoesnotchange.Figures11.2

. , .

8/12/2019 Bag Molding

8/12

Fig.11.2:SetupforPressureBagMoldingProcess

8/12/2019 Bag Molding

9/12

Fig.11.3:SetupforVacuumBagMoldingProcess

8/12/2019 Bag Molding

10/12

ResinTransferMoldin

ResinTransferMoldin commonl knownasRTM isawet rocess where

fibersandresinareplacedinthemoldseparately.

F rst, ayerso ers ormats a r cs, arep ace nt emo ayer y

layer.Onceallthelayersoffibershavebeenplaced,pressurizedresinis

introducedinthemoldtoimpregnatethefibers.Themoldcavity,into

which

resin

flows,

has

inlets

for

resin,

and

also

vents

for

air.

, .

However,incertainapplications,curingmayoccuratroomtemperature.

Inthismethod,lowviscosityresinsarechosen,assuchresinswetfibers,

andalsoenterinterfibergapsrelativelyeasily.

8/12/2019 Bag Molding

11/12

ResinTransferMoldin

RTMensureim rovedcontroloverfiberorientation asfluid ressurein

resinisnotsufficientlylargeenoughtodislodgefibersfromtheirintended

location.

Vacuumassistedresintransfermolding(VARTM)isavariationofRTM,

wherevacuumpressureaidsflowofresin,andalsowettingoffibers.

Compared

to

RTM,

VARTM

is

significantly

less

expensive.

8/12/2019 Bag Molding

12/12

e erences

1. Analysis and Performance of Fiber Composites, Agarwal,. . an rou man, . ., o n ey ons.

. ec an cs o ompos te ater a s, ones, . ., c raw

Hill.

3. Engineering Mechanics of Composite Materials, Daniel, I.

. , ., .