ABSTRACT
The term melt fracture was coined by John Tordella in 1956 [1] to describe a critical stressphenomenon.ForPE,acrylicresins,nylon,andPTFE,roughextrudatesand extrudateswithshapesthatdidnotconformtothecrosssectionsoftheopeningsof extrusiondieswereobserved.Heconsideredthatthesepolymermeltextrudatesshow smallcrackscausedbymeltfracturingthatinitiatesandpropagatesindieinlets,at and above a critical pressure. Flow visualization experiments may have inspired the fractureterminology.Tordella’sconjecturewasthatduring¦ow,asubstantialelastic strainisimposed.Theinabilityofthepolymermelttodeformelasticallyresulted inthefractureofthemelt[2,3].Polymermeltfracture,ormeltfractureforshort, hasbeenusedeversincetodescribecollectivelyallkindsofextrudatedistortions. The subsequent visual inspection of extrudates has led to the introduction of many descriptivetermsfordistortionssuchashaze,matte,lossofgloss,orangepealor orangeskin,sharkskin,pickhammered,wavy,screwthread,helical,pulsating,slipstick,spurt,gross,orchaotic.Alternatively,termshavebeende–nedthatassociate thephenomenawithaperceivedorigin.Structuralturbulenceandelasticturbulence [4,5] are notable examples. The latter nomenclature is of doubtful appropriateness, sinceturbulenceisbasedonthedissipationofenergyineddies,whichdoesnotoccur inpolymermelts[3,6].Overall,semanticshavede–nitelynotcontributedtotheclarityoftheissueandsomecontroversymayremainwithanyde–nition.Certainly,the commonuseofmeltfractureisinsuf–cienttocaptureallphenomena,andfracture may not always be related to its origin. As a more neutral and general terminology, theterms“melt¦owinstability”[6,7]or“extrudatedistortion”areused.Theformer focusesontheorigin,whereasthelatterde–nesthe–nalresult.Inthischapter,the term“meltfracture”isusedlooselytorefertoanyformofextrudatedistortion. Surfaceandvolumedistortionsoftheextrudateareconsideredastwodistinctive types of melt fracture (Table 5.1).
