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1、Chapter6Webformingcontractionratiorespectively(seeFig.41).Figure41.Fiberorientationanisotropyatdifferentthicknesslevelsofapapersample.FEXtwin-wirerollformingexperiments,60g/m2,hardwood/softwood-mixture,700m/min.Headboxcontractionratio8.5.Differentmix-wires
2、peeddifferences:0m/min(squares),+45m/min(circles),?75m/min(triangles)35.Fromthefigureitcanbeconcludedthat,withalowheadboxnozzlecontractionratioandnospeeddifferencebetweenmixandwires,thereisalowdegreeofanisotropythroughoutthepapersample.Fiberorientationanis
3、otropycanbeobtainedintwoways:-Byusingalowcontractionrationozzleandapositiveornegativespeeddifferenceduringdewatering.-ByusingahighcontractionnozzleandnospeeddifferenceduringdewateringAtsheetcenter,aslightanisotropyminimumappearsinthespeeddifferencecase.Thi
4、scouldbeexplainedbythepresenceoffiberflocsinthemix,whichwouldfrompurelygeometricalreasonsbesqueezedtowardthecenterduringdewatering,andonwhichorientedshearwouldhavelessdirectionaleffectsthanonfreelymovingfibers.Thelowerorientationtowardthepapersurfaceshasbe
5、ensuggestedtodependontheshortdrainagetimeforthelayersclosetothewireandthusontheshorttimesfortheorientedsheartoalignthefibers34.However,thedegreeoforientationanisotropyfollowsthesamepatternwithoutspeeddifference,atahighnozzlecontractionratio.Aspreviouslydis
6、cussedinconnectionwithFig.17thelowerdegreeofanisotropytowardthepapersidescouldthereforealternativelybeaneffectoflocalflowdisturbanceswhenthejethitsthewires,andtheevolvingdewateringpressuredeceleratesthemixbetweenthewires.Therecouldalsobelessinitialfiberori
7、entationanisotropyonthetwosidesofthemixjet,asaresultofboundarylayereffectsalongtheupperandlowerheadboxlips.TheassumptionofaconstantdewateringpressureduringrollformingresultedinthejetspeedrequiredforminimumfiberorientationanisotropylistedinTable5.Asalreadym
8、entioned,thepressureintheinitialdewateringphaseonlygraduallyreachesthestaticvalueassumedinTable5.Thismeansthat,inreality,therearedifficultiestodeterminewhatjetspeedtoactuallyaimfortogenerateminimumanisotropy.
