如何解决挤出机真空口冒料问题

HOWTOFIXVENT-FLOWPROBLEMS如何解决真空冒料问题廖家志翻译ByChrisRauwendaal,PresidentfromRauwendaalExtrusionEngineeringInc.Puttingoneortwoventsbetweenthefeedthroatanddieisagoodwaytoremovemoisture,trappedair,andothervolatilesfrommeltedplasticasitmovesthroughanextruder在落料管和口模之间设置一到两个排气口，是除掉通过挤出机的塑料熔体中的湿气、夹带的空气以及挥发性气体的好方法。Fig.1—Addinganundercutdownstreamoftheventopeningallowsroomforthebankofrollingmelttoexpandandpreventsventflow图1在排气口的下阶增加一个底切口，给滚动的熔体堆留出膨胀空间，以阻止冒料Fig.2—Thevent-portdiverteratleftshowstwodesignfaults:(1)Ithasanundercutaheadofthevent,makingtheopeningtoobigandallowingtoomuchresidencetimeforswelling.(2)Itdoesn’tletanoperatorseehowfullthescrewflightsareintheventarea.Atright,thediverterwasmodifiedbyfillingintheupstreamundercutandchangingthetangentialdesigntoaradialoneforincreasedvisibility.图2-左边的排气口分流原件显示出两个设计缺陷：（1）它在排气口上游有一个底切口，这使得开口太大，给熔体膨胀留出了更多的时间；（2）它没有让操作者完全看到排气区的螺杆。在右边，分流原件做了改正，填补上游底切口，将原来的切线设计变更为圆弧设计以增加可视性。Puttingoneortwoventsbetweenthefeedthroatanddieisagoodwaytoremovemoisture,trappedair,andothervolatilesfrommeltedplasticasitmovesthroughanextruder.Butopeningsinthebarrelcanalsomakeforproblems.Oneofthemostcommondifficultiesisplasticflowingoutofthevent.Evenasmallamountofventflowhurtsremovalofvolatiles,soproductqualitysuffers.Alotofventflowcanplugtheventandforceashutdownoftheextruder.在喂料口与机头之间设置1～2个排气口可除去熔融挤出物料中的水分及其他挥发份。但机筒开口常常会出现一些问题，最常见的问题是从排气口冒料。即使少量的冒料也会影响挥发份的排出，使产品质量受到影响；大量冒料就会堵塞排气口，甚至导致停机。Twothingscauseventflow:Improperscrewdesigncausesmelttobackupunderthevent.Or,apoorventdesign“scrapes”passingmeltintotheport.Thefirststepistolookintotheventporttoseeifmeltisbackingupinthescrew.Inmostventedextruders,youcanactuallyseethescrewturnandthemeltmoveforward.(Besuretowearsafetyglasses!)Meltformsarollingbankagainstthepushingflankofthescrewflights,whichshouldfillnomorethan50%ofthechannel.Ifthescrewismorethan50%full,screwdesignisprobablycausingtheventflow.Ifthescrewislessthan50%full,thenthescrewisworkingcorrectlyandtheventportand/orinsertordiverterisprobablycausingtheventflow.冒料一般有两个原因：一是螺杆设计不合理，导致排气口处物料倒流；二是排气口设计不合理，在熔融物料通过排气口时被“挂住”。查找原因首先从排气口看螺杆中的物料是否倒流，在大多排气式挤出机中，都能看到螺杆旋转和熔体向前运动（要戴安全眼镜！）。熔体在螺槽里形成一个滚动的熔体堆，这个熔体堆占据螺槽的体积不到50%。如果螺杆的充满度大于50%，可能引起排气口冒料;小于50%时,螺杆可正常工作。冒料可能是由于排气口或分流元件设计不合理引起,如图1所示。FIXINGSCREWPROBLEMS解决螺杆引起的冒料问题Aventedextruderrequiresaspecialscrewgeometrywithmultiplestagestokeepthemeltundertheventportsatzeropressuresoitdoesn’tflowout.Anextruderwithoneventneedsatwo-stagescrew;anextruderwithtwoventsneedsathree-stagescrew.Eachstagehasitsownzero-pressuresectionfollowedbycompressionandmetering/pumpingsections.Thefirststagestartswithazero-pressurefeedsection.Subsequentstagesstartwithazero-pressureextractionsection,wheretheventislocated.排气式螺杆多采用多阶设计，此设计的优势在于排气口处为常压，物料不致流出。有一个排气口的挤出机要求有2阶螺杆,有2个排气口需要3阶螺杆。每1阶有常压段、压缩段及计量段，第1阶起始段为常压加料段，第2阶为常压排气段,也就是排气口所在的位置。Twoissuesareimportantinscrewdesignforaventedextruder.First,plastichastobecompletelymeltedwhenitreachestheextractionsectioninordertopermitvolatilestoescape.Second,conveyingcapacityinthesecondstagemustbegreaterthanthatinthefirststage,sothatthescrewchannelatthebeginningofthesecondstageisstarved.Thiskeepszeropressureunderthevent.对于排气式挤出机螺杆设计有2个主要的问题：第一，在到达排气段时，物料必须全部熔融，以排出挥发份；第二，第2阶螺杆的送料量必须大于第1阶螺杆，以使第2阶起始处螺槽未充满，这样就可使排气口处保持常压。Meltbacksupintheextruderwhenthereverseoccurs—i.e.,whenthefirststagepumpsmoreplasticthanthesecondcanhandle.Tosolvetheproblem,youhavetoeitherreducepumpingcapacityinthefirststageorincreasepumpingcapacityinthesecondstage.当第1阶螺杆的送料量比第2阶螺杆大时,挤出机中的熔体就会发生倒流。要解此决此问题,就得减少第1阶的送料量或增加第2阶的送料量。AQUICKERFIXThefastestandeasiestansweristochangeprocessconditionsratherthanreworkthescrew.Forexample,Reducingtemperaturesincreasesfrictionandviscousstressalongthebarrelorscrew,whileraisingtemperaturesreducesit.Increasingfrictionorviscosityalongthebarrelincreasesconveying,whileincreasingfrictionorviscosityalongthescrewsurfacereducesconveying.解决冒料的最简便快速的方法就是改变工艺条件。如降温、增大沿机筒或螺杆的摩擦及剪切应力，同时通过升温也可以减少摩擦和剪切应力。增大沿机筒表面的摩擦或粘度以增加输送量，同时增加螺杆表面的摩擦和粘度也可以减少输送量。（上海北京顺德注：降温或升温都是调节手段，要视具体情况选择升温或降温）Theconveyingcapacityofthefirststagecanbereducedbyseveralmeans:Increasingbarreltemperaturesinzones2and3.Adjustingbarreltemperature,usuallydownward,atthebeginningofthefirststage.Coolingthescrewinthefirststage.Starve-feedingtheextruder.Adjustingthetemperatureofthefeedhousing(bytrialanderror).第1阶的送料量可通过以下几种方式来降低:(1)升高机筒2区及3区的温度。(2)冷却第一阶螺杆。(3)采用饥饿法喂料。(4)调节喂料仓的温度(需反复实验)。Alternatively,toincreasetheconveyingcapacityofthesecondstageonecouldtrythesesteps:Reducebarreltemperaturesinthesecondstage.Increasescrewtemperatureinthesecondstage.Increasedietemperature.Openthediegapordierestriction.Changeadirtyscreenpack.Reducethenumberofscreensinascreenpack.Useascreenwithlargerholes.增加第2阶的送料量可采用如下方法:(1)降低第2阶机筒的温度。(2)升高第2阶螺杆的温度。(3)升高机头温度。(4)增大机头间隙或减少机头阻力。(5)减少过滤网的数量。(6)使用空隙较大的过滤网。Itmaynotbepossibletodetermineaheadoftimewhetherincreasingordecreasingthetemperatureoftheinitialfeedzonewillgivebetterresults.Bothshouldbetried,andtheresultscarefullymonitored.Iftheproblemcan’tbesolvedbychangesinoperatingconditions,moreexpen