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Current Trends in ContainerBoard ManufacturingAbstractRecent developments in packaging products have created a marketfor new types of corrugatedboards, namely miniflute and microflute boards. These very thincorrugated boards are manufactured out of light basis weight linerboardand corrugated medium.To be able to produce lightweight container board economically, machine speed has to be increased remarkably. In most casesliner and fluting are made ofrecycled fiber. This creates an ad- ditional dilemma: how to producelight basis weight paper fasterthan ever out of lower grade furnish that, in addition to includingweak fibers, contains a considerable amount of ash and stickies.Runnability thus becomes a major challenge.While the basis weight of linerboard and fluting are reduced,quality requirements are becoming stricter. Higher paper machineand corrugator speeds require a stronger and more homogeneous paper web. As new packaging products are mostly used for consumer packages, visual appearanceand suitability for multicolorprinting also create additionalquality requirements, especiallyfor linerboard.To meet these new challenges,Metso Paper has developed a new paper machine generation calledthe OptiConcept for Container Boards.Development ofPackaging ProductsMiniflute and microflute boardshave gained an expanding footholdin the consumer packagemarket over the past few years,which is traditionally considered boxboard territory. While conventional corrugated boards are usedfor shipping boxes, mini and microflute boards find their applicationsin various types of consumer packages. Fragile items, fastfood, medical products, cosmetics, electronics, etc. are more andmore often packed in a corrugated board box.Microflute board differs from traditional corrugated board insome respects. First of all, theheight of E, F, G and N flutes isclearly lower, typically 0.5…1.2mm, than that of conventional A,B, and C flutes at 2.6…5.0 mm(Fig.1). In addition to lower flute height, microflute boards favorlower basis weight linerboard and corrugated medium. Moreover,the top liner furnish often differsfrom traditional linerboard. It ismost commonly made of graphics board, white top linerboard, orcoated white top liner, all of which make for a high-quality printing surface. When the inside of a boxwill also be printed, the back lineris made of white furnish as well.In non-printed cases the back lineris made of testliner or brown kraftliner. The corrugated mediumis made of recycled furnish orsemi-chemical stock.SpeedAs linerboard and corrugatedmedium are sold by weight andnot by area like boxboard, the reduction of basis weight automaticallymeans the need to increaseproduction speeds (Fig.2).A traditional fourdrinier forming section, either single or multiwire,has a maximum speed of about900 - 1100 m/min., depending onthe application. The most important reason for this speed limitationis random basis weight variation. Shear forces between theslurry on the forming table and surrounding ambient air cause random basis weight variation,which increases as machinespeed increases. An openfourdrinier forming table is therefore more sensitive to randomvariations than closed formingusing a gap former (Fig.3).Machine speed also dictatesthe type of a size press. When machine speed surpasses approximately900 m/min, turbulence inthe size pond becomes unacceptable. Sizing agent pickup also becomesmore difficult to control. Asize press with film applicationcan overcome these limitations ofa pond size press.RunnabilityThe runnability of a containerboard machine is affected bymany factors, such as stock furnish, machine concept and design, machine ventilation, electricdrive system, cleanliness of the machine, and machine clothing, automation, chemicals, etc. Two subjects should be emphasized: furnish treatment and the machine concept.The recycled furnish on themarket today has a higher ash content, shorter average fiber length, and contains more stickies than was the case just a few years ago. All of these influence board properties and also the runnabilityof the machine. The higher theash content and the shorter thefiber length, the lower will be thesheet strength. As a rule of thumb,ash content should be kept below 10%.Stickies get machines andclothing dirty, and create dewatering problems at the formingand press sections. The sticking ofthe sheet to stickies on press feltsand dryer canvases, as well as onthe surfaces of rolls and dryer cylinders, promotes sheet breaks, especiallywith weaker sheets. It ishighly advisable not to save onthe cleaning of furnish when investing in a waste paper plant.The money invested will have afast payback in the form of fewersheet breaks and higher machine efficiency.The most important machinesections in terms of runnabilityare the press section and dryer section. If the machine includes asize press, attention should alsobe paid to the sheet run after sizing.The open draw, combinedwith web release from the centerroll, creates a critical runnability issue on a combination press withhigh speed and weak web (Fig.4).The biggest stress component atthis location is the centrifugalforce, which increases by thesquare of speed increase. Thestrength of the web at this positionis a function of fiber propertiesand web dryness. It is obviousthat if web strength cannot be increased, the non-supported runbetween the press and dryer sections will become a bottleneck athigher speeds. In the case of apress section with double-feltednips and a fully closed web runfrom the forming section to thedryer section the stress components at the transfer point are extremely low, thus offering a transfersolution and excellent runnabilityfor weak webs (Fig.5).The single-tier dryer sectionwas designed to support the paper web between dryer cylinders. Because the single-tier sectionhas lower drying efficiency andneeds more space than a conventional two-tier section, combineddryer section arrangements aretoday mostly used for optimized runnability and drying efficiency. Runnability after the size pressis a subject that sometimes creates the biggest hindrance to machine speed increase, especiallywith low basis weight webs of recycled furnish. If the web moisturecontent drops below 60% after asize press, special arrangementsfor web support have to be considered. Board QualityThe strength-related quality requirements of linerboard aremeasured with such tests as thebursting strength, tensilestrength, Scott Bond, and the RingCrush test. In some market areasSTFI or SCTcd (Shot Column Test,or Short span Crush Test) are alsoused instead of RCTcd. For corrugated medium the most commontests are CMT (Concora MediumTest) and RCTcd (Fig.6). Bursting strength is clearly an in-planeproperty that benefits from separately formed plies. Low headboxconsistency and a high jet/wire ratio also enhance bursting strength. A low MD/CD tensile ratioand low headbox consistencyare preferred for good RCTcd. Onthe other hand, high CMT favorshigh headbox consistency and ahigh MD/CD tensile ratio. High headbox consistency also increases Scott bond.As can be seen, some of the measurements mentioned aboveare contradictory. This means thata process condition which is beneficial in terms of one measure is counterproductive with respectto another. The machine conceptand related process should therefore be versatile enough to enableany required adjustments.In most container board machines,a size press is needed forthe required strength properties.As mentioned earlier, a pond size press cannot be used at high speeds. On the other hand, a filmsize press that allows high speedsis typically meant for surface sizing rather than deep penetration.Surface sizing will not be a problem with RCTcd as surface characteristics are essential for RingCrush. Trial runs also show that reduced size solids content may increase penetration. This will improve Scott Bond tests and CMT.In addition to strength-relatedtests, printing creates some additional requirements for linerboard.Print quality is influencedby surface finish, optical appearance, ink absorption, permeability, formation, and caliper. Surface characteristics depend on a varietyof factors, including the natureof the fibers used and their treatment during manufacture, wetend chemicals, and forming and finishing processes. One of themost important factors affectingprint quality is the surface structureof paper, generally referred toas surface smoothness. Finishing processes, such as calendering, coating or glazing, can be used to achieve a given surface structure. These factors also influence the absorbency and permeability of paper, and eventually ink laydown characteristics during printing.To obtain a good print result,ink absorbency must be balanced between the factors shown inFig.7. Not much is known aboutthese interactions, however.Last but not least, excellent MDand CD basis weight profiles, moisture, and fiber orientation area necessity when producing highquality linerboard and corrugatingmedium efficiently. Obviousrunnability problems will otherwise exist not only with the papermachine itself but also with converting machines.Machine ConceptSelectionForming SectionWhenever lightweight containerboard needs to be produced athigh speeds, a gap former forming section is a must. A gap formeris also recommended for slower machines if the SR-value of thestock is high (Fig.8). Dependingon board grades and basis weight ranges, combined forming section alternatives may also be considered. If only corrugated medium willbe produced, a single gap formeris the right choice for the forming section (Fig.9). In some cases, depending on market requirements, low-grade testliner canalso be produced with this forming concept. In that case the headboxwill be of two channel type.The basis weight range of a single gap former is limited, however, and depends on stock furnish (Fig.10). For high-quality testliner, aforming section with two gap formers is needed because ofhigh strength and good formation requirements (Fig.11). Withthis concept proper layer puritywill be reached, along with excellent coverage, formation, andclearly better bursting strength.In the case of white top liner, agap former–fourdrinier combination can be considered (Fig.12).This concept is recommended when virginlong fiber is used forthe top ply, whichneeds low headboxconsistency for goodformation and inplanefiber orientation.For the ultimateformation of whitetop ply, a top-dewateringunit, the Sym-Former MB, may stillbe added on top ofthe top ply fourdrinier.Press SectionMachines dedicatedto high speeds andlight basis weights canbe equipped with acombination press,such as the SymPressB(Fig.13). It offers theadvantage of a closedsheet run through allthree press nips, adouble felted 1st nip,and a shoe nip in the last press position. It also needs relatively little space in the machine hall. Asthe press concept includes onlythree felts, its dewatering capacityis limited and it cannot be usedfor heavy basis weights. On theother hand, the open draw afterthe press section limits the runnability of very light basis weights.The other press option forhigh-speed production is a press section with two straightthrough presses, such as the OptiPress (Fig.14). Its closed web runand four felts offer maximal runnability without any practical limitationson dewatering.Dryer SectionDryer section development follows closely the evolution ofprinting paper machines. Singletier dryer groups are preferred inthe early part of the dryer sectionin order to give full support to the weak wet web. Double-tier dryergroups are preferred at the latterpart of drying for higher drying efficiency. This SymDry concept (Fig.15) offers excellent runnability with high drying efficiency. Italso offers control of moisture on both sides to control curl. Machine hall length will be shorterwith SymDry than with a full single- tier dryer section. The dryerfabric gives 100% support to the paper web in a single-tier dryer section. Fluttering may appear, though, because of the rotation of cylinders. To minimize fluttering, Metso Paper has developed a series of blow boxes (Fig.16). Specially designed boxes can be locatedat various positions on thepress and dryer sections. They create a vacuum that counters opposing forces at the right locationsin order to keep the paperweb in contact with felts and fabrics, thus improving runnability.Size PressMetso’s size press for high-speed machines, the OptiSizer, uses filmtype application (Fig.17). Whenboth high speed for light basisweights and penetration of sizefor heavy basis weights are needed,a press can be built to use bothpond and film application. Therun of the web from the size pressto the after-dryer section, as wellas the layout of the after-dryersection itself, should be considered case by case because of var-ying runnability requirements.CalenderCalendering is the most importantpart of the papermakingprocess with respect to surface smoothness. In addition to smoothness, calendering also influencescaliper and density.Some paper strength tests maythus be affected. The amount ofnips can be reduced by applyingheat and an extended dwell timein the calender, thus minimizingthe loss of caliper. Single-nip hot calendars, the OptiHard and OptiSoft, meet modern calenderingrequirements for linerboard. Depending on stock furnish and thesurface requirements for final paper,a calender nip can be eitherhard or soft. As smoothness requirements for white top linerand coated WTL are higher thanfor brown grades, two hot nips areoften recommended. No calenderis needed for corrugated medium. SummaryThe development of packagingproducts subjects containerboard makers to simultaneousdemands for higher board qualityand higher production rates.Metso Paper’s new machine generation, called the OptiConceptfor Container Boards (Fig.18), isthe solution to meeting these requirements. It is designed forhigh-speed operation with highefficiency. It is also the key to improved board quality for modernmicroflute boards and multicolorprinting.Generated by Foxit PDF Creator © Foxit Software For evaluation only.。