"Visual appeal linerboard" is one of the fastest growing value-added segments of the containerboard andcorrugated market. There is continued pressure to upgrade quality. For a growing number of users ofcorrugated containers, "plain brown boxes" are neither plain nor brown anymore. Many customers whomade the switch from brown to mottled white over the past decade are now seeking a surface with betteroptical uniformity. The primary motivators for growth in visual appeal liner is external to the corrugatedindustry:1. Residual demand for better print quality at an economical cost in corrugated containers;2. Changes in retailing--specifically, continued growth of mass merchandising leading to lesspersonal selling and more dependence on product information provided on the package--four sided,multicolor graphics to provide product differentiation at the point of sale.This level of quality is difficult to obtain with flexo printing on the board or box after it has been laminatedon the corrugator. The flexo printing process has been subject to numerous improvements over the pastdecade. To improve fidelity, plate makers have reduced thickness from 0.067 to 0.045 inch (1.7 to 1.14mm). In some cases, foam material is being added as a layer on the impression cylinder, a "shockabsorbing" layer, or under the plate. Photo-polymer plates have improved dot accuracy. Optimized inktransfer, targets for color accuracy and tone, and improved register are now parts of the direct print processin many plants. Printing on corrugated boxes, not preprint, has improved to include 150-line screen andprocess color. Today, many box makers are looking to achieve "preprint" graphic quality through directprint on corrugated. Achieving this may require a different combination of sheet properties and surfacecharacteristics. Some of the grades that perform well on preprint presses, such as coated SBS and coatedSUS, are problematic in direct print. The major problem area is the lack of drying in direct print operations.On a case-making machine, belts grip the sheet immediately after printing. If the surface cannot absorb theink vehicle (water) rapidly, there will be belt marks on the printed surface.Paper machine improvements have made lighter weight high performance liners a reality. The use ofbleached fiber as top liner has an unfavorable effect on sheet structural properties, especially at lowerweights. Additionally, bleached fiber usage can complicate papermaking. Since the amount of white fiberrequired per unit area to get a given brightness and optical uniformity is constant regardless of overall basisweight, the bleached fiber ratio increases as sheet weight decreases. Problems with delamination arecommon when trying to make white top liner at low basis weights. A number of box plants coat on thecorrugator, applying premixed coatings. These coatings are latex based and have large proportions oftitanium dioxide pigments. Most of the coaters used are rod coaters. With this combination, it is necessaryto apply heavy coat weights to obtain optical uniformity. Some plants have found it necessary to reducecorrugator speed when coating. Most corrugated plant managers would prefer not to coat on the corrugatorand resort to it out of economic necessity. There is a growing need for a range of coated linerboard gradesto meet the requirements of a changing marketplace. The coated surface produced needs to be scuffresistant, glueable with both aqueous and hot melt adhesives, and direct flexo printable withoutsupplementary drying. The coating must not adversely affect the strength properties of the linerboard basestock. It is practical to coat any basis weight of linerboard. Both natural kraft and recycled fiber linerboardcan be coated. Smoothness and sizing of the linerboard can impact the amount of coat weight and theformulation required. Two approaches are in use today in both on-machine and off-machine coatingoperations:1. Single coating application over a mottled white or white top linerboard2. Double or triple coating of brown linerboard.The next phase of growth will require coated linerboard that is printable by offset. Miniflute and microflutecorrugated produced with new technology glue machines can make 26 to 36-point combined corrugatedthat can be printed and converted in standard folding carton operations. The resulting boxes are not onlystronger, but the stack weight is 35 - 40% less than the same caliper of multi-ply recycled paperboard.Digital printing is the next likely revolution in printing boxes. Producing a coated surface suitable for highdefinition digital printing is likely to be the next big challenge for linerboard coaters.
展开▼
