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FLEXO Magazine : October 2013
defines the percentage of solid (resin) and vehicle in a low vis- cosity adhesive compound. The percentage of solids hinges on a specific formulation but is slightly flexible. By adjusting the percentage of solids, within certain limits, it is possible to influence coating weight. Low Viscosity Adhesives: These adhesives involve com- pounds formulated around a vehicle and the actual adhesive. Low viscosity adhesives are liquid at room temperature and at the time of coating. A list of the most significant coating technologies to be considered for this category of adhesives will include: • Flexo: A coating station design that allows for a high degree of adhesive blending. A flexo station can be configured as a three-, four- or five-roller setup. Increas- ing the number of rollers will increase the complexity of the design but will also improve its ability to calibrate the adhesive layer. A closed-chamber doctor blade should obviously be avoided • Rotogravure: Adhesive is contained in a set of two trays— a smaller one in contact with the rotating engraved cyl- inder and a larger one used to capture splash and over- flows. A positive oscillating doctor blade removes excess adhesive and limits transfer to the actual cell volume. Coating weight is adjusted by the engraving specification of the roller and by adjusting the percentage of solids. Adhesive is circulated, by means of a pump, between an adhesive reservoir and the coating station • Offset Gravure: This station, also known as “semi-flexo,” is configured with two rollers. The actual coating roller is a steel-and-chromed roller motorized and turning in the web direction, at a surface speed equal to web speed. The second roller, rubber coated, is also independently motorized and can turn clockwise or counterclockwise and at a surface speed equal, less than, or greater than the coating roll. By adjusting those variables it is possible to calibrate the coating weight High Viscosity Adhesive: These adhesives are high viscosity at room temperature and, in the case of solventless adhesives, also during the coating stage. • Solventless: This is a coating head design that properly doses adhesives that are 100 percent solids and classi- fied as high viscosity at the time of coating. The design is based on multiple rollers turning at an increased rate of surface speed to provide proper dosing. The adhesive passes through several rollers, all turning at different speeds. Adhesive is laid down at a differential rate of speed, set between two motors. drY LAMinAting The dry lamination process goes hand-in-hand with low viscosity adhesives. The vehicle can be either water or a combination of solvents. During dry lamination, the vehicle is removed from the coated compound in a drying oven before the lamination nip (See Picture 02-1). Picture 02-2 demonstrates a layout for machines used in the process. Dry lamination is by far the most-used configuration in adhesive lamination. The coating heads used are determined by the adhesive and coating weight. Substrates & Adhesives: Because dry lamination is the most-used lamination configuration, it is regularly seen in ap- plication on virtually every material. Every day, thousands of laminators around the world convert: • Plastic films • Foil • Paper • Foams The most interesting developments in the area of substrates have involved thicknesses. The ability to control web handling at very high levels of accuracy, thanks to new digital technolo- gies in motors and drives, has allowed for substrates to get thinner and thinner. This has been a very useful development that has allowed converters to create customized solutions targeting energy efficiency and weight reduction. Dry bond adhesive chemistry can be water- or solvent- based and include polyether and polyester urethanes, acryl- ics and polyesters. The adhesives are typically applied at 50 to 100 centipoise (CPS) and can range in application solids from 25 percent to 50 percent, depending on the chemistry. 40 FLeXo OCTOBER 2013 www.flexography.org SL Lamination, Dry bond Lamination, Wet Bond Lamination, PSA, Cold Seal, Primers, Heat Seals, Lacquering, Thermoplastics, Energy Cured, more coatings... Flexible packaging, Pharmaceutical, Security, Thermal insulation, Photovoltaic, Windows film, Tamper evident, Wiring, Labels, Wine caps, and more industries... NORDMECCANICA Spa Strada dell’Orsina, 16 29100 Piacenza - Italy phone 0039 0523 596411 telefax 0039 0523 612051 firstname.lastname@example.org NORDMECCANICA N.A., Ltd. 250H Executive Drive, Edgewood 11717 New York - Usa phone 001 631 2429898 - telefax 001 631 2429899 email@example.com Nordmeccanica N.A. Ltd is a full service company for USA, CANADA and MEXICO www.nordmeccanica.com