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FLEXO Magazine : November 2010
Technologies & Techniques Flexo Printing Directly onto Extruded Flexible Tubes A new option for Tube Decorators By Harald gavin Printing photo-like images directly onto extruded flex- ible tubes for cosmetics or personal care packaging has long been a wish of packaging designers. Stun- ning images can be offset or screen printed directly onto flexible tubes, but neither process can print photographic quality images, despite the fact that both are capable of doing so on flat sheetfed or web-fed substrates. Recent ad- vances in servo technology make direct flexo printing onto flexible tubes possible, and machine design has fulfilled the wish of packaging designers. Traditionally, flexible tubes are dry-offset printed. Today, high-speed offset printing presses can print up to nine colors onto tubes at speeds of more than 200 tubes per minute (tpm). Halftone images and color gradients can be offset printed, but the dots tend to be deformed, which prevents printers from creating photo-like images. Offset-printed transfers a thin layer of ink onto tubes, and the resulting images can look dull on dark-colored tubes. Screen printing transfers a thicker layer of ink onto tubes. Therefore, screen printed images can look very vibrant on dark tubes and—an additional advantage—consumers can feel the ink layers when they touch the package. Advances • Flexo is now being used to produce high quality im- ages on flexible tubes. • Both sides of a flexo print unit must be moved inde- pendently. • High quality print depends on a constant gap be- tween plate and impression roller. FigurE 1. Radial runout of a mandrel. All art courtesy isiMAT gmbH siebdruckmaschinen. in screen printing inks (particularly UV inks), mesh manufac- turing and machine design enable screen printers to print outstanding images, but the resolution of the screen printing process is not sufficient to print photo-like quality onto tubes. A combination of offset printing and screen printing can be used to create images with color gradients and bold colors. The disadvantage of this combination is the need for two printing machines and therefore changeover times are longer and costs are higher. The quality of flexo and the ability to produce photo-like images caused designers of tube printing machines to explore ways of using flexographic print stations in a tube printer. High- quality printing has always held a special attraction for cosmet- ics companies marketing personal care products. The first attempts were disappointing; looking at label presses did not give an indication on how to overcome the inherent problems presented by the configuration of a tube printing machine. High-quality tube printing requires a constant gap width between flexo plate roller and impression roller. Flexo plate rollers and impression rollers are usually aligned in pairs on a multi-color narrow web press. The effect of an impression roller’s radial runout can be minimized by setting this pair for optimum printing performance. In contrast, a multi-color tube printing machine is fun- damentally different. Tubes are transferred to the different printing stations by a horizontal rotary indexing ring. A ring can have 22 stations, and each station holds a mandrel (see Figure 1). Tubes are pushed onto the mandrels and the man- drels rotate in the print station. The mandrels act as impression roller during the print pro- cess. Measurements showed that gap width variations should not exceed the range of ±0.05mm (the diameter of a normal human hair). Industrial manufactured mandrels that are cantilevered in the station heads of an indexing ring can have a radial runout of up to ±0.15mm. In a worst case, the runout of a mandrel could cause a change in gap width by several times the maximum permissible distance. A variation like this in the set gap width between a rotating flexo plate roller and a rotating mandrel would vary the printing pressure and a consistent 96 FLeXO noveMBeR 2010 www.flexography.org FLX_Nov10_mech.indd 96 11/1/10 2:27 PM
Sustainable Fall 2010