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FLEXO Magazine : November 2010
Technologies & Techniques 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 printing pressure around the circumference of a mandrel would not be achievable. An assumption was made: the effect of the radial runout could be virtually eliminated by moving the flexo print as- sembly (plate, anilox roll and ink chamber) in such a way that the gap width is kept constant. A test rig was built and the assumption proved to be correct. However, building a flexo printing unit with moveable flexo printing assembly was not straightforward. Investigating the mandrels showed that radial runout at the base is smaller than the radial runout at the tip. In addition, the maximum runout at the tip and the maximum runout at the base occur at different rotational angles. Therefore, the two sides of a flexo printing assembly have to be moved independently. Two servomotors are required to independently move each side of a flexo printing assembly. In addition, radial runout is specific to each mandrel. A flexo printing assembly would need to be moved in a differ- ent way for each mandrel. This means that the servo drives of the two motors in a flexo print station have to have movement profiles that are specific to that mandrel. These profiles have to be changed when another mandrel is indexed into the station; at maximum production speed, the profiles have to be updated 120 times per minute for each station. A measuring station is built into the machine to determine the radial runouts at the tip and the base of a mandrel. These measurements have to be repeated only when the mandrels are changed for printing onto tubes of a different diameter. The measurements are used to calculate two movement profiles, one for each side of a flexo printing assembly. A servo control system uses these movement profiles to control two motors that move the sides of the flexo printing assembly independently. A patent application has been filed for this Dynamic Roller Positioning system (DRP system). Setting the printing pressure with an active Dynamic Roller Positioning system ensures that even small dots are perfectly printed. When the system is switched off then there are some areas that are perfectly printed, some areas are not printed at all, and other areas where dots are squashed (Figure 4). Printing a complete image without a Dynamic Roller Po- sitioning system requires higher printing pressure, but then Figure 2. Radial runout of a mandrel. The green graph shows the base of mandrel, while the black graph shows the tip. Figure 3. The two sides of a flexo printing unit have to move independently from one another. www.flexography.org novembeR 2010 FLeXO 97 FLX_Nov10_mech.indd 97 11/1/10 2:27 PM
Sustainable Fall 2010