Classic Pad Printing
Open Ink System
The open ink system represents the first industrial application of pad printing. With this method, the ink is supplied in an open container (small inkwell or tray). From there, the cliché is coated with ink using an ink spatula. Surplus ink is then removed by the doctor blade pressing down and moving backwards across the cliché. As a result, ink remains only in the cliché cavities. This ink is now taken up by the pad and transferred to the part to be printed.
The open system was standard practice until the 1970s. However, it presents a number of challenges to the user: The solvents in the ink can easily evaporate in the open well. Consequently, the consistency and, above all, the viscosity of the ink changes – and with it printing behavior and quality. To keep viscosity stable and therefore maintain printing quality, the ink must be regularly stirred or diluted. Furthermore, printing must be carried out in a very clean environment, to prevent dirt getting into the open ink container. Similarly, changing ink in an open system is more expensive and complicated.
For basic applications, the open system is now considered outdated in comparison with the sealed ink system. It is now only used to meet special requirements, for example, to realize particularly large print images or when using special inks (or other print media) which may only be processed using the open system.
Sealed Ink System
In the 1980s, the open ink system was refined and a sealed ink system developed. Here, the ink is supplied in an inverted cup which stands on the cliché, thereby completely sealing its opening. The doctor blade in this case is integrated into the ink cup, so that ink application and removal (doctoring) occurs in a single operation. The doctor blade functions simultaneously as a sealing ring. As a result, the cup is pressed onto the cliché and completely (“hermetically”) sealed.
Since the solvent in such sealed systems hardly evaporates, the viscosity of the ink remains constant for longer and stirring or diluting is not required as often. The sealed system also protects the ink against dirt or contamination. Furthermore, the sealed system saves one cycle, thereby making it slightly faster. It is also easier to handle when changing inks. And last but not least, the sealed ink system is generally more environmentally friendly, as it produces fewer emissions.
These advantages have made the sealed system the method of choice on the market when it comes to standard applications. It also represents the state of the art for today’s stand-alone machines.
Sealed Ink System with Crosswise Doctor Blade
When using a sealed ink system, the size of the ink cup puts fundamental constraints on the size of the print image. In turn, sealed ink cups are limited in size due to their construction.
However, it is still possible to considerably increase print image sizes in one dimension: Namely, by using a crosswise doctor blade.
With the crosswise doctor blade system, the ink cup is drawn transversely across the fixed cliché. The diameter of the ink/doctoring cup therefore restricts the size of the print image in only one dimension; the width of the print image, on the other hand, is basically unlimited.
Rotary Pad Printing
The Fastest Pad Printing Method
Classic Rotary Pad Printing
In the 1980s pad printing was perfected to enable the printing of closures. The key aspect here was high volume and therefore high speeds. A continuous (“circular”) process is generally the more suitable solution for this purpose.
In the case of classic rotary pad printing, both the cliché and the pad therefore take the form of cylinders (or are mounted on cylinders). This enables a continuous procedure which runs at a considerably higher speed and meets the volume requirements of modern closure manufacturing.
The rotary pad printing method is particularly suited to integration in complex, automated systems in which printing is only one part of the process, but one which must keep pace with the continuity and speed of the system as a whole.
The positioning of several printing units in sequence is ideal for multicolor printing.
Double-sided Rotary Pad Printing
Double-sided rotary pad printing permits fast, synchronous printing of front and back of a substrate in a single operation. To do this, two cliché cylinders and two pad rollers are positioned (symmetrically) opposite each other; the substrate is fed between the two and printed.
Double-sided rotary pad printing is also ideal for multicolor printing. In this case, the desired number of pairs of printing units is positioned in sequence.
Circular Pad Printing
360° Maximum Speed and Maximum Precision
Clocked Rotary Circular Pad Printing (360°)
Increasingly ambitious designs, initially for bottle tops then also for other substrates, called for high-quality 360° circular printing solutions. In response, the rotary pad printing method has been refined to enable 360° pad printing.
Primarily, the innovation takes the form of a freely rotating workpiece jig. This is used to transport the part for 360° printing to the pad roller, where it is rotated once on its axis (360°) and then rolled off by the rotating pad roller. This is a clocked process, owing to the way the substrate is fed through the printer. Although cliché and pad roller run continuously, transport of the workpiece jigs is interrupted while a workpiece is being printed.
Continuous Rotary Circular Pad Printing (360°)
Clocking of the workpiece slows down the 360° printing process. In order to accelerate it, clocked rotary circular pad printing has been refined further.
What is innovative about this new continuous rotary circular pad printing process is the replacement of the pad roller by a pad belt. This means the workpiece jigs can be transported to the pad belt in one continuous movement which continues uninterrupted as the workpiece rolls off.
Continuous circular pad printing can therefore be integrated once more into automated systems requiring high speeds and levels of precision.