The DFTA Flexographic Printing Technology Center makes promising advancements in flexo printing processes
Flexography (also known simply as flexo) is a high-pressure printing process involving flexible relief plates. Only the protruding areas of the plate come into contact with screen rollers and the material being printed on. Depending on the nature of the print and the amount of pressure required, printing plates sometimes adopt irregular shapes, similar to the way a roughly treaded or damaged car tire can become squashed slightly during travel. Uncoiling can result in vibrations in the printing machine, and in extreme situations the material being printed on is left with horizontal stripes. Researchers at the DFTA Technology Center, a Steinbeis Transfer Center based at Stuttgart Media University, have now registered a “Full Contact” flexo printing process for patent. The new technique simplifies the use of printing machines and drastically reduces the degree of contortion resulting from printing plate production. The technique has every potential to make flexo printing even more competitive.
The ultimate indication that somebody has mastered a specialist discipline is that they observe and analyze a process and come up with a new solution. The DFTA sees this as one of its most important tasks. As a result, a team of researchers has been looking closely at deformations in printing plates during printing. A string of tests showed that the only way to eradicate residual stripes after printing was to avoid “process resonance” by adjusting printing speeds.
The question was, could another technique be found to address disharmonious printing? The DFTA experts started by looking at the fundamentals of high-pressure printing processes. Differences between printing surfaces and non-printing surfaces already emerge when printing ink is applied. By only allowing the upper surfaces of anilox rollers to touch the raised relief areas on a plate (which takes on ink), only these parts of the plate can transfer ink to the substrate. This is of course nothing new to the experienced printing expert. But the researchers decided to think one step further. The experts conducted a series of experiments to raise the entire non-printing surface on the printing plate – the relief base – and position it so high that there is virtually “full contact” in the printing gap between the printing plate and the substrate. Another way to think about this is that the relief depth is reduced to such an extent that the entire base area of the relief along the printing line comes into contact with the printed surface. This full contact between the plate cylinder and the printing medium allows the feed to roll off extremely harmoniously, because protrusions on the plates are hardly detectable (less than a hair’s breadth) so they no longer get in the way.
Vibrations and the horizontal stripes these leave on the print should thus be a thing of the past. At the same time, a whole range of other advantages are gained. The main kind of printing plate used in the European flexo printing market is made from photopolymer materials. The manufacturing times or costs for these materials are largely dictated by the depth of monomer materials used to create relief, and these have to be washed out or removed using thermal techniques. In the DFTA experiments, only one tenth of a millimeter of relief depth made a huge difference in shortening the time needed for the main exposure of plates, washing, developing heat, and drying. This time saving is passed on to the printing houses, which also save solvents and energy, and thus free up resources.
One knock-on benefit of flatter relief formations on photopolymer printing plates is invaluable: improvements in the accuracy of multicolor prints. The experiments being carried out at the DFTA Technology Center are still underway, but the experts have the impression that the lower relief on flatter printing plates will not allow the usual expansion differences to occur on prints (caused by outline effects). It would also appear that the photopolymer layer is still relatively thick (with a plate of 1.14mm thickness, even after subtracting the thickness of the carrier layer, the photopolymer thickness is still 0.85mm) and this helps to minimize supposedly unavoidable distortions in parts of the printing plates used for multicolor printing. Overall, this improves accuracy.
The DFTA experts found it was also easier to carry out certain parts of the printing process itself. If there are vibrations and stripes resulting from the printing process, these can be kept within a tight window and managed more easily. Lateral adjustments can be reduced for each print and this makes each pass easier to control when feeding prints through the rolls, because less time is invested in trying to make changes – something that was also previously impossible to guess beforehand for each printing effect. This benefit is probably due to the fact that all printing plates come into uniform contact with the cylinder. The printing quality is on a par with conventional printing techniques, although the researchers have also been looking into new experimental screen methods. These should help achieve even more significant improvements in quality, and this will still be in line with standard image resolutions with digital photopolymer plates of 2540dpi.
“We believe the function of our alternative flexo printing process is linked closely to how precisely the extremely low relief depth can be adhered to across all areas of the printing plate. This is because this is the only way to keep the relative pressure uniform when there’s full contact between the printing plate and the material,” surmises Prof. Dr. Martin Dreher, director of the Steinbeis Transfer Center. When producing photopolymer plates, it is not possible to simply control the timing and thus achieve low relief depths, especially if the depths are nominally determined by light exposure behind the plate. The new procedure cannot cope with thickness variances in the relief base of }40μm or more, which can be normal for back exposure but are not so much of a problem with conventional processes. As a result, the project team has developed a reliable alternative which makes it possible to adhere to tolerances of only around }10μm.
The biggest challenge now is how to design the transverse gap needed when using printing plates. The design options examined in the experiments to date did not satisfy the experts, but they do still have a number of options up their sleeves. The task now is to design the gap in such a way that it covers the entire area during full contact. This is because if the process does leave a gap, this could cause vibration again in the system. On the other hand, however, the edges must not protrude too far upwards or else unwanted oblique lines will be printed.
If seamless round printing plates can be used, the researchers will have achieved their goal and will have found a way to use the process without restriction. This will mean there is nothing stopping the experts registering their technology for patents. The next challenge will be to conduct testing to discover which areas of the flexo printing market the new technique is particularly well suited to.