In close cooperation with its suppliers, STI Group has elevated flexo post-printing to a new level with a MASTERFLEX-HD from BOBST

At STI Corrugated GmbH’s Alsfeld plant in the German state of Hesse, the BOBST MASTERFLEX-HD is the beating heart of a high-quality flexographic printing process used by the company for corrugated packaging and displays. It was the first press of its kind on the German market. Over the years, STI Group has gradually optimized its entire production process in cooperation with its suppliers. As a result, the MASTERFLEX-HD, which boasts technical availability of at least 97%, is also used to produce numerous items of packaging and displays that previously required offset printing.

“Without the MASTERFLEX-HD we would no longer be able to cater to the needs of the market,” stresses plant manager Andreas Goldbach. To prove his point, he holds up a large-format corrugated box for a lawnmower produced by a well-known company. The pictures of the yardwork machine are of almost photo-realistic quality. And the surfaces are strikingly even and smooth. Seen from a distance of one or two meters, it's hardly possible to distinguish between this result and the quantity offered by offset printing.

Carsten Geisel sums up the level of flexographic post-print quality achieved at the Alsfeld factory: “The MASTERFLEX-HD allows us to achieve a defined level of quality that is consistently reproducible across large print runs.” As a project manager at STI Pro Grafik GmbH, the in-house prepress operation, he advises customers on how to optimize their print motifs for flexo direct printing.

Giving customers all the options
“In conjunction with our offset and digital printing presses, we are able to provide our customers with every option in their production process, and to always select the printing process for their orders that best meets their requirements,” STI Group Marketing Director Claudia Rivinius is pleased to note. The Group has produced a “Look Book” Book of printed samples to illustrate which printing method produces the best results.

This pattern book clearly shows the possibilities afforded by the various printing and finishing processes on various substrates. Every pattern in the book is of impressive quality. Only once they are juxtaposed does it become visible to the layperson that there are still differences in print quality, depending on the material and process used. But there is no question about it: among the samples shown, flexographic printing using the MASTERFLEX-HD can only be distinguished from offset printing by very careful scrutiny.

The limits of flexographic printing shifted onto corrugated board
“Starting with the printing materials and inks, then moving on to the printing plates and the logistics systems, and finally to the press itself – over the years we have gone about optimizing the entire process with our suppliers step-by-step, and have acquired comprehensive practical know-how,” explains Geisel. “As a result, the MASTERFLEX-HD allows us to achieve what is most probably a unique quality-to-cost ratio in flexo post-printing on behalf of our customers,” Goldbach is convinced. This in turn allows STI Group to set up and complete orders in a single processing step on the press, where previously there was no alternative to offset printing with subsequent lamination of the prints onto the corrugated board.

The press runs at capacity in three-shift operation, practically around the clock, with such high utilization helped by the fact that it can also be used to print on cardboard with thicknesses of down to 0.5 mm for folding cartons and crates. Goldbach mentions another application: “We have installed a pile turner in front of the MASTERFLEX-HD in order to be able to print with high quality on the insides of both the corrugated board formats produced on our own corrugated board machine and on the offset-laminated corrugated board produced in Lauterbach, located 20 km away. Using the MASTERFLEX-HD, we reliably print at least 8,000 sheets per hour.”

In other words, the product range at STI Group's Alsfeld factory basically starts where many other corrugated board plants run up against their limits. “Last but not least, this has allowed us to significantly expand our geographic coverage,” Rivinius concludes. Thus, the production site benefits greatly from this high-quality production process.

On the way to zero-defect production
Because there are still no national or international standards in place to govern flexo post-printing, STI Group went ahead and launched its own quality management system – and not only in order to standardize its internal process. The quality management system also simplifies communication with the company's customers. “Agencies in particular are grateful for our specifications, as they are able to base their production data on them from the start,” explains Geisel. Not only that, but errors in the production of printing plates are virtually impossible thanks to the standardization, he adds. For large lot sizes and printing plate costs of around €500 per square meter, he says, these days no one can afford to have to re-make plate sets from scratch again due to incorrect or missing specifications.

Plant manager Goldbach explains the progress made in the plant's flexo post-printing process from another perspective: “With the MASTERFLEX-HD, we have made major strides towards our ambition of zero-defect production.” For example, the MASTERFLEX-HD not only provides efficient ways for controlling production quality, but also gives ways to seamlessly check production quality using the integrated iQ300 camera – and to document quality at the customer's request. “Compared to previous methods, where we were only able to check the quality on a spot basis, with the MASTERFLEX-HD there are no more times not subject to verification.” Waste sheets are automatically ejected from the process at speeds of up to 10,000 sheets per hour. Goldbach explains that in conjunction with the Registron system, which automatically adjusts the press as soon as the iQ300 detects deviations from preset tolerances, this guarantees constant, uniform quality of production. From the perspective of STI Group, this is a unique selling proposition of the MASTERFLEX-HD. “If the iQ300 is switched off,” Goldbach explains, “our shift managers are required to justify and document this.” Generally speaking, he explains, the machine operators are obliged to work with the quality assurance systems.

Advantages of an offline solution
STI Group first discussed the idea of replacing an older flexographic printing press with the MASTERFLEX-HD in 2009. Geisel recalls the key considerations at that time: “With an offline solution, our experts would be able to concentrate fully on the printing process, and we would be able to operate two die cutters.” Were the people at the plant already fully aware at that time of the quality that could be achieved with the press? “We went to BOBST in 2010 with a highly detailed specification in which we also defined our stringent requirements for register accuracy of no more than plus or minus 0.25 mm,” Goldbach recalls. Initial tests were performed in 2011 at the BOBST Competence Centre in Mex, Switzerland. After the purchase decision and after the factory acceptance test in the offering of 2012, the press was finally commissioned in Alsfeld around the turn of the year.

Quality-determining factors
The high production quality starts with the automated feed logistics and with the pile turner. The pile turner ensures gentle handling of the materials without mechanical damage of any kind if the materials need to be printed on both the outside and the inside. STI Group is able to work with or without pallets in its process.

“The MASTERFLEX-HD is designed for high print quality by definition,” says Goldbach. For a wide range of different materials – from F flute to BC flute, from offset-laminated corrugated board to cardboard down to 0.5 mm in thickness – the Direct Drive Feeder guarantees true-to-register feed without loss of caliber. This way, it supports accurate throughput of the materials through the entire flexographic printing press. This applies equally to horizontal flutes. Geisel adds: “The precision of the print-to-die register is determined in the feeder.” In addition, he goes on, electrostatic sheet cleaning by means of the vacuum device in the feeder reliably clears any dust deposits off the sheets even as they are transported through the printing press at high speed and high accuracy by the castor vacuum transport system. In addition, the actual positions of the sheets is detected by means of photocell sensors in front of each printing unit; these values are then compared to the target values and the orientation of the printing plate is adjusted as necessary.

The printing units themselves offer ink cooling, viscosity control and register control. There is also variably adjustable intermediate drying, which supports high quality wet-on-wet printing in particular. Finally, the anilox rolls and the printing plates are cleaned by automatic washing systems. “As a rule, we document all order parameters, thus ensuring reproducibility of the results achieved,” Geisel reveals.

Ease of operation and thus also short set-up times were decisive criteria that prompted STI Corrugated GmbH to purchase the MASTERFLEX-HD high-performance flexographic printing press from BOBST.

Order change within minutes
The reliability of BOBST’s technology as well as the short set-up times offered by the MASTERFLEX-HD were also decisive criteria informing the investment decision. Both of these factor significantly in STI Group's achievement, for several years running now, of the press’s expected technical availability of at least 97%. “Orders with lot sizes of 4,000 sheets are finished after a maximum of 30 minutes. Smaller batch sizes go even faster,” emphasizes Goldbach. For this reason rapid retooling is a fundamental requirement. Depending on how many inks are to be used in filling a print order, thanks to the seven printing units, machine operators are able to retool the free printing units even as the press is still processing jobs on the other units. Each printing unit is equipped with an automatic rapid-change system for the anilox rolls; a transport carriage that is integrated into the printing unit features a lifting device that allows the anilox rolls to be replaced in just a few minutes. The Start & Go system for automatic register adjustment offers a high degree of convenience during setup as well. In addition, all the machine operators have to do is enter the grades of the corrugated boards and the thicknesses of the printing plates – after that, the press automatically adjusts the distances between the anilox rolls and the printing plates, as well as the counter-pressure to the printing sheets. Finally, the Start & Go system performs a nearly 100% printing impression adjustment. The motorized angle adjustment and the pneumatic quick-release system makes the machine operator's life easier when swapping out the printing plates.

In case something should go wrong, BOBST Service is there to help STI Group employees with its the Helpline Plus service. Helpline Plus allows the experts to log into the machine via the Internet, perform troubleshooting, and clear faults immediately depending on what they find.

STI Group continues to submit suggestions to BOBST concerning further process optimization, Geisel says. “They are always open to our suggestions. They also took our ideas into consideration when designing the press.” The covers of the anilox rolls, which are now part of the standard configuration of the MASTERFLEX-HD, are one example of this.


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