An image setting unit for many applications – Lüscher installed a MultiDX device at Schreiner

The MultiDX! hybrid direct exposing unit (Source: Ansgar Wessendorf)

Developed by Swiss company Lüscher Technologies, the new MultiDX! UV laser diode image setter is a flexible device for the direct imaging of various printing formes. The Schreiner Group decided to purchase this unit for direct imaging plates for the letterpress and offset processes and also for producing flat screen printing meshes for its hybrid label printing presses.

Written by Ansgar Wessendorf

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This article was first published in NarrowWebTech, issue 3-2017.

The beta version of the MultiDX device was installed at Schreiner Group in 2015 and has been in everyday production for more than a year. For Lüscher, this family-owned business operating in the printing and film converting industry is an ideal and well known partner for further development and general optimization of the hybrid direct laser exposing unit under real production conditions.

With headquarters in Oberschleißheim near Munich, the Schreiner Group is a worldwide manufacturer of special label solutions with locations in Germany, China and the USA.

The product portfolio encompasses products like self-adhesive functional components, RFID labels, printed electronics and security solutions for product, brand and document protection. Sectors served include the pharmaceutical industry, medical technology, the automotive and electronics industries, mechanical engineering and logistics, as well as telecommunications providers, banks and government agencies.

Universal direct imaging

With the patent pending MultiDX! flat-bed exposing unit Lüscher responded to the requirement of the Schreiner Group for an universal and flexible device for the imaging of printing formes for the offset and letterpress processes and also for meshes for rotary screen printing. “In addition, we also use letterpress plates which do not contain the ablative black LAMS layer”, says Markus Petratscheck, responsible for the prepress department at Schreiner Group. Moreover, some letterpress plates are not flexible and therefore cannot be fixed on the drum of a conventional CtP exposing unit. For the production of complex special labels Schreiner Group relies on the benefits and possibilities offered by the respective print and converting processes of hybrid label presses. In addition, the Group appreciates the high flexibility of the MultiDX! unit. For example, the space-saving device is also used as a backup system for the exposing of screen printing meshes.

(Left to right): Markus Petratscheck, head of the prepress department for Schreiner Group, Urs Bachofner, vice president sales & marketing for Lüscher, and Alexander Tamfall, head of the forme production department for Schreiner Group
(Source: Ansgar Wessendorf)

Adapted machine construction

Due to the flat bed construction layout of the MultiDX!, rigid printing formes supplied with different carrier materials and of various shapes and sizes are efficiently exposed on a single unit to the requested quality level. As the printing formes remain static throughout the direct image setting process, balancing problems due to different size and thickness of the formes do not occur with MultiDX!

Furthermore, the flat-bed construction of the exposing device is provided with a flexible clamping system adapted to the needs of Schreiner Group and offering perfect positioning accuracy of the images on offset and letterpress plates and also on screen printing meshes. This, in addition, contributes to exact and stable register accuracy during the whole production run, which has an impact on make-ready times and waste rates.

UV laser diodes for direct exposing

For the direct image setting process fibre-coupled UV laser diodes working at wavelengths of 405 nm and 375 nm respectively are used. The switch from one wavelength to the other is easily done by the push of a button.

“Conventional offset plates and screen printing meshes are directly exposed at a wavelength of 405 nm, whereas letterpress plates require very short wave UV light at a wavelength of 375 nm”, states Urs Bachofner, vice president sales & marketing. The MultiDX! unit is provided with a total of 48 diodes. The maximum repeat length that can be processed by the compact unit is 800 mm (32”) x 600 mm (24”) x 50 mm (2”).

At the final stage of construction, the MultiDX! is able to monitor up to 128 UV laser devices.

The flexible clamping system for offset and letterpress plates and also for screen printing meshes (Source: Ansgar Wessendorf)

Laser diodes with a shorter wavelength

In 2007, Lüscher introduced the first CtP unit provided with UV laser diodes working at a wavelength of 405 nm which had been used to image conventional offset plates and photo reactive layers and emulsions. By contrast, the photopolymer relief printing plates used at the time showed no reaction to light of this wavelength. The next logical step was the development of a diode technology constantly emitting UV light at a wavelength of 375 nm, which was introduced into the market in 2016. This wavelength corresponds to the reaction range of market available photopolymer materials for the direct imaging of relief plates.

To achieve the above mentioned step, complex challenges had to be overcome mainly in the field of direct 3D imaging of relief plates. This was solved with the development of a specialized software package further optimized in detail with the support of the Schreiner Group.

Software for 3D imaging

The X!Direct software developed by Lüscher meets the requirements of 3D modulation for relief plates:

  • Support of the highlight areas and small graphic elements like lines and small fonts
  • Maintaining mid-tones and shadow areas for a clean printout

In this context the following challenges had to be met: The generation of crosslinked polymers is a direct result of the level of energy input. If this level falls below a certain limit in certain areas of the plate, crosslinking is insufficient and as a result the whole polymer ayer is removed during the washout process. When the necessary level of energy input is reached, all “exposed” parts are rendered insoluble and therefore cannot be removed during the washout process.

In addition, also the original thickness of the plate remains unchanged. Between these two extreme positions an area exists, where the real local plate thickness after the washout process is determined by the level of energy input.

For the development of the MultiDX!, complex challenges had to be overcome mainly in the field of direct 3D imaging of relief plates. This was solved with the development of an up-to-date and highly specialized software package (Source: Ansgar Wessendorf)

The working principles

The algorithm of the software scans the entered TIF file “on the fly” and also identifies graphic elements which may be much smaller than the pre-adjusted limit. This results in a sufficient level of base generation for the dots if this is required for this area.

Base generation means the support of a given number of pixels around the graphic element to be exposed. This process requires just a fraction of energy necessary to generate the respective plate elements according to the specifications of the TIF file. As a result, a variable support of the dot is generated which is also called a bumpup to strengthen the dots in the highlight areas.

 Conclusion

With the MultiDX! Lüscher meets the requirements of Schreiner Group for a universal and flexible hybrid direct exposing unit for offset and letterpress plates and screen printing meshes. This not only reduces costs but also contributes to a very stable and reproducible forme making process.

Background: Lüscher – Plate imaging technology

Headquartered in Bleienbach, Switzerland, Lüscher Technologies AG is manufacturer of imaging systems for various printing processes and is part of the international Heliograph Holding. The company developed the MultiDX! Image setter which facilitates the use of different types of laser sources in one machine. The MultiDX! integrates fibre-coupled laser diodes offering the required UV wave length spectrum. The actual number of laser diodes depends on the performance capability requested by the customer, whereas local upgrading to increase the output is possible at any time. The latest development from Lüscher is the hybid direct image setter. The company’s exposing units are mainly used in the security and label industries. According to Lüscher, they enjoy a market share of about 80% in the field of security printing.

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