Soap and water, c'est tout

At drupa 2008, Asahi Photoproducts Europe premiered a new printing plate called DEF that can be digitally imaged, is said to be suitable for all types of ink and can be washed out with water. The new photopolymer plate has been successfully trialled by the Witzhave, Germany, based label converter Rako Etiketten since September 2007.

A water washable printing plate needs to accomplish a balancing act. It should be unsusceptible to moisture in the ambient air during handling and storage, or at the hands of the operator, while simultaneously permitting washing with water.

Addressing this need, the black surface layer on Asahi’s plate is water soluble and the residues of the mask layer can be removed from the water bath following the washout process. The flexo plate based on water washable polymers is resistant to all common printing inks – particularly to solvent based systems. At Rako Etiketten, the new DEF plates are predominantly used with UV inks, says Stefan Behrens, head of preprint at Rako.

The Rako company is currently using plates that are 1.14 mm thick but Asahi aims to expand the range to 2.84 mm. The maximum plate size is 900 x 1,200 mm, which corresponds to the format of the CDI Spark system for laser imaging. The main- and post-exposure are performed using an Asahi AFP 912 EDLF system. Depending on the size of the printing plates, six or nine formes can be produced in a single pass. Asahi has also developed a new unit for washing out the new flexo plates and an ordinary soap solution can be used for washing.

Fast is the word

The new plate can be processed within an hour. This includes the entire manufacturing process, such as imaging, exposure, and washout, drying and post-exposure. Rako also discovered that the time required for imaging the DEF plate was about 30 per cent lesser in comparison with other digital plates. Martin Wohlschlegel, branch manager at Asahi, attributes this to a modification in the chemical composition of the black layer. Encouraged by the favourable results, Asahi is now considering using the modified mask layer for the other digital flexo plates in its range.

During beta testing, the printers were also impressed with the quick trapping of the printing ink at the anilox roller and good transfer to the substrate, which allowed for fast makeready times and waste reduction.

No swelling

Rako also achieved good results in terms of register, which is particularly vital in multiple-up repeat jobs. Even with six and eight colour jobs produced on one of the 420 mm wide presses with perhaps eight copies next to each other and a range of 635 mm, the plates provided true register after several mounting operations, it is claimed.

Stefan Behrens says: “In contrast to flexo plates that are washed out in solvent, no swelling occurs with the DEF printing plate.” The washout agent utilised is water with commercially available soap – sodium carbonate, commonly known as soda. The used soap solution in Witzhave is pumped directly from the washout system into a 1,000 litre tank. A disposal company collects the stored liquid as soon as the tank is sufficiently full.

Although it would certainly be permissible to discharge the soap solution into the normal sewer system, Asahi recommends a professional disposal operation. Martin Wohlschlegel explains: ”There is at least a certain risk that during the production process monomers will be washed out. Commissioning a disposal firm will prevent such substances being able to enter the natural environment, even in small quantities.”

Filmless production

Stefan Behrens also sees the new Asahi printing plate as an opportunity for changing over to filmless plate making in the future, a goal that Rako Etiketten has been pursuing for quite a time.

In the case of small impression cylinder circumferences, as inevitably encountered in narrow web printing, the water washable photopolymer plates offer the requisite flexibility for fitting snugly around the tight radii of the cylinders involved. “In comparison, the flexo plates available to date with a mask layer for digital imaging, which are usually washed out in solvents, exhibit a significantly higher rigidity,” says Asahi, “In the printing process, these plates therefore cause repeated problems due to edge lifting.”

Stefan Behrens believes the DEF plate meets three important requirements: “The new printing plate can be digitally imaged and also quickly washed out in water, and in addition it is so flexible that it fits snugly against the tight radii of the small impression cylinders.“