Production of Flexographic and Relief Printing Forms

The following technologies are used in production of flexographic and relief printing forms. Depending on the template used – negative one or positive one, the photo-polymer forms are further divided into analogous and digital (laser-type) ones. For production of analogous forms, there are used negative foils (printable areas are transparent and non-printable ones - black), and the negative image on the surface of digital forms is generated on the plate before exposure, when the laser beam controlled by the computer software „burns in” the image template on the plate coating.

The flexographic and relief printing forms are produced from the light-sensitive polymer, which polymerises into rubber-type substance when irradiated by the UV light of some wavelength in time of plate exposure. Non-exposed polymer areas are washed off or removed in any other way, and the printing relief is obtained, which transports ink from the ink section onto the printing surface.

	Exposure from Bottom. This process is intended for photo-polymer sensibilisation and for forming the substrate needed for image details. Selection of the correct exposure duration greatly influences both the relief depth, and the success of the main exposure and the form quality.
	Negative Placing. In this process, the protective film is removed from the upper side of the polymer and the negative foil is placed on it, with its transparent and opaque areas corresponding to printable and non-printable areas of the image. It is very important at this stage to have no air bubbles and dirt between the polymer and the negative foil, which may introduce distortions in time of exposure. With this purpose, the equipment has installed air-removal pumps.
	Main Exposure. At this stage, the light-sensitive polymer is exposed to the UV light through the negative foil. The transparent areas of the negative pass the light, which conditions the polymer in these places and activates the polymerisation reaction, producing tough chains of polymer molecules.
	Washing. In time of washing stage, the non-exposed and non-polymerised substance is washed off, and the relief printing form is obtained.
	Drying. In time of drying, the washing liquid is removed, which has penetrated into the polymer in time of washing.
	Final Exposure. In time of this process, the entire form is irradiated with the UV light in order to finish the polymerisation process and to harden the form.

Depending on the nature of washing liquid, the forms are further divided into washable with thinners and washable with water.

The technology of photo-polymers washable with thinners is the oldest one and most widely used. Selection of photo-polymers of this type on the market is very wide and diverse, there are developed different types of polymers for different inks (spirit-based, water-based, UV) and for different surfaces, nevertheless, this technology has several essential deficiencies – thinners evaporating in time of processing pollute environment and affect workers’ health, and the very process is rather long.

Photo-polymers washable with water are comparatively new on the market, however they enable to avoid negative impact to the nature and worker health, additionally, the process of printing form production takes around three times less of time than that with photo-polymers washable with thinners. This is due to the fact, that the polymers swell less when washed with water and it takes less time and energy to dry them, and this fact is important when ensuring constant thickness of the plate.

The Du Pont company has developed the unique photo-polymer plate Cyrel® FAST, where the polymer from the non-exposed areas is removed by heating the plate up to the defined temperature and collecting the melted polymer by the special absorbing material. This method is distinguishing by high economy and gives practically no impact to the nature. The main drawback of this technology is that it is used exclusively by the Du Pont company and there is no other alternative until now.

Principal Cyrel FAST operation scheme

One more technology of producing the flexographic printing forms is the direct engraving. When using this technology, the printing forms are obtained by forming the image relief on the surface of the rubber or some special polymer and burning out the non-printable areas by means of the laser. The main advantage of this technology is exclusion of such intermediate processes as exposure, washing, drying, however, this technology is popular only in the specific market, where the highest printing quality is not required. The main drawback is the need to have the powerful laser to burn (evaporate) out large areas of the form surface, and the more powerful laser, the larger its focused point; this means, it is impossible to reconstruct small details. Nevertheless, there are intensive R&D works in this field with purpose to develop the new materials suitable for engraving with lasers of lower power.