Research and Development

In the offset printing method based on a planographic printing plate, a thin water membrane is formed on a non-image portion of the printing plate to repel oily ink, whereby ink is received only on the lipophilic image portion. Then an ink layer formed like an image on the surface of the printing plate is transferred to paper through a rubber roller (Fig. 1).

In a commpnly employed planographic printing plate, an aluminum plate having a hydrophilic surface provided with microscopically roughening (also called graining) is used as a substrate.

Konica Minolta Co., Ltd. has developed a printing plate substrate having a hydrophilic layer formed on the polyesterf substrate capable of replacing the aluminum grain substrate. Use of a film substrate allows a printing plate to be supplied in a rolled form. This permits a space-saving, highly automated plate-making and printing system to be configured. A rolled process-less printing plate (printing plate eliminating the liquid development processing) made by Konica Minolta Co., Ltd. is used in the Truepress 344 manufactured by Dainippon Screen Incorporated Co., Ltd.

A hydrophilic layer used in the printing plate substrate is required to have both a high degree of hydrophilicity for forming a uniform thin water membrane, and a wear resistance for withstanding stress during tens of thousands of printing operations. A high degree of hydrophilicity of the hydrophilic layer can be achieved by a synergistic effect between the hydrophilicity ensured by material structure and water retentivity ensured by surface form control. The hydrophilic layer is formed by metallic oxide particles bonded by a porous organic/inorganic composite binder, and is mainly made up of a highly hydrophilic metallic oxide. Metallic oxide particles having a microscopically roughened structure on the surface are adopted as metallic oxide particles, and the particle diameter is assigned with an adequate distribution, thereby forming a multiple roughened structure where the wavelength components ranging from several tens of nanometers to a few µm are superimposed (Fig. 2).

A uniform microscopically roughened structure on the surface causes an increase in the specific surface area. This arrangement ensures further improvement in the water retentivity of the highly hydrophilic metallic oxide layer. Further, a high-strength protruded structure arranged at an interval of several tens of µm receives the distributed form of the stress applied to the plate surface, whereby the wear resistance o the entire hydrophilic layer is improved.