Flexofraphic
Flexofraphic printing
Flexography is a variation of high printing using flexible elastoplastic relief printing forms and liquid paste-like colours. The colour is applied on the form with the help of rastor (anilox) roll. Elasticity of the printing form gives a possibility to imprint any materials including polyethylene, polypropylene, foil and combed paper.
Advantages of flexography
The technology of flexographic printing is distinguished for its simplicity and low costs and at the same time for its high productivity; it does not reqire many workers as machines have a simple colouring apparatus. This method ensures high quality of imprints on various materials comparable to the quality of offset and deep printing. The printing is environmentally-friendly; water- and alcohol-based colours are used. Energy intensity of the equipment is low (less by 40-50% compared to the offset method). Flexographic printing is noted for its on-the-line production – all the operations starting with unreeling and ending with cutting and seaming of final manufacturers are done during one skip. The necessity of production room is 50-60% less than for offset printing. This kind of printing requires less investments than other traditional ones – printing and processing are operated on machines which are at that less expensive than offset ones. Due to machines unitizing, side operations are eliminated; waste percent is low owing to simplified construction of the colouring apparatus. The possibility of using UV-approved colours increases the quality of products and speed of printing.
Forms for flexographic printing.
Forms for flexographic printing can be made of rubber and dycril. The most popular are forms from dycril which were initially created and introduced on the market by DuPont company. The technology of the forms manufacturing is as follows: negative photo forms are spreaded onto a photopolymer with UV-emanation, light-struck areas are polymerized while light-non-struck areas are washed off with a special solution. There are dozens of forming plates brands. They differ from each other by the level of firmness, resistance to solvents, lacquers, washing-off solutions. Thickness and firmness are the main characteristics of the forms. Hardness of one and the same plate, as it were, increases at decreasing of thickness. Thinner and less hard ones reproduce a raster full stop better but are more difficult to work with. Harder forms for smooth imprinted material are better to use during raster printing than strokes and texts printing. The usual thickness of the flexographic form is 2.84mm, 2.54mm and so on. Not only does the form apply the colour onto the imprint but serves as a deckle as well.
Forms are fixed on the printing rollers with the help of special two-sided adhensive tape (two-sided Scotch tape). During the printing process with the same deformation of elastic printing elements, pressure on the solid plate and on the small element is different what can lead to quality loss. In order to avoid it, raster and solid plates are printed in one section while the stroke image and the text in the other. Photo forms should be negative and readable from the emulsion side; moreover, they should content necessary elements: crosses for register, proofing bars and others. Form assemblage on cylinders during multicoloured printing takes much time. During the process of photo forms manufacturing such factors as spread, for instance, should be taken into account. Moreover, during fixing the form onto the printing cylinder lengthening of the image (distortion) occurs because of the form curve; the distortion depends on thickness of the form, the Scotch tape and diameter of the roller. In order to compensate such distortions the negative is reduced. The diameter of a minimum raster full stop should be no less than thickness of an anilox raster cell.
Peculiarities of printing on different materials.
When printing on polymer films, they should previously be processed before printing to ensure safe paint attaching (especially if paints with low percent of solution or without it are used0. One of the variant of such processing is a method of corona discharge, that is running the film through the field of high tension and firing it with ionized air molecules. The quality of the processing is controlled with a test pencil and liquid; the liquid should not run or gather into a drop. Printing on cardboard requires equipping the printing machine with dust retaining devices. The surface of the cardboard is porous, therefore the printing is better to carry out on laminated cardboard.
