The evolution of plates for printing

Photo Polymer plate was invented to process the plates in the shortest possible time at lower cost besides eliminating entire pre processing time. Those plates can not be reused also. There was another reason to glee over introduction of  Photo Polymer plates which did not require dampening the plate during print process. The printing process with Photo Polymer plates which had the image areas in relief and needed non engagement of dampening system was called Dry Offset Printing which in strict sense was the combination of Letterpress and Offset Printing process. 

Conventional plate graining machine grinds the plate inside a tub in heave action (moving front and back) with glass balls and fine sand mixed with water put over the plate. 

In short the graining process is nothing but roughening of the plate surface to remove previous images and to create fresh micro pits called grains on the same surface to enable the plates reusable several times by erasing the previous images. Such an action – graining to create micro pits on surface – provides better latitude for technical requirement such as ink/water balance on press, cause less trouble with dirt and hickeys, and less tendency for dot slur which mar the quality of image on print.

Emergence of Anodized Aluminium and Paper plates :
Prior to the introduction of Photo Polymer plates, the outdated technology of mechanical graining process  got converted into electrolytic graining called anodizing process by which thin tin sheet was electrolytically deposited with a thin layer of rough surfaced aluminium to use as printing plate as against the conventional Zinc and Aluminium plates. When anodized metal plate was found to function as good as grained plate, the annodization process replaced the mechanical plate graining process thus paving way for the introduction of Anodized aluminium plates for use as printing plates for small Offset printers on small to medium size machines. When anodized aluminium plate was introduced, simultaneously one more intermediary plate with Paper base was also introduced for use on small Offset machines.

Anodization is a process by which extremely hard aluminium oxide is electrolytically deposited on the surface of a grained aluminium or tin sheet which further converts the plate surface into corrosion resistance aluminium. The anodized layer has several micro pores or micro pits, the pattern of which could be compared to a honeycomb. Thus the Anodized Aluminium plate which undergo the annodization process turns the metal surface into twin quality material such as water-receptive and oil receptive one. The anodized plates behave similar to conventional aluminium plates and are economical for small offset presses, easy to use and work well on the press. These plates were improvised version of Paper based plates. Anodized plates are also reusable by recycling them for a few times by performing chemical graining (removing the old image by etching process) instead of subjecting them to mechanical graining. The process of erasing and re imaging even for few times provided not only economic advantages but also environmental benefit as well.

As said earlier the development of Anodized plates which were primarily meant for short-run copies was instrumental in the development of plates’ called Wipe on Plates and ‘Pre sensitized plates’ which eliminated several intermediate processes. While the Wipe on plates were meant for making plates using Negative, the Pre Sensitized plates were made available to work both with Negative and Positives. While the Wipe on Plates were to be hand coated with special sensitizer solution, the Pre Sensitized plates were ready to use  (directly exposable with negative or positive) pre sensitizer coated plates which needed no hand coating of sensitizer.

What is Wipe on Plate ? :
Before the introduction of the Pre sensitizer coated plates Wipe on plates were introduced  processing printing plates in the quickest possible time eliminating certain pre exposure equipments. Since a ready to use sensitizer solution which was made available  was swiped (wiped) on the anodized aluminium plate by hand with a plain cotton wool or smooth soft cloth dampened with the sensitizer solution such processed plates were called Wipe on Plates. The coating could not be expected to be uniform, but the hand coated plate dried in few minutes after swiping the plate with the sensitizer solution, allowed it to dry, exposed with negative or positive and then washed with water and a special solution supplied along with ready to use sensitizer to make ready the plate for printing. The Wipe on plates were easy and quick to process, but the reproduction of the halftone images were not as good as normal processed plates because of non uniform coating by hand. The plates processed as above, called Wipe on Plates helped the small printers to quickly prepare plates for short run copies at short intervals. Such type of plates were not made available for printing by Letterpress, Gravure or by Intaglio and only for Offset printing plates they were available. 

With the passage of time in order to produce plates which can give several thousand copies without quality loss, use of multi layer metallic plates made ready by electrolytic process were introduced for Offset printing. Initially they too were negative and positive working plates, but could not be made available in the form of Pre sensitized plates. The electroplating process based plates had to undergo various steps. The concept of use of multi metal plates gained momentum and two types of multi metal plates were in use. One was called Bi metal plate meaning combination of two metals, while the other one was called Tri metal plates- a combination of three metals.