Guide to Printing Students – 25
-Few objective Questions and answers-
Written by : N.R. Jayaraman
107) What is Wipe on plate?
Wipe on Plates is an advanced version in Surface Plate making. Instead of coating the plate with sensitizer solution on a whirler, the coating is done by hand using a sponge or a wade of cotton wool soaked in the sensitizer solution. The sensitizer solution is supplied by the manufacturers in two parts as A and B solution. As and when the plate is required to be prepared, the A and B solutions are instantly mixed and used without waiting for cooling time. Since the process of coating is done by wiping the solution over the plate, it has come to be called Wipe on Plate making process. Normally the Wipe on plates require finer grained metal plates for producing uniform coating.
108) What are the advantages of Wipe on plates and how is a Wipe on plate processed ?
The processing of Wipe on plate is very simple. In this process of Plate making, the instantly mixed light-sensitive coating is applied by wiping over the plate the Cotton wool or smooth Sponge soaked in the sensitizer solution. Once the coating is allowed to dry in the yellow lighted room(Semi dark room) the negative will be attached to it at appropriate position, similar to the one as done in Surface plate making, locked in the vacuum plate holder and exposed against the U V light source and then the special lacquer is applied over the exposed areas followed by inking the image areas by hand with a small swab of cotton wool. Once the greasy ink is applied over the image areas and rubbed well to dry, the inked plate is washed with plain water or special solution if supplied any. Finally the plate is gummed and kept ready for printing. Thus all steps as mentioned for the Surface Plate making remains the same except the coating.
An additional advantage of the wipe-on plates is that it can be used to produce good halftone reproduction images even on inexpensive, non-sensitized paper plates (instead of metal plates) used for short runs on small offset presses. The small time printers prefer the use of Wipe on plate making process when they have no whirler facility to coat the plates. This process also leads to faster production of plates at much cheaper rates than the Surface Plate making process because the ready to use sensitizer solution kept in dark box can be taken out at any time and used instantly. The quantity of solution used will also be very small, being applied by hand. They are economical to use and meant for short to medium run copies. Very high quality tonal reproduction may not be possible with these plates as uniform hand coating can not be done.
109) What is pre-sensitized plate ?
The required printing plates for Offset Printing are made by exposing the plates with Negatives or Positives and then processed with certain chemicals in both Surface plate making process as well as Wipe on Plates processing. Both the processes are time consuming as the solutions are to be prepared before processing. In order to quicken the process of plate making, special sensitizer coated ready to expose Zinc or anodized Aluminum plates are made available by the manufacturers with certain period of shelf life, minimum being one year. The Pre sensitized plates are to be simply taken out from the boxes and exposed with negative or positive and developed with special solution which forms the images on the plates. For this purpose both negative working as well as Positive working Pre sensitized plates are made available in Such ready to expose plates, one more improved version in the Wipe on Plate process is called Pre sensitized plates. In Pre sensitized plates both negative and positive working plates are made available in both Zinc and Aluminum metals. They are economical to use and meant for short to medium run copies.
110) What is Photo Polymer plate ?
A still advanced version of Pre sensitized plate is Photo Polymer plates which also comes as negative and positive working plates. The metal plate meant for Offset printing is ready to use, pre-coated sensitized Polymer material, supplied in various thicknesses. No special developer is needed to process them. The plates are to be taken out from the boxes and after exposure to very high UV light emitting light source, the exposed plate is simply washed with water or alcohol solution. They are then dried and preserved for printing.The light passing through the negative or positive polymerizes the image areas and harden them to become insoluble in water or alcohol while the non image areas which are not affected by light gets washed away with water or alcohol thus giving relief images for printing. No gumming is required after processing. The plates have excellent acid and abrasion resistance, good ink receptiveness, run for million copies retaining sharp image, have longer shelf life, does not get affected by the atmospheric changes thereby maintaining dimensional stability etc.