17. Set off : While printing on any smooth paper or coated stock of paper, it may so happen that the ink from the previously printed sheet may transfer some amount of ink from its surface on to the back of the next printed sheet which will sit on it in delivery pile. Such an undesirable ink transfer is called the Set Off. This happens due to the wet ink which failed to dry quickly. Therefore suitable ink to match the paper stock is to be selected. In the high speed machines between each printed sheets some unprinted sheet of low quality will be inserted to prevent the good paper getting spoiled. In letterpress machines Anti set Off spray used to be employed to spray the anti set off powder which will prevent ink transfer.
19. Shrink wrap: Packing the material in tight pack with the use of plastic or PVC or Polythene film is called Shrink Wrap. The material to be packed is wrapped around by the plastic film and pushed into the heat chamber. Due to heat the plastic film slightly melts and shrinks into a bundle. The heat chamber is so designed that it does not burn away the film, at the same time ensure that the film gets shrunk tightly and adheres with its own surface . The shrink wrapping is done by automatic machines which automatically wraps the material with a layer of thin plastic film and shrink wrap it. This clear shrink wrap is used for packaging of many products including bundling of Currency notes.
20. Surface Plate :- There are few processes by which the plates for Offset Printing is prepared. Out many techniques employed to make a printing plate for Offset machine is the Surface plate making and the other is deep etch plate making process. The Surface plate making process involves coating of Alluminium or Zinc plates with light sensitive albumen, or gelatin or Casein material. The plate is then exposed with a negative. The image areas become hardened and insoluble to water or other liquids, while the non image areas remain un affected. The plate is then applied with an ink which makes the hardened areas ink receptive. While the non image areas with the coating remain unaffected. Then the inked plate is developed with water and all the non image area coating, which remained unhardened gets washed away leaving the hardened image areas on plate. The plate is then gummed for using it on the Offset machine. Since the image areas lie in the same plane with non image areas, the process is called Surface plate.
