The simple offset blanket quiz - The Noel D'Cunha Sunday Column

Ans: Metal cylinders were used with lithographic stone and while the paper sheets were fed manually, the paper would get a print with the help of the impression cylinder. This impression cylinder was covered with a specially treated cardboard which was later changed to rubber. The accident was: When the person who was manually inserting the paper would miss the sheet then the impression from the stone would be transferred to the impression cylinder having the specially treated cardboard and then the next sheet which was inserted would get the print on both sides – One from the stone and the other from the impression cylinder. What was noticed was: the print from the impression cylinder was sharper and superior. This was the beginning of offset printing.

Ans: Ira Rubel, a paper mill owner from America who had this accident in 1905. Ira Rubel thought of modifying the press to what we call today offset printing.

Ans: Conventional (no sponge layer); and Compressible (with sponge layer). In the past hundred years, we have many more sub species of blankets which have a Fabric Back; Polyester Back; Metal Back; Self Adhesive; No Back; Sleeves.

Ans: Unpack new blankets and lay them flat or hang them in the rack; always store the new blankets – face to face; and blankets should be kept in a dark and cool place away from direct sunlight plus electric motors etc. Ensure that the humidity in the room should also not be more than 65%. Always use FIFO policy for blankets. Never store too many blankets (maximum 15) when laying flat one over the other. And do not store for more than 12 months, if possible.

Ans: On the machine treat a blanket as though it is precious. Do tell the operator it is a part of him that helps boost the hygiene levels. Clean the blanket, regularly. Also use good solvents to clean mixed with water. Please nb: the water helps as a carrier for removing the muck besides helping a strong solvent to reduce it’s negative effect. Always use a rejuvenator or conditioner regularly. And finally, use a deglazer periodically, since it helps in removing the embedded ink resins.

Ans: Check the undercut of the machine being used and then calculate the thickness of the blanket. On deducting B from A, you get the total thickness of packing (Adding 0.05 mm to the total thickness of packing and over pack, in about 2000 impressions, the blanket tends to lose its gauge and you would tighten the same). Undercut of machine - 2.30 mm Thickness of blanket - 1.95 mm Packing thickness - 2.30 - 1.95 = 0.35 + 0.05 = 0.40 mm

Ans: Remember to adjust the pressure between the impression cylinder and the blanket cylinder depending on the stock being used.

Ans: This happens because the blanket is too tight for the cylinder. The first thing to do is, to reduce tension of the top blanket and reduce width of under-packing by 6-10 mm.

Ans: The possible cause is, the first colour ink has transferred to the second colour blanket and sensitises the second colour plates. There are two things you must do. One, run blankets at a lower squeeze pressure; and two, use better desensitised plates.

Ans: Use stiffer ink or a less absorbent paper or faster drying agent. Also reduce press speed and apply litho varnish.

Ans: The cause could be: the coating is not suitable for offset; or the ink is too stiff or there is too much back pressure. Therefore soften the ink slightly; reduce the back pressure to a minimum; and change paper and use harder blanket.

Ans: Wet wash the plate or use a non-binding lacquer, re-adjust rollers and pressure and/or reduce blanket packing. Since in this case, the gum adheres to the surface of lacquer or the image has worn off the plate.