Guide to printing students
-Few objective Questions and answers-
Written by : N.R. Jayaraman
The articles are meant to help the students gain knowledge and effectively compete in the interviews and nothing else. Please note that these are not to be published by anyone for commercial purpose without the written permission from the author since the theme and illustrative examples have been compiled exclusively by the author for the benefit of the student community. THIS WILL BE PERIODICALLY PUBLISHED.
37) What types of inks are used in various printing processes ?
Various types of inks used in the printing industry includes the following:
- Rotary ink
- Heat set ink
- Quick set inks
- Moisture-set ink
- Water-washable ink
- News paper ink
- Job ink
- Cold-set ink
- Ultraviolet- (or UV-) curable ink
- Colour shift inks called OVI (optically varying inks)
- Photo chromic inks
38) What is the difference between Quick set and Heat set inks ?
The heat set ink is quick-drying ink that dries by evaporation in high temperature. The printed sheets pass through a heat chamber to allow the solvent in the ink to quickly evaporate thus facilitating quick drying. Heat set inks are used in both in Offset and in Letterpress and meant especially for very high speed web machines printing magazines, catalogs, newspaper etc.
The Quick set Ink is one of the important inks used in printing to prevent set off problem in delivery pile. Quick set inks are used mainly for printing coated and enamel paper stocks, and used on high speed presses and these inks generally dry by Absorption cum Oxidation cum Polymerization process as they contain balanced Oil, Solvent and Carrier vehicle ratio. Quick set Intaglio inks is another new development in the Printing industry as they are extensively used by the Bank Note and Currency printing Presses where the speed of the Intaglio machine will require rapid drying on delivery pile at the same time retain their relief on printed sheets, and do not cause set off problem.
39) What is called dedusting unit and anti static rod ? Why are they necessary?
The paper packed after manufacture in the mills generally contains dust and debris on their surface attracted from the atmosphere. Those settled dust and debris gets removed while jogging the paper lots before piling them for printing. However some of the dust and debris continue to remain struck on the surface as if they have been embedded while the fact is they remain embedded due to the static electricity generated on them while the sheets gets rubbed over each other. If the partially embedded particles are not removed from the surface of the paper, they may get released along with the inked image from the blanket cylinder surface which in turn get struck to the surface of the plate cylinder. Those particles getting struck on the plate cylinder will then on not accept the ink from the inking roller and cause voids on the successive print images. They are called fluff or debris. Therefore such partially embedded particles are to be removed from the surface of the paper before they get printed, and in order to remove them the static from the paper surface need to be removed first. This is done by the dedusting unit with high pressure vacuum unit fitted along with anti static rods to remove the static and to sucks away those released debris. The rod which continuously release anti static energy energy while the paper travel below them is called anti static rod and the unit that sucks away the released particle is called dedusting unit.
40) What is called Chalking in printing ?
This is one of the defects attributed to Ink-Paper relationship. The images on the printed stocks appear to fade away or show dusty image after the sheets are printed. The deterioration on the printed image is caused by ink carrier vehicle that gets absorbs into paper too fast leaving the ink pigments in semi wet condition on the paper surface or are not getting properly absorbed into the paper thus causing improperly-dried ink getting rubbed off from the surface of the paper during travel in the press. This powdering defect is due to the ink that has not been properly formulated to suit the paper. The other possible reasons for this defect could be acetic paper, fountain solution too acetic (in wet offset ),too soft an ink which emulsifies etc.
41) What is the difference between perfecting machine and both side printing machines ?
Perfecting machine means simultaneously printing both sides of the paper with perfect registration in one single pass on the printing machine. Both sides printing machine prints both sides of the paper in one pass, however impression on front and back taking place not simultaneously but at a short spell away from each other.
42) What is Doubling in printing ?
It is a Printing defect that occurs due to the blankets getting loosened during impression in the printing machine. The improper cylinder packing, grippers not holding the sheets firmly, worn out plate or blanket cylinder gears, static charge (Static electricity) on paper, use of different thickness blankets on the same machine etc also causes this defect. The image will appear blurred or shadowed.
43) What is meant by fluff or debris?
Fluff or debris is a printing defect caused by lint and dust particles partially embedded on the paper getting released and struck on the surface of the plate cylinder.
If the partially embedded particles are not removed from the surface of the paper, they may get released along with the inked image from the blanket cylinder surface which in turn get struck to the surface of the plate cylinder. Those particles on the plate cylinder will not accept the ink from the inking roller and cause voids on the successive prints. They are called fluff or debris.
to be continued...............8