Equipment & Solutions for Newspaper and Commercial Printers
Monday, January 16, 2017
RUBBER ROLLER MAINTENANCE & PROBLEMS
Any printing machine is desired to deliver optimum performance. In any machine the printing section is considered the important section, the process of ink considered the important section, the process of ink transfer to the surface becomes the vital point for quality outputs. Thus in a way the "Rubber rollers" which actually transfer the ink can be treated as the heart of the printing machine.
The print quality can be heavily hampered if proper attention is not paid towards the rollers, as it is directly associated in ink transfer. But that's not all the rubber rollers not maintained properly leads to other indirect losses, some of them are :-
Heavy downtime due to defective printing caused by faulty rollers.
Frequent replacement of rollers.
Plate wear due to incorrect quality of rollers.
Wastage of proper printed matter due to uneven inking.
Scumming due to defective dampeners.
This number can go on increasing, but the message if this exercise is to emphasize the importance of the Rubber Rollers. Without proper care they can eat up a substantial amount of the profit and many time it is not even recognized!!!
The most expensive thing in printing is "DOWNTIME" of the machine. It can cost a substantial amount of money per day and one of the common reasons for it is Defective Rollers or Rollers requiring change.Thus it is very necessary for printers who strive for quality should understand the methodology of proper use and care of the Rubber Rollers.
PRINTING PROBLEMS CAUSED BY FAULTY ROLLERS
Scum- Scumming is the appearance of unwanted inked are in the non-image area. One of the important reasons for scumming is inadequate supply of water to the plate. Dampening rollers should be soft & should be capable of supplying very thin even layer of moisture to the plate. Hard or uneven Dampening Rollers will provide inadequate or uneven water giving rise to scumming.
Image wear- A hard dampening or Plate Inking roller will cause rapid wearing of plate.
Dot Size- A hard or glazed roller gives dot elongation or dot again. This effects in getting sharp image with full color value.
Skidding-Due to accumulation of dried ink film, roller becomes glazed. Glazed rollers are poor in traction of ink. Thus when they strike the plate which is having greater surface speed than a roller a skid is produced, in turn causing a streak across the plate.
Stripping-Ink Rollers refusing to hold ink is called stripping. These are two main reasons for stripping. There are two main reasons for stripping. Loss of affinity for ink in some areas is due to (a) Film of dried ink on the rollers, (b) Desensitization of copper rollers.
Uneven Corrosion- Rollers with roughed up sides does not allow full size printing. Hence accurate setting and proper inking is not possible on these rollers.
Belling- Dampening rollers tend to "bell" at the edges due to constant pull of the hose on the edges. The hose tends to shrink while drying and thus exerts a pull towards the center. It upsets the pressure and distribution of water. It causes bowing of roller and uneven wear of roller.
Swelling-A roller absorbing ink, cleaning solvents and chemicals becomes tacky or swollen. It gives rapid drying of ink. A good roller should have low ink absorption rate.
Hickeys-These are white spots appearing in solid image areas. They are caused by hardened specks of ink, dried gum, dirt etc. and hamper the print quality.
Ghosting Repeat-It is a faint image outline in printed matter caused by poor ink film recovery on the plate by rollers. It is an indication of poor ink storage capacity in the roller.
Worn of Journal ends- This gives uneven inking, skidding, due to run-out or excessive play between the journal ends of bearings of a roller.
HOW TO MAINTAIN YOUR ROLLERS IN TOP CONDITION
Rollers should be cleaned thoroughly with a roller wash especially at the end of every day's work. This will prolong the life of the roller and minimize the hardening effect.
Clean the rollers frequently before the ink starts to dry. Accumulation of dried ink film will cause Glazing.
Rollers should never be left for long periods in contact because" flats" can form on the soft rollers. Moreover if wash-up has not been completely clean the rollers can stick at contact points with the risk of pieces being top of the surface of the roller when the machine is turned.
Many rollers become useless because of longtime and/or improper storage. Spare rollers should be rotated with the rollers on the press, at least every six months. This acts as a sort of seasoning, reliving stresses that are inbuilt in roller. The replaced rollers should be cleaned thoroughly and after dusting them with talcum powder and covering them up with the black or brown paper, should be rested horizontally on the journals. The rubberized portion should never contact any other surface. The storage place should be away from direct sunlight and heat.
While printing smaller jobs on larger machines printers tend to release ink only to cover the print area. This causes the sides of the rollers to run dry generation high temperature and causing heavy wear on the roller ends due to abrasion.
While fitting the bearings, take care that you do not bend or damage the journal ends by hammering or forcing. Check up the journal ends/bearing seats, sockets and brackets for wear or damage.
Use the correct technique for mounting the hose on a Dampening Roller. Excessive force may damage the roller by tearing the soft roller from the metal core.
To avoid "bulging" or "belling" of dampening Rollers place a disc/collar of fibers/plastic, with a slightly smaller diameter than the rubber, at both rubber ends and then sew on the hose ends. Any pressure or pull exerted now will be on the disc and not on the rubber sides.