A ready beckoner of common printing problems associated with faulty rubber roller.
SCUM: This is the appearance of unwanted inked areas in the non-image areas one of the important reasons being – inadequate supply of water to the plate. To prevent this, the DAMPING ROLLERS should be soft (22/25 SH) and resilient. They should apply a very thin even film of moisture to the plates on non-image areas. Hard roller being stiff on the surface, are not able to apply an adequate layer of water, giving rise to scumming. The fear of scum makes you run your machines at lower speed – loosing on invaluable production hours.
SKIDDING / STREAKING: When the roller becomes glazed due to accumulations of a dried ink film, it looses its frictional potential and slows down due to poor fraction. Thus, when the roller strikes the plate whose cylinder speed is greater than that of the slower glazed roller, a skid is produced. This, in turn, causes a streak across the plate.
STRIPPING: Occasionally, parts of the ink rollers will refuse to hold ink. This is known as STRIPPING. There are two main reasons for stripping- loss of affinity for ink in some areas is due to (a) a film of dried ink on the roller (b) desensitization of copper rollers.
IMAGE WEAR: A hard damping/plate inking roller tends to cause a rapid weakening of the plate image due to abrasion.
BELLING: The DAMPING HOUSE being made of cotton fabric, it tendes to exert a constant pull at the edges of the damper roller as it tends to shrink while drying. This is called belling. Belling of roller edges upsets the pressure distribution and a harder setting is required for the middle portion to transfer sufficient moisture to the plate. This results in excessive and uneven wear of damping roller covers.
SWELLING: Rollers, which readily absorb printing ink oils and cleaning solvents, become tacky and swollen. A good quality roller should have a low ink absorption rate and should also be resistant to chemicals and solvents especially in roller washes. Although its structure should not be affected by ink or solvents, a roller should have good ink acceptability while allowing easy transference of ink.
UNEVEN LINKING: Rollers with run out, uneven roller surface or depression on the roller surface results in faulty inking showing weak areas.
WORN OFF JOURNAL ENDS: This will give uneven inking, skidding or wipe due to run out or excessive play between the journal ends and the bearings.
PROBLEMS DUE TO IMPROPER SETTING: No matter how good the roller are, if they are not set properly – they can be the cause of many troubles including scum, streaks, filled half-tones, filled screen, ragged solids, retarded ink drying and excessive damping. This will have a cumulative effect on the job and will result in the loss of color value and poor quality impressions.
HICKEYS: These are the white spots, which usually appear in solid image areas. Hickeys are caused by the hardened specks of ink, dried gum, dirt etc. which form imperfections on the printed sheet by showing as a solid speck with a halo around it.
