It is the raw rubber that gives the rubbery material its general characteristics. The vulcanization agents are added to this for the subsequent vulcanization method.
Plasticizers (liquid oils) play a critical role as they – in addition to the extenders and the vulcanization agents – affect the future degree of hardness of the material. The more plasticizers are added, the softer the rubber after the vulcanization operation.
Shore Hardness
The Shore hardness of a roller determined with a Shore meter indicates the resistance against penetration of a needle taking the
form of either a conic section (Shore A) or a point (Shore D), pressed with a defined force (1 kp) for a period of three seconds
against the rubber surface. Soft rollers are measured with Shore A, and hard rollers with Shore D.
Inking rollers should be adjusted to be oil-friendly .
In an inking system having conventional inks, rollers of Shore A hardness approximately 30° and 35° are used. For UV inks, rollers with Shore A hardness of 25° and between 40° and 45° are used.Dampening rollers ought to be water-friendly (hydrophilic). For alcohol dampening systems, Shore A hardness of between 25° and 30° is suitable.
In direct dampening systems, hard rubber rollers are used.
Roller Preservation
For cleaning the rollers it is advised:
1. Use a water miscible wash to remove the top layer of solvent-soluble and water-soluble particles from the nap of the roller.
2. Apply a non-grit roller paste to get rid of the insoluble particles.
3. Finally, wash a second time by using a water-miscible wash to get rid of the remaining soluble particles.
Glazing
Over time, rollers absorb particles from the printing ink, the paper coating as well as the dampening agent which block their pores. This contamination results in a smooth shiny surface. Unsuitable detergents and hard water contribute to this effect. Transport of ink and dampening solution is substantially inhibited due to this fact.
Problems with irregular rollers
A worn out roller, a roller with damaged bearings, rollers set with too much pressure or, a roller that has not been efficiently cleaned, could cause:
* color inconsistency
* reduced solids, or mottling
* streaks
* dirty print
* abnormal dot gain
Besides the obvious charges of inferior print quality, including wasted paper and ink, bad or poorly maintained rollers can cause excessive press downtime, raised running costs, and shorter roller life.
Storing of rollers
When possible, don't expose rollers to direct natural or artificial light.
• The special paper that the rollers come delivered in prevents light entering ,thus helps retain the rubber’s characteristics.
Due to this, do not remove the paper before applying the roller in the printing press.
• Always put the roller with its pivots on a framework support: Putting it down directly onto the rubber can cause permanent deformations (pressure marks).
• Keep rubber rollers in a cool and dry place.
• Under certain circumstances ozone causes microscopic scratches on the rubber surface. For this reason never store rollers in the vicinity of motors or other electrical instruments.
Rollers Used In Offset Printing