The blanket for Offset printing is very important part of process of printing and has direct affect of quality of printed image. It has to be constructed to meet very high demands not only to to transfer a quality image from the plate to the blanket, but can function at high speeds with different papers.
At the same time, it also has to convey dampening solution evenly, and correct irregularities in thickness in the printing stock. The most important features that blanket for offset printing must have are:
– Tensile Strenght – Blankets should only be tightened around the cylinder with just as much force as they need to not move on the run, nothing more. The reason being is that the tensile strength, or the ability of the woven fabric to withstand the pull around the cylinder, is only so strong. The basic idea is to have the blanket stretch as little as possible for two reasons:
1.) The blanket can rip.
2.) Blanket height is lost as more torque is applied to the mounted blanket.
– Solvent Resistance – Blanket must resist the tendency to swell, crack or distort when coming in contact with chemicals (inks, dampening solution, …) because this will result with distortion in the image.
-Caliper– Allways check caliper of blanket before putting her on machine. Check all 4 corners of the blanket and accept tollerance no more than + or – 1/1000 of an inch. If caliper is irregular it will lead to dot gain or can even cause tension.
– Compressibility – compressibility is the single most important factor influencing dynamic performance on press and print quality. With reference to offset blankets, compressibility is defined as the volume reduction capacity of a substrate under load. Offset blanket compressibility is a very important factor in the printing process. Having the proper range of compressibility will prevent excessive printing pressures and should help to lengthen the lifetime of the press, blankets, and plates. Furthermore, the compressibility factor also allows the blanket to recover sufficiently and quickly after smashes without resulting in distorted print quality. And, last but not least, it considerably shortens make-ready times.
-Surface Release – Typically, the smoother a blanket is, the better the image it will reproduce on the paper. The problem is that the smoothness will not release the ink as easily as if it had a rough surface. There must be a balance.
-Stretch – There is usually at least one layer inside the blanket that consists of a sewn fabric. The is literally the strength of the blanket that holds it together. It is sewn in such a way that the most strength is given to the circumferential property. When tightening the blanket, a certain amount of stretch will occur as these threads pull on one another. However overdoing it will cause your blanket height to go down and possibly rip the blanket.
-Squareness – Blanket manufacturers will sometimes send blankets that are not completely square. Or possibly, they were not square to how the fabric was sewn. This will cause major tension and slur problems so allways must be a quality check before mounting blanket on sheetfeed press.
The two main component of blanket for offset printing are :
The offset blanket surface structure, profile, and hardness are extremely important and contribute significantly to the printing performance of an offset blanket. Additionally, surface imperfections will certainly cause printing problems; therefore, production standards are set to guarantee the highest quality of printing surface. It is not an easy task to develop a suitable rubber compound for the printing surface of an offset blanket utilized for high quality offset printing. The difficulty is due to the conflicting chemical and mechanical requirements which can be found during the printing operation.
The blanket surface must exhibit a “dual” personality. The surface rubber compound must have the capacity to take the maximum amount of ink possible from the printing plate without distorting the image and transfer it almost half way around the cylinder to the printing stock. This precise operation must be done at very high circumferential speeds. The tack of the printing surface must remain low in order to ensure a minimum build-up of paper dust, dirt, and ink.
The Carcass – The carcass basically refers to everything sandwiched underneath the surface layer.
Layers in the offset blanket have two designations: a fabric layer and a compressible layer. The fabric layer consists of strong quality thread woven at right angles. Each direction is called a warp or weft. The warp is considered the strongest and so the blanket is constructed in such a way that the warp wraps in the circumferential direction around the cylinder. This direction receives the most stress when it is tightened against it. Comparably, very little stress is put laterally on the blanket. The compressible blanket contains a layer that is sponge like in design. This layer can have a closed or open cell design. The open cell design allows air to flow throughout the blanket and has its applications. However the more popular design is the closed cell since it retains its compressible state longer and has an increased resistance to smashing over a longer period of time. Good ink transfer, paper smash resistance and good dot gain are only a few of the the qualities a good sheetfed blanket needs to provide but they can make the difference when you decide, it’s not the cost per blanket but the cost per copy which should influence your choice.