The rubber printing blanket is an all-rounder medium which comes into contact with every component relevant to the printing process. It has to transfer type and picture elements as accurately as possible from the printing plate to the printing stock. This is a challenging task. Take a quick look at the deciding parameters in the following summary. Not only should a rubber printing blanket be able to take on ink well, but also to dispense it in the same way.
Furthermore, it also has to convey dampening solution evenly with all the chemical ancillary products and restrain depositing on the areas not to be printed on the blanket, so that it is not transferred to the printing stock.
By the action of the dampening film remaining on the rubber blanket, paper dust or powder build-up can be minimized.
However, the printing ink should be completely transferred to the substrate. Absolute uniformity without stripes and structures must be obtained here, so that the lowest possible dot gain can be achieved. Constant process stability is attained through high edge definition. Solid areas should be transferred evenly and without mottling.
The specific characteristics of the blanket used are also responsible for the rapid release of the substrate. So that material stress is as low as possible, we need a good quick release (QR) value. By low adhesion (good QR value) registration problems are minimized.
In order to ensure an absolutely homogeneous transfer of dots and solid areas, a blanket needs a shock absorber (compressible layer), to compensate for any unevenness of the substrate. The aim is to transfer dots and solid areas almost as if the printing substrate had a flat surface. This is why printing blankets have different structural construction. There are two-ply, three-ply and four-ply rubber blankets. Those layers consisting of fabric and intermediate layers, depending on the different qualities, are filled with overpressure.
In order to achieve a long and stable process life, the roughness of the surface layer, as well as physical and chemical resistance, is an important quality indicator. Outdated storage (maximum 12 months), however, can render the quality advantages useless. In this connection, please also refer to the chapter on storage of blankets.
The cleaning agents can have both a positive and negative effect on the durability.
The aging process and thus the printing characteristic curve are significantly influenced by the number of rotations. Here statistics should be maintained, so that it is clear as to whether a cheaper blanket also works effectively.
If there are any irregularities in the printed image, the following should be excluded:
Wrong packing (height or hardness) may be responsible for slurring and ghosting
The packing height can be quickly measured with a dial gauge. The hardness is determined by the number and type of underpacking. Furthermore, it must be ensured that the printing blanket was tightened with the correct torque for the printing machine used.
Only then can optimum compressibility and durability be guaranteed.
If a mottled print cannot be attributed to printing stock or ink, defects in the surface or construction of the rubber blanket may be the cause.
Here is a test for a fast check. A single proof from the printing press to be tested, which should include a solid area without dampening, is often indicative for the quality of the printing blanket. Wrong tonal value reproduction that is above or below process standards for offset printing, is often an indication of problems in the printing process or wrong compression.
Registration difficulties can result from the reasons stated above by the interaction between the rubber blanket, substrate, ink and the quantity of fountain solution.
Here again is a summary of the consequences of wrong compressive stresses:
Compressive stress is too high:
Increased dot gain, vibration strips, ghosting, high ink laydown caused by excessive build-up of the printing blanket temperature, unnecessary strain on the machine bearings and thus faster wear, poor paper release, wrinkling, fan out, increased sinking of the rubber blanket, reduced mileage of the blanket. Printing plates breaking at the canal.
Compressive stress is too low:
Too low tonal value increases, slurred solids print, pushing, linting, register variations.
In web offset, an even web tension must be achieved, so that optimal paper transport takes place. Register and print repeat accuracy are significantly influenced by the printing blanket. The transport properties of a blanket can be positive, neutral or negative.
Blanket attachment is done with different bar systems. The different varieties of shapes, materials and adapter systems are supplied by us. Through custom conversion of your blankets, we can match all dimensions you require. The bar width, the bar length, the clamping aid (stencil template) – simply everything.
Ask our expert advisers who will be glad to offer assistance.