Mounted on a cylinder, it is the basic element for reproducing the images on the printing medium
The blanket is the key element for offset lithography to reproduce the images we want to transfer to the printing substrate with maximum accuracy. It is a semi-rigid surface, made of rubber and other materials, which, mounted on a cylinder, receives the image to be printed from the plate and transfers it to the substrate.
The earliest blankets were made of pure rubber, which allowed to get a much higher quality image to be reproduced than lithographic plates, but also caused technical problems. With the invention of synthetic rubber in the 1930s, these problems disappeared, and this gave the final push to the new printing technique..
Later, the arrival to the market of Expancel microspheres improved the strength and compressibility of the blankets. The innovation from the Swedish company Nouryon consists of small thermoplastic spheres filled with gas which, when heated, expands, while the blanket softens, giving a dramatic increase in volume to the whole without changing its weight and maximising its properties..
How is a blanket made?
The blanket consists of a reinforced fabric backing made of cotton, with polyester yarns to increase its strength. The rubber compound is mounted on the casing, which is pre-stretched to simulate the tension it will endure when it is ultimately mounted on the press. The strength of the fabric must be greater lengthwise than across its width, in order to avoid deformations that could impair the quality of the print.
To get an idea of the structural complexity of the blanket, the casing can be made up of several layers of fabric, and the rubber structure can have as many as eighty individual layers. Throughout the production process, it is essential to control environmental factors such as temperature and humidity to keep the thickness of the structure within acceptable tolerances. Finally, the blanket is vulcanised to improve its strength and dimensional stability.
It must be noted that the blanket must fulfil two essential functions: transfer, with the highest quality, the image to be printed from the plate to the support and absorb shocks caused by incidents (paper jams between cylinders, for example) or irregularities in certain substrates.
There are two basic types of blankets for offset printing: conventional or hard. The hard ones are used in applications that require higher than normal printing pressures; and compressible blankets, more frequently used, as they offer more resistance to deformation, without losing quality, and have a longer useful life.
Accuracy is a core element in offset printing. Therefore, blankets should not be mounted on the press unless all the details relating to technical specifications are known. Gauge, quadrature, and cylinder alignment are critical mechanical parameters. Cylinder ring pressures must be checked and adjusted regularly to keep them within the required specifications.
Here we list a few maintenance tips that must we followed in order to maintain the properties of the blanket and extend their service life:
- Check automatic washing systems and perform preventive maintenance.
- Avoid solvents that can damage the surface of the blanket.
- Watch out for possible accumulations of ink in the printing cylinders.
- Avoid excessive pressure, as it would shorten the useful life of both the blanket and the plate.