➤ Big questions surround the job done by a printing blanket on a newspaper press, and its influence on process standardisation. What effect does the sinking of the printing blanket have on the dot gain value and delta E values? What is the correlation between the compressibility and required nip forces? How do face compound polarities and surfaces affect printing quality? What are the feeding characteristics of a blanket and their significance? These are questions Stephan Peters, sales and technical manager for Conti-Air Asia Pacific, has sought to address in talks at technical forums in the region. “What it all comes down to how significant a blanket is for the standardisation of offset printing processes,” he says. Peters says its importance as a quality factor in printing process is often underestimated, “although it transfers the printed dot and thus the quality to the paper”. It also absorbs vibrations from the machine and compensates for different paper grades and qualities. “In the printing process, the blanket affects a series of key variables and parameters which reflect the dot gain value of the printing machine, which in turn is the basis for the ICC profile of the machine and therefore complete digital workflow,” he says. “These gains should be stable throughout the life of a printing blanket in order for the desired printing quality to be reproduced.” That makes blankets an important product for printing to the ISO 12467-2 (heatset offset) and ISO 12467-3 (coldset newspaper) standards. “With its Conti-Air brand, ContiTech is one of the world’s leading manufacturers of printing blankets,” says Peters. “The sole German manufacturer of blankets, the high-tech company which belongs to Continental AG, has the know-how to have a major impact on and improve process stability.” Blanket characteristics such as sinking, nip speeds, face compound polarities and grinding technologies, different compressibility and face compound manufacturing process, as well as various fabric types are all quality features that are specifically controlled by Conti-Air, as they have a direct impact on the printed dot and thus the dot gain value. The three-ply (1.69 mm) or four-ply (1.95 mm) printing blanket is a composite consisting of a solid rubber, a compressible rubber and various fabric plies and grades. Conti-Air fabrics, which are woven exclusively in Germany, determine the stretch resistance vital for tensioning the blanket. Factors affecting dot gain include the elasticity of the face rubber compound and the characteristics of the compressible layer. ContiTech was the first to produce a blanket face compound and compressible layer using solvent-free and environmentally-friendly calendering technology. Pressure in the nip The compressibility of the printing blanket affects the pressure between the plate and blanket cylinders as well as the blanket and impression cylinders in the printing nip. For optimum ink transfer, the industry strives for pressure between 70-100 N/cm2. Compressibility is determined by the design of the blanket carcass and the product process used for the compressible layer based upon the microsphere or blowing agent: “For some 120 carcasses available from various manufacturers, little attention is paid to the compressibility of a printing blanket when determining the underpacking,” says Peters. “This is the same as if all shock absorbers for cars and other vehicles had the same stiffness and maximum load ... be no choice between soft or stiff when tuning a chassis.” He says height loss should be as little as possible, to ensure dot gain stability during the lifetime of the blanket, and keep delta E values within ISO 12647-2. Peters questions the practice of overpacking as dot gain will vary until the blanket has reached its final height. Composition and construction also affects feeding characteristics, while the polarity of the face compound – with characteristics to suit the application – grinding technology and the surface are also important. “The shape and uniform structure of the printed dot should be the same on the paper,” says Peters, “and as far as possible independent of the paper grade used. “This complex task places high demands on modern printing blankets, which as a key link in the offset printing process standards are often underestimated. “Key parameters such as sink and feeding characteristics, polarity, compressibility and face compound technologies can be adjusted by ContiTech to ensure a constant dot gain value over the life of a blanket. “This constant dot gain value is, in turn, the basic requirement for determining the ICC profile of the machine and for certification in accordance with ISO 12647.” • Based in Hanover, ContiTech is the world’s largest rubber and plastics technology specialist in the non-tyre rubber sector. It has a workforce of around 24,000 and is a division of automotive supplier Continental Corporation. Conti-Air Asia Pacific 32/211 Soi Nuan-chan 12 Nuan-chan Road, Klong-kum Bueng-kum, Bangkok 10230 Thailand conti-air@pegras.com www.contiair.com