Registered in England No. 1725990 © Pomeroy Pressroom Products Ltd 2010 Website design by: AH Designs Printing Blankets Vegra Rapid Washes Q & A / Advice - Blankets: The blanket is a fundamental part of the offset lithography process and its construction and condition can dramatically affect the printed result. All too often, however, the offset blanket is considered to be of minor importance and, as a consequence, is the cause of many costly printing and downtime problems. Although modern blankets offer better performance than in the past, there is now a greater range available, making the correct choice of blanket ever more difficult. It is therefore crucial for a printer to understand the basic concept of a blanket, what it is designed to do and how different types of construction can affect the printed result. While many printers take much care and consideration in specifying inks, fountain solutions and washes, the blanket is often forgotten. In essence, an offset blanket transfers ink from the plate to the paper and, during this operation, must ensure a consistent image quality, while giving good ink and paper release. The choice of blanket is determined primarily by the stock being used. So for rough papers, a soft surface which easily deforms to the irregularities of the paper is advisable, while for a smooth surface a hard blanket will tend to give better results. Beyond this simple definition, however, their are many other factors which can affect the choice, these include release characteristics, which increase with the hardness of the blanket, dot reproduction and the resistance to abrasion. Blanket Construction: There are basically two types of blanket construction:  conventional and compressible. Both of these consist of two essential components, the carcass and the surface layer. Conventional blankets have carcasses which have changed very little over many years. They usually contain two or three piles of fabric, bonded with adhesive to form a laminate of a required thickness. The laminated section provides the stability and flexible strength required to mount the blanket on to the cylinder and, in addition, gives a degree of compressibility. The compressible blanket gives printers the flexibility to utilise various types of stock with minimal press adjustment. By comparison, compressible carcasses incorporate a layer of microcellular sponge between two of the fabric piles, to increase dramatically the degree of compressibility. This layer is usually between 0.25mm and 0.5mm thick and allows the n;anket to recover rapidly from indentations suffered on the press. In general terms, the greater performance of compressible blankets has made them popular. They tend to give better dot reproduction, are more resistant to smashes and are able to handle a wide variety of paper, board or tinplate without press adjustments. The surface layer of the blanket is designed to transfer ink from the plate to the paper and is typically manufactured from a number of proprietary compounds, the choice of which depends on the application: It should have the following characteristics: o Be of precise and uniform thickness. o Be free from blemishes or flaws. o Be of uniform hardness. o Be receptive to ink. o Be resistant to swelling after applications of cleaners and solvents. o Be resistant to blistering and glazing. o Be able to transfer and release ink. o Be able to release the sheet. Specifying the correct blanket: Many printers rely on the adivce of their supplier when choosing a blanket. Nevertheless, there are a variety of factors which the supplier should be made aware of before they can recommend an appropriate blanket, these are: o The type of press (sheetfed or web fed); and if blanket to blanket. o The type and quality of image to be produced. o The substrate to be used. o The ink or coating to be used. o The diameter and width of cylinders. o The cylinder undercut. o The speed of the press and risk of minor smashes. o If multiple or single printing units are being used. o The packing or type of underblanket used. o Whether the press is non-bearer or bearer to bearer type. Getting the best from your blanket: Once the correct blanket has been chosen, there are a few basic rules which will increase the performance and longevity of the blanket. Theses are: o Always store blankets flat and face to face. o If packing or underblankets are used, check the total blanket height and follow the press manufacturer’s instructions. o If possible, seal blanket edges to prevent the ingress of solvents. o Follow press manufacturer’s instructions for press settings. o Use good quality blanket washes. o Wash blankets on a regular basis. Blanket problems: Some of the most common blanket problems include: Hickies and pinholes, Specks or holes on the image surface. Caused either by pinholes in the blanket, which are often a result of a smash or, if a solid dot with a halo,  by paper dust or dried ink on the surface of the plate. Blanket glazing, The blanket has a glazed, hard shiny surface and fails to transfer ink correctly. Usually caused by the use of low grade cleaning products which leave ink residues, or sometimes drive gum arabic emanating from the fountain solution. Mottled print, Recognisable when halftone areas appear to be mottled or bruised. Can be due to lack of pressure, particularly with compressible blankets or, if both blanket and pressure are correct, by poor paper. Face removal, This is due to the surface layer coming away from the carcass. Predominately caused by a build-up of ink outside the sheet or web area.