Rubber rollers in today's printing processes.
Paper and ink are the raw materials. Printing presses provide the means to combine the materials into finished products. Without exception, these presses share a common requirement. They are all dependent upon the unique chemical and physical properties which rubber rollers possess. Without them, the presses could not function.
Four processes are widely used to produce printed products. They include lithography, flexography, gravure and letterpress. Over the years, the relative popularity of each of the processes has been driven by the perceived quality of the work that the process could produce, and by the cost of production associated with that process.
Letterpress is the oldest of printing processes. Ink is applied to raised metal images (type) which are then pressed onto paper to produce the printed image. Letterpress type has an appearance similar to the letters on an old typewriter. Pieces of metal type (each containing an individual letter) can be hand set to create complete lines or full pages of copy. Specialty machines called linotypes can cast complete lines of type from molten lead. Illustrations must be engraved by hand or by a photoengraving process. Letterpresses have evolved from hand operated machines on which each sheet was printed individually, to highly automated machines which print thousands of images on either flat sheets or a continuously running paper roll called a web. Running a letterpress is simple and economical. It is capable of producing high quality work while using basic, low cost inks. In today's printing marketplace, the high cost of preparing the type and the lengthy press make-ready time more than offset the operational economy. As recently as the early 1960s, letterpress was the most widely used printing process in the world.
Gravure is a process wherein a printing cylinder that contains an image area comprised of a pattern of tiny holes (called cells) runs partially immersed in ink. The ink fills in the cells, and excess ink is scraped off the OD of the cylinder by a blade. The image is transferred to the paper (normally a continuous web) by pressing the paper against the printing cylinder with a rubber covered roller. Gravure inks are free flowing liquids that are highly volatile and dry quickly. Gravure produces very high quality multicolor work at very high speed. It has been the process of choice for high volume, high resolution multicolor printing for much of the twentieth century. For many years, the high gloss magazine section of many Sunday newspapers was printed on high speed rotary gravure web presses, and was often referred to as the "rotogravure" section. Despite its high cylinder preparation and make-ready costs, gravure remains a viable, cost effective process for producing high quality, high volume magazines and catalogs.
Flexography is a process in which ink is applied to a raised image area that has been fashioned upon the face of a rubber printing plate. As in the letterpress process, the image is pressed against paper in order to provide a printed image. The process is similar to a common rubber stamp. For many years, flexography was considered to be a process which produced an inferior product which was suitable only for very basic single color work on corrugated cartons and simple labels. Its attributes included simplicity, combined with low equipment and operating cost. Because the process requires a minimal amount of pressure to produce an image, it found early acceptance as a means to print pressure sensitive substrates. With the advent of laser engraved plates, more stable plate materials and improved inks, flexography has matured into a high quality multicolor process which rivals offset lithography. Flexography has made substantial inroads into the packaging market which previously had been virtually the exclusive province of offset lithography.
Lithography is a process in which the image and non-image areas of the job are generated on the same plane of a photosensitive or thermal sensitive printing plate. In other words, the image is neither raised as in letterpress or flexography, nor is it depressed as in gravure. Lithography is based on the fact that oil, which is a basic component of printing ink, and water do not mix.
The construction of the undeveloped printing plate, combined with the imaging process, produce a virtually flat plate upon which the image areas are ink receptive and water repellent, while the non-image areas are water receptive and ink repellent. The actual printing process requires the use of a dampening solution (predominantly water) as well as ink. Water is applied to the plate, where it is attracted to the non-image area and repelled from the image area. When ink is applied to a dampened plate, the ink sticks to the image area only. The plate is pressed against a thin rubber sheet called a blanket. This causes the image to be transferred to the blanket, which in turn is pressed against the paper, creating the printed image. It is possible to print directly from the plate to the paper, but the addition of the blanket as an interim step produces a clearer, sharper image. Since the paper is not printed directly from the plate, the printing link is considered to be off-set. Hence the term "offset lithography." Offset lithography produces a very high quality product at speeds that previously were possible only in the gravure process. In addition, offset lithography provides the benefits of inexpensive plates and plate processing combined with fast press make-ready. Because of the necessity to balance ink and water coverage on the printing plate, offset lithography generally requires press operators with a higher degree of skill than is required by the other processes. High product quality, low preparation cost and high running speeds have made offset lithography the process of choice over the last several decades. Offset lithography currently accounts for more than 80% of the world's printing volume.
Each of the processes presents different requirements for the rubber rollers that it utilizes.
Letterpress and offset presses generally utilize inks whose primary components include color pigment and a vehicle which is often a vegetable or hydrocarbon based oil. A small segment of the industry utilizes ultraviolet cured inks to produce certain products. Instead of oils, UV inks utilize photosensitive monomers as a vehicle. Both the letterpress and offset lithography processes require the use of solvents for cleaning rollers and press components. Historically, these solvents contained high VOC materials such as long chain hydrocarbons. In recent years, there has been a migration to the use of low VOC solvents.
In addition, offset lithographic presses utilize dampening solutions which are largely water, but contain wetting agents, gum arabic and acid. Until recent years, the most common acid in use was phosphoric, but efforts to reduce phosphates have driven a changeover to other acids such as citric and malic.
Rollers used in the letterpress and offset lithographic processes are generally constructed with buna-nitrile, PVC-nitrile blends or urethane. Presses running UV curable inks utilize PVC-nitrile blends, and EPDM. Roller hardnesses range between 20 Shore A and 50 Shore A, depending on the function of the roller.
The primary use of rubber rollers in the gravure process is the roller which presses the paper against the engraved gravure cylinder. While there is no substantial chemical exposure, these rollers must be highly abrasion resistant, able to withstand high temperatures, and in most modern machines, be electrically conductive to accommodate a high voltage electrostatic charge which is used to assist thorough ink transfer from the cylinder to the paper. Materials used in these rollers include Hypalon, buna-nitrile and urethane. Shore A hardnesses range from 50 to 95, depending on ap-plication and speed.
The flexography process utilizes rubber rollers as ink fountain rollers and, to a lesser degree in more modern machines, as ink doctor rollers. Depending upon the type of work and the type of substrate, flexographic presses run a variety of inks. These include the solvent based inks which contain acetates, esters and ketones, water based inks including those containing aliphatic hydrocarbon solvents and glycols. Materials used in these rollers include bunanitrile, EPDM and urethane. Hardness ranges from 55 to 85 Shore A.
Since lithography is the pre-eminent process used in today's printing industry, let's examine a lithographic press and see what it is that makes rubber rollers so indispensable to the process.
Rubber rollers perform two critical functions in press operation. The primary function is to distribute and transfer quantities of ink and dampening solution to the appropriate portions of the printing plate. The other major function performed by rubber rollers is transport of the substrate (generally paper) through the press.
A closer look at the inking and dampening rollers shows that typical inkers and dampeners are constructed by alternating rubber rollers with hard rollers which are typically nylon or copper. The hard rollers are generally gear driven by the press, while the rubber rollers are generally friction driven by contact with adjoining rollers. The diameters of rollers used throughout the inker are normally varied in order to provide non-repeating ratios which serve to even out longitudinal ink distribution. In addition to being driven rotationally, the hard rollers are often driven in a side to side oscillation in order to provide a more even lateral distribution of ink.
The product delivered by a printing press is normally either a flat printed sheet or a printed and folded product known as a signature. The number of sheets or signatures to be printed in any given job may be less than one hundred or as many as several million. In every job, each of the primed pieces must be indistinguishable from the others when viewed with the naked eye. This feat is routinely accomplished at speeds as high as 3,000 ft./min. (approx. 34 miles/hr.). It is possible only when a precise amount of ink is applied to the printing plate with each revolution of the press. The amount of pressure applied between rollers and from rollers to the printing plate plays a critical part in controlling the precision distribution that is required.
Pressure between rollers is a function of rubber hardness and force applied to the roller(s). There is no practical method of directly measuring the pressure that a roller exerts on an adjacent roller or on the printing plate. Over the years, pressmen and press manufacturers have learned that a procedure known as "striping" will yield a reasonably accurate indication of the pressure being exerted by a roller. Assuming that the hardness of the rubber roller is within its design parameters, the procedure provides a quick, easily visible indication of the amount of pressure being applied.
If we examine a typical nip point between a stationary rubber roller and an adjacent hard roller we see that, as the center distance is reduced, the rubber roller covering is displaced and conforms to the outer diameter of the hard roller. The resulting area of contact between the two rollers is the "stripe" that is measured by the pressman in order to determine the comparative amount of pressure being exerted. Press manufacturers publish "stripe" settings and Shore A hardness recommendations for each machine that they manufacture. Much of this body of information has come from years of experimentation throughout the industry.
In order to transfer ink or dampening solution through the roller train, it is necessary for the fluid being transferred to generate sufficient hydraulic force to overcome the force being applied to the roller and to further displace the rubber by a precise amount, thus creating an opening for the fluid to pass through. An appropriately sized opening will allow a precise amount of fluid to enter the nip under pressure, and the fluid film passing through the nip will be split with half of the film staying with the supplying roller and half of the film being taken away by the receiving roller. Softer rollers obviously require less force to displace and will allow more fluid to pass. Too much fluid can cause printing problems known as mottling and scumming. Too little fluid can cause color variation and color density problems. If a roller hardens in use by 10 Shore A points, it can generate as much as 70% more pressure than it did initially.
Historically, the primary challenge to rubber roller manufacturers has been resistance to pressroom chemistry. Recommended hardness for most lithographic printing applications is normally in the range of 22 to 35 Shore A. Softening the rubber compounds to achieve these hardness ranges requires the use of plasticizers, which in many cases tend to chemically bond to high VOC solvents and subsequently flash tiff with them. Roller shrinkage and hardening have been the cause of considerable expense to the printing industry for many years. The costs include the obvious necessity of replacing the rollers which have hardened, and also the production time lost to resetting rollers as shrinkage occurs. Roller manufacturers have expended a considerable amount of effort and expense to improve the chemical stability of their products and to extend their useable lives.
As the industry has begun to migrate to low or no VOC solvents, a new set of challenges has faced the roller manufacturers. The low VOC solvents tend to cause swelling in the rubber rollers, and the swelling tends to be non-uniform. Localized swelling, particularly at the ends of the rollers where the ratio of surface area to rubber volume is greater, creates roller setting problems wherein the pressman cannot adjust the roller to obtain a uniform stripe across the roller. The non-uniform stripe is an indication of uneven pressure which is most often addressed by increasing overall pressure until the least heavily loaded portion of the roller meets the minimum stripe recommendation. This creates an over pressure situation in the areas where the roller is swollen, and generally has a negative effect on the quality of the printed work. Excess heat is generated which in turn aggravates the swelling condition until the roller suffers hysteresis failure (blowout) in the affected area. In recent years, roller manufacturers have addressed this situation via compound modification, mechanical changes in roller design and educational materials to assist pressmen in dealing with these new challenges.
As press speeds have increased (as much as 100% in the past ten years), the issues of heat and mechanical accuracy have gained importance as challenges to be faced by robber roller manufacturers. At current press speeds, a three inch diameter rubber roller will be compressed and released more than 7,500 times per minute. The roller must fully recover from each of these compressions within approximately .008 seconds in order to ensure consistent ink distribution. In most cases, this performance is required 24 hours a day, seven days a week at operating temperatures ranging from 80 to 120 [degrees] F. Diameter variations of .004 in. caused by either manufacturing imperfections or by heat expansion can affect pressure by as much as 50% and consequently cause ink or water distribution difficulties which are unacceptable to the printer.
In summary, the printing industry has presented the rubber roller manufacturers with a number of new challenges within the last decade. The industry, has seen more change in the past decade than in the previous thirty years, The largest paris of the answers to these challenges have been or are being resolved by new developments in the compounding of the rubber covering materials. The developments include advances in chemical and heat resistance, rebound speed and machinability of materials.
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|Comment:||Rubber rollers in today's printing processes.|
|Date:||Oct 1, 1999|
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