Introduction
The rollers in a printing machine are designed for a specific task: to supply a properly prepared film of ink to the plate. Without the inclusion of high quality rollers, printing becomes a more difficult process. In this section, we discuss rollers and blankets and how they interact with UV printing and coating.

Roller rubber:

What is it?
Specially formulated for each particular end use, rubber rollers vary in length, thickness, composition and final physical properties. To understand the complexity of rubber rollers, it is worth reviewing composition of a product critical to the printing process.
Roller rubber is typically an elastic material that is capable of returning to its size and shape after being under pressure and in contact with other objects or materials. The base materials are commonly called elastomers, a word derived from the two root words “elastic” and “polymer”. A polymer is a term used to describe a chemical whose molecules have many repeating structural units (like a chain). It is necessary to add other chemicals to polymers to change them from a “plastic” to an “elastic” condition. While there is a varied choice of ingredients to alter the final polymer, modified polymers are referred to as compounds.
Note: Not all elastomers are necessarily rubber as other matter such as polyurethane and polyvinyl chloride (plastics) qualify as elastomers.

Roller formulation
Roller manufacturers’ specifications should be adhered to if the optimum performance is to be obtained from a set of new rollers for a specific process.
The wash-up procedure is especially important as it has great influence on the expected life of the roller material. There is a considerable difference between simply removing ink from rollers and the thorough washing of rollers to ensure their continuing high performance. New rollers ink-up and perform well, and the wash-up at the end of the run is completed quickly and efficiently. These characteristics can only be retained throughout the useful life of the rollers by proper wash-up and maintenance procedures.

UV roller application
The market has certainly come a long way since rollers and UV came together. Initially, rubber materials were rigid in their formulation, but, through experimentation, rollers have progressed to meet the stress levels associated with UV printing.
Two types of rubber material are normally used within the UV market place:
EPDM and BUNA (commonly known as “nitrile”). The two products are totally different. For example, nitrile can be used quite successfully with UV inks providing it is of a low swell formulation, but not with UV varnish unless it is an aqueous varnish. On the other hand, EPDM can be used with UV inks and UV varnish. The difference between the two products from a machine setting point of view is the shore hardness factor, or the duro factor. EPDM for example, is usually restricted to a minimum shore hardness of 40 versus nitrile formulations which can be reduced down to approximately 30-35 for UV printing.
 

Conventional printing and UV printing
It is important to understand how these two rubbers react when printing UV and conventional. Firstly, EPDM is not suitable to run with conventional inks as it has a very poor resistance factor to ASTM No.1 or No.3 oil, aromatics, toluene and hexane. Nitrile has a poor resistance to ketones and chlorinated solvents, such as methylene chloride.

Dual purpose roller
The dual purpose roller, which is extensively used today, is an optional roller which allows a printer to change between UV and conventional inks.  As an optional roller, it has limited life due to the change from UV to conventional ink on a daily or hourly basis. To maximize roller life, it is recommended that only one process be used. As many companies initially run UV for specific jobs only, and maintain 80-90% of their conventional workload, this satisfies the use of the dual roller. When changes become more frequent than is practical, it is recommended that the company run either UV or conventional on a specific set of rollers.

Recommended wash-up systems
As previously stated, certain products are not acceptable for either EPDM or nitrile. Therefore, it is essential to stay away from any product that tends to swell rollers unnecessarily and use only recommended washes.

UV application
We have found that rollers returned at the end of their life period are softer than they were when supplied. This indicates the harshness of the products that are used within the UV process. This can be broken down into two areas as follows:
UV ink in many instances is not washed off the press as it will not dry under normal circumstances as compared with conventional ink. This ink, however, does penetrate the rubber surface when left on for long periods of time. Therefore, companies are required to evaluate the cost of roller replacement on a more regular basis should they not wish to wash-up daily against the cost of wash-ups and inks that would be wasted by washing up. Many companies build the cost of rollers into their jobs over a given period of time.

Wash - conventional
When using conventional printing inks or UV inks, it is important that the second (or flush) step of your UV roller wash be compatible with your conventional printing system. Such products are available. In addition it is important that washes do not contain any methylene chloride as this product is prone to swell rollers excessively.

Blankets for UV use
The early use of UV curing products caused ink, roller and blanket manufacturers to review the technology of their products. Blankets especially were the target of some very critical comments. The technology to produce more suitable products was found and blankets generally became more appropriate for the task. Products from blanket manufacturers are now eminently suited to the UV printing process.
 

Compressible blankets are available which have materials such as EPDM and nitrile rubber included in their composition. Some of the blankets which were earlier available caused problems as they were susceptible to cuts from the paper and board stock being used. Materials such as PVC and other plastics also were extremely hard on these blankets.
Most printers today rely on the technological advances of blankets which feature high resistance to UV inks, varnishes and coatings.
 

Mounting blankets
In the mounting of a blanket some guidelines should be observed. If the blanket is simply placed on the press and tightened, within a few thousand sheets problems may begin to appear. Therefore, it is better to use a progressive method of tensioning the blanket to ensure long life and better resistance to smashes. The blanket should be tightened initially, then run under pressure for several minutes, and retensioned using the devices provided. This process is repeated after a couple of minutes under pressure and again after about one minute. The result is a blanket which is properly and evenly tensioned around the cylinder. It is also more resistant to smashes than one tensioned tightly initially and retightened after fifteen minutes or so under pressure. Nothing is gained in terms of quality printing if the blanket is overtightened. In fact, the rear part of the blanket may be somewhat thinned with the plies distorted by overtensioning.

Washing UV blankets
Blankets which are being used for UV printing and coating should only be washed with UV compatible solvents. If the solvents used are of a conventional-type, curing problems may be experienced if the UV inks absorb these petroleum solvents. It should be noted that not all UV products are identical in composition. Even though quality UV products contain components which are specified by safety standards, some washes, inks and coatings contain aggressive substances which can adversely affect rollers and blankets, causing swelling and embossing.

Wash-up blankets
Whether your system includes an automatic blanket wash-up system or not, blankets should be treated with a renewer on a regular basis. This treatment should be performed each day on presses that are washed more than once a day, or once a week on presses that are washed once a day. A good renewer will not only lubricate your blanket, but maintain an excellent surface for ink acceptance.

Conclusion
The selection of proper materials for rollers and blankets is essential for consistent, trouble-free results. Fortunately, the manufacturers of these products are able to supply products to suit most applications. They have no control however over the inks and coatings used, which can vary considerably in their effect on rollers and blankets. Materials that are very aggressive should be used only sparingly if a reasonable life is to be expected from the rollers and blankets.

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