- December 01, 2000, David J. Bentley, RBS Technologies
Impression rolls play a very important part at various points in many different converting operations. They are usually small rolls that are perhaps 3-4 in. in diameter. The rolls have a rubber surface. Their construction is either all rubber or, perhaps more commonly, a rubber-encased metal shaft. The job that these rolls do sounds very simple: They keep a moving web or substrate in contact with another roll.
The two most common uses for impression rolls are at applicator stations and at laminating stations.
At an application station, an impression roll presses the moving web against another roll that has a covering of fluid ink, adhesive, or coating. This other roll may be an etched metal roll, a smooth metal roll, or a roll with a rubber surface. Its liquid coating transfers to the substrate to be coated as a result of the impression roll forcing the substrate to contact the coating.
At a laminating station, the impression roll causes the secondary or uncoated web to contact the adhesive-coated web as it travels around a heated metal roll.
In both instances, the rubber impression roll is under a force that causes the roll to exert pressure onto the web on which it is acting.
The amount of pressure that the impression roll exerts is important. Converting lines usually have adjustable clamp-type devices that allow the operator to control the level of pressure. Clamping down on the impression roll causes it to exert more pressure. Easing up on the clamp reduces the level of pressure. For an applicator station, the amount of pressure exerted must be sufficient to ensure that all of the coating transfers from its initial surface onto the substrate.
Insufficient pressure will result in transfer of only part of the adhesive, coating, or ink. The ensuing low coating weight obviously would be unsatisfactory and cause problems in the converted product.
Excessive pressure can be equally undesirable. In this case, all the liquid film will transfer, but the substrate may abrade on the surface and become deformed or may even wrinkle. Again, the converted product is unacceptable.
For a laminating station, the impression roll must provide sufficient pressure to ensure complete contact of the coated product onto the uncoated substrate so that complete wetting occurs. Insufficient pressure obviously will cause unacceptably low bond values. Excess pressure can cause wrinkling.
Many converters are not aware of the importance of the hardness of the rubber impression roll. A good hardness is usually 60-90 measured as Shore Durometer A. Measuring the hardness involves placing an instrument containing a blunt point onto the rubber surface. The point is under pressure, and this pressure will cause the point to deform the roll. A gauge on the instrument measures the degree of deformation.
High deformation equals a soft roll and gives a low reading for hardness. A high reading indicates the rubber is very hard and does not deform easily. Converters should check the hardness of their rubber impression rolls periodically to be certain they are within the acceptable range needed for the particular job. Depending on the particular rubber used to manufacture the roll, it may have a tendency to become hard after a certain period of use.
Many rubbers will degrade upon exposure to oxygen or solvent vapors and other contaminants in the atmosphere of a converting operation. As they become harder, the impression rolls do not deform as well and therefore do not exert the requisite amount of pressure.
As an impression roll initially starts to become harder, an operator can compensate for this tendency by increasing the amount of pressure resulting from the clamp. As the roll becomes increasingly harder, the amount of force will become increasingly greater. Eventually, the roll will become extremely hard, and the pressure exerted through the clamping will reach a maximum. At this point the operation definitely needs a new impression roll.
Converters should work closely with the suppliers of their rubber impression rolls so they use the proper roll in the correct way.
David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at firstname.lastname@example.org.