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Choosing the Right Roll Covering

The roller industry uses the widest range of elastomers of any segment of the rubber industry. This is largely due to numerous physical properties required as well as the variety of solvents and chemicals rollers are exposed to. Rollers are used in a variety of Industries from paper and printing industries to steel and plastics. These applications have all their own unique parameters that must be met. Whether it is solvent or chemical resistance or hardness and modulus requirements, each compound must posses certain attributes to be and effective roll covering.

Rollers are covered with elastomeric compositions typically to create a uniform pressure zone called "nip". The properties of roll covering must be constant and not degrade in use. The selection of the roll covering material must not change appreciably in use. This will largely be determined by solvent and chemical resistance as well as the dynamic properties of roll covering.

Elastomeric materials, or rubber compounds, can be modified, or tailored to some degree to provide flexibility within their own inherent limitations. A separate topic, not to be covered here, has to do with the interesting subject of formulating rubber. By the proper selection of various compounding ingredients, elastomeric materials can be altered to vary hardness, strength, and other characteristics.

Selecting the right roll covering is basically a process of matching the requirements of the application with the characteristics of available elastomeric materials.

The following is intended to be a guide in making the right match. It consists o a series of questions which, when answered, will help narrow the choice.

1) What worked?
As most roller manufacturers are involved in the recovery of old, worn out rollers, it is a valid question to ask what worked. As the saying goes, "If it ain't broke, don't fix it". Unless the customer is looking for some-safest course to follow is to use the same type of covering as was working on the roller previously. If you do not know what the previous covering was, cut off a sample from the roller and send it to Hevea Laboratory for Identification.

2) Obviously, knowing what the material was that didn't work is valuable information, atleast to help eliminate those types of compounds from future considerations. A word of caution however, is that sometimes a roller covering doesn't work because of misuse, abuse or because a wrong hardness of rubber waas used in the first place.

3) What does it do?
Understand your customer's application to the degree he will let you. The more you know about what the roller's function is, the better you can match the covering.

4) How hot is it?
Each elastomer has its own temperature limitations; if the roller operates at elevated temperatures, it is important to know how hot. Some hidden aspects to the "how hot" question are:
a) Is exposure to high temperature intermittent or continuous?
b) Does the roller surface come into direct contact with the heat source, such as a hot metal strip?
c) Is the actual surface temperature of the rubber as hot as the surrounding environment?
d) Is the roller being cooled internally?

5) What comes in contact with the roller?
Inks pints, coatings, solvents, or chemicals? Again, each elastomer has its own inherent chemical characteristics which dictate where it can and cannot be used satisfactory. Give special consideration when blends are more severe in their attack on rubber than the individual chemicals.

6) How much?
If the roller surface is exposed to something, it is important to know how much. Is it one chemical or a bend? Is it diluted? If so, to what level? What is it diluted with? Is it direct contact of vapors only? MSDS sheets are helpful, but they are usually written in general terms and do not give exact percentages, often leaving off key items. Knowing both the nature of what comes into contact with the roller and at what temperature, is extremely helpful.

7) How fast?
Speed, in terms of R.P.M., is important. Roller applications usually involve a "nip" region, where whatever comes into contact with the rubber creates a distortion in the rubber surface momentarily, not unlike a tire on a car at rest. As the roller rotates, the rubber is continuously distorted at the point of contact, then recovers. The faster the roller turns, the more frequent the rubber flexes. Many roller applications involve at least two "nip" regions; thus, the rubber flexes twice per revolution. The dynamic properties of some elastomers are better than others. For example, NE > BN > XNBR.

8) How did it fail?
A wealth of information can be gleaned from inspecting the worn-out covering. Did it crack, shrink, glaze, abrade, cut, or swell?

Also, how long did the roller last before it failed? Some applications for rollers experience life expectancy of days or weeks due to the specific conditions, others may last for years. But asking "how did it fail and how long did it last", may lead to a better selection.

9) How hard?
Hardness is usually dictated to roll company by either the equipment manufacturer or the customer. It is generally determined by the function the roller serves in the application itself. Use caution in recommending a specific hardness unless prior knowledge is available as a guide. Keep in mind, due to age, the hardness of the old worn-out covering your customer requested that you remake with their "same" material, is probably considerably harder than it was as a new roller. Cut off a piece and try to get a better reading on harness from a new surface not exposed to the working environment.

10) What is unique?
One of the most interesting aspects to the roller business is the variety of roller applications. There are all different types and sizes, operating conditions, functions, elastomer, hardnesses, and colors. Because each application is usually unique, define what it is. Electrical properties, release to hot adhesives, hydrolytic stability etc.,

Lastly, in order to make the right selection one must be familiar with the characteristics of each of the elastomers. To make the right selection ask questions, do your homework and you will come out with good grades.

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