The functions of rubber rolls in various applications are diverse, demanding different properties from the rubber that is used. This, in turn, means that rubber rolls must be made for specific applications, which makes standardization difficult.
The design of rubber rolls involves consideration of general factors such as the work place, the working conditions, the desired performance, and the required durability. .
Depending upon the application, following are the important properties of Rubber, to be considered.
The proper type of rubber selection depends on careful study of the chemicals and solvents that will come in contact during service. There can not be all-purpose Rubber, suitable for all application. The correct choice of the rubber to be used for coating that will offer optimum chemical resistance is sometimes very difficult because so many factors, especially under actual service conditions, can affect the behavior of the rubber. These factors include the nature of the chemicals and solvents, their concentration, the degree of purity and nature of suspended solids, the extent and continuity of contact with rubber coverings, the effect of chemical absorption and swelling, the effect of temperature, the effects on the rubber compounding ingredients, and the effects of pressure and stress.
Basically Rubber can be attacked by either solvent swelling or chemical attack. In case of solvent swelling, test temperature can affect both penetration rate as well as swelling value.
Also, effect of pressure and stress during actual working condition can not be ignored, due to which results of normal immersion test may not give desired information regarding resistance.
Variations in compounding and curing will alter resistance properties, to minor extent. Incase of swelling by solvents, reduction of available rubber content by increasing compounding ingredients will reduce solvent swelling.
Selection of plasticizer also play important roll, as extraction of plasticizer by chemical may result in alteration of Hardness and Physical properties, thus affecting functionality, so selection of non-extractable plasticizers are advisable. In general, higher coating thickness will withstand chemical attack better than less coating thickness. Similarly, attack will be more pronounced if just surface comes in contact with chemical compared to dipping a roll in chemical bath.
The temperature encountered during service must be considered for the selection of Rubber. The roll service temperature is raised not only by ambient temperature of service area but also by heat build up in the rubber due to high speed rotation under load. Because of poor heat conductivity of rubber, surface temperature cannot be used as a criterion of heat build up inside the coating, this difference can be as high as 30-35 OC.
Apart from chemical resistance and heat resistance, the rubber selection should consider its ability to withstand working conditions such as load, shock, shearing, abrasion etc. For example, Silicone Rubber can not be used in many applications due to poor physical properties, or NBR, XNBR, PU should replace general purpose NR, where high nip pressures are encountered.
As compression deflection characteristics are used for all applications, Hardness and Thickness of coating are important criteria.
The hardness of the roll is the middle reading of three taken 120° apart around the centerline. The allowable limit of any specified hardness usually is ±5 Shore A.
The hardness variation arises not only from dispersion defects but also from non-uniformity in heating during vulcanization. In heavy rolls (e.g., paper mill rolls), the crown will not hold with satisfactory uniformity if there is excessive variation in hardness. Again the softer areas may cause wet streaks, while the harder spots give a dry surface. Similar anomalies may be observed in printing rolls, particularly ink transfer rolls or spreader rolls, wherein softer areas may cause more deposition while the harder spot gives a finish which is not distinct.
It is quite normal to set the hardness of rubber between 30 and 85 Shore A and the thickness between 10 and 35 mm, but there are exceptions such as foam rolls for printing (with a hardness of 15 Shore A) and platen rolls for typewriters (with a hardness higher than 90 Shore A).
When several rubber materials meet the requirements, such factors as cost, life, grinding interval, and loss due to the exchange of rolls should be taken into consideration.
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