220.127.116.11.1 Rubber Roll Covers
Rubber roll covers are the covers with the longest tradition. The hardness ranges from very soft elastomeric covers to hard rubber. Both natural rubber and synthetic rubber polymers are used. Typical rubber formulations consist of approximately 8 to 15 ingredients, such as polymers, fillers, processing materials, tackifiers, anti¬degradant components, colorants, activators and vulcanizing agents. The possibil¬ity of compounding standard formulations as well as high performance formula¬tions makes rubber an excellent material for roll covers.
Hard rubber covers are used for guide rolls and hard press rolls. For guide rolls the compounds are cost optimized due to the large number of guide rolls in a paper machine and the relatively low mechanical demands. Therefore a large amount of conventional inexpensive fillers is used. Depending on the application, single layer designs are usually used. Bonding of the covers to the metal shell is a combination of shrink fit of the hard rubber and chemical bonding, resulting in good corrosion resistance, which is the main reason for applying the cover.
For hard press roll applications the demand on surface quality and abrasion resistance is higher, therefore more expensive fillers and combinations of fillers are used. Release properties of the cover can be adjusted by addition of polymer fillers like PTFE. Two layer designs are frequently used, a cost optimized hard rubber bonding layer and a performance optimized functional layer.
Soft press roll covers are formulated for good dynamic properties in combina¬tion with high wear resistance. Chemical resistance and swelling characteristics are adjusted by selection of the polymer. The higher loads of the nipped positions require two or three layer designs in order to withstand the shear forces in opera¬tion. Bonding is achieved by chemical bonding supported by the shrink fit when using a hard underlayer.
for example result in good chemical resistance. Properties like heat build up, dampening, wettability, roughness, and low compression set for marking resis¬tance must be considered for development of the compound in these applica¬tions.
Most roll cover manufacturers carry out their own rubber compounding because of the variety of the formulations, the demand for high quality and the small batch sizes. Mixing of the compounds is frequently done on open mills (Fig. 6.9) due to the small batch sizes, the high number of different compounds as well as hardness variations and the good quality of this compounding procedure. The use of inter¬nal mixers is not always justified due to high equipment costs.
After mixing the rubber compound, it is strained to remove impurities and converted into either a feeding strip for an extruder or a calendered sheet for direct application.
The surface of the roll is carefully prepared by cleaning and sandblasting before the application of chemical adhesives or rubber cements.
There are three basic methods for applying the rubber cover to the roll core:
• Extrusion: an extruded strip of rubber is spirally wound around the rotating roll body (Fig. 6.10)
. • Knott method: a narrow strip of calendered rubber is spirally wound around the rotating roll body
. • Hand build: large calendered sheets of rubber are manually applied on the roll.
With each of these build up methods, the desired layers of different materials are applied. The choice of application method depends mainly on the compound and the requirements for cover homogeneity. The hand build method, as an example, is the most sophisticated and most expensive build method. For these reasons it is only used for applications with extremely high demands on surface quality such as sizing or coating.
Vulcanisation of the covers is done in steam autoclaves, where the rubber poly¬mer is crosslinked to the elastomeric network. The cover is finished by mechanical tooling, drilling and grinding to the required geometrical dimensions.