Do you use centrally cored rolls or peripherally drilled rolls in your two-roll mixing mills? Or a combination of both?

Roll selection for a mixing mill is of decisive importance for the quality of many high-tech products manufactured by the rubber industry. Open two-roll mills in rubber processing are recommended when quick cooling for the batch being mixed is sought, say for example in final mix compounds.

Generally, these rolls are made of Chilled Cast Iron (CI) through a process of vertical casting. Chilled CI has greater resistance to deflection and uniform heat transfer characteristics. Depending on the presence or absence of alloy, the hardness of the outer working (chill) zone could be in the range of 460-650 HV with a thickness 12-20 mm.

Basis the application, manufacturers take extreme care on the properties of the rolls that include breaking strength from journals and core material, thermal conductivity, surface quality and wear resistance of the roll, overall machining and surface quality.

As these rolls operate at high speeds, precise concentricity with proper balancing of rolls is a prerequisite for efficient utilization of material and energy. The surface quality of the rolls is crucial for the products to be produced. The smoother and more precise the rolls, better the product.

Viscous deformation of the rubber compounds occurs between the rolls of mills during mixing and mastication. This generates heat that needs to be removed through effective cooling. Hence, water circulation passages for cooling are an essential feature of the roll design in rubber mills. These passages allow a pre-defined circulation of the cooling agent (mainly water) and ensure that the temperature can be kept within a prescribed tolerance over the entire face length of the rolls.

Two designs are normally available – centrally cored rolls and peripherally drilled rolls. Peripherally drilled rolls are possible for diameter greater than 150mm (or 6 inches). The cross-section of a centrally cored roll is easy to visualize. But, ever wondered how the insides of a peripherally drilled roll looks like?

Well here is with a sectional view with water flow.

The water entering into the roll is cooler (blue colour) and as the heat transfer occurs, the water temperature rises gradually (red colour at exit).

The efficiency of heat transfer is relatively higher in the case of peripherally drilled rolls than in centrally cored rolls due to close proximity of the water channels to the roll surface. In peripherally drilled rolls, the passages for heat exchange are provided approximately 25mm under the roll surface and can vary nominally between manufacturers. Reputed roll manufacturers like Walzen Irle, Leonhard Breitenbach and Karl Buch, in their decades of existence, have built their own standards. Roll manufacturers are also available in China, Taiwan and India for various sizes.

The manufacturing processes and costs involved in producing a peripherally drilled rolls is relatively high, hence they are priced higher than cored rolls. Your choice of peripherally drilled or cored rolls depends on the quality of rubber processing required in mixing mills and the marketability of your rubber products for a price that profitably covers your investment.

Any state-of-the-art Calender in rubber processing also use peripherally drilled rolls for its stated advantages.

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