Friction in Mixing Mills: Optimizing Compounds
Mixing mills are primarily used for producing rubber compounds. To achieve homogeneity, a certain amount of friction is required in the roll nip. But what exactly is friction, and when does it become relevant for you?
Friction describes the ratio of different rotational speeds of the rolls. The back roll of a rolling mill typically rotates 10 to 30% faster than the front roll..
But be careful - it's not just about the speed of the rolls! It's important to understand the difference between rotational speed and friction.
Rotational Speed vs. Friction: The Key Differences
Rotational speed refers to the speed at which each individual roll turns. It's measured in revolutions per minute (RPM). Friction, on the other hand, is the result of the speed differential between the two rolls.
This creates frictional or shear forces in the roll nip between the rolls and the rubber compound. These forces are necessary for the mixing process to occur. Due to the higher speed of the back roll, the material bank in the roll nip remains in constant rolling motion. The back roll functions as the friction roll. It continuously tears off parts of the material and spreads them into the roll mill sheet circulating on the front roll.
The friction heats the rubber compound and ensures homogeneous mixing of the individual components. At the same time, it prevents the material from adhering to the back roll, where the operator would no longer be able to access the batch.
The speed differential indicates the friction ratio. For example, with a back roll that rotates 20% faster than the front roll, there would be a friction ratio of 1:1.2..
Constant vs. Variable Friction: When Each Option Makes Sense
We distinguish between a fixed or variable friction ratio.
A fixed friction ratio means that the speed differential between the two rolls remains constant. The rotational speeds of both rolls are designed so that the same difference always exists.
This design approach is used when special process stability is required. An example of this would be production mills, whose rolls are typically driven by coupling gears.
A variable friction ratio, however, means that the ratio of the rolls' rotational speeds can be changed during operation.
This design is particularly advantageous for changing material properties, different production phases, or recipes. Laboratory rolling mills are often designed for such universal use. For this purpose, each roll has its own electric motor as well as a continuously variable transmission.
When the rolls rotate at the same speed, this is called synchronous operation. No shear forces are created in the roll nip, and thus no friction occurs. This design approach is used when the rubber should not be further kneaded but needs to be rolled out for further processing steps or storage, for example.
Retrofitting for variable friction is either impossible or only possible with great effort and cost due to the design principles. For a variable friction ratio, a rolling mill with individual drives like the DEGUMA neo premium mixing mill is therefore recommended.
Selecting Friction Ratios: How to Get It Right
Machines with variable friction are significantly more complex for operators. Experienced machine operators may have it "in their feel," but when choosing the right friction, several factors must be considered.
Roll diameter and spacing must be taken into account, as well as factors like the resulting temperature, end product requirements, and production rate. In particular, however, the type of rubber, its toughness and hardness affects the mixing behavior or required friction through different physical properties. This naturally also applies to added fillers and additives.
For soft rubber compounds, friction ratios between 1:1.2 and 1:1.5 are suitable. For hard compounds, lower friction between 1:1.1 and 1:1.2 is recommended. But these are only rough rules of thumb! In breakdown mills, frictions of 1:2 or even 1:4 are chosen for breaking down cold bales and slabs.
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