Two-Roll Mills have been in existence since the time mixing of rubber started for various applications. It is said familiarity breeds contempt. And, hence this could be a reason why we consider two-roll mills as a low technology product and automatically assume that anybody can manufacture them.
How far is this true?
There are multiple instances today in the industry where two-roll mills are not meeting the expectations of the customers. The fault lies on either side – end users failed to draft the technical specifications properly and manufacturers failed to understand the expectations properly. After all a two-roll mill is only a two-roll rubber mixing mill, isn’t it?
How complicated can it be?
Developments in Two-Roll Mill (Rubber Mixing Mill)
The fact that each component going into the mill needs to maintain the physical and dimensional integrity is taken for granted. Any mismatch can lead to recurring problems resulting in downtime and causing loss of production. Let’s examine few other developments.
Developments in mixing mill.
Stock Blender: A main disadvantage in mixing on a two-roll mill is that the process is dependent on the operator skill. Stock blenders reduce this operator variable in the mixing process. Use of a stock blender improves mixing quality, reduces human interference and gives consistent productivity.
Drives: The new mills that are manufactured today adopt compact direct drive where the gearbox output shaft is coupled directly to the roll. Bull gear-pinion that consumed more power, made noise and vibration has become obsolete.
Now we also use uni-drives where two output shafts from the same gear-box drive the two rolls. The external connecting gears are absent here making the mills appear more aesthetic and compact.
Added to above are independently driven rolls where each roll is connected to an individual gearbox taking power from individual motors. The advantage is that variable friction ratio can be obtained for different recipes.
A hydraulic drive is another advancement that saves power though comes with higher initial capital investment.
Rolls: Cored rolls have been replaced by peripherally drilled rolls for efficient and precise heat transfer leading to a better mixing quality. When we look at an optimal investment, we can opt for front roll to be peripherally drilled as the rubber compound adheres itself on to the front roll while mixing.
Nip-gap adjustment: A motorized or hydraulically actuated nip-gap adjustment improves precision and facilitate faster nip setting for multiple recipes. This can further be automated to have ‘preset nip gaps ’obtained at the press of a button on the operator panel. Nip Gap can be controlled / measured in electronic devices.
Stock Guides: The movement of stock guides can be made pneumatically or hydraulically actuated for faster movement and precision. The shape of the stock guides in such uses may be modified to make the rubber flow always to the centre of the rolls so that the compound does not seep sideways prolonging the rubber mixing process further.
Safety and automation are other areas of development and depending on the end-user’s requirement a high level of customization is possible.
Concluding, a two-roll mill is no more as “simple” rubber machinery as it is considered to be; and a proper planning of the requirements and drafting of specifications can help you acquire optimized technology for your rubber mixing process.
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