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Tag Archives: Rubber Mixing Mill


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Mixing Mill – A Story Of Woes to Wows

Two-Roll Mixing Mills have been in existence since the time mixing of rubber started for various applications. It is still a sacred machinery for the rubber processors. Hence, we had to think thrice or even more before we decided to write a feature on this machinery in our digital publications.

The theme ‘Mixing Mill: A Story of Woes To Wows!!’, I think, is a true reflection of what this popular rubber machinery has morphed into. Modern mixing mills are rich in features, safe and automated as compared to ancestors. (I think you will agree that our edition’s cover page sums up this contrast well!).

Please click on the below image to go to the digital edition of this special supplement.

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Our previous issue of a concise Knowledge On-The-Go Special Supplement to give you useful information in a concise and timely manner was a success with some exciting reviews. Thanks to each one of you – our readers and advertisers.

I hope you will find this edition too informative and interesting. Please do let me know.

(Disclaimer: The pictures and statements in our special supplements including Know Your Supplier editions are shared with us by the respective companies and/or sources are mentioned appropriately. Rubber Machinery World does not independently verify them nor will vouch for their genuineness that they share with us, hence will not be liable for any misrepresented data)

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Should you buy a Stock Blender for your Open Mill?

The Stock Blender is an add-on accessory mounted on top of an open two-roll mill. It aids in rubber blending by pulling the mix compound from the front roll of the mill and guiding it to the nip gap between the two mill rolls.

A Mixing Mill With Stock Blender and Hydraulically Operated Mill Guides

Structurally, this machine assembly consists of a blender roll, a carriage unit with guide rolls, pressure roll and rear roll along with required automation.

The Blender Roll is a hollow roll with water passage for internal cooling. The surface of this roll is hard chrome plated, mounted on antifriction bearings at both ends and operates on a variable speed motor. The VFD allows you to tune for every compound or recipe, thus enabling the speed of the stock blender to follow mill speed. This feature is crucial to ensure that compound does not sack-in or tear-off when taken up and through the stock blender.

On the assembly frame is also a hollow pressure-roll mounted on antifriction bearings and has a swivel operating arrangement. Pneumatic cylinders actuate this movement. The Carriage with screw and guide rollers is equipped with two limit switches at either end of travel. These limit switches signal the carriage traverse motor to reverse at extreme ends during to & fro travel, while guiding flow of rubber to mill nip via blender roll.

A stock blender helps in homogeneous mixing and repeatability of the mixing operations. Many companies use them on the open mill (also known as dump mill) below the internal mixer. After masterbatch mixed in an internal mixer, curatives are added and final homogenization done on these mills. Because the mill helps incorporate undispersed curative or fillers that might have fallen off the ram or drop-door when the batch was dropped. In a different scenario, if you need to drop the mix batch at a higher dump temperature, you could use the stock blender to take some heat out of the batch and then add the curatives on the mill. Reduction in temperature is obtained by allowing the batch from the mill roll airborne to pass over the water-cooled blender roll before it returns back to the nip gap. When these mill rolls (centrally cored or peripherally drilled design) are properly cooled, the stock-blender allows you to reduce the batch’s temperature continuously for lower viscosity compounds.

Stock Blenders are also used on pre-warming mills before feeding the rubber sheet to a Calender.

A Stock blender enhances efficiency and productivity through reduced operator fatigue. Cutting down hot rubber many times, rolling the sheets and feeding them back into the nip gap of the mill rolls is a physically demanding task for the operator. Stock blenders eliminate this completely and improve your batch to batch consistency. The operator also benefits from a safe work-environment.

The operators, though need to be trained well. For an open mixing mill the stock blender can be used only when the batch has achieved a certain stage of homogeneity. When a batch from the internal mixer is dropped on an open mill with stock blender, the operator must pass the complete batch through the nip before diverting the batch to the stock blender. Else, there is a risk that only a part of the batch turns round the stock blender. A big lump,which is neither cooled nor mixed, would continue to ride and turn on top of the nip. This lump does not pass the nip until the operator evacuates the compound to second mill or batch off.

The stock blender designs vary among manufacturers. Hence, not paying attention to the design features can result in owner’s or operator’s nightmare. For example designs with ledges behind the rolls can cause compound to accumulate and these must be cleared out regularly to prevent contamination. Similarly, a ball screw with tube casing for carriage movement is superior to a conventional screw design and eliminates contamination from lubrication of the screw. Compact designs and increased level of automation with programmable carriage traverse movements make the stock blenders a joy to work with a high level of safety.

Summarizing, buying a good robust stock blender increases the mixing capacity of an open mill, improves the batch cooling, reduce mixing time and improves dispersion. With a trained operator, you can extract the best returns out of your investment in this rubber mixing machinery accessory.

 

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Developments in Two-Roll Mill

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.

16”x42” Mill

Picture of a 16”x42” Mill. Image: Bainite Machines Pvt Ltd.

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.

(Note: This co-authored article by Prasanth Warrier first appeared in Dec 2011 issue of IRJ) 

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