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5 Answers to the Most Frequently Asked Questions about Rubber Calendering

In an earlier post on Calender staying unpopular for 100 years since its invention, I had introduced you to this oldest rubber processing technology.

Recently, an industry friend confided that for many rubber good manufacturers, a Rubber Calender continues to be amongst the “mysterious” rubber machinery.

This statement, I found interesting. Is it true?

I let you to be the judge and let me know. However, even if a small share of rubber goods producers aspires to know more on Rubber Calendering, this post should help. Here it is:

5 Answers to the Most Frequently Asked Questions about Rubber Calendering

1. What is Rubber Calendering?

Calendering is a mechanical process by which rubber is pressed into textiles (cloth, fabric, tire cord) forming composite sheets.

Calender From Comerio

2. What are the different types of Rubber Calenders?

A calendar is heavy-duty rubber machinery consisting of two or more rolls that revolve in opposite directions. The Classification of Rubber Calenders is based on

  1. The Number of rolls
  2. The Position or Orientation of the rolls

Hence you will see various rubber calender manufacturers offering you,

  • 2-Roll Horizontal Type Calender
  • 2-Roll Vertical Type Calender
  • 2-Roll Inclined or Tilted Type Calender
  • 3-Roll Vertical Type Calender
  • 3-Roll Offset Stack Calender
  • 4-Roll Vertical Type Calender
  • 4-Roll Offset Stack Calender (Inverted ‘L’ Type)
  • 4-Roll Offset Stack Calender (‘S’ Type)
  • 4-Roll Offset Stack Calender (‘Z’ Type)

(You will hear more on Calender construction and their various types in my next few posts.)

3. What are the different types of Rubber Calendering?

Calendaring is classified based on what you are calendaring – Fabric calendaring and Steel cord calendaring. The adhesion of rubber to the fabric or steel cord is critical to final performance. And the rubber compound that you coat the fabric is different from the one used to make the tread or that which coats the cords.

4. Where is Rubber Calendering adopted?

For example, you use calendared textiles for casing and cap plies and chafers in tyre industry. And you use calendered steel cord for belts. The sheets you produce by this calendering process falls into two categories: either fabric inserted, or unsupported (do refer my earlier post). Calendering is also well adopted in plastic industry (say, PVC sheeting).

5. What are the functions of Rubber Calendering? And which Rubber Calenders to use?

Calendering can help you perform

  • Skim Coating or Topping

Here, rubber is coated on both sides of the substrate (i.e your fabric or steel cords). This operation strengthens the adhesion between rubber and the substrate.

Your process decides whether you use a 3-Roll Calender or a 4-Roll Calender. (Let’s cover more on this in future post lest we digress).

  • Frictioning

Here, you use calendering process to force rubber into the fabric weave. Frictioning imparts good adhesiveness. You deploy 3-Roll Calender, where the top and bottom rolls have a lower speed than the middle roll. 4-Roll Calenders cannot be used for Frictioning, because obtaining a speed difference between fabric and compound is difficult.

  • Rolling

Here, you just allow hot rubber compound to pass through a 2-Roll Calender and get a continuous sheet of rubber of thickness 3 – 4 mm.

  • Embossing & Profile Calendering

If you produce tread sections of cycle tires or involved in hand building of foot wear, you may choose a Calender with “patterns” on the rolls to produce profiled treads or patterned sheets.

A step forward of Rolling, here you press a heavy engraved roll against hot rubber compound. You would consider this if your sheet needs a”design” look. The engraved design of the rolls is transferred to your rubber sheets.

Summarizing, Rubber Calendering, one of the oldest rubber processing technologies, is a mechanical process by which rubber is pressed into textiles (cloth, fabric, tire cord) forming composite sheets.And Calendering operations need critical checks to ensure you get high quality products.

Do you agree?


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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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