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What I Wish I Knew a Year Ago About The Different Types Of Rubber Calendering

Our readers who read 5 Answers to the Most Frequently Asked Questions about Rubber Calendering, would recall my apprehensions, I had on the topic.

However, to my surprise, I admit that my industry friend was right when he said that for many rubber good manufacturers, a Rubber Calender continues to be amongst the “mysterious” rubber machinery.

And I am glad that I was wrong. Because, if not, I would have not written the post that many readers found useful. Thank you Henry!

Rubber Calendering is classified based on what you are calendering

  1. Fabric calendering and
  2. Steel cord calendering.

Well, that’s the basics. Let’s explore a bit further.

What I Wish I Knew a Year Ago About The Different Types Of Rubber Calendering

Fabric Calendering:

The materials that go into your fabric calendering are your softened rubber compound (from mills) and Nylon or Polyester from the Dip Unit.

Rubber Calender Collage 1

A collage of images from different source in the web.

In this process, the tension, temperature and humidity are critical parameters of your fabric cords before they are calendared with the rubber compound. And hence is the reason you will observe that the fabric cord is kept in a room (where temperature and humidity are controlled well) once this raw material arrives in your shop floor.

This textile is arranged in a flat and parallel manner. Under proper tension they are continually pressed through two rolls. Simultaneously, you add the rubber compound to the opening area or nip between the rolls. As a result, a thin layer of rubber is applied into top of and the bottom layers of the fabric.

Then you make this continuous sheet of fabric and rubber go through many additional rolls to ensure that rubber is penetrated properly between the cords achieving the desired adhesion between rubber and the fabric.

The sheets are cut at required angles so that the cords are set at predetermined angles across the sheet.

Even inner liners for tyre manufacturing are calendered the same way into sheets of required thickness and then cut into appropriate widths for use in tire construction.

Finally, your calendered fabric sheets are wound into rolls with layers of woven fabric liner to prevent the surfaces of sheets from sticking together.

Steel Cord Calendering:

In the tire industry, steel cord calendering is for the radial tires.

Here, the body plies and reinforcing strips incorporate polyester cord that is coated in an adhesive liquid. This cord is passed between large, heated rolls of a rubber calender machine. Woven fabric is similarly prepared and calendered for the anti-chafing strips.

Since your rubber compound will not adhere to bare steel, the steel cord wires for the steel belts are coated with a very thin layer of brass. These high-tensile brass-coated rubber-encased steel cords (multi-strand cables) become the steel belts.

You may note that the steel cords come in various arrangements like cross-section of pairs, triplets and so on.

The brass-coated steel wires used in the manufacture of tire components are also extremely moisture sensitive. Hence, they must be protected in a temperature and humidity controlled environment – right from procurement, to during shipment and at the tire plant location. More importantly, any exposure to moisture can result in corrosion and a breakdown in rubber adhesion when calendered.

At your plant, these wires are stored in an environmentally controlled “Creel Room” until it is processed in your rubber calender. You pass a preset number of steel cords under proper tension from the creel room on rolls through aligning combs into the calender where the wires are coated with a thin sheet of skim stock rubber. For maximum adhesion, the rubber should also penetrate these steel cords. Maximum adhesion also means least rusting.

These steel cords are cut at specified angles and widths for use in tire building process.

Rubber Calender Collage 2

A collage of images from different sources in the web.

In ground reality, the distance between the creel room and your rubber calender (varying between 20 – 60 feet depending on your layout) is usually not environmentally controlled and, hence the wire may be exposed to moisture prior to its being encapsulated in rubber.

This problem is worsened by slowdowns, temporary shutdowns, humidity spikes and failure to adequately control temperature and humidity within the creel room. Once the belt wire becomes contaminated with moisture, it becomes more difficult to obtain proper adhesion of the rubber to the brass-coated wire.

The strongest possible bond between the rubber and the belt wire is critical in the construction of your steel belted radial tires.

Next, how do you judge the quality of a calendered sheet?

Well, you could quantify the quality of your fabric calendered sheet in terms of your pre-desired

  • Thickness of the sheet
  • Spacing between cords
  • Number of cords and
  • Penetration of rubber into the space in between cords.

Summarizing, Rubber Calendering is classified into Fabric Cord Calendering and Steel Cord Calendering based on what you are calendering. Each of these is different yet similar in operations and sensitive to environment influence for your high quality product.

Let me know your views.


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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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A Starter’s Guide – Special Edition

This edition of Knowledge On-The-Go Special Supplement features two key machinery used in the rubber industry viz. Autoclave and Compression Moulding.

Autoclave is a vulcanizing equipment, and the other is the equipment used in your most common rubber moulding technique – compression moulding.

Our previous issue on Mixing Mills tracked the advancements of a very popular rubber machinery. This edition is aimed at fulfilling the curiosity of the new generation of business leaders in rubber goods manufacturing – who are either taking over from their parents or those young entrepreneurs setting up their own venture.

Hence, the theme – ‘A Starter’s Guide’.

Download Your PDF Here

I hope you find this special supplement different and informative.

(Disclaimer: Any pictures and quoted 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 actions, hence will not be liable for any misrepresented data)


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If you are an equipment supplier and would like your organization to be promoted on Rubber Machinery World, please see the opportunities on Partner Me or Contact Me at rubbermachineryworld@gmail.com for your customized offering.


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The Only Reason Why Rubber Calender Was Unpopular for 100 years Since it’s Invention

Calendering is one of the oldest rubber processing technologies. The Rubber machinery that helps you do calendering is known as a Calender (…obviously!!).

The word ‘calender’ itself is a derivation of the word κύλινδρος kylindros – the Greek word that is also the source of the word ‘cylinder’.

In 1836 Edwin M. Chaffee, of the Roxbury India Rubber Company, patented a four-roll calender to make rubber sheet. He worked with Charles Goodyear with the intention to “produce a sheet of rubber laminated to a fabric base”. Despite this development, calendering as a process became popular only after the 1930’s.

And, so the Calender was never ‘popular’ when it was first invented over 2 centuries back. Here’s why.

Rubber Calender 1

Collage By Rubber Machinery World

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

In this process, you pass pre-selected fabric and rubber through a series of rolls to flatten, smoothen and sandwich the materials.

Depending on your end-use, the calendered sheets could have multiple layers “sandwiched” together.

A Rubber Calender can help you get either ‘supported’ or ‘unsupported’ calendered sheets.

  • Unsupported’ sheets contain only layers of rubber that has been joined together.
  • On the other hand, a ‘supported’ sheet has textile fabric or steel cord coated with a film of rubber (aka “skim stock rubber”) on both sides and into the material. Supported rubber sheets give you higher strength or tear resistance.

Coating of fabrics has been done for almost 200 years. Steel cord and fabric cord topping is a process in your tire manufacturing. Specifications and tolerances for calendered cords are very tough. So you will find it difficult to fulfill the thickness variation across the calendered ply and the cord density.

The calender never did become very popular when it was first invented mainly because it was difficult to adjust the desired gap between rolls. Consequently, it was difficult to get an accurate rubber sheet thickness.

Hence, the process did not become popular till the calender machines became easier to adjust (i.e until the 1930’s).

Rubber Calender Line

Since then, features have evolved. Modern calenders’ can achieve tolerances around ±0.005mm and their lines are adaptive for most sophisticated precision requirements of your product.

What do you think?


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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.

Download PDF here

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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If you are an equipment supplier and would like your organization to be promoted on Rubber Machinery World, please see the opportunities on Partner Me or Contact Me at rubbermachineryworld@gmail.com for your customized offering.


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Machinery Is The Key To Rubber Processing – Dr. Chakravarty

Machinery is the key to rubber processing and brought significant changes in rubber industry, says Dr. S.N.Chakravarty in an exclusive interview with Rubber Machinery World. Celebrating his 50th year in Rubber Industry working in different countries including India, he shares his experience.

Know A Rubber Leader

Dr. Chakravarty has many achievements to his credit. After working in Bayer AG, Germany for many years he became Technical Manager of Bayer (India) Ltd. and then of Modi Rubber Ltd. Subsequently he started his own Consultancy Organisation in Rubber and related field.

He is the past Chairman of Indian Rubber Institute and Rubber Committee (PCD14) of Bureau of Indian Standard (BIS), Fellow of Plastics & Rubber Institute, PRI (UK), Indian Rubber Institute (IRI),  Indian Chemical Society (ICS), Indian Institute of Chemical Engineers (IIChE), Inst. of Chemist and Member of Rubber Division, American Chemical Society (USA). He represented India in TC 45 / ISO & was a member of IRCO, UK Committee, Convenor of 1st Int. Rubber Conf., Rubber Con ’93 Delhi in 1993 and member of ITTAC for many years.

Dr. Chakravarty has been Hon. Professor of Indian Institute of Technology, Delhi and Kharagpur, visiting Faculty to different Universities & Institutes. He was the first President of Asian Rubber Forum to which all Asian countries like Japan, Korea, China, India, Malaysia, Sri Lanka, Nepal etc. are members.

Dr Chakravarty has more than 106 Publications in National & International journals of repute. His work in the area of Rubber Chemistry & Technology and educational activity are well recognised all over the country & abroad. Dr. Chakravarty has traveled widely over the world and has command over different languages.

Recently he was awarded the “TRiLA Life Time Achievement Award, 2015” by Tyre Times at the recently concluded Tyre expo in Chennai.

So, now you would understand my predicament when I had to limit to 10 questions to capture his vast knowledge, leadership and expertise to feature him in your Know Your Rubber Leader Series!

Fortunately (for me), he also shares his expertise through articles on this portal. (Read his earlier posts – Mixing & Mix Design and Injection Moulding Of Rubber Product)

Below are some thoughtful snippets of his interview while you could read the complete interview by clicking on the image below.

Know A Rubber Leader - Dr.Chakravarty

Click on this image to read the complete interview

  1. Would you say that rubber processing has undergone change in the last 4.5 decades that you have been with rubber industry? What were the drivers for this change?

Surely, rubber processing has undergone substantial changes over the last about 5 decades, This is mainly due to availability of developed machinery – Mixer, Extruder, Calender, Presses etc., along with electronic / microprocessor controls, as well as development of rubber technology & ingredients used in compounding. Machinery is the key to rubber processing.

  1. What role has machinery played in this change?

Machinery played many fold role in rubber industry. These developments of machinery have made it possible to

  • Improve production output (impact on cost)
  • Superior dispersion & homogeneity – improvement in property level.
  • Lowering of wastage – impact on cost
  • Dimensional accuracy of components
  • Accuracy in working – improvement in quality
  • Effective Quality Control (QC)
  • Energy saving – very important factor
  • Pollution Control, better housekeeping, health factor for the working people in this industry.

xxxxx

Read Complete Interview or Download PDF Here

I hope you find this interview insightful. Let me know your thoughts.


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An Introduction to Rotocure

A Rotocure is a shortened name of Rotary Curing Press. This machinery is designed for the continuous vulcanization of technical rubber products.

So, if you are into manufacturing of rubber belts, roof covering, rubber belts with textile inserts, sealing plates, membrane and ship textile, conveyor belts, flat belts, floor covering, etc you would have heard of this equipment (if you are not yet using one in your production line).

As a technical rubber goods producer, vulcanization is critical to your manufacturing. In an earlier post, I had introduced you to Autoclave for vulcanizing of products that you cannot achieve on a standard press-based compression molding process. You install a Rotocure to create continuous vulcanization process where the sheet is heated and vulcanized.

While every technical rubber goods manufacturing plant may not have a Rotocure, those who do have one positions their rotary curing press after the calendar.

Rotocure Line:

A typical Rotary Curing Press Line consist of following component machines – A Winding Let-off Unit, Rotocure, Cooling Unit, Equipment fo edge trimming of product and a Winding-up equipment.

Rotocure Image Collage

Rotocure Line: A Collage of Individual Images From The Web

From Winding Let-off Unit, your semi-finished calendered rubber material is led on to the lower roll and from there into pressing space between the heating drum and thrust pressing belt, where it is pressed and vulcanized.

Operating A Rotocure:

Your main vulcanizing machine is always the Rotocure in the set-up and consists of a large, steam-heated, revolving steel drum. This is partly encompassed by an endless steel band. The steel band encompasses approximately 2/3 of the circumference of the drum through the action of two conducting rollers and a tensioning pulley. The drum is heated by means of steam to a temperature of 150 – 200°C. In addition, there is a heating plate bent round approximately half of the surface of the drum and which gives some additional heat, but primarily it prevents loss of heat.

When you operate a Rotocure Equipment, you need to ensure that all the various parts work in precise alignment. It is  crucial for you to maintain all process parameters in their correct tolerance. Because a slight change in the heated roll or in the steel belt that are both in charge of the smooth vulcanization of the sheet can cause your sheet to be unevenly vulcanized. This leads to irregularities and subsequently rejection of your end-product.

Your calendered rubber material is placed between the drum and the band at the lower roller. With the aid of the tensioning pulley, the band is strained so that a pressure (say approximately 0.3 MPa) is obtained between the band and the drum.

Rotocure - Pelmar

A Fully Refurbished Rotocure From Pelmar Engineering, Israel

The linear speed of this drum can vary from a 0.04-200 m of cured material per hour, all depending on your material’s thickness and rate of vulcanization.

The drum, which is interchangeable, can have completely smooth or patterned surfaces and normally has a diameter of 700-1500 mm and a width of between 1200-2500 mm.

For a normal speed range of 5 – 50 metres/hour and a contact length against the drum of 3 minutes, you may achieve a curing time between 4 – 35 min for your specific compound.

As a user, your will quickly realize that the main advantages of a well-manufactured and correctly operated rotary curing press (or rotocure) machine are its high hourly output, long lifespan, low energy consumption and high reliability.

On the other hand, you could be challenged by the limitation of your product thickness and the pressure sought during vulcanization.

As is normal with other categories of rubber machinery, you get both New and Used Rotocure’s in the market. When you shortlist your suppliers, you could also discuss with Pelmar Engineering Ltd for Used Rotocure or Pracsol Chemicals And Machinery for New Rotocure – more of their details on this site is in our interviews section.

Summarizing, a Rotocure is an important rubber machinery if you are into manufacturing of technical rubber products. I look forward to your thoughts on this continuous vulcanizing machine.


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