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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.

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Here’s Why TCU Is An Irresistible Ancillary For Your Rubber Machinery And Worth Your Investment.

TCU or Temperature Control Units are ancillary equipment to your rubber machinery that control process temperatures during rubber processing. During rubber processing, you continually seek optimal temperatures for the best product quality.

A TCU achieves temperature control by circulating water or an oil based fluid through the process application (in a closed loop). This results in heating of the application and temperature control of the process.

Temperature control units service a single process at any one time. In rubber industry, water is most popular circulating media.

For example, you have Three-Zone Water TCU’s for your internal mixer where One Zone supplies water to the Rotors, Second Zone to Drilled Sides (or Mixing Chamber) and Third Zone to Discharge Drop Door (and Floating Weight, RE Plates as appropriate to the mixer model). Usage of TCU in an intensive internal mixer is found to improve carbon black dispersion, eliminate first-batch-effect, maintain batch-to-batch consistency of your rubber mix compound, increase productivity, and reduce machine stress (which in turn increases the life of your internal mixer).

Similarly, you have 4 or 5 Zone TCU’s for your Extruders, where each of the zones supply water to screw, forward and rear barrels, die head, feed roll, etc as is required for your rubber extrusion process. Or you may have Two-Zone TCU for open two-roll mixing mills where each zone supplies water to your peripherally drilled or cored rolls. Most Rubber Calenders also have TCU supplying water to the rolls to control the calendering process temperature.

TCU for a 3-Roll Calender

3-Zone TCU for a 3-Roll Vertical Calender – Image from Bainite Machines Website

A properly installed, operated, and maintained TCU system gives you years of reliable operation.

Each TCU is a self-contained system consisting of a centrifugal pump, electric heater, cool/vent solenoid valve, electrical control, including a PID microprocessor controller and thermocouple along with standard safety devices like a mechanical over temperature safety thermostat, a pressure relief valve, motor overload protection, a low pressure cut out switch, etc.

Temperature Control Units come both in portable and robust sizes (depending on the application) with a variety of different control instruments, heater sizes, and cooling vales.

A Compact TCU

Compact TCU – Image from AEC Website

And you are right when you imagine that the portable TCU’s are visually appealing and popular because of their compact size.

At your installation site, you need to provide adequate water (from portable or central chillers or towers) to these TCUs from your plant water supply source. Check your manufacturer’s operations manual to verify the recommended water pressure requirement. This is important because lower than recommended water pressure can cause pump cavitations while excess pressure can damage internal components.

The pump in your TCU is used for rapid circulation of a relatively small amount of water. This in turn ensures close and uniform temperature differences between the delivery and the return lines.

You use temperature control units, when you want to preheat a process to the desired operating temperature. The heater and  cooling valve works together to give fast and accurate response to bring the water up to desired temperature or to change the settings when needed. This controls your circulating water temperature.

Here is an animation, that I found on the website of Sentra Temperature Controllers. It is quite informative and I hope you too like it. (Click on the picture to start animation)

Advantage Make TCU

Upon reaching your desired operating temperature the TCU can continue to add heat or convert into a cooling device by exchanging a small amount of circulated water with cooling water from an external source. Modern TCU have the controls wherein cooling water is precisely metered into the system by the cooling valve.

Most TCU’s in rubber industry for your machinery has temperature control from 30° to 300°F and designed for easy maintenance.

Summarizing, rubber processing under optimal temperature conditions is a must for your final product quality. And the features of TCU makes it irresistible as an efficiency enhancing ancillary equipment, which control process temperatures on your rubber machinery. Thus worth your investment.

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Peripherally Drilled Rolls or Centrally Cored Rolls?

Do you use centrally cored rolls or peripherally drilled rolls in your two-roll mixing mills? Or a combination of both?

Roll selection for a mixing mill is of decisive importance for the quality of many high-tech products manufactured by the rubber industry. Open two-roll mills in rubber processing are recommended when quick cooling for the batch being mixed is sought, say for example in final mix compounds.

Generally, these rolls are made of Chilled Cast Iron (CI) through a process of vertical casting. Chilled CI has greater resistance to deflection and uniform heat transfer characteristics. Depending on the presence or absence of alloy, the hardness of the outer working (chill) zone could be in the range of 460-650 HV with a thickness 12-20 mm.

Basis the application, manufacturers take extreme care on the properties of the rolls that include breaking strength from journals and core material, thermal conductivity, surface quality and wear resistance of the roll, overall machining and surface quality.

As these rolls operate at high speeds, precise concentricity with proper balancing of rolls is a prerequisite for efficient utilization of material and energy. The surface quality of the rolls is crucial for the products to be produced. The smoother and more precise the rolls, better the product.

Viscous deformation of the rubber compounds occurs between the rolls of mills during mixing and mastication. This generates heat that needs to be removed through effective cooling. Hence, water circulation passages for cooling are an essential feature of the roll design in rubber mills. These passages allow a pre-defined circulation of the cooling agent (mainly water) and ensure that the temperature can be kept within a prescribed tolerance over the entire face length of the rolls.

Two designs are normally available – centrally cored rolls and peripherally drilled rolls. Peripherally drilled rolls are possible for diameter greater than 150mm (or 6 inches). The cross-section of a centrally cored roll is easy to visualize. But, ever wondered how the insides of a peripherally drilled roll looks like?

Well here is with a sectional view with water flow.

Peripherally Drilled Roll

The water entering into the roll is cooler (blue colour) and as the heat transfer occurs, the water temperature rises gradually (red colour at exit).

The efficiency of heat transfer is relatively higher in the case of peripherally drilled rolls than in centrally cored rolls due to close proximity of the water channels to the roll surface. In peripherally drilled rolls, the passages for heat exchange are provided approximately 25mm under the roll surface and can vary nominally between manufacturers. Reputed roll manufacturers like Walzen Irle, Leonhard Breitenbach and Karl Buch, in their decades of existence, have built their own standards. Roll manufacturers are also available in China, Taiwan and India for various sizes.

The manufacturing processes and costs involved in producing a peripherally drilled rolls is relatively high, hence they are priced higher than cored rolls. Your choice of peripherally drilled or cored rolls depends on the quality of rubber processing required in mixing mills and the marketability of your rubber products for a price that profitably covers your investment.

Any state-of-the-art Calender in rubber processing also use peripherally drilled rolls for its stated advantages.

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Rubber Sheeting Battery – A Simple Processing Machinery

Rubber Sheeting Battery

Image of A Rubber Sheeting Battery

Sheeting Battery is a machinery used during the process to produce rubber sheets from latex. This processing machinery is introduced at a stage after coagulation of latex.

The coagulated latex is processed between a pair of rolls, which are either grooved (corrugated) or smooth (plain). Hence, they are like two-roll mixing mills used in the rubber processing. In fact just visualize many miniature mixing mill rolls working in tandem fixed on a common frame to produce a sheet.

In its simplest form, the Sheeting Battery could be a mechanical chain-driven type. Or a compact motor-driven automatic-sheeter in the upgraded version. The customer’s selection depend on the operating conditions at his rubber plantation.

In an automatic-sheeter, the first few set of rolls have smooth (or longitudinally grooved surface to grip the wet slabs better). The nips between the rolls are so designed to narrow on each successive pair of rolls. This is to reduce the thickness of the coagulated slab of rubber passed through them. Rubber sheets thus processed are of uniform thickness and enhances the quality of dry sheets later on. The weight of each rubber sheet is around 550g while it can go up to 1.2Kg in wider-roll sheeting battery with higher power.

Fresh water is sprayed on each set of rolls on the sheeting battery to prevent them from drying and avoid tearing the sheets. Fresh water also washes away the acidic serum in the coagulated latex as much as possible.

The last mill rolls perform the finishing operations. These rolls have diamond-shaped indentations or grooves that impart the characteristic ‘ribbing’ to the sheet produced. Ribbed Smoked Sheets (or RSS) evolve after the smoking process is completed of the dried rubber sheets from this machinery.

Rubber sheeting using this machine saves labour and time. For example, a single operator (or labour) produce 350-750 sheets per hour, depending on the machinery selected, with minimal efforts and training.

Few manufacturers based in Kerala, Southern part of India, manufacture Rubber Sheeting Battery for domestic and international markets.

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