Calendering

Introduction:

Calendering is the process of making thin sheet of rubber and also lamination of such sheet on to other substrate like fabric,cord warp sheet by passing through set if rollers under controlled Conditions.

A calendar comprises of a number of rows or bowls held in frame. Rolls rotate in opposite directions either with even speed or with a friction ratio.

Types Of Rubber material used

Elastomers that can be used include:

·        Neoprene ·        SBR ·        Natural ·        Hypalon ·        Urethane ·        EPDM          Viton           Fluroelastomer         Butyl          Nitrile         Silicone Fabric Materials Rayon Cotton Polyester Blends Monfilament Blends Kevlar Fiberglass Nylon Square Woeven Polyester Fiberglass Nomex

·

Advantage of Calendering:

1.Accurate rubber sheet gauge across the full length.
2.Frictioning operation for rubberized fabric
3.Skim coating and toping operation for rubberized fabric.
4. To obtain free of defect components like air bristles,rough surface.etc.

The basic rubber calender consist of the two to three hardened and accurately machined metal 

rollers known as Rolls rotating in bearing journal boxes which are set in rugged iron frames.One bowl in each pair is equipped with nip adjustment to control the thickness of processed material.Adjacent pairs of rolls rotating in the same or opposing directions form a nip where the material being processed is squeezed into sheets or laminated to form desired product.A calender, depending on the number and the design of its bowls is capable of sheting,frictioning,coating,profiling and embosing.

The rubber calenders are based on 

1.The number of rolls

2.The position or orientation of the rolls.

The various rubber calender types are

•  Two roll horizontal type calender
• Two roll vertical type calender.
• Two Roll inclined or tilted type calender
• Three  Roll vertical type calender
• ThreeRoll Offset stack calender
• Four Roll Vertical Type calender 
• Four Roll Offser stack calender(Inverted L type)
• Four Roll Offset Stack calender (S type)
• Four Roll Offset Stack calender(Z type)

There are other types of special calenders available

TWO ROLL CALENDER

The Two roll calenders are used to produce strips and profiles often in combination with extruder feeding which they are commonly referred to as Roller dies

                                 Two roll vertical

                   Two Roll Inclined

The Three Roll Calender:

The three roll calender also known as universal calender, It is highly flexible in usage.The three roll vertical calender with 24 in((60.96 cm) diameter) x68 in (172.72 cm)) rolls have been used for mass production of Tyres, belts.etc

Three roll calenders have been widely used for processing of mechanical rubber goods.

ThreeRoll Calender

Three Roll Inclined Calendering machine

Four Roll Calender

The four roll calender with Z and L are used mainly in the tyre .plants.The four roll permit simultaneous application of rubber compound to both tyre cord fabrics and steel cords.The four bowls permit simultaneous application of rubber compound to both sides of tyre cord fabrics and steel cords.

Four Roll Vertical Calendering Machine.

Inverted L Type Calendering Machine

Z type Offset Calendering Machine

S type calendering machine                      

Roller die

SHEETING:

This operation utilises a two roll calender in horizontal or vertical operation.The feed material either in strip or pig form is fed into one side of the nip and is squeezed by the rolls thereby emerging as a sheet.The thickness control is accomplished by use of the adjustable nips and may be further refined by automatic control system using thickness sensors.There will some variation in  the thickness of the product  and to avoid it three basic techniques are used.

1.Grinding or crowning of the roll faces to Compensate for deflection.The rolls are mechanically ground so that the diameter at the center is larger than that at the ends of the roll face.Under load the crowned bowls defelct so that the gap between them is more uniform.Grinding and regrinding after service wear must be carried out at normal running temperature if the crowning is to be correct.

2.Roll crossing where the roll axis can be mechanically deflected from a planar relationship.This effet will create a progessively greater gap between the roll from centre to ends,Because roll crossing is a deliberate misalignment of the horizontal axis of the rolls, it is also known as axis skewing.

3. Roll bending whereby the roll is physically bent by the application of force to the outside of each roll journal bearing.Positive bending causes an increase in roll convexity while negative bending causes an increase in concavity.

In order to minimise air entrapment and blistering, the thickness of each sheet is generally limited.To build up the required thicknes of the final sheet, two or more plies of calendered sheet are usually laminated on the bottom rollo of a three roll calender.

Frictioning:

• Frictioning involves rubbing or wiping an elastomeric compoun into a substrate of textile or metallic cords, which may or maynot be held together by pick threads or fill yarns or the substrate may consist of a square woven fabric like hose ducks orbelt ducks.
• Usually a three roll calender is employed wherein th rubber sheet is formed between the upper and middle bowls while the resulting sheet is simultaneouslu being frictioned into the substrate between the middle and bottom rolls.
• In this operation the upper and middle rolls  may be moving with either friction or even surface speed but he middle and bottom rolls will only be run with friction or unequal surface speeds so that the rubber is effectively wiped into the substrate being carried on the bottom roll.

Coating:

• A coating or skim coating operation is similar to that dscribed for frictionin except that the middle and bottom rolls will run at even surface speed.
• This process produce a heavy deposit or thin skin depending on th product.Generally multipurpose calenders such as three roll unit are equipped with even and fricitom gearing arrangements so that a number of combinations of roll speed ratios are possible.
  
  

A four roll calender can simulataneouslu apply a rubber coating onto both sides of a fabric.Very sophisticated devices are usually incorporated into the calender design to control thickness and width of the individual sheets and the resulting laminate.

Profiling:

Many rubber products require uncured components which are not rectangular in cross section.In such cases at least one calender roll may have a peripheral design cut into its surface to produce cross section.

Embossing:

Some rubber products are made from uncured components which must have a surface design that cannot be economically formed by subsequent moulding.i.e Cover strip around the sole of canvas shoes.The method of producing such strips is similar to profiling in that the required design is engraved into the calender roll or shell as a mirror image of the design itself.

Feeding device:

The rubber compound has to be heated normally up tp 80 - 100 degree celsius before it is fed to the calender.The feeding can be done either through series of mill or by cold feed extruder.

A cold feed extruder preferable a pin extruder can be placed in front of the calender to produce strips which is fed to the calender via conveyor belt.

LET- OFF AND WIND UP UNITS:

Calendered sheet material has to be taken up in some type of a wrapping cloth which can be a fabric or a holland cloth.To handle that and to wind up the calendered sheet let off and wind up stations for  bothe linear fabrics and sheeeted rubber are needed.cooled rolls for cooling the sheet are also needed.

For the rubber coating fabrics in large volumes when manufacturing tyres,hoses and conveyor belts,an extensive production line is needed both before and after the calender.With such a production  both sides of the fabric has to be rubber coated.

The most efficient process is achieved using four roll calender where both sides are coated at the same time

Coating line of fabrics