Saturday, December 11, 2021

Objectives of calendaring process, types of calendars, structure and working principle of calendaring machine

 Calendaring process:
  Calendaring is a mechanical fabric finishing process. An improved fabric lustre results after the calendaring process. The shape of the cross-section of yarn present in the fabric gets almost round. When the fabric is processed through a calendering machine, the cross-section of the yarn present in the fabric gets elliptical. Why does this change in the yarn's cross-section take place? When the calendaring process of fabric is carried out, the fabric passes between many rollers nips. The fabric surface is compressed under controlled conditions of time, temperature and pressure. The round shape of the yarn gets elliptical due to compressing action between rollers nips. You can see the below picture:
Effects of the calendaring process:
When the fabric is processed in the calendaring machine, the below changes result in the fabric:
1. Fabric thickness gets reduced after the calendaring process.
2. The lustre of the fabric gets improved. A massive increase in the fabric lustre results after calendaring.
3. When the shape of the yarn changes from round into the elliptical, the yarn coverage area increases. This increased coverage area helps to increase the cloth cover. 
4. The handle of the fabric is also improved and the smooth silky feel of the fabric surface has resulted after the calendaring process.
5. Since the cloth or fabric cover increases after the calendaring process so that the air porosity gets decreased.
6. Increased cloth or fabric cover helps to reduce the yarn slippage in the fabric.

Types of calendaring machines: 

  The types of calendaring machines getting used in the industry are given below:

Swizzing calendar: 

   Swizzing calendars usually consist of seven to ten bowls. The rollers are mounted in a vertical plane - in a closed, solid cast iron frame with basic bearings. If two metallic rollers run together, they can damage the fabric. One hard metallic roller and one composition softer roller are used to run together to minimise the chances of fabric damage. The non-metallic roller is called a calendar bowl(non-metallic roller). The fabric passes between the many roller nips. This process is carried out at ambient temperature. The smoothness and lustrous fabric appearance result after this process.




 The following parameters have to be accurately controlled if the calendering process is to be consistent on a day-to-day basis: 
(1) Pressure, and distribution of pressure, across the nip.
(2) Temperature of the bowls.
(3) Speed, and relative speed, of the bowls.
       
 Friction calendering 

  This machine is also called a glazing calendar. Maximum surface change results after calendaring process in this machine. This machine gets heavier than swizzing calendar machine. The smooth metallic roller rotates faster than a softer composition bowl. There may be the peripheral speeds of metallic roller three times more than softer composition bowl.
  The fabric enters the nip and tends to stick to the softer bowl. The faster-moving metal bowl ( roller) then imparts a glaze or highly lustrous surface to the fabric. 
  The cloth handle can become quite papery and thin. 

  All the calendar parameters, as well as the correct presentation of the fabric, are required to achieve desired results. 
  The moisture content is the most important factor which directly affects the handle after calendaring in this machine.
 
Schreiner calenders:

  ‘Schreinering’ is a form of embossing. Fine lines are engraved on the hard metallic roller surface. These fine lines are transferred to the fabric when the fabric passes through the nip between the heated engraved roller and a filled bowl. 
  The metal bowl(roller) is engraved with very fine lines at an angle so that when the fabric is calendared these lines are impressed on the surface of the fabric. With the correct cloth construction and the correct line direction of the engraving, a soft lustrous handle can be achieved. A production problem with these calendars is the ease of damage to the engraved bowl and also the pick-up of lint in the very fine engraving, which naturally spoils the optical effect. With the correct cloth, and engravings of up to 500 lines per inch at an angle of 20° to the weft, extremely lustrous fabrics can be obtained. Nowadays, plain fabrics are given an imitation Schreiner finish using a bowl with only 150 to 200 lines per inch. 

  Process conditions For any Schreiner line, the result is governed by: 
1- The moisture content of fabric: 
 The dry fabric cannot be finished satisfactorily and it is essential that the moisture content be not less than the standard regain, which is 9–15 % for cotton, for example; this is usually ensured by pre-damping; 
3 - Temperature – 120–160 °C. 
3 - Nip pressure – 3.5–5.0 Bar
4 - Speed – 2–10 m min–1
 

Embossing calenders 

   The embossing calendar usually consists of two bowls; the top metal bowl is engraved with a suitable pattern and the softer composition bowl has a surface that will accept the embossing pattern. the filled bowl has to be first impressed with a specific, deeper, reversed version of the design on the steel roller for true embossing. this can only be done if the filled bowl is positively driven at the same peripheral speed so that the impression remains in the register. These bowls are specially made with super-resilient properties and often both the bowls are heated. The embossed bowls are still, quite expensive to produce and the embossing process can be quite slow. Originally, these calendars were used to produce imitation leather cloth and book cloths. A moiré embossed effect can be produced by an embossing roller but of course, there will always be a repeat to the effect. The application of an embossed crepe design on easily deformed viscose fabrics gave a whole family of creping effects.

Chasing calenders 
  As many generations of finishers have been aware, when two or more layers of fabric are passed through a nip, useful changes to the fabric handle occur. One of the early cotton finishes was that of beetling, which was related to felting in wool. Repeated compression with wooden hammers produced a soft handle on linen fabrics. As linen was regarded as one of the best fibres in existence, there were attempts to finish cotton with linen properties. Multiple passages through the various nips gave linen-like slub effects and, with some fabrics, watermarking and moiré effects. As a result, calenders were and are made with multi-bowl configurations of different bowl types – some heated and many with clever loading arrangements – with the number of bowls related to the commercial status of the company. The cloth path allows the fabric to pass from nip to nip from top to bottom of the stack; it is then returned to the first nip and passes down the stack as a double layer until it emerges at the bottom for rolling up. One of the problems with this type of arrangement is that it is batch-wise. Although clever cloth passage arrangements are available to give continuous running, they are very space consuming. Chasing calendars have been produced, usually with five bowls. The fabric is run progressively through the nips but it is allowed to run onto the top bowl and build up. Again, this is a slow batch-wise process but gives special thready finishes. The majority of calenders now in use for commercial production require high throughput. They tend to be of simple construction but will be extremely well-engineered with full control of all the parameters discussed. Two- or three-bowl calendars are the norm but they are capable of producing special effects to satisfy aesthetics or high technical specification


Moiré calenders 

  The moiré effect is an optical effect produced when a tightly woven fabric with very fine yarns is subjected to a surface pressure that distorts the weave structure by yarn movement or yarn self-compression. It resembles the watermarking effect produced by repeated damp rolling techniques. The moiré effect can only be produced when the fibre being treated is capable of being deformed; for example, wool does not produce bold patterns because it has good resilience and springs back after deformation in the calender nip. The moiré effect is in demand for a number of styles, including heavily moiréed cotton and synthetic fibres for curtaining use where the moiré effect is produced on plain dyed materials. Similar fabrics are used for wall coverings to produce subtle patterns that hide any wall imperfections. Acetate and viscose fabrics are given a moiré treatment and used in presentation boxes and as the lining in purses and handbags. Several moiré calendars have been produced, having two or three bowls. Cloth feed arrangements allow two layers of cloth to be passed, one on top of the other, through the heated nip to give the desired effect. The finish is usually carried out twice with the cloth turned over for the second run. This evens out the gloss produced by the metal bowl. Some extremely attractive results have been produced and there are many jealously guarded secrets as to how the best moiré is produced. Clever cloth guiding devices to give traversing movements and so alter the effect are recorded. The use of a Schreiner calendar with cloth traversing also gives a watermarking pattern. Moiré effects are still produced for special uses and the refurbishment of ‘stately homes’ often requires the reproduction of finishes as they used to be produced, and this can tax the ingenuity of many a works finisher

 Structure of modern calendaring machine:
Passage of a Modern calendar machine:


                            
Inlet unit: 
It contains a tension device & brake roll for even and proper feeding of fabric to the machine.
Metal Detector: 
To detect metal particles in the fabric for avoiding damage to calendar rolls and fabric.
Cat Walk: 
To avoid dust & dirt particles coming in contact with the fabric. 
Calendaring Unit
This Contains One steel roll, a plastic-coated roll & one cotton roll.
Steel roll is heated with Thermic fluid. Hydraulic oil is supplied in a plastic roll to give enough pressure to the steel roll.  The cotton roll is used to increase the weight of the fabric.
Inlet feeding unit:
Three different passages of fabric: 

Arrangement of plastic and steel roll       
Fabric feeding in calendar unit
Outlet batching device
Cooling rollers: These rollers are used to cool the fabric after passing from the calendar unit. 
Batching / plaiting device: This device is used to wind the fabric or plaiting the fabric in a trolley.

Calendaring process parameters:

S.No

Parameters

Settings

1

Steel Roll Temperature

30°C-150°C

2

Top Line pressure

50-300 Ns

3

Rear Line pressure

50-300 Ns

4

Speed Of Machine

10-120 m/m

5

Hydraulic Oil Temperature

27°C-35°C

6

Batching arm pressure

1-2 Bar

 
Operating sequence of calendaring machine: 
  
 The sequence of operation of the calendaring machine is given below:
1-Switching on the Power                            
2-Open Thermic oil and steam valve 
3-Checking the proper passage of fabric 
4-Understand and follow the instruction from the lot card and programme book. 
5-Switch ON the power and then open compressed air, cooling water valve and thermic oil. 
6-Check the quality and lot number of the fabric before Loading on the machine 
7-Transport the fabric to be run, to the inlet feeding unit of calendering machine using a hydraulic hand puller or electric truck. 
8-Stitch the two ends, (ie) one end of the fabric is to be finished and the other one leader fabric in the machine. 
9-Ensure straightness of fabric without crease. 
10-Observe the defect in the fabric before and during the process and report to the shift in charge if any irregularities are observed.
11-Set all the important parameters in the machine.
12-Set the Speed of the machine according to the instructions of the supervisor which varies based on the effect of calendaring required.
13- do not allow selvedge folds and improper feeding to the inlet of the machine. 

14-Set the temperature of steel roll as per supervisor’s instructions (30 – 150 degrees C), if the temperature is higher, smoothness and lustre of the fabric become more. 
15-Check by hand to feel the outlet fabric and if it is not matching with the standard specified inform to supervisor. 
16-Check for various process damages in the finished fabric like stains – dust, chemicals, rust, handling stains, crease, water dropping, oil, grease, etc.
16-If the machine is to be stopped for a long time, put the leader fabric on the machine and cool the calendaring unit immediately without delay.  
17-Properly batch the outlet fabric without any crease.

Please click on the below link to watch the full topic in Hindi:



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