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 Bar4 - 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
unitOutlet batching deviceCooling
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.
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