It is a mechanical process that finishes the fabric, by passing it between sets of rollers and applying heat and pressure. The outside of the rollers can be smoother or engraved to give the perfect finish to the fabric, the structure of the rollers varies from hardened chromium plated steel to elastic thermoplastic rollers. By varying the rollers, adding any additional chemical treatment and temperature, a variety of calendared finishes can be produced like glazed or moiré fabrics.
Calendaring is done for many purposes but the main objectives are:
● To give softening to the face side of the fabric
● To increase fabric lustre or glaze
● To give silk like appearance
● To close the open threads
● To decrease the air permeability
● To increase the fabric clarity
● To flatten the slubs
● To modify the fabric surface by embossing
In general a calendaring machine has 2 to 7 rollers with most common being the 3 bowl rollers. Less number of bowls is used for lightweight fabrics whereas more numbers of bowls are used for calendaring heavy weight fabrics. The bowls are made with alternating hard steel and elastic. The elastic bowl is usually made from compressed paper or compressed cotton, however a lot of modern calendaring machines are made with Nylon 6 covering. This provision is given so that there remain resiliency between the two consecutive rollers and the compression remains uniform. Heating arrangements are made via steam circulation chamber.
The process parameters that can be controlled during calendaring are speed of the fabric, speed of the rollers and the surface of rollers. Different types of calendared effects are: surface glazing, ciré effect, moiré effect, Schreiner effect and embossing effect. Different treatments are given to the fabric before calendaring. For example, to obtain a glazed finish like chintz (polished cotton), the fabric is first saturated in starch or wax or resin solution and is dried before calendaring. Starch or wax gives a temporary finish to the fabric but if the fabric is saturated in resin, it gives a durable glaze. Calendared fabrics with their glossy or wet look are produced in the same manner as glazing. The fabrics are coated with a wax or resin before being calendared with heated rollers. When thermoplastic fibres are used, the fibre surface that comes in contact with the metal rollers melts and flattens slightly and produces a highly polished fabric. Examples of calendared fabric are taffeta, satin or tricot, silk or silk blend fabrics.
Moiré fabrics have characteristic water marked look that is produced during the calendaring process. The effect is developed using either a moiré embossing roller or by a high compression calendaring of two layers of ribbed-base fabric in a single pass. One popular method of preparing moiré fabric features using rollers that have been engraved with a design. The fabric is run between the engraved rollers with some sections of the fabric squeezed to reveal the f inished design that has a watery look. This type of application is often used to create material styles that are ideal for evening gowns, clutch handbags, and other types of formal apparel and accessories for women.
Schreiner finishes on fabrics produce soft lustre and hand by flattening the yarns and surface of a fabric through calendaring. The Schreiner calendar has a metal roller engraved with 200-300 fine diagonal lines per inch that are visible only under a magnifying glass. A Schreiner finish is used on cotton sateen and table damask to make them more lustrous and on nylon tricot to increase its cover. Fabric embossing can also be carried out in the calendaring machine in which the fabrics can have a pattern imprinted or embossed into it. The embossed pattern is created by passing the fabric between a heated embossing roller and a shaped paper roll. The degree of lustre given to the fabric can be modified by heating the chilled iron roll, by changing the pressure at the nip, by changing the speed of the machine, by carrying the moisture percent present in fabric. Various designs can be created by using a 2-bowl embossed roller or 3-bowl embossed roller arrangement.