Finishes that are imparted with the help of physical operations/machinery are termed as mechanical finishes. These finishes will not alter the chemical structure but may change the shape, surface and structure of fabrics.
Calendering: Calendering is a mechanical finish essentially used for finishing plain fabrics to impart more lustre. Calendering is performed with sets of rollers revolving against each other imparting smooth to textured effects in fabrics. Several calendering types are used for finishing:
Simple Calendering.
Friction Calendering
Schreiner Calendering
Moiré Calendering
Embossed Calendering
Generally, most of the calenders have three rollers. Two/five/seven rollers may also be present in few calenders. Highly polished steel rollers revolve against a soft roller/cloth covered/solid paper rollers, but never revolve against the metal roller.
Simple Calendering: It is a final finish, just like domestic ironing, except that the pressure exerted on the fabric is much greater. This machine consists of two or more highly polished, vertically stacked rollers that are hydraulically compressed against each other. In operation, steel rollers are heated. Slightly dampened fabric passes between the rollers with a high pressure of around 140 kg/cm2
The purpose is to flatten the fabric and thus make it smooth and more lustrous and impart a more supple hand.
Friction Calendering: This calendar imparts more friction on the surface of a fabric and produces a glazed surface. The finish may be temporary or permanent based on the type of material used for finishing. If the fabric is impregnated with starch or waxes the finish will be temporary. If resins are used for finishing before calendering, the finish will be durable. To impart the finish, the fabric is first passed through the finishing solution and dried partially. Then it is passed through the calendar machine. The speed of the metal rollers is very high when compared to the speed of the paper roll. Thus the metal rollers polish the surface of the fabric by friction created by the speed of the metal rollers.
Schreiner Calendering: The primary purpose of the special schreiner calendar is to produce soft and deep-seated lustre rather than shine in fabrics. The calendar has a metal roller engraved with a series of fine lines ranging from 250 to 350 per inch. The direction of the lines makes roughly around 260 to the construction of the fabric almost parallel to the yarn twist. The small roller is hollow and has the heating arrangement from inside. The large engraved roller will revolve against the small heated roll. The fabric is passed between the rollers and the design of the engraved roller is impressed on the fabric producing a soft silk like lustre. Cotton sateens and damasks are finished by this process. When resins are used prior to finishing, the effect produced will be durable.
Moiré Calendering: It is a special calendar used to produce a moiré/watery effect on the surface of the fabric, primarily known for producing watery silks. Later with the advent of manmade fibres, the finish became more durable due to the thermoplastic nature of the fibres. Two methods are in vogue for production of moiré on fabrics. Two ribbed fabrics are placed with their faces together and slightly offgrain to each other and are either stitched or held together along the selvage and then fed through a hot metal calendar at a high pressure of around 8 to 10 tons. This pressure along with heat flattens the fabric as per the rib pattern of the opposite layer of the fabric. Thus fabrics are impressed with flat raised pattern and light reflects in the form of moiré pattern.
On thermoplastic fabrics, the moiré pattern is impressed at high temperature ona single rib fabric. The moiré pattern is engraved on a calendar roll which is heated during finishing. When the manmade fabrics pass through this calendar moiré pattern is impressed on fabric which is durable.
Embossed Calendering: Embossing refers to creation of patterns with flat and raised surface for the purpose of producing interesting texture and enhancing aesthetics in fabrics. Embossing when done on cellulosics, the finish is temporary. The finish is durable on resin coated cellulosics and manmade thermoplastic fabrics. The embossed calendar essentially consists of two rolls – one is a metal roll which is hollow and engraved with an embossed pattern. The second roll us a solid paper roll double the size of the metal roll. To produce the engraved designs, the fabric is drawn between the two rolls.
Two types of embossed designs are possible – flat and raised. For producing flat designs, simply the fabric is drawn between the heated engraved roller and the paper roll. The hot engraved roll which has the design in the form of raised portion impresses on the fabric producing engraved designs. For producing designs, paper roll is soaked in water and partially dried. It is made to revolve against the engraved roll producing the embossed pattern on the paper roll. After attaining sufficient temperature, the fabric is drawn between the two engraved surfaces of the metal and paper rolls to produce clear raised embossed designs that show the impressions even on the wrong side.
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