1.Calendering Finishing
The luster of a fabric is mainly determined by the reflection of light on its surface. When light irradiates the fabric surface, a certain amount of reflected light is generated. The stronger the reflected light, the higher the fabric luster. To enhance the luster of polyester fabrics, calendering or schreinering finishing can be applied. Calendering is a mechanical treatment method, but its luster effect has poor durability. However, if the fabric is first padded with resin prepolymer, followed by pre-drying and tentering before calendering, relatively durable luster can be achieved.
(1) Principle and Methods of Calendering
The light reflection of a fabric is determined by its surface characteristics. After wet processing such as pre-treatment, dyeing and printing, the yarns of the fabric become more curved, leading to reduced surface smoothness and flatness. Meanwhile, the fluff on the fabric surface causes diffuse reflection of light, which impairs the fabric luster. Calendering finishing is a process that utilizes the plasticity of fibers under hot and humid conditions to flatten the fabric surface or press dense parallel diagonal lines on it. This makes the fabric smoother, reduces light diffuse reflection, and thereby enhances the fabric luster. According to different finishing requirements, calendering processes can adopt different temperatures, with the following three commonly used methods:
① Hot Calendering Method (150~200℃): Flattens the fabric surface and imparts a certain degree of uniform luster.
② Light Hot Calendering Method (40~80℃): Ensures soft hand feel of the fabric without affecting yarn density, and gives the fabric a slight luster.
③ Cold Calendering Method: Flattens the yarns, arranges them more closely, and closes the interlacing pores of the fabric, resulting in relatively poor luster effect. The effect of calendering finishing largely depends on the roll surface material, nip pressure and calendering temperature.
(2) Commonly Used Calenders
Based on the different combinations of soft and hard roll nips, as well as variations in pressure, temperature and fabric feeding methods, calendering equipment can be classified into three types: ordinary calenders, friction calenders and stack calenders.
① Ordinary Calenders: An ordinary calender consists of 3~6 soft and hard rolls, forming various soft-hard nips to meet the finishing requirements of different fabrics. Conventionally, when the fabric passes through hard nips and the nips between hard and soft rolls, the yarns are flattened, the surface becomes smooth, the luster is enhanced, and the hand feel turns stiff. This calendering method is called plain calendering. When the fabric is calendered by the combination of two soft rolls, the yarns are slightly flattened, presenting a soft luster and a supple hand feel, which is known as soft calendering. By adjusting the combination of rolls with different hardness, as well as parameters like pressure, temperature and fabric guiding methods, fabrics can obtain different luster effects after calendering. Plain calendering and soft calendering are widely used in the finishing of polyester-cotton blended fabrics to improve their appearance and style.
② Friction Calenders: A friction calender usually has three rollers. The bottom roller is made of cast iron, the middle one is a soft roller, and the top roller is a friction roller made of heatable chrome-plated steel with mirror-polished surface. Friction calendering achieves a polishing effect by making the surface linear speed of the friction roller higher than that of the fabric passing through the nip. The speed difference causes friction on the surface of the processed fabric, thereby imparting a strong luster to the fabric.
③ Stack Calenders: A stack calender is composed of 5~7 rollers and is equipped with a fabric guide frame with a set of 6~10 guide rollers. It calenders fabrics in stacks (3~6 layers) through the same nip. The mutual pressing between fabric layers creates a ripple effect on the fabric surface, makes the yarns round and uniform, ensures soft hand feel and clear lines, and gives the fabric a luster similar to linen fabrics. After stack calendering, polyester/cotton poplin fabrics can have improved poplin effect, with more prominent weft yarn weave grains.
(3) Calender Rollers
Calender rollers are mainly divided into two categories: hard rollers (metal rollers) and soft rollers.
① Hard Rollers: Hard rollers of calenders are mostly made of metals such as chilled cast steel and medium carbon steel, with a general roller diameter of 200~300mm. The roller core is hollow and heatable, and common heat sources include steam, electric heating and gas. Polished steel rollers, i.e., friction rollers, are generally chrome-plated and undergo mirror polishing and flattening treatment.
② Soft Rollers: Soft rollers are mostly made of fibrous materials such as cotton and paper sheets under high pressure. They have a relatively large diameter, typically ranging from 350~600mm. In recent years, Germany has developed polyamide plastic elastic rollers (Nipco rollers). These rollers are made by casting polyamide plastic into cylindrical shape through molding process and fixed on steel shafts with bolts. Oil pressure is applied inside the rollers to solve the problem of rolling deflection, so as to ensure uniform pressure on the fabric.
(4) Analysis of Calendering Process Factors
The effect of fabric calendering is closely related to process conditions such as fabric moisture content, applied pressure, finishing temperature and fabric speed. In addition, it also has a certain correlation with the finishing agents contained in the fabric.
① Fabric Moisture Content: The moisture content of the fabric during finishing has a significant impact on the hand feel, luster and formability of the finished product. The higher the fabric moisture content, the stiffer the hand feel of the finished product after luster finishing, but the better the formability and luster, and the thinner the finished product. On the contrary, dry fabrics can obtain a soft hand feel. Generally, for fabrics to be subjected to schreinering, friction calendering, hot calendering and embossing, a relatively high moisture content (usually 10%~15%) is required during finishing to achieve strong luster and stiff hand feel. For fabrics undergoing plain calendering and stack calendering, a lower moisture content is preferred to prevent wrinkling caused by elongation during processing.
② Finishing Temperature: Different calendering temperatures result in different calendering effects. The cold calendering method gives the finished product a soft hand feel and a gentle luster; light hot calendering can impart a slight luster and a slightly stiff hand feel to the finished product; the hot calendering method endows the finished product with strong luster and stiff hand feel.
③ Pressure: The greater the pressure applied during finishing, the better the finishing effect, but the hand feel of the finished product also becomes stiffer with the increase of pressure. The selection of pressure should be compatible with conditions such as fabric speed, finishing temperature and fabric moisture content.
④ Fabric Speed: The fabric speed should be determined according to the fabric type, fiber material, gram weight and the roller arrangement of the calender. It is mostly controlled within the range of 10~60m/min, but sometimes it needs to be as low as below 10m/min during embossing finishing.
(5) Calendering Finishing Processes
① General Calendering Process Flow: Fabric padding with slurry or softener solution → Drying → Tentering → Calendering.
② Combined Finishing Process: Combining calendering with anti-shrinkage, anti-wrinkle, water-repellent, oil-repellent and coating finishing can improve the durability of fabric luster. For this reason, researchers have developed durable oil-shiny water-repellent polyester/cotton fabrics. Such fabrics are required to have oil-like luster with good durability and excellent water-repellent performance. Therefore, a combination of chemical finishing and mechanical finishing is needed. Its finishing process flow is as follows:
Padding with resin water-repellent agent (pick-up rate 60%~70%) → Pre-drying → Tentering and drying (controlling moisture content at 12%~16% when fabric is off the machine) → Friction calendering → Curing (150~155℃, 4~5min) → Friction calendering → Winding.
Schreinering Finishing
Schreinering finishing involves passing the fabric through the nip formed by a heated (schreinering) roller engraved with dense parallel diagonal lines and a soft roller. After calendering, parallel diagonal lines consistent with the twist direction of the main yarns are formed on the fabric surface. These lines reflect light regularly, endowing the fabric surface with a soft luster similar to silk. The principle of schreinering finishing is basically the same as that of calendering finishing. Most schreinering machines are of the two-roll calender type, but some are made into three-roll type. To achieve a good schreinering effect, the angle and density of the parallel diagonal lines formed on the fabric surface should match the twist angle and linear density of the fabric yarns. This requires that the diagonal direction of the engraved lines on the schreinering roller should be as consistent as possible with the twist direction of the main yarns on the fabric surface, that is, the diagonal angle of the schreinering embossing should be based on the twist direction of the main yarns on the fabric surface.
Embossing Finishing
Similar to calendering and schreinering finishing, embossing finishing of textiles utilizes the plasticity of fabrics under hot and humid conditions. A hard steel roller engraved with patterns is used to press the fabric, creating embossed pattern effects and local luster effects on the surface.
(1) Embossing: Most embossing machines are of the two-roll type, with soft and hard rollers as mother-child rollers. The steel roller has convex patterns, and the soft roller has concave patterns. The circumferential length of the hard steel roller maintains an integer ratio with that of the soft roller, such as 1:2 or 1:3, and they are connected by gears to ensure that the soft and hard rollers operate at the same linear speed. The pattern pressed on the soft roller closely matches the pattern engraved on the hard roller.
(2) Heat Embossing: Also known as light embossing, it is composed of an embossing roller (a copper roller used in printing machines) and a nitrile rubber roller (with a rubber layer thickness of about 25mm). Only the hard roller is engraved with shallow concave patterns, and the applied pressure is relatively small. After finishing, the embossed parts of the fabric show a glossy appearance.
Post time: Jan-26-2026