Stenter

In chain mercerising, the weft threads of the fabric are kept under tension on a clip stenter. In a chainless merceriser the fabric dimensions are controlled by a series of rollers. The fabric is usually woven slightly wider to allow for some weft shrinkage. Yarn is mercerised in hanks between two movable rollers which create the required tension whilst knitted constructions may be mercerised in either slit or tubular forms. Chain and chainless mercerising have been compared [276]. 

There is a so-called dry mercerisation process [275] in which the fabric is padded with caustic soda liquor at 20-25 °C and then dried in a stenter at about 130 °C. An immersion time in the pad trough of 7-10 seconds is sufficient but the goods need a total saturation time in the alkaline liquor of 30-40 seconds, i.e. from the nip to entry into the drying zone. 

This stretching, similar to the drawing of fibres, which promotes orientation and crystallite formation, is called biaxial orientation. It gives the film added strength and gas-barrier properties. In some processes, monoaxial (uniaxial) drawing is employed, e.g., polypropylene, which is then slit into thin strips and fabricated into heavy duty sacks, carpet backing, etc. The stenter process is used to make biaxial oriented poly(vinylidene dichloride) ("ding" film), polyester, polyamide and polypropylene films. 

A great part of the variation in the final properties of a textile is adjusted for by finishing procedures, for example, wrinkle resistance, soil repellence, hydrophobic properties, flame retardance, and antimicrobial properties [69]. In many cases chemicals are added by padding/ squeezing followed by drying/fixation, for example, in a stenter. 

Selvedge glues, based on a non-thermoplastic resin dissolved in a non-flammable solvent to be applied on slitting machines for circular knits or at the stenter avoids curling of selvedges... 

Stenter Through-circulation Tenter, range (textiles)... 

Continuous sheeting. Direct-type, continuous operation includes stenter, Yankee, impingement. Not applicable Not applicable Not applicable Not applicable Not applicable Not applicable Generally high capacity. Different es are available for different requirements. Suitable for drying without contacting hot surfaces Not applicable... 

Stenters (Tenters) and Textile Dryers These are the basic type of dryer used for sheets or webs in the textile industry. The sheet is held by its edges by clips (clip stenter) or pins (pin stenter), which not only suspend the sheet but also keep it taut and regulate its width—a vit consideration in textile drying. Drying is by convection hot air is introduced from one or both sides, passes over the surface of the sheet, and permeates through it. Infrared panels may also be used to supply additional heat. A schematic diagram of the unit is shown in Fig. 12-88. A typical unit is 1.4 m wide and handles 2 to 4 t/h of material. 

Method A Heat setting Scouring -> Bleaching -> Weight reduction Dyeing —> Drying on stenter. 

A worsted polyester/wool blended fabric is prepared by crabbing or blowing and then Scoured Dry Heat-set (on pin stenter) -> Brushed and Cropped Press or Decatise. 

Stenters are widely used for stretching, drying, heat-setting and finishing of fabrics (Fig. 8-3). Woven and knitted fabrics of polyester and nylon fibres and... 

Figure 8-3. Stenter for drying, finishing and heat-setting knitted and woven fabrics (Courtesy of Monti s.p.A.).

Figure 8-4. Multi-layer stentering machine (Courtesy of Bruckner GmbH), ventional entry and exit of single-layer stenter it is possible to obtain operating flexibility. The main advantages are the avoidance of longitudinal distortions and the ease of operation and maintenance with this ground level arrangement. The multi-layer stenter is available in versions with 3 or 5 fabric passes (layers).

The shrinkage in length and width may be as much as 10% and this must be taken into consideration in determining the overfeed and the adjustment of the width. Most of the stenters make provision for overfeeds - 5% to + 40%. If the entry rollers feed the cloth at a slower speed, the cloth will be actually pulled by the... 

On steam injection stenters the heating medium is a mixture of super-heated steam and air. The superheated steam required for the purpose can be generated inside the chamber by injection of saturated steam. In this mixture total heat capac-... 

For texturised nylon, after relaxing heat-setting is normally done at 150-160 C for 30 sec in a hot air stenter with an overfeed of 10-15%. The overfeed can be increased to 15-20% depending upon the type of stretch. The heat-setting tempera-... 

These fibres cannot be heat-set in the conventional sense since the fibres are readily stretched or deformed at temperatures above 75 C. The degree of stability, however, can be obtained by passing these fabrics through a hot air stenter at about 120 C. Temperatures above 120 C may cause discolouration of the fabrics. For blended fabrics containing acrylic and modacrylic fibres higher heat-setting temperatures may be required. Knitted fabrics produced from a feeder blend of acrylic and textured polyester fibres, are heat-set at about 160 C for 30 sec. 

Polyester/long-staple fibres are also used in the linen industry, where the yams may be of either the stretch-broken or unbroken type, but are more commonly of the latter. Heat-setting of such fabrics may be carried out at 180 C for 30 secs on the hot air pin stenter, allowing up to 2% weft shrinkage with overfeeding only to compensate for any warp shrinkage that has occured in the previous process. 

Heat-setting of polyester/silk fibres blended fabrics can be carried out on stenter at 190°C for 30 secs. The introduction of up to 50% polyester does not effect the characteristics drape and handle associated with silk fibres. 

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