Knitted fabrics heat-setting

Most of the textured apparel and industrial yams are woven or knitted directly into fabric. The carpet BCF yams can be tufted directly off package into loop pile or velvet constmctions. For the textured saxony constmctions, the BCF and the spun staple yams must be ply-twisted and heat-set. The heat-setting temperature for nylon-6 and nylon-6,6 is 180—220°C in hot—dry atmosphere, and 120—140°C in saturated steam. The yams are twist-set in pressurized autoclaves or continuously on the Superba and Suesson machines (121). Before setting the twist, the yam is heated and relaxed for predevelopment of the bulk. 

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.).

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. 

Fabrics containing elastomeric fibres can be heat-set. A tricot or sleek-knit fabric knitted from nylon 6 and polyurethane may be satisfactorily heat-set at 180-185 C for 20-25 sec. Slightly higher temperatures (190 C) may be used for nylon 6,6. However, the required width and stretch characteristics of the finished fabric determine the precise conditions to be employed with a particular fabric [29]. 

Natural fibres, such as wool or cotton, have a curled or irr ular shape. Yams and fabrics made from these fibres are bulky and have a high thermal insulation and a pleasant grip and appearance. Many methods have been developed to give synthetic yams and fibres simUar properties. This is called texturisation. In texturisation processes unannealed yams or fibres are usually deformed by twisting, stuffing or knitting. They are then heat-set in the deformed state, which makes the deformation more or less permanent Most of those processes can be applied to split fibres but we shall only deal with techniques in which orientation effects play an important role. 

An explanation for this inereased energy transfer is that tlie specific heat of steam/air mixture is almost twice of that of air alone. Also by using steam/air mixture, heat exchange at the exterior of the fiber is increased, but the heat conduction from the surface to the interior of the fiber remains imchanged. Thus warp knitted fabrics and lightweight woven like taffeta and georgette require 3-4 seconds with steam/air mixture, compared with 15 seconds if hot air alone is used. Heavier fabrics, especially woven require 9 seconds with rapid heat setting (with steam/air mixture) and 30 seconds with hot air. 

When such yarns are knitted or w oven into fabric, the filaments tend to return to the configurations in which they were originally heat-set. Contraction lakes place in the direction of the yarn axis, and this in turn converts the smooth fiat fabric into a stretch fabric and gives the surface a textured appearance. These fabrics or the garments made from them, whatever the process used to produce the yams, may be given additional... 

PLA processes and products are compared and are comparable to PET both are melt spun, both are available in filament and staple form, weaving and knitting set-up conditions are similar, fabrics can be heat treated to give dimensional stability, and they are both dyed with disperse dyes. However, both fibers are unique and they do behave differently. It is therefore important that processors acknowledge this fact, and that the operating details are refined accordingly. 

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