Methods of Heat Setting

Heat Setting:
Heat setting is a heat treatment by which shape retention, crease resistance, resilience and elasticity are imparted to the fibres. It also brings changes in strength, stretchability, softness, dyeability and sometimes on the colour of the material. All these changes are connected with the structural and chemical modifications occuring in the fibre. Normally heat setting done for lycra fabric.

Methods of Heat Setting:

Contact method
In this method the fabric is run in contact with a heated metal surface. Some machines are composed of metal rollers having gas fired cores and are filled with a liquid known as diatherm to uniformly distribute the heat. Sometimes enclosed rollers are heated with high temperature steam.

Steam-setting method
Short staple polyester yarns including polyester/cotton blends are normally set by relaxation in saturated steam. The most effective means of stabilising these materials are to steam at 107~ on the ring spinners tube and soft dyeing packages under minimum tension. Sewing threads receive special setting treatments, designed to confer stability whilst preserving their high tensile properties. Polyester garments, garment lengths and hosiery are also stabilised by steaming in much the same way as for yams. Nylon can be set in saturated steam at temperatures above 1OO°C in an autoclave by batchwise process.

Hydro-setting method
The hydro-setting or aqueous heat-setting of polyester is done with hot water in a high temperature liquor circulating machine at about 130°C. A typical cycle may require 30 min. Water (or steam) promote swelling of fibre and may cause some hydrolysis in the ester groups in polyester chain. Nylon fabric can be hydro-set in hot water since the swelling action assists in weakening or breaking intermolecular bonds.

Heat-setting using stenter frame
Stenter machine are widely used for stretching, drying, heat-setting and finishing of fabrics (Fig). Woven and knitted fabrics of polyester and nylon fibres and their blends are normally heat-set on pin-stenter in hot air. 
Figure. Stenter for drying, finishing and heat-setting knitted and woven fabrics
An alternative to the pin stenter is the clip machine. The fabric is held into the chains either by pins mounted into a base plate or by clips in which the fabric edge is clamped between two smooth surfaces. Stenters that are used for setting only have a light pin chains whereas stenters used for both drying and setting (finishing) are provided with a heavy combined pin and clip chain.

Selective infra-red emitters method
Polyester can be heat-set by exposing the material under selected areas of magnetic spectrum of infra-red rays. The wavelength of the radiation source must be chosen with respect to the absorption band of the fibre i.e. a particular infra-red wavelength is chosen for a particular fibre. For example, in the case of polyester the selective infra-red radiation wavelength is the region of 1 to 4 g.


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