Method and apparatus for stretching tubular material
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of stretching continuously moving tubular fabric in a wet state and to apparatus for carrying out the method. 
     2. Description of the Related Art 
     In order to improve the dimensional stability of tubular textile material, it is known for the textile material to be stretched in the wet state and then to be dried. Until now, this stretching of the tubular material has been carried out with the aid of mechanical stretching means (width holders) in which the tubular material is gripped in its edge region and is stretched widthways by gradual moving apart of the two gripping zones. 
     The principal disadvantage of this method is that the tubular material is considerably more stretched in the immediate proximity of the gripped edge region by the greater traction exerted there than in the central region between the gripped zones. Thus, in this known method, the tubular material is stretched unevenly, so that optimum improvement of the dimensional stability is not achieved. 
     For another purpose, namely for laying textile material in rope or tubular form during wet treatment (such as dyeing, washing, rinsing), it is also known to blow air into the tubular material in order to open out the creases which have formed (U.S. Pat. No. 4,318,209). However, no expansion forces are exerted on the material beyond the simple opening out of the tube. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to make further developments to a method of the type set out in the Field of the Invention in such a way that uniform stretching of the tubular material over its whole width is achieved without any mechanical action impairing the surface of the material. 
     This object is achieved according to the invention. 
     In the method according to the invention, the tubular material is introduced into the stretching zone with a lead. With a delivery speed higher than the removal speed, a greater length of tubular material is therefore introduced into the stretching zone than is removed. In addition, compressed air (in sufficient quantity and under sufficient pressure) is sprayed into the stretching zone, and the tubular material is sealed at the beginning and at the end of the stretching zone against the escape of air. 
     In the tests on which the invention is based, it was found that the extent of the stretching of the tubular material can be influenced very sensitively and accurately by regulating the delivery speed and/or the removal speed. If the width of the tubular material is measured after the stretching zone, then by regulation of the delivery and/or removal speed, the tubular material can be stretched accurately to the desired width. In contrast to the previously known mechanical method of stretching, in the pneumatic stretching method according to the invention, an absolutely uniform stretching of the tubular material over the whole width is achieved, since the same expansion forces act on all peripheral points and no zones of the tubular material are gripped. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a schematic view of an apparatus for practicing the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The schematically represented apparatus serves for stretching tubular material 1 in a stretching zone 2, the beginning and end of which are each defined by a pair of rollers 3, 4 and 5, 6 respectively. One roller of each pair, namely the rollers 4 and 6 respectively, is driven. In addition, the speed of rotation of the roller 4 of the pair of rollers provided at the beginning of the stretching zone is regulable. 
     The rollers 3 and 4 are arranged in a fluid tank 7 which is preferably filled with water and into which the tubular material 1 is led over a guide roller 8. 
     For spraying compressed air into the tubular material 1, a nozzle element 9 which butts against the outside of the tubular material 1 is provided at the beginning of the stretching zone 2, is supplied with compressed air by a pump 10 and contains at least one nozzle opening (not shown). The nozzle element 9 can either be arranged stationary or can run freely with the tubular material. In the latter case the openings of the nozzle element 9 which are not covered by the tubular material can either be sealed by a stationary seal or (accepting a certain loss of compressed air) can remain open. 
     Two guide rollers 11, 12 are arranged some distance before the rollers 5 and 6 located at the end of the stretching zone and cause the inflated tubular material 1 to run into the pair of rollers 5, 6 at a defined aperture angle which is not too great. In this way, the formation of creases in the tubular material 1 as it passes the pair of rollers 5, 6 is prevented. 
     After leaving the stretching zone 2, the tubular material 1 passes a reel 13, for example. The tubular material 1 is led between the pair of rollers 5, 6 and the reel 13 in a flat (planar) state and passes a measuring arrangement 14 which measures the width of the tubular material 1 which was stretched in the stretching zone 2 and is now in two layers. 
     The method according to the invention can be carried out using the illustrated apparatus, for example, in such a way that the roller 6 which removes the tubular material 1 from the stretching zone 2 is driven at a fixed speed so that the tubular material leaves the stretching zone at a predetermined removal speed. By contrast, the roller 4 which is provided at the beginning of the stretching zone 2 is driven as a function of the signal from the measuring arrangement 14 at such a speed that the tubular material 1 runs into the stretching zone 2 at a delivery speed which is higher than the removal speed. Furthermore, if compressed air is sprayed into the tubular material in sufficient quantity and at sufficient pressure through the nozzle element 9, then by regulating the drive speed of the roller 4 it is possible for the degree of stretching of the tubular material 1 taking place in the stretching zone 2 to be influenced very sensitively and accurately (and with quite a small delay in response). If the delivery speed is increased, then the stretching becomes greater, and the reverse applies. In the stretching zone 2 the fluid applied to the tubular material 1 in the fluid tank 7 seals the interior of the tubular material 1 against an escape of air. In addition, at the beginning and at the end of the stretching zone 2, the rollers 3, 4 and 5, 6 effect the necessary sealing of the stretching zone. 
     The following example should serve for further explanation of the invention. A tubular knit fine-ribbed cotton material comes as raw material for finishing with a tube width of 36 cm. The desired tube width of the finished material is 30 cm. 
     The material is treated in a bleaching plant, then rinsed, and then because of the longitudinal stretching has a tube width of 25 cm. 
     This material runs at 20 m/min into the gap formed by the rollers 3 and 4. The fluid level in the tank 7 is chosen so that the material coming out of the gap can in turn be fully wetted with water. 
     Air is passed to the wet fluid-covered tubular material by the pump 10 at a pressure approximately 1000 mm of water via a stationary pipe which is provided with a bore of 3 mm. In this way, the tubular material is filled with air. The rollers 11 and 12 ensure that the tubular material reaches the gap formed by the rollers 5 and 6 without distortion of the curvature. The pair of rollers formed by the rollers 5 and 6 run at a speed of 17 m/min. This results in a speed differential between the two  pairs of transport rollers of 15%. 
     The material is squeezed hard between the rollers 5 and 6 so that it has a minimal water content, for example 70%. The material passes the measuring arrangement 14 with a tube width of 31 cm. In the subsequent drying, the material shrinks a little more lengthways and widthways so that the desired finished width of 30 cm is achieved and the length of material has low residual shrinkage values.