Patent Publication Number: US-6663823-B2

Title: Process for making nonwoven fabric and apparatus used for this process

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for making a nonwoven fabric including the steps of melt-extruding thermoplastic synthetic resin fiber and then thinning the fiber by stretching this fiber under hot air blast. This invention relates also to an apparatus used for this process. 
     It is well known to feed continuous fibers of melt blown thermoplastic synthetic resin onto a conveyor belt and thereby to form a nonwoven fabric. According to this well known process, a melt-extruder is provided with a plurality of nozzles arranged in an array adapted to discharge continuous fibers which are stretched and thinned under hot air blast in molten or semi-molten state. 
     In the case of the well known process, the continuous fibers extending parallel one to another in an orderly manner immediately after discharged from the nozzles may be deflected under the effect of the hot air blast and fused one with another in molten or semi-molten state before these continuous fibers reach the conveyor belt. Consequently, a plurality of fibers may be bundled together and/or a plurality of fibers may be intertwined together to form a fibrous lump as the fibers are cooled Such fibrous bundles and/or lumps necessarily result in the nonwoven fabric presenting uneven fiber distribution, rough touch and more or less spotted appearance. While such defects of the nonwoven fabric are not remarkable so far as the nonwoven fabric is of component fibers each having a diameter less than 3 μm, such defects can no more neglected and its functional value as well as its commercial value will correspondingly go down if the fiber diameter is as large as in order of 10˜20 μm. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a process and an apparatus enabling a nonwoven fabric to be made without an anxiety-of forming bundles and/or lumps of continuous component fibers even if each of these component fibers has a relatively large diameter. 
     According to a first aspect of this invention, there is provided a novel process for making a nonwoven fabric and, according to a second aspect of this invention, there is a novel apparatus used to exploit the novel process. 
     Specifically, according to the first aspect of this invention, there is a process for making a nonwoven fabric comprising the steps of stretching and thereby thinning continuous fibers of thermoplastic synthetic resin discharged from a melt-extruder under hot air blast and then placing these continuous fibers upon conveyor means. 
     The process further comprises, there being provided a guide box located between the extruder and the conveyor means, the guide box having an upper end opening spaced apart from nozzles of the extruder by a predetermined distance and adapted to receive the stretched and thinned continuous fibers and a lower end opening formed adjacent the conveyor means and having a width larger than the upper end opening as viewed in a travelling direction of the conveyor means and suction means located under the conveyor means so as to in opposition to the guide box, the steps of putting the stretched and thinned continuous-fibers in order within the guide box so that the stretched and thinned continuous fibers flow in well ordered manner from the upper end opening toward the lower end opening, then secondarily further stretching, thinning, cooling the continuous fibers at a flow velocity of the fibers maintained or increased in vicinity of the upper end opening, and placing the continuous fibers upon the conveyor means. 
     According to the second aspect of this invention, there is provided an apparatus adapted to stretch and thereby to thin continuous fibers of thermoplastic synthetic resin discharged from a melt-extruder under hot air blast and then to place these continuous fibers upon conveyor means, the apparatus comprising a guide box located between the extruder and the conveyor means, the guide box having an upper end opening spaced apart from nozzles of the extruder by a predetermined distance and adapted to receive the stretched and thinned continuous fibers and a lower end opening formed adjacent the conveyor means and having a width larger than the upper end opening as viewed in a travelling direction of the conveyor means and suction means located below the conveyor means so as to be opposed to the guide box with the conveyor means therebetween, the suction means being capable of putting the continuous fibers in order, the continuous fibers being subjected to the hot air blast and then secondarily further stretching, thinning and cooling the continuous fibers by maintaining or accelerating a flow velocity of the continuous fibers in vicinity of the upper end opening. 
     The apparatus and the process according to this invention for making the nonwoven fabric enable the continuous fibers discharged from the melt-extruder to be introduced into the guide box in the well ordered condition substantially similar to the condition in which the continuous fibers have been discharged. In addition, the apparatus and the process according to this invention enable the velocity of the continuous fibers immediately after discharged to be maintained or increased so that the continuous fibers may be effectively stretched and thereby thinned while they are gradually cooled. In this way, this invention provides the nonwoven fabric free from any bundle and/or clump of the fibers and offering a comfortable touch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the apparatus according to this invention; 
     FIG. 2 is a sectional view taken along line II—II in FIG. 1; and 
     FIG. 3 is a diagram illustrating the important part of FIG. 2 in an enlarged-scale. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Details of a process and an apparatus according to this invention for making a nonwoven fabric will be more fully understood from the description given hereunder with reference to the accompanying drawings. 
     FIG. 1 is a fragmentary perspective view of an apparatus  1  according to this invention for making a nonwoven fabric. The apparatus  1  comprises a melt-extruder  2 , an endless belt  3  running below the extruder  2 , a guide box  4  located between the extruder  2  and the endless belt  3  and a suction box  6  located to be opposed to the opposed to the guide box  4  with the endless belt  3  therebetween. The endless belt  3  runs in a direction indicated by an arrow Y and made of breathable material in order to ensure the suction box  6  may properly act upon the guide box  4  as the endless belt  3  travels immediately above the suction box  6 . The guide box  4  is supported by a pair lateral supports  7  in vertical movable manner. 
     FIG. 2 is a sectional view taken along line II—II in FIG.  1 . The extruder  2  is provided within its head  11  with a plurality of extruding nozzles  12  adapted to discharge thermoplastic synthetic resin fibers  13  in molten state which are then stretched and thereby thinned under hot air blast  15  injected from hot air nozzles  14 . 
     The guide box  4  underlying the head  11  has a pair of side walls  16  lying in the vicinity of transversely opposite sides of the belt  3  as viewed in FIG. 1, a front wall  17  and a rear wall  18  as viewed in the running direction of the belt  3 . The guide box  4  presents a trapezoidal cross-section and has an upper end opening  21  immediately underlying the extruding nozzles  4  and a lower end opening  22  formed adjacent the upper surface of the belt  3 . The lower end opening  22  has a width larger than that of the upper end opening  21  as viewed in the running direction of the belt  3 . In the vicinity of the lower end opening  22 , the front and rear walls  17 ,  18  are respectively provided on outer surfaces thereof with front and rear rollers  23 ,  24 . These rollers  23 ,  24  rotate in the travelling direction Y of the belt  3  as the belt  3  runs. These rollers  23 ,  24  are vertically movable slightly but sufficiently to substantially close a gap defined between the lower ends of the front and rear walls  17 ,  18  and the belt  3 . In other words, the front roller  23  is mounted on the front wall  17  so that a gap defined between the lower end of the front wall  17  and fibrous web  31  being conveyed on the belt  3  may be substantially closed by the front roller  23  and the rear roller  24  also is similarly mounted on the rear wall  18 . The rollers  23 ,  24  have their upper halves protected by covers  26 ,  27  extending outward from the front and rear walls  17 ,  18 , respectively. 
     The suction box  6  is connected to a vacuum pump (not shown) via a pipe  28 . A suction effect of the suction box  6  exerted on the guide box  4  enables the outside air to be forcibly introduced into the guide box  4  through the relatively small upper end opening  21  toward the lower end opening  22 . The outside air into the guide box  4  in this manner serves not only to keep the fibers  13  discharged from a plurality of the extruding nozzles  12  arranged transversely of the belt  3  in well ordered condition but also to maintain or increase, in the vicinity of the upper end opening  21 , a velocity of the fibers  13  after discharged. In this way, the fibers  13  can be further stretched and thereby thinned in the upper end opening  21 . In addition, the fibers  13  are cooled in perfectly or substantially well ordered condition and collected on the belt  3 . Consequently, it is not apprehended that the fibers  13  might be broken before collected on the belt  3  or fused together to form fibrous bundles and/or intertwined together to form fibrous lumps. 
     The fibers accumulated on the belt  3  are converted to web  31  which is conveyed in the direction Y through a small gap between the front wall  17  of the guide box  4  and the belt  3  and then taken up in the form of nonwoven fabric  32 . Outside the front wall  17 , the roller  23  is rotating in the direction Y as the roller  23  is kept in contact with the upper surface of the web  31 . The presence of the front roller  23  reliably prevents the outside air from entering into the gap defined between the front wall  17  and the belt  3 . 
     FIG. 3 is a diagram illustrating the important part of FIG. 2 in an enlarged-scale. The illustrated embodiment uses the extruding nozzles  12  each having a diameter=0.45 mm and arranged at a pitch=1 mm. 
     The illustrating embodiment uses the guide box  4  of which the upper end opening  21  has a width b=10˜100 mm, the front and rear walls  17 ,  18  define an angle of intersection θs=30˜60°. The guide box  4  is spaced apart from the extruding nozzles  12  by a dimension a=10˜200 mm and lies at a level=50˜400 mm as measured from the belt  3 . 
     For the apparatus  1  shown in FIGS.  1 ˜ 3 , styrene-based elastomer of tri-block type having MFR=70 g/10 min (2.16 kg at 230° C.) was used as raw material for the continuous fibers  13  which was discharged at a rate=0.13 g/min/hole at a resin temperature=270° C. under hot air blast injected at a rate=2.5 Nm 3 /min. Operation of the apparatus  1  at a suction air blast rate=42 m 3 /min and a belt speed=3.3 m/min resulted in a fiber velocity=662 m/min immediately after discharged and a fiber velocity=1062 m/min in the vicinity of the upper end opening  21 . Thus the nonwoven fabric  32  was obtained in which the component fibers  13  are evenly distributed and neither the bundles nor the lumps of these component fibers  13  are present. Operation of the apparatus  1  with the guide box  4  eliminated therefrom resulted in the nonwoven fabric which was observed to comprise continuous fibers each having an average diameter of 17 μm and to have a plurality of fibrous bundles and/or lumps.