Abstract:
A plant and method for making continuous yarns made of silicone material comprises at least an extrusion station, into which the material is introduced in an amorphous condition, and extrusion means which cause the material to exit from the extrusion station along an extrusion axis. The plant also comprises a vulcanization station, located downstream of the extrusion station, at a determinate distance therefrom, in which the continuous yarn is vulcanized in a direction of treatment. The plant also comprises a drawing unit, disposed downstream of the vulcanization station.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §120 to and is a divisional application of U.S. patent application Ser. No. 13/125,266, filed on Apr. 20, 2011, entitled “Plant for Making Continuous Elastic Yarns Made of Silicone Material,” which claims priority to International Patent Application No. PCT/IB2009/007164 filed on Oct. 20, 2009, the disclosures of which are incorporated herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The present invention concerns a plant for making continuous elastic yarns of silicone material usable in the medical field, that of protective technical fabrics and in those industrial fields for which elastic yarns are required with a section of less than 0.7 mm, advantageously 0.3-0.4 mm, and even as little as 0.2-0.1 mm, resistant to certain chemical agents and/or to high or low temperatures, and in particular cases usable as electric conductors. In particular, the present invention can be used to make so-called elastic yarns of silicone rubber. 
         [0004]    The present invention also concerns the continuous elastic yarn obtained using the plant. 
         [0005]    Here and hereafter in the description and claims, the term elastic yarn means generally a continuous extruded product, either filiform or with a different profile, for example packings, with a count higher than a bead, filament, thread or profile, that is, the multi-filament type. 
         [0006]    Description of Related Art 
         [0007]    Plants are known, for the extrusion of silicone material used to make extruded products of various section and thickness. 
         [0008]    Known plants normally extend horizontally and comprise, in succession, feed means that feed the material in the form of paste, an extrusion station, in which the extruded product is made, and a vulcanization station. 
         [0009]    This type of known plant has the disadvantage that the extruded product, at the end of the extrusion process, is deposited on a conveyor belt or strip that normally advances at a speed substantially equal to the speed of extrusion. 
         [0010]    Since this feed occurs horizontally, or substantially horizontally, the extruded product being made is affected both by the extrusion pressure and also the force of gravity, under the action of which the extruded product tends to modify its section, flattening before entering the vulcanization station, so that, at entrance to the vulcanization station, the extruded product no longer has the correct section but is deformed, mainly due to the extrusion pressure. 
         [0011]    This problem is accentuated when the extruded products to be made are particularly fine, for example in the case of silicone based elastic yarns with a diameter of less than 0.7 millimeters, to as little as 0.2 and 0.1 mm. 
         [0012]    This problem also makes it very difficult to have a precise control on the value of section obtained, and also it does not allow to guarantee a uniform section over the length of the yarn. 
         [0013]    Moreover, known plants do not allow to extrude materials such as silicone rubber, with a reduced section and in the liquid state, or at least not totally solid, because the elastic yarn exiting from the extrusion station would not be firm enough to be deposited on a conveyor belt or to be held by clamping means and drawn through the vulcanization station. 
         [0014]    Another disadvantage of known plants is that the type of oven usually used in the vulcanization station are normally moved manually on each occasion, to be moved closer to or away from the vulcanization station, depending on the type of extruded product to be processed, but this interrupts the production process and increases both the economic cost and the production times, and also does not guarantee precision positioning. 
         [0015]    Known solutions for producing extruded products made of silicone material work on thicknesses having a slightly larger size than those sought in the present invention, such as for example DE-C2-38 44 60. 
         [0016]    Other solutions work on products in which the field of use is completely different from the production of elastic yarns according to the present patent application, such as DE-A 1-101 45 945, which concerns the production of multi-layer optical cables, or W-A1-2008/074622, which concerns the production of windscreen wiper brushes, that is, with values of size and quality that cannot be compared with those required in continuous elastic yarns which are the object of the present patent application. 
         [0017]    Purpose of the present invention is to achieve a plant for the continuous production of elastic yarns made of silicone material that overcomes the shortcomings of the state of the art and allows to make elastic yarns, simply and economically, with a section of much less than 0.7 mm, advantageously less than 0.4 mm and even 0.2-0.1 mm. 
         [0018]    The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages. 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea. 
         [0020]    In accordance with the above purpose, a plant for making continuous elastic yarns made of silicone material comprises at least an extrusion station, a vulcanization station located downstream of the extrusion station at a determinate adjustable distance therefrom, and at least a drawing unit, located downstream of the vulcanization station and able to impart on the elastic yarn exiting from the extrusion station a drawing value in a ratio comprised between 2 and 6 (100-500%) before vulcanization. 
         [0021]    The extrusion station is provided with an extrusion body, in which the material is able to be introduced in an amorphous condition, and extrusion means, able to allow the material to exit from the extrusion station along an extrusion axis, conditioning the shaping thereof. 
         [0022]    The extrusion means are conformed so as to confer on the extrusion axis a substantially vertical direction, so that the elastic yarn exiting, disposed with a substantially vertical axis, is drawn due to the effect of the drawing unit, with the desired drawing ration and set according to at least the final thickness to be obtained, at least along the adjustable distance between the extrusion station and the vulcanization station. 
         [0023]    The drawing action determines on the one hand the desired lengthening of the elastic yarn and thinning to the desired value of thickness, and on the other hand the uniformity and regularity of its section; it also orients in a more regular manner the molecular chains inside the elastic yarn, allowing to obtain values of lengthening and force above the standard of the base product. 
         [0024]    The use of a drawing unit with a high drawing ration also allows to eliminate the deformation effect of the force of gravity that acts upstream of the vulcanization station, which in known solutions with at least partly vertical extrusion, determines deformations and inaccuracies in the section. 
         [0025]    Moreover, in particular in the case of very thin extruded products, it reduces the effect of the heat abatement caused by the introduction of the yarn inside the vulcanization oven, before the vulcanization treatment is completed. 
         [0026]    In this way, continuous yarns of any length can be produced, and they have no surface imperfections, with constant technical-elastic characteristics for their whole length and with no deformation in section. 
         [0027]    A plant according to the present invention can therefore be used to make elastic yarns with controlled and uniform sections even much less than 0.7 mm, as little as 0.4-0.3 mm and less. 
         [0028]    Furthermore, since no horizontal movement is provided, it is also possible to introduce into the vulcanization station elastic yarns that are still in a liquid, plastic or at least not solid state. 
         [0029]    According to a variant, the plant provides a collection unit, located downstream of the vulcanization station, and able to collect the yarn exiting from the vulcanization station. 
         [0030]    In another variant, the collection unit comprises said drawing unit so that the elastic yarn, in the segment between the exit from the extrusion means and the entrance to the vulcanization station, reduces its section to the desired value while still keeping the regularity and quality of the section shape. 
         [0031]    According to a variant, a plant according to the present invention comprises a controlled cooling station, located downstream of the vulcanization station and upstream of the drawing and possible collection unit. 
         [0032]    The cooling station is able to cool the elastic yarn to a temperature near the environmental temperature, so as to prevent the finished product from continuing the crystallization process also after collection and storage. 
         [0033]    According to a variant, the vulcanization station comprises a zonal vulcanization oven, each zone being characterized by a heating system: high temperature, ultraviolet rays, microwaves or other. 
         [0034]    According to another feature of the present invention, the vulcanization station is selectively movable in a vertical direction, and is heat adjustable according to the type of elastic yarn treated. This solution allows to select the temperature of the various zones of the vulcanization oven with a simple programming of the plant, reducing manpower and equipping times. 
         [0035]    According to another variant, the cooling station is also selectively movable in a vertical direction and is heat adjustable according to the type of extruded product treated. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0036]    These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawing wherein: 
           [0037]      FIG. 1  is a lateral schematic view of a plant for making elastic yarns made of silicone material according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    With reference to the attached drawing, a plant  10  according to the present invention is used to make extruded products of silicone material, in this case continuous elastic yarns  30 , with a thickness of less than 0.7 mm, advantageously 0.3-0.4 mm, as little as 0.1 mm, of silicone rubber. 
         [0039]    The plant  10  comprises feed means  11 , an extrusion station  12 , a vulcanization station  16 , a cooling station  17  and a drawing unit and collection  26  of the yarn  30 . 
         [0040]    The feed means  11  feed the extrusion station  12  and are automatic. 
         [0041]    In this case, the feed means  11  comprise a hopper  22 , dosing pumps, able to introduce into the hopper  22  the correct quantity of material according to the capacity of the plant, and static and dynamic mixers, able to mix the material so as to prevent the formation of air bubbles. 
         [0042]    The feed means  11  also provide a device able to keep the level of material inside the hopper  22  constant, so as to also keep constant the exit pressure of the material from the hopper  22 . 
         [0043]    According to a variant, in the case of material in the form of paste, the feed means  11  comprise a roller or a pair of rollers cooperating with each other, which feed the material in the form of a strip. 
         [0044]    According to another variant, in the case of a continuous strip of material, the feed means  11  comprise a conveyor belt on which the strip is disposed. 
         [0045]    The extrusion station  12  comprises an extrusion body  31  and extrusion means  15 . 
         [0046]    The extrusion body  31  is provided with compression means  13 , dosing means  14  and a device  25  to distribute the material. 
         [0047]    The compression means  13  compress the material and thrust it toward the dosing means  14 . In this case, the compression means  13  comprise a worm screw, sized according to the capacity of the plant  10 , and a degassing zone, to eliminate the possible formation of air bubbles. 
         [0048]    According to a variant, the compression means  13  provide a piston device. 
         [0049]    The dosing means  14  are able to dose the material compressed by the compression means  13  so as to feed the device  25  to distribute the material in a uniform and continuous manner. 
         [0050]    In this case, the dosing means  14  comprise a dosing pump with gears and a plurality of pressure transducers, coordinated with the compression means  13 . 
         [0051]    The transducers detect the pressure at entrance and at exit to/from the dosing means  14  and, according to this detection, send a signal to the compression means  13  to regulate the speed of the material and to keep a constant delivery through the dosing means  14 . 
         [0052]    The device  25  to distribute the material feeds the extrusion means  15  uniformly. In this case, the extrusion body  31  has a vertical elbow-type conformation. The material enters the extrusion body  31  in a horizontal direction of feed, due to the effect of the pressure exerted by the compression means  13 , and exits from the extrusion means  15  along an axis of extrusion D, with a substantially vertical direction, due to the effect of said pressure and the force of gravity. 
         [0053]    According to the conformation of the draw-plate, the yarn  30  obtained at exit from the extrusion means  15  can have a variable number of beads or filaments, for example comprised between 2 and 30. 
         [0054]    The draw-plate comprises corresponding perforations having a lead-in to facilitate the entry of the material and a bevel at exit to facilitate the detachment of the material. 
         [0055]    According to a variant, the extrusion body  31  has a horizontal conformation. 
         [0056]    According to a variant, the extrusion body  31  has a vertical conformation. 
         [0057]    According to a variant, upstream of the draw-plate the extrusion means  15  comprise at least a filter and at least a pre-drawing plate, advantageously two filters and two pre-drawing plates. This is particularly advantageous to prevent obstruction of the perforations of the draw-plate and to distribute the material better, in the case where the extrusion means  15  include a draw-plate each, equipped with a plurality of perforations for the simultaneous production of a corresponding plurality of elastic yarns. 
         [0058]    The presence of the filters upstream of the draw-plate is extremely important so as to be able to break the memory effect that the extrusion screw imparts to the material. The filters are chosen of a size such as to prevent the yarn, descending vertically, from twisting, knocking and from preventing the spinning. 
         [0059]    The extrusion station  12  also comprises a heating and cooling device  24  for the automatic heat-regulation of the various zones of the extrusion station  12 , so as to keep the ideal conditions of extrusion temperature and pressure. 
         [0060]    The elastic yarn  30  exiting from the draw-plate advances vertically, drawn by the drawing and collection  26  unit. In this case, the collection unit  26  comprises drawing means  18 , such as a motorized roller, which exert a predetermined drawing force on the extruded product  30  and guarantee that the extruded product  30  advances toward the vulcanization station  16 , maintaining a substantially vertical direction, without horizontal displacements. 
         [0061]    Furthermore, as a consequence of the drawing force of the drawing means  18 , the section of the continuous yarn  30  is reduced to the desired size, with a drawing factor that can vary, according to needs, from 2 to 6, in this way reducing the section of the elastic yarn  30  proportionally, yet keeping the quality and uniformity of the section shape high, and orienting the molecular chains in a more regular way. 
         [0062]    The vulcanization station  16  comprises a zonal oven  27 , each zone being characterized by a heating system: high temperature, ultraviolet rays, or microwaves. 
         [0063]    Inside the oven  27 , the elastic yarn  30  is treated in a direction of treatment “T”, substantially vertical and parallel to the axis of exit “D” of the yarn from the extrusion station  12 . 
         [0064]    The oven  27  is selectively movable vertically, with known mechanical or pneumatic devices, to be positioned at different distances “L” from the draw-plate according to the type of elastic yarn  30  and the relative crystallization and vulcanization times thereof. 
         [0065]    In particular, the variation in the distance “L” between the exit from the extrusion station  15  and entry into the vulcanization oven  27  allows to vary the drawing conditions on the continuous yarn  30  according to the section to be obtained, both in the shape and thickness and also in orientation of the molecular chains, apart from, obviously the type of yarn  30  itself. 
         [0066]    In the case of continuous elastic yarns  30  according to the invention, the distance “L” typically varies between 400 and 1500 mm, in particular according to the final thickness of the elastic yarn  30  to be obtained. 
         [0067]    The vulcanization oven  27  is heat-adjustable so as to vary the vulcanization temperature according to the type of elastic yarn  30  treated. 
         [0068]    The cooling station  17 , located downstream of the vulcanization station  16 , is able to cool the elastic yarn  30  to a temperature near the environmental temperature so as to prevent the final product from continuing the crystallization process after collection and storage. 
         [0069]    The cooling station  17  is heat-regulated so as to guarantee that ideal cooling temperature conditions are maintained, according to the type of elastic yarn  30 . 
         [0070]    The collection unit also comprises, apart from the afore mentioned drawing means  18 , surface treatment means  19 , control means  20  and collection means  21 , of a known type. 
         [0071]    The surface treatment means  19  are able to treat the external surface of the extruded product  30  in order to confer upon it determinate physical characteristics. 
         [0072]    The control means  20  of the extruded product  30  comprise a roller associated with control devices and are able to control the physical characteristics of the extruded product  30 , such as the thickness, uniformity of section, elasticity and others. 
         [0073]    The collection means  21  are able to collect and pack the elastic yarn  30  for storage. The collection means  21  comprise an automatic reel-winder or a device to cut the elastic yarn  30  and insert it into containers. 
         [0074]    It is clear, however, that modifications and/or additions of parts may be made to the  10  as described heretofore, without departing from the field and scope of the present invention.