Abstract:
A screw  12  for an extruder  10  rotatably supported on the cylinder of the extruder and carrying roll-heated rubber material supplied from a hopper port  13   s  provided at the rear part of the cylinder  11 , wherein the height of the portion of the flight portion  12   z  of the screw  12  located below the hopper port  13   s  is set lower than the height of the flight portion  12   a  on the downstream side of the screw  12 , whereby the pulsation of extruded matter can be reduced while maintaining a high discharge rate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a screw for use in extruders and, particularly, to the structure of a screw for reducing the pulsation of an extruded product and a production process therefor. 
     2. Description of the Prior Art 
     In the prior art, to manufacture a rubber member which requires a predetermined sectional form, such as a tire tread or a side wall, an extruder comprising a screw is generally used. For instance, in the step of extruding a tire tread, a kneaded and blended rubber material for treads is heated with a warming mill to be softened, supplied to the hopper  51  of an extruder  50  shown in  FIG. 3 , carried to the end of a cylinder  53  and extruded from the nozzle  55  of a head  54  attached to the end of the cylinder  53  by a screw  52  to be molded into a predetermined sectional form, cooled and cut to a required length. 
     However, as the above extruder  50  generally has a short length (L/D), the shape of the rubber material to be molded is easily affected by the revolution of the screw  52 . Therefore, when the extrusion rate (delivery rate) is increased by changing the structure of the screw, pulsation caused by delivery becomes large. 
     For example, when a high-delivery type screw  62  having a double-thread structure as a whole and a single-thread structure only on the hopper port  51   s  side is used as shown in  FIG. 4 , the delivery rate becomes higher than in the prior art. However, as all the rubber material supplied by the above single-thread structure portion is forced into the cylinder  53 , a change in pressure applied from the rubber material to the flight portion  62 F of the screw  62  become large in the vicinity of the hopper port  51   s . Therefore, the pulsation of the delivered extruded product A becomes large, thereby increasing a gauge fluctuation. 
     Particularly when the extruded product is a tread, the above gauge fluctuation exerts a bad influence upon the uniformity and balance of tires. Therefore, the development of a screw capable of suppressing pulsation even when the extrusion rate is increased is desired. 
     It is an object of the present invention which has been made in view of the above problem of the prior art to provide a screw for extruders capable of reducing the pulsation of an extruded product while a high delivery rate is maintained. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a screw for use in an extruder for carrying a rubber material supplied from a hopper port at the rear of the cylinder of the extruder by the screw, molding it into a predetermined sectional form and extruding it from a nozzle attached to the end of the cylinder, wherein 
     the height of a flight on an upstream side of the screw is made lower than the height of a flight on a downstream side. 
     According to a second aspect of the present invention, there is provided a screw for use in an extruder for carrying a rubber material supplied from a hopper port at the rear of the cylinder of the extruder by the screw, molding it into a predetermined sectional form and extruding it from a nozzle attached to the end of the cylinder, wherein 
     the height of a flight portion located below the hopper port of the screw is lower than the height of a flight portion on a downstream side and continuously changes in a peripheral direction at a predetermined angle. 
     According to a tenth aspect of the present invention, there is provided a screw, wherein a portion where the height of the flight portion is lower than the height of the flight portion on a downstream side has an angle of 180° or less. 
     According to a third aspect of the present invention, there is provided a screw, wherein the number of threads on an upstream side of the screw is made smaller than the number of threads on a downstream side. 
     According to a fourth aspect of the present invention, there is provided a screw, wherein the interval of the threads on an upstream side is made wider than the interval of the threads on a downstream side. 
     According to a fifth aspect of the present invention, there is provided a screw, wherein the diameters of the threads on an upstream side of the screw are made larger than the diameters of the threads on a downstream side. 
     According to a sixth aspect of the present invention, there is provided a screw, wherein the height of the flight located below the hopper port is made 2 to 6% smaller than the diameter of the screw. 
     According to an eleventh aspect of the present invention, there is provided a process for producing a screw for use in an extruder for carrying a rubber material supplied from a hopper port at the rear of the cylinder of the extruder by the screw, molding it into a predetermined sectional form and extruding it from a nozzle attached to the end of the cylinder, the process comprising cutting away a peripheral portion of a flight portion located below the hopper port of an existing screw at a predetermined angle so that the height of the flight portion continuously changes in a peripheral direction to produce a screw having the flight portion located below the hopper port lower than the height of the flight portion on a downstream side. 
     According to a seventh aspect of the present invention, there is provided a process for producing a screw for extruder, wherein the amount of the peripheral portion cut away is 2 to 6% of the diameter of the screw. 
     According to an eighth aspect of the present invention, there is provided a process for producing a tire rubber member by using the screw according to any one of the second to sixth and ten aspects. 
     According to a ninth aspect of the present invention, there is provided a tire rubber member manufactured by using the screw according to any one of the second to sixth and ten aspects and having a gauge fluctuation of 0.15 mm or less. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the constitution of an extruder according to an embodiment of the present invention; 
         FIGS. 2(A) and 2(B)  are partially enlarged views of a screw for use in the extruder according to the embodiment of the present invention; 
         FIG. 3  is a diagram showing the constitution of an extruder of the prior art; 
         FIG. 4  is a diagram showing the constitution of a screw for use in an extruder having a high delivery rate; and 
         FIG. 5  is a diagram for explaining the gauge fluctuation of an extruded product. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described hereinunder with reference to the accompanying drawings. 
       FIG. 1  is a diagram of an extruder  10  having a screw for extruders according to an embodiment of the present invention. Reference numeral  11  denotes a cylinder as a container,  12  a screw which is rotatably fitted in the cylinder  11 ,  13  a hopper for injecting a rubber material heated with an unshown warming mill into the cylinder  11 ,  14  a head mounted to the end of the above cylinder  11 , and  15  a nozzle attached to the end of the head  14  and having an opening  15   a  for molding the material into a predetermined required sectional form and detachable from the head  14  so that it can be exchanged according the type of an extruded product. 
     In this embodiment, a screw having a double-thread structure as a whole and a single-thread structure only on the hopper port  13   s  side near the inlet of the cylinder  11 , in which a peripheral portion of a flight portion  12   z  is smoothly cut away at an angle θ so that the height “h” of the screw flight portion  12   z  crossing the hopper port  13   s , that is, located below the hopper port  13   s  becomes smaller by a predetermined distance “d” than the diameter D of the screw  12  as shown in  FIGS. 2(   a ) and  2 ( b ) is used as the screw  12 . More specifically, the above “d” is set to around 4% of the diameter D of the screw  12 . The above θ is set to around 90° at maximum. The height of the flight portion  12   a  on a downstream side is the same as the diameter D of the screw  12 . 
     Since the return of the material occurs in the vicinity of the hopper port  13   s  thereby, all the rubber material injected from the hopper port  13   s  is not forced into the cylinder  11 . Therefore, a change in pressure applied to the above flight portion  12   z  in the vicinity of the hopper port  13   s  is reduced and the extrusion pressure is made uniform. Accordingly, the delivery rate of the extruded product extruded from the opening  15   a  of the nozzle  15  is made uniform, thereby making it possible to reduce the gauge fluctuation of the extruded product. 
     When a tread was actually extruded and molded by using an extruder equipped with the above screw  12 , the delivery rate was the same as the delivery rate of a screw  62  for high-delivery type extruders shown in  FIG. 4  and (about 20%) higher than that of the prior art and still the gauge fluctuation of the tread was half (0.15 mm) of the gauge fluctuation (0.3 mm) when the above screw  62  was used. 
     In this embodiment, since the height of the flight portion  12   z  located below the hopper port  13   s  of the above screw  12  for extruders which carries a rubber material supplied from the hopper port  13   s  at the rear of the cylinder  11  of the extruder  10  is made lower than the height of the flight portion on a downstream side, the pulsation of the extruded product can be reduced while a high delivery rate is maintained. 
     When a tire rubber member is manufactured by using the above screw  12 , it is possible to manufacture a high-precision tire rubber member having a gauge fluctuation of 0.15 mm or less. 
     In the above embodiment, the gauge fluctuation of the extruded product is reduced while the delivery rate is maintained by improving a high-delivery type screw  62  of the prior art. The present invention is directed not only to the above screw  62  but also to a screw having another structure such as a single-thread structure as a whole. 
     In the above embodiment, the screw  12  having a double-thread structure as a whole and a single-thread structure only on the hopper port  13   s  side is used. The thread structure of the screw is not limited to this. When a screw having a smaller number of threads on an upstream side than the number of threads on a downstream side, for example, two threads on an upstream side and three threads on a downstream side is used, the delivery rate of the extruded product is made uniform and the gauge fluctuation of the extruded product can be reduced. 
     Since the delivery rate of the extruded product can be made further uniform by making the interval of threads on an upstream side of the screw wider than the interval of threads on a downstream side or by making the diameters of the threads on an upstream side of the screw wider than the diameters of the threads on a downstream side, it is possible to further reduce the gauge fluctuation of the extruded product. 
     In the above embodiment, the peripheral portion of the flight portion  12   z  located below the hopper port  13   s  of the screw  12  for existing extruders is cut away. When a screw for extruders is to be newly manufactured, the height of a flight portion located below the hopper port  13   s  of the screw is made smaller by around 4% than the diameter of the screw  12 . 
     The height of the flight portion  12   z  located below the hoper port  13   s  may be made 2 to 6% smaller than the diameter of the screw. When the above value “d” is smaller than 2%, the material rarely returns and the pulsation of the extruded product cannot be reduced fully. When the value is larger than 6%, the rubber material cannot be forced into the cylinder  11  smoothly, the delivery rate cannot be made uniform. 
     INDUSTRIAL FEASIBILITY 
     As described above, according to the present invention, since the height of the flight located below the hopper port of the screw for extruders is made lower than the height of the flight on a downstream side, the extrusion pressure and delivery pressure of the screw can be made uniform and the gauge fluctuation of the extruded product can be reduced while a high delivery rate is maintained. 
     When a tire rubber member is manufactured by using the above screw, a tire rubber member having an extremely small gauge fluctuation can be obtained.