Abstract:
A die is mounted within an extrusion die assembly to pivot with respect to the die body and thereby adjust the concentric position of the die with respect to the tip and the product to be coated. Spherical surfaces are formed in an intermediate support module to engage mating surfaces formed on the die holder. The spherical surface surround the die holder and are clamped together by screws which are limited in tightening, to avoid binding the pivotal movement between the spherical surfaces and the die holder.

Description:
RELATED APPLICATION 
   This application is a continuation in part application of pending application Ser. No. 09/189,286, filed Nov. 10, 1998 and now U.S. Pat. No. 6,382,944 B1. 

   BACKGROUND OF THE INVENTION 
   Extrusion dies are frequently used to provide a tubular coating to a wire or other product which provides a generally cylindrical substrate. A typical die assembly  101  for performing this function is shown in FIG.  1 . The basic assembly  101  consists of a die body  102 , a tip  103 , a die holder  104 , and a die  105  held in place by end caps  106  and  114 . The tip  103  is mounted within an axial bore of the die body  101  and is itself constructed with an axial bore  107  having a downstream exit portion  108 . In operation, a wire or other tubular product  116  is fed through bores  107  and  108  to exit axially at  115 . An annular extrusion passage  109 , surrounds exit  115 , and is positioned to receive flowing plastic and apply it, in a tubular layer, to the product  116  at exit  115 . 
   The die assembly  101  is an assembly of machined parts, each having its own manufacturing tolerance. These tolerances tend to multiply with the assembly of the components. It is therefore a difficult task to maintain the desired coaxial relationship between the product and the extruded layer. An adjustment mechanism is generally needed to insure concentricity of the die and pin so as to provide an even thickness of the applied layer. This is generally accomplished simply by mounting the die holder  104  for adjustment, along x and y coordinates. The adjustment may be actuated through adjusting screws  112 . As shown in  FIG. 1 , the adjustment motion of the prior art is radially towards and away from the axis  117 , of the assembly  101  as a torque is applied to an adjustment screw  112 . This may require a significant adjustment torque because of the high thrust loads on the die holder during operation. 
   It is a purpose of this invention to provide a simple mechanism for adjusting the relative position of the die with respect to the pin to apply a consistently concentric layer of plastic about the circumference of the product, while reducing the torque required to adjust the position. 
   SUMMARY OF THE INVENTION 
   The die assembly of this invention may be generally described as a cross head die in that it receives molten plastic from an extruder which enters the die passages in a direction that is transverse to the longitudinal axis of the assembly. The plastic must then be turned to flow downstream axially towards an annular tapered extrusion channel. The extrusion channel surrounds an axially extending passage through which a wire or tubular product may be directed to receive a cylindrical layer from the extrusion passage. In order to apply a cylindrical layer of constant radial depth, the position of the die relative to the tip must be precise. Since the degree of positioning accuracy cannot be maintained during assembly of the components, an adjustment mechanism is provided which allows the relative position of die and tip to be adjusted after assembly. 
   The die body of this invention is constructed with a recess at its downstream end to receive the die holder which supports the die in a fixed relation. The die holder and die body recess engage to allow a pivot motion between the two components. To accomplish the pivot motion, the recess is constructed with a spherical internal surface which mates with a spherical external surface constructed on the die holder. These surfaces engage and operationally cooperate to allow the die holder to pivot in a substantially universal motion. To actuate the adjustment, adjusting screws are provided in the die body which engage the die holder at a position axially displaced from the spherically engaged surfaces of the die body and die holder. This provides a mechanical advantage proportional to the distance that the adjustment screws are displaced from the fulcrum formed by the spherical surfaces and results in a reduction of the torque which must be exerted on the adjustment screws to correct the relative position of the die and tip. In this manner the die and tip may be accurately maintained in a concentric relation. 
   In an alternate embodiment of this invention, the spherical surfaces of the die holder seat are constructed on an intermediate support module. The intermediate support modules comprises a pair of seat elements each having spherical surface portions to receive the spherical surfaces constructed on the die holder. The seat elements are assembled around the die holder and secured together by screws. By operation of the screws is designed to limit their tightening to provide a clearance. The assembled intermediate support module is positioned within a recess in the die body and secured by an end cap. This prevents the end cap from being overtightened and causing binding of the adjustment mechanism. 

   
     DESCRIPTION OF THE DRAWING 
     The invention of this application is described in more detail below with reference to the Drawing in which: 
       FIG. 1  is a sectional view of an extrusion die assembly showing the adjustment mechanism of prior art; 
       FIG. 2  is a sectional view of an extrusion die assembly showing the adjustment mechanism of this invention; 
       FIG. 3  is an enlarged sectional view of the adjustment mechanism of this invention; and 
       FIG. 4  is an enlarged sectional view of an alternate embodiment of the adjustment mechanism of this invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An extrusion die assembly  1 , constructed in accordance with the subject invention, is shown in FIG.  2 . The assembly  1  receives plastic from an extruder (not shown) and supplies it to a tapered annular extrusion channel  9  where it is extruded and applied to a wire  16 . The general function of the die head  1  is to receive plastic at upstream inlet  13  and distribute it to downstream outlet  15  in a flow pattern that is evenly dispersed about the extrusion channel  9 . 
   Extrusion die assembly  1  consists of components which are assembled in alignment with the axis  17  and cooperate to form an extrusion passage  9 . Die body  2  is a generally cylindrical element having an internal axial bore  18  having openings at its upstream and downstream ends. A tip  3  is assembled within the bore  18  and extends to the outlet  15  at the downstream end of the die body  2 . Bore  18  is constructed with a recess  19  concentric with the bore  18 . Die holder  4  is assembled within recess  19  and is constructed with a surface  11 , which is tapered inward to form the exit  15  of the extrusion channel  9 . A similarly shaped surface  10  is constructed on the downstream end of the tip  3  in a manner which provides a clearance with the surface  11  to form an annular tapered channel  9  in the assembled position. A die  5  may be removably fixed within the die holder  4  to complete the extrusion channel  9 . As best shown in  FIG. 3 , the exit  15  of the extrusion passage is formed by the cooperation of the die  5  and the tip  3 . The dimensions and configuration of the exit  15  is dependent on the relative position of these members. Tip  3  is constructed with an axial bore  7  having a downstream end  8  to provide a passage through which a tubular product, such as a wire  16  may be passed to receive a layer of plastic issuing from extrusion passage  9 . In order to deposit a cylindrical layer of plastic on an elongated product in a uniform thickness, the die  5  and tip  3  must be positioned concentrically. The components of the die assembly may be secured in the assembled position by caps  6  and  14 . The caps are attached to the die body  2  means of threads or by bolts. 
   Because of the accumulated tolerance errors within the assembly  1 , it is necessary to provide a means by which the concentric position of the die  5  may be adjusted with respect to the pin  3 . The overall motion required for adjustment is small, i.e., the difference between dimensions x and y as shown in FIG.  3 . To provide this motion, the inner surface of the recess  19  is provided with a spherical seat and the outer surface of the die holder  4  is constructed with a mating spherical contour. More specifically a spherical surface  20  is machined into the recess  19  at its upstream side while a similar surface  21  is machined in the downstream side of the recess  19 . Mating surfaces  22  and  23  are machined on the die holder  4  to engage the surfaces  20  and  21  respectively. 
   Although a full spherical engagement could be constructed, it has been found that, by employing a suitable clearance, sufficient motion can be allowed with the partial spherical engagement shown. This reduces the machining required and simplifies the manufacture of the adjustment means of this invention. 
   The spherical relationship between the engaging surfaces of the die holder  4  within the recess  19  allows the die to be pivoted within the extrusion passage  9 , thereby adjusting the dimensions and configuration of the exit  15 . To accomplish the adjustment, adjusting screws  12  are provided in the cap  6  at a distance z downstream of the center  24  of the spherical surfaces  20  through  23 . The distance z provides a mechanical advantage through the leverage of the force exerted by the adjustment screws  12  on the die holder  4 . The lever arm  2  also provides an adjustment movement which is maximized at the downstream end of the die holder  4  with a relatively small movement occurring at the upstream end of the element  4 . In this manner an accurate and sensitive adjustment can be made, even under the loads placed on the components during operation of the extrusion die  1 . 
   Alternate Embodiment 
   In the embodiment shown in  FIGS. 2 and 3 , the die holder  4  is supported between die body  2  and cap  6 . Cap  6  is secured to die body  2  by means of multiple screws, one of which is shown as screw  35  in FIG.  4 . The operative cooperation between cap  6  and die body  2  clamps the die holder  4  in place. Care must be taken to avoid binding the adjustment movement of die holder  4  within its spherical seat. A bound condition may result by the over tightening the clamping force exerted on die holder  4 . 
   In some applications, therefore, it may be advantageous to employ an intermediate support module  30 , as shown in FIG.  4 . The purpose of the module  30  is to provide a replaceable mounting arrangement which can be manufactured independently to construct the mating spherical surfaces in a simpler and more accurate method. In addition the module  30  is constructed with means to limit the clamping forces on the die holder  4 . 
   Intermediate support module  30  is constructed having an upstream seat element  32  and a downstream seat element  31 . The inner surface  40  of seat element  32  is spherically shaped to form a partial spherical seat. Similarly seat element  31  has an inner surface  41  which is also spherically shaped to form an second portion of the spherical seat. Partial spherical surfaces  40  and  41  cooperate to form a spherical mating surface for engaging the spherical surface  42  constructed on die holder  4 . Support module  30  is assembled by sequentially arranging the die holder  4  into engagement with seat surface  40  of seat element  32 . The assembly is completed by engaging the seat surface  41  of seat element  31  over the exposed spherical surface  42  of die holder  4 . The module  30  is held in the assembled conditions by multiple screws  36 . Screws  36 , as shown in  FIG. 4 , fit through a countersunk bore  37  of seat element  41  to engage an aligned threaded bore  34 . The screws  36  are tightened to secure the assembly. A clearance  33  is maintained by the action of screws  36 . This is accomplished by designing the bore  34  to limit the tightening, so as to maintain a predetermined clearance  33 . The assembled module  30  may be secured by positioning within the recess  19  of die body  2  and tightening the cap  6  into engagement. In this instance it can be observed that the action of screws  36  will prevent the over tightening of bolts  35 . This will effectively avoid binding the movement of die holder  4 .