Abstract:
A method of extending the life of an extruder screw by affixing tiles of wear-resistant material along the crest of the extruder screw flighting threads, and optionally grinding the hardened material to remove any sharp edges. In another configuration of the present invention, plates of wear-resistant material are secured to the leading edges of the extruder screw threads to further prolong the life of the extruder screw.

Description:
CLAIM OF PRIORITY 
   This application is a continuation of U.S. Nonprovisional patent application Ser. No. 11/122,707 filed on May 5, 2005, now U.S. Pat. No. 7,513,676 issued on Apr. 7, 2009, which claims priority from U.S. Provisional Patent Application No. 60/569,366 entitled “EXTRUDER SCREW WITH LONG WEARING SURFACES” filed on May 8, 2004, which are incorporated herein by reference for all purposes. 
   CROSS-REFERENCE TO RELATED APPLICATION 
   This application is also related to U.S. patent application Ser. No. 11/122,664, entitled “DIE FOR EXTRUDED MATERIALS,” filed May 5, 2005, on behalf of Edward Williams, now U.S. Pat. No. 7,316,667 issued on Jan. 8, 2008, which is incorporated by references for all purposes. 

   TECHNICAL FIELD 
   The invention relates generally to extrusion equipment, more particularly to extrusion screws, and most particularly to extrusion screws that have been modified for extended wear and durability. 
   BACKGROUND 
   Extrusion is a well-known process that is used in many industries. A variety of products from foods to optical fibers, plastics and metals are produced by extrusion. In the extrusion process, the materials to be extruded are fed into the extruder, a type of giant mixer. The materials are mixed, heated if appropriate, and fed into a barrel that contains an internal rotating screw. The screw moves the liquid or viscous material being extruded (the “extrudate”), out the end of the extruder, through a die having the shape of the desired product. The extrudate will take the shape into which it is formed coming out through the extruder die. The extrudate is generally continuous, and is often cut into desired lengths after being extruded out the die. Extrudate is formed into an almost infinite variety of shapes, ranging from tubes to sheet material. 
   It has been found that the outside edges of the screw threads in the extruder, which mix the extrudate and push it out the end of the extrusion barrel, tend to be more subject to wear than other parts of the screw threads. When the outside edges of the screw threads wear sufficiently, extrudate flows between the screw threads and the barrel, which interferes with the normal extrusion process. To this end, steps have been taken to extend the durability on those parts of the screws that see additional wear. Such steps have included applying a coating of various liquid substances that harden on the screw or applying an additional layer of solid wear-resistant material to the portions of the extruder screw subject to wear. However, it has been found that materials that project out from the surface of the screw are subject to shear during use, and the materials can experience fatigue cracks. Subsequently, pieces of the material can break off and end up being incorporated in the extrudate being mixed. Often this necessitates the disposal of batches of extrudate which are contaminated with pieces of wear-resistant material or coating. 
   Another solution has been to cut grooves in the crests of the extruder screw threads, and insert other materials, such as molybdenum, in those grooves, as shown in U.S. Pat. No. 5,135,378 to Catton (“Catton”). The Catton process is, however, very complex, involving cutting a groove, laying in a first material along the sides and at intervals as bridges across the width of the groove, then laying in a second material in a liquid state that hardens to form a mechanical bond with the extruder screw. The added material is then ground to form a smooth surface. 
   Another solution, as disclosed in U.S. Pat. No. 6,346,293 to Douris et al. (“Douris”), is to weld a bead of a hardened material along part of the width of the crest of the extruder screw threads, and then weld beads of a different hardened material along either side of the first bead of hardened material. Subsequently, the hardened materials are ground to the required tolerances. Because of the many steps involved in welding two separate materials to the crest of the screw, because of the need for the materials that will bond to each other as well as to the screw thread, and because of the grinding the hardened material, the process is very involved and time consuming. Thus, the Douris process is rather expensive. 
   Therefore, a method and/or apparatus for extending the life of extruder screws is needed by making them more resistant to wear wherein the method does not compromise the structural integrity of the screw, nor produce fragments that contaminates the extrudate, and is not overly complex, involved or expensive. 
   SUMMARY 
   The present invention, accordingly, provides a method of applying a plurality of tiles to a flighting crest of at least one thread of an extruder screw. A groove is formed along the crest. Each tile is then inserted into the groove, wherein the tiles are in a spaced-apart arrangement. The tiles are then secured into the groove. In one preferred embodiment of the present invention, the groove is formed in the extruder screw when the extruder screw threads are cast. In yet another preferred embodiment of the present invention, the groove is cut into the crest after the extruder screw has been cast. In yet another alternative embodiment of the present invention, the tiles are further ground to remove any protruding edges and to fit within the tolerances of an extruder barrel. Another alternative embodiment of the present invention further comprises inserting a spacer into spaces between the tiles; and securing the spacers. 
   In another preferred embodiment of the present invention, plates of a wear-resistant material are secured to at least one leading edge of the extruder screw threads in a spaced-apart arrangement. 
   In one embodiment of the present invention, the tiles are made of carbide or tungsten-carbide. In another preferred embodiment of the present invention, the step of securing further comprises brazing, soldering, or gluing. 
   Additionally, the present invention also provides an apparatus for extruding. An extruding screw having at least one thread with a crest and having a groove formed therein is provided. Also, there are a plurality of tiles formed of a hardened materials secured within the groove in a spaced-apart arrangement. In an alternative embodiment of the present invention, the hardened material is carbide or tungsten-carbide. In another alternative embodiment of the present invention the tiles are secured by brazing, soldering, or gluing. In another preferred embodiment of the present invention, spacers secured between the tiles. 
   Additionally, in yet another preferred embodiment of the present invention, a plurality of plates made of the hardened material is secured to the crest in a spaced-apart arrangement. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a perspective view of an extruder screw embodying the features of the present invention; 
       FIG. 2  is a side view of an extruder screw embodying the features of the present invention; and 
       FIG. 3  is an enlarged cross-sectional view of a portion of an extruder screw taken along line  3 - 3  of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   In the discussion of the FIGURES the same reference numerals will be used throughout to refer to the same or similar components. In the interest of conciseness, various other components known to the art, such as extrusion devices and extruder die plates have not been shown or discussed. 
   In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details not considered necessary to obtain a complete understanding of the present invention have been omitted, in as much as such details are considered to be within the skills of persons of ordinary skill in the relevant art. 
   Referring to  FIGS. 1-3  of the drawings, the reference numeral  100  generally designates an extruder screw of the present invention. The system  100  includes a cast screw  10  and tiles  20 . The tiles  20  are typically made of carbide, tungsten-carbide, or some other hard, long-wearing material, and the cast screw is made of a cast metal or allow, such as steel. 
   As can be seen in  FIG. 1 , the wear-resistant layer is comprised of a series of tiles  20  spaced slightly apart from each adjoining tile  20 , rather than from a single piece of material. The purpose of spaced-apart tiles  20  is to provide sufficient flexibility so that the tiles  20  can move slightly in relation to each other and the screw  10 , and therefore withstand shear and fatigue cracking that can occur when the extruder screw is moving extrudate through the extruder. Thus, this spacing and the resulting ability of the tiles  20  to move slightly prevents damage to the tiles  20  that results in pieces of the tiles  20  being broken off and falling into the extrudate. 
   In one arrangement of the present invention, generally flat tiles  20  are affixed to the length of the flighting crest  12  of the threads  14  of the extruder screw  10 . The tiles  20  are generally brazed, soldered, or glued to the flighting crest  12  of the extruder screw threads  14 . Preferably, as can be seen in  FIG. 1 , some of the tiles  40  bonded to the leading face  18  of the extruder screw threads  14  have an edge that is generally coextensive with the flighting crest  12 . 
   In another arrangement of the present invention, the tiles  20  have a generally flat upper surface, and a tapered base (not shown). The tapered base portion (not shown) of the tiles  20  fit into a groove (not shown) that has been cut or cast into the flighting crest  12  in the screw threads  14 . The tiles  20  are generally brazed, soldered, or glued into the groove  16  in the flighting crest  12 . 
   In another embodiment of the present invention, as seen in  FIG. 1 , thin plates of a hardened material  40  are affixed to the leading edge  18  of the screw  10 . Preferably, the plates  40  are bonded to the leading edge  18  of the screw  10 . Some examples of bonding are brazing, soldering, or gluing. The plates  40  are spaced slightly apart from one another, rather than a single piece of material being affixed to the leading edge  18  of the screw  10 . The spaced-apart plates  40  should be sufficiently close to each other so as to provide a generally continuous surface across the leading face  18  and flighting crest  12  of the thread. The purpose of spaced-apart plates  40  is to provide sufficient flexibility so that the plates  40  can move slightly in relation to each other, and therefore withstand shear and fatigue cracking that can occur when the extruder screw is moving extrudate through the extruder. This spacing, and the resulting ability of the plates  40  to move slightly, prevents damage to the plates  40  that results in pieces of the plates  40  being broken off and falling into the extrudate. In some arrangements of the present invention, spacers  42  are inserted into the spaces between the plates  40 . These spacers  42  are typically made of a soft material, such as copper, that will move in response to movement of the plates  40  and/or the screw  10 , and preferably have a different expansion and contraction rate than the screw  10  or the plates  40  for increased flexibility. If necessary, the plates  40  can be ground to achieve a smooth surface of the desired dimensions. 
   As can be seen in  FIG. 2 , some arrangements of the present invention also employ spacers  30  in conjunction with the tiles  20  used along the flighting crest  12 . Spacers  30  are inserted into the spaces between the tiles  20 . These spacers  30  are typically made of a soft material, such as copper, that will move in response to movement of the tiles  20  and/or the screw  10 , and preferably have a different expansion and contraction rate than the screw  10  or the tiles  20  for increased flexibility. 
   After the tiles  20 , and if used, spacers  30 , have been affixed to the extruder screw flighting crest  12 , they can be ground and polished to fit the tolerances of the extruder. 
     FIG. 3  shows a cross-section of an extruder screw  100  of the present invention after tiles  20  have been secured to the extruder screw flighting crest  12  and been ground to tolerance. 
   It is understood that the present invention can take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or the scope of the invention. 
   Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.