PROCESS FOR PRODUCING A DOME-SHAPED EXTRUSION DIE

Abstract: This application pertains to a process for making a seamless, dome-shaped extrusion die (30), comprising the steps of: hydroforming a metal sheet into the shape of a dome, said being seamless; annealing the hydroformed metal sheet; and forming a plurality of extrusion openings (38), in the dome.

Description: TITLE PROCESS FOR PRODUCING A DOME-SHAPED EXTRUSION DIE BACKGROUND OF THE INVENTION The present invention pertains to an improved process for fabricating a perforated metal dome-shaped die for use in a screw-type extrusion granulating apparatus. Certain commercially available dome extrusion dies, such as those available for use with certain double dome extruders, are produced from perforated flat metal sheet which is cut such that it can be formed into a dome shape with three equidistant longitudinal slits. The dome shape can be secured by a combination of forming the flat sheet over a domed surface and then welding each slit closed. Although dies of different diameters can be produced, many limitations are inherent in the dome dies resulting from this process. The larger commercially available dies of about 220 mm diameter suffer an unacceptably high rate of catastrophic failure through breakage when used in extrusion applications subjecting them to typical operating pressures of more than 1 x (10 6 ) Pascals and temperatures of from 25 to 75°C. These limitations are initially due to weakness at the welds, the strength of which are prematurely exceeded through normal operation of the extrusion apparatus. In addition, large diameter dies generate stresses which exceed the yield strength of typical materials, such as 304 stainless steel, leading to premature failure of the metal away from the welds. This renders the extrusion operation costly and impractical. Additional limitations involve the range of materials which can be used to form the dome dies, the hole patterns possible, and the thickness of the metal relative to the diameter of the holes. Materials are limited to those which are easy to shape and which can be welded. Such materials consist mainly of metals such as 304 and 316 stainless steels. The extrusion holes are normally punched in the metal sheet with hole size limited to diameters no greater than the thickness of the metal sheet. Hole roundness is deformed in the process of cutting, shaping and welding the domes leading to irregularities in the final extruded products. U.S. Patent 5,240,400 describes a dome-shaped die useful with a screw- type extrusion granulating apparatus. The extrusion openings of the die are of a cross-sectional dimension approximately the same as the thickness of the die. SUMMARY OF THE INVENTION This invention pertains to a process for making a seamless, dome-shaped extrusion die, comprising the steps of: hydroforming a metal sheet into the shape of a dome, said dome being seamless; annealing the hydroformed metal sheet; and forming a plurality of extrusion openings in the dome to yield a seamless, dome- shaped extrusion die. This invention also pertains to dies which are generally hemispherical and to dies where the extrusion openings are generally circular and the thickness of the die is within a range of about 0.7 to about 3.3 times the diameter of the extrusion openings. This invention further pertains to dies where the diameter of the extrusion openings is less than the thickness of the die and to dies where the thickness of the die is within a range of about 1.2 to about 3.3 times the diameter of the extrusion openings. This invention also teaches a process of die manufacture comprising annealing the seamless, dome-shaped extrusion die, and the process wherein the metal sheet is formed from a metal selected from the group consisting of a 300 series stainless steel, aNITRONIC® series austenitic stainless steel, a nickel based alloy, and a heat treatable carbon steel. Also disclosed is the process wherein the thickness of the metal sheet is about 2 millimeters or less and wherein the extrusion openings are of a diameter in the range of about 0.4 to about 1.5 millimeters. This invention further pertains to the process of wherein the extrusion openings are arranged in a generally equilateral triangular relationship to one another as well as the case where the extrusion openings are arranged in sections with each section separated from another section by areas with no extrusion openings, said areas with no extrusion openings being in a longitudinal segment, concentric circular, or geodesic pattern. In addition this invention pertains to processes further comprising electropolishing the seamless dome- shaped extrusion die to improve the internal surface smoothness of a plurality, preferably all, of the extrusion openings as well as the inside surface of the die itself, to the process further comprising coating the seamless dome-shaped extrusion die with titanium nitride using a vapor deposition process, and to the process further comprising electropolishing the seamless dome-shaped extrusion die followed by coating the die with titanium nitride using a vapor deposition process. Further, this invention pertains to dies prepared by the processes described above. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a double dome/ring set constructed using the prior art process of welding a pre-perforated metal sheet. Figure 2 shows the configuration of a pre-perforated metal sheet useful in constructing the dies of Figure 1. Figure 3 shows a double dome/ring set prepared using the process of the present invention. Figure 4 shows a die of the present invention having an area without extrusion openings which area is useful in the prevention of impingement of granules when a double dome/ring set is used during operation of a screw-type extrusion granulating apparatus. Figure 5 shows a preferred extrusion opening pattern in a longitudinal segment configuration. Figure 6 shows a preferred extrusion opening pattern in a concentric circular configuration. For illustrations purposes, only a portion of the die is shown here as having the extrusion openings. In use, the extrusion openings would generally continue to the edge of the die. Figure 7 shows a preferred extrusion opening pattern in a geodesic configuration. DETAILED DESCRIPTION The following description pertains to the process of making a dome- shaped extrusion die and the die made by such a process. Dies of the present invention are suitable for producing granules of a very small cross-sectional dimension. The present invention overcomes the significant limitations of the conventional process for fabricating dome dies which conventional process uses a welding process. Figure 1 shows a double dome/ring set 10 of the prior art which consists of two dies 12 each mounted on cylindrical ring 14 which ring is welded to die plate 16. As can be seen in Figures 1 and 2, the conventional process uses a perforated flat sheet 22 which is cut and then welded into dome-shaped die 12 which has welds 18 extending from the edge of the die to a short distance from the top of the die. Each die 12 has a plurality of extrusion openings 20. Figure 3 shows a double dome/ring ring set 30 comprising two seamless dome-shaped extrusion dies 32 of the present invention each welded to cylindrical ring 34 which ring is welded to die plate 36. In the present process, an unperforated dome-shaped die is produced in a pressure molding process, such as by hydroforming, where a blank metal sheet is placed over a male mold and through the application of hydraulic pressure is formed into a seamless dome- shaped die. Generally, hydroforming is done in a series of applications. During each application, the metal is exposed to the hydraulic pressure whereupon it is forced in directions approaching the shape of the mold. Each subsequent hydroforming application forces the metal to more closely approach the shape of the mold until it eventually conforms to the shape of the mold. After each hydroforming application, an annealing step is generally required to remove stress in the metal hydroformed sheet since the metal is work-hardened during the present process. Suitable types of annealing include bright annealing and flame annealing. Preferably, the hydroformed sheet is subjected to bright annealing where the temperature is controlled and charring does not occur. For example, 304 stainless steel can be annealed at about 1900°F (1038°C), and NITRONIC® 50 stainless steel can be annealed at about 2000°F (1040°C). (NITRONIC® is a registered trademark of Armco Steel Corporation, Baltimore, MD). The pressure at which the hydroforming applications are performed, the number of hydroforming applications, the number of annealing steps needed, and the temperature at which the annealing is performed are dependent upon the type of metal and the thickness of the metal used in the metal sheet. In addition, the number of hydroforming applications and the number of annealing steps should be of a sufficient number to substantially relieve the stress in the metal. Hydroforming services are available from Kosempel Manufacturing Company, Philadelphia, PA. Alternatively, a spinning process can be used to prepare the present dome- shaped die. Spinning can be done on a lathe and a disc of metal forced stepwise over a spinning mold in the desired shape. Stamping is another method which can be used to form a seamless, dome-shaped die of the present invention. Preferably, the dome-shaped portions of the die are of a hemi-spherical configuration, but dome-shaped configurations of other curvatures are also contemplated. For example, any die whose dome-shaped portion is a portion of a hollow spheriod, whether a circular spheriod, hemisphere, an oblate, or a prolate spheroid, or any other curved surface of a body of revolution, is contemplated to be within the scope of this invention. Preferred is a generally hemispherical dome-shaped extrusion die. The present process is advantageous for producing robust domes which are seamless. By "seamless" is meant there are no welds in the portion of the die where the extrusion openings are found. Welds are major weak points in known extrusion dies. With seamless dies, stress is more evenly distributed over the dome structure during use greatly increasing the life of the die, uptime of the extrusion process and overall utility of the extrusion apparatus. Equipment capacity is increased because the seamless dome-shaped dies of the present invention are able to withstand higher operating pressures during the extrusion of materials. Thus, the overall cost effectiveness of a granulating system using dies of the present invention significantly increases because shutdown and replacement or repair of dies is lessened or eliminated. Using the present process, dies can be produced from a wide range of materials of varying thickness with versatile extrusion opening patterns and extrusion opening size options. The greatest advantages are realized with dome- shaped die of approximately 220 mm diameter, where operating stresses are two or more times as great as compared with smaller domes 110 mm or 80 mm in diameter. The larger capacity domes are advantageous because a die with twice the diameter can provide four times as much throughput. Although 220 mm is the largest sized die for screw-type extrusion granulating apparatus currently available, the preparation of dies with diameters greater than 220 mm is possible using the present process. Highly reproducible parts with accurate dimensions can be produced using the present process as compared with the conventional welding process which conventional process produces less precise parts. For the preferred embodiment wherein the dome-shaped die is generally hemispherical, the accuracy with which the seamless dies approach perfect half spheres can be critical since extrusion rate and operating load are optimized when the extrusion wiping blade of the screw- type extrusion granulating apparatus can be brought to within 1 -2 millimeters of the inside surface of the dome throughout its full range of rotation. As shown in Figures 3 and 4, a plurality of extrusion openings 38 are formed in the dome of the present invention to yield a seamless, dome-shaped extrusion die 32. In the extrusion of moistened powders, it has been a "rule of thumb" that the metal sheet thickness limits hole size which can be punched to diameters less than the thickness of the sheet. For example, if 1 mm holes were desired, metal sheet up to only 1 mm thick could be used. However, by drilling the extrusion openings, such as with a laser, as is done in the present invention, the diameter of the extrusion opening is not so limited. Thus, formation of the extrusion openings of the present invention are preferably by drilling, most preferably by laser. A laser can be precisely aimed perpendicular to the dome- shaped die at any position on its surface. The diameter of the extrusion opening is much less limited by metal thickness or metal type than the hole diameter using the conventional welding process, and the pattern of the extrusion openings is limited only by a designer's imagination and the specific function to be performed by the part being perforated. Extrusion openings are selected to be of a size corresponding to the desired diameter of the granules to be produced from the moistened powder material. Lasers are available from a variety of sources. Preferred are 5 axis lasers, such as those available from Lasercraft, Inc., Elyria, Ohio and Versatile Fabrication Co., Muskegon Heights, Michigan. Preferably the laser is used with computer aided design software which guides the movement of the laser to preprogrammed positions for cutting, such as is available from Delcam Pic, Birmingham, United Kingdom. Extrusion openings can be examined under a microscope to determine the best drilling conditions, e.g., laser voltage, pulse frequency and pulse duration. High hole quality is qualitatively defined as those extrusion openings with a combination of a high degree of roundness, sharp edges, smooth internal surfaces and the least amount of metal debris or uncut portion for both entry and exit sides of the opening. Preferably, the extrusion openings are of a diameter in the range of about 0.3 to 1.2 mm. The process of the present invention can further comprise an annealing step after formation of the extrusion openings. This annealing step can be used to remove any stresses remaining in the dome-shaped extrusion die. The outside surface, inside surface or both surfaces of the dome-shaped extrusion dies of the present invention can be polished to remove any surface debris that may have accumulated around the drilled extrusion openings, or surface charring from annealing, and to achieve a surface finish with roughness value between 32Rs and 125Rs, 8 (10" 4 ) and 32 (10" 4 ) mm, or N6-N8 using an alternate designation, to enable wiping of the material to be extruded by an extrusion wiping blade which blade forms the tip of a conveying screw of a screw- type extrusion granulating apparatus through the extrusion openings with appropriate levels of friction. A relatively smooth surface for the inside surface of the die and for the walls of the extrusion openings is desirable to reduce average working loads, undesirable temperature rise through the dies and unwanted changes or variability in product quality (smoothness of the extrudates). Dome surfaces, including the walls of a plurality, preferably all, of the extrusion openings themselves, can be improved through a process of electropolishing. Unmounted seamless dome-shaped dies, complete with drilled extrusion openings, can be fitted with an electrode system and immersed in a suitable conductive bath comprising an acidic chemical reagent. The application of current leads to preferential removal of metal where surface imperfections exist, resulting in a controlled removal of metal leaving a substantially smoother surface and no significant change in extrusion opening or surface geometry. In addition, the polished dome surfaces, inside and out, can be further improved using a vapor deposition process which puts down a titanium nitride uniform coating no more than 5 ten thousandths of an inch (0.0013 cm). The coating confers upon the walls of a plurality of the extrusion openings and external dome surfaces additional improved properties such as increased hardness, reduced surface friction and improved cleanability. These improvements additionally lead to lower operating pressures through the die openings, reducing stresses in the metal, and further increasing dome life and utility. Electropolishing can be performed at Able Electropolishing in Chicago, Illinois and titanium nitride coating can be performed at Richter Precision, Inc. located in East Petersburg, PA. As shown in Figure 3, one, or preferably two, dome-shaped extrusion dies can be each mounted on and welded to cylindrical metal ring 34. The dome/ring set 30 can then be welded or bolted to die plate 36 which conforms to a screw- type extrusion granulating system with each dome-shaped extrusion die in axial alignment with an extruder screw (not shown). Alternatively, cylindrical ring 34 can be formed as an extension of dome-shaped extrusion die 32 and a portion of this extension adjacent the edge can be formed into a flange. This flange can then be bolted to die plate 36. The present invention produces a robust seamless dome-shaped extrusion die with a longer operating life than a dome-shaped extrusion die made using the conventional pre-perforated sheet and weld process. The longer life of the dies of the present invention is due partly to the elimination of welds in the dome-shaped die and partly to the increased flexibility of the domes which are no longer constrained by welds. By using the present process to prepare a dome-shaped extrusion die, it is possible to use a wider range of materials including stronger materials, in the construction of such dies than could be used in the conventional pre-perforated welding technique. In addition to 304 and 316 stainless steel useful in the conventional process, metals useful in the present invention include austenitic stainless steel, such as the NITRONIC® series stainless steels (NITRONIC® is a registered trademark of Armco Steel Corporation, Baltimore, MD), nickel based alloys including INCONEL® and MONEL® (registered trademarks of the International Nickel Company), and HASTELLOY® (registered trademark of Hynes International, Kokomo, IN) and heat treatable carbon steels. Use of the above additional materials confers improved properties of corrosion resistance, hardness and strength. In particular, nickel based alloys increase the life of the die of the present invention over the life of a die prepared from non-nickel based alloys when the die is used to extrude materials containing ionic chlorides and where operating temperatures exceed 50°C. Under these conditions, the 300 series stainless steels are subject to stress corrosion cracking to which nickel based alloys are resistant. Normal extrusion operating pressures can exceed 2.75(10 6 ) to 3.5(10 6 ) Pascals at the dome surface. This generates stresses in the metal which approach or even exceed the yield strength of 304 or 316 stainless steel. Fabrication of seamless dome-shaped extrusion dies of the present invention from the NITRONIC® series steels, such as for example NITRONIC® 30, 40 or 50, provides dies with increased strength up to two times that of dies formed with a 300 series stainless steel. Along with additional strength, thicker materials useful in the present invention enable the use of extrusion openings with diameters not only greater than the thickness of the metal used in construction of the die but with diameters less than the thickness of the metal used in construction of the die. For example, 0.7 mm diameter extrusion openings cannot be punched in 304 stainless steel which is 1.2 mm thick as must be done if fabrication is to be performed via the conventional welding technique. This thickness limitation does not exist for the present process. A metal sheet having a thickness of 1.2 mm can easily be hydroformed and annealed into a blank hemisphere and then laser drilled with 0.7 mm extrusion openings using the present process. Preferably, the thickness of the die of the present invention is within a range of about 0.7 to about 3.3 times the diameter of the extrusion opening. In another preferred embodiment, the thickness of the die lies within a range of about 1.2 and about 3.3 times the diameter of the extrusion openings. Preferably, the thickness of the metal sheet used to prepare the die is less than 2 mm; most preferably from about 0.8 to about 2 mm. A preferred embodiment is where the thickness of the die is about 2 mm or less and the extrusion openings have a diameter in the range of about 0.4 to about 1.5 mm. Another advantage of the present invention is that the extrusion openings are formed after the dome-shaped die is formed and so are not subject to distortion during formation of the dome. This enables the production of a more uniform extruded product. In addition, there is no significant limitation to the choice of extrusion opening pattern or to allowance for unperforated sections of the surface. Although any pattern or random arrangement of extrusion openings is acceptable in the present invention, preferably the extrusion openings are arranged in a uniform pattern with the extrusion openings having a generally equilateral triangular relationship to one another and wherein the distance between opening centers is about one and one half that of the opening diameters. A second pattern is preferably superimposed over the triangular pattern of extrusion openings. Preferred examples of this second pattern can follow a longitudinal segment pattern (see Figure 5), concentric circular pattern(see Figure 6), geodesic configuration pattern(see Figure 7), or a combination of such patterns. Unperforated areas of the dome-shaped extrusion die can increase functionality of the extrusion domes. Unperforated areas can form part of any of the second patterns described above, for example separating sections in a longitudinal segment pattern. In addition, as can be seen in Figures 3 and 4, a preferred embodiment is to provide at least one section 40 of each dome near its base having no extrusion openings present, an wherein at least one of such unperforated sections would face the section having no extrusion openings of another die so that when the dies are assembled into a multiple dome/ring set, such as a double dome/ring set, extruded granules would not impinge on each other as they exit through the extrusion openings of the dies. Thus unperforated section 40 acts to prevent clumping of extruded granules and loss of throughput between multiple dome-shaped extrusion dies. Recycle rates of extruded material are reduced using the seamless dies of the present invention. Extrudate appearance and strength are superior using a die of the present invention made from NITRONIC® steel over a conventional welded dome. Stronger granules result in less granule fracture resulting in less recycled material. Another effect of stronger granules is a more consistent product bulk density evident when using a die of the present invention made from NITRONIC® steel. In addition, such granules are more consistent (smoother) in appearance. The present invention also concerns a seamless, dome-shaped extrusion die prepared by the above-described process. Preferred embodiments for such a die are as described above for the process of making such a die. Dies of the present invention are useful in a screw-type extrusion granulating apparatus for extruding a moistened powder material into granules comprising a housing defining a screw chamber for receiving said moistened material, a rotatably driven conveying screw disposed in said screw chamber for conveyance of said moistened powder material through said screw chamber, and a die. Such a screw- type extrusion granulating apparatus is described in U.S. Patent 5,240,000, incorporated by reference herein. A die made by the process of the present invention represents an improvement to such an apparatus. Preferred embodiments for such an improved apparatus are as described above for the process of making such an improved die. Conventional extrusion granulators of the type described in U.S. Patent 5,240,000 consist primarily of a single or twin conveying screw arrangement driven by a motor, a screw housing in which the conveying screw is disposed, an extrusion blade or blades incorporated into the tip of the conveying screw, and a die plate attached to the front end of the screw housing in axial alignment with the screw. In operation, powdered raw material previously moistened and well mixed by kneading machines is inserted into the screw housing from a feed hopper. The raw material is then forced to transfer forwardly to the front end of the screw housing by means of the conveying screw. During this process, the raw material is transferred by the conveying screw and is delivered toward the forward end of the screw chamber and the die by means o f the rotation of the conveying screw. When the material reaches the spacing between the forward end of the conveying screw and the interior surface of the die, the material is forcibly extruded continuously through the extrusion openings of the die by the combination of a plowing effect on the material layered over the interior die surface created by the extrusion blade and conveyance of the material by the conveying screws. EXAMPLE 1 A hemisphere shaped mold 8.35 inches (21.21 cm) in diameter was created and used to hydroform ten (10) unperforated domes from 304 stainless steel sheet, 1.2 mm in thickness. Two annealing steps were conducted during the hydro- forming process to relieve stresses in the metal. These annealing steps involved flame annealing the hydroformed metal until it appeared red hot. The final die parts were polished to remove any charring. A five axis laser drilling machine was programmed using software from Computer Aided Design to cut approximately 36,000 holes, 0.7 mm in diameter in each of four of the above hydroformed domes. Laser voltage was set at 300 watts and oxygen was used as the assist gas. Holes were cut perpendicular to the surface of the dome from the inside out. Hole to hole spacing was based on an equilateral triangular pattern, with hole centers 1.5 mm apart. A larger circular pattern approximately 32 mm in diameter consisting of about 350 holes was first cut at the top of the dome. Twelve longitudinal segments were then cut, each one starting just under the topmost circular pattern and traveling down to the dome base. Each triangular section contained approximately 3,000 holes. In total the twelve sections covered the hemisphere surface leaving unperforated strips approximately 1-2 mm in width between sections. The inside surface of the dome was mechanically polished to remove any metal debris remaining after laser cutting. The inside dimensions of the completed dome conformed to the hemisphere shape of the hydroform mold within 1-2 millimeters out of a total diameter of 220 millimeters. Two dome-shaped dies, as prepared above, were mounted on and welded to 304 stainless steel rings which were in turn welded to a die base plate. This dome/ring set was mounted on a Fuji Paudal TDG220 Double Dome extruder and put into service in the extrusion of CLASSIC®25DF herbicide (CLASSIC® is a registered trademark of E. I. du Pont de Nemours and Company). The dies produced quality product and lasted 35 hours before breaking. Dies of the same material, similar thickness (1.0 mm) and similar extrusion opening diameter size (0.65 mm) but made using the conventional pre-perforated welding process lasted an average of 10 hours before breaking. 10 EXAMPLE 2 Two unperforated, hydroformed domes from the ten described in Example 1 were laser drilled in similar fashion to those from Example 1 , except for two differences. The Computer Aided Design system was reprogrammed to cut 36,000 holes 0.8 mm in diameter in each hemisphere, while maintaining the hole pattern of Example 1 ; and a longitudinal section measuring 2.5 cm x 11 cm was left unperforated adjacent the edge of each die. The two dome-shaped dies were mounted in a double dome/ring set with the unperforated sections facing each other; impingement of granules against each other was reduced and conversion increased. EXAMPLE 3 Unperforated domes were prepared as in Example 1 except the metal sheet was formed from NITRONIC® 50 austenitic steel obtained from Pittsburgh Flatroll Co. Three intermediate annealing steps instead of two were required to relieve stresses in the metal. To optimize hole quality, ten holes using differing laser settings were drilled in a small metal coupon made of NITRONIC® 50 steel. The holes were examined under a microscope for comparative quality and the best drilling conditions chosen including drilling holes from the center toward the outside edge, powering the laser to 300 watts and using oxygen as the assist gas. High hole quality was qualitatively defined as those holes with a combination of the highest degree of roundness, sharpest edges, smoothest internal surfaces and the least amount of metal debris or uncut portion for both entry and exit sides of the hole. EXAMPLE 4 Forty unperforated hemispheres made from NITRONIC® 50 steel were prepared as described in Example 3. Ten of these were then perforated using a 5 axis laser. Approximately 20,000 holes were cut in each hemisphere excepting a 2.5 cm x 11 cm longitudinal section at the base of each hemisphere. Hole diameters were 1.0 mm each, hole centers were 2.0 mm apart and the hole to hole pattern was that of an equilateral triangle. The larger pattern with which the holes were cut, was in concentric bands starting at the top of the dome and extending toward the base. The resulting dome surfaces maintained a hemispherical shape since stresses of cut versus uncut sections were evenly distributed over their surfaces. The two perforated domes were then mounted on rings and an extruder plate such as in the assembly shown in Figure 3. Two additional complete dome sets were also formed. 11 EXAMPLE 5 Two dome sets from Example 4 were used to extrude AUTHORITY® 75DF herbicide (AUTHORITY® is a registered trademark of the FMC Corporation). AUTHORITY® extrusion was carried out and the domes produced 150,000 and 130,000 lbs (68,000 and 60,000 kg) of product, respectively without failure. Extrusion testing stopped after this amount of material was extruded; therefore, it was not determined in this test how much more material could have been extruded until failure might have occurred. Prior to the use of these extrusion dies, two 220 mm diameter welded hemisphere dies, each 1.0 mm thick, with 1.0 mm diameter punched die holes, lasted from 15,000 to 30,000 lbs (6,800 to 14,000 kg) before failure occurred. EXAMPLE 6 Samples of NITRONIC® 50 Steel plate, 1.2 mm thick, were perforated by laser cutting. Two 0.7 mm diameter holes and two 1.0 mm diameter holes were produced. Separate buttons of the steel plate 10 mm in diameter were cut so that each laser hole was at the center of each separate button. One button with a 0.7 mm hole and one button with a 1.0 mm hole were vapor coated with titanium nitride over all surfaces, inside and out. A "Zwick" materials testing machine, available from Zwick Gmbh and Co., Ulm, Germany, was outfitted so that a small sample of moistened extrusion powder could be compressed or effectively "extruded" through the center hole in each button. The forces and force patterns were measured for all four coupons. An important parameter indicating the ease with which a material extrudes through a diehole is known as the "impulse," measured in Newton-seconds, is calculated from the above mentioned force curves. Lower impulse values correlate with relatively lower extrusion pressures. Impulse values are shown in the table below. Diehole Type Impulse (newton-seconds ^ ) 0.7 mm, uncoated 8200 0.7 mm, coated 400 1.0 mm, uncoated 6610 1.0 mm, coated 600 EXAMPLE 7 The dome set from Example 5 through which 150,000 lbs (68,000 kg) of material was extruded, was cleaned and then coated using the titanium nitride coating process mentioned in Example 6. An additional 150,000 lbs (68,000 kg) 12 of AUTHORITY® 75DF was produced using this coated die set before dome failure occurred. It was observed that cleanability of the holes was much improved compared with the dome prior to coating. EXAMPLE 8 Two unmounted hemisphere domes from Example 4 were put through an electropolishing process through which several ten thousandths of an inch of metal was removed from all exposed surfaces of the dome die including the internal wall surfaces for all dieholes. All surfaces of the dome, including the internal surfaces of the holes were markedly improved in smoothness, based on visual observation under 10-fold magnification. 13

Claims: WHAT IS CLAIMED IS: 1. A process for making a seamless, dome-shaped extrusion die, comprising the steps of: hydroforming a metal sheet into the shape of a dome, said dome being seamless; annealing the hydroformed metal sheet; and forming a plurality of extrusion openings in the dome to yield a seamless, dome-shaped extrusion die. 2. The process of Claim 1 wherein the die is generally hemispherical. 3. The process of Claim 1 wherein the extrusion openings are generally circular and the thickness of the die is within a range of about 0.7 to 3.3 times the diameter of the extrusion openings. 4. The process of Claim 3 wherein the diameter of the extrusion openings is less than the thickness of the die. 5. The process of Claim 4 wherein the thickness of the die is within a range of about 1.2 to 3.3 times the diameter of the extrusion openings. 6. The process of Claim 1, further comprising annealing the seamless, dome-shaped extrusion die. 7. The process of Claim 1 wherein the metal sheet is formed from a metal selected from the group consisting of: a 300 series stainless steel, a Nitronic® series austenitic stainless steel, a nickel based alloy, and a heat treatable carbon steel. 8. The process of Claim 1 wherein the thickness of the metal sheet is about 2 millimeters or less. 9. The process of Claim 8 wherein the extrusion openings are of a diameter in the range of about 0.4 to 1.5 millimeters. 10. The process of Claim 1 wherein the extrusion openings are arranged in a uniform pattern with the extrusion openings having a generally equilateral triangular relationship to one another. 11. The process of Claim 10 wherein a second pattern is superimposed over the triangular pattern, said second pattern selected from the group consisting of a longitudinal segment pattern, a concentric circular pattern, a geodesic configuration pattern, and a combination of such patterns. 12. The process of Claim 11 further comprising electropolishing the walls of the extrusion openings and the inside surface of the seamless dome-shaped extrusion die. 13. The process of Claim 12 further comprising coating the seamless dome-shaped extrusion die with titanium nitride using a vapor deposition process. 14 14. The process of Claim 11 further comprising coating the seamless dome-shaped extrusion die with titanium nitride using a vapor deposition process. 15. A seamless, dome-shaped extrusion die prepared by the process of Claims 1-14. 16. At least two seamless, dome-shaped extrusion dies each prepared by the process of Claim 1 , each die having at least one section with no extrusions openings, such section located near the base of each dome and at least one of such sections of each dome facing the section with no extrusion openings of another dome. 15

Inventors: GLEICH STEVEN IRWIN, SWANSON LARS D, GLEICH, STEVEN, IRWIN, SWANSON, LARS, D.
Applicants: DU PONT, GLEICH STEVEN IRWIN, SWANSON LARS D, E.I. DU PONT DE NEMOURS AND COMPANY, GLEICH, STEVEN, IRWIN, SWANSON, LARS, D.
Application year: 1999
Main classification: B30B11
IPC codes: B21D26, B23P15, B30B11