Patent Publication Number: US-7718115-B2

Title: Method of forming hollow part

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a forming method and, more particularly, to a method of forming a hollow part. 
     BACKGROUND 
     Due to heightened environmental concerns, exhaust emission standards for machines have become increasingly stringent. To comply with these emission standards, machine manufacturers have increased the operating temperatures of the machines. The increased operating temperatures sometimes melt and/or warp hollow plastic parts of the machine, such as, for example, tanks, which may have complex features. Metal parts do not melt or warp at the increased operating temperatures. But, it is difficult to form hollow metal parts with complex features. 
     One way to form hollow metal parts is described in U.S. Pat. No. 2,390,160 (the &#39;160 patent) issued to Marvin on Dec. 4, 1945. The &#39;160 patent describes a method of forming hollow cylindrical objects from non-compacted metal powder. The method includes mixing the metal powder with a volatile organic solvent and a binder to form a slurry. Additionally, the method includes supplying a predetermined quantity of the slurry to a retaining shell held within a centrifuge. The method also includes rotating the shell to centrifugally distribute the powder to form a hollow cylindrical shape and simultaneously evaporate the solvent. In addition, the method includes removing the shell with the formed object therein. The method also includes sintering the object under suitable conditions of time, temperature and atmosphere for decomposing the binder and causing the particles of metal in the object to sinter together and form a hollow cylindrical object. 
     Although the method of the &#39;160 patent may be used to form hollow cylindrical objects from non-compacted metal powder, using the method of the &#39;160 patent may do little to form non-cylindrical hollow parts. Moreover, although the volatile organic solvent of the &#39;160 patent may evaporate rapidly while the shell of the &#39;160 patent is rotated, the volatile organic solvent may be subject to regulation and may be a potential health hazard. 
     The disclosed methods are directed to overcoming one or more of the problems set forth above and/or other problems in the art. 
     SUMMARY 
     In one aspect, the present disclosure may be directed to a method of forming a hollow part from a mixture. The method may include rotationally molding the mixture into a green part. Additionally, the method may include debinding the green part into a brown part. The method may also include sintering the brown part into the hollow part. 
     In another aspect, the present disclosure may be directed to another method of forming a hollow part from a mixture. The method may include rotationally molding the mixture into a green part. Additionally, the method may include debinding the green part in to a brown part. The debinding may include connecting a cavity interior to the green part to an atmosphere exterior to the green part. The debinding may also include exposing the green part to a catalyst. The method may also include sintering the brown part into the hollow part. 
     In yet another aspect, the present disclosure may be directed to a method of forming a hollow part from a mixture including a metal. The method may include rotationally molding the mixture into a green part. Additionally, the method may include debinding the green part in to a brown part. The debinding may include connecting a cavity interior to the green part to an atmosphere exterior to the green part. The debinding may also include exposing the green part to a catalyst. The method may also include sintering the brown part into the hollow part. The sintering may include heating the brown part to a temperature sufficient to fuse particles of the metal to each other. The temperature may also be below a melting point of the metal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial illustration of an exemplary disclosed hollow part; 
         FIG. 2  is an illustration of an exemplary disclosed mixture; 
         FIG. 3  is a cross-sectional illustration of an exemplary disclosed green part; 
         FIG. 4  is a cross-sectional illustration of an exemplary disclosed brown part; 
         FIG. 5  is a cross-sectional illustration of the hollow part of  FIG. 1 ; 
         FIG. 6  is a pictorial illustration of the mixture of  FIG. 2  within an exemplary disclosed mold; 
         FIG. 7  is a cross-sectional illustration of a green part being debinded into the brown part of  FIG. 4 ; and 
         FIG. 8  is a cross-sectional illustration of the brown part of  FIG. 4  being sintered into the hollow part of  FIGS. 1 and 5 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary hollow part  10 , which may not be subject to melting and/or warping at high temperatures. Hollow part  10  may be a unibody hollow part and may or may not include a complex geometry. For example, hollow part  10  may include radiussed edges  15  and complex curves  20 . Hollow part  10  may embody, for example, a tank, a duct, or another hollow part that may or may not be located within close proximity to a heat source. The heat source may be, for example, a combustion engine of a machine. 
     As illustrated in  FIG. 2 , hollow part  10  may be formed from a mixture  25 . A method of forming hollow part  10  (hereafter the “method”) may include rotationally molding mixture  25  into a green part  30  (referring to  FIG. 3 ). Green parts are parts that have been molded and not debinded. The method may also include debinding green part  30  into a brown part  35  (referring to  FIG. 4 ). Brown parts are parts that have been at least partially debinded and not sintered. In addition, the method may include sintering brown part  35  into hollow part  10  (referring to  FIG. 5 ). 
     Mixture  25  may include, as illustrated in  FIG. 2 , a metal  40 . Metal  40  may include an elemental or alloyed metal. For example, the elemental or alloyed metal may include tungsten, rhenium tantalum, osmium, molybdenum, iridium, ruthenium, niobium, hafnium, nickel, iron, tin, cobalt, copper, uranium, stainless steel, stains less steel, brass, ferrochromium, ferrovanadium, ferrotungsten, aluminum bronze, magnesium bronze, or constantan. Alternatively, the elemental or alloyed metal may include another material having a high melting point. The high melting point may be sufficiently high to inhibit melting and/or warping of hollow part  10  formed from metal  40 . Although metal  40  is represented in  FIG. 2  by spheres, it should be understood that these spheres merely represent a powder or pellet form of the elemental or alloyed metal. This powder or pellet form of the elemental or alloyed metal may include particles with an average size conducive to sintering. For example, the average size of the particles may be less than  50  microns. 
     Mixture  25  may also include, as illustrated in  FIG. 2 , a binder  50 . Binder  50  may include a polymer in the form of pellets  55 . Alternatively, the polymer may be in the form of a powder. The polymer may have a melt flow rate conducive to rotational molding. This melt flow rate (as measured according to ISO 1133) may be greater than 0.1 in 3 /10 minutes measured at 190 degrees Celsius employing a 2.16 kilogram weight (hereafter “greater than 0.1 in 3 /10 minutes”). For example, the polymer may include polyethylene, nylon, PVC plastisol, polypropylene, polyoxymethylene, or another polymer with a melt flow rate greater than 0.1 in 3 /10 minutes. The polymer may also have a melting point below the melting point of metal  40 . Additionally, the polymer may carbonize at a temperature below the melting point of metal  40 . Although pellets  55  are illustrated as spherical, it is contemplated that pellets  55  may have other shapes. 
     As illustrated in  FIG. 2 , metal  40  and binder  50  may be mixed to form mixture  25 . Although  FIG. 2  illustrates metal  40  and binder  50  as distinct and separable components of mixture  25 , it is contemplated that mixture  25  may be a homogeneous mixture. Metal  40  and binder  50  may be homogeneously mixed by way of extrusion. Specifically, a single screw or multi-screw extruder may pressurize and/or heat metal  40  and binder  50  together into blended pellets (not shown). These blended pellets may then be pulverized into powder form. Alternatively, the blended pellets may not then be pulverized into powder form. In yet another alternative, metal  40  and binder  50  may be homogeneously mixed by way of another method known in the art. As illustrated in  FIG. 2 , mixture  25  may include unequal amounts of metal  40  and binder  50 . Specifically, mixture  25  may include an amount M of metal  40  and an amount B of binder  50 . It is contemplated that amounts M and B may both represent volumes. Amount M may be at least one fourth as large as amount B. In other words, mixture  25  may include by volume at least 20% metal  40 . In some embodiments, amount M may be larger than amount B. For example amount M may be 1.5 times as large as amount B. In other words, mixture  25  may include by volume 60% metal  40  and 40% binder  50 . 
     As previously discussed, mixture  25  may be rotationally molded into green part  30 . Rotationally molding (also known as rotomolding) a mixture into a part may include forming a part from the mixture using a hollow mold that is rotated about one or more axes. The rotational molding of mixture  25  into green part  30  may be similar to rotational molding of polymers. The rotational molding of polymers is known in the art. In particular, the rotational molding of mixture  25  into green part  30  may include placing mixture  25  into a mold  60  (referring to  FIG. 6 ), sealing mold  60 , heating mixture  25 , rotating mold  60 , cooling mixture  25 , unsealing mold  60 , and removing green part  30  from mold  60 . 
     As illustrated in  FIG. 6 , mold  60  may include two or more components  65 . When components  65  are separated (as illustrated), mixture  25  may be placed on an interior surface  70  of one or more components  65 . Each interior surface  70  may be equivalent to an exterior surface  75  of green part  30  (referring to  FIG. 3 ). Thus, when components  65  are joined together to form mold  60 , an interior surface  80  of mold  60  (a combination of each interior surface  70 ) may be equivalent to an exterior surface  85  of green part  30  (a combination of each exterior surface  75 ). Furthermore, although components  65  are illustrated without moving parts, it is contemplated that components  65  may have moving parts. These moving parts may improve a spreading of mixture  25 . 
     After placing mixture  25  on interior surface  70 , mold  60  may be sealed. This sealing may include joining components  65  to each other. This joining may be by way of bolt, screw, clamp, buckle, caulk, glue, or other joining mechanism. Mold  60  may then be rotated about one or more axes. For example, mold  60  may be rotated about an axis x, an axis y, and an axis z. It is contemplated that mold  60  may simultaneously be rotated about axes x, y, and z. Alternatively, mold  60  may be rotated about one or more of axis x, y, or z at a time. Before or while mold  60  is rotated, mixture  25  may be heated. In some embodiments, mixture  25  may be heated before it is placed within mold  60 . The heating may be by way of convection, conduction, induction or another form of heating known in the art. The heating may continue until a temperature of mixture  25  rises above the melting point of binder  50 . As binder  50  melts and mold  60  rotates, mixture  25  may spread in one or more directions along interior surface  80 . The heating and rotating may continue until mixture  25  spreads approximately evenly along interior surfaces  70 . Spreading evenly means coating interior surfaces  70  with a layer of mixture  25  having a consistent depth as measured from each interior surface  70 . If components  65  have moving parts, these moving parts may be moved during the heating and rotating to promote the spreading of mixture  25  to certain interior surfaces  70 . Alternatively and whether or not components  65  have moving parts, some interior surfaces of mold  60  (not shown) may be configured so that the layer of mixture  25  has a varied depth. This varied depth may, for example, be caused by one or more protrusions from these interior surfaces. 
     Once mixture  25  has spread approximately evenly along interior surfaces  70  (excluding interior surfaces configured so that the layer of mixture  25  has a varied depth), the heating of mixture  25  may cease while the rotating of mold  60  may continue. As the rotating of mold  60  continues, mixture  25  may cool. As mixture  25  cools, mixture  25  may solidify into green part  30 . It is contemplated that green part  30  may be capable of supporting itself once fully solidified. In other words, a cavity  87  (referring to  FIG. 3 ) interior to green part  30  may not collapse when the rotating is discontinued. Therefore, the rotating may be discontinued when green part  30  has sufficiently solidified (i.e., when a temperature of green part  30  has decreased below the melting point of binder  50 ). 
     After the rotating of mold  60  has been discontinued, mold  60  may be unsealed. This unsealing may include separating components  65 . Once mold  60  is unsealed, green part  30  (referring to  FIG. 3 ) may be removed from mold  60 . Green part  30  may then be debinded into brown part  35 . Debinding a part may include removing at least a portion of a binder from the part. In particular the debinding of green part  30  into brown part  35  may include machining one or more holes  90  (referring to  FIG. 7 ) into green part  30 , placing green part  30  in a debinding mechanism  95  (referring to  FIG. 7 ), and removing brown part  35  from debinding mechanism  95 . 
     As illustrated in  FIG. 7 , hole  90  may connect cavity  87  to an atmosphere  97 . Atmosphere  97  may include any fluid or fluids exterior to green part  30 . Hole  90  may be sufficiently large to allow atmosphere  97  to flow into cavity  87 . Hole  90  may be circular. Alternatively, hole  90  may be another shape. For example, if hollow part  10  is a tank, hole  90  may be shape configured to temporarily or permanently be joined to a filling or draining apparatus for the tank. 
     Debinding mechanism  95  may embody a heater and may include a catalyst  105  and a fan  110 . Catalyst  105  may be an acid capable of dissolving binder  50 . It is contemplated that catalyst  105  may be liquid or gaseous in form. Fan  110  may be positioned within debinding mechanism  95  to circulate catalyst  105 . When green part  30  is placed in debinding mechanism  95 , green part  30  may be positioned such that hole  90  faces fan  110 . Thus, fan  110  may circulate catalyst  105  via atmosphere  97  into cavity  87 . This circulation may be through hole  90  or exterior surface  85 . In other words, catalyst  105  may pass through exterior surface  85 . While green part  30  is in debinding mechanism  95 , debinding mechanism  95  may heat green part  30 . The combination of the heating of green part  30  and the exposure of green part  30  to catalyst  105  may result in a debinding of a portion of binder  50  from green part  30 . In other words, a portion of binder  50  may be removed from green part  30 . Only a portion  113  of binder  50  may remain. The removed portion of binder  50  may be more than three times as large as portion  113 . In other words, the removed portion of binder  50  may include more than 75% of the amount of binder  50 . 
     As previously discussed, brown part  35  may be sintered into hollow part  10 . Sintering a part may include heating the part to a temperature below the part&#39;s melting point until the part&#39;s particles fuse to each other. In particular, the sintering of brown part  35  into hollow part  10  may include placing brown part  35  in a heating mechanism  115  (referring to  FIG. 8 ), heating brown part  35 , and removing brown part  35  from heating mechanism  115 . During the heating of brown part  35 , brown part  35  may be supported by a fixture (not shown). This fixture may prevent the heating of brown part  35  from undesirably deforming brown part  35 . 
     As illustrated in  FIG. 8 , heating mechanism  115  may embody a heater. It is contemplated that heating mechanism  115  and debinding mechanism  95  may together embody a single integral component. Heating mechanism  115  may heat brown part  35  to a temperature sufficient to fuse the particles of metal  40  to each other. This temperature may be below the melting point of metal  40 . The heating of brown part  35  may result in a carbonizing of portion  113 . The heating of brown part  35  may also result in the particles of metal  40  fusing to each other. When the particles of metal  40  fuse to each other, brown part  35  may shrink into hollow part  10 . Hollow part  10  may occupy less than 90% of a volume of brown part  35 . In other words, the sintering of brown part  35  into hollow part  10  may shrink a volume of brown part  35  by more than 10%. 
     INDUSTRIAL APPLICABILITY 
     The disclosed method may be applicable to a mixture, which may be rotationally molded to form a hollow part. This hollow part may have complex features and may be capable of withstanding high temperatures without melting and/or warping. Thus, the hollow part may be efficiently located within close proximity to a heat source such as, for example, a combustion engine of a machine. 
     It is contemplated that the method of forming hollow part  10  may be conducive to forming hollow parts  10  from metal  40 . These metal hollow parts  10  may be capable of withstanding higher temperatures than similar plastic hollow parts. Specifically, metal hollow parts  10  may not melt and/or warp at the higher temperatures. Moreover, the method of forming metal hollow parts  10  may be environmentally friendly as it may require no volatile organic solvents. 
     Additionally, it is contemplated that the method of forming metal hollow parts  10  may efficiently yield unibody metal hollow parts  10 . In particular, the method may be highly repeatable as it requires no welding or casting. Also, the lack of welding and casting may minimize quality control issues. Additionally, the unibody structure of metal hollow parts  10  may minimize possible leak points. 
     It is also contemplated that the method of forming hollow parts  10  may produce hollow parts  10  with low stress radiussed edges  15 . These low stress radiussed edges  15  may maximize the durability of hollow part  10 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the methods of the present disclosure. Other embodiments of the methods will be apparent to those skilled in the art from consideration of the specification and practice of the methods disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.