Patent Publication Number: US-2010111745-A1

Title: Method of producing composite materials through metal injection molding

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/898,736 filed Jan. 31, 2007 entitled METHOD OF PRODUCING COMPOSITE MATRIALS THROUGH METAL INJECTION MOLDING and which is incorporated herein by reference. 
    
    
     BACKGROUND ART 
     The invention relates to metal injection molding, and more specifically, to a method of impregnating particles into a metal injection molded item, following the debinding process and prior to the sintering process in order to alter the characteristics of the item. 
     Metal injection molding is a process that has been developed and improved upon for many years. As with all products produced through a given process, there is a constant desire to improve the characteristics of the finished product by improving or altering the chosen process. Metal injection molding is no exception. 
     It is not uncommon in the field of metal injection molding to perform post-sintering processes to a finished part to enhance that part&#39;s characteristics, such as strength, durability, and hardness. Such treatments may include, for example, heat treatment, HIP, cryogenic freezing, plating, mechanical processing, etc. Yet, all of these processes take place after the completion of the metal injection molding process to enhance the characteristics of parts. Accordingly, it will be apparent that there continues to be a need to achieve some of the same, additional or even better finished product characteristics, through in-situ improvements or alterations to the known metal injection molding process itself. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings which form part of the specification: 
         FIG. 1  is a diagram of an injection molding process, according to an embodiment of the present invention. 
         FIG. 2  is a depiction of a debinding process, according to an embodiment of the present invention. 
         FIG. 3  is a depiction of an impregnation process, according to an embodiment of the present invention. 
         FIG. 4  is a depiction of a sintering process, according to an embodiment of the present invention. 
         FIG. 5  is a 50× magnification photograph of a polished cross-section of a “finished” part processed in accordance with an embodiment of the present invention. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. 
     BEST MODES FOR CARRYING OUT THE INVENTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the invention, describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. 
     As shown in  FIGS. 1-5 , a first embodiment of the present invention comprises a method for impregnating a metal injected material to produce a composite article  10 . Generally, the method includes the steps of forming a “green part”  12  from a feedstock  14 , removing a binder  16  from the green part  12  to form a “brown part”  18 , and impregnating the brown part  18  with select particles  20  to form a finished composite article  22 . 
     Initially, a select composition  24  of one or more finely powdered metal(s), combines with a binder  16  to form the feedstock  14  ( FIG. 1 ). In the embodiment of  FIG. 1 , the feedstock is a mixture of three to eight micron stainless steel powder and Acetel plastic binder, which can be obtained from BASF under the trade name “Catamold” and has the stock code of 316-L. However, the powdered metal can include any type of metal including elemental metals, one or more metal alloys, or any combination thereof. In addition, the binder  16  can comprise any type of plastic, wax, moisture, or other suitable material. 
     An injection molding machine  28 , injects the feedstock  14  through a nozzle  30  into the mold  32 , preferably at a temperature of approximately 190° C. The machine  28  applies a pressure of approximately 900 bar at approximately 128° C. for approximately three seconds to compress the injected feedstock  14  into a semi-rigid part, or what is known in the industry as a “green part”  12  or “green state”. When compressed, the binder  16  binds together the green part  12 . Preferably, this entire injection molding stage has a cycle time of approximately 20 seconds. Those skilled in the art will recognize that the pressure, temperature, and time of compression can vary according to each application. For example, the pressure exerted on the feedstock  14  in the mold  32  during the compression phase varies with each application, but the pressure typically falls in the range of 500 psi to 1800 psi. After compression, the green part  12  ejects from the mold  32  and transfers to a debinding oven for removal of the binder  16 . 
     Debinding typically consists of heating the green part  12  in the presence of a catalyst to dissolve the binder  16 , leaving only enough material to just hold the finely powdered metals or metal alloys together. In the embodiment of  FIG. 2 , the debinding oven heats the green part  12  to approximately 110° C. and exposes the green part to Nitric acid having concentration of 98%. The green part  12  remains in this environment until achieving a weight loss of preferably at least 7%. After debinding, the article is often referred to as a “brown part”  18 . The removal of the binder  16  yields a network of generally interconnected microscopic cavities  34  and interstitial voids or crevices in those areas of the brown part  18  where the binder  16  has been removed. These cavities  34  may range in size from sub-micron to 20+ microns. However, smaller and larger cavities may also be present. 
     After debinding, the particles  20  are introduced to the surface  36  of the brown part  18  for impregnation. The particles  20  can comprise any type of material that provides desired characteristics or enhancements to the finished article  22 , including metal, non-metal, plastic, ceramic, natural, or synthetic materials. Preferably, the particles  20  are suspended in a liquid solution  38 , such as water or alcohol. In this way, the particles  20  migrate into the cavities  34  of the brown part  18  as a part of the liquid solution. The liquid solution may also include surfactants, such as fluorochemicals, silicone compounds, or soaps to lower the surface tension of the solution and allow the solution to infiltrate the network of cavities  34  more easily. The particles  20  should be generally sized smaller than the size of the cavities  34  to allow the particles  20  to infiltrate the network of cavities  34 . In the embodiment of  FIG. 3 , the particles  20  of aluminum oxide (Al 2 O 3 ) are preferably about  40  nanometers. However, the particles  20  can be sized to accommodate each individual application and its particular network of cavities  34 . Many applications will utilize particles sized within a range of about 20 nanometers to about 80 nanometers. 
     The brown part  18  submerges or dips in the solution  38  for impregnation of the particles  20 . Applicant believes that the impregnation of the particles  20  into the brown part  18  is enhanced naturally through capillary action. Further, the rate and extent of the particle incorporation may be adjusted by subjecting the brown part  18  to a pressure above atmospheric, or alternative by placing the brown part in a vacuum while submerged or coated with the liquid solution. In the embodiment of  FIG. 3 , a vacuum imparts a pressure P of approximately 3 psia below atmospheric pressure to the brown part  18  for a predetermined time period, such as overnight. After removal of the impregnated part  40  from the solution, the impregnated part  40  dries until the weight loss of the part  40  reaches equilibrium, thereby, indicating nearly all the moisture has been removed. 
     As shown in  FIG. 5 , the particles  20  impregnate into the surface of part  22  at a depth of about 0.01″. However, the depth of impregnation varies according to a number of factors, such as the size of the cavities, the size of the particles, the pressure used during impregnation, and the duration of the soak. The density of the particles  20  is about 30% by volume. Again, the density of the particles  20  varies according to a number of factors, such as the size of the cavities the size of the particles, the pressure used during impregnation, and the duration of the soak. Also, the particles can impregnate the entire part  22 . 
     Following the impregnation of the particles  20 , the impregnated part  40  is sintered, preferably using liquid phase sintering, in an oven at temperatures great enough to fuse but not melt the impregnated part  40 . The sintering process traps the particles  20  in the part and completes the metal injection molding process to form a finished article  22 . During sintering, the particles  20  do not necessarily or fully chemically bond to the brown part  18 . Depending on the materials, the particles can either mechanically bond or chemically bond to the brown part. In the embodiment of  FIG. 4  the impregnated part  40  is preferably sintered in a hydrogen reduction atmosphere at approximately 1350° C. for one hour. However, sintering may subject the impregnated part to temperatures of up to 2500° F. or more, in an environment in which the impregnated part is exposed to one or more gases to further improve the sintering process. Other processes may be used in place of sintering. In another alternate embodiment, an annealing technique could alternately be used in place of the sintering process. 
     If desired, the finished article  22  may also be subjected to post-sintering processes, such as heat treatment, HIP, cold forming, nitriding, carburizing, or other thermo-mechanical or mechanical processing. 
     Applicant believes that parts processed through this novel method will provide unique and otherwise desirable characteristics to completed articles, such as improved heat and chemical resistance, improved strength, corrosion and cracking resistance, anti-microbial, and anti-fungal properties that are vital in industries such as the medical and pharmaceutical fields. In addition, the novel method will produce cermet materials that can re-passivate their surface. 
     Applicant has discovered that using the novel method disclosed herein, the impregnation of submicron particles and/or nanometer sized particles of metal oxides or carbides into a brown part having a feedstock comprised of metal, metal alloys or non-metals, will result in a finished article having a composite composition not achievable using other methods. For example, the novel method permits a composite material with both metal and non-metal materials. 
     Changes can be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. Other methods exist to impregnate the particles  20  into the brown part  18 . In alternate embodiments, for example, the particles may be applied as a powder directly to the surface of the brown part. Alternatively, as another example, the particles may be suspended in a semi-gaseous state and applied to the brown article in an environmental chamber.