Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Reexamination Certificate
2000-03-01
2003-07-22
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
C419S036000, C264S113000, C264S128000, C264S642000, C264S651000, C264S669000, C425S078000, C425S084000
Reexamination Certificate
active
06596224
ABSTRACT:
BACKGROUND
This invention relates to ceramic and metal containing parts that are formed from powder, and more particularly to such parts made from fine powder. Structural ceramic parts are generally made of fine powders, typically on the order of 1 micron in size. Such powders are difficult to spread dry, as they tend to agglomerate. Use of such fine powders offers several advantages. First the use of fine powders enhances sinterability. Sintering is a solid state diffusion process and will be enhanced by increasing the surface area to volume ratio of the powder in any green part that is subsequently sintered. As is known, surface roughness can be no smaller than the powder size. Thus, using fine powders also enables overall part quality to be improved. Smaller powders also mean that the minimum feature size that can be specified is also improved. Lastly, smaller powders allow thinner layers to be used in any layered fabrication technique, which helps eliminate slicing defects such as stair-stepping.
A processing technique that uses powders has become known as “three-dimensional printing” (“3D Printing”) and is described in general in numerous patents, including: U.S. Pat. No. 5,204,055, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,340,656, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Sachs, Haggerty, Cima, and Williams; U.S. Pat. No. 5,387,380, entitled THREE-DIMENSIONAL PRINTING TECHNIQUES, by Cima, Sachs, Fan, Bredt, Michaels, Khanuja, Lauder, Lee, Brancazio, Curodeau, and Tuerck; U.S. Pat. No. 5,490,882, entitled PROCESS FOR REMOVING LOOSE POWDER PARTICLES FROM INTERIOR PASSAGES OF A BODY, by Sachs, Cima, Bredt, and Khanuja; and U.S. Pat. No. 5,660,621, entitled BINDER COMPOSITION FOR USE IN THREE-DIMENSIONAL PRINTING, by James Bredt. All of the foregoing 3D Printing patents are incorporated herein fully by reference.
3D Printing is also disclosed and discussed in, co-assigned applications, including: U.S. Ser. No. 08/600,215, filed Feb. 12, 1996, entitled CERAMIC MOLD FINISHING TECHNIQUES FOR REMOVING POWDER, by Sachs, Cima, Bredt, Khanuja, and Yu; U.S. Ser. No. 08/596,707, filed Feb. 5, 1996, entitled HIGH SPEED, HIGH QUALITY THREE DIMENSIONAL PRINTING, by Sachs, Curodeau, Fan, Bredt, Cima, and Brancazio; U.S. Ser. No. 08/856,515, filed May 15, 1997, entitled CONTINUOUS INK-JET DROPLET GENERATOR, by Sachs and Serdy; U.S. Ser. No. 08/551,012, filed Oct. 31, 1995, entitled ENHANCEMENT OF THERMAL PROPERTIES OF TOOLING MADE BY SOLID FREE FORM FABRICATION TECHNIQUES, by Allen, Michaels, and Sachs; U.S. Ser. No. 08/831,636, filed Apr. 9, 1997, entitled THREE DIMENSIONAL PRODUCT MANUFACTURE USING MASKS, by Sachs and Cima; and U.S. Ser. No. 60/060,090, filed Sep. 26, 1997, entitled REACTIVE BINDERS FOR METAL PARTS PRODUCED BY THREE DIMENSIONAL PRINTING, by Sachs, Yoo, Allen, and Cima (provisional application). All of the foregoing 3D Printing patent applications (and provisional application) are incorporated herein fully by reference.
Basically, the 3D Printing process is a Solid Freeform Fabrication (SFF) process, which allows parts to be created directly from computer models. Other SFF processes that are commonly used include stereolithography (SLA), selective laser sintering (SLS), laminated object manufacturing (LOM), and fused deposition modeling (FDM). These processes all differ from traditional machining, since material is added to the desired part, as opposed to material removal in milling, turning, and boring.
A typical implementation of the 3D Printing process begins with the definition of a three-dimensional geometry using computer-aided design (CAD) software. This CAD data is then processed with software that slices the model into many thin layers, which are essentially two-dimensional. A physical part is then created by the successive printing of these layers to recreate the desired geometry. An individual layer is printed by first spreading a thin layer of powder and then printing binder to adhere the powder together in selected regions to create the desired layer pattern. The growing part is lowered by a piston and a new layer of powder is spread on top. This process is repeated until all the layers have been printed. The binder joins powder together within a layer and between layers. After printing is complete, the unbound powder is removed, leaving a part with the desired geometry. Typically the part is a green part that will experience further processing, such as sintering. However, in some circumstances, the part may be a final part.
This traditional 3D Printing layer generation technique relies on the powder being flowable in order for smooth layers of uniform density to be created.
There are many different powder and binder systems, based on metal, or ceramic or polymer powder. The part can be sintered or infiltrated to full density. Because 3D Printing is an additive manufacturing process, many geometries are possible that are not feasible with traditional machining, such as undercuts and internal cavities. Furthermore, many materials can be used in the 3D Printing process, as long as they can be obtained in powdered form. Currently, work has been done using metal, polymer, ceramic, and glass-ceramic powders. Using these materials, a wide variety of parts have been produced. This includes the direct printing of metal parts, injection molding tooling, casting shells, and structural ceramics. Parts, such as tooling, can incorporate conformal cooling channels to surfaces to decrease cycle time and residual stresses in parts made with such tooling. Other types of parts can also include such channels. Using the 3D Printing process, it is also possible to make individual parts with regions composed of varying materials (functionally gradient materials). This can be achieved by printing different materials into selected regions of an individual layer. This extra degree of freedom allows designers to vary the material properties within a single part.
Despite the many advantages of using fine powders mentioned above, they are difficult to use in the known 3D Printing process for a variety of reasons. Fine powder particles tend to stick to each other, forming agglomerates, due to various reasons, including Van der Waal's attractive forces, and moisture. The particles also tend to stick to any other bodies they come into contact with, including powder piston walls and the powder spreader bar. Low flowability also occurs because very fine particles are typically irregularly shaped, increasing friction. Poor flowability combined with powder adherence to the spreader bar makes it difficult to spread smooth layers. The low flowability of the powder also inevitably leads to uneven densification within layers and consequently, the resulting green body. Further, it is difficult to print into fine powders, without problems of ballistic ejection and erosion due to the binder jet impinging on the power bed surface.
Another problem with forming parts from powder relates to packing density and sintering. Much work has been done with spherical granules, typically ranging from 30-100 &mgr;m in size. The granules are actually agglomerates of submicron powder bound together-by an organic phase with a typical packing density of about 50% of theoretical. As a result, the packing density of the resulting green part is too low to be sintered directly (typically 30-35% of theoretical). An iso-static pressing step is required to increase the green body packing density. After iso-static pressing, alumina green parts fabricated with spray dried powders exhibit packing densities ranging from 59-63% of theoretical, depending on whether the cold iso-static pressing (CIP) or the warm iso-static pressing (WIP) process is used. This is adequate to achieve full density during sintering.
However, use of these processes introduces several problems. Before sintering, green parts are quite fragile and easy to damage. Pressing requires a relatively large amount of part handling. Further, the pressing step can introduce density gradie
Caradonna Michael A.
Cima Michael J.
Grau Jason
Moon Jooho
Sachs Emanuel M.
Jenkins Daniel J.
Weissburg Steven J.
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