Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
1998-05-26
2003-04-29
Jenkins, Daniel J. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C419S005000, C264S610000
Reexamination Certificate
active
06554882
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to a new rapid tooling (RT) sintering method based on rapid prototyping (RP) technology and powder compositions therefor. Rapid tooling (RT) technologies are based on established techniques related to rapid prototyping (RP).
The formation of metal parts, especially those that are intricately or irregularly shaped such as mold cavities for the injection molding of thermoplastics, gears, sprockets, or threaded parts, may involve many process steps. Often, the formation process involves the steps of casting, machining, heat treatment for hardening, tempering, polishing and plating of the parts. Significant volume changes during casting usually require time consuming and expensive processing. Before the creation of rapid prototyping techniques, a skilled technician might take many weeks or months to machine a regular prototype model. The resulting prototype was likely to have dimensional inaccuracies due to human error.
The term “prototype” defines a positive model of an article and has essentially the same dimensions of compression or injection molded articles. The prototype is usually more porous than the molded article, but otherwise it is visually essentially indistinguishable from the molded article, and functions in essentially the same manner as the molded article. A prototype is used for complete evaluation of the form, design, and performance of a desired molded article.
Using a variety of technologies, it is now possible to generate prototype models in a few days or weeks. These models are termed “rapid prototyped models” (RP models) and are usually based on CAD-Solid Modeling technology, which is well known in the art. “CAD” is an abbreviation for Computer Aided Design in which the design is displayed three-dimensionally on a cathode ray tube. CAD-Solid Modeling is the method by which these three-dimensional computer models may be transformed into three dimensional solid positive models, i.e., RP models.
An example of CAD-Solid Modeling is the process of stereolithography which is well known in the art and disclosed, for example, in U.S. Pat. No. 4,575,330, issued Mar. 11, 1986 to Hull, which is hereby incorporated by reference. Generally, a photopolymerizable liquid resin is subjected to selective bombardment of radiation to form a step-wise laminar buildup of the desired object. The rapid prototype comprises a resin, which has limited strength and heat durability, but has dimensions which accurately represent the computerized data from which it was created.
With the advantages of speed and accuracy, rapid prototyping technologies advanced quickly after their introduction in the early 1980's, and a large market has been created for rapid prototyping equipment and services. The arrival of more sophisticated solid modeling systems based on computer generated models is expected to increase the design complexity of part geometry.
The next logical step forward after the creation of rapid prototyped models is to create “rapid tooling” (RT) which would be used to create production versions of the rapid prototype models. RT is a process of quickly and efficiently manufacturing a negative mold (“tool”) from a positive RP model. By “negative” it is meant that the mold will impart its form to fluid, powder or other substance and the fluid, powder or other substance adopts the dimensional form of the prototype. A “positive” mold will impart its form to a fluid, powder or other substance, but a “negative” reproduction of the prototype will be formed.
When the fluid, powder or other substance is stabilized or solidified, the RP can be removed leaving a tool. The tool has the negative dimensional characteristics of the prototype and can be used to form molded articles in mass production. An example of this process is injection molding. This process involves molding metal, plastic, or non-plastic ceramic shapes by injecting a measured quantity of the molten or non-polymerized material into the tool.
Ideally, rapid tools can be created in a short time and at a low cost. However, due to the limitations of strength and heat durability of RP photopolymerized resins which are made by stereolithography, most of the negative molds (tools) cannot be applied to real productivity due to warping, shrinking and melting of the resin RP when the negative mold is made. Thus, the formation of intricately or irregularly shaped parts, such as mold cavities for the injection molding of thermoplastics, gears, sprockets, or threaded parts, may involve many processing steps which are time consuming and expensive. Often, the formation process involves the steps of casting, machining, heat treatment for hardening, tempering, polishing and plating of the parts. Significant volume changes during casting may require expensive and careful processing. Machining and polishing can be particularly difficult for intricate or irregular shapes. The formation of parts from very hard, corrosion resistant alloys greatly increases the difficulty in processing.
A number of techniques have been developed to create rapid tools from rapid prototypes. The two technologies which are most popular are the Selective Laser Sintering (SLS) process and the Keltool process. The SLS process, as disclosed in U.S. Pat. No. 5,342,919, issued Aug. 30, 1994 to Dickens, Jr. et al., describes a laser-sinterable powder product which is prepared, deposited and leveled into a thin layer. Following a pattern obtained from a two dimensional section of a 3-D CAD model, a CO
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laser sinters the thin layer of the target region and generates a first slice of sintered powder in a two-dimensional shape. Subsequent layers are applied and sintered until the part is complete. The part or prototype is nearly fully dense and may be further refined by drying, firing and copper infiltration.
Thus, a rapid tool capable of running a large number of injection molded parts can be created. The turnaround time for creating production quality parts can be as little as two weeks with this process. However, the cost of this operation is reported to be very high and the accuracy of the product is still poor due to the dimensional gaps surrounding the periphery of each layer. Preparing different types of molds, such as ceramic or ceramic metal parts is also difficult. Further, because the infiltration and sintering are separate processes, some distortion and shrinkage will occur.
The Keltool process was developed more than 20 years ago. During this process a Room Temperature Vulcanizing (RTV) mold is created from a master part by pouring silicone rubber around a thermoplastic pattern, which may be based on a resin CAD-stereolithography technology. After the silicone rubber is solidified, the master part is removed from the silicone rubber by cutting the silicone rubber along a parting line. This step is difficult and time consuming. Another difficultly is the removal of the RTV mold from the green compact. Typically, the mold is heated in a furnace to burn out the silicone rubber, which causes pollution. The formed RTV mold is then filled with a metal powder/binder mixture to form a green compact which is sintered and further treated.
Another method of producing dense metal molds and parts from rapid prototyped models is disclosed in U.S. Pat. No. 5,507,336 issued Apr. 16, 1996 to Tobin. In this patent, which involves numerous steps, a resin RP pattern is placed in a steel tube and a ceramic slurry and a binder is cast around the critical surfaces thereof. The RP pattern is burned out, preferably by heating in a furnace at 1100° F. for three hours, and metal powder is cast around the critical surfaces of the ceramic member. An infiltration metal is placed over the powder and the apparatus is placed in a furnace at 2100° F. for at least one hour. The ceramic member is then removed and what is formed is a metal part or a mold half suitable for mating with another mold half to form a mold for casting multiple parts.
This procedure suffers from numerous draw backs. First, the final sintered metal part or mol
He Zongyan
Zhou Gongyao
Akin Gump Strauss Hauer & Feld L.L.P.
Drexel University
Jenkins Daniel J.
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