Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-08-01
2001-07-17
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S252000, C075S254000, C075S255000, C075S248000, C075S249000, C419S010000, C419S036000, C419S040000
Reexamination Certificate
active
06261336
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a composition for forming molded metal containing articles using a molding feedstock having improved stability.
2. Description of the Related Art
Injection molding is a well known process for forming thermoplastic molded articles, such as plastic bottles or containers. Other types of useful injection molded articles are formed from ceramic powder (ceramic injection molding, or “CIM”) or metal powder (metal injection molding, or “MIM”), rather than a thermoplastic material. Powder injection molding generally involves injecting a moldable fluid composition, comprising a combination of a ceramic or metal powder, a gel forming binder and a solvent into a mold of a predetermined shape under conditions sufficient to form a shaped article, referred to as a “green body.” For example, see U.S. Pat. No. 5,746,957 which teaches a process for forming ceramic and/or metal articles from a composition comprising a ceramic and/or metal powder, a polysaccharide binder and a gel strength enhancing agent. After forming a green body, it is most typically required to sinter the article to densify the article so that it may have useful strength and other physical and mechanical properties.
There are various reasons for which metal or metal containing, ceramic molded articles may be desirable. For example, a metal article may be highly conductive, or a metal component may impart superior strength or durability to an article. Generally, metal injection molded articles may be formed from any pure metal or metal alloy in powder form. Powder injection molding applications typically use wax or polymeric binders, or a water-based binder and solvent for a variety of metals, for example stainless steels or nickel based superalloys. However, it is well known that certain metals and metal compounds oxidize when contacted with water. For example, iron reacts with water to formiron oxide, or rust, and copper reacts with water to form copper oxide. Therefore, it has been a problem in the art to form metal containing compositions using a water based binder while preventing oxidation of the some metals.
To illustrate, U.S. Pat. No. 6,013,723 teaches a composition for injection molding an article incorporating a fatty acid metal salt lubricant. This lubricant is used to inhibit the formation of rust when iron containing compounds are incorporated within the composition. Also, U.S. Pat. No. 5,906,813 teaches a composition for injection molding an article incorporating an antioxidant additive. This antioxidant is used to inhibit the formation of rust when iron containing compounds are incorporated within the composition. Further, U.S. Pat. No. 5,378,428 provides a method of preserving an article wherein the article and an oxygen absorbent composition are sealed inside a container. The oxygen absorbent composition serves to prevent rusting of the article.
SUMMARY OF THE INVENTION
The invention provides a composition for forming molded articles comprising:
a) at least one metal powder or a combination of at least one metal powder and at least one ceramic powder;
b) a gel forming polysaccharide binder; and
c) sodium silicate.
The invention also provides a process for forming molded articles comprising:
A) forming a fluid composition comprising:
i) at least one metal powder or a combination of at least one metal powder and at least one ceramic powder;
ii) a gel forming polysaccharide binder;
iii) sodium silicate; and
iv) a solvent; and
B) molding the composition under conditions sufficient to form a solid molded article.
The invention further provides a process for forming molded articles comprising:
A) forming a fluid composition comprising:
i) at least one metal powder or a combination of at least one metal powder and at least one ceramic powder;
ii) a gel forming polysaccharide binder;
iii) sodium silicate; and
v) a solvent;
B) molding the composition under conditions sufficient to form a solid molded article; and
C) sintering the molded article.
The invention still further provides articles produced by the processes of the invention.
It has now been unexpectedly found that adding sodium silicate to a composition comprising a corrosive metal, such as iron or copper, and a water based binder, will suppress oxidization of the metal for a significant time. Moreover, it has been found that adding a borate compound with the sodium silicate will further increase the resistance of the composition to oxidation. Thus a corrosion resistant binder composition is produced for forming injection molded articles through use of a sodium silicate solution and/ or a mixture of sodium silicate solution and potassium tetraborate tetrahydrate as a corrosion inhibiting additive. The corrosion inhibitor prevents metal oxidation when a metal containing powder is mixed with a water based binder, providing stability to the article and preventing generation of hydrogen gas. Specifically, the sodium silicate alone serves as a corrosion inhibitor for about three weeks, while including a borate compound will extend this inhibiting property to a duration of about four months. By preventing oxidation, the corrosion inhibitor also prevents the concurrent generation of hydrogen gas and consumption of moisture essential to the binder composition. This improves the stability of the feedstock molding parameters and thus promotes uniformity in molded articles formed from the feedstock composition.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a process and composition for forming corrosion resistant feedstocks to be used for molding articles wherein the composition comprises at least one metal powder or a combination of at least one metal powder and at least one ceramic powder, a water based gel forming binder and a sodium silicate corrosion inhibitor. Preferably a borate such as potasium tetraborate tetrahydrate is also present as a corrosion inhibitor. This composition is combined with a solvent and molded into a self-supporting shaped article. The article is then preferably heated at a high temperature to sinter the particles thus densifying the material to have useful physical and mechanical properties. The molded article may or may not be corrosion resistant in its final sintered form. The corrosion inhibitor is added to the metal powder to prevent the feedstock from oxidizing. Since the inhibitor is gone after sintering, the final product may oxidize just as any oxidizable metal would. Adding sodium silicate and or mixture of sodium silicate and potasium tetraborate tetrahydrate to the composition allows a water based binder to be employed without causing significant oxidation when the metal powder in the binder is contacted with water present in the aqueous based polysaccharide binder.
To form the moldable composition, the powder element is mixed with the gel forming binder, a solvent such as water, sodium silicate and optionally a borate such as potasium tetraborate tetrahydrate such that a homogeneous composition is formed. This composition is then shredded and either molded through well known techniques or stored for future use. In general, metallic powders suitable for use in molding systems comprise either elemental powders, semi-elemental powders, pre-alloyed powders or mixtures thereof. An elemental powder is generally composed of individual metal element only. For example, an iron or nickel powder or a mixture thereof. A semi-elemental powder is generally a powder composed of more than one metal element, such as a semi-elemental ferrochrome powder comprised of 50% iron and 50% chrome. A mixture of elemental and semi-elemental powders is referred to as an elemental/semi-elemental powder, such as mixing a ferrochrome powder with an iron powder to form stainless steel. A pre-alloyed powder is a powder composition that has been formed from an existing metal alloy. For example, solid high or low carbon steel or super alloys having the desired composition can be melted and pulverized to form a powder. Combining different types of metall
Behi Mohammad
Glandz George A.
LaSalle Jerry C.
Criss Roger H.
Mai Ngoclan
Rutgers The State University of New Jersey
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