Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-06-20
2001-07-17
Sanders, Kriellion (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S405000, C524S413000, C524S428000, C524S430000, C524S439000, C524S442000, C527S300000, C106S162720
Reexamination Certificate
active
06262150
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to a composition and a process for forming sintered, molded articles having improved dimensional 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 metal powder (metal injection molding, or “MIM”) or ceramic powder (ceramic injection molding, or “CIM”), 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.” After forming a ceramic or metal green body, it is often desirable to sinter the article to remove any residual solvent and density the article. However, sintering frequently causes undesirable cracks or distortions in the article.
In order to remedy this problem, it has been suggested to improve the mechanical strength of the fluid composition. This is accomplished through increasing its solid content. In particular, when using a powder containing composition, increasing the powder content will increase the mechanical strength of the composition and decrease the probability that a shaped article will crack during sintering. To the contrary, a composition having a lower powder content will cause cracking and shrinkage in molded articles formed therefrom during sintering.
To increase the amount of powder a composition can retain, it may be necessary to augment the applicable binder with at least one viscosity reducing additive. The binder generally comprises a water soluble component or a polysaccharide that combines with the powder to form a flowable gel. Optimally, the additive will serve to increase the powder holding capacity of the binder, decrease the porosity of a molded article, and result in sintered articles having superior mechanical and physical properties.
Compositions of the prior art have generally been unable to form green bodies having sufficiently high density and dimensional stability. Various attempts have been made using different additives to increase the solid holding capacity, and thus the mechanical strength, of a ceramic or metal composition. For example, U.S. Pat. No. 5,746,957 teaches a process for forming ceramic and/or metal articles comprising a ceramic and/or metal powder, a polysaccharide binder, a solvent and a gel strength enhancing agent comprising a borate compound. The borate compound is incorporated to increase the quantity of solid powder material that the composition can retain, thus increasing the density of the product. U.S. Pat. No. 5,950,063 teaches a powder injection molding process using a composition comprising a powder and a binder incorporating additives such as coupling agents, antioxidants and surfactants. Also, U.S. Pat. No. 4,734,237 provides a process for injection molding metallic or ceramic articles from a mixture comprising a metal or ceramic powder, a gel-forming material having specific desirable properties and a gel-forming material solvent. The specific gel-forming material allows for increased solid retention in the moldable composition.
SUMMARY OF THE INVENTION
The invention provides a composition for forming molded articles comprising:
a) at least one metal powder, at least one ceramic powder, or a combination thereof;
b) a gel forming polysaccharide binder; and
c) a sugar.
The invention further provides a process for forming molded articles comprising:
A) forming a fluid composition comprising:
a) at least one metal powder, at least one ceramic powder, or a combination thereof;
b) a gel forming polysaccharide binder;
c) a sugar; and
d) a solvent; and
B) molding the composition under conditions sufficient to form a solid molded article.
The invention also provides a process for forming molded articles comprising:
A) forming a fluid composition comprising:
a) at least one metal powder, at least one ceramic powder, or a combination thereof;
b) a gel forming polysaccharide binder;
c) a sugar; and
d) 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 would be desirable to provide a composition which enables a greater quantity of solid to be retained with in the composition. It has now been unexpectedly found that adding a sugar to a combination of at least one metal or ceramic powder, or combination thereof, and a polysaccharide binder reduces the viscosity of the composition and increases the solids loading. By increasing the solids loading, the sugar additive allows the formation a stable green body having increased shape retention, and a particle density sufficient to prevent cracking and deformation of the body during sintering. This also reduces the production cost of the injection molded components since less agar binder is used or replaced by less expensive glucose binder. Also, expensive supporting equipment is not needed and the components are not as sensitive to handling. When the green body is ultimately sintered, the sugar and solvent are burned away, leaving behind a high density, high strength ceramic or metal article. Sugar is generally inexpensive, commercially available as glucose that reduces the viscosity of the polysaccharide binder, and thus increases the amount of powder it can retain while in a fluid state. When the desired binder to sugar ratio is attained, the composition can be molded and sintered to produce an article having increased density compared to articles formed through prior art processes. The present invention also allows for the reduction of the amount of binder used in the composition, reducing the cost of forming the composition.
REFERENCES:
patent: 4734237 (1988-03-01), Fanelli et al.
patent: 5746957 (1998-05-01), Fanelli et al.
patent: 5950063 (1999-09-01), Hens et al.
patent: 6146560 (2000-11-01), Behi et al.
patent: 6171360 (2001-01-01), Suzuki et al.
Behi Mohammad
Duyckinck Richard
Fanelli Anthony
Criss Roger H.
Honeywell International , Inc.
Sanders Kriellion
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