Binder removal in selective laser sintering

Abstract

A method of fabricating an article, such as a prototype part or a tooling for injection molding, by way of selective laser sintering, using a composite powder system of a metal and/or ceramic powder with a polymer binder comprising thermoplastics and thermoset polymers, and a metal hydride powder to form a "green" article. After removal of unfused material from the green article it is placed in an oven or furnace in a non-reactive atmosphere such as, for example, nitrogen or argon, for subsequent heat treatment to decompose and drive off the binder and sinter the metal substrate particles prior to infiltration by a metal with a lower melting point. During the critical step of decomposing the binders, the metal hydride begins to decompose also and releases an in-situ concentration of hydrogen gas that creates the reducing conditions necessary to thoroughly decompose the polymer fragments so that the hydrocarbon fragments can escape the skeleton structure of the article. It has been found that even with higher loadings of binders, leading to higher desired green strengths, the decomposition of the metal hydride eliminates the blistering phenomena associated with high loadings of some binders.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The field of solid freeform fabrication (SFF) of parts has, in recent years, made large improvements in providing high strength, high density parts for use in the design and pilot production of many useful articles. "SFF" generally refers to the manufacture of articles in a layer-wise fashion directly from computer-aided-design (CAD) databases in an automated fashion, as opposed to conventional machining of prototype articles from engineering drawings. As a result, the time required to produce prototype parts from engineering designs has reduced from several weeks, using conventional machinery, to a matter of hours.

[0003] 2. Description of the Relevant Art

[0004] One example of an SFF technology is the selective laser sintering process practiced by systems available from 3D Systems, Inc. of Valencia, Calif. According to this technology, articles are produced in layer-wise fashion from a laser-fusible powder that is dispensed one layer at a time. The powder is fused, or sintered, by the application of laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. After the fusing of powder in each layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete. Detailed description of the selective laser sintering technology may be found in U.S. Pat. No. 4,863,538 and U.S. Pat. No. 5,017,753, both assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508, expired, to Housholder; all incorporated herein by this reference in pertinent part. The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including nylons, polystyrenes, and composite materials such as polymer coated metals and ceramics. Examples of composite powder materials are described in U.S. Pat. No. 4,944,817, U.S. Pat. No. 5,076,869, and in U.S. Pat. No. 5,296,062, all assigned to Board of Regents, The University of Texas System, and incorporated herein by this reference.

[0005] A related SFF technology, referred to as 3-Dimensional (3D) Printing, is described in U.S. Pat. Nos. 5,340,656 and 5,387,380. From a computer (CAD) model of the desired part, a slicing algorithm draws detailed information for every layer. Each layer begins with a thin distribution of powder spread over the surface of a powder bed. Using a technology similar to ink-jet printing, a binder material selectively joins particles where the object is to be formed. A piston that supports the powder bed and the part-in-progress lowers so that the next powder layer can be spread and selectively joined. This layer-by-layer process repeats until the part is completed. Following a heat treatment, unbound powder is removed, leaving the fabricated part.

[0006] As this technology has evolved, SFF has increasingly been used not only to make prototype parts but also to make final useful parts as well as tools or molds that can be used to make multiple parts. A developing trend is to fabricate such parts, tools, or molds with an "indirect" process that uses a powder of metal and/or ceramic particles either coated by or blended with a polymer, from which a "green" article is fabricated by selective laser sintering to bind the particles to one another. The green article is then heated to a temperature above the decomposition temperature of the polymer, which both drives off the polymer and also binds the metal and/or ceramic substrate particles to one another to form an intermediate porous article. The porous article can then be infiltrated with another material such as a lower melting temperature metal to give a fully dense article with desirable properties. The green article can also be fabricated with 3D printing.

[0007] Some examples of the use of these approaches for functional applications are described, for example, in U.S. Pat. Nos. 5,433,280, 5,544,550, and 5,839,329 to Smith et al. These describe the use of selective laser sintering a tungsten carbide-polymer composite powder to generate a "green" drill bit which is then infiltrated in a furnace cycle with a copper alloy to generate a fully functional drill bit for down hole oil exploration. Another commercial application of these indirect approaches is a product called ProMetal by ExtrudeHone. Utilizing the 3D Printing technology described above, ProMetal builds metal components by selectively binding metal powder layer by layer. The finished structural skeleton is then sintered and infiltrated with bronze to produce a finished part that is 60% steel and 40% bronze and is used for injection molding tools or final metal parts. Another commercial example is 3D Systems' ST-100 system, which uses selective laser sintering of a steel polymer composite powder to generate a green article which is subsequentially put through a furnace cycle that removes the polymer binder and infiltrates the metal skeleton with bronze to create a functional fully dense article that can also be used for injection mold tools or final parts.

[0008] As is well known in the art, the structural strength of the green article is an important factor in its utility, as weak green articles cannot be safely handled during subsequent operations. Another important factor in the quality of a prototype article is its dimensional accuracy relative to the design dimensions. However, these factors of part strength and dimensional accuracy are generally opposed to one another, considering that the densification of the powder that occurs in the sintering of the post-process anneal also causes shrinkage of the article. The polymer content of a metal and/or ceramic composite powder described above could be increased in order to provide higher green part strength, but the shrinkage of the part in post-process anneal would increase accordingly. As a result, compromises between article strength and dimensional stability must be made in the design of the composite powder system.

[0009] Some drawbacks of conventional composite powders incorporating thermoplastic polymer binders have been observed. In the post-process anneal of green articles using such binders, creep deformation has been observed as the article is heated to a temperature above the glass transition temperature of the polymer binder, but below the decomposition temperature at which the binder is released. The viscosity of the polymer decreases to such an extent that the metal or ceramic substrate particles slide past one another under the force of gravity. Not only do the dimensions of the article change as a result of this creep deformation, but also this dimensional change is not uniform in that taller features deform by a larger extent than do shorter features. This non-uniformity in deformation precludes the use of a constant shrinkage correction factor in the selective laser sintering fabrication of the green part, further exacerbating the difficulty of achieving dimensionally accurate articles of high density and strength.

[0010] Creep deformation has been observed to deform not only the height but also the shape of vertical features such as sidewalls. For example, vertical walls of mold cavities formed by selective laser sintering of polymer-coated metal powders, and having a thickness of 0.75 inches and a height of 1.5 inches, have been observed to bow outwardly as a result of creep deformation. The dimensional accuracy of the infiltrated final part is, of course, severely compromised by such deformation.

[0011] To address this tendency of creep deformation, another prior art technique was developed that combined the use of a thermoplastic binder with a thermoset binder. This is described in U.S. Pat. No. 5,749,041. In this approach a "green" part is formed by the selective laser sintering of a metal-polymer composite powder, in which the polymer binder is a thermoplastic polymer. Following its fabrication, the green article is infiltrated with a thermosetting material prior to heating the part. The thermosetting material may be an aqueous emulsion of a cross-linkable polymer with a cross-linking agent, or may instead be an aqueous emulsion of only the cross-linking agent. In the first case, the cross-linking agent reacts with the cross-linkable polymer in the infiltrant to form a rigid skeleton for the green article; in the second case, the cross-linking agent reacts with the polymer binder of the green article to form the rigid skeleton. Following the formation of the rigid skeleton, the article may be heated to decompose the polymer and sinter the metal substrate particles, followed by infiltration with a metal for added strength. This prior art approach enabled a solution to the creep deformation problem but added significant time to the post processing of the part to dry out the article after the aqueous infiltration step.

[0012] Another approach used commercially to avoid the aforementioned drying step was to incorporate both a thermoplastic and thermoset binder in the formulation of the metal-polymer composite article. In one successful version a phenolic type thermoset was combined with a wax binder to give a system that gave adequate initial green strength and a more rigid skeleton for the green article. The green strength of this system though, while improved, still has resulted in unacceptable failure rates due to breakage of green parts in handling. Thus the search for stronger green part systems has continued. The trend has been to use more polymer binder materials over time.

[0013] As the amount and complexity of binders has increased in these metal and/or ceramic polymer composite approaches, there has been increased difficulty in removing all of the polymer system binders during the decomposition and burn-out phase. The decomposition of the polymer into smaller fragments should be complete enough to ensure that the bulk of the hydrocarbon fragments can escape the article skeleton before the infiltrating metal (copper or bronze, for example) enters the skeleton. If all of the hydrocarbon fragments do not escape, trapped ones can lead to a phenomena of blistering on the surface of the final article. In some systems the presence of too much residual carbon can also impede the infiltration process. The presence of a reducing atmosphere, such as hydrogen or forming gas helps the polymer degradation greatly but is a more expensive alternate than an inert nitrogen atmosphere.

[0014] Accordingly, there is a need for improving the efficiency of the decomposition and removal of the polymer binder systems during the oven or furnace cycle.

BRIEF SUMMARY OF THE INVENTION

[0015] It is therefore an aspect of the present invention to provide a method of fabricating high density and high strength articles and tooling via SFF techniques from a metal and/or ceramic and polymer composite powder with improved initial green strengths.

[0016] It is a further aspect of the present invention to provide such a method using a composite powder that improves dimensional accuracy.

[0017] It is a further aspect of the invention to provide such a method while avoiding blistering phenomena even in nitrogen atmospheres.

[0018] The invention may be incorporated into a method of fabricating an article, such as a prototype part or a tooling for injection molding, by way of selective laser sintering. According to the present invention, the selective laser sintering of a metal-polymer composite powder, in which the polymer binder may be a thermoplastic polymer or a combination of thermoplastics and thermoset binders, forms a "green" part. In addition, a metal hydride powder is added to the metal-polymer composite formulation. After removal of unfused material from the green part it is placed in an oven or furnace in a non-reactive atmosphere such as, for example, nitrogen or argon for subsequent heat treatment to decompose and drive off the binder and sinter the metal substrate particles prior to infiltration by a metal with a lower melting point. During the critical step of decomposing the binders, the metal hydride begins to decompose also and releases an in-situ concentration of hydrogen gas that creates the reducing conditions necessary to thoroughly decompose the polymer fragments so that the hydrocarbon fragments can escape the skeleton structure of the article. It has been found that even with higher loadings of binders, leading to higher desired green strengths, the decomposition of the metal hydride eliminates the blistering phenomena associated with high loadings of some binders.

[0019] The invention also includes a preform green article formed by selective laser sintering comprising a composite powder, the composite powder being fused and comprising metal and/or ceramic particles, polymer particles and metal hydride particles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other goals and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings, wherein:

[0021] FIG. 1 is a flow diagram illustrating a method of fabricating an article according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] According to the preferred embodiments of the present invention, three-dimensional articles of complex shapes may be made with high dimensional accuracy and good part strength both in its green state and also as a finished article. It is to be understood that, while the present invention is particularly useful in the fabrication of prototype injection molds and tooling, the present invention may also be used to advantage in the fabrication of prototype parts, such as used in the modeling of mechanical systems. Indeed, it is contemplated that the selective laser sintering process and the method of the present invention may be used to manufacture end use articles and parts therefor, particularly in custom or very limited runs, as economics permit. As such, the use of the term "article" hereinbelow will be used to refer either to a part (prototype or end-use), or to tooling for injection molding, thus encompassing various eventual uses of the article.

[0023] Referring now to FIG. 1, a method of fabricating an article according to a first embodiment of the invention will now be described in detail. According to this embodiment of the invention, the method begins with process 10, which is the selective laser sintering of a composite powder to form a "green" article. The term "green" refers to the intermediate state of the article, prior to its densification as will be described hereinbelow. The composite powder used in process 10 according to this embodiment of the invention is a metal and/or ceramic powder blended with or coated by a polymer binder system and also includes a metal hydride powder. The polymer binder system may use thermoplastics, thermosets, or a combination thereof.

[0024] Selective laser sintering process 10 is preferably performed in a modern selective laser sintering apparatus, such as the VANGUARD system available from 3D Systems, Inc. As described in the above-referenced patents, process 10 fabricates the green article in a layer wise fashion, by dispensing a thin layer of the powder over a target surface, preferably in a controlled environment, and then applying laser energy to selected locations of the powder layer to fuse, or sinter, the powder thereat. According to the present invention, wherein the powder is a composite powder of metal or ceramic particles, polymer binder particles and metal hydride particles, and the powder particles are fused to one another by the melting and cooling of the polymer binder, rather than by sintering of the metal substrate particles (which would require very high laser power). The selected locations of the powder layer correspond to those portions of the layer in which the article is to be formed, as defined by a computer-aided-design (CAD) data base representation of the article. After the selective fusing of a layer, a subsequent layer is disposed over the previously processed layer, and the selective fusing is repeated in the new layer at locations of the layer corresponding to the CAD "slice" of the article to be formed therein. Those portions of a layer that overlie fused portions of the powder in the prior layer are bonded to the fused portions in the prior layer, such that a solid article results. The unfused powder in each layer serves as a support medium for subsequent layers, enabling the formation of overhanging elements in the article. As a result of process 10, the green article is formed to the desired size and shape.

[0025] It is contemplated that the particular settings and operating parameters of the selective laser sintering system used in process 10 may be readily selected by one of ordinary skill in the art. These parameters include such items as the laser power, laser scan rate, ambient chamber temperature, layer thickness and the like. Typically, the values of these operating parameters are optimized for a given commercially-available powder, such as the composite powder described above, according to documentation provided by the system manufacturer.

[0026] Other thermal-based additive processes may alternatively be used to form the green article. For example, it is contemplated that process 10 may be performed by the layer wise masked exposure of the composite powder to light, so that the portions of the powder to be fused are exposed to the light and the unfused portions are masked therefrom.

[0027] Upon completion of process 10, process 12 is then performed to remove the unfused or unsintered powder from around the article in the conventional manner. Such removal is commonly referred to as "rough break-out", and generally involves the mechanical removal of the unfused powder to yield the green article. Further surface finishing of the green article may be performed at this time, if desired.

[0028] Upon completion of process 12, process 14 is then performed. In process 14 the green article is placed in an oven or furnace, usually packed in inert powder packing made up of alumina or silica powders to provide support during the subsequent heating steps. A lower melting infiltrant material is placed in the oven or furnace in contact with the green article. During process 14 the temperature of the oven or furnace is slowly raised to a first temperature high enough to begin to decompose the polymer binders present. At these temperatures the metal hydrides present also begin to break down and release hydrogen gas in the immediate environment of the decomposing polymers, the resulting reducing atmosphere accelerating the breakdown of the polymer fragments into smaller fragments. This simultaneous breakdown of polymers and release of hydrogen leads to a much more complete removal of residual carbon from the article skeleton, thereby reducing the likelihood of a later problem in these types of systems, that is surface blistering of the final infiltrated article due to residual carbon material forced to the surface during final infiltration.

[0029] After process 14, process 16 is performed; the temperature of the oven or furnace is raised to increase the temperature of the article further to begin a preliminary sintering of the composite articles to form a rigid skeleton. This now stronger article is often referred to as the brown part or brown article.

[0030] After process 16, continuing to raise the temperature of the oven or furnace performs process 18 when the infiltrant placed in the oven or furnace in contact with the article melts and infiltrates the brown article, resulting in a fully dense article.

[0031] A preferred example of a composite powder to be used in the selective laser sintering process that is useful in connection with this embodiment of the invention has a substrate of a stainless steel powder, such as spherical particles of 420, -53 micron, specification 2290 stainless steel powder; a polymer binder system made up of approximately 1% by weight of Ceracer 126A wax, available commercially from Shamrock Specialty Products Group of Shamrock Technologies, Inc. of Newark, N.J.; approximately 1% by weight of Atofina 3501 UD natural nylon, available commercially from Atofina Chemicals, Inc. of Philadelphia, Pa.; and approximately 1.25% by weight of a G-P 5546 phenolic, available commercially from Georgia-Pacific of Atlanta, Ga. The polymer binder is preferably blended with the metal powder substrate particles. In addition, the preferred composite powder includes approximately 1% by weight of Monico titanium hydride powder -44 micron, available commercially from Monico Alloys of Los Angeles, Calif.

[0032] It should be recognized that other waxes, polyamides, and phenolics could be combined into workable systems for the purposes of this invention. In addition, other thermoplastics could be substituted for the polyamide and other thermosets for the phenolic. Potential metal hydrides that can be employed in the present invention include titanium hydride, nickel-metal-hydride, magnesium hydride, lithium aluminum hydride, calcium hydride, sodium hydride, and sodium borohydride and combinations thereof.

[0033] While the present invention has been described according to its preferred embodiments, it is of course contemplated that modifications of, and alternatives to, these embodiments, such modifications and alternatives obtaining the advantages and benefits of this invention, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein.

Claims

We claim:

1. A method of fabricating an article, comprising the steps of forming a green article by the selective laser sintering of a composite powder, wherein said composite powder comprises metal and/or ceramic particles, polymer particles, and particles of a metal hydride.

2. The method of claim 1 wherein said metal hydride particles are selected from the group consisting of titanium hydride, nickel-metal-hydride, magnesium hydride, lithium aluminum hydride, calcium hydride, sodium hydride, and sodium borohydride and combinations thereof.

3. The method of claim 1 further comprising using a steel powder in said composite powder.

4. The method of claim 3, further comprising using titanium hydride as the metal hydride.

5. The method of claim 2 wherein said polymer particles are selected from a group consisting of thermoplastic and thermoset polymers.

6. The method of claim 5 further comprising using polyamide polymers.

7. The method of claim 5 further comprising using phenolic polymers.

8. The method of claim 1, further comprising after said forming step, heating said green article in a first heating step to a first temperature to decompose said polymer binder and said metal hydride.

9. The method of claim 8, further comprising after said first heating step heating said article in a second heating step to a second temperature, the second temperature being above said first temperature, to sinter said composite particles to one another, forming a brown article.

10. The method of claim 9, further comprising during said second heating step, infiltrating said brown article with a second material.

11. The method of claim 10 further comprising using a copper alloy as said second material.

12. A preform green article formed by selective laser sintering comprising a composite powder, the composite powder being fused and comprising metal and/or ceramic particles, polymer particles and metal hydride particles.

13. The green article according to claim 12 wherein the metal hydride particles are selected from the group consisting of titanium hydride, nickel-metal-hydride, magnesium hydride, lithium aluminum hydride, calcium hydride, sodium hydride, and sodium borohydride and combinations thereof.

14. The green article according to claim 12 wherein said composite metal powder further comprises a steel powder.

15. The green article according to claim 14 wherein said composite powder further comprises titanium hydride.

16. The green article according to claim 13 wherein said polymer particles are selected from the group consisting of thermoplastic and thermoset polymers.

17. The green article according to claim 16 wherein said thermoplastic polymers further comprise polyamides.

18. The green article according to claim 17 wherein said polymer particles further comprise phenolics.