CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) of U.S. provisional patent application Serial No. 60/557,791 filed March 30, 2004, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a selec¬ tive laser sintering (SLS) process to form articles of manufacture, and to polymeric particulate mate¬ rials used in the SLS process.

BACKGROUND OF THE INVENTION

Selective laser sintering (SLS) is a pro¬ cess wherein an article of manufacture is built layer by layer. The SLS process is based on using a laser beam to selectively fuse a predetermined por¬ tion of a layer of particulate material . The par- ticulate material exposed to the laser beam is heated to a temperature above its melting point, after which adjacent molten particles are fused together, as are adjacent layers of fused material. The SLS process alternatively has been termed a selective laser melting (SLM) process. As used herein, the term "SLS" is defined as encompas¬ sing a selective laser sintering process and a selective laser melting process. Selective laser sintering generally is performed according to the following steps : 1. A piston of a process chamber is lowered by one layer thickness, and simultaneously, a piston of a reservoir containing a particulate material is raised; 2. The particulate material is intro¬ duced into the process chamber, and a leveling roller spreads the particulate material evenly over a build surface of the process chamber; 3. The particulate material is heated to a temperature near, but below, its softening point; 4. A laser beam then traces a predeter¬ mined cross section of the article on the layer of heated particulate material in the build surface, thus forming a solid layer; and 5. Steps 2 through 4 are repeated until manufacture of the article is completed. The particulate material in areas that have not been sintered by the laser remains unfused and in place, and acts as a natural support for the article being manufactured.. Accordingly, no need exists for support structures during manufacture of the article. After cooling, the article of manufac- ture and the unfused particulate material are re¬ moved from the process chamber, and the article of manufacture is separated from the unfused particu¬ late material, i.e., "breakout." Figures 1 and 2 are schematics illustrat- ing the laser-sintering process for the manufacture of an article having a complex shape. Clausen et al . U.S. Patent No. 6,110,411, incorpoxated herein Iby reference, provides a detailed description of the selective layer sintering process . The unfused particulate material typically is recycled to the reservoir containing the particu¬ late material, and reused in subsequent selective laser sintering cycles to manufacture additional articles of manufacture. However, it is known that the performance of the particulate material can change as the material is subjected to a plurality of SLS cycles. These changes in physical properties can adversely affect the article of manufacture, e.g., cause "orange peel" effects and diminished me¬ chanical properties. To avoid such adverse effects, the particulate material is discarded after a number of SLS recycles, which increases raw material costs.

SUMMARY OF THE INVENTION

The present invention is dir-ected to an improved SLS process, and to polymeric particulate "materials used in the SLS process. The polymeric particulate material is laser sinterable and over¬ comes several disadvantages associated with present- day polymeric particulate materials used in SLS pro¬ cesses. The polymeric particulate materials used in the present method have improved stability for re¬ cycling in an SLS process, and therefore provide articles of manufacture of high and uniform quality. It is theorized, but not relied upon here¬ in, that a change in physical properties of a par- ticulate material can be attributed, at least in part, to chemical reactions induced by repeated heating of the particulate material over a rrumber of SLS processes using particulate material recycled from one SLS cycle to the next. One polymeric particulate material common¬ ly used in SLS processes is nylon-12, as disclosed in U.S. Patent No. 6,245,281, for example. .Nylon-12 is polylaurolactam, and also is known as PA- 12. Additional polymers used as a polymeric particulate material in an SLS process include, but are not limited to, other nylons, such as nylon-11 (PA-Il), polyacetals, polyethylenes, polypropylenes, ion- omers, polycarbonates, elastomeric block copolymers, and polystyrenes. The most widely used polymeric particulate materials in SLS processes, e.g., PA-12, exhibit a disadvantage during recycling. While not intending to be bound by any theory, it is proposed tϊiat the performance of PA-12 decreases after recycling through a number of SLS cycles because the molecular weight and solution viscosity of PA-12 increase dur¬ ing thermal exposures encountered during recycling through a plurality of SLS processes. This molec¬ ular weight increase can lead to gelling of partic- ulate material particles, especially at particle surfaces, which prevents proper processing of the particulate material after several SLS cycles. Therefore, such polymeric particulate materials are not ideally suited for SLS processes because fresh and recycled particulate materials can produ.ce articles of manufacture of different quality, and possibly regions of (differing quality within a single article. More particularly, the present invention is directed to an SLS process utilizing a polymeric particulate material having improved stability, and that substantially retains the physical properties of virgin particulate material, i.e., particulate material unprocessed through an SLS process, after undergoing one or more SLS cycle. A polymeric particulate material utilized in the present SLS process essentially .retains its physical properties over a plurality of SLS cycles, and, therefore, pro¬ vides articles of manufacture having properties essentially equal to an SLS manufactured article solely from virgin polymeric particulate material. As a result, articles made from virgin and recycled particulate material have essentially uniform and consistent properties. Accordingly, one aspect of the present in- vention is to provide an SLS process wherein the particulate material can be recycled through a plur¬ ality of SLS cycles, and the resulting articles of manufacture from eacli cycle have excellent breakout, layer-to-layer adhesion, surface finish, and strength. The articles, prepared at least in part from recycled particulate material, also exhibit consistent properties because each successively prepared article has properties essentially equal to a previously prepared, article. Another aspect of the present invention is to provide a particulate material that maintains its physical properties and chemical identity, e.g., melting point, solution viscosity, and molecular weight, over a plurality of SLS cycles. The reten¬ tion of physical properties of the particulate mate- rial allows SLS production of articles using re¬ cycled particulate materials, wherein the article has essentially the same properties as an article manufactured solely frrom a virgin particulate mate¬ rial . We have discovered that polymeric particulate materials widely used in SLS processes can be modified to maiLntain their physical and chemical properties in a number of different ways. For example, we have (discovered that the polymeric particulate material can be stabilized by positioning nonreactiv^e endcaps on a polymeric par¬ ticulate material. Trie nonreactive endcaps prevent, or at least retard, criemical reactions induced by SLS heating steps, ancL maintain a more constant molecular weight of the particulate material over a plurality of SLS cycles. The endcapped polymeric particulate material therefore maintains an essen¬ tially constant molecular weight and an essentially constant solution viscosity over a plurality of SLS cycles to facilitate the SLS process and improve the quality and uniformity of the manufactured articles. Yet another aspect of the present inven¬ tion is to further stabilize an endcapped polymeric particulate material, and further improve the SLS process and articles prepared thereby, by subjecting the polymeric particulate material to an extraction step to remove low molecular weight components from the endcapped particulate material _ Still another aspect of the present inven¬ tion is to provide a method of manufacturing an article of manufacture using selective laser sinter¬ ing comprising the steps of (a) depositing a quantity of a particu¬ late material from a reservoir into a process chamber; (b) leveling the particulate material to form a build surface of the material; (c) then directing an energy beam over a predetermined target area on the build surface of the material causing a portion of the particulate material to fuse and form an integral layer; (d) repeating steps (a) to (c) to form additional integral layers that are integrally bonded to adjacent layers to form a. three-dimension¬ al article; (e) separating unfused particulate material from the article; and (f) recycling the particulate material of steps (e) to the reservoir to manufacture a second article of manufacture; ■ wherein a solution viscosity and/or molec¬ ular weight of the unfused particulate material re¬ mains essentially constant over ten repeating cycles. Yet another aspect of the present inven- tion is to provide an article manufacture by an SLS process using recycled particulate material, at least in part, said article having properties essen¬ tially equivalent to an article prepared in an SLS process solely from an identical virgin particulate material . These and other novel aspects and advan¬ tages of the present invention will become apjparent from the following detailed description of the pre¬ ferred embodiments taken in conjunction with the figures.

BRIEF DESCRIPTION OP THE FIGURES

Figs. 1 and 2 are schematics illust.rating the laser sintering process; and Fig. 3 contains plots of solution viscos¬ ity for an endcapped and an uncapped polymeric par- ticulate material over ten SLS cycles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIIMENTS

Present-day polymeric particulate mate¬ rials utilized in SLS processes typically contain chemically reactive groups. Examples of such par- ticulate materials include DURAFORM PA, DURAFORM GF, EOSINT PA, and RILSAN PA-Il. In particular, the most commonly used polymeric particulate materials are PA-12, i.e., polylaurolactam, and PA-Il, which contain reactive amino (-NH2) and carboxyl (-COOH) end groups. It is theorized that these reactive end groups can undergo a condensation reaction at elevated temperatures encountered in the SLS process to increase the molecular weight of the polymeric particulate material, including chain extension and/or branching. An increase in molecular weight of a poly¬ meric particulate material impacts the SLS process and articles of manufacture resulting therefrom. In particular, an increase in molecular weight of the particulate material that occurs over repeated SLS cycles affects the solution viscosity of the poly¬ meric material. The solution viscosity is an i_mpor- tant property of the particular material in an SLS process because too high of a solution viscosity leads to insufficient flow out and article density and, therefore, insufficient mechanical properties. This negates an important benefit of the SLS process, i.e., that only the natural flow of a molten particulate material is required to produce a dense article. No additional compaction forces, such as injection molding, need to be applied. In addition, highly viscous particulate materials associated with a high molecular weight provide articles having a rough surface and orange peel- effects, which adversely affect the surface of the article. Therefore, an important feature of ttie present SLS method is to utilize a polymeric partic¬ ulate material having an essentially constant solu¬ tion viscosity over a plurality of, e.g., ten, SLS cycles. As used herein, the term an "essentialLly constant solution viscosity" is defined as a solu- tion viscosity of a particulate material after a plurality of, e.g., ten, SLS recycles that has in- creased no more than 20% over the solution viscosity of the identical virgin particulate material as described below. In preferred embodiments, the solution viscosity increases no more than 10% over a plurality of SLS cycles. In other preferred embodiments, the solu¬ tion viscosity of the recycled particulate material (after 10 cycles) is no more than 0.15, and more preferably 0.10, greater than the solution viscosity of virgin particulate material. It should be understood that the solution viscosity typically remains essentially constant over more than ten SLS cycles, for example, twenty to thirty SLS cycles. Accordingly, the particulate material has optimal stability for recycling through an SLS apparatus in cases wherein virgin particulate material is not added to recycled particulate method, and in cases when virgin particulate mate¬ rial is added to recycled particulate material . As used herein, the term "essentially con¬ stant molecular weight" is defined as a molecular weight of a recycled polymeric particulate material after ten SLS cycles that is no more than 50%, pref¬ erably no more than 25%, greater than an identical virgin polymeric particulate material. More pref¬ erably, the molecular weight of the recycled poly¬ meric particulate material (after 10 cycles) is no more than 20% greater than an identical virgin poly¬ meric material . The solution viscosity and the molecular weight of the recycled particulate material after ten SLS cycles is determined as follows. An article of manufacture is prepared using a 3D Systems Van¬ guard HS apparatus. The Vanguard HS apparatus con¬ tains two reservoirs for a particulate material, and about 15 kilograms of particulate material is intro¬ duced into each reservoir. A quantity of the particulate material is introduced into the process chamber of the appara¬ tus. A sufficient quantity of the particulate mate- rial is introduced into the process chamber to pro¬ vide a layer of particulate material about 2.5 cm deep. The particulate layer then is leveled to form a build surface. The surface area of the process chamber is about 38 cm by 32 cm. The particulate material in the process chamber is heated to about 10C° below its melting point, then a laser beam is directed over a pre¬ determined area on the build surface to form a first fused layer of particulate material. Next, the pro- cess chamber, which is positioned on a piston, is lowered about 0.1 mm, and a new layer of particulate material of 0.1 mm height is introduced 'into the process chamber and leveled. The newly added par¬ ticulate also is heated and subjected to a laser beam to form a second fused layer, that also is fused to the first fused layer. This process is repeated until articles of manufacture weighing about 500 g have been prepared. At this time, the process chamber contains about 15 kg of heated, but unfused, particulate material. The reservoirs con¬ tain about 14.5 kg of particulate material that is remixed with the previously heated, unfused partic¬ ulate material. The new blend of particulate material from the unfused particulate material of the first SLS cycle and the reservoirs is sieved, then recycled in the above-described SLS process to form a second article of manufacture. This second article is pre¬ pared from particulate material that a portion of which undergone one SLS cycle. The particulate material in the reservoirs again is set aside and remixed with the unfused material that has been subjected to two SLS cycles and used to manufacture a third article of manufacture. The SLS cycling is continued until the unfused particulate material has undergone ten cycles without adding virgin partic¬ ulate material . After each SLS cycle, a portion of the un¬ fused particulate material is tested for solution viscosity using the DIN-EN-ISO Norm 307 method for polyamides, and for molecular weight using GPC (gas- phase chromatography) method. An essentially constant solution viscosity can be achieved by improving the stability of the virgin particulate material, e.g., by reducing or eliminating chemical reactions that can occur during an SLS cycle. We have discovered that the stability of the polymeric particulate material is adversely affected by the presence of reactive end groups during the SLS process. We have also discovered that inactivating these reactive end groups alleviates this instability problem. For example, we have discovered that thermal stability can be imparted to a polymeric particulate material by endcapping the polymeric particulate material used in the preparation of an article of manufacture. The endcapped polymers have a substantially in¬ creased stability for use in an SLS process, and eliminate heat-induced reactions during the SLS pro¬ cess that increase polymer molecular weight and ultimately reduce article quality. It is theorized, but not relied upon here¬ in, that the polymer at the surface of the particles can undergo chemical reactions to increase the molecular weight of the polymer at the particle sur¬ face. Particle softening, and increases in molec- ular weight at particle surfaces, lead to particle agglomeration that adversely affect the SLS process . By providing a more stable polymeric particulate material, these surface changes are substantially reduced or eliminated, and the SLS process using recycled particulate material is facilitated. In particular, articles made from recycled particulate material over a plurality of SLS cycles have con¬ sistent and uniform properties. We have discovered that polymeric particulate materials widely used in SLS processes can be modified to maintain their physical and chemical properties in a number of different ways. For example, we have discovered that a polymeric particulate material useful in the present SLS process can be endcapped to prevent reactive groups from further chain propagation and branching. The particulate material can be endcapped using monofunctional organic compounds or multifunctional or-ganic compounds. Preferably, endcapping is achieved using a monofunctional organic compound. An endcapping compound typically is a low molecular weight compound, e.g., a compound containing one to about eight carbon atoms . For example, an amino end group on a polymeric particulate material can be endcapped using a monocarboxylic acid, an acid anhydride, an ester, an acid halide, an aldehyde, a ketone, an epoxy- containing compound, or other compound having a functional group capable of reacting with an -NH2 giroup. For particulate materials having a carboxyl end group, the' polymer can be endcapped using an amine, alcohol, ester, epoxy-containing compound, or other compound having a functional group capable of reacting with a -CO2H group. The amine typically is a secondary amine. A hydroxyl end group on a polymeric particulate material can be endcapped using an alkyl halide, for example. The endcapped, polymeric particulate material can be prepared in a number of different ways. For example, the endcapped polymer can be prepared using the same polymerization method used to prepare the uncapped polymer, except an endcapping compound is present in the monomer mixture. In a more preferred method, the polymerization process is allowed to proceed until a predetermined molecular weight is achieved. Then the endcapping compound is added to the polymer- ization reaction. This method is preferred because the molecular weight and physical properties of the polymer can be more precisely controlled for effective use in an SLS process. A more precise molecular weight range of the polymeric particulate material is preferred in SLS processes because a polymer having a more narrow molecular weight range exhibits a more definite melting point. The endcapped polymeric particulate mate- rial can be either a granular form of the polymer or a precipitated form of the polymer. In SLS proces¬ ses, a large difference between the melting point and the solidification point of the polymeric par¬ ticulate material is preferred. The precipitated form of the polymeric particulate material is pre¬ ferred because the precipitated form has a higher melting point and higher melting enthalpy than the granular; form. In addition, the endcapped polymeric particulate material can be used as is, or can be glass filled, as is known in the art. The chemical identity of the endcapped polymeric particulate material is not limited, ex¬ cept the material must be capable for use in an SLS process. Therefore, the endcapped polymeric partic- ulate material can be an endcapped version of any polymer having functional groups and useful in an SLS process. The endcapped polymeric particulate material therefore can be, for example, a polylactam or other; nylon, an ionomer, or a polyacetal. A preferred endcapped particulate material is an endcapped PA-Il or PA-12, i.e., a polylauro- lactam, having endcapped amino and carboxyl groups. Such polymers are prepared by Degussa AG, Marl, Germany, as VESTOSINT X1546 LV (low viscosity) , VESTOSINT X1546 HV (high viscosity) , and VESTOSINT X4175. The properties of these endcapped polymeric particle materials are summarized in the following Table 1, and compared to a conventional uncapped PA-12, i.e., DURAFORM PA, manufactured by The VaI- spar Corporation AG, CH-8627 Grύningen, Switzerland and distributed by 3D Systems, Valencia, CA. 11 Solution viscosity determined by PIN-EN-ISO Norm 307, incorporated herein by reference.

Regardless of the chemical identity of the

endcapped particulate material, the material prefer¬

ably has a melting point of about 1700C to about

2100C, and more preferably about 1800C to about

2000C. To achieve the full advantage of the present

invention, the particulate material has a melting

point of about 1800C to about 195°C.

The particulate material can comprise one

or more endcapped polymers. Each particulate mate¬

rial preferably has a melting point of about 1700C

to about 2100C. In a mixture of polymeric partic¬

ulate materials, it is preferred that the melting

points of the polymers are within 10C°, and more

preferably 5C° or less, of one another. To further improve the properties of an article of manufacture prepared in an SLS process using an endcapped particulate material, the poly¬ meric particle material optionally is extracted to remove low molecular weight monomers, oligomers, and impurities from the endcapped polymer. In particular, as previously discussed, polymeric particulate materials used in an SLS pro¬ cess have the disadvantage of increasing molecular weight under thermal exposure. This molecular weight increase is theorized to lead to gelling, which prevents proper melting after several cycles in use. Therefore, such particulate materials are not ideally suited for SLS processes because the virgin and recycled particulate material produce different quality parts. Extraction of low molecular weight com¬ ponents from the endcapped polymeric particulate material is believed to provide improvements because these components potentially are reactive, and have a lower melting point and higher- vapor pressure than the particulate material . These low molecular weight compounds, therefore, can cause problems in an SLS process, such as fuming below and at the melting temperature of the particulate material. The low molecular weight compounds also can migrate to areas where they form agglomerates with partic¬ ulate material that is not exposed to the laser beam, and can produce undesirable deposits on the laser sintering equipment. Extraction of the endcapped polymer par¬ ticulate material can be achieved using any method known in the art, for example, Soxhlet extraction or counter- current extraction. U.S. Patent Nos . 3,423,183; 6,326,457; 6,320,021; 6,32S,460; 6,160,086; 4,978,743; and 3,639,659, each incorpo¬ rated by reference, disclose extraction techniques for polyamides and other polymers . In one method, a polymer was extracted -with ethanol for about four hours using a Soxhlet extraction apparatus. It was found ttiat ethanol extraction removed about 2 to about 3 wt% of low molecular weight components from the polymer. Using an extracted and endcapped polymeric particulate material in an SLS process reduced fuming during the SLS cycles, and the manufactured articles had im¬ proved color, i.e., yellowing of the articles after sun exposure was substantially reduced . Optional ingredients also can be added to the endcapped particulate material, for example, to facilitate the SLS process or to improve the es¬ thetics of the manufactured article. These optional ±ngredients are particulate materials and include organic and inorganic materials, such as fillers, optical brighteners, UV stabilizers, and coloring agents. Inorganic optional materials are preferred. An optional ingredient is added to the endcapped polymer particular material in a suffi¬ cient amount to perform its intended function, with- out adversely affecting the endcapped material or an article prepared therefrom. Optional ingredients have a particle size in the range of tlαe particle size of the endcapped material. Each optional in¬ gredient is milled, if necessary, to the desired median particle size and particle size distribution. Each individual optional if present at all, typically is added to the endcapped polymer particular material in an amount of about 0.1% to about 150%, by weight, of the endcapped material . The total amount of optional ingredients added to the endcapped material ranges from 0% up to about 150%, by weight. Each optional ingredient must be compat¬ ible with the particulate material in order to pro¬ vide a strong and durable article of manufacture. The optional ingredient, therefore, can. be inorganic filler that imparts additional strength to the arti¬ cle of manufacture. One optional ingredient is a coloring agent, for example a pigment or a dye, like carbon black, to impart a desired color to the article of manufacture. The coloring agent is not limited, as long as the coloring agent does not adversely affect the particulate material or an article prepared therefrom, and is sufficiently stable to retain its color under conditions of the laser sintering pro¬ cess and during exposure to the laser. The optional ingredient also can be a second polymer that modifies the properties of the particulate material . One useful second polymer is a nylon, such as nylon 6, nylon 66, nylon 11, nylon 610, nylon 12, nylon 612, copolymerized nylons, and mixtures thereof . One nonlimiting example of a nylon that can be included in the present invention is the RIlLSAN line of PA-Il resins, available from Elf Atoch-im. To illustrate the new and unexpected re¬ sults actiieved by utilizing an endcapped polymeric particulate material in an SLS process, an endcapped polymer, i.e., VESTOSINT X1546, was used to manufac¬ ture three-dimensional articles of manufacture. The article was manufactured using a 3D Systems Vanguard HS apparatus . The endcapped polymer was heated to about 1OC° below the melting point of the polymer prior to fusing a portion of the polymer with a laser. Articles made using an endcapped polymer in the Vanguard HS apparatus included T-bars foar tensile strength and flexibility tests, quadratic short pipes, gear box casings, pyramid platforms. The quadratic pipes were positioned in the corners of the pirocess chamber, and show orange peel effects when the particulate material becomes defective after a plurality of SLS cycles. It is believed that present-day polymeric particulate materials, e.g., DURAFORM PA-12, have recyclability problems. Our studies have shown that these problems are attributable, at least in part, to an increase of the molecular weight of the PA-12 nylon. In turn, the increase in molecular weight causes an increase of the melt viscosity, which then in turn causes a surface distortion problem termed "orange peel." It has been found that when the molecular weight of a polymeric particulate material increases by about 75%, orange peel effects can be expected. An SLS process utilizing an endcapped polymer substantially overcame the orange peel problem because molecular weight increases were con¬ trolled. Zn one test, an endcapped PA-12 (i.e., VESTOSINT 1X1546) and an uncapped PA-12 (i.e., EOS 2200 PA) were subjected to repeated SLS cycles. The solution viscosity was determined using a standard method in the art, i.e., DIN-EN-ISO Norm 307. The plots in Fig. 3 show that the solution viscosity of the endcapjoed polymer remains essentially constant over twelve SLS cycles (i.e., about 1.55) . In con- trast, the uncapped polymer demonstrated a solution viscosity increase from 1.55 to 1.9 over the twelve SLS cycles . Fig. 3 shows that the uncapped material substantially increased in molecular weight over the 12 cycles, whereas the endcapped material maintained. an essentially constant molecular weight. The stable solution viscosity of the endcapped polymer provided articles produced after several SLS cycles having good surface quality and homogeneous article density, for both unfilled and glass-filled end- capped PA-IL2. Zt further was found that after thirteen SLS cycles using an endcapped polymeric particulate material, orange peel was reduced. In addition, the density of the articles made using an endcapped material was more constant, and does not decrease with recycling of the particulate material . In total, the tests show that using an endcapped polymeric particulate material in a plur¬ ality of SLS cycles, SLS provides substantial bene¬ fits, including improved breakout of unfused partic- ulate material from the article of manufacture, homogenous article density , improved layer-to-layer adhesion, reduced orange peel, and articles prepared from virgin and recycled particulate materials having properties and qualities essentially equal to an article prepared solely from a virgin particulate material . Obviously, many modifications and varia¬ tions of the invention as hereinbefore set forth can be made without departing from the spirit and scope thereof and, therefore, only such limitations should be imposed as are indicated by the appended claims.