MXPA97007363A

MXPA97007363A - Composite plastic material for laser select sinterization

Info

Publication number: MXPA97007363A
Application number: MXPA/A/1997/007363A
Authority: MX
Inventors: F Forderhase Paul; Pmcalea Kevin; E Ganninger Mark; W Kuning Frederic; J Magistro Angelo
Original assignee: Dtm Corporation
Priority date: 1995-03-31
Filing date: 1997-09-26
Publication date: 1998-02-01

Abstract

A composite powder specially adapted for use in selective laser sintering is disclosed. The composite powder includes a polymer powder dry blended with a reinforcing powder, wherein the polymer powder has a substantially lower melting temperature than that of the reinforcing powder. In the case where almost completely dense pieces are to be formed, the first constituent powder is preferably a semi-crystalline powder, for example, nylon 11 of an appropriate composition to form almost completely dense pieces when used without combining in the selective laser sintering If porous pieces are desired, the polymeric powder is an amorphous powder such as polycarbonate, polystyrene, acrylates, styrene / acrylate copolymers. The reinforcing powder is preferably made of glass microspheres, preferably coated to improve wetting and adhesion with the polymer powder when selective laser sintering is carried out. In addition to improving the rigidity and heat resistance of the piece produced, the composite powder expands the process window in relation to that provided by the non-combined powder, provides improved dimensional accuracy in the produced part and facilitates abrupt detachment and Smooth or even finish of the product

Description

"COMPOSITE PLASTIC MATERIAL FOR SELECTIVE LASER SINTERIZATION" This application is related to co-pending application Serial No. 08 / 298,076, filed on August 30, 1994, assigned to DTM Corporation, and incorporated herein by this reference.

TECHNICAL FIELD OF THE INVENTION This invention is in the field of producing three-dimensional objects such as prototype parts through the selective laser sintering of powders, and is directed more specifically to materials for use in selective laser sintering.

BACKGROUND OF THE INVENTION Recent advances have been made in the field of producing three-dimensional objects, such as prototype parts and finished parts in small quantities, directly from computer-aided design (CAD) databases. Different technologies are known to produce these parts, particularly through the use of additive processes, as opposed to subtractive processes such as conventional machine-working. An important additive process for the production of these objects is selective laser sintering, developed and popularized by DTM Corporation. According to the selective laser sintering process, a powder is explored in the form of layers by a directed energy beam such as a laser beam to melt the powder at selected locations corresponding to the cross sections of the object. The molten locations within each layer adhere to the melted portions of the previously melted layers, so that a series of layers processed in this manner results in a finished part. The computer control of the energy beam scan therefore allows the direct transfer of a design into a computer-aided design (CAD) data base on a physical object. This method, and the apparatus for carrying out the same, are described in further detail in U.S. Patent No. 4,247,508 issued January 27, 1981; U.S. Patent Number 5,252,264 issued October 12, 1993; and U.S. Patent Number 5,352,405 issued October 4, 1994; all of which are assigned to DTM Corporation and incorporated herein by this reference. A further detail is also provided in U.S. Patent Number 4,863,538 issued September 9, 1989; U.S. Patent Number 5,017,753 issued May 21, 1991; U.S. Patent Number 4,938,816 issued July 3, 1990; U.S. Patent Number 4,944,817 issued July 31, 1990; U.S. Patent No. 5,076,869 issued December 31, 1991; U.S. Patent Number 5,296,062 issued March 22, 1994; and U.S. Patent Number 5,382,308 issued January 17, 1995; all of which are assigned to the Board of Regents, The University of Texas System and are incorporated herein by this reference. Further refinements in the selective laser sintering process and advanced systems and machines for performing selective laser sintering are described in U.S. Patent No. 5,155,321 issued October 13, 1992, commonly assigned herein. U.S. Patent Number 5,155,324 issued October 13, 1992 and International Publication WO 92/08566, all of which are incorporated herein by reference. As described in the patents referred to above and in US Pat. No. 5,156,697 issued October 20, 1992; U.S. Patent Number 5,147,587 issued September 15, 1992; and in U.S. Patent No. 5,182,170 issued January 26, 1993, all also assigned to the Board of Regents, The University of Texas System, incorporated herein by this reference, may be processed in accordance with this method., different materials and combinations of materials, including these materials, plastic materials, waxes, metals, ceramics and the like. In addition, as described in these patents and applications, the parts produced by selective laser sintering can have configurations and features that are complicated enough so as not to be capable of manufacturing by conventional subtractive processes, such as machine styling. This complexity is allowed by the natural support of protruding cast portions of the object that is provided by the unmelted powder remaining in the previous layers. Specifically, US Patent Number 5,382,308 to which reference has been made above and its related patents disclose multi-material powder systems useful in selective laser sintering. These multi-material powders include mixtures of powders of materials with different melting temperatures (or bond or dissociation), for example, a mixture of glass powders with alumina powders. This patent also describes several examples of coated powders, where one material is coated with another. By way of further background, U.S. Patent Number 5,342,919 issued August 30, 1994, assigned to DTM Corporation and incorporated herein by this reference, discloses powder systems that are especially useful in the manufacture of an article almost completely dense by selective laser sintering. An example of this powder is a nylon powder 11, which has a number average molecular weight within the range of 75,000 to 80,000, a molecular weight distribution within the range of 1.2 to 1.7 and which is milled to produce particles that they have a sphericity greater than 0.5 and a certain particle size distribution. By way of still additional background, the use of plastic matrix composite materials including a plastic and reinforcing materials, are widely used in the plastics molding industry. Examples of reinforcing materials common in this field include carbon, glass and any of the other relatively inexpensive fillers or fillers. These reinforcements, in the form of microsphere fiber or in particles, are typically mixed with thermoplastic polymers in a mold compound. This mold compound is typically extruded and sliced or formed into an appropriate configuration for injection molding for the production of reinforced parts. As is well known in the field of injection molding, the resulting part is generally stiffer and more resistant than would be an injection molded part similarly configured of a non-reinforced thermoplastic. It is also known that the coefficient of thermal expansion (CTE) of the reinforced molded parts is less than that of the molded plastic parts not reinforced, reducing the effort of molding and improving the dimensional accuracy of the molded part. Furthermore, it is also already known that, since the reinforcing material is generally less expensive than the thermoplastic material, these composite materials for injection molding are less expensive than the non-reinforced thermoplastic for injection molding. Of course, the mixed powder as conventionally used for injection molding is unsuitable for use in the selective laser sintering process. The selective laser sintering process is mainly a thermal process, since the object is formed by sintering or other melting of the powder at selected locations of a layer, which receive the directed energy from the laser beam sufficient to reach the melting temperature or Sintering Those portions of each powder layer that do not receive the laser energy must remain unmelted and therefore must remain at a temperature lower than the melting or sintering temperature. further, the temperature of the powder that receives the laser energy will usually be higher than the temperature of the underlying previous layers (melted or not melted). As such, there are significant thermal gradients present on the surface of the powder target in the selective laser sintering process. It has been observed that these thermal gradients can result in distortion of the object being produced, thus requiring precise thermal control of the selective laser sintering process in order that the objects produced accurately fill the design. One cause of this distortion is the cambering and shrinkage of the object due to the thermal shrinkage of the sintered layer as it cools from the sintering temperature to its post-sintering temperature.; In addition, shrinkage may occur due to the reduction in volume of the molten powder as it passes through the phase change from liquid to solid. In any case, the reduction in volume of the sintered powder will cause the upper part of the object to contract. Since the underlying layers have already contracted and are immersed in the non-molten powder (which is a relatively good thermal insulator), a stress stress is induced on the surface, and the object may become curved. Another source of distortion in the production of objects by selective laser sintering is the undesired growth of the piece that is occurring beyond the volume defined by the laser beam. As is well known, the size of the laser beam area can be made quite small so that the resolution of the particularities or characteristics in the object can be quite well defined. However, the conduction of heat from the molten locations can cause the powder to the outside of the scan to sinter to the directly sintered portion, causing the molten cross-section to "grow" beyond the laser scan area and therefore both beyond the dimensions of the design. The growth between the layers may also occur if sufficient heat from the sintering remains in the molten portion so that the newly distributed powder is sintered to the sintered portions of the previous layer only upon dispersion. It has also been observed that the presence of this growth makes it more difficult to remove the non-sintered powder from the finished part (this removal is referred to in the art as "abrupt detachment"). It is therefore an object of the present invention to provide a material that can improve the robustness of the selective laser sintering process. A further object of the present invention is to provide a material that reduces the distortion effects such as curvature and growth, in the selective laser sintering process. A further object of the present invention is to provide a material that allows the production of almost completely dense parts of the selective laser sintering. A further object of the present invention is to provide a material that improves the efficiency with which the part produced can be finished, for example, by sanding. Other objects and advantages that are provided by the present invention will become apparent to those skilled in the art having reference to the following specification, together with their drawings.

COMPENDIUM OF THE INVENTION The invention can be implemented in a powder useful in the selective laser sintering which is a compound of multiple constituents. In accordance with the preferred embodiment of the invention, a constituent of the composite powder is a semicrystalline powder such as nylon 11; the other powder constituent is a reinforcing material, such as glass, having an average particle size that is somewhat smaller than the particle size of the semicrystalline powder. The composite powder is formed of a mixture of, for example, approximately equal percentages by weight of these two constituents, with the semicrystalline material having a sintering temperature considerably lower than the reinforcing material. The use of this composite powder in the selective laser sintering provides an improved process window along with reduced distortion of the produced part, easier abrupt detachment and finishing ability or improved part finishing. The semicrystalline polymer results in the part that is almost completely dense. Alternatively, the lower temperature constituent may be an amorphous polymer, and a more porous finished part will be produced.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a selective laser sintering apparatus for producing three-dimensional objects of a powder in the form of layers, with which the preferred embodiment of the invention can be put into practice. Figure 2 is a micrograph of a composite powder according to the first embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION An apparatus for carrying out selective laser sintering according to the present invention will first be described, in relation to the simplified schematic representation illustrated in Figure 1. The preferred apparatus for performing selective laser sintering in accordance with the present invention is the SI TERSTATION® 2000 selective laser sintering system which can be obtained from DTM Corporation, to which generally corresponds the simplified schematic representation of Figure 1. The apparatus of the schematic representation of Figure 1 includes a camera 2 (doors fronts and top of chamber 2 not shown in Figure 1 for reasons of clarity) within which the selective sintering process is carried out. White surface 4, for purposes of the present disclosure, refers to the upper surface of the thermally fusible powder (including portions sintered previously if present) placed in the piston 6 of the part; the sintered and unsintered powder placed in the piston 6 of the piece will be referred to herein as bed 7 of the piece. The vertical movement of the piston 6 of the piece is controlled by the motor 8. The laser 10 provides a beam that is reflected by mirrors 12 controlled by a galvanometer (only one of which is shown for reasons of clarity) in the manner in which it is described in the North American Patents to which reference has been made in the foregoing. Referring again to Figure 1, the supply of the thermally fusible powder is achieved in the apparatus of Figure 1 through the powder piston 14, controlled by the engine 16, by the counter-rotating roller 18. As described in U.S. Patent No. 5,017,753 to which reference has been made above, the counter-rotating roller 18 transfers the raised dust over the floor of the chamber 2 to the target surface 4 in a uniform and level manner . As described in US Pat. No. 5,252,264 to which reference has been made above, it is preferred to provide two powder pistons 14 on either side of the piston 6 of the part, for efficient and flexible powder delivery purposes. The control of the thermal conditions on the surface 4 of white has been observed to be important to avoid the effects of distortion, such as curvature and growth, to which reference is made in the foregoing. In an apparatus such as that shown in Figure 1, preferred techniques for controlling these thermal conditions include the downward flow of a temperature controlled gas (e.g., nitrogen) through the target surface 4, such as it is described in U.S. Patent Number 5,017,753 incorporated hereinbefore. In addition, radiant heaters may also be preferably used in order to uniformly raise the temperature of the target surface 4 to a desired temperature, as described in US Pat. No. 5,155,321 previously incorporated. As described herein, heating the powder on the target surface reduces the thermal gradients (ie, the thermal "shock") on the target surface, which occurs when a subsequent layer of the powder is applied to the newly coated layer. sintered; these thermal gradients, if excessive, can cause the previous layer to bend or otherwise cope. As described in U.S. Patent No. 5,342,919 incorporated hereinbefore, semicrystalline materials such as nylon 11 have been used in the production of parts by selective laser sintering. An example of the conventional nylon powder 11 that is particularly well suited for selective laser sintering is the LASERITE® LNF5000 nylon composite obtainable from DTM Corporation, which has been found to be especially beneficial in the production of selective laser sintering. of the pieces "almost completely dense". The term "almost completely dense" means for the purposes of this description, that the produced piece imitates the flexural modulus and the maximum effort to the limit (kilograms per square centimeter) that it would have if it were completely dense (ie, as if it would have been isotropically molded). According to the preferred embodiment of the invention, the thermally fusible powder used in the apparatus of Figure 1 is a composite powder, namely a mixture of powder powder of a polymer powder and a reinforcing powder. The polymer powder has a lower melting or softening temperature than the reinforcing powder, so that the application of laser energy to the composite powder will cause the particles of the polymer powder to bind to each other and the particles of the powder reinforcement, without causing any fusion or significant change in the phase of the reinforcing particles. As mentioned above, this powder is a "dry-blended" powder in such a manner that the individual particles of each of the polymer powder and the reinforcing powder are freely separated from and do not combine with each other. According to the preferred embodiment of the invention, the polymer powder is preferably a semicritaline polymer of a type which provides signs of a critaline order under X-ray examination, and which shows a defined crystalline Tm melting temperature as well as a temperature Tg of vitreous state transition. Examples of semicrystalline polymeric powder materials useful in connection with the preferred embodiment of the invention include nylon, polybutylene terephthalate (PBT) and polyacetal (PA). As described in the copending application incorporated in the above Serial No. 08 / 298,076, materials such as polyacetal, polypropylene, polyethylene and ionomers, may alternatively be used as a semicritaline polymeric constituent of the composite powder in accordance with the present invention. The preferred semicrystalline powder material according to this embodiment of the invention is a nylon powder 11, wherein the average particle size is in the order of 50 microns. Still more preferably, the nylon constituent 11 of the powder according to the preferred embodiment of the invention is an unmixed polymer having a differential scanning calorimetry (DSC) fusion peak that does not overlap with its recrystallization peak. DSC, when measured at a scanning regime of 10 ° C to 20 ° C per minute, a crystallinity within the range of 10 percent to 90 percent (measured by DSC), and an average number-average molecular weight within the range of about 30,000 to 500,000 and a molecular weight distribution Mw / Mn within the scale of 1 to 5. An additional detail related to the composition and attributes of the constituent of the semicrystalline powder of the composite powder according to the embodiment present of the invention is disclosed in the co-pending US application Serial No. 08 / 298,076 filed on August 30, 1994, assigned to DTM Corporation and incorporated in the present through this reference. As mentioned above, an example of this nylon powder 11 is the LASERITE® nylon compound LNF5000 which can be obtained from DTM Corporation. The melting temperature of the nylon powder 11 according to the preferred embodiment of the invention, is approximately 186 ° C. The reinforcing powder constituent of the composite powder according to the preferred embodiment of the invention is preferably a glass powder consisting of glass microspheres (ie, particles essentially having a spherical shape), having an average particle size inside. of the order of 35 microns. Although the preferred glass is a glass powder A that can be obtained from Potters Industries Inc., it is believed that the specific glass composition is not critical since other types of glass can be used. The glass glass microspheres preferably have a coating that is compatible with the chemistry of nylon to provide good wetting and adhesion. An example of this coating is an amino-functional silane. The melting temperature of the glass microspheres according to this embodiment of the invention is in the order of 704 ° C. The composite powder of the preferred embodiment of the present invention is, as mentioned above, a combined mixture of the nylon powder 11 described above with the glass microspheres. Preferably, the composition of the blended mixture is from 50 percent to 90 percent by weight of the nylon powder 11 described above, with from 10 percent to 50 percent by weight of the coated glass microspheres described above. . The weight percentage of the glass powder is limited by the packing limitations of the glass microspheres, and the ability of the material at low temperature (e.g., nylon powder) to reliably adhere to the compound in a mass when sinterize; on the other hand, if less than 10 weight percent of the glass reinforcing material is used the amount of the reinforcing material is so small as to provide little advantage. The specific scale of the percentage of the composition of the constituents will depend to a certain degree on the particle size of the reinforcing powder. The composite powder can be produced through the use of conventional mixing equipment such as a conventional V mixer. A particularly beneficial example of the powder compounded in accordance with the preferred embodiment of the invention is 50 weight percent of the nylon powder 11 described above (50 micron average particle size) and 50 weight percent of the coated glass microspheres. (average particle size of 35 microns); this composition has been found to provide excellent total dimensional prediction capacity (ie, uniform and isotropic shrinkage) when subjected to the selective laser sintering process in combination with excellent mechanical properties such as stiffness and strength. The isotropic nature of the shrinkage behavior of the sintered composite powder according to this preferred embodiment of the invention is believed to be due to the essentially spherical configuration of the particles of the reinforcing material in the composite powder. It will be noted that this example essentially maximizes the amount of glass reinforcing material in the composite, while providing excellent adhesion of the sintered powder. Figure 2 is a micrograph of this exemplary composite powder, and its non-sintered condition. In Figure 2, the spherical shaped bodies are the glass microspheres, while the irregularly shaped bodies are the nylon particles 11. The variations in the specific percentage composition of the composite powder within the scales specified above, They can be beneficial for specific applications. Alternatively, other materials such as the polymeric constituent in the composite powder may be used, if the part does not need to be formed to be almost completely dense. These materials include certain amorphous polymers such as polycarbonate, polystyrene, acrylates and styrene / acrylate copolymers, which can serve as the polymeric constituent of the composite powder in those cases where a porous piece is desired. In addition, in the alternative, it is also proposed that other discontinuous organic or inorganic reinforcing materials be used in the composite powder. During operation in accordance with the present invention, the apparatus of Figure 1 supplies the composite powder to the chamber 2 through the powder cylinder 14; the composite powder is placed inside the chamber 2 by a partial upward movement of the powder cylinder 14 provided by the motor 16. The roller 18 (preferably provided with a scraper to prevent accumulation, having not been shown in Figure 1 on scraper for reasons of clarity) disperses the composite powder within the chamber by transfer from the powder cylinder 14 towards and through the blank surface 4 on the surface of the bed 7 of the piece above the piston 6 of the piece, the manner described in U.S. Patent Number 5,017,753 which has been referenced above and U.S. Patent Number 5,252,264. At the time when the roller 18 is providing the composite powder from the powder piston 14, the target surface 4 (whether or not an anterior layer is placed thereon) is preferably below the floor of the chamber 2 by a small amount, for example of .102 millimeter, defining the thickness of the powder layer to be processed. It is preferred, for uniform and complete distribution of the composite powder, that the amount of the composite powder that is provided by the powder cylinder 14 be greater than that which can be accepted by the cylinder 6 of the piece, so that a certain excess of Roll movement powder 18 through surface 4 of white; this can be achieved by the upward movement of the piston 14 of the powder by means of a greater amount than the distance below the floor of the chamber 2 where the surface 4 of white is placed (e.g., .254 millimeter versus .102 millimeter). It is also preferred to subordinate the counter-rotation of the roller 18 to the movement of the roller 18 inside the chamber 2, so that the ratio of the speed of rotation to the speed of movement is constant. Further, during operation, after the transfer of the composite powder to the surface 4 of the target, and the return of the roller 18 to its original position near the powder piston 14, the laser 10 (e.g., a CO2 laser) ) selectively sinters the portions of the composite powder on the target surface 4 corresponding to the cross section of the layer of the piece to be produced, in the manner that has been described in the North American Patents to which reference has been made in the foregoing. A particularly beneficial method for controlling the sintering mechanism of the thermal selective laser by controlling the scanning of the laser beam is disclosed in U.S. Patent No. 5,352,405 incorporated hereinbefore. After completing the selective sintering of the specific layer of the composite powder, the piston 6 of the piece moves downwards by an amount corresponding to the thickness of the next layer, waiting for the deposition of the next layer of the powder composed of the roller 18 which is going to be added to the bed 7 of the piece. As mentioned above, the thermal parameters within the selective laser sintering apparatus are of importance in the production of the part. For the example where the composite powder is a 50/50 nylon 11 (percent by weight) and the glass microspheres coated in accordance with the preferred example of the invention described above, the nominal operating parameters used for producing the parts in a selective laser sintering system of SINTERSTATION® 2000, obtainable from DTM Corporation, are the following: feeding temperature: 110 ° C bed temperature of the piece: 190 ° C CO2 laser powder: 3 watts Current flow rate from 5 to 10 descending: liters / min The process continues until the part to be produced is completed, after which the part and the surrounding non-molten composite powder are removed from the apparatus, the non-molten composite powder is then removed from the part at another station (a process which is commonly referred to as "abrupt detachment"). In accordance with the preferred embodiment of the present invention which will be described below, when the piece is formed of a polymer-based composite powder, the process is completed by uniformly finishing the piece by means of sanding or a similar operation, In order to obtain the desired surface finish. Parts have been produced through selective laser sintering of the composite powder of nylon 11 and glass microspheres described above in accordance with the preferred embodiment of the invention. The micrographs of these parts have shown that the resulting parts are almost completely dense in the manner described in US Patent Number 5,342,919 to which reference has been made above and Copending Application Number 08 / 298,076. In addition, the incorporation of the glass microsphere reinforcement material has been found to increase in part the stiffness and heat resistance, while reducing the ductility of the non-reinforced nylon parts. The following table lists measured attributes of the sintered parts produced from the composite powder in accordance with the preferred embodiment of the invention, and for similar parts produced from the non-reinforced nylon composite LASERITE® LNP5000: Table Property Compound Powder Compound LASERITE® DTUL (0.45 MPa) 188 ° C 163 ° C DTUL (1.82 MPa) 134 ° C 44 ° C Stress Resistance (to the limit) 49 MPa 36 MPa Voltage Module 2828 MPa 1400 MPa Modulus of Bending 4330 MPa 870 MPa For the purposes of this table, DTUL measurements were made in accordance with Test Method Number D648 of the American Society for Testing Materials, stress and modulus measurements were made in accordance with Test Method Number D638 of the American Society for the Materials Test, and the flexural modulus measurements were made in accordance with Test Method Number D790 of the American Society for the Testing of Materials. It should be noted that, as expected, the impact strength and elongation at break of the formed parts of the composite powder are somewhat lower than those of the non-reinforced nylon powder. In addition, it has been observed that several extremely important and unexpected advantages of using the powder in accordance with the preferred embodiment of the invention have been raised in the sintering of the selective laser. First, the use of the composite powder in accordance with the preferred embodiment of the invention has been observed to allow a wider processing window, measured in temperature than in the case of non-reinforced nylon powder 11. Specifically, it has been observed that a powder composed of 50 weight percent of the nylon powder 11 with an average particle size of 50 microns and 50 weight percent of the glass microspheres coated with an average particle size of 35 microns can be distributed above the surface 4 of white (Figure 1) at a temperature up to 10 ° C higher than the temperature at which the essentially pure nylon 11 can be distributed; The limit of these temperatures is the so-called agglomeration temperature, which is the temperature at which dust particles begin to weakly adhere to one another. The ability to distribute at a higher powder temperature not only reduces the thermal gradients discussed above by allowing a powder to be distributed at a higher temperature above the newly sintered layer, but allows the temperature of the bed 7 of the piece to be lower for the composite powder in an order of 2 ° C to 4 ° C, relative to the non-reinforced nylon powder. Also, it has been found that a lower laser power can be used to selectively synthesize the composite powder of the preferred embodiment of the invention from that required to sinter the non-reinforced nylon powder 11; for example, it has been observed that 2 to 4 watts less laser power can be used to sinter the composite powder relative to the non-reinforced nylon powder 11. This reduction in laser power reduces the thermal variability in the molten cross section of the powder, as well as the temperature difference between the laser irradiated powder and the adjacent non-sintered powder on the target surface 4; Both of these effects serve to reduce the incidence of curvature or warping of the piece that is being produced. Since the difference between the temperature of the compound powder that is distributed and the temperature of the bed 7 of the piece is reduced when the composite powder is used in accordance with the preferred embodiment of the invention, the present invention reduces the incidence of warping of the piece. integral. In addition, the amount of curvature observed for pieces made of composite powder is about half the curvature observed for similar constructed pieces of unrefined nylon powder 11; likewise, the linear shrinkage observed for the composite powder when sintering is about 3 percent as opposed to the 4 percent linear shrinkage observed for the unreinforced powder. It is believed that the reduction in linear shrinkage for the composite powder according to the preferred embodiment of the present invention reduces the tendency of the sintered powder to develop stresses and warpage during cooling from the production temperature to the release temperature. These attributes allow a wider process window for the temperature of the bed 7 of the part in the apparatus of Figure 1, when the composite powder is used in accordance with the preferred embodiment of the invention. For example, the use of the above described composite powder has been observed to allow the temperature of the bed 7 of the piece to vary through the scale of 3 to 4 degrees centigrade; in contrast, the use of a pure nylon powder 11 (i.e., unreinforced powder) allows a process window for the temperature of the bed 7 of the piece of only about 1 ° C. It has been observed that the parts formed by the sintering of the selective laser of the composite powder according to the preferred embodiment of the invention are easier to detach from the non-sintered powder than the formed parts of the selective laser sintering of the nylon powder 11 do not reinforced. This is believed to be due to the reduced incidence of undesired growth (ie, sintering of the powder from the outside of the laser scan to the scanned portions) that is observed for pieces formed by sintering the selective laser of the composite powder, according to the present invention, with respect to the pieces formed from the non-reinforced nylon powder 11. It is believed that several factors are responsible for the reduction in growth. First, the presence of reinforcing glass particles in the composite material reduces the amount of sinterable material at specific temperature conditions, thereby reducing the degree of any growth by reducing the available sinterable material. In addition, the lower temperature of the bed of the piece and the lower laser energy that can be used in relation to the composite powder of the preferred embodiment of the invention is also believed to contribute to this reduction in undesirable growth. Another unexpected advantage of the preferred embodiment of the invention relates to the finishing ability of the piece produced by sintering the selective laser of the composite powder. As is well known in the art, the pieces produced by selective laser sintering are usually finished or finished smooth, for example, by sanding, after peeling from the non-sintered powder. The pieces produced from the composite powder described above, have been found to be easier to finish or finish in this way, requiring an order of half the time of sanding and effort for finishing, in relation to pieces formed of nylon not reinforced. Even though the invention has been described here in relation to its preferred embodiment, it is of course proposed that the modalities of and alternatives to this modality, with these modifications and alternatives obtaining the advantages and benefits of the invention, will be evident to those persons with knowledge of the technique that have reference to this specification and its drawings. It is proposed that those modifications and alternatives be within the scope of the invention as claimed hereinafter.

Claims

R E I V I N D I C A C I O N E S:

1. A method for producing a three-dimensional object comprising the steps of: applying a layer of a composite powder to a target surface, the composite powder comprises: from about 50 percent to about 90 percent by weight of a semi-crystalline polymer powder having a melting peak and a recrystallization peak, as shown in the differential scanning calorimetry traces, which do not overlap when measured at a scanning rate of 10 ° C to 20 ° C per minute- of about 10 percent to 50 weight percent of a reinforcing powder, dry blended with the polymer powder, and having a melting temperature considerably higher than the melting temperature of the polymer powder; directing the energy at selected locations of the layer corresponding to a cross section of the object to be formed in that layer, to melt the powder composed therein; repeat the steps of application and direction to form the object as layers; and remove the unmelted dust from the object.

2. The method of claim 1, wherein the composite powder contains about 50 percent by weight of the polymer powder and about 50 percent by weight of the reinforcing powder. The method of claim 2, wherein the polymer powder has an average particle size that is greater than the average particle size of the reinforcing powder. 4. The method of claim 1, wherein the reinforcing powder comprises glass. The method of claim 4, wherein the reinforcing powder comprises essentially spherical glass particles. 6. The method according to claim 5, wherein the virio particles are coated. The method of claim 15, wherein the polymer powder comprises a polymer selected from the group consisting of nylon, polybutylene terephthalate, polyacetal, polypropylene, polyethylene and ionomers. The method of claim 2, wherein the polymer powder comprises nylon 11. The method of claim 16, wherein the polymer powder has an average particle size that is greater than the average particle size of the powder reinforcement. SUMMARY OF THE INVENTION A composite powder specially adapted for use in selective laser sintering is disclosed. The composite powder includes a polymer powder dry blended with a reinforcing powder, wherein the polymer powder has a substantially lower melting temperature than that of the reinforcing powder. In the case where almost completely dense pieces are to be formed, the first constituent powder is preferably a semicrystalline powder, for example, nylon 11 of a suitable composition to form almost completely dense pieces when used without combining in the selective laser sintering; if porous pieces are desired, the polymeric powder is an amorphous powder such as polycarbonate, polystyrene, acrylates, styrene / acrylate copolymers. The reinforcing powder preferably consists of glass microspheres, preferably coated to improve the wetting and adhesion with the polymer powder when selective laser sintering is carried out. In addition to improving the rigidity and heat resistance of the piece produced, the composite powder expands the process window in relation to that provided by the non-combined powder, provides improved dimensional accuracy in the produced part and facilitates abrupt detachment and Smooth or even finish of the production piece.