Sinterable semi-crystalline powder and near-fully dense article formed therewith

#This invention is in the field of rapid prototyping, and is more particularly directed to materials for producing prototype parts by way of selective laser sintering.

#This invention relates to a synthetic resinous powder product to be laser-sintered in a selective laser sintering machine, such as a SINTERSTATION 2000 system manufactured and sold by DTM Corporation. The laser-sinterable powder (referred to as "sinterable powder" herein) is "designed" or "tailored" to incorporate specific physical properties uniquely adapted to form a bed (of powder) upon which a sintering laser in the infra-red region is directed.

#Prior art sinterable powders are unable to yield a sintered part which, for most purposes, appears to be a duplicate of one which is isotropically molded. Moreover, conventional sinterable powders form a bed which generally lacks the ability to provide the exigent heat transfer characteristics which determine whether a sintered part will be distorted, even if it is successfully completed. Since a layer of particles typically rolled out of the feed bed and onto the part bed of a selective laser sintering machine, is about 8 mils (200 .mu.m) such powders used had a maximum particle diameter which was less than 200 .mu.m and whatever "fines" were generated in the course of grinding the powder to the desired mesh size, irrespective of the distribution of particle sizes in the powder.

#The foregoing and additional objects and advantages of the invention will best be understood by reference to the following detailed description, accompanied with schematic illustrations of preferred embodiments of the invention, in which illustrations like reference numerals refer to like elements, and in which:

#FIG. 1 is a schematic illustration of a bed of uniform spheres packed in a bed.

#FIG. 2 is a schematic illustration of a bed of large spheres and very small ("too-small") spheres, showing that the too-small particles fit within the interstitial spaces between larger particles, and produce a bed of higher bulk density and correspondingly higher pressure drop.

#The selective laser sintering machine specifically referred to herein uses a 10.6 .mu.m CO.sub.2 laser, but any other infra-red laser-generating source may be used, as well as excimer lasers and neodymium glass lasers which generate in the near-infra-red. A preferred laser is a Synrad Model C48-115 CO.sub.2 laser. Powders are sintered using a 10.6 .mu.m laser in the range from 3 to 30 watts operated at a ratio of power/scan speed in the range from 0.075 watts/cm/sec to 0.3 watts/cm/sec, preferably in the range from 0.1-0.2 watts/cm/sec using a beam width in the range from 0.23-0.9 mm, preferably from 0.4-0.6 mm. Particularly with Nylons 6, 11 and 12, PBT and PA, the selective laser sintering machine is preferably operated at a fluence in the range from 1 mJ/mm.sup.2 to 100 mJ/mm.sup.2, more preferably from 15 mJ/mm.sup.2 to 45 mJ/mm.sup.2, where fluence (milliJoules/mm.sup.2) is the measure of the energy of the laser beam delivered to a defined area of powder. The laser is typically operated with a beam width of 0.6 mm and a power ranging from 3-22 watts, preferably from 5-10 watts, at a scanning speed ranging from about 76.2 cm/sec to 178 cm/sec.

#Among the requirements of a preferred semi-crystalline or substantially crystalline powder uniquely tailored to yield, when sintered, a porous but near-fully dense article, are the following:

#Free-flowing or Non-caking: The powder is freely flowable and does not cake when stored for up to 24 hr, at a temperature in the range from 2.degree. C. to 20.degree. C. below its caking temperature; in particular, the powder does not cake even when held from 1-8 hr in the part bed at T.sub.s, at a slightly higher temperature than its storage temperature in the feed bed which latter is lower than T.sub.s of the powder. A determination as to whether a powder will meet the free-flowability requirement is made by running the time-temperature ASTM flow test referred to hereinabove.