BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the lamina-by-lamina formation of three- dimensional objects through application of the principles of stereolithography, and more specifically to enhancements in the formation of layers of an unsolidified material in preparation for formation of successive laminae of the three-dimensional object.

2. Description of Related Art Stereolithography is one of three classes of technologies presently commercially used in the Rapid Prototyping and Manufacturing (RP&M) industry. These technologies are described in a number of references described herein.

Stereolithography is used to create three-dimensional objects based on the successive formation of layers of a fluid-like medium adjacent to previously formed layers of the medium and the selective solidification of those successive layers to form and adhere successive laminae of the three-dimensional object. Each layer is selectively exposed to prescribed stimulation according to cross-sectional data representing successive slices of the three- dimensional object.

One specific stereolithography technology is known simply as stereolithography and it uses a liquid medium that is selectively solidified by exposing it to prescribed stimulation. The liquid medium is typically a photopolymer and the prescribed stimulation is typically visible or ultraviolet electromagnetic radiation. Liquid-based stereolithography is disclosed in various patents, applications, and publications of which a number are briefly described in the Related Patents, Applications and Publications section hereinafter. Another stereolithography technology is known as Selective Laser Sintering (SLS). SLS is based on the selective solidification of layers of a powdered medium by exposing the layers to infrared electromagnetic radiation to sinter or fuse the particles. SLS is described in US Patent No.

4,863,538 to Deckard. A third stereolithography technology is known as Three Dimensional Printing (3DP). 3DP is based on the selective solidification of layers of a powdered medium which are solidified by the selective deposition of a binder thereon. 3DP is described in US Patent No. 5. 204,055 to Sachs.

The present invention is primarily directed to stereolithography using liquid-based building materials or medium. The present invention presents techniques for improving the accuracy of the layer formation process (i. e. the coating process) and/or improving the formation time of those layers.

Over the years various techniques and devices have been proposed to enhance the coating process in stereolithography. One of these improvements has been called deep dipping and was disclosed in US patent 4,575,330. Subsequent techniques included the use of various devices that would be swept at or somewhat above a desired working surface of the building material to deposit material, spread material and/or strike off excess material. These devices included rigid doctor blades, flexible doctor blades, rakes, brushes, films, counter rotating rollers, and the like. Other devices included various dispensing devices such as slot coaters (i. e. curtain coaters) and apertured coaters. Some of these coating devices intentionally dispensed excess material with the excess portion to be swept off while others attempted to dispense exactly the amount of material needed to form a layer. Some dispensers and spreaders were used in combination with one another. Various parameters associated with these devices were made controllable to improve coating characteristics. For example, the speed of sweeping a spreader or dispenser was made controllable. The vertical spacing from the bottom of the coating device to the working surface (commonly known as"blade gap") was made controllable. The vertical spacing from the bottom of the coating device to the last formed lamina (commonly known as"blade clearance") was made controllable. The number of sweeps of the coating device was made controllable. Even though many coating devices were proposed and parameters controlled, problems continued to exist with regard to the quality of the coating formed and/or the time it took to form coatings. These problems were especially acute when attempting to coat over trapped volume regions. Trapped volumes are regions containing unsolidified material, which are effectively bounded from below and on the sides by transformed material. The unsolidified material within the trapped volume is effectively isolated from unsolidified material outside the trapped volume such that isolated regions do not quickly come to an equilibrium state where a common surface level is attained for each region. A region may be defined as an effective trapped volume based on the time it takes material in the trapped volume to transition from a first level to a second level which matches the level of the material outside the trapped volume. If the time is greater than some desired time, the region may be considered to be a trapped volume. The desired time may be on the order of the time to be spent during the coating process (e. g. less than 60,20,10,5 or even 2 seconds). Trapped volumes are further discussed in US Patent No. 5,174,931, and US Patent Application Nos. 08/854,950 and 08/790,005. Thus a need still remained for enhanced

techniques that could form more accurate coatings in less time.

In recent years, further advances to coating techniques have been made. Techniques have been proposed that limit the sweeping of coating devices to limited regions of the working surface. These limited portions of the working surface are in one way or another determined based on the maximum extents of the object being formed or on the maximum extents of groups of object cross-sections. These techniques speed up the coating process by removing the time that was wasted in causing the coating device to traverse regions that did not require coating. Other techniques have been proposed that control the coating device to sweep at a high speed when away from the exposure region (which is utilized in forming a specific object) while controlling the coating device to sweep at a slower, more appropriate, speed when over the exposure region. These so called"smart"sweeping techniques are described in US Patent Application No. 08/790,005 to Almquist et al. and Japanese Patent Publication 8-15760 B2 to Narukawa et al.

Other advances have included the ability to automatically recognize geometric features of the objects as they are being formed. Such techniques are described in US Patent Application No. 08/854,950. It has been proposed that these techniques be used in automatically controlling various coating parameters to allow optimization of the coating process in an object geometry dependent manner.

Further advancements have involved the use of vacuum forces or electrostatic forces to draw unsolidified material from the working surface into an applicator. These devices, and the like, might be called vacuum applicators, electrostatic applicators, or more generically, balanced flow applicators since they extract material from the same working surface onto which they are to form coatings. More particularly, these balanced flow applicators tend to continuously extract material from the working surface even during the process of applying material to the working surface. Unlike other applicators or dispensers, these balanced flow devices do not typically continue to dispense material when it is not needed, instead they tend to release material only as needed. These devices, in particular the vacuum applicators, have been demonstrated to be much more effective at forming uniform coatings over all object geometries than most, if not all, other coating devices commercially utilized. Unfortunately, this device may tend to release too little material when sweeping over liquid regions and/or may tend to drag or push material out of these regions. Such liquid regions occur within trapped volumes and are problematic since, by definition, trapped volumes are regions of liquid material that are effectively isolated from other regions. Thus, if too little material is deposited by the applicator within a trapped volume region or if material is removed from the region, the region tends to become depressed in height as successive layers are formed. Thus,

even with the advantages of the balanced flow applicators. a need remains for enhancements to improve the coating accuracy attainable from the overall coating process without significantly impacting the overall time required to form the coatings. A vacuum applicator is disclosed in Figures 9a-9k of US Patent Application No. 08/790,005 while an electrostatic applicator is described in WO 95/25003 to Fockele et al.

As with doctor blades, positive flow dispensers, and the like, commercially used balanced applicators still suffer from a problem known to those of skill in the art as"leading edge bulge". Leading edge bulge is a phenomenon that causes a bulge in coating thickness to occur over a coated portion of a previously formed lamina. The previously formed lamina may be the immediately preceding lamina or may be one of the other laminae preceding it. The bulge is typically formed about 2-12 mm from the initial edge of the previously formed lamina that is encountered by the smoothing device when sweeping. The bulge may initially be mm in height, or more, and mm in width, or more. Over time the bulge decreases in height and increases in width. Unfortunately, an excessive amount of time is required to make the leading edge bulge recede in thickness to a desired level.

Other phenomena, such as scoop out, drag out, and under flow, are believed to exist when using various smoothing devices including balanced applicators. Scoop out or drag out is a phenomenon where the movement of the applicator/smoothing device tends to remove material from regions that are swept over. Underflow is a phenomenon that occurs when a height difference exists in front of and behind the applicator/smoothing device. Material can tend to flow from the region having the higher level to the region having a lower level.

3. Additional Related Patents, Applications and Publications The patents and applications in the following table are hereby incorporated by reference herein as if set forth in full. The gist of each patent and application is included in the table to aid the reader in correlating specific types of teachings to specific references. It is not intended that the incorporation of subject matter be limited to those topics specifically indicated, but instead the incorporation is to include all subject matter found in these applications and patents. The teachings in these incorporated references can be combined with the explicit teachings described herein in many ways to derive enhanced aspects of the instant invention. Such combinations will be apparent to those of skill in the art in view of the teachings herein. For example, the coating techniques described herein may be used in combination with various coating techniques and devices described in these references. Data manipulation techniques described in these references may be utilized in combination with the enhanced coating techniques of the instant invention to derive enhanced inventions related to partial or even full automation of the coating techniques of the instant invention. As a further example, the various hardware configurations disclosed in these references may be used in conjunction with the specific embodiments disclosed herein.

Table 1: Related Patents and Applications Patent No./Inventor Subject AppiicationNo. 4,575,330 Hull Discloses fundamental elements of stereolithography. 4,999,143 Hull, et al. Discloses various removable support structures a licable to stereolithography. 5, 058, 988 Spence Discloses the application of beam profiling techniques useful in stereolithography for determining cure depth and scanning speed, etc. 5,059,021 Spence, et al. Discloses the utilization of drift correction techniques for eliminating errors in beam positioning resulting from instabilities in the beam scanning system 5,076,974 Modrek, et al. Discloses techniques for post processing objects formed by stereolithography. In particular exposure techniques are described that complete the solidification of the building material. Other post processing steps are also disclosed such as steps of filling in or sanding off surfacediscontinuities. 5, 104, 592 Hull Discloses various techniques for reducing distortion, and particularly curl type distortion, in objects being formed by stereolithography. 5, 123, 734 Spence, et al. Discloses techniques for calibrating a scanning system. In particular techniques for mapping from rotational mirror coordinates to planar target surface coordinates are disclosed. 5,133 987 Spence et al. Discloses the use of a stationary mirror located on an optical path between the scanning mirrors and the target surface to fold the optical path in a stereolithography system. 5,174,931 Almquist, et al. Discloses various doctor blade configurations for use in forming coatings of medium adjacent to previously solidifiedlaminae. Discloses the use of multiple wavelengths in the 5,182,056 Spence, et al. exposure of a stereolithographic medium. 5,182, 715 Vorgitch et al. Discloses various elements of a large stereolithographic system. 5,184,307 Hull, et al. Discloses a program called Slice and various techniques from app. no. for converting three-dimensional object data into data 07/331, 644 descriptive of cross-sections. Disclosed techniques include line width compensation techniques (erosion routines), and object sizing techniques. The application giving rise to this patent included a number of appendices that provide further details regarding stereolithography methods and systems. 5,209,878 Smalley, et al. Discloses various techniques for reducing surface discontinuities between successive cross-sections resulting from a layer-by-layer building technique. Disclosed techniques include use of fill layers and meniscus smoothing. 5, 234, 636 Hull, et al. Discloses techniques for reducing surface discontinuities by coating a formed object with a material, heating the material to cause it to become flowable, and allowing surface tension to smooth the coating over the object surface. Discloses a technique for minimizing curl distortion byb 5,238,639 Vinson, et al. balancing upward curl to downward curl. 5,256,340 and Allison, et al. Discloses various build/exposure styles for forming 08/766,956 objects including various techniques for reducing object distortion. Disclosed techniques include: (1) building hollow, partially hollow, and solid objects, (2) achieving more uniform cure depth, (3) exposing layers as a series of separated tiles or bullets, (4) using alternate sequencing exposure patterns from layer to layer, (5) using staggered or offset vectors from layer to layer, and (6) using one or more overlapping exposure patterns per layer. 5 321,622 Snead, et al. Discloses a computer program known as CSlice which is used to convert three-dimensional object data into cross-sectional data. Disclosed techniques include the use of various Boolean operations in stereolithography. 5,597,520 and Smalley, et al. Discloses various exposure techniques for enhancing 08/428,951 object formation accuracy. Disclosed techniques address formation of high resolution objects from building materials that have a Minimum Solidification Depth greater than one layer thickness and/or a Minimum Coating Depth greater than the desired object resolution. 08/722,335 Thayer, et al. Discloses build and support styles for use in a Multi-Jet Modeling selective deposition modeling system. 08/722,326 Earl, et al. Discloses data manipulation and system control techniques for use in a Multi-Jet Modeling selective deposition modeling system. 08/790,005 Almquist. et al. Discloses various coating techniques for use in stereolithography. Disclosed techniques include 1) an ink jet dispensing device, 2) a fling coater, 3) a vacuum applicator, 4) a stream coater, 5) a counter rotating roller coater, and 6) a technique for deriving sweep extents. 08/792, 347 Partanen, et al. Discloses the application of solid-state lasers to stereolithography. 08/792,347 Partanen, et al. Discloses the use of a pulsed radiation source for solidifying layers of building material and in particular the ability to limit pulse firing locations to only selected target locations on a surface of the medium. 08/855, 125 Nguyen, et al. Discloses techniques for interpolating originally supplied cross-sectional data descriptive of a three- dimensional object to produce modified data descriptive of the three-dimensional object including data descriptive of intermediate regions between the originallyoriginallysupplied cross-sections of data. 08/854, 950 Manners, et al. Discloses techniques for identifying features of partially formed objects. Identifiable features include trapped volumes, effective trapped volumes, and solid features of a specified size. The identified regions can be used in automatically specifying coating parameters and or exposure parameters. Discloses techniques for forming objects utilizing low- 08/920,428 Kruger, et al. resolution materials that are limited by their inability to reliably form coatings less than a Minimum Coating Depth. Coating techniques are described which can be used when the thickness between consecutive layers is less than leading edge bulge phenomena.

The following two books are also incorporated by reference herein as if set forth in full: (1) Rapid Prototyping and Manufacturing: Fundamentals of Stereolithography, by Paul F.

Jacobs; published by the Society of Manufacturing Engineers, Dearborn MI; 1992 ; and (2) Stereolithography and other RP&M Technologies: from Rapid Prototyping to Rapid Tooling; by Paul F. Jacobs ; published by the Society of Manufacturing Engineers, Dearborn MI; 1996.

SUMMARY OF THE DISCLOSURE An object of the present invention is to provide a more accurate coating process and apparatus for stereolithography.

A second object of the invention is to provide a more accurate coating process and apparatus while minimizing the time involved in the formation of layers.

A third object of the invention is to provide a more accurate coating process and apparatus for coating over effective trapped volume regions.

A fourth object of the invention is to provide a desired level of accuracy in forming coatings in effective trapped volume regions while minimizing the time involved in the formation of layers that include these trapped volume regions.

A fifth object of the invention is to provide further automation of the object formation process.

A seventh object of the invention is to limit leading edge bulge accumulation.

A first aspect of the invention provides extra time to form coatings over particular regions of a partially formed object while minimizing time spent coating over other regions of the partially formed object.

A second aspect of the invention provides reduced coating device sweeping speeds when approaching or crossing over at least some liquid to solid transition regions on a previously formed layer/lamina.

A third aspect of the invention provides a method for forming at least a portion of a three-dimensional object, including forming a successive layer of unsolidified building material adjacent to any immediately preceding lamina in preparation for forming a successive lamina of the three-dimensional object. The formation of a successive layer includes sweeping a coating device over the immediately preceding lamina. The successive layers are exposed to prescribed stimulation to form the successive lamina of the three-dimensional object and to adhere the successive lamina to any immediately preceding lamina. The formation and exposure of layers are repeated a plurality of times to form the portion of the three-dimensional object from a plurality of adhered laminae. During formation of at least one of the layers of unsolidified material the coating device is swept over a selected portion and an unselected portion of the immediately preceding lamina wherein the coating device spends a greater average time per unit length over the selected portion than an average time spent per unit length over the unselected portion.

A fourth aspect of the invention provides a method for forming at least a portion of a three-dimensional object, including forming a successive layer of unsolidified building

material adjacent to any immediately preceding lamina in preparation for forming a successive lamina of the three-dimensional object. The formation of a successive layer includes sweeping a coating device over the immediately preceding lamina. The successive layers are exposed to prescribed stimulation to form the successive lamina of the three-dimensional object and to adhere the successive lamina to any immediately preceding lamina. The formation and exposure of layers are repeated a plurality of times to form the portion of the three-dimensional object from a plurality of adhered laminae. During formation of at least one of the layers, the coating device is swept over a selected portion and an unselected portion of the combined extents of the immediately preceding lamina and a plurality of previously formed laminae wherein the coating device spends a greater average amount of time per unit length over the selected portion than an average amount of time spent per unit length over the unselected portion.

A fifth aspect of the invention provides a method for forming at least a portion of a three-dimensional object, including forming a successive layer of unsolidified building material adjacent to any immediately preceding lamina in preparation for forming a successive lamina of the three-dimensional object. The formation of a successive layer includes sweeping a coating device over the immediately preceding lamina. The successive layers are exposed to prescribed stimulation to form the successive lamina of the three-dimensional object and to adhere the successive lamina to any immediately preceding lamina. The formation and exposure of layers are repeated a plurality of times to form the portion of the three-dimensional object from a plurality of adhered laminae. During formation of at least one of the layers, the coating device is swept over the immediately preceding lamina and pauses the sweeping of the coating device while over at least a portion of the immediately preceding lamina.

A sixth aspect of the invention provides a method for forming at least a portion of a three-dimensional object, including forming a successive layer of unsolidified building material adjacent to any immediately preceding lamina in preparation for forming a successive lamina of the three-dimensional object. The formation of a successive layer includes sweeping a coating device over the immediately preceding lamina. The successive layers are exposed to prescribed stimulation to form the successive lamina of the three-dimensional object and to adhere the successive lamina to any immediately preceding lamina. The formation and exposure of layers are repeated a plurality of times to form the portion of the three-dimensional object from a plurality of adhered laminae. During formation of at least one of the layers, the coating device is swept over a range including at least a minimum extent and a maximum extent of the immediately preceding lamina and wherein a selected portion of the range is swept with a larger number of sweeps than an unselected portion of the range.

A seventh embodiment of the invention provides a method for forming at least a portion of a three-dimensional object, including forming a successive layer of unsolidified building material adjacent to any immediately preceding lamina in preparation for forming a successive lamina of the three-dimensional object. The formation of a successive layer includes sweeping a coating device over the immediately preceding lamina. The successive layers are exposed to prescribed stimulation to form the successive lamina of the three- dimensional object and to adhere the successive lamina to any immediately preceding lamina.

The formation and exposure of layers are repeated a plurality of times to form the portion of the three-dimensional object from a plurality of adhered laminae. During formation of at least one of the layers the coating device is swept over a range including at least a minimum and a maximum extent of the immediately preceding lamina and wherein a selected portion of the range which is over the immediately preceding lamina is swept at a slower speed than an unselected portion of the range which is over a different portion of the immediately preceding lamina.

Other aspects of the invention provide apparatus for implementing the methods of the above noted aspects.

A first preferred embodiment of the invention provides a coating device that is swept above an immediately preceding lamina of an object being formed, which lamina includes an effective trapped volume region, where the coating device (e. g. balanced applicator) is made to pause over the effective trapped volume region for a period of time to allow extra material to be transferred into the trapped volume region.

A second preferred embodiment of the invention provides a coating device that is swept above an immediately preceding lamina of an object being formed, which lamina includes an effective trapped volume region, where the coating device (e. g. a balanced applicator) is made to slow down (as compared to a sweeping speed elsewhere over the immediately preceding lamina or over a combination of the immediately preceding lamina and other previously formed laminae) over at least a portion of the effective trapped volume region to allow extra material to be transferred into the trapped volume region.

A third preferred embodiment of the invention provides a coating device that is swept above an immediately preceding lamina of an object being formed, which lamina includes an effective trapped volume region, where the coating device (e. g. balanced applicator) is made to sweep back and forth above at least a portion of the trapped volume region more times than it sweeps over at least some other portions of the immediately preceding lamina or over a combination of the immediately preceding lamina and other previously formed laminae to allow extra material to be transferred in to the effective trapped volume region.

A fourth preferred embodiment of the present invention focuses on minimizing material removal from a trapped volume region rather than focusing specifically on allowing sufficient time to transfer material into a trapped volume region.

A fifth preferred embodiment of the present invention provides a coating device that is swept multiple times over a limited portion of a preceding lamina to reduce coating inaccuracies over a region anticipated to be susceptible to leading edge bulge related coating errors.

Alternatives to the above embodiments may apply the extra coating time techniques (e. g. pausing and/or slowing down and/or extra moment techniques) to other object geometries; such as shallow regions that have restricted flow but which would not typically be considered trapped volumes.

Various alternatives to the above embodiments including combinations thereof are discussed hereafter. Many other alternatives and modifications will be apparent to those of skill in the art upon review of the teachings herein.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 a and 1 b depict a schematic side view of a stereolithography device that stacks laminae in a Z-axis and includes a vacuum applicator coating device which extends in a Y-axis and sweeps along an X-axis.

Figure 2 depicts a schematic side view of a sample object being stereolithographically formed including transition regions along an X-axis that may be considered when setting coating parameters.

Figure 3a depicts a plot of coating-device, sweeping speed versus transition regions along the X-axis where the speed is set to appropriately form coatings over regions solidified on the immediately preceding lamina.

Figure 3b depicts a plot of coating-device, sweeping speed versus transition regions along the X-axis where the speed is set to appropriately form coatings over trapped volume regions.

Figure 3c depicts a plot of coating-device, sweeping speed versus transition regions along the X-axis where the speed is set to appropriately form coatings over both regions solidified on the immediately preceding lamina as well as trapped volume regions where the motion of the coating device is paused over a portion of the trapped volume region to allow extra time for transferring material into the trapped volume.

Figure 4a-4c depict plots of coating device sweeping speeds versus transition regions

along the X-axis, where the speed is varied to optimize coating formation over both regions solidified on the immediately preceding lamina as well as trapped volume regions.

Figure 5 depicts a plot of coating device sweeping speed versus transition regions along the X-axis, where the speed is varied to optimize coating formation over both regions solidified on the immediately preceding lamina as well as trapped volume regions where the motion of the coating device is paused over a portion of the trapped volume region.

Figure 6 depicts a plot of coating device sweeping speed and number of sweeps versus position over the immediately preceding lamina where extra sweeps are used over a portion of the trapped volume region.

Figure 7 depicts a plot of coating-device, sweeping speed and number of sweeps versus position over the immediately preceding lamina where extra sweeps and slower sweeping speed are used over a portion of the trapped volume region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Introduction Figure I a and 1 b depict schematic side view representations of a preferred stereolithography apparatus 1 (SLA) for use with the instant invention. The laminae in Figures 1 a and I b are stacked one on top of the other along a Z-axis and only one horizontal dimension, parallel to the X-axis, is shown. The preferred SLA includes container 3 for holding building material 5 (e. g. photopolymer) from which object 15 will be formed, elevator 7 and elevator driving means (not shown), elevator platform 9, exposure system 11, coating device 13, coating device driving means (not shown), and at least one computer for manipulating object data (as needed) and for controlling the exposure system, elevator, and coating device.

The basic elements of a preferred stereolithography system are described in US Patent No. 4,575,330.

A preferred exposure system 11 includes a source of prescribed stimulation for transforming the building material and a device or system for selectively applying the prescribed stimulation to the working surface. The source of stimulation is typically a laser whose beam is directed into a beam expander. From the beam expander. the beam is directed into a pair of computer controlled XY rotatable scanning mirrors. The laser is preferably a UV laser of the frequency tripled Nd: YV04 type or of the gas AR+ or HeCd type. The rotatable scanning mirrors are preferably of either the motor driven or galvanometer type. Other preferred scanning systems involve the use of flood of radiation in combination with

transmissive or reflective light valves such as LCDs, deformable mirrors and the like. Preferred exposure systems 11 are described in several of the patents and applications referenced above including numbers 5,058,988; 5,059,021; 5,123,734; and 08/792,347.

Preferred control and data manipulation systems and software are described in a number of the patents referenced above, including numbers 5,184,307; 5,321,622; and 5,597,520.

A preferred coating device is a balanced applicator. A vacuum applicator is particularly preferred. This applicator seems to operate as a combination of a dispenser and what may be called a siphon pump. Some material is dispensed from the interior of the applicator onto an immediately preceding lamina. Furthermore, when the applicator is positioned over a given set of locations, it is believed that a flow path is created that can allow transfer of material from locations with a higher surface level to locations with a lower surface level in much the same way a siphon equalizes the surface level in two containers. As noted above, this type of vacuum applicator is described in US Patent Application No. 08/790,005.

This preferred applicator is filled with building material by the application of a slight vacuum to the applicator bar such that building material is drawn into the applicator from the surface (19) or near the surface of the building material 5 held in container 3. This preferred applicator includes coater or applicator bar 13, regulated vacuum pump 17, and vacuum line 19 connecting the bar 13 and the pump 17.

Other highly preferred applicators include those that do not dispense a fixed quantity of material per unit time but instead dispense or remove material on a demand basis. Such applicators include various other embodiments of the vacuum applicator and counter rotating roller embodiments found in the'005 application as well as the electrostatic applicator mentioned above. Preferred coating devices might include methods and apparatus for intentionally changing the amount of material being retained in the applicator (e. g. by varying the vacuum level) so that excess material is removed from the working surface or additional material is added when a need for such control is detected or anticipated.

Other coating devices may also be used in conjunction with the instant invention, including various dispensing devices, spreading, and/or strike off devices known in the art including those found in the'005 application.

Other components of a preferred SLA (not shown) may include a liquid level control system, a build chamber, an environmental control system including a temperature control system, safety interlocks, a viewing device (e. g. a window or monitor and camera system), and the like.

SLAs on which the instant invention can be utilized are available from 3D Systems,

Inc. of Valencia, California. These SLAs include the SLA-250 using a HeCd laser operating at 325 nm, the SLA-3500 and SLA-5000 using a solid state laser operating at 355 nm.

Preferred building materials are photopolymers manufactured by CIBA Specialty Chemicals of Los Angeles, California, and are available from 3D Systems, Inc. These materials include SL 5170 for use with the SLA-250, SL 5190 for use with the SLA-3500, and SL 5195 for use with the SLA-5000.

The typical operation of an SLA involves alternating formation of coatings of material (i. e. layers of material) and the selective solidification of those coatings to form laminae of the object. This alternating process is continued until the object is formed from a plurality of adhered laminae. The process begins with the elevator platform 9 immersed one layer thickness below the upper surface 19 of the photopolymer 5 and having a first coating with a thickness equal to one layer thickness existing above the elevator platform. The first coating of photopolymer is selectively exposed to a beam of prescribed stimulation (e. g. UV radiation) which cures the material to a desired depth to form an initial lamina adhered to the elevator platform. This initial lamina corresponds to an initial cross-section of the object to be formed or corresponds to a support structure that may be used to adhere the object to the platform.

After formation of this initial lamina, the elevator platform and adhered initial lamina are lowered a net amount of one layer thickness into the liquid. As the material is typically very viscous and the thickness of each layer is very thin (e. g. mm), the material typically does not readily form a coating over the last solidified lamina when it is located only one layer thickness below the surface. To form the required coating, a coating device is typically swept at or somewhat above the surface of the resin (i. e. work surface of the material) to aid in the formation of a fresh coating. The coating formation process may involve the sweeping of the coating device one or more times at a desired speed. After formation of this coating, the layer is solidified by a second exposure of the medium to prescribed stimulation according to data representing a second cross-section of the object or second cross-section of a support structure. This process of coating formation and solidification is repeated over and over again to form a plurality of laminae (21,23,25,27,29,31, and 33). This formation process continues until all laminae of the object are formed.

The process of coating formation is illustrated in Figures 1 a and 1 b, where Figure I a depicts the partially formed object 15 after formation of an immediately preceding lamina (labeled as 33) and after it is lowered one layer thickness below the desired working surface of the unsolidified material but before sweeping of coating bar 13 has begun. Figure I b depicts formation of the layer after the coating device has partially swept (from left to right) over the immediately preceding lamina such that a coating has been formed to the left of the coating

device but has not yet been formed to the right. After completion of sweeping so that the entire immediately preceding lamina has been passed over, a short delay may occur to allow gravity and surface tension to smooth out any irregularities in the coating prior to applying the prescribed stimulation to the coating.

A coating device may be swept over the entire working surface of the building material or alternatively it may be swept over a reduced part of the working surface.

Techniques for reduced area or length sweeping are disclosed in the above referenced'005 application. In such techniques, the coating device is swept beyond the regions to be coated and to be exposed in forming the next lamina so that the imperfections in the coating that may exist in proximity to the device do not negatively impact the exposure region and so that the region to be exposed is not in the shadow of the coating device. Additionally, the coating device may sweep an extra increment to provide a safety margin for relative or absolute positioning errors that might be involved in defining the coater position or the exposure position. Furthermore, such techniques may involve the use of encoders, feed back systems, or other devices, manual and/or automatic set up routines to correlate the position of the coater device to the location of the object being formed.

In some building techniques, incomplete solidification of some or all object cross- sections may occur. Alternatively, in some processes an object lamina associated with a given layer (i. e. a lamina whose location should be positioned, relative to the rest of the object, at the level corresponding to that layer of material) may not be exposed or may be only partially exposed in association with that layer (i. e. when that layer is located at the surface of the liquid). Instead, that lamina may be formed in whole or in part in association with a subsequently formed layer wherein the quantity of exposure applied to this subsequent layer causes solidification in the material at the level of the associated lamina. In other words, the layer that is associated with a given lamina may not be the layer in association with which the lamina will be solidified. It may be said that the layer in association with which a lamina or portion of a lamina is formed, is that layer which is located at the surface of material at the time it is solidified; while the layer with which a lamina is associated, is that layer which corresponds to the dimensionally correct location of the lamina relative to the rest of the object.

Issues related to delayed exposure of layers in whole or in part are discussed in more detail in US Patent No. 5,597,520, and US Patent Application Nos. 08/428,951 and 08/920,428.

We next turn our attention to specific preferred embodiments of the instant invention that will be described in view of the preliminary information and background provided above.

The headers associated with the following embodiments are intended to aid with the reading of this disclosure but are not intended to isolate or limit the applicability of the teachings herein to

those individual embodiments in connection with which explicit disclosure is made.

Embodiment 1 Pause Coating in Trapped Volume Regions The first preferred embodiment is directed to enhancing the coating of an immediately preceding lamina that includes an effective trapped volume region. Effective trapped volumes are known in the art and have been described briefly above and in a number of previously referenced patents. In particular, US Patent Application Serial No. 08/854,950 describes effective trapped volumes and how they may be automatically detected. In this first preferred embodiment a vacuum applicator is swept above the surface of an immediately preceding lamina and is made to pause for a predetermined period of time within the confines of the trapped volume region prior to completing its sweeping. Briefly pausing the applicator allows extra time for material to be transferred from the applicator into the trapped volume regions without using a more time consuming approach (e. g. by sweeping the applicator at a slower rate over its entire path) or less accurate approach (e. g. sweeping the applicator at a typically faster rate required to achieve a reasonable coating over non-trapped volume regions).

This embodiment is further explained with the aid of Figure 2. Like elements in Figures 1 and 2 are identified with like reference numerals. In the example of Figure 2, laminae 15a and 15b represent a support structure that is typically used in spacing an object from platform 9. Laminae 15c and 15d represent the lowest laminae of the object and in this example they form floor 32 of trapped volume 30. Laminae 15e-15j represent closed or effectively closed cross-sectional rings of solidified material, these rings, in combination with the floor 32, form effective trapped volume 30.

Left and right pointing arrow 50 depicts the direction that the vacuum applicator will be swept when forming of a coating over the last solidified laminae of the object. Reference lines 40a-40o depict various transitional regions of the object that may be encountered and considered as the applicator sweeps along its path and forms a coating of fresh material.

The effective trapped volume associated with laminae 15j (i. e. the immediately preceding lamina) is the region between lines 40i and 40n. The object region to be coated over to the right of the trapped volume extends from lines 40n to 40o. The object region to be coated over to the left of the trapped volume extends from 40i to a location positioned between 40g and 40a inclusive. Trapped volume 30 may be considered to contain several regions, where each region may be distinguished by its depth. It may be useful to distinguish these regions so that different coating parameters may be used in each region. Such subtlety however, will not be further considered in this embodiment.

In this first embodiment, a vacuum applicator is made to sweep over the combined

extents of the object plus some additional increment deemed appropriate to ensure that the applicator's influence does not fall within the combined extents of some number of previously formed lamina and within the region to be exposed next. The applicator's region of influence includes such things as: (1) the physical presence of the applicator and its hardware, (2) the shadow region of the applicator and its hardware, (3) any meniscus regions or other regions associated with the applicator which may cause disruptions in the desired working surface, (4) any additional increment necessitated by limitations in knowing the true position of the applicator and or correlating the applicator's position to the exposure positions of the laminae.

Of course, some of these influences may fall within the combined extents, such as shadowing, so long as they do not fall within the exposure region of the next lamina.

In this first embodiment the applicator is swept over at least the above noted combined extents according to standard parameters for sweeping non-trapped volume regions. It has been found that higher sweeping speeds can typically be used over non-trapped volume regions than over trapped volume regions using a vacuum applicator while still achieving acceptable coatings of material. The trapped volume region 40i to 40n is identified manually either before beginning object formation, or during object formation, or is identified automatically for example according to the techniques discussed in the above noted'950 application. The coater parameters are made to deviate from standard parameters at some position within the trapped volume so as to cause the coater to stop or pause its sweeping operation for a predetermined period of time. This predetermined period of time may be a fixed number or it may be related in some manner to the size/shape of the trapped volume regions. For example, pause time may be based on the length of the trapped volume from 40i to 40n.

Determination of optimal or at least acceptable pause times may be empirically determined by those of skill in the art. The determination of acceptable pause times may be derived according to the following procedure: A number of test objects may be formed with trapped volumes of different widths, lengths, and depths. In forming layers over these different regions, the applicator is paused and observations made to determine the amount of pause time needed to allow sufficient filling of the trapped volume regions. A table of desired pause times for a plurality of trapped volume configurations can be created based on the times ascertained from the test objects. When preparing to coat over a trapped volume region, the region is correlated to one or more of the definitions in the table and the longest correlated pause time is selected as a coating parameter to use in the actual coating process. The entries of the table, in addition to being based on trapped volume size, may be based on the number of trapped volume layers separating successive uses of pausing if it is desired not to use a pause

in the formation of each layer.

Determination of optimal sweeping speeds, optimal gaps between the bottom of the coating device and the working surface, optimal clearances between the bottom of the coating device and the top of the immediately preceding lamina, and other coating parameters for different object geometries can also be determined by those of skill in the art. Such determinations can be made based on procedures analogous to that noted above for determining pause time and the results can be placed in appropriate tables.

Pause time and other coating parameters can be set manually by an operator either for an entire object or on a range-by-range basis. Alternatively, the above noted tables may be used automatically by the system to set coating parameters without manual intervention.

Once the predetermined pause time has lapsed, the coater device is made to continue its sweeping so as to finish the coating process. A delay may be utilized after sweeping is complete to allow time for minor imperfections in the coatings to settle out or at least to reach a tolerable level (e. g. 0.25,0.5 or maybe even one layer thickness). Once a coating of desired accuracy is formed, exposure occurs to form the lamina.

As noted above, it is preferred that the vacuum applicator be used in forming each of the successive layers necessary to form the laminae of the object. It is further preferred that during formation of each layer over a lamina containing a trapped volume region, that the vacuum applicator be briefly paused while it is positioned over the trapped volume region to allow extra filling to occur. The length of pause may be the same from layer to layer or it may be varied based on specified criteria.

The technique of this embodiment is further illustrated with the aid of Figures 3a-3c.

Figures 3a-3c depict plots of sweeping speed versus position parallel to the X-axis over the partially formed object shown in Figure 2. Figure 3a depicts sweeping over the last formed lamina at a first relatively high constant speed FSH. This speed might be appropriate for forming adequate coatings over non-trapped volume regions, but not for forming coating over trapped volume regions. Figure 3b depicts sweeping over the last formed lamina at a first relatively slow speed FSL. This speed might be appropriate for forming adequate coatings over trapped volume regions, but slows the process unnecessarily over non-trapped volume regions.

For illustrative purposes only, the slow sweeping speed is depicted to be about one half the higher speed. Figure 3c represents an optimized coating process according to the teachings of this invention. Extra time is allowed for forming the coating over the trapped volume region without simultaneously using excessive coating time over non-trapped volume regions. As depicted, sweeping occurs at the higher speed FSH appropriate for coating over non-trapped volume regions in combination with a pause that occurs at some point within the trapped

volume region located between transitions 40i-40n. The trapped volume length (TVL) along the X-dimension is the difference between the coordinates of 40n and 40i. It is anticipated that the combination of the sweeping time and pause time will be no more than the amount of time the coating device would be within the trapped volume region if swept at the slower speed FSL depicted in Figure 3b. It is believed that a paused coating device (e. g. a vacuum applicator) will more efficiently transfer material from outside to inside the trapped volume than a moving device. Ignoring acceleration and deceleration issues, the approximate upper limit on pause time, ULPT, may be determined from the following equation, ULPT <= (TVL/FSL)- (TVL/FSH).

An alternative to a single pause over a trapped volume region is to use two or more pauses over the trapped volume region. The combined time for all pauses may be approximated by the total time required if a single pause was made. In other words, if two pauses are utilized, the pause time PT, for each may be about PT/2. Alternatively, one pause could be longer and the other could be shorter.

If the immediately preceding lamina includes two or more independent or substantially independent trapped volumes, pausing may occur one or more times over each trapped volume region.

Further alternatives might involve the use of higher speed sweeping as depicted in Figures 3a and 3c over each lamina but use of pauses in trapped volume regions over only some containing trapped volumes. For example, every second, third, or fifth or higher order lamina containing a trapped volume may be coated using a pause. In this alternative, it is preferred that the error associated with forming a coating over a single lamina is not significant but that accumulated errors, if allowed to build up, would become significant and therefore periodic pauses are preferably used to minimize these accumulated errors.

Even further alternatives of this first embodiment might use a level detection system to determine surface level within the trapped volume to determine the extent to which each layer will benefit by being formed using a pause. Based on the extent of benefit, use or non-use of a pause may be decided. Alternatively, the detection may be used to determine how long pause times should be instead of relying on preset time periods. Further alternatives could utilize a combination of detection and preset time to determine pause times. For example, if one wants to emphasize speed of formation, one might use the shorter of a predetermined time and a detected level to trigger when a pause should end.

Embodiment 2 Slowed Coating in Trapped Volume Regions The second preferred embodiment of the invention is similar to the first embodiment with the exception that instead of pausing the sweeping motion of the coating device within a trapped volume region, the sweeping motion of the coating device is slowed when it is located within at least a portion of a trapped volume region. This slowed sweeping allows more time for material to be transferred from the applicator into the trapped volume region. If a balanced applicator is used, as opposed to a positive flow applicator, extra material will be transferred to regions where the level is too low and little or no extra material will be transferred to regions that are at the right level or have excess material. In fact, with a balanced applicator, excess material in a region may even be removed by action of the applicator.

Acceptable or even optimal sweeping speed within a trapped volume can be determined empirically by those of skill in the art. It is anticipated that the speed of sweeping over the trapped volumes in this embodiment, in order to achieve a desired coating accuracy, may need to result in the applicator being positioned over the trapped volume for a somewhat longer time than that required when coating using the pause technique of the first embodiment.

However, it should be further noted that in other circumstances the time may be substantially the same or reduced.

Figures 4a-4c depict examples of slowed sweeping SS4a, Sus46, SS4c over a trapped volume region 40i-40n. As with Figures 3a-3c, Figures 4a-4c depict plots of sweeping speed versus position over the partially formed object of Figure 2. Figure 4a depicts that when the coating device is swept over non-trapped volume regions it is swept with a first speed Fus,,.

This first speed is preferably appropriate for forming coatings over non-trapped volume regions. In the trapped volume region, a second speed SS4a of sweeping is used which is about half that of the first speed. This second speed 8843 is the same as the speed FSL illustrated in Figure 3b This halving of the speed allows extra time to transfer material to the trapped volume region. The speed of sweeping over the trapped volume is preferably selected such that sufficient time is allowed for the trapped volume region to be appropriately coated. In this Figure it is noted that the period of acceleration when transition from the faster to the slow speed, and vice-a-versa, occurs outside the trapped volume region.

Figure 4b indicates that the second speed Sus46 is approximately one third of the first speed FSH, but that the acceleration from the first to second speed, and vice-a-versa, occurs within the trapped volume region. As a result of this slower second speed and the accelerations being within the trapped volume region, approximately the same amount of time is spent within the trapped volume as for the example of Figure 4a. It is within the level of skill in the art to determine what speeds SS4b and time periods at those speeds will yield an

overall desired time period for the coating device to remain within the extents of the trapped volume.

Figure 4c indicates that the second speed SS4c is approximately one fourth the first speed FSH. In this example, the trapped volume region is swept, in part, with the first speed FSH, the second speed SS4c, and under deceleration and acceleration between the two speeds.

It is anticipated that the total time the coating device spends within the trapped volume should be about the same as for the examples of Figures 3c, 4a, and 4b. It is within the skill of those in the art to determine what combinations of speeds and associated time period will yield the overall desired period of time. It is furthermore within the skill of the art to determine whether the optimal overall time period to remain within the trapped volume should be different between the examples of this embodiment and that of the first embodiment.

As opposed to using only a pause or only a slowed sweeping speed, an alternative embodiment can use a combination of these two approaches. This alternative preferably provides a balanced applicator that is swept above an immediately preceding lamina of an object being formed, which lamina includes an effective trapped volume region, where the vacuum applicator is made to both slow down and pause over at least a portion of the effective trapped volume region to allow extra time for material to be transferred into the trapped volume region.

This alternative is exemplified with the coating speeds shown in Figure 5. Figure 5 depicts sweeping the majority of the non-trapped volume regions of the partially formed object with a first speed FSH while sweeping the trapped volume region 40i-40n with a combination of the first high speed FSH, relatively weak deceleration Dw (yielding effective sweeping at a speed less than FS,), a pause P, and a relatively weak acceleration Aw.

Though not necessary, the example of Figure 5 depicts a preferred supplemental technique where the speed of sweeping upon exiting a trapped volume is smaller than a speed of sweeping upon entering a trapped volume. This lack of symmetry in coating speed is believed to be advantageous as it may minimize coating time while simultaneously minimizing the amount of scoop out of material from the trapped volume, and minimizing leading edge bulge deposits that may occur over the immediately preceding lamina as the applicator crosses out of the trapped volume.

Other alternatives involve the use of progressively decreasing coating speeds as the trapped volume is traversed.

Other alternatives to the present embodiment may involve alternatives discussed above in association with the first embodiment.

Embodiment 3 Back and Forth Coating in Trapped Volume Regions A third preferred embodiment of the invention provides a balanced applicator that is swept above an immediately preceding lamina of an object being formed, which lamina includes an effective trapped volume region, where the balanced applicator is made to sweep back and forth above at least a portion of the trapped volume region more times than it sweeps over at least some other portions of either the immediately preceding lamina or the immediately preceding lamina in combination with other previously formed laminae.

An example of sweeping speeds utilized in this embodiment are illustrated in Figure 6.

Figure 6 depicts a plot of sweeping speed versus position over the partially formed object of Figure 2. As can be seen, the entire immediately preceding lamina is swept with a coating device at a first high coating speed FSH which is the same as that depicted in Figures 3a, 3c and 4a-4c. Over a portion of the trapped volume region, two additional sweeps at the same speed are indicated at slightly offset positions for illustrative purposes. A first of these extra sweeps occurs in the opposite direction from the primary sweeping direction. The second of these extra sweeps occurs in the same direction as the primary sweeping direction. In the illustration it is assumed that the accelerations and decelerations are high enough that their impact can be ignored. If it is preferred, it is within the skill of the art to account for these acceleration issues in view of the teachings herein. In this example, it is believed that the coating device may need to spend about the same amount of time over the trapped volume region as utilized in Figures 4a-4c.

An alternative embodiment combines the techniques of the second embodiment with those of the third embodiment. This alternative involves extra sweeps being utilized over a trapped volume region in combination with the use of slowed sweeping speeds. This alternative is depicted in Figure 7 that depicts a plot of sweeping speed versus position over the partially formed object. The coating device makes a single sweep at speed FSH across both the trapped volume and non-trapped volume region; however, when passing over the trapped volume region, the coating device pauses sweeping in the primary direction and sweeps <BR> <BR> <BR> <BR> backward at a second reduced speed SS7 for a partial width of the trapped volume then sweeps<BR> <BR> <BR> <BR> <BR> <BR> back in the primary direction at the reduced speed SS7 for the same distance then starts sweeping again at first speed FSH to complete the sweeping process Other Alternatives to the third embodiment, as well as to the other embodiments, might involve use of different sweep speeds in each direction and further might involve different sweep speeds before and after the trapped volume. more than two or three passes over the trapped volume, and/or a different number of passes over different portions of the trapped volume. Other alternatives might involve a combination of the first and third embodiments;

first, second, and third embodiments ; or any other combination of alternatives disclosed herein.

For example, when coating over an immediately preceding lamina containing a trapped volume, a first sweep may be performed at a slow rate over the trapped volume region. A second sweep in the opposite direction may be performed at a slow rate over the trapped volume region. The speed of the two sweeps may be set to allow adequate filling of the trapped volume region. A third sweep may then be performed where the sweeping direction is the same as that for the first sweep. The third sweep may be performed with at an appropriate speed to form a desired coating over a region solidified on the immediately preceding lamina while minimizing the amount of material which is removed from the trapped volume area due to the movement of the coating device. Other alternatives might involve sweeping the full vat extent, higher speed sweeping away from the object or away from the region occupied by a plurality of previously formed laminae, or a plurality of passes over non-trapped volume regions (may be the same as or different from the number of passes over trapped volume regions). Other alternatives might involve the use of other balanced applicators (e. g. counter rotating rollers, electrostatic applicators, and the like), other non-balanced applicators known in the art (e. g. curtain or sheet coaters, apertured coaters, spin coaters, ink jet coaters, and the like), spreading devices known in the art (e. g. doctor blades, brushes, and the like) either alone or in combination with each other.

Embodiment 4 Paused or Slowed Sweeping to Minimize Removal of Material From Trapped Volume Regions A fourth embodiment of the present invention does not focus specifically on allowing sufficient time to transfer material into a trapped volume region, but instead focuses on the applicator minimizing material removal from a trapped volume region. Removal of material can occur by dragging or pushing material out of the trapped volume region as the coating device approaches or crosses over the boundary of a trapped volume region. This embodiment is similar to the other embodiments discussed above in that it is primarily directed to improving the coatings formed in trapped volume regions and preferably uses a balanced recoater.

Minimization of the negative impact of material removal from trapped volumes can occur in several ways. For example, a first way is by allowing time for any leading edge wave in front of the applicator to settle out prior to it being forced over the edge of the trapped volume. This first way may be achieved by pausing the coating device when approaching an exiting edge of a trapped volume and/or by slowing the sweep speed when approaching the edge of the trapped volume so as to prevent, or at least significantly limit the amount of

material removed from the trapped volume. The second way is by pausing and/or significantly slowing the movement of the applicator within the trapped volume at or near its edge so that time is allowed for the coating device to transfer material back into the trapped volume that its motion may have caused to be removed. Experiments have indicated that this removal phenomenon can account for up to 0.1 mm, or more, of material thickness being removed from a trapped volume. As such, implementing steps to reduce the occurrence of, or impact of, this phenomenon on coating accuracy can lead to significant improvements in the overall process.

In this embodiment, the exiting edge (relative to the direction of sweeping) of each trapped volume associated with each lamina or with selected laminae are automatically identified, for example by the techniques disclosed in the previously referenced'950 application. A predetermined distance from these edges is selected where the applicator will be made to slow down and/or pause. This predetermined distance may be as large as 50 mm, or more, as small as 25 mm, 12 mm, or even set to zero. The coating device is made to slow down or pause at the predetermined location. It is then made to move slowly until the trapped volume is completely exited by the coating device or at least until the leading edge of the coating device has exited the trapped volume. After exiting the trapped volume, the coating device may be made to speed up to a more standard speed such as FSH, as noted in the other embodiments.

As with the previous embodiments, this embodiment may use pause times and or sweeping speed/distance combinations to allow material to be transferred into the trapped volume near the exiting edge of the trapped volume. In addition, this embodiment may be able to use shorter total sweep times while still securing adequate coating levels within trapped volume regions, as the time may be primarily required to shift material around within the trapped volume as opposed to transferring material into the trapped volume region.

In this embodiment, as with the previous embodiments, the exiting edge of the trapped volume may be the actual exiting edge or it may be an effective edge. The edge on which the pause or initial slowing is based may be an effective edge, while the actual edge may be the edge which dictates when sweeping speed may be increased. The actual boundary, extent, or exiting edge of a trapped volume may be defined as the locations where the trapped volume is bounded by solidified material on the immediately preceding lamina. The effective boundary of a trapped volume may exist within a trapped volume region as the result of solidified material existing on one or more layers below the immediately preceding lamina. This solidified material on previously formed lamina may effectively block the timely transfer of material from one side to the other. This material may effectively partition a trapped volume into two effectively independent, relatively deep, trapped volumes or it may alternatively

partition the trapped volume into one deep section and one shallow section. The effectiveness of such partitioning decreases with the depth below the surface of the most recent solidified material. Material that is solidified within 0.5 mm to I mm or even 2 mm, or more, of the immediately preceding lamina may form an effective partition.

If slowed movement is to be used prior to exiting the trapped volume, but after the initial pause or slowing, appropriate speeds for such movement can be empirically determined by those of skill in the art by forming test objects and experimenting with different speeds.

Similarly, appropriate predetermined distances, pause times, etc. can be empirically determined by those of skill in the art. It is believed that pause times on the order of one to five seconds should be adequate in most circumstances to allow bulges of material existing in front of the sweeping device (i. e. leading bulges) to dissipate. It is believed that predetermined distances on the order of 25-50 mm should be adequate to ensure that leading bulges are not pushed out of the trapped volume. It is further believed that exiting speed on the order of 2.5-12 mm/sec will be slow enough to minimize build up and removal of leading bulge material.

Though the above enhancements to coating techniques have been specifically directed to trapped volume regions, these techniques are believed to be applicable to other object regions. For example, these techniques of providing extra coating time can be applied to shallow regions that are not actually trapped volumes but simply have reduced flow paths. In shallow regions, reduced speed techniques and/or extra sweep techniques are preferred especially where these techniques will be utilized over large portions of these shallow regions.

Of course, in all the above embodiments, pauses, slower sweep speeds, and/or extra passes over portions of the non-trapped volume regions can also occur while still at least partially achieving the decreased coating time objective. In achieving this objective, the coating device can spend an average amount of time per unit length over the trapped volume region which is greater than the average amount of time per unit length spent over the non- trapped volume regions formed by the immediately preceding lamina or the combination of the non-trapped volume regions of the combination of the immediately preceding lamina and a plurality of previously formed laminae. More broadly stated, the average time spent over a selected portion of the immediately proceeding lamina (or combination with one or more previously formed laminae) is greater than the average time spent over a non-selected portion of the immediately preceding lamina (or combination with one or more previously formed laminae) which is coated over. Alternatively, in achieving the objective, the average speed over the trapped volume is less than the average speed over the non-trapped volume regions of the previously formed lamina or laminae.

The techniques disclosed herein can be looked at in an opposite perspective where it is considered that solidified regions on the immediately preceding lamina are coated using shorter average times per unit length or unit area than average times used for other regions (e. g. trapped volume regions) per unit length or unit area.

Embodiment 5 Slowed Sweeping and/or Back and Forth Sweeping to Minimize Leading Edge Bulge The fifth embodiment of the present invention in its various alternatives use variable speed sweeping with controlled acceleration and/or back and forth sweeping to reduce the height of leading edge bulges that typically form at the leading edges of laminae that are encountered by a coating device (e. g. a balanced applicator) during the coating process.

A first alternative of the fifth embodiment controls the speed of the coating device (e. g. balanced applicator) over solidified regions of a preceding lamina (e. g. the immediately preceding lamina) so that when sweeping over the leading edge of the preceding lamina the coating device is made to move more slowly than when sweeping over at least some subsequently encountered portions of the preceding lamina.

It is believed that the leading edge bulge may result from removal of building material that wet the trailing edge of the sweeping device. Shear forces are believed to increase rapidly as the coating device moves from a region of relatively deep liquid (e. g. away from the object being formed) to a region of shallow liquid (e. g. over a portion of the object being formed and particularly over a portion of the immediately preceding several laminae). This increased shear force is believed to be responsible, at least in part, for tearing away at least a portion of the wetting material and depositing it on the previously solidified lamina. It is further believed that the magnitude of the shear force is related to the speed at which the coating device sweeps over the leading edge of the lamina.

As such, it is believed that sweeping over the leading edge of the lamina with a reduced speed can result in a smaller amount of material being deposited near the leading edge of the lamina. If the speed of sweeping is ramped up as the coating device moves further from the leading edge, it is believed that though the same, or nearly the same, amount of material may eventually be deposited, this amount of material will be spread over a larger area thereby resulting in a decreased height of the bulge on initial deposition. This decreased height will hopefully result in less coating error and/or reduced time in forming a coating with a desired accuracy.

For example, the speed of the applicator may be within the range of about 1 mm-6 mm per second as the trailing edge of the coating device encounters the leading edge of a

solidified region on a previous lamina. After crossing the leading edge, the speed of the coating device may be increased to 12-50 mm, or more, per second. The acceleration may be made to occur over a desired distance to allow the bulge to be deposited over a large area. The desired distance, for example, may be from 6 mm to 50 mm, or more, in length.

An alternative to ramping up the sweeping speed after the trailing edge of the coating device crosses over the leading edge of the lamina, or after it crosses a certain distance beyond the leading edge of the lamina, the speed may be rapidly transitioned from a slower speed to a higher speed. The length over which the transition occurs may be, for example, within the range of 0.5-6 mm. This rapid transition may result in a higher bulge being located in a smaller region of the lamina than with a longer ramping approach, however, this approach may be useful in minimizing build up of the bulge from lamina-to-lamina if the location of the transition is varied between successive laminae or groups of laminae.

For example, when sweeping over a first lamina in a first direction, a first transition may occur at a specified location relative to the leading edge of the first lamina resulting in deposition of a bulge at a first position. When sweeping over a second lamina (which may or may not consecutively follow the first lamina) the transition location may be specified so as to cause deposition of the bulge at a second position offset by at least a desired amount from the first position.

It is anticipated that the first deposition position and second deposition position will be offset from the first transition location and second transition location respectively, in the respective directions of sweeping by some amount which may be dependent on the acceleration and the initial and final speeds of sweeping. As such, if sweeping over the first and second lamina is in the same direction, if accelerations are similar, and if initial and final velocities are the similar, it is anticipated that the first and second transition locations should be offset by one another by approximately the desired amount or more. Alternatively, if the sweeping directions are opposite to one another, the transition locations should be determined based on the opposite directions of shift between the first and second transition locations and the first and second desired deposition positions. If the separation between first and second positions are sufficiently offset, little or no build up in bulges can occur from lamina-to- lamina : thus improving accuracy and reliability of the coating formation process.

A third alternative can combine the offsetting of deposition positions and broadening of deposition widths of the first two alternatives.

In a forth alternative a coating with reduced leading edge bulge may be obtained over a lamina by sweeping with a first sweep in a first direction, followed by a second sweep in a second opposite direction, and further followed by a third sweep in the first direction. It is

proposed that the second sweep may be used to redistribute leading edge bulge material which was deposited in the first sweep so as to reduce the size of the initially deposited leading edge bulge more rapidly than it would have been reduced without the second sweep. In this alternative, a first side of the coating device acts as its leading edge during a first sweep in a first direction while the second side acts as its trailing edge. When sweeping in an opposite second direction, the second edge acts as the leading edge while the first edge is the trailing edge.

In this alternative, it is preferred that the first sweep proceed far enough over the solidified portion of the lamina to deposit the leading bulge of material from its second side on to the leading edge of the solidified portion of the previously formed lamina. It is also preferred that the coating device stop its sweeping motion prior to the second side completing its sweep over the solidified portion of the lamina. It is more preferred that the coating device stop its motion in the first direction prior to the first edge completing its sweep over the solidified portion of the lamina.

After sweeping in the first direction, the coating device sweeps in the second direction.

The sweep in the second direction progresses far enough so that at least the second side sweeps over the region where the bulge was believed to be deposited. Preferably, the sweeping in the second direction stops prior to the second side sweeping beyond the edge of the solidified portion of the lamina. It is possible for the coating device to sweep somewhat beyond the edge of the lamina prior to halting its motion. After completing its sweep in the second direction, the coating device is again made to sweep in the first direction to complete its sweeping over the previously formed lamina. When sweeping in the second direction while using a balanced coating device, the force used to draw material into the device may be varied. In particular, it is thought that increasing the amount of material in the device (or at least the force attempting to draw material in) while sweeping in the second direction might be useful in inhibiting redeposit of a leading edge bulge once sweeping in the first direction is reinitiated.

In a fifth alternative, additional back and forth sweeps, beyond those used in the fourth alternative, may be used in reducing the leading edge bulge.

In a sixth alternative, multiple back and forth sweeps may occur over different portions of the lamina if it is believed that multiple leading edge bulge regions may exist over the lamina.

In a seventh alternative, the back and forth sweeping of the fourth to sixth alternatives may be used in combination with the speed changes and transition location variations of the first three alternatives.

While particular illustrative embodiments have been presented above, many other embodiments and modifications will be apparent to those of skill in the art upon review of the teachings herein. As such, the invention should not be considered to be limited to the embodiments presented, but instead should be limited only by the scope of the claims presented hereafter.