Stereolithographic method and apparatus for production of three dimensional objects with enhanced thermal control of the build environment

Abstract

A rapid prototyping and manufacturing (e.g. stereolithography) method and apparatus for making three-dimensional objects with enhanced control of the temperature of the build environment. A modulated control signal is supplied to a heating element based on a difference between a desired build temperature and a detected temperature. The modulated signal result in the heater supplying, on average, more energy to the build chamber when the temperature difference is large and supplying, on average, less energy to the build chamber when the difference is small. The detection of the build chamber temperature may occur at one or more locations. If multiple locations are used, the detected temperatures may be averaged, or otherwise combined, to derive a single difference value that will be used to control the heater. Alternatively, the multiple detected temperatures may be used to produce multiple difference values that can in turn be used to control the quantity of heat applied directly to different locations in the build environment by either using multiple heaters or multiple controllable ducts.

Description

FIELD OF THE INVENTION

This invention relates to the formation of three-dimensional objects using a Rapid Prototyping and Manufacturing (RP&M) technique (e.g. stereolithography). The invention more particularly relates to the formation of three-dimensional objects using improved thermal control of the building environment.

BACKGROUND OF THE INVENTION

1. Related Art

Rapid Prototyping and Manufacturing (RP&M) is the name given to a field of technologies that can be used to form three-dimensional objects rapidly and automatically from three-dimensional computer data representing the objects. RP&M can be considered to include three classes of technologies: (1) Stereolithography, (2) Selective Deposition Modeling, and (3) Laminated Object Manufacturing.

The stereolithography class of technologies create three-dimensional objects based on the successive formation of layers of a fluid-like material adjacent to previously formed layers of material and the selective solidification of those layers according to cross-sectional data representing successive slices of the three-dimensional object in order to form and adhere laminae (i.e. solidified layers). One specific stereolithography technology is known simply as stereolithography and uses a liquid material that is selectively solidified by exposing it to prescribed stimulation. The liquid material is typically a photopolymer and the prescribed stimulation is typically visible or ultraviolet electromagnetic radiation. The radiation is typically produced by a laser through other sources of radiation are possible such as arc lamps, resistive lamps, and the like. Exposure may occur by scanning a beam or by controlling a flood exposure by use of a light valve that selectively transmits or reflects the radiation. Liquid-based stereolithography is disclosed in various patents, applications, and publications of which a number are briefly described in the Related Applications section hereafter.

Another stereolithography technology is known as Selective Laser Sintering (SLS). SLS is based on the selective solidification of layers of a powdered material by exposing the layers to infrared electromagnetic radiation to sinter or fuse the powder particles. SLS is described in U.S. Pat. No. 4,863,538, issued Sep. 5, 1989, to Deckard. A third technology is known as Three Dimensional Printing (3DP). 3DP is based on the selective solidification of layers of a powdered material which are solidified by the selective deposition of a binder thereon. 3DP is described in U.S. Pat. No. 5,204,055, issued Apr. 20, 1993, to Sachs.

The present invention is primarily directed to stereolithography using liquid-based building materials (i.e. medium). It is believed, however, that the techniques of the present invention may have application in the other stereolithography technologies for the purposes of maintaining more uniform building environments.

Selective Deposition Modeling, SDM, involves the build-up of three-dimensional objects by selectively depositing solidifiable material on a lamina-by-lamina basis according to cross-sectional data representing slices of the three-dimensional object. One such technique is called Fused Deposition Modeling, FDM, and involves the extrusion of streams of heated, flowable material which solidify as they are dispensed onto the previously formed laminae of the object. FDM is described in U.S. Pat. No. 5,121,329, issued Jun. 9, 1992, to Crump. Another technique is called Ballistic Particle Manufacturing, BPM, which uses a 5-axis, ink-jet dispenser to direct particles of a material onto previously solidified layers of the object. BPM is described in PCT publication numbers WO 96-12607, published May 2, 1996, by Brown; WO 96-12608, published May 2, 1996, by Brown; WO 96-12609, published May 2, 1996, by Menhennett; and WO 96-12610, published May 2, 1996, by Menhennett. A third technique is called Multijet Modeling, MJM, and involves the selective deposition of droplets of material from multiple ink jet orifices to speed the building process. MJM is described in U.S. Pat. No. 5,943,235 issued to Earl et al. on Aug. 24, 1996 and U.S. patent application Ser. No. 08/722,335, filed Sep. 27, 1996, by Leyden et al. now abandoned (both assigned to 3D Systems, Inc. as is the instant application).

Though, as noted above, the techniques of the instant invention are directed primarily to liquid-based stereolithography object formation, it is believed that the techniques may have application in the SDM technologies to maintain more uniform building environments.

Laminated Object Manufacturing, LOM, techniques involve the formation of three-dimensional objects by the stacking, adhering, and selective cutting of sheets of material, in a selected order, according to the cross-sectional data representing the three-dimensional object to be formed. LOM is described in U.S. Pat. Nos. 4,752,352, issued Jun. 21, 1988, to Feygin, 5,015,312, issued May 14, 1991, to Kinzie, and 5,192,559, issued Mar. 9, 1993, to Hull et al.; and in PCT Publication No. WO 95-18009, published Jul. 6, 1995, by Morita.

Various techniques for maintaining build temperature have been proposed. In particular various techniques have been described in European patent publication 376 571 B, published Feb. 8, 1995, by Takano et al. This publication describes the use of induction heating to heat the resin held in a vessel to a desired temperature, where the control of the heater is based on a detected temperature of the resin.

Even in view of the teachings of the above noted reference, a need remains in the art for improved techniques for controlling the temperature of the build environment.

2. Other Related Patents and Applications

The patents, applications, and publications mentioned above and hereafter are all incorporated by reference herein as if set forth in full. Table 1 provides a listing of patents and applications co-owned by the assignee of the instant application. A brief description of subject matter found in each patent and application is included in the table to aid the reader in finding specific types of teachings. 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 teachings of the instant application in many ways. For example, the references directed to various data manipulation techniques may be combined with the teachings herein to derive even more useful, modified object data that can be used to more accurately and/or efficiently form objects. As another example, the various apparatus configurations disclosed in these references may be used in conjunction with the novel features of the instant invention.

TABLE 1______________________________________Related Patents and ApplicationsU.S. Pat. No.Issue DateApplicationNo.Filing Date Inventor Subject______________________________________4,575,330 Hull Discloses fundamental elements of stereo-Mar 11, 1986 lithography.06/638,905Aug 8, 19844,999,143 Hull, Discloses various removable supportMar 12, 1991 et al. structures applicable to stereolithography.07/182,801Apr 18, 19885,058,988 Spence Discloses the application of beam profil-Oct 22, 1991 ing techniques useful in stereolithography07/268,816 for determining cure depth and scanningNov 8, 1988 velocity, etc.5,059,021 Spence, Discloses the utilization of drift correctionOct 22, 1991 et al. techniques for eliminating errors in beam07/268,907 positioning resulting from instabilities inNov 8, 1988 the beam scanning system5,076,974 Modrek, Discloses techniques for post processing ob-Dec 31, 1991 et al. jects formed by stereolithography. In partic-07/268,429 ular exposure techniques are described thatNov 8, 1988 complete the solidification of the building material. Other post processing steps are also disclosed such as steps of filling in or sand- ing off surface discontinuities.5,104,592 Hull Discloses various techniques for reducingApr 14, 1992 distortion, and particularly curl type distor-07/339,246 tion, in objects being formed by stereolitho-Apr 17, 1989 graphy.5,123,734 Spence, Discloses techniques for calibrating a scan-Jun 23, 1992 et al. ning system. In particular techniques for07/268,837 mapping from rotational mirror coordinatesNov 8, 1988 to planar target surface coordinates are disclosed5,133,987 Spence, Discloses the use of a stationary mirror lo-Jul 28, 1992 et al. cated on an optical path between the scan-07/427,885 ning mirrors and the target surface to foldOct 27, 1989 the optical path in a stereolithography system.5,141,680 Almquist, Discloses various techniques for selectivelyAug 25 1992 et al. dispensing a material to build up three-07/592,599 dimensional objects.Oct 4, 19905,143,663 Leyden, Discloses a combined stereolithography sys-Sep 1, 1992 et al. tem for building and cleaning objects.07/365,444Jun 12, 19895,174,931 Almquist, Discloses various doctor blade configurationsDec 29, 1992 et al. for use in forming coatings of medium07/515,479 adjacent to previously solidifiedApr 27, 1990 laminae.5,182,056 Spence, Discloses the use of multiple wavelengths inJan 26, 1993 et al. the exposure of a stereolithographic medium.07/429,911Oct 27, 19895,182,715 Vorgitch, Discloses various elements of a large stereo-Jan 26, 1993 et al. lithographic system.07/824,819Jan 22, 19925,184,307 Hull, Discloses a program called Slice and variousFeb 2, 1993 et al. techniques for converting three-dimensional07/331,644 object data into data descriptive of cross-Mar 31, 1989 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.5,192,469 Hull, Discloses various techniques for formingMar 9, 1993 et al. three-dimensional object from sheet material07/606,802 by selectively cutting out and adheringOct 30, 1990 laminae.5,209,878 Smalley, Discloses various techniques for reducingMay 11, 1993 et al. surface discontinuities between successive07/605,979 cross-sections resulting from a layer-by-layerOct 30, 1990 building technique. Disclosed techniques include use of fill layers and meniscus smoothing.5,234,636 Hull, Discloses techniques for reducing surfaceAug 10, 1993 et al. discontinuities by coating a formed object07/929,463 with a material, heating the material to causeAug 13, 1992 it to become flowable, and allowing surface tension to smooth the coating over the object surface.5,238,639 Vinson, Discloses a technique for minimizing curlAug 24, 1993 et al. distortion by balancing upward curl to07/939,549 downward curl.Mar 31, 19925,256,340 Allison, Discloses various build/exposure styles forOct 26, 1993 et al. forming objects including various techniques07/906,207 for reducing object distortion. DisclosedJun 25, 1992 techniques include: (1) building hollow, par-and tially hollow, and solid objects, (2) achieving08/766,956 more uniform cure depth, (3) exposing layersDec 16, 1996 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, Discloses a computer program known asJun 14, 1994 et al. CSlice which is used to convert three-dimen-07/606,191 sional object data into cross-sectional data.Oct 30, 1990 Disclosed techniques include the use of vari- ous Boolean operations in stereolithography.5,597,520 Smalley, Discloses various exposure techniques forJan 28, 1997 et al. enhancing object formation accuracy. Dis-08/233,027 closed techniques address formation of highApr 25, 1994 resolution objects from building materialsand that have a Minimum Solidification Depth08/428,951 greater than one layer thickness and/or aApr 25, 1995 Minimum Recoating Depth greater than the desired object resolution.08/722,335 Thayer, Discloses build and support styles for use inSep 27, 1996 et al. a Multi-Jet Modeling selective deposition modeling system.5,943,235 Earl, Discloses data manipulation and system con-Aug 24, 1999 et al. trol techniques for use in a Multi-Jet Model-08/722,326 ing selective deposition modeling system.Sep 27, 19965,902,537 Almquist, Discloses various recoating techniques forMay 11, 1999 et al. use in stereolithography. Disclosed tech-09/790,005 niques include 1) an ink jet dispensing de-Jan 28, 1997 vice, 2) a fling recoater, 3) a vacuum appli- cator, 4) a stream recoater, 5) a counter rotat- ing roller recoater, and 6) a technique for deriving sweep extents.5,840,239 Partanen, Discloses the application of solid-state lasersNov 24, 1998 et al. to stereolithography. Discloses the use of a08/792,347 pulsed radiation source for solidifying layersJan 31, 1997 of building material and in particular the ability to limit pulse firing locations to only selected target locations on a surface of the medium.6,001,297 Partanen, Discloses the stereolithographic formation ofDec 14, 1999 et al. objects using a pulsed radiation source where08/847,855 pulsing occurs at selected positions on theApr 28, 1997 surface of a building material.08/855,125 Nguyen, Discloses techniques for interpolating origi-May 13, 1997 et al. nally supplied cross-sectional data descrip- tive of a three-dimensional object to produce modified data descriptive of the three- dimensional object including data descriptive of intermediate regions between the original- ly supplied cross-sections of data.5,945,058 Manners, Discloses techniques for identifying featuresAug 31, 1999 et al. of partially formed objects. Identifiable fea-08/854,950 tures include trapped volumes, effectiveMay 13, 1997 trapped volumes, and solid features of a specified size. The identified regions can be used in automatically specifying recoating parameters and or exposure parameters.5,902,538 Kruger, Discloses simplified techniques for makingMay 11, 1999 et al. high-resolution objects utilizing low-resolu-08/920,428 tion materials that are limited by their inabil-Aug 29, 1997 ity to reliably form coatings of a desired thickness due to a Minimum Recoating Depth (MRD) limitation. Data manipulation techniques define layers as primary or sec- ondary. Recoating techniques are described which can be used when the thickness be- tween consecutive layers is less than a lead- ing edge bulge phenomena.09/061,796 Wu, et al. Discloses use of frequency converted solidApr 16, 1998 state lasers in stereolithography.09/154,967 Nguyen, Discloses techniques for stereolithographicSep 17, 1998 et al. recoating using a sweeping recoating device that pause and/or slows down over laminae that are being coated over.09/484,984 Earl, Entitled "Method and Apparatus for Formingfiled et al. Three-Dimensional Objects Using LineJan 18, 2000 Width Compensation with Small Feature Retention." Discloses techniques for forming objects while compensating for solidification width induced in a building material by a beam of prescribed stimulation.09/246,504 Guertin, Entitled "Method and Apparatus for Stereo-Feb 8, 1999 et al. lithographically Forming Three Dimensional Objects With Reduced Distortion." Discloses techniques for forming objects wherein a de- lay is made to occur between successive exposures of a selected region of a layer.09/248,352 Manners, Entitled "Stereolithographic Method and Ap-Feb 8, 1999 et al. paratus for Production of Three Dimensional Object Using Multiple Beams of Different Diameters." Discloses stereolithographic techniques for forming objects using multiple sized beams including data manipulation techniques for determining which portions of lamina may be formed with a larger beam and which should be formed using a smaller beam.09/248,351 Nguyen, Entitled "Stereolithographic Method and Ap-Feb 8, 1999 et al. paratus for Production of Three Dimensional Objects Using Recoating Parameters for Groups of Layers." Discloses improved tech- niques for managing recoating parameters when forming objects using layer thicknesses smaller than a minimum recoating depth (MRD) and treating some non-consecutive layers as primary layers and treating inter- mediate layers there between as secondary layers.09/426,416 Bishop, Entitled "Rapid Prototyping Apparatus withFeb 8, 1999 et al. Enhanced Thermal and Vibrational Stability for Production of Three Dimensional Objects." Discloses an improved Stereo- lithographic apparatus structure for isolating vibration and/or extraneous heat producing components from other thermal and vibration sensitive components.09/247,120 Everett, Entitled "Stereolithographic Method and Ap-Feb 8, 1999 et al. paratus for production of Three Dimensional Objects Including Enhanced Control of Pre- scribed Stimulation Production." Discloses techniques forming objects using varying production of prescribed stimulation (e.g. UV radiation) and enhanced scanning con- trol. Production is reduced or eliminated dur- ing non-exposure periods (e.g. recoating, z-wait, and pre-dip delay). Production is set to a desired level based on the type of expo- sure that is desired. Scanning speed is set based on a number of criteria. Transition be- tween successive exposure vectors may be made with multiple intervening non-exposure vectors. The laser power is set using an AOM in combination with a temporary detection of beam power.09/427,119 Kulkarni, Entitled "Stereolithographic Method and Ap-Feb 8 ,1999 et al. paratus for Production of Three Dimensional Objects Including Simplified Build Prepara- tion." Discloses techniques for forming objects using a simplified data preparation process. Selection of the various parameter styles needed to form an object is reduced to answering several questions from lists of possible choices.______________________________________

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 Mich.; 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 Mich.; 1996.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improved control of process chamber (i.e. build chamber) temperature.

A first aspect of the invention is to provide a method of forming a three-dimensional object from a plurality of adhered laminae by exposing successive layers of a material to prescribed stimulation, including (1) providing a build environment in which to form the three-dimensional object; (2) sensing the actual temperature of the build environment at at least one location during formulation of the object; (3) deriving a difference between the actual temperature and a desired temperature; (4) operating a heating element to apply a heat to the build environment in proportion to the difference so as to substantially maintain the build environment within a predefined temperature range; (5) forming a layer of material adjacent to any last formed layer of material in preparation for forming a subsequent lamina of the object; (6) exposing the material to the prescribed stimulation to form a successive lamina of the object; and (7) repeating the acts of forming and exposing a plurality of times to form the object from a plurality of adhered laminae.

A second aspect of the invention is to provide an apparatus for forming a three-dimensional object from a plurality of adhered laminae by exposing successive layers of a material to prescribed stimulation, including: (1) a build environment in which to form the three-dimensional object; (2) a sensor for sensing the actual temperature of the build environment at at least one location during formation of the object; (3) a device for deriving a difference between the actual temperature and a desired temperature; (4) a heating system, comprising a heating element and a control element for causing the heating element to apply heat to the build environment in proportion to the difference so as to substantially maintain the build environment within a predefined temperature range; (5) a coating system for forming a layer of material adjacent to any last formed layer of material in preparation for forming a subsequent lamina of the object; (6) an exposure system for selectively applying prescribed stimulation to the material to form a successive lamina of the object; and (7) a computer programmed to operate the coating system and the exposure system to form the object from a plurality of adhered laminae.

A third aspect of the invention is to provide a method of forming a three-dimensional object from a plurality of adhered laminae, including: (1) providing a build environment in which to form the three-dimensional object; (2) sensing the actual temperature of the build environment at at least one location during formation of the object; (3) deriving a difference between the actual temperature and a desired temperature; (4) operating a heating element to apply a heat to the build environment in proportion to the difference so as to substantially maintain the build environment within a predefined temperature range; (5) forming a lamina of the object adjacent to and adhered to any previously formed lamina; and (6) repeating the act of forming a plurality of times to form the object from a adhered laminae.

A fourth aspect of the invention is to provide an apparatus for forming a three-dimensional object from a plurality of adhered laminae, including: a build environment in which to form the three-dimensional object; (2) a sensor for sensing the actual temperature of the build environment at at least one location during formation of the object; (3) a device for deriving a difference between the actual temperature and a desired temperature; (4) a heating system, comprising a heating element and a control element for causing the heating element to apply heat to the build environment in proportion to the difference so as to substantially maintain the build environment within a predefined temperature range; (5) means for forming a lamina of the object adhered to any previously formed lamina; and (6) means for repeatedly operating the means for forming a plurality of times in order to form the object from a plurality of adhered laminae.

Additional aspects of the invention will be clear from the embodiments of the invention described below in conjunction with the Figures associated therewith. Further aspects of invention involve the practice of the above referred to aspects of the invention in combination with one other.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1a and 1b depict side views of a stereolithography apparatus for practicing the instant invention.

FIG. 2 depicts a block diagram of the temperature control system of a preferred embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIGS. 1a and 1b depict schematic representations of a preferred stereolithography apparatus 1 (SLA) for use with the instant invention. The basic components of an SLA are described in U.S. Pat. Nos. 4,575,330; 5,184,307; and 5,182,715 as referenced above. The preferred SLA includes container 3 for holding building material 5 (e.g. photopolymer) from which object 15 will be formed, elevator 7 and driving means (not shown), elevator platform 9, exposure system 11, recoating bar 13 and driving means (not shown), at least one computer (not shown) for manipulating object data (as needed) and for controlling the exposure system, elevator, and recoating device. The exposure system is used to selectively apply prescribed stimulation to the surface of the material from which the object is to be formed.

FIG. 1a depicts the partially formed object as having its most recently formed lamina lowered to a position approximately one layer thickness below the desired level of the upper surface of the building material 5 (i.e. desired working surface). As the layer thickness is small and the building material very viscous, FIG. 1a indicates that the material has not flowed significantly across the last formed lamina even after lowering the platform 9. FIG. 1b depicts the coating bar 13 as being swept part way across the previously formed lamina and that the next layer of building material has been partially formed.

A preferred exposure system is described in several of the patents and applications referenced above including U.S. Pat. Nos. 5,058,988; 5,059,021; 5,123,734; 5,133,987; U.S. Ser. Nos. 08/792,347; 09/147,120. This preferred system includes a laser, a beam focusing system, and a pair of computer controlled XY rotatable scanning mirrors of either the motor driven or galvanometer type.

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

A preferred recoating device is described in U.S. patent application Ser. No. 08/790,005 as referenced above and includes recoater bar 13, regulated vacuum pump 17, and vacuum line 19 connecting the bar 13 and the pump 17.

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, and the like. A preferred temperature control system will be further described hereafter.

SLAs on which the instant invention may be utilized are available from 3D Systems, Inc. of Valencia, Calif. These SLAs include the SLA-250 using a CW HeCd laser operating at 325 nm, the SLA-3500, SLA-5000, and the SLA-7000 each using a solid state lasers operating at 355 nm with pulse repetition rates of 22.2 KHz, 40 KHz, and 25 KHz, respectively. Preferred building materials are photopolymers manufactured by CIBA Specialty Chemicals of Los Angeles, Calif., and are available from 3D Systems, Inc. These materials include SL 5170, SL 5190, and SL 5530HT.

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 an object from a plurality of adhered laminae. The process may conceptually be viewed as beginning with the elevator platform 9 immersed one layer thickness below the upper surface 20 of the photopolymer 5. The coating of photopolymer is selectively exposed to prescribed stimulation (e.g. a beam of UV radiation) which cures the material to a desired depth to form an initial lamina of the object adhered to the elevator platform. This initial lamina corresponds to an initial cross-section of the object to be formed or corresponds to an initial cross-section of supports 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 material.

As the material is typically very viscous and the thickness of each layer is very thin (e.g. 4 mils to 10 mils), the material may not readily form a coating over the last solidified lamina (as shown in FIG. 1a). In the case where a coating is not readily formed, a recoating device may be swept at or somewhat above the surface of the building material (e.g. liquid photopolymer) to aid in the formation of a fresh coating. The coating formation process may involve the sweeping of the recoating bar one or more times at a desired velocity.

After formation of this coating, the second layer is solidified by a second exposure of the material to prescribed stimulation according to data representing a second cross-section of the object. The process of coating formation and solidification is repeated over and over again until the object is formed from a plurality of adhered layers (21, 23, 25, 27, 29, 31, and 33).

It is preferred that objects be formed in a build chamber (sometimes referred to as a process chamber or process module) portion of an SLA. The temperature of this build chamber is preferably controlled so as to maintain the building material and surrounding air at a desired temperature. The uniformity of the temperature of the environment surrounding the object during its formation (i.e. the build environment) has a direct role in the accuracy of the objects formed and possibly in the reliability of the processes used informing those objects. The preferred temperature is typically a few degrees above ambient (e.g. 28-38.degree. C. assuming ambient is 25.degree. C.). The desired temperature is preferably above ambient so that the actual temperature may be appropriately maintained using only a heater and natural heat loss. As an alternative, the temperature may be maintained at ambient or below where a refrigeration system and possibly a heater would be employed in the process of maintaining the temperature at the desired level.

In a first preferred embodiment a heater and fan are supplied at the rear of the build chamber behind a container for holding the building material (e.g. photopolymer). An air temperature sensor is supplied in the build chamber. The sensor is preferably located in the air within the build chamber near the surface of the building material (e.g. within five inches) but other locations may also be used. The fan is preferably operated continuously even when power is not being supplied to the heater. In an alternative, the fan may be controlled to turn on and off along with the heater or be controlled to turn on somewhat before the heater and be controlled to turn off somewhat after power is supplied to the heater. The heater is operated in an appropriate manner in response to temperature as detected by the sensor and a desired set point temperature. The heater is controlled to nominally keep the build chamber/environment temperature at a desired level. The heated air is directed to a ducting system that distributes it under the container of building material. The power supplied to the heater, and the heat produced by the heater, may be controlled in several ways. A first way operates the heater in either a full power state or an "off" state and obtains an average desired power output by modulating a duty cycle. That is by modulating the period of "on" time as a portion of total time (i.e. "on" time divided by the total of "on" time pulse "off" time). A second way supplies power on a continuous basis but only at a desired level that is somewhere between 0% of maximum power and 100% of maximum power.

According to a preferred embodiment of the invention, when the temperature is detected to be below the desired set point the heater is turned on. The heater, however, may not be maintained in the "on" state for the entire duration of time that the detected temperature is too low. As the actual temperature nears the set point temperature, "on" and "off" cycling occurs based on a criteria that sets the time average of power supplied to the heater based on the difference between the actual temperature and the desired temperature levels.

FIG. 2 depicts a block diagram of the temperature control system of the first preferred embodiment. Software 100 and appropriate logic 102 are used to obtain a desired voltage from a digitally controlled potentiometer 104. The desired voltage is indicative of a desired chamber temperature (i.e. temperature set point). Element 106 indicates that the output from the digital potentiometer is modified to obtain an appropriate analog signal that can be used by an analog differencer amplifier 112 for comparison to the output from element 110.

Element 108 provides a sensor located in the build chamber and is used to provide an output from which the temperature in the chamber can be ascertained. A preferred sensor is a thermistor, but other sensors may be used. Element 110 modifies the output from element 108 to put it in an appropriate form for use by the analog differencer amplifier 112. The actual temperature may be sensed continuously or periodically. In the present embodiment temperature sensing is done continuously. Signals from elements 106 and 110 are provided as input to the analog differencer amplifier 112 from which an analog difference signal is generated and passed on to comparator 120. The output of amplifier 112 is indicative of the temperature difference between the set point temperature and the measured temperature. If the signal voltage is zero, the difference between set point temperature and actual temperature is zero.

Elements 114, 116, and 118 provide a slowly varying voltage to an analog comparator 120. This slowly varying voltage ramps up and down between a low value V.sub.L and a high value V.sub.H over a relatively long period (e.g. 30 seconds). This ramping signal is used to vary power control in relationship to the temperature difference derived by differencer 112. This varying voltage is obtained by a control signal being generated by a combination of a clocking mechanism 114 and appropriate logic 116. This control signal is applied to a digitally controlled potentiometer to obtain the slowly varying output voltage.

The analog comparator 120 compares a signal representing the temperature difference, derived from the differencer 112, to the slowly varying ramp voltage. If the temperature difference signal is greater than V.sub.H, the comparator 120 drives the heater fully (i.e. the duty cycle is 100%). When the temperature difference is below V.sub.L, or is negative, the comparator does not drive the heater at all (i.e. on time duty cycle of 0%). The temperature difference is continuously supplied to the analog comparator 120.

If the temperature difference signal is between the V.sub.L and V.sub.H, the comparator output is a digital pulse width modulated signal. The signal is "more-on" when the temperature difference is closer to the maximum V.sub.H of the ramp. It is "more-off" the closer the temperature difference is to the minimum V.sub.L of the ramp. This may be conceptually understood by considering that the "on" time from the comparator is related to the percentage of time that the temperature difference signal is larger than the slowly varying ramp signal.

In the present embodiment V.sub.H is set to correspond to a temperature difference of about 0.5 to about 1.0.degree. C. Above this difference, the comparator sends a signal for the heater to be fully on. Below this difference, modulation occurs until the difference falls below a value corresponding to V.sub.L at which point the heater remains off until V.sub.L is again exceeded. Appropriate values for V.sub.L, V.sub.H, and ramp period, as well as other variables may be empirically determined. The optimal values may vary based on specific characteristics of the stereolithographic system. Such characteristics may include heat loss in the system, volume of air being heated, heater power, and the like. In the first preferred embodiment, the heater power is approximately 800 watts.

The signal from the comparator is sent to logic element 124. A software enable signal is provided by element 122 and sent to logic element 124 where a Boolean "and" operation is performed on the comparator signal and the enable signal. The result of this operation is passed to a solid state relay 126 that provides electric power from 130 and 132 to chamber heater 128. The software enable signal provides an initial signal that controls when the heater may initially be turned on when the machine is first powered up. The signal then maintains the heater in an "enable" state during the rest of the time the machine is powered up. It is preferred that the solid state relay implement zero-crossing AC switching so as to minimize noise.

Many alternatives to the above embodiment are possible. Several such alternatives are discussed herein next.

In an alternative embodiment, the desired chamber temperature set point may be hard coded or set in various other manners known to those of skill in the art. The ramping function may produce linear increasing ramps and decreasing ramps or the ramping may be based on a non-linear function. The comparator may produce a linear or non-linear modulation signal when the temperature difference is between values V.sub.L and V.sub.H.

The analog processing and analysis may be replaced by digital processing. The ramp generation of elements 114-118 and the comparator 120 may be replaced by a look-up table having duty cycle information and possibly time period information based on the temperature difference that has been determined.

In an alternative embodiment, the heat supplied by the heater may be supplied at a variable rate that is based on a temperature difference between the set point and an actual measured temperature but only at certain times when enabled by a modulated signal. The modulated signal may be applied using software 122 and logic 124. Alternatively, the modulated signal may be generated by hardware. This modulated signal causes the heater to be enabled only a portion of the time that the comparator provides a positive signal for the heater to be activated. For example, this additional modulation may operate with a 50% or greater duty cycle with a period of a few seconds.

In a further alternative, a variable amount of heat may be supplied without the use of any on-off modulation. Instead, a variable current may be supplied to the heater based on the temperature difference supplied by amplifier 112. This variable current may be based on a look-up table correlating output heat to input current or electrical power.

In a further alternative, the temperature may be measured at more than one location. In this alternative, the measured temperatures may be averaged or otherwise combined to yield a single value for comparison to a set point temperature. It is preferred that temperature measurements be made in the air or other atmospheric environment of the build chamber; however, in alternative embodiments the temperature of the building material or of some other solid or liquid component in the build chamber may be measured. If the temperature of the building material is measured it may be preferable to measure, it near its upper surface than its lower surface.

In a further alternative, the heat flow may be variably directed to selected locations in the build chamber based on multiple temperature measurements. This selective directing of heat flow may be implemented based on ducting and selective control of airflow through those ducts (e.g. by controlling the flow rate of one or more fans). Alternatively, selective application of heat may occur via two or more heaters located in selected portions of the build chamber.

Heat detection may occur by any well known sensing means. For example, thermocouples, thermistors, and IR detectors, thermometers, optical detection, and the like, may be used. Heat may be supplied by production of resistive heat through a current carrying component, by induction heating, by IR transfer (i.e. radiation and absorption), by heat transport by gas or liquid flow, conduction, and the like. All or a portion of the heat required for the process chamber may be supplied to the build chamber by directing heat from peripheral heat producing electrical components of the SLA such as one or more power supplies, drivers, computers, radiation generator(s), scanning devices, and the like. Peripheral heat producing components are those components of the SLA that provides some other services to the SLA operation than merely heat production.

In further alternatives, the variation in heat supplied to the building environment may be based on ranges of temperature difference. In other alternatives, the heat supplied may be provided by specification of a desired pulse width and period or based on duty cycle. In some embodiments, the heat supplied may not go completely to zero when the temperature difference goes to zero as it may be anticipated that heat loss to the outside environment will result in some continuous, but probably small, power input needs.

The proportional control of heating in the preferred embodiment provides a build environment that is substantially free of large thermal gradients that can adversely affect the stereolithographic process. The preferred proportional control implementation relies on the principle of pulse width modulation (PWM), where the active state of a control signal is time modulated in proportion to some feedback mechanism (e.g. thermistors). The PWM signal is integrated by the heating element to provide an averaged heat output to the build chamber that does not include any rapid thermal transients.

The the methods and apparatus in the above embodiment and its alternatives can be modified according to various teachings in the above incorporated patents and applications. As noted previously, it is believed that the teachings herein can be applied to other RP&M technologies in order to achieve better process control.

Though a preferred embodiment as been described and illustrated and several alternatives purposed, many additional embodiments and alternatives will be apparent to those of skill in the art upon review of the teachings herein. As such, these embodiments are not intended to limit the scope of the invention, but instead to be exemplary in nature.

Claims

1. A method of forming a three-dimensional object from a plurality of adhered laminae by exposing successive layers of a material to prescribed stimulation, comprising:

providing a build environment in which to form the three-dimensional object; sensing the actual temperature of the build environment in at least one location during formation of the object;

deriving a difference between the actual temperature and a desired temperature;

supplying heat to the build environment via a heating element controlled by a modulated signal, the modulated signal turning the heat on and off according to a selected pulse width or duty cycle that varies in relation to the difference so as to substantially maintain the build environment within a predefined temperature range;

forming a layer of material adjacent to any last formed layer of material in preparation for forming a subsequent lamina of the object;

exposing the material to the prescribed stimulation to form a successive lamina of the object; and

repeating the acts of forming and exposing to form the object from a plurality of adhered laminae.

2. The method of claim 1 wherein the heat is supplied by the heating element that is controlled in response to the temperature difference.

3. The method of claim 1 wherein at least a portion of the heat is supplied by peripheral heat producing components.

4. The method of claim 1 wherein the predefined temperature range is within 0.5.degree. C. of a desired set point temperature.

5. The method of claim 1 wherein the variation occurs based on ranges of temperature difference.

6. The method of claim 1 wherein the pulse width and duty cycle are not zero when the difference is zero.

7. An apparatus for forming a three-dimensional object from a plurality of adhered laminae by exposing successive layers of a material to prescribed stimulation, comprising:

a build environment in which to form the three-dimensional object;

a sensor in the apparatus for sensing the actual temperature of the build environment in at least one location during formation of the object;

a device connected to the apparatus for deriving a difference between the actual temperature and a desired temperature;

a heating system connected to the apparatus, comprising a heating element and a control element for controlling the heating element to cause the heating element to apply heat to the build environment in proportion to the difference so as to substantially maintain the build environment within a predefined temperature range the control element responding to a received modulated signal and further turning the heating element on and off using a pulse width or duty cycle that varies in relation to the temperature difference;

a coating system cooperative with the apparatus in the build environment for forming a layer of material adjacent to any last formed layer of material in preparation for forming a subsequent lamina of the object;

an exposure system cooperative with the apparatus for selectively applying prescribed stimulation to the material to form a successive lamina of the object; and

a computer cooperative with the apparatus programmed to operate the coating system and the exposure system to form the object from a plurality of adhered laminae.

8. The apparatus of claim 7 wherein the control element uses a pulse width or duty cycle that varies based on ranges of temperature difference.

9. The apparatus of claim 7 wherein the control element uses a pulse width and duty cycle that are not zero when the difference is zero.

10. A method of forming a three-dimensional object from a plurality of adhered laminae, comprising:

providing a build environment in which to form the three-dimensional object;

sensing the actual temperature of the build environment in at least one location during formation of the object;

deriving a difference between the actual temperature and a desired temperature;

controlling a heating element to apply heat to the build environment by use of a modulated signal that turns heat on or off according to a selected pulse width or duty cycle that varies in proportion to the difference so as to substantially maintain the build environment within a predefined temperature range;

forming a lamina of the object adjacent to and adhered to any previously formed lamina; and

repeating the act of forming a plurality of times in order to form the object from a plurality of adhered laminae.

11. A stereolithography apparatus for forming a three-dimensional object from a plurality of adhered laminae, comprising:

a build environment within a build chamber in which to form the three-dimensional object;

a sensor in the apparatus for sensing the actual temperature of the build environment in at least one location during formation of the object;

a device connected to the apparatus for deriving a difference between the actual temperature and a desired temperature;

a heating system connected to the apparatus, comprising a heating element and a control element for causing the heating element to apply heat to the build environment surrounding the build object in the build chamber in proportion to the difference so as to substantially maintain the build environment within a predefined temperature range;

means cooperative with the build chamber for forming a lamina of the object adhered to any previously formed lamina; and

means for repeatedly operating the means for forming a plurality of times in order to form the object from a plurality of adhered laminae in an accurate and reliable fashion.

12. A method of forming a three-dimensional object from a plurality of adhered laminae by exposing successive layers of a material to prescribed stimulation, comprising:

providing a build environment in which to form the three-dimensional object;

sensing the actual temperature of the build environment in at least one location during formation of the object;

deriving a difference between the actual temperature and a desired temperature;

supplying heat to the build environment via a heating element in relation to the difference so as to substantially maintain the build environment within a predetermined temperature range, at least a portion of the heat being supplied by peripheral heat producing components;

forming a layer of material adjacent to any last formed layer of material in preparation for forming a subsequent lamina of the object;

exposing the material to the prescribed stimulation to form a successive lamina of the object; and

repeating the acts of forming and exposing to form the object from a plurality of adhered laminae.