US 20050013890 A1
A solid imaging apparatus and method produces an integral three-dimensional object from a multiplicity of cross sectional portions of the object by selectively exposing successive layers of a liquid photoformable composition to actinic radiation. The apparatus includes a vessel for containing the composition so as to present a free surface, and a movable platform disposed within the vessel below the free surface. Part of the composition is transferred above the free surface by lowering and raising a dispenser at predetermined positions located away from the platform. A doctor blade contacts the composition transferred above the free surface, and then moves over the platform to form a substantially uniform layer of the composition.
1. In an apparatus for fabricating an integral three-dimensional object by selectively exposing successive layers of a liquid photoformable composition to actinic radiation, said apparatus including an imaging means for exposing said layers, a vessel for containing a fixed amount of said composition so as to present a free surface at a substantially constant position relative to said imaging means, and a movable platform disposed within said vessel below said free surface, the improvement in said apparatus comprising:
a means to transfer a part of said composition above said free surface; and
a layering means for contacting the composition transferred above said free surface and moving over said platform to form a layer of said composition.
This is a continuation of application Ser. No. 07/884,030 filed May 18, 1992, which is a continuation-in-part of application Ser. No. 07/804,269, filed Dec. 5, 1991, now abandoned, which is a continuation of application Ser. No. 07/488,095, filed Mar. 1, 1990, now abandoned.
1. Field of the Invention
This invention pertains to a solid imaging method and apparatus for fabricating an integral three-dimensional object from a multiplicity of cross sectional portions of the object. More particularly, the cross sectional portions correspond to solidified portions of contiguous layers of a photoformable composition. The method and apparatus use a dispenser in a coating station, which transfers part of the photoformable composition over the free surface of the composition so that a doctor blade may produce a uniform liquid layer.
2. Description of Related Art
Many systems for production of three-dimensional modeling by photoforming have been proposed. European Patent Application No. 250,121 filed by Scitex Corporation Ltd., on Jun. 6, 1987, discloses a three-dimensional modeling apparatus using a solidifiable liquid, and provide a good summary of documents pertinent to this art. U.S. Pat. No. 4,575,330, issued to C. W. Hull on Mar. 11, 1986, describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction. Successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, are automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process. U.S. Pat. No. 4,752,498, issued to E. V. Fudim on Jun. 21, 1988, describes an improved method of forming three-dimensional objects, which comprises irradiating an uncured photopolymer by transmitting an effective amount of photopolymer solidifying radiation through a radiation transmitting material which is in contact with the uncured liquid photopolymer. The transmitting material is a material which leaves the irradiated surface capable of further crosslinking, so that when a subsequent layer is formed it will adhere thereto. Using this method, multilayer objects can be made.
A publication entitled “Automatic Method for fabricating a three-dimensional plastic model with photohardening polymer” by Hideo Kodama, Rev. Sci. Instrum. 52(11), 1770-1773. November 1981, describes a method for automatic fabrication of a three-dimensional plastic model. The solid model is fabricated by exposing liquid photo-forming polymer to ultraviolet rays, and stacking the cross-sectional solidified layers. A publication entitled “Solid Object Generation” by Alan J. Herbert, Journal of Applied Photographic Engineering, 8(4), 185-188, Aug. 1982, describes an apparatus which can produce a replica of a solid or three-dimensional object much as a photocopier is capable of performing the same task for a two-dimensional object. The apparatus is capable of generating, in photopolymer, simple three-dimensional objects from information stored in computer memory. A good review of the different methods is also given by a more recent publication entitled “A Review of 3D Solid Object Generation” by A. J. Herbert, Journal of Imaging Technology 15: 186-190 (1989).
Most of these approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photoformable composition with a laser beam according to a desired pattern and building a three-dimensional model, layer by layer. In addition to various exposure techniques, several methods of creating thin liquid layers are described which allow both coating a platform initially and coating successive layers previously exposed and solidified.
The aforementioned methods of coating, however, are not capable of ensuring flat uniform layer thickness or of producing such layers quickly, or they do not effectively prevent damage or distortion to previously formed layers during the successive coating process and they involve coating only liquid formulations of preferably low viscosity. Furthermore, they omit to recognize very important parameters involved in the coating process such as the effects of having both solid and liquid regions present during the formation of the thin liquid layers, the effects of fluid flow and rheological characteristics of the liquid, the tendency for thin photoformed layers to easily become distorted by fluid flow during coating, and the effects of weak forces such as hydrogen bonds and substantially stronger forces such as mechanical bonds and vacuum or pressure differential forces on those thin layers and on the part being formed.
The Hull patent, for example, describes a dipping process where a platform is dipped below the distance of one layer in a vat. then brought up to within one layer thickness of the surface of the photoformable liquid. Hull further suggests that low viscosity liquids are preferable but, for other practical reasons, the photoformable liquids are generally high viscosity liquids. Motion of the platform and parts, which have cantilevered or beam regions (unsupported in the Z direction by previous layer sections) within the liquid, creates deflections in the layers, contributing to a lack of tolerance in the finished part. In addition, this method is rather slow.
U.S. Pat. No. 2,775,758, issued to O. J. Munz on Dec. 25, 1956, and the Scitex application describe methods by which the photoformable liquid is introduced into a vat by means of a pump or similar apparatus such that the new liquid level surface forms in one layer thickness over the previously exposed layers. Such methods have the aforementioned disadvantages of the Hull method except that the deflection of the layers during coating is reduced.
The patent issued to Fudim describes the use of a transmitting material to fix the surface of a photopolymer liquid to a desired shape, assumably flat, through which photopolymers of desired thickness are solidified. The transmitting material is usually rigid and either coated or inherently nonadherent to the solidified photopolymer. The methods described by Fudim do not address the problems inherent in separating such a transmitting material from a photopolymer formed in intimate contact with the surface of the transmitting material. Whereas the effects of chemical bonding may be reduced significantly by suitable coatings or inherently suitable films, the mechanical bonds along with hydrogen bonds, vacuum forces, and the like are still present and in some cases substantial enough to cause damage or distortion to the photopolymer during removal from the transmitting material surface.
Methods utilizing doctor blades and/or material supply mechanisms have been proposed in such publications as Japanese Patent Application Publication numbers 61-114817, 61-114818, and 61-116322. However, these methods require an exact amount of material or photoformable composition to be added in the vessel every time a layer has to be formed. Also, they require the doctor blade or smoothening blade to have a length equal to the width of the vessel in order to properly operate. Because of this, the systems described in these patents have restrictions necessarily confining the photosensitive material between the doctor blade and part of the vessel at all times. Thus, it becomes very difficult to form a uniform layer in one continuous pass of the doctor blade without ending up with an excess or shortage of material at the end of the pass. In other words, the doctored layer may be either lacking a part of it at the end of one doctoring operation or it may have an excess of material, which will be very difficult to redistribute in order to achieve the proper thickness and uniformity, due to the confined nature of the arrangement. Also, the doctor blade has a tendency to create wave motion in the material surrounding the previously exposed layer causing a disturbing effect, particularly on parts of the previously exposed layer which are partially unsupported.
Thus, it is one of the objects of the present invention to provide an apparatus and a method for fabricating an integral three-dimensional object from a multiplicity of cross sectional portions of the object, the cross sectional portions corresponding to solidified portions of contiguous layers of a photoformable liquid composition, in a fast and uniform manner. Another object of the present invention is to provide a gentle way of raising part of the photoformable composition above the surface of said composition and in front of the doctor blade. Use of a pump to recirculate a liquid of the nature used in solid imaging or stereolithography does not present a viable solution because the viscosity and mainly sensitivity of such compositions cause blockage of the paths and seizure of the pumping operation at an unacceptably high rate. Premature polymerization within the higher-shear components of the pump seem to be the most probable cause of this problem.
The present invention comprises a solid imaging apparatus and method for fabricating an integral three-dimensional object by selectively exposing successive layers of a liquid photoformable composition to actinic radiation. The apparatus includes a vessel for containing the composition so as to present a free surface, and a movable platform disposed within the vessel below the free surface. Part of the composition is transferred above the free surface by lowering and raising a dispenser at predetermined positions located away from the platform. A doctor blade contacts the composition transferred above the free surface, and then moves over the platform to form a substantially uniform layer of the composition.
The present invention is directed to a solid imagine method and apparatus for fabricating an integral three-dimensional object from a multiplicity of cross sectional portions of the object. More particularly, the cross sectional portions correspond to solidified portions of contiguous layers of a photoformable composition. The method and apparatus use a dispenser in a coating station, which transfers part of the photoformable composition over the free surface of the composition so that a doctor blade may produce a uniform liquid layer.
The radiation beam 12 passes through the modulator 14, preferably an acousto-optical modulator. The modulated radiation beam 12′ passes in turn through the deflection means 16 or scanner, which comprises two mirrors 20 and 22, each mirror having an axis (not shown) allowing reflection of the beam to a free surface 46 in X and Y directions, the X and Y directions being perpendicular to each other and parallel to the free surface 46. The mirrors 20 and 22 may rotatably move around their corresponding axes by means of motors 24 and 26, respectively, for controllably deflecting the beam in a vector scanning mode, in the X and Y directions, towards predetermined positions of a photoformable composition 40 contained in a vessel 44 of the coating station 71. As the beam is deflected by the deflection means 16, it assumes an acceleration from zero level to a maximum acceleration, and a velocity from zero level to a maximum constant velocity. The velocity and intensity of the beam remain proportional to each other, so that the exposure remains substantially constant. The beam 12″ exposes preselected portions of the composition to a substantially constant depth as described below.
For the purpose of this invention, the radiation beam 12″ may be not only a focused beam from a laser, but also light from any other light source, modified in a number of different ways. For example, it may be transmitted through any type of variable optical density photomask such as a liquid crystal display, silver halide film, electro-deposited mask etc., or reflected off of any variable optical density device, such as a reflective liquid crystal cell. Also, the deflection means may be any other type of scanner, such as a raster scanner, for example.
The coating station 71 comprises a vessel 44 for containing the liquid photoformable composition 40. A substantially flat platform 41 is disposed within the vessel 44 and adapted to be positioned under the free surface 46 of the composition 40. The platform 44 has sides, such as a left L and a right R side. A placement means 42 is provided for controllably varying the distance between the free surface 46 of the composition 40 and the platform 41 through a supporting arm 42′. Although the placement means 42 is shown in
In operation of the preferred embodiment of this invention, the radiation means 10 provides a radiation beam 12 having an intensity as aforementioned. The radiation beam 12 passes through a modulator 14, where its intensity may be modulated from a zero intensity level to a maximum intensity level having a value less than that of the unmodulated beam intensity, due to energy losses. The modulated radiation beam 12′, having somewhat decreased intensity due to losses, passes in turn through the deflection means 16 having a two-mirror 20 and 22 assembly, each mirror separately driven by a different motor 24 and 26, respectively. Mirror 20 deflects the beam in a X direction, while mirror 22 deflects the beam in a Y direction, the X direction being perpendicular to the Y direction. Electrical feedback regarding the relative movements of the mirrors 20 and 22 is provided by the deflection means 16 to the computer 34 through line 54. This feedback, being correlatable to the velocity and average residence time of the beam 12″ on the predetermined portions of the thin layer 48, is processed by the computer 34, and it is fed to the modulation means 14 as a control command through line 50 in order to modulate the intensity of the radiation beam 12, so that the product of the intensity of the beam 12″ and the average residence time at each position of the predetermined portions of layer 48 remains substantially constant. Thus, the exposure level, being by definition the product of these two parameters, remains substantially constant. By maintaining the exposure level constant over the predetermined portions of each contiguous thin layer, the thickness of the layers is also kept substantially constant. This correction or compensation is very important, especially at unsupported portions of the thin layers, where swollen edges will appear as a result of overexposure due to the low initial velocity at the edges in vector scanning. The higher the intensity of the beam 12″, or the higher the photosensitivity of the photoformable composition, the more severe this problem becomes in the absence of means to maintain the exposure level constant. Such exposure control is also necessary in raster scanning or in systems incorporating overscanned vector schemes, the difference being that the edges of the image may be underexposed due to lack of exposure contribution from adjacent non-exposed regions. In these cases, modulation means are utilized to ensure that edge regions of the image receive substantially the same exposure as non-edge regions. In any event, the radiation beam 12″ is controllably directed towards the photoformable composition 40.
The platform 41, which has a substantially flat upper surface 41′, is initially placed within the vessel 44 in a way that the flat upper surface 41′ is contained within the free surface 46 of the composition 40. In sequence, the platform 41 is lowered in the composition 40 by the thickness of the layer 48. The dispenser 43, which is preferably kept at least partially dipped under the free surface 46 of the photoformable composition 40 when not in motion, is raised and starts dispensing liquid composition 40 between the doctor blades 73 and 73′. The doctor blade 73 then produces a uniform liquid layer 48 on top of the substantially flat surface 41′ of platform 41. In
After this fast imaging step, the platform 41 is lowered again by the thickness of the layer 48. The dispenser 43, which was now kept partially dipped under the free surface 46 of the photoformable composition 40 at the left side L of the platform, is raised and starts dispensing liquid composition 40 between the doctor blades 73 and 73′. The doctor blade 73′ then produces a uniform liquid layer 48 on top of the platform 41 and previously photoformed layer as the assembly of blades 73 and 73′ and dispenser 43 now moves towards the right side R of the platform 41. When the assembly of the doctor blades 73 and 73′ and the dispenser 43 reaches the right side R of the platform 41 they stop again, and the dispenser 43 is dipped in the composition 40 under the free surface 46. A short time may be allowed again, if necessary, for the free surface 46 to reach equilibrium and assume the desired uniformity. At least a portion of the liquid layer 48, now being on top of the previously imagewise exposed layer, is exposed imagewise to the laser beam 12 a. The above steps are repeated until all contiguous layers have been produced and the three dimensional object has been completed. All the above steps are coordinated by the computer 34 in a conventional manner.
In the present invention, the equilibrium level of the free surface 46 always remains substantially constant, regardless of the distance moved by the platform, because the amount of photoformable composition 40 within the vessel 44 remains the same since no additional composition 40 is added. The composition needed for successive layers 48 is transferred above the free surface 46 by lowering and raising the dispenser 43 at predetermined positions alongside the platform 41. Since the dispenser 43 dips under the free surface 46 and directly transfers part of the composition above the free surface 46, the temporary level of the free surface 46 will be lowered, relative to the previous equilibrium level, due to transfer of some of the composition and the dispenser 43 above the free surface 46. However, after the dispenser 43 is again dipped into the composition 40 below this temporary free surface 46, the free surface 46 quickly returns to its equilibrium level. Consequently, the equilibrium level of the free surface 46 will always remain substantially the same, thereby ensuring that the distance between the deflection means 16 and the free surface 46 remains substantially constant. It is critically important that this distance remain substantially constant in order that the laser beam 12″ remain focused precisely at the surface 46 of the composition so as to achieve dimensionally photoformed layers. Even though a typical photoformable composition 40 may change in volume upon polymerization by shrinking approximately one (1) percent, in practice such a change in volume is not significant and does not require any fine adjustments in the equilibrium level of the free surface 46 or the adding of additional composition 40, particularly when the mass of the object being fabricated is less than thirty (30) percent of the mass of the composition 40 in the vessel 44. Usually, the mass of the fabricated part is between one (1) and five (5) percent of the mass of the composition in the vessel 44. Du Pont's SOMOSä solid imaging materials are sufficiently close to “ideal” such that no fine turning of the equilibrium level of the free surface 46 is necessary during the fabrication process. It is also significant in the present invention that the dispenser 43 allows the temporary level of the free surface 46 to be lowered while the doctor blade 73 moves across the platform 41, so that the doctor blade 73 minimizes any type of wave motion in the composition surrounding the previously exposed layer, thereby preventing any such wave motion from disturbing the previously exposed layer, particularly those parts of the exposed layer which are partially unsupported.
In operation, referring back to
After the dispenser 43 has been raised, both the dispenser 43 and the doctor blade 73 move forward with the dispenser 43 leading and the doctor blade 73 following. The distance between the previously solidified layers 11 and the doctor blade 73, when the doctor blade 73 is passing above the solidified layers 11, is maintained constant and corresponds to about the thickness of the layer 48. After a full pass, a short time may be allowed for the surface 46 of the composition 40 to stabilize, after which the step of exposing image wise is performed. The speed of travel of the assembly of dispenser 43 and doctor blade 73 should be lower than a certain limit in order to avoid air entrapment in the form of bubbles. This limit depends on the rheological and foaming characteristics of the photohardenable composition 40. With the photohardenable compositions employed by the applicants, speeds of less than 1 inch per second, and preferably about 0.5 inch per second are adequate to cause only minimal air entrapment.
The dispenser 43 may be shaped like a trough as shown in
In another embodiment of the present invention, illustrated in
Still another embodiment is shown in
Dispenser 1243, in a different embodiment shown in
Still another form of dispenser 1343 is shown in
As shown in
The dispenser 1643 shown in
The front of the doctor blade 1743, as shown in
Another embodiment, shown in
In still another embodiment illustrated in
Two doctor blades 2173 and 2173′ may be used, one on each side of the dispenser 2143, as illustrated in
The embodiment illustrated in
In the cases where the dispenser and the doctor blade are separate units, it is often desirable to dip the dispenser in a particular manner. For example, as shown in
In most cases it is desirable for the dispenser 43 to be in the dipped position while exposing the layer, so that material still held by the dispenser will be in the container and will not change the level of the free surface of the composition and of the layer. As mentioned before, it is important for the dispensed liquid to be very close to the free surface of the composition in order to avoid splashing and entrapment of air in the form of bubbles. Of course, one can work uder vacuum, thereby making the height at which the dispenser operates immaterial.
Since all these devices are controllable by a computer, one can arrange the delivery cycle as well as their speed of operation in order to obtain maximum efficiency and uniformity. Depending on the viscosity and other properties of the photoformable liquid composition, somewhat different conditions may be needed to obtain optimal results. Sensors such as ultrasonic, infrared, and the like may be used to give feedback to the computer regarding the build-up in front of the doctor blade and regulate, accordingly, the delivery through the above mechanism.
The photoformable compositions which can be used in the practice of the instant invention are any compositions which undergo solidification under exposure to actinic radiation. Such compositions comprise usually but not necessarily a photosensitive material and a photoinitiator. The word “photo” is used here to denote not only light, but also any other type of actinic radiation which may transform a deformable composition, preferably a liquid, to a solid by exposure to such radiation. Cationic or anionic polymerizations, as well as condensation and free radical polymerizations and combinations thereof are examples of such behavior. Cationic polymerizations are preferable, and free radical polymerizations even more preferable. Photoformable compositions containing thermally coalescible materials are of even higher preference.
A liquid thermally coalescible photoformable composition is a composition which solidifies upon exposure to actinic radiation without attaining necessarily its ultimate physical properties, particularly with respect to their adhesive and cohesive characteristics. However, it develops adequate integrity to be handled until such time when further treatment is provided. The composition is considered as coalescible when it comprises particulate matter in dispersed form, which particulate matter undergoes coalescence under a certain set of conditions, such as increased temperature for example. Coalescence is the transformation of a dispersed phase to a cohesive continuous solid phase.
Preferably the photoformable composition comprises a thermally coalescible polymeric cohesive material, a photoformable monomer, and a photoinitiator. Preferably the photoformable material comprises an ethylenically unsaturated monomer. Upon exposure to the actinic radiation, the exposed areas of the photoformable composition must remain thermally coalescible after removing the unexposed areas. This is important to improve both adhesion in the joining surfaces between the layers and cohesion within the layers for a multilayer integral three dimensional object. Actually, cohesive bonds are formed at the joining surfaces by the thermally coalescible material, providing superior properties to the structure of the final three dimensional object. It is also very important to prevent substantial overgrowth of infra posed surfaces, as it will be discussed below.
In the case of photoformable compositions which are not based on coalescible materials, post treatment after the exposure step is not required. In the case where a coalescible material is an essential component of the formulation, further heat treatment is needed for the object to attain its ultimate strength. In such cases, when all the layers of the three dimensional object have been formed by the method described above,, the unexposed portions of the composition may be removed by any conventional means, such as shaking the object, blowing gas towards the object, and the like. Further removal may be achieved by rinsing the object with poor, noncoalescing solvents. Water, alcohols, and in general polar solvents are poor solvents for non-polar compositions and vice-versa. As long as the solvent under consideration does not extract excessive amounts of materials from the exposed area or cause the object being rinsed to swell within the rinsing time, it is considered to be a poor, non-coalescing solvent. The object then is thermally coalesced in order to develop high cohesive and adhesive strength. This step may be performed in an oven, such as a convection, IR or microwave oven. Optimum temperature and time are dependent on the individual composition. Typically the temperature range is 100°-250° C. and the time range is 5-30 minutes. However, temperature and times outside these regions may be used.
A very important group of thermally coalescible materials are plastisols. Plastisols are fluid mixtures, ranging in viscosity from pourable liquids to heavy pastes, obtained by dispersing fine particle size polymeric resins in nonvolatile liquid thermal plasticizers, i.e., materials which are compatible with the polymer or resin and increase its workability and flexibility but have no substantial solvent activity for the resin or polymer under ordinary conditions of storage (e.g. room conditions). When the plastisol has been formed into a desired shape, e.g., by molding or coating, it can be heated to coalesce the polymeric resin particles and the nonvolatile liquid constituent, thereby forming a homogeneous solid mass. Volatile diluents can be added to plastisol dispersions to modify their viscosity and to achieve desirable handling characteristics in coating or other forming operations.
A dispersion that contains no more than 10% volatile diluent is regarded as a plastisol. Since the plasticizer used in the case of plastisols acts as a plasticizer to solvate the polymer only at temperatures higher than storage temperatures, it may also be called a thermal plasticizer. The most widely used plastisols on a polyvinyl chloride homopolymer in a plasticizer.
The following photohardenable composition was made by mixing thoroughly the following ingredients:
An automobile distributor cap of excellent quality was made by using this photohardenable composition, and the method and apparatus as described hereinabove. The double doctor blade arrangement with the dispenser as shown in