APPARATUS FOR BOTTOM-UP STEREOLITHOGRAPHY WITH AN LCD LIGHT SOURCE WITH LED MATRIX AND TANK WITH ELASTIC MEMBRANE BOTTOM WITH REDUCED AND VARIABLE THICKNESS, AND METHOD OF USE

Abstract

An apparatus for 3D printing of the bottom-up photo-curing type, including an LCD light source with LED matrix, above which is positioned a tank containing a liquid photo-curing material, within which is immersed an extraction plate that moves with reciprocating rectilinear motion along a direction perpendicular to the bottom of the tank, from a position at a distance from the bottom of the tank equal to the thickness of a layer which can be obtained by photo-curing of said photo-curing liquid material, the bottom of the tank formed by an elastic membrane transparent to the radiation of said light source. The tank, light source and other components can be utilized in various methods of bottom-up photo-curing 3D printing, implemented using such an apparatus.

Claims

1-4. (canceled)

5. A bottom-up, photo-curing 3D printing apparatus, comprising an LED matrix LCD light source, above which a tank is arranged, the tank containing a photo-curing liquid material, inside which an extraction plate is immersed, provided with means of movement with alternating rectilinear motion along a direction perpendicular to the bottom of said tank, from a position at a distance from the bottom of said tank equal to the thickness of a layer obtainable by photo-curing of said photo-curing liquid material, the bottom of said tank made of an elastic membrane transparent to the radiation of said light source, said tank positioned in correspondence with an opening of a support plate, said apparatus comprising means for the relative movement of said light source with respect to said elastic membrane, from a position in which the display of said light source is in contact with said elastic membrane, to a position in which the display of said light source is separated from said elastic membrane, wherein the distance between the LEDs of said LED matrix is
dL=√(ETOT/πδLED) wherein ETOT is the nominal energy of the LEDs being used and SLED is the set density of energy and the distance between said LED matrix and said display is defined according to the emission diagram of said LEDs, and, in correspondence of the display, given the emission angle (a) of each LED, is
dLCD=d.Math.L cotg(α/2) to be apt to avoid the use of diaphragms and collimators, and the thickness of said membrane is chosen according to the distance between said LED matrix and said display and is determined as a function of an acceptable error, expressed as a function of the size p of the single pixel of said display and equal to p/2, and is determined by equation
D=p/2cotg(α/2) to introduce a diffusion system, equal to the acceptable error, apt to help compensate the aliasing phenomenon.

6. The 3D printing apparatus according to claim 5, wherein said light source is coupled, with possibility of rotation around a hinge axis, to said support plate, and the opposite side of said light source is coupled to a handling system.

7. The 3D printing method of the bottom-up photo-curing type, implemented by the apparatus of claim 5, comprising the following sequence of steps: a) forming of a solid layer on an extraction plate by photo-curing of a liquid photo-curing material contained within a tank, in the space between an extraction plate and an elastic membrane that forms the bottom of said tank, in which an LCD display is in contact with the underside of said elastic membrane; b) distancing said LCD display from said elastic membrane; c) lifting said extraction plate with progressive detaching of said elastic membrane; d) returning said light source to its initial position, with said LCD display in contact with said elastic membrane; and e) lowering said extraction plate down to a position in which the last layer of photo-cured material is at a distance of one layer to be formed with respect to said elastic membrane.

8. The 3D printing method of the bottom-up photo-curing type, implemented by the apparatus of claim 5, comprising the following sequence of steps: a) forming of a solid layer on an extraction plate by photo-curing of a liquid photo-curing material contained within a tank, in the space between an extraction plate and an elastic membrane that forms the bottom of said tank, in which an LCD display is in contact with the underside of said elastic membrane; b) distancing said elastic membrane, and said tank, and at the same time of said extraction plate, from said LCD display with progressive detaching of said elastic membrane from said display; c) returning said elastic membrane, and said tank said to their initial positions, in contact with said display, said extraction plate remaining motionless, with progressive detaching of said elastic membrane from the last layer of photo-cured material of the object being formed; and d) lowering said extraction plate down to a position in which the last layer of photo-cured material is at a distance of one layer to be formed with respect to said elastic membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0082] The invention is now described, by way of example and without limiting the scope of the invention, according to a preferred embodiment, with reference to the accompanying drawings, in which:

[0083] FIG. 1 shows a perspective view of an apparatus for bottom-up stereolithographic 3D printing with a light source of the LCD type with LED matrix and an independent extraction tank with an independent elastic membrane bottom with reduced and variable thickness according to a first embodiment of the invention, in a first position, suitable to allow a curing step of a layer of a three-dimensional object to be printed;

[0084] FIG. 2 shows a perspective view of a portion of the apparatus of FIG. 1, in a second position, to allow the detachment from the bottom of a newly formed layer of a three-dimensional object to be printed;

[0085] FIG. 3 shows an exploded perspective view of the apparatus of FIG. 1;

[0086] FIG. 4 shows a perspective view of an apparatus for bottom-up stereolithographic 3D printing with a light source of the LCD type with LED matrix and an independent extraction tank with an independent elastic membrane bottom with reduced and variable thickness according to a second embodiment of the invention, in a first position, suitable to allow a curing step of a layer of a three-dimensional object to be printed;

[0087] FIG. 5 shows a perspective view of a portion of the apparatus of FIG. 4, in a second position, to allow the detachment from the bottom of a newly formed layer of a three-dimensional object to be printed; and

[0088] FIG. 6 shows an exploded perspective view of the apparatus of FIG. 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0089] Referring preliminarily to FIGS. 1-3, the elements of an apparatus for bottom-up stereolithographic 3D printing with a light source of the LCD type with LED matrix and an independent extraction tank with an independent elastic membrane bottom with reduced and variable thickness according to the invention essentially comprise a tank 10 (which can be considered as a consumable), with a bottom 11 constituted by a membrane made of free-field elastic material; a light source 12 of the LCD type with LED matrix, provided with a display 13 or LCD matrix and a LED matrix 14, distanced from the display 13 by means of a support structure 15; and an extraction plate 16 with a respective movement and support system 17, the extraction plate 16 being designed for housing on its lower surface the first layer of the object to be printed, obtained by photo-curing of the photo-curing liquid material by the effect of the radiation of the LCD light source 12, as well as for progressively extracting the object from the tank 10, with the alternative lifting and partial lowering movement typical of bottom-up photo-curing type 3D printing systems.

[0090] The tank 10 and the light source 12 are coupled to the rest of the apparatus by means of a support plate 18, which has a hole for the passage of the radiation coming from the light source 12 and directed through the bottom 11 of the tank 10 in order to obtain the curing of the layers which will form the object to be printed.

[0091] In particular, the bottom 11 of the tank 10 consists of a membrane of elastic type (elastic membrane in free field), inserted with preload (that is, with a certain degree of tension) between the walls of the tank 10.

[0092] The light source 12, as mentioned, is of the LCD type with LED matrix, or more precisely, with LED backlighting of the so-called “luminous mat” type, to allow, by means of a dedicated microprocessor, dynamically acting on the various portions of backlighting, optimising them on the basis of each individual frame being reproduced, thus significantly improving the contrast. Between the LED matrix 14, which forms the “light mat”, and the LCD display 13, there is a support structure 15, the function of which is to maintain the defined distance between the LED matrix and the LCD display 13.

[0093] A first side of the body of the light source 12 is coupled with a possibility of rotation about a hinge axis to the support plate 18, while the remaining sides of the body of the light source 12 are free with respect to the support plate 18. The body of the light source 12 is connected to a movement system (not shown), which rotates the body of the light source 12 about the hinge axis, moving the LCD display 13 away and subsequently bringing the LCD display 13 close to the bottom 11 of the tank 10.

[0094] Thanks to this configuration, each phase of polymerisation of the liquid resin by exposure to the radiation of the light source 12 to obtain a layer of the object being printed, is accomplished while the LCD display 13 is rigidly attached to the bottom 11 of the tank 10, allowing the system to behave like a classic bottom-up photo-curing 3D printing machine, wherein the layer being formed is compressed between two rigid bodies, with the consequent advantage of a high compression and precision of the layer (there is no problem of the string that would be generated by an elastic membrane without reference), but at the same time, the suction cup effect would be generated.

[0095] In order to counteract the occurrence of the suction effect, in the following phase the light source 12 is rotated around the hinge axis, detaching itself from the elastic membrane forming the bottom 11 of the tank 10, which instead remains attached by the suction effect to the newly cured layer. Subsequently, the extraction plate 16 is raised to detach the layer from the elastic membrane of the bottom 11 of the tank 10. The elastic membrane triggers the peeling phenomenon by gently peeling off from the newly formed layer. The removal of the LCD display 13 from the base of the elastic membrane of the bottom 11 of the tank 10 allows detachment of the newly formed layer, reducing/eliminating the suction cup effect previously described. In the next step, the elastic membrane, detaching itself from the newly formed layer, returns to its rest position. Subsequently, the extraction plate 16 descends towards the bottom 11 of the tank 10, returning to the position of printing the next layer. Finally, in the last step, the body of the light source 12 is rotated around the hinge axis to return to its starting position, so that the formation of the next layer can be started.

[0096] It is evident that the printing process described allows the suction cup effect to be reduced/removed, allowing a gentle removal of the elastic membrane of the bottom 11 of the tank 10 from the newly formed layer, thanks to the peeling effect resulting from the progressive moving away of the extraction plate 16 from the bottom 11. At the same time, when forming the layer, the position of the LCD display 13, in contact with the bottom 11 of the tank 10, allows a layer to be made with high compression and precision.

[0097] Preferably, an arrangement is applied to the interface between the LCD display 13 and the elastic membrane forming the bottom 11 of the tank 10 which results in a greater adhesion between the LCD display 13 and the elastic membrane which is greater than that established between the elastic membrane and the last formed layer of an object being printed, inducing a peeling phenomenon between the LCD display 13 and the elastic membrane forming the bottom 11 of the tank 10. This arrangement could, by way of example, comprise a pressure/decompression system, or the presence of a layer of adhesive component arranged between the LCD display 13 and the elastic membrane forming the bottom 11 of the tank 10.

[0098] This arrangement has the result of increasing the suction cup effect between the LCD display 13 and the bottom 11 of the tank 10, thanks to which, after each step of polymerisation of the liquid resin due to the exposure to the radiation of the light source 12 in order to obtain a layer of the object being printed, in the following step, when the body of the light source 12 is rotated around the hinge axis, due to the fact that the adhesion force between the rigid support the LCD display 13 and the elastic membrane is greater than the adhesion force generated between the elastic membrane and the newly cured layer, the display 13 tends to carry the elastic membrane with it, allowing a controlled detachment (reverse peeling) of the elastic membrane from the display 13, with a consequent reduction of the mechanical stress to which the elastic membrane is subjected. Moreover, the movement of the elastic membrane away from the newly formed layer, which results from the fact that the elastic membrane tends to follow the LCD display in its movement, generates a volume below the newly cured layer, which is filled by the liquid resin, thus increasing the filling speed of the space between the newly formed layer and the elastic membrane (refresh), making it unnecessary to move the extraction plate 16 away from the bottom 11 of the tank 10, and then bring it closer again to proceed with the formation of a new layer.

[0099] Alternatively, according to the invention, the movement of the light source 12 with respect to the bottom 11 of the tank 10 can be achieved by a translation movement.

[0100] In particular, according to this alternative embodiment, referring to FIGS. 4-6, the body of the light source 12′ is mounted on a motor which allows the translation of the light source 12′, and in particular of the LCD display 13′, from a first position, in contact with the bottom 11, to a second position, wherein the bottom 11 is free.

[0101] In the printing step, the light source 12′ is initially positioned under the extraction plate 16, so that the LCD display 13′ is interposed between the elastic membrane of non-stick material forming the bottom 11 of the tank 10 and the LED matrix source 14′. Once in position, the irradiation and generation of the first layer of the object to be printed is performed. After the first layer is formed, the light source 12′ is moved to a second position, which is not under the extraction plate 16. The extraction movement of the extraction plate 16 is then performed in a condition in which the elastic membrane of non-stick material constituting the bottom 11 of the tank 10 behaves like a free-field membrane. At this point, the light source 12′ is again moved to the position below the extraction plate 16, for the formation of a second layer of the object to be printed. The process described above is continued until the object is printed.

[0102] It is clear that, by using this method, the suction effect is not only contained but definitively eliminated, like a suction cup attached to a sheet of glass, which rather than being deformed on one side to allow air to enter (peeling phenomenon), is actually moved to the edge of the sheet of glass. On this occasion, the perpendicular component, which opposes the detachment of the object from the elastic membrane and generates the suction cup effect, is effectively cancelled out.

[0103] Moreover, according to this embodiment of the 3D printing apparatus according to the invention, it is also possible to achieve a further technical effect. In fact, by translating the LCD display 13′ and keeping the bottom 11 of the tank 10, which is made of flexible non-stick material, stationary, no mechanical stress is generated on the lower surface of the last formed layer of the object being manufactured, since during the translation step of the light source 12′ the newly cured layer and the membrane of non-stick material forming the bottom 11 of the tank 10 remain stationary.

[0104] In particular, as shown in FIGS. 4-6, according to an alternative embodiment of the invention, the LCD display 13′ has an extension equal to half that of the bottom 11 of the tank 10 and the body of the light source 12′ is mounted on a motor which allows the translation of the light source 12′, and in particular of the LCD display 13′ from a first position, in contact with a first half of the bottom 11, to a second position, in contact with the remaining half of the bottom 11.

[0105] According to a further alternative embodiment of the invention, movement of the light source 12 relative to the bottom 11 of the tank 10 may be achieved by a movement of the tank 10 rather than of the light source 12.

[0106] In particular, according to this alternative embodiment, not shown, following the curing step of a layer of the object to be formed, in which the membrane of non-stick material is directly above the LCD display or matrix, the extraction plate and the tank 10 move in an integral fashion upwards to resolve the suction cup effect. The tank with the membrane of non-stick material return to the position above the LCD display, while the extraction plate remains stationary, allowing a second step of removal of the suction cup effect, achieved by peeling between the membrane and the newly cured layer. Finally, the extraction plate is lowered towards the bottom of the tank, to the distance required for the formation of a further layer of the object to be printed.

[0107] This alternative embodiment has the advantage of overcoming a technical limitation, which is extremely complex to resolve, that is typical of prior art embodiments, whereby the mechanical repositioning of the light source 12, 12′ between one layer and another must be extremely precise, otherwise the light source would cure layers that are not aligned with each other, compromising the printing quality. The solution according to the latter embodiment of the invention has, on the other hand, the purpose of solving the problem of repositioning the light source between one layer and another, as well as reducing the cost of making the apparatus.

[0108] This embodiment allows a number of advantages to be pursued: [0109] the resolution of the suction effect is split between the step of lifting the tank and LCD display and the subsequent step of moving the tank away from the extraction plate, splitting the peeling phases in two (rather than resolving the suction effect at once); this reduction in the peeling effect resulting in higher printing quality. In addition, this embodiment solves the technical problem of repositioning the LCD display in its original position.

[0110] The configurations described make it possible to use a light source that is mechanically detached from the tank and, above all, a particularly thin non-stick material, that is negligible in thickness compared to the projection of the light rays. In fact, the thickness of the non-stick material is so thin compared to the light path of the LEDs that the distortion introduced is negligible.

[0111] According to the invention, and as will be described below, the possibility of having an elastic membrane whose thickness is released from mechanical considerations makes it possible to choose the thickness of the elastic membrane constituting the bottom 11 of the tank 10 as a function of the optical paths, in order to introduce a natural anti-aliasing effect, which is able to increase the surface quality of the objects to be printed, as will be explained below.

[0112] As previously mentioned, in bottom-up stereolithographic 3D printing systems using an light source of the LCD type with LED matrix, collimation systems are introduced in order to compensate for image distraction problems. In order to make the collimators effective, a large portion of the light emitted by the individual LED must be eliminated through the use of diaphragms, chosen according to the emission pattern of the LED itself.

[0113] Although the lens on top of the LED source can be chosen to have a narrow beam, in order to allow the collimator to work correctly, the actual usable angular portion of light is extremely small. This is one of the reasons why the percentage of radiation actually usable in the active part does not exceed 20% of the light actually delivered by the LED matrix, with a considerable loss of efficiency on the one hand and of curing performance on the other, not to mention the thermal problems that arise from having to increase the number of LEDs in the matrix (hence the reduction in the distance dL between the LEDs in the matrix).

[0114] Using this strategy, through diaphragms and collimators, in addition to the loss of energy and therefore of system efficiency and performance, an even more limiting problem is introduced, which to date is one of the major limitations for this type of light source, that is to say, it is extremely complex to achieve satisfactory uniformity of illumination, particularly in the transition between one diaphragm and another (so-called “black dots”). This affects the polymerisation of the single layer, which is not uniform and therefore leads to an inevitable loss of quality in the printed objects.

[0115] The possibility of determining the thickness of the elastic membrane of non-stick material forming the bottom 11 of the tank 10 with a particularly thin thickness, that is, without being influenced by limitations of a mechanical nature in the choice of thickness, allows the thickness to be chosen in such a way as to make it possible to eliminate the use of diaphragms and collimators, in order to obtain greater light intensity and at the same time a greater quality of light distribution at the polymerisation interface. In fact, by exploiting the emission diagram of the LEDs and their overlap, by correctly sizing the distance dL between the LEDs of the matrix and the distance dLCD between the LED matrix and the LCD display, it is possible to eliminate the problem of the black dots in the LED matrix transitions.

[0116] In particular, it is possible to define two of the three variables to be characterised (dL and dLCD), which depend on the emission diagram of each individual light source and the power density to be obtained on the display, which is a function of the density of LEDs installed in the matrix; in fact, as the number of LEDs increases, the power at the interface will increase, but the distance dLCD will decrease. Finally, by working appropriately on the D dimension (the thickness of the non-stick material above the LCD display), it is possible to achieve maximum performance levels with regard to reducing the aliasing effect.

[0117] Thus, following the solution according to the invention, it is possible to define one of the 3 variables to be characterised (dL), namely the distance between the LEDs of the emission matrix, which is dependent on the emission diagram of each individual light source. Finally, it will be seen that by working appropriately on the other two variables dLCD (the distance between the LED matrix and the LCD matrix) and D (the thickness of the non-stick material placed above the display), it will be possible to obtain the maximum performance, relative to the reduction of the aliasing effect.

[0118] Imposing, consequently, the geometrical conditions, for the definition of the distance dL it is imposed that the distance between two LEDs, defined on the one hand as a function of the emission diagram, on the other hand determines the value of the thickness of the non-stick layer, once the desired error deviation has been imposed, as described below. The distance dLCD between the LED matrix and the LCD display is a function of the emission profile of the individual LED. In particular, the distance dL between two LEDs of the LED matrix is defined according to the desired energy density and the nominal energy of the LEDs used, and is:

δ.sub.LED=E.sub.TOT/πdL.sup.2

from which it follows that

dL.sup.2=E.sub.TOT/πδ.sub.LED

dL=√(E.sub.TOT/πδ.sub.LED)

[0119] After defining the distance dL between two LEDs of the LED matrix, it is possible to define the distance dLCD of the LCD display, or LCD matrix, from the LED matrix, as a function of the opening angle α of the emission diagram of the individual LED; for the energy on the display to be uniformly distributed, the distance dLCD must be such that, at the display (or LCD matrix), the emission cone of one LED intersects the axis passing through the centre of the adjacent LED. In trigonometric terms, it will therefore be:

dLCD=dL.Math.cotg(α/2)

[0120] A parameter p, representing the size of a single pixel in the LCD matrix, is also defined, and an acceptable error of half a pixel (p/2) is imposed, precisely to introduce a diffusion system, equal to half the maximum permissible resolution, which helps to compensate for the aliasing phenomenon. The thickness D of the non-stick membrane can therefore be appropriately dimensioned by choosing a value that allows the introduction of a diffusion equal to p/2.

[0121] In particular: [0122] dL and dLCD being constant and defined according to the type of LED emission diagram, [0123] the resolution being known and, therefore, the value of the pixel p being defined and constant, one can proceed to calculate the value D, equal to the thickness that must be imposed on the elastic membrane of non-stick material forming the bottom 11 of the tank 10 in order to introduce an error equal to half a pixel. In particular, the dimension D (the thickness of the non-stick material placed above the LCD display) is expressed by the relationship

D=p/2cotg(α/2)=p/2(cos(α/2)/sin(α/2))

[0124] It is evident that the error trend is linear with respect to the thickness D of the anti-aliasing material: as the thickness of the membrane increases, the error introduced also increases and therefore the anti-aliasing effect, while, on the contrary, as the thickness of the membrane decreases, the diffusion decreases to the advantage of precision.

[0125] In conclusion, it is possible to summarise the features of the invention as follows.

[0126] The use of an LCD tilting or translating system, wherein the non-stick membrane can remain fixed during the detachment step, or the system wherein the non-stick membrane moves away while the light source remains fixed, makes it possible to use a non-stick membrane with variable thickness, allowing different intensities of diffusion, and therefore of antialiasing, to be obtained depending on the printing result to be obtained. By using thicker membranes (although still extremely thin), such as 250 micron Teflon membranes, the antialiasing effect can be increased, resulting in smoother surfaces. On the other hand, by working with 90 and 125 micron Teflon membranes, the geometric precision of the printed object can be increased.

[0127] In conclusion, the advantages of the solution according to the invention are as follows: [0128] reduction of the suction cup effect: by resolving the suction cup effect between the LCD display and the non-stick material, no mechanical stresses are introduced during the formation of each newly cured layer. [0129] increase in the light source performance and efficiency: by being able to work with extremely thin membranes, it is possible to build LCD-type light sources with an LED matrix that do not use collimators, diaphragms or reflectors, increasing the energy available for layer polymerisation by a factor of four, and thus reducing the polymerisation time and thermal problems for management of the LED matrix; [0130] better uniformity of illumination: by not having to use collimation systems, and working directly on the distance of the LEDs in the matrix, a better result of uniformity of light irradiation on the polymerisation interface can be obtained, eliminating the problem of the black dots.

[0131] The solution according to the invention, by allowing the use of non-stick materials, such as Teflon, with an extremely reduced but variable thickness, makes it possible to vary the diffusion errors introduced, and consequently makes it possible to have a system for compensating the aliasing phenomenon, which is characteristic of digital projection systems.

[0132] Moreover, the solution according to the invention makes it possible to achieve a reduction in the cost of making light sources. In fact, the possibility of working with systems without collimators, diffusers or reflectors allows a significant reduction in the cost of implementation, and a considerable reduction in the number of LEDs in the matrixes, while also simplifying the associated thermal regulation systems.

[0133] The invention is described by way of example only, without limiting the scope of application, according to its preferred embodiments, but it shall be understood that the invention may be modified and/or adapted by experts in the field without thereby departing from the scope of the inventive concept, as defined in the claims herein.