METHOD AND APPARATUS FOR ISOTROPIC STEREOLITHOGRAPHIC 3D PRINTING WITH A VARIABLE SPEED AND POWER HYBRID LIGHT SOURCE

Abstract

An apparatus for 3D printing of the bottom-up photo-curing type, including a first source of photo-curing radiation, of the DLP type, having a predetermined wavelength, and a second source of photo-curing radiation, of the laser type, having a wavelength equal to that of the first source, the second source using laser deflection and a polarisation coupling optic, the first source having linear polarisation oriented according to a predetermined angle, and the second source having linear polarisation oriented according to an angle orthogonal to that of the first source; the second source having variable irradiating flux power and the laser deflection having variable speed, the irradiating flux power and speed of the laser deflection controlled by predictive software as a function of the time required for photo-curing of each layer by the first source. Embodiments also relate to a method of 3D printing of the bottom-up photo-curing type using the apparatus.

Claims

1-8. (canceled)

9. A 3D printing apparatus of the bottom-up photo-curing type, comprising: a tank containing a photo-curing liquid material, inside which at least one extraction plate is immersed, provided with means of movement with alternating rectilinear motion, in 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 obtainable by photo-curing of the photo-curing liquid material, the apparatus for 3D printing further comprising a first source of a photo-curing radiation, of the DLP type, with a predetermined wavelength, a second source of a photo-curing radiation, of the laser type, with a wavelength equal to that of the first source of photo-curing radiation, of DLP-type, the second source of photo-curing, of laser type, provided with laser deflection means, and a polarizing beam combiner of the radiation of the first source of photo-curing radiation, of the DLP type and of the radiation of the second source of photo-curing radiation, of laser type, the first source of photo-curing radiation, of the DLP type, having linear polarization oriented according to a given angle, or associated with a polarizer configured to allow passage of only the portion of radiation of the first source of photo-curing radiation, of the DLP type, which has linear polarization oriented according to a given angle, and the second source of photo-curing radiation, of the laser type, having linear polarization oriented according to an angle orthogonal to that of the first source of photo-curing radiation, of the DLP type, or associated with a polarizer configured to allow passage of only the portion of radiation of the second source of photo-curing radiation, of the laser type, which has linear polarization oriented according to an angle orthogonal to that of the first source of photo-curing radiation, of the DLP type; the bottom of the tank formed by a material that is transparent to both radiations used for photo-curing, the second source of photo-curing radiation, of the laser type, having variable radiating flux power and the laser deflection means having variable speed, the radiating flux power and the speed of the laser deflection means controlled by a predictive software as a function of the time required for the photo-curing of each layer by means of the first source of photo-curing radiation, of the DLP type.

10. The 3D printing apparatus according to claim 9, wherein the material transparent to both the radiations used for the photo-curing is borosilicate glass or quartz.

11. The 3D printing apparatus according to claim 9, wherein the laser deflection means comprise a galvanometric head.

12. The 3D printing apparatus according to claim 9, wherein the second source of photo-curing radiation, of laser type, comprises a variable power diode.

13. The 3D printing apparatus according to claim 9, wherein the predictive software is a CAD-CAM/Slicer software.

14. A 3D printing method of the bottom-up photo-curing type, implemented by the apparatus of claim 9, comprising the following steps: a) lowering the extraction plate to a position where the last cured layer, or in its absence the lower surface of the extraction plate, is at the distance of a layer to be formed with respect to the bottom of the tank; b) proceeding with the irradiation and the generation of one cured layer of the object to be printed; c) lifting the extraction plate, with progressive detachment of the bottom of the tank from the cured layer; and iteratively repeating the steps a)-c) until completion of the object to be formed, each iteration conducted by setting the speed of the laser deflection means so that it goes through the contour of the layer to be formed in a time equal to the time required for the photo-curing of the same layer by the first source of photo-curing radiation, of the DLP type, at the same time by setting the power of the irradiating flux of the laser according to the set speed of the laser deflection means.

15. The 3D printing method of the bottom-up photo-curing type according to claim 14, wherein, for each layer n being printed, the following conditions are met:
v.sub.lasern=L.sub.shapen/t.sub.laser=L.sub.shapen/t.sub.DLP
P.sub.lasern=dE.sub.laser/dL.sub.shape.Math.v.sub.lasern wherein v.sub.lasern is the laser scanning speed for the layer n, L.sub.shapen is the length of the contour of the layer n, baser is the time taken by the laser to scan the contour of the layer n, t.sub.DLP is the time of persistence of the DLP image for the photo-curing of the layer n, P.sub.lasern is the power of the laser source for the layer n, dE.sub.laser is the useful energy density to be transferred for the curing process and dL.sub.shape is the portion of L.sub.shapen covered in the time dt.sub.lasern.

16. The 3D printing method of the bottom-up photo-curing type according to claim 15, wherein, for each layer being printed, the following condition is met:
dE.sub.laser/dL.sub.shape=k wherein k is a constant value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] 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:

[0059] FIG. 1 shows a perspective view from above of a stereolithographic 3D printing apparatus of the isotropic type, with a hybrid light source of variable speed and power according to a first embodiment of the invention, and

[0060] FIG. 2 shows a perspective view from above of a stereolithographic 3D printing apparatus of the isotropic type, with a hybrid light source of variable speed and power according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0061] With reference to FIGS. 1 and 2, the elements of an apparatus for the stereolithographic 3D printing of isotropic type, with hybrid light source at variable speed and power according to the invention essentially comprise a tank 10 (which can be considered as a consumable element), designed to contain a liquid photo-curing material, the tank being equipped with a bottom 11, transparent to the radiation of a monochromatic DLP light source 12 and to the radiation of a monochromatic laser light source 13, arranged under said tank 10. The tank 10 is arranged above a support plate 14, which in the part below the bottom 11 of the tank 10 has an opening, which can be left open or can be covered with a sheet of a rigid material transparent to the radiation of the monochromatic DLP light source 12 and to the radiation of the monochromatic laser light source 13.

[0062] In particular, the bottom 11 of the tank 10 may consist of an elastic membrane of non-stick material.

[0063] The apparatus further comprises an extraction plate 15 with a respective handling and support system 16, the extraction plate 15 being designed for housing on its lower surface the first layer of the object to be printed, obtained by photo-curing of the liquid photo-curing material due to the effect of the radiation of the monochromatic DLP light source 12 and the radiation of the monochromatic laser light source 13, as will be explained in greater detail below, as well as progressively extracting said object from the tank 10, with the alternative lifting and partial lowering movement typical of 3D printing systems of the bottom-up photo-curing type.

[0064] The monochromatic light source DLP 12, suitably calibrated in terms of focus and projection distance, performs the task of curing the inner portion of each layer of the object being made, with an energy density and dwell time characteristic of each type of resin and layer thickness.

[0065] In particular, according to the invention, the monochromatic laser light source 13 is provided with a variable power diode 17 and a device designed to deflect the laser beam generated by the diode in two dimensions, in particular a variable speed galvanometric head 18, suitably calibrated in order to cure only the edge of each single layer simultaneously with the DLP source and with the same power densities and timing.

[0066] Moreover, in order to obtain isotropic objects, the monochromatic DLP light source 12 and the monochromatic laser light source 13 must also have the same wavelength, that is to say, they must have equal energy.

[0067] In order to achieve the spatial superposing of the two light beams (coaxial) whilst maintaining the same wavelength, the two beams must be polarisation-coupled using a polarising filter.

[0068] In general, assuming that it is possible to identify (or breakdown) the polarisation states of the two light beams along two directions orthogonal to each other and orthogonal to the direction of propagation of each of them, the invention proposes the use of an optic, commonly referred to as a polarisation coupling optic (polarising beam combiner), capable of transmitting one of said polarisation states (typically the so-called “p” polarisation) and reflecting the state orthogonal thereto (typically the so-called “s” polarisation).

[0069] According to the invention, referring to FIG. 1, the method and the relative apparatus for stereolithographic 3D printing of an isotropic type, based on the use of a hybrid light source, comprising a first source 12 of photo-curing radiation, of the DLP type, having a predetermined wavelength, and a second source 13 of photo-curing radiation, of the laser type, having a wavelength equal to that of said first source 12 of photo-curing radiation, of the DLP type, are spatially superimposed along a same direction of propagation by using a polarisation coupling optic 19, at the same time one of said light sources having linear polarisation oriented according to a predetermined angle and the other having orthogonal linear polarisation, in such a way that the polarisation of the light beam of one of said light sources is perpendicular to the incidence plane on said coupling optic (s-pol), the light beam being reflected, whilst the polarisation of the light beam of the other is parallel to the same plane (p-pol), the light beam being therefore transmitted. According to the embodiment shown by way of example in FIG. 1, the light beam 20 of the first source 12, of the DLP type, is parallel to the plane of incidence on the coupling optics 19 and is transmitted and the light beam 21 of the second source 13, of the laser type, is perpendicular to the plane of incidence on the coupling optics 19 and is reflected, the two beams superposing spatially, thanks to the polarisation coupling, to form a beam 22, maintaining the same wavelength.

[0070] The orientations of the polarisations of the two light beams shown in FIG. 1 are purely indicative, that is to say, they can be reversed or oriented at different angles. Similarly, the position of the first source 12, of the DLP type and of the second source 13, of the laser type, with respect to the polarisation coupling optics 19 may be inverted, the polarisation coupling optics 19 being oriented accordingly, with the aim of spatially superimposing the beams coming from the two light sources, in such a way that they have linear polarisations orthogonal to each other.

[0071] Typically, the DLP type sources used for this application emit either a linearly polarised beam in a given first predetermined direction, or a randomly polarised beam, whilst laser type sources are laser diodes which emit a linearly polarised beam in a second predetermined direction. In order to obtain the spatial superposition of the two beams, laser and DLP, it is necessary that they strike the polarisation optics with linear polarisation, one oriented perpendicular to the plane of incidence (s-pol) and the other parallel to it (p-pol). If the polarisation axes of one or both beams are not linear or are not oriented according to this definition, it is always possible to correct their orientation by using so-called “polarising” optics.

[0072] If the polarisation of the beam is linear, but oriented at an angle not adequate for striking on the polarisation optics, it is possible to rotate the orientation with a λ/2 foil with a suitably oriented optical axis. If the beam polarisation is not linear, but is circular, it is possible to transform it into linear by using a suitably oriented λ/4 foil. If the polarisation of the beam is random, it is possible to linearize and orient it by using a polariser, that is to say, a device which works on a principle similar to that of the coupling optics, but orientated in such a way that the mixed polarisation of the starting beam is broken down into its orthogonal components s and p, one of which will be reflected by the optics and the other transmitted. Depending on convenience, one of the two polarisations into which the original one has been split will be diverted to a beam collecting device (target, sensor . . . ) and will not contribute to the 3D printing process, whilst the other one will be effectively directed towards the coupling optics.

[0073] FIG. 2 shows, by way of example, an apparatus for stereolithographic 3D printing of an isotropic type, based on the use of a hybrid light source, according to an embodiment wherein the first source 121 of photo-curing radiation, of the DLP type, with predetermined wavelength, is not polarised, the light beam 201 of that light source 121 being polarised by a polariser 23, the mixed polarisation of the light beam 201 being split into a portion of the beam 203 with s-pol polarisation, which is deflected in the direction 24 towards a beam collecting device (not shown . . . ), and into a portion of the beam 202 directed towards the coupling optics 19. Indifferently, according to a different embodiment, not shown but always realised according to the invention, the source of photo-curing radiation, of laser type, may not be polarised, or both sources may not be polarised.

[0074] The polarisation beam coupling technique provides an additional and innovative advantage for a stereolithographic 3D printing apparatus of the isotropic type, with a hybrid light source, according to the invention with respect to apparatuses of a similar type according to the prior art, such as, for example, those in which the coupling of the light beams takes place in wavelength with a dichroic filter. In fact, unlike the latter, a polarisation coupling optic does not require any type of coating, and is able to guarantee the maximum degree of transmissivity for the p-polarisation and the maximum degree of reflectivity for the s-polarisation, when the beams striking on it are at the same wavelength. An example of such a type of polarisation coupling optics is the so-called Brewster foil.

[0075] The advantage of the possibly to use, if necessary, optics which do not require coating becomes apparent when the wavelength of at least one of the beams (more specifically that of the laser source, as it has a higher density) is in the UV range. In fact, the UV radiation, if it has a sufficient intensity, can trigger a phenomenon of surface degradation of the coating at its interface with the surface on which it is deposited (UV-induced optical damage), which actually creates blackening which worsens over time, as the coating itself absorbs and emphasises the UV radiation striking it.

[0076] Similarly, a Brewster foil may be used as polarisation optics along the path of one or both light beams of the hybrid light source of a stereolithographic 3D printing apparatus according to the invention, to filter out only the linear polarisation component of interest for the purpose of beam superposition in the coupling optics.

[0077] Another advantage of an apparatus for stereolithographic 3D printing with a hybrid light source according to the embodiment of the invention shown in FIG. 2 consists in the fact that, since only a portion of the beam 202 of the light beam 201 passes through the polariser 23, said polariser 23 can be used for the calibration of the stereolithographic 3D printing apparatus with hybrid light source according to the invention, so as to superpose with precision the working area covered by the light beams coming from the two light sources and avoid expensive vision systems which would prevent, for example, real-time verification of the system.

[0078] In order to guarantee correct operation of the radiation of the monochromatic DLP light source 12 and of the radiation of the monochromatic laser light source 13, the 3D printing apparatus according to the invention is equipped with a hybrid software having a hybrid slicer capable of generating for the same layer of the three-dimensional model, on the one hand, the monochromatic image to be projected with the DLP source and, on the other hand, the vector path relative to the edge of each individual layer. Once the energy density has been set, which is constant for each resin and for the thickness of each layer, the software must be able to generate in advance a sequence of instructions capable of defining the speed (a function of the curing time and the size of the path) and the power of the laser for each individual layer, as described below.

[0079] Using a standard DLP light source (projector), t.sub.DLP is defined as the image persistence time for polymerisation and P.sub.DLP is the power generated by the same projector with a predetermined wavelength (usually UV).

[0080] As is well known, for each type of resin and for each thickness of each layer associated with the same resin, we have:

t.sub.DLP=constant;

P.sub.DLP=constant;

[0081] that is, throughout the entire process of forming the object, having fixed the thickness of each layer, the power of the projector and the persistence time of the image associated with the n-th layer do not vary, which is why a DLP type three-dimensional printer is said to be time invariant to the volume of the object being printed.

[0082] The t.sub.laser time is then defined as the time taken by the laser to scan the inside to be cured, and laser P.sub.laser the characteristic power of the laser source at a fixed wavelength, equal to that of the DLP projector. As explained above, the wavelength of the two light sources must be the same in order to obtain an object with isotropic characteristics.

[0083] In accordance with the invention, the following condition is imposed

t.sub.DLP=t.sub.laser=constant

that is to say, the condition is set that for each layer the laser travel time to cure the side edges of the layer is equal to the persistence time of the image produced by the DLP projector. In other words, a condition is created whereby, whilst the DLP is curing the inside of the layer, the laser is simultaneously and in the same amount of time curing the side edges of the same layer.

[0084] This condition, if met, is necessary but not sufficient for isotropic printing, even in continuous mode. In order to guarantee this condition, v.sub.lasern is defined as the laser scanning speed of the nth layer, and L.sub.shapen is the edge length of each individual image n. Obviously, for each individual layer, the edge of the layer to be cured may change as the shape of the three-dimensional object changes (that is, when printing a cone, the edge length tends to decrease linearly with each successive layer).

[0085] Finally, the first mathematical condition underlying the solution according to the invention is defined. Where the speed being defined as

v=s/t

in order to travel the entire length of the edge L.sub.shapen of the n layer in time baser, the speed v.sub.lasern must be equal to:

v.sub.lasern=L.sub.shapen/t.sub.laser

and with the condition for isotropic printing

t.sub.DLP=t.sub.laser=constant

the following is therefore obtained:

v.sub.lasern=L.sub.shapen/t.sub.laser=L.sub.shapen/t.sub.DLP

that is, for each layer, as a function of the length of the edge, the speed of the galvanometer head 18 must vary linearly, in order to guarantee that the scanning time of the laser for the curing step of the edge is equal to the time taken by the DLP source to cure the inner portion of the image.

[0086] Having defined the first condition, it can be seen that, changing the route of the edge L.sub.shapen for each layer and having to maintain the scanning time of the laser t.sub.laser constant and equal to the curing time of the projector t.sub.DLP throughout the entire printing process, it is necessary to work on the v.sub.lasern speed of the galvanometric scanning head. However, in order to obtain isotropic printing, the percentage of completion of chemical cross-linking between the inner part of the layer and the edge must remain homogeneous, which means that the power density to be transferred per unit area must be constant; hence the second condition:

dP.sub.DLP=dP.sub.laser=constant

and as defined above, this varies from resin to resin and for each thickness of the layer, and remains constant throughout the printing process. The condition is therefore imposed that the energy transfer, defined as the amount of photons transferred in the unit of space and time, remains constant, which leads to the condition

E=P.Math.t

where E is the useful energy to be transferred for the curing process, P is the power of the light source at constant wavelength and t is the energy delivery time. Thus, in the unit surface area, turning to the concept of density, we have:

dE=P.Math.dt

[0087] At this point, the first condition defines the speed of the laser as linearly dependent on the path to be scanned, so the persistence time of the laser on the surface unit is reduced in an inversely proportional manner.

[0088] If the isotropy condition is to be met, we have:

dE.sub.laser=dE.sub.DLP=constant

which is a characteristic condition for the entire printing process and is constant for each resin and each layer thickness:

dE.sub.laser=P.sub.n.Math.dt.sub.lasern

from which it follows that, for each layer, we have

dt.sub.lasern=dL.sub.shapen/v.sub.lasern

and therefore

dE.sub.laser=P.sub.n.Math.dL.sub.shapen/v.sub.lasern

from which the second mathematical condition underlying the solution proposed according to the invention is lastly defined.

[0089] Having therefore imposed that the energy density delivered by the laser must be equal and constant for each resin and for each layer thickness, we obtain:

P.sub.lasern=dE.sub.laser/dL.sub.shape.Math.v.sub.lasern

wherein dE.sub.laser/dL.sub.shape=k, where k is a constant value from which it is evident that, as the path to be scanned increases, and thus increasing the speed of the galvanometric head, which must in any case maintain the condition of temporal constancy of the scanning, in order to keep the transferred energy density unchanged, the laser power must vary linearly with respect to the speed.

[0090] For example, imagining that it is necessary to cure a layer with a certain path of the edge, if the second layer has twice the length of the edge, in order to keep the time unchanged, the speed must double, and therefore, since the persistence time is half the previous one, the power of the light source must also double.

[0091] In order to obtain an isotropic printing, which is continuous in all directions, without the aliasing effect, resolution unchanging with respect to the dimensions of the printing plate, the invention proposes an apparatus for 3D printing by photo-curing of bottom-up type, like the one described above with reference to FIG. 1, which comprises a hybrid light source, provided with a monochromatic DLP projector and a suitably calibrated laser source, with variable speed and power, as well as a predictive software capable of satisfying the following conditions for each layer being printed:

v.sub.lasern=L.sub.shapen/t.sub.laser=L.sub.shapen/t.sub.DLP

P.sub.lasern=dE.sub.laser/dL.sub.shape v.sub.lasern

wherein dE.sub.laser/dL.sub.shape=k for each layer, where k is a constant value.

[0092] In conclusion, by using a hybrid source and software as described above, the objectives of the invention can be achieved: [0093] isotropic printing in XY [0094] isotropic printing in Z (if continuous printing) [0095] eliminating aliasing effect [0096] possibility of continuous printing in Z.

[0097] 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.