Additive manufacturing method for an optical article with improved surface quality

Abstract

The invention relates to a method of manufacturing an optical article with smoothed top surface having an accurate optical surface including the steps of: - manufacturing an optical article by polymerizing a polymerizable material during an additive manufacturing process, said additive manufacturing process being performed such that the optical article comprises an unfinished top surface, said unfinished top surface having a relief pattern formed by traces of the additive manufacturing process, having at least one depression with regard to a mean curved surface of the surface and being constituted of non-polymerized material, - spinning the article with the unfinished top surface such that said non-polymerized material spreads out to substantially cover the entire top surface of the article and forms a layer of non- polymerized material, - polymerizing the layer of non-polymerized material.

Description

[0001]ADDITIVE MANUFACTURING METHOD FOR AN OPTICAL ARTICLE WITH IMPROVED SURFACE QUALITY FIELD OF THE INVENTION The present invention relates to methods of manufacturing an optical article, for example ophthalmic lenses, by additive manufacturing. BACKGROUND OF THE INVENTIONIt is known to use an additive manufacturing technology to manufacture an optical articlesuch as a lens.For example, additive manufacturing methods comprise three dimensions (3D)-printingprocesses that build objects layer by layer based on a digital model. These methodsencompass various technologies such as inkjet printing or stereolithography and theirvariants.For example, a lens may be produced by successively depositing droplets of liquidcomposition and curing them forming layers of composition. This method, commonlyreferred as inkjet printing, generally controls the shape of the layers by controlling theposition and volume of the deposited droplets whereas the curing step is usually global.Other additive manufacturing methods may also be used such as stereolithography (SLA)and its variants, in which the curing step is usually selective, namely the object is generallyformed layer by layer by selectively hardening the resin with light where the object shouldform. Digital light processing stereolithography (DLP-SLA) is an additive manufacturing method using vat technology. According to DLP-SLA, an image of a slice of the part of the opticalarticle to print is projected on a liquid resin vat surface in order to print a layer. After eachlayer is formed, the part is moved vertically to print a new layer. The part can be moved above or inside the vat. The printing includes a support for the part. At the end of the process, the part of the optical article is removed from the vat. The optical article is thenwashed to remove residual liquid resin.It is possible to manufacture a complete optical lens layer by layer by additivemanufacturing, or to manufacture a complementary optical member which is adjoined byadditive manufacturing to a starting optical member (or existing optical base), using amethod sometimes referred as “build over” technology. Such a build over technology isdisclosed for instance in international applications WO2015004383A1 and WO2020115061A1.Due to the limitations of printing resolution, the resulting object often exhibits a discretizedor stepped pattern across the three dimensions, especially in slanted or curved areas ofthe optical article. As a result, the printed article has rough surface. This pattern thereforedoes not allow to obtain the desired optical function when the optical article is anophthalmic lens or a part thereof. This is because the roughness is too great, renderingthis surface unacceptable for optical application.One known solution is to smooth the surface of the article, for example by mechanical treatment such as polishing or surfacing. This mechanical treatment removes part of themanufactured article, resulting in material waste and potentially altering the original printedgeometry of the optical article. Another solution is disclosed in EP 3842221A1. The surface of the optical article is coated with a curable coating liquid having exactly the same refractive index of the object. However, this method requires a very thick coating layer to cover all the roughness, which can lead to increased manufacturing costs. Furthermore, the original optical function andthe original printed geometry of the optical article may be altered.Thus, there is still a need for a method to reduce the surface roughness of an optical articleproduced by additive manufacturing, without generating waste and / or altering,compromising the original optical function and / or the original printed geometry of theoptical article. INVENTION SUMMARY The present invention aims to remedy all or some of the above disadvantages of the state of the art. Specifically, the invention aims to provide a method to better reduce the surface roughnessof an optical article that has been produced by additive manufacturing in a way that avoidswaste and preserves both the original optical function and / or printed geometry of theoptical article. To solve these problems, the invention provides a method of manufacturing an optical article with smoothed top surface having an accurate optical surface including the steps of: -step a): manufacturing an optical article by polymerizing a polymerizable materialduring an additive manufacturing process, said additive manufacturing process being performed such that the optical article comprises an unfinished top surface, said unfinished top surface having a relief pattern formed by traces of the additive manufacturing process,having at least one depression with regard to a mean curved surface of the surface andbeing constituted of non-polymerized material,- step b): spinning the article with the unfinished top surface such that said non-polymerized material spreads out to substantially cover the entire top surface of the articleand forms a layer of non-polymerized material, -step c): polymerizing the layer of non-polymerized material.No additional polymerizable material is dispensed, is applied to the unfinished top surfaceafter step a) or during step b).In a preferred embodiment of the present invention, an organic solvent or mixture oforganic solvents is dispensed during the spinning of the article. These solvents are capableof solubilizing in particular, the non-polymerized material present on the surface of the optical element after step a). This invention enables to keep the initial geometry printed during the additive manufacturing process and / or improve the surface smoothness of the optical article. As a result, the optical function of the optical article is improved. This method is compatible with various additive manufacturing processes such as DLP-SLA or inkjetprinting, making it easy to integrate into industrial processes. It also eliminates the needfor the conventional cleaning step and creates no waste.BRIEF DESCRIPTION OF THE DRAWING The present invention will be better understood in the light of the following examples, given by way of illustration, with reference to: -Figure 1 which represents the surface profile of the concave top surface of the lensof Comparative Example 1; -Figure 2 which represents the surface profile of the concave top surface of the lensof Comparative Example 2; -Figure 3 which represents the surface profile of the concave top surface of the lensof Comparative Example 3 -Figure 4 which represents the surface profile of the concave top surface of the lensof Example 1; -Figure 5 which represents the surface profile of the concave top surface of the lensof Example 2; -Figure 6 which represents the surface profile of the concave top surface of the lensof Example 3. -Figure 7 which compares the transparency of the lens of Comparative Example 3(Figure 7A) and of the lens of Example 3 (Figure 7B);- Figure 8 which compares the transparency of the lens of Comparative Example 3(Figure 8A) and of the lens of Example 3 (Figure 8B). DETAILED DESCRIPTION OF THE INVENTIONThe method of the present invention is a method of manufacturing an optical article withsmoothed top surface preferably having an accurate optical surface, said method includingthe steps of: -step a): manufacturing an optical article by polymerizing a polymerizable materialduring an additive manufacturing process, said additive manufacturing process being performed such that the optical article comprises an unfinished top surface, said unfinished top surface having a relief pattern formed by traces of the additive manufacturing process, having at least one depression with regard to a mean curved surface of the surface and being constituted of non-polymerized material, -step b): spinning the article with the unfinished top surface such that said non-polymerized material spreads out to substantially cover the entire top surface of the article and forms a layer of non-polymerized material, -step c): polymerizing the layer of non-polymerized material.Preferably, the method of the present invention is a method of manufacturing an optical article with smoothed top surface preferably having an accurate optical surface, said method including the steps of: -step a): manufacturing an optical article by polymerizing a polymerizable materialduring an additive manufacturing process, said additive manufacturing process being performed such that the optical article comprises an unfinished top surface, said unfinished top surface having a relief pattern formed by traces of the additive manufacturing process, having at least one depression with regard to a mean curved surface of the surface and being constituted of non-polymerized material, -step b): spinning the article with the unfinished top surface such that said non-polymerized material spreads out to substantially cover the entire top surface of the article and forms a layer of non-polymerized material, wherein no further polymerizable material is dispensed to the unfinished top surface after step a) or during step b), -step c): polymerizing the layer of non-polymerized material.Optical articles include, but are not limited to, ophthalmic lenses, lenses for opticalinstruments, in photography or astronomy, optical sighting lenses, ocular visors and optics of lighting systems, screens, glazing, windshields, sport masks, face shields, goggles, optical coatings or adhesives. The optical article is preferably an ophthalmic lens. In the present disclosure, the term of “ophthalmic lens” is used to mean a lens adapted to a spectacle frame to protect the eye and / or correct the sight. Said lens can be a non- corrective ophthalmic lens or a corrective ophthalmic lens. Examples of ophthalmic lens include, but are not limited to, afocal, unifocal, bifocal, trifocal, progressive, plano, solarand Fresnel lenses or any other kind of lenses having a discontinuous surface such aslenses with microstructures. In the present disclosure, the optical articles have preferably at least one optical function. This optical function is defined as the optical response of this optical article. In other words, the optical function is defined as any modification in the propagation and transmission of an optical beam through the optical article, whatever the angle of incidence of the incident optical beam and whatever the geometric extent of an input diopter illuminated by the incident optical beam. Said at least one optical function may be simple or complex. More specifically, in the ophthalmic field, the optical function is defined as the distribution of wearer power and astigmatism characteristics and of higher-order aberrations associated with the optical article for all the directions of gaze of a wearer of this optical article. That of course assumes that the geometric positioning of the optical article with respect to the eye of the wearer has been predetermined. As used herein, the term “top surface” refers to the outward-facing surface of the opticalarticle in the final assembled product. In the present disclosure, the term ‘top surface'should be understood as referring to the entire surface of the optical article, including theentire periphery of the optical article, in particular when the article is an ophthalmic lens.The top surface of the optical article can be convex or concave. In particular, the top surface of the optical article can be the front of the lens or the back of the lens. <<Step a>> The polymerization of the polymerizable material during the step a) of the methodaccording to the present invention can be performed by UV-visible irradiation or thermaltreatment. Preferably, this polymerization is carried out by an UV irradiation and more preferably by using a Light-Emitting Diode (LED) Lamp.The additive manufacturing process is preferably a 3D printing method, more preferablyinkjet printing, stereolithography (SLA) or digital light processing stereolithography (DLP- SLA), still more preferably stereolithography (SLA) or digital light processing stereolithography (DLP-SLA). The polymerizable material is for example a photocurable resin based on (meth)acrylate, urethane acrylate or thio-acrylate monomers or oligomers, more preferably based on acrylate monomers or oligomers.As mentioned above, the optical article at the end of the step a) shows an unfinished topsurface having a relief pattern formed by traces of the additive manufacturing process, and having at least one depression with regard to a mean curved surface of the surface, In other words, the optical article exhibits a top surface with a too high roughness, rendering this article unacceptable for optical application.The surface of the optical article is unacceptable for optical application, for example, whenthe observation of an image through said optical article is perceived with significant loss ofcontrast or when the formation of an image through said optical article is obtained withadversely affecting the quality of the image. In other words, the surface of the optical article is acceptable for optical application when the optical article is transparent, namely when the observation of an image through said optical article is perceived with no significant loss of contrast, that is, when the formation of an image through said optical article is obtained without adversely affecting the quality of the image. In the present disclosure the surface roughness of the top surface of the optical article ischaracterized using standard roughness parameters, such as the arithmetic averageroughness (Ra) and the maximum height of the profile (Rz), as defined in ISO 4287. Theseparameters provide a quantitative evaluation of the topography of the surface. The measurements are typically carried out using a surface profilometer, such as a FormTalySurf (FTS) stylus instrument, commercialized by Taylor Hobson Company.According to standard definitions, the arithmetic average profile roughness (Ra) is an amplitude parameter that characterizes a surface based on the vertical deviations of theroughness profile from a mean line. It is defined as the arithmetic average of the absolutevalues of roughness profile ordinates. The Ten Point Height of irregularities (Rz)represents the average value of the absolute values of the heights of five highest-profilepeaks and the depths of five deepest valleys within the evaluation length.At the end of the step a), the surface of the optical article, also named as the initialroughness, is typically characterized by a Ra value equal to or less than 3.5 µm and greaterthan 1 µm and / or a Rz value equal to or less than 20 µm and greater than 3 µm. Preferably,the initial roughness of the optical article is characterized by a Rz = 17 ± 2 µm and a Ra =3.0 ±0.5 µm.By “unfinished” we mean that the top surface is constituted of non-polymerized material.We mean that the surface has a conversion rate of polymerization lower than that desiredfor the finished optical article. As a result, the top surface of the optical article has a non-homogeneous layer of non-polymerized material. This layer can have a thickness between 50 µm and 1 mm. For example, with DLP-SLA technology, this unfinished top surface corresponds to thenon-polymerized material that remains on the surface of the optical article once it has been removed from the vat. In the state of the art, this layer of non-polymerized material or at least partially polymerizedmaterial is usually removed by a cleaning step, typically by dipping the article in a bathcontaining solvents.According to the present invention, this layer is not removed by such cleaning step.The method according to the present invention comprises the use of centrifugal force to homogeneously spread the non-polymerized resin on the surface of the optical article. <<Step b>>The method according to the present invention comprises a step b) of spinning the articlewith the unfinished top surface. In the present method, the spinning step (step b) is performed directly after the additive manufacturing step. In particular, in the case of DLP-SLA additive manufacturing, the spinning is carried out immediately after the optical article has been removed from the vat, while the surface still contains non-polymerized material. Importantly, the method of the present invention does not include any step of adding polymerizable material after step a) or during step b). The smoothing effect is achieved solely by redistributing the existing non-polymerized material present on the surface of the article at the end of the additive manufacturing process.The spinning step can last from few seconds to few minutes, in particular from 2 secondsto 5 minutes and preferably from 2 seconds to 1 minute. For example, the spinning stepcan last 10 seconds or 30 seconds. An advantage of the method according to the present invention is that it does not require any additional processing step or separate workstation on the production line. The spinning step is integrated directly after the additive manufacturing step, which simplifies the overall manufacturing process. This results in a reduction of production time and operational complexity. In addition, the method reduces the risk of handling errors or contamination that could occur when transferring the optical article between separate processing stations. The spin speed applied is selected such that the non-polymerized material spreads substantially over the entire top surface of the article, forming a continuous layer of non- polymerized material. In particular, the speed of the spinning can be equal to or less than 5000 RPM, preferably 4000 RPM, more preferably 3000 RPM, still more preferably 2000 RPM, even more preferably equal to or less than 1400 RPM.In particular, the speed of the spinning can be equal to or less than 2000 RPM, preferablyequal to or less than 1400 RPM, more preferably equal to or less than 1200 RPM, still more preferably equal to or less than 600 RPM, even more preferably equal to or less than400 RPM. The spinning speed is preferably equal to 2000 RPM, 1400 RPM, 1200 RPM,800 RPM, 600 RPM, 400 RPM, 300 RPM or 200 RPM.The spinning speed can also be equal to or greater than 200 RPM and equal to or less than 2000 RPM. Preferably, the spinning speed can also be equal to or greater than 200 RPM and equal to or less than 1400 RPM. In particular, the spinning speed can be between 200 RPM to 2000 RPM, 200 RPM to 1400 RPM, 200 RPM to 1200 RPM, 200 RPM to 600RPM, 200 RPM to 400 RPM, 400 RPM to 600 RPM, 400 RPM to 800 RPM, 400 RPM to1200 RPM, 400 RPM to 1400 RPM, 400 RPM to 2000 RPM, 600 RPM to 800 RPM, 600RPM to 1200 RPM, 600 RPM to 1400 RPM, 600 RPM to 2000 RPM, 800 RPM to 1200RPM, 800 RPM to 1400 RPM or 800 RPM to 2000 RPM.In a particularly preferred embodiment, at least two different speeds can be applied duringthe spinning step, the second speed being higher than the first speed. As a result, the layerof non-polymerized material has a more uniform thickness across the entire top surface ofthe optical article. In other words, the step b) of spinning may include two distinct spinningphases.Each step of spinning can last from few seconds to few minutes, in particular from 2seconds to 1 minute and preferably from 2 seconds to 1 minute. For example, the spinningat each speed can last 10 seconds or 30 seconds. For example, the first speed is equal to or less than 1200 RPM, preferably equal to or less than 600 RPM and the second speed is equal to or less than 2000 RPM, preferably 1400 RPM, more preferably 1200 RPM. The first speed is preferably 200 RPM, 300 RPM, 400 RPM or 600 RPM, more preferably 400 RPM and / or the second speed is preferably 400RPM, 600 RPM, 800 RPM or 1200 RPM, more preferably 600 RPM.For example, the first speed is 600 RPM and the second speed is 1200 RPM or the firstspeed is 200 RPM and the second speed is 400 RPM or the first speed is 400 RPM andthe second speed is 600 RPM or the first speed is 300 RPM and the second speed is 800RPM. In particular, the first speed is equal to or greater than 300 RPM and equal to or less than 1200 RPM, more preferably the first speed is equal to or greater than 400 RPM and equal to or less than 1200 RPM, still more preferably the first speed is equal to or greater than400 RPM and equal to or less than 600 RPM, even more preferably the first speed is 400RPM. <Use of Solvent(s)> In a preferred embodiment of the present invention, a solvent or mixture of solvents isdispensed during the spinning of the article with the unfinished top surface. In suchpreferred embodiment, step b) further comprises the supply of solvent(s).Thus, a solution comprising at least a first organic solvent is dispensed to the unfinished top surface during the spinning, notably to sufficiently fill up the at least one depression insaid unfinished surface. This solution solubilizes the non-polymerized material. Therefore,the viscosity of the solubilized material is lower than that of the non-solubilized material. As a result, the solubilized non-polymerized material spreads more easily on the top surface of the optical article. For example, when the polymerizable material is a resin, the viscosity of the solubilized non-polymerized resin is preferably equal to or less than 400 cP. The first organic solvent in the solution may have an evaporation rate of equal to or greater than 0.8 and equal to or less than 5, preferably equal to or less than 3.0 and more preferably equal to or less than 1.5. The evaporation rate of the solvent can be measured according to ASTM D3539 standard. The solution may also comprise a second organic solvent, said second organic solventshowing preferably an evaporation rate lower than the evaporation rate of the first organicsolvent.The first and second solvents useful for the present invention may be independentlyselected in the group formed by 1-methoxy-2-propanol, isopropyl alcohol, isobutyl alcohol, methanol, diethylene glycol monobutyl ether, 1-tetradecene, water, ethanol, acetone, butanone, toluene, xylene, chloroform, chlorobenzene, benzene, trichlorobenzene, diiodooctane, octanedithiol, glycerol, methylethyl ketone, hexane and naphta. The solution may further comprise one or more additives selected in the group formed ofsurface additives, surfactants, rheology additives, defoamers, air release additives andadhesion additives.The solution of solvent(s) dispensed during the spinning of the article material is free ofpolymerizable material, free of any hardenable liquid coating. The term “free ofpolymerizable material” means that the solution contains less than 1 weight%, preferablyless than 0.5 weight%, more preferably less than 0.2 weight%, and most preferably lessthan 0.1 weight%, of polymerizable material, the “weight%” being relative to the totalweight of the solution. In a particularly preferred embodiment, the solution contains nodetectable traces of polymerizable material. In a preferred embodiment the solution dispensed during the spinning of the articleconsists of a solvent or a mixture of solvents and optionally one or more additives selectedin the group formed of surface additives, surfactants, rheology additives, defoamers, air release additives and adhesion additives.According to this embodiment wherein a solvent is used, the spinning speed during stepc) is typically between 400 RPM and 2000 RPM. In contrast, when no solvent is usedduring step c), the spin speed may be as high as 5000 RPM.According to this embodiment wherein a solvent is used, step b) of spinning may includetwo distinct spinning phases. Thus, step b) may comprise:- a first spinning phase, during which the article with the unfinished top surface is rotatedat a first speed, for example of equal to or less than 1200 RPM, preferably equal to or lessthan 600 RPM, while simultaneously dispensing the solution of solvents onto the unfinished top surface of the article; -a second spinning phase, during which the article at a second speed, for example ofequal to or less than 2000 RPM, preferably 1400 RPM, such that said non-polymerizedmaterial spreads out to substantially cover the entire top surface of the article and forms a layer of non-polymerized material;Wherein the second speed being preferably higher than the first speed.In this embodiment, the solvent solution is preferably dispensed only during the firstspinning phase, i.e during the first part of the spinning step at the lower speed.The first and second speed are defined as above. In particular, the first speed is preferably200 RPM, 400 RPM or 600 RPM, more preferably 400 RPM and / or the second speed is preferably 400 RPM, 600 RPM or 1200 RPM, more preferably 600 RPM. In case the manufacturing process is stereolithography (SLA) or digital light processing stereolithography (DLP-SLA), the method according to the present invention further comprises at least one step of removing the article with the unfinished top surface fromthe vat containing the polymerizable material before the step of spinning the article.<<Step c>> The method according to the present invention advantageously comprises a step c) ofpolymerizing the layer of non-polymerized material.This polymerization can be performed by UV-visible irradiation or thermal treatment. Preferably, this polymerization is carried out by an UV irradiation, more preferably by a broad-spectrum UV irradiation, and still more preferably by a metal halide lamp.In one embodiment, the method may further comprise a step of waiting with no motion ofsaid article between the step of spinning and the step of polymerizing. <<The optical article thus obtained>> A further object of the present invention is the optical article, more particularly the ophthalmic lens obtained by the method defined above. According to the present invention, the top surface of the optical article (obtained after step c) is considered smooth when the surface has a Rz value equal to or less than 3 µm and a Ra value equal to or less than 1 µm.The optical article obtained by the method defined above typically has a top surface havinga Rz value equal to or greater than 0.10 µm and equal to or less than 3 µm, more preferablyequal to or less than 2 µm, and a Ra value equal to or greater than 0.02 µm, preferablyequal to or greater than 0.05 µm and equal to or less than 1 µm. In other words The opticalarticle obtained by the method defined above has a top surface with an Rz value ranging from 0.10 µm to 3 µm, preferably up to 2 µm, and a Ra value ranging from 0.02 µm, preferably 0.05 µm, to 1 µm. The optical surface of the optical article obtained by the method according to the presentinvention is thus accurate since the desired optical function of the optical article is obtained.The optical article obtained after step a), in particular the top surface of the optical articleobtained after step a), may have an initial geometry. Examples of initial geometry include,but are not limited to, the curvature of the lens, the complex surface geometry of aprogressive addition lens and the microstructured surface of a lens. In particular, the initialgeometry can include intentionally designed relief patterns such as structures,microstructures or notches which are not artifacts of the additive manufacturing processbut were deliberately integrated into the top surface of the optical article during the additive manufacturing process. This initial geometry can serve the optical function of the optical articleSuch initial geometry is maintained after step c) of the method according to the presentinvention. EXAMPLES Hereinafter, the present disclosure will be described by way of examples. However, the present disclosure should not be construed as being limited to these examples. Characterization methodsThe surface roughness of the concave surface of the lenses was characterized by using asurface profilometer, a Form TalySurf (FTS) stylus instrument commercialized by TaylorHobson Company. The stylus had a 2 µm diamond point and was moved on the receivingsurface at a 1 mm / s speed with a constant pressure. The Roughness average (Ra) andthe Ten Point Height of Irregularities (Rz) were measured for each surface profile.As mentioned above the arithmetic average profile roughness (Ra) is an amplitude parameter that characterizes a surface based on the vertical deviations of the roughness profile from a mean line. The arithmetic average profile roughness (Ra) is the arithmetic average of the absolute values of roughness profile ordinates. The Ten Point Height of irregularities (Rz) is the average value of the absolute values of the heights of five highest-profile peaks and the depths of five deepest valleys within the evaluation length.The measurement error for Ra and Rz is + / - 0.8 nm.Moreover, the lenses were all manufactured with a notch of 400 µm in depth and 0.4 mmin width on their top surface. In order to characterize the preservation of this initial geometryof the lenses, the depth and the width of the notch on each top surface of the lensesobtained after step c) were measured using the same surface profilometer as described above.For each lens, the absolute relative deviation of the depth and of the width (σ) weredetermined using the following formula:σ (Depth) = (|Depthreference – Depthlens|) ÷ DepthReference, with Depthreference = 400 µmσ (width) = (|Widthreference – Widthlens|) ÷ WidthReference, with Widthreference = 0.4 mmThe initial geometry of the lenses is considered preserved (“saved”) if the absolute relativedeviation of the depth and of the width are equal to or lower than 100%, preferably equal to or lower than 75%. Comparative Example 1 with no cleaning stepA lens was manufactured by DLP-SLA using a resin of acrylate monomers (see Table 2).Each layer of the lens was polymerized by an UV irradiation at λ= 385 nm using a Light- Emitting Diode (LED) Lamp. When all layers had polymerized to make the printed lens (each layer having a conversion rate of the polymerization of the resin between 60% and80%), the lens was removed from the vat containing the resin. At this point, there was alayer of non-polymerized resin on the top surface of the lens. The lens was then left to staywith no motion for 1h30 and then a post UV irradiation with a metal halide lamp was appliedfor 3 min to polymerize the resin. The concave (Comparative Example 1) top surface ofthe lens was characterized. The results are shown in Table 3 and Figure 1 represents thesurface profile of the concave top surface of the lens (Comparative Example 1). Comparative Example 2 with one cleaning stepA lens was manufactured by digital light processing stereolithography (DLP-SLA) using aresin of acrylate monomers as in Comparative example 1 (see Table 2). Each layer of thelens is polymerized by an UV irradiation at λ= 385 nm using a Light-Emitting Diode (LED)Lamp. When all layers had polymerized to make the printed lens (each layer having aconversion rate of the polymerization of the resin between 60% and 80%), the lens was removed from the vat containing the resin. At this point, there was a layer of non-polymerized resin on the top surface of the lens. The lens with non-polymerized resin onits top surface was immersed (i.e dipped) in a commercially available cleaning aqueousbased solution (a mixture of water, butyl diglycol and tetradecene-1 commercialized under the name PLM-403-SUB by PostProcess Technologies) to remove all the non-polymerized resin.The surface roughness after cleaning (Comparative Example 2) is shown in Table 3 andFigure 2 represents the surface profile of the concave top surface of the lens (Comparative Example 2). Comparative Example 3 with one cleaning stepA lens was manufactured using a process substantially identical to that of the lens ofComparative Example 2. In particular, the same resin of acrylate monomers was used andthe same commercially available cleaning aqueous based solution (a mixture of water, butyl diglycol and tetradecene-1 commercialized under the name PLM-403-SUB byPostProcess Technologies) was used to remove all the non-polymerized resin. However,the printer used for digital light processing stereolithography (DLP-SLA) was different from the printer used in Comparative examples 1 and 2. The surface roughness after cleaning (Comparative Example 3) is shown in Table 3 and Figure 3 represents the surface profile of the concave top surface of the lens (Comparative Example 3). Examples 1 and 2 (according to the present invention, with solvent during thespinning step)Three lenses were manufactured by DLP-SLA using the same resin of acrylate monomersas in Comparative examples 1 and 2 (see Table 1 below). For each lens, each layer of thelens was polymerized by an UV irradiation at λ= 385 nm using a Light-Emitting Diode (LED) Lamp. When all layers had polymerized to make the printed lens, the lens was removed from the vat containing the resin. At this point, there was a layer of non-polymerized resin on the top surface of the lens.Each lens with non-polymerized resin on their top surface was then spinned in order tospread the non-polymerized resin homogeneously on the surface. During the rotation ofthe lens at a first specific speed (600 and 400RPM in example 1 and 2 respectively), about1 mL of solvent was dispensed on the top surface of the lens (either IsoPropyl Alcohol(IPA)). Then, the speed was increased to a second specific speed (1200 or600 RPM, inexample 1 and 2 respectively). As a result, the non-polymerized resin spreads out to substantially cover the entire top surface of the lens.After the spinning, a post UV irradiation with a metal halide lamp was applied for 3 min topolymerize the resin.The surface roughness of the concave top surface of the lenses obtained in Examples 1and 2 is shown in Table 3 below. Figure 4 represents the surface profile of the concavetop surface of the lens of Example 1, Figure 5 represents the surface profile of the concave top surface of the lens of Example 2.The surface roughness of lenses Example 1 and 2 was compared with the surfaceroughness of lens Comparative Example 2 with the following formulae:^(Rz, µm)=(RzComp Ex 2-Rzn)÷RzComp Ex 2^(Ra, µm)=(RaComp Ex 2-Ran)÷RaComp Ex 2wherein Rzn and Ran corresponds to the values Rz and Ra for Examples 1 and 2.Example 3 (according to the present invention, without any solvent during thespinning step) A lens was manufactured by DLP-SLA using the same resin of acrylate monomers as inComparative examples 1, 2 and 3 and in Examples 1 and 2 (see Table 1) and using thesame printer as in Comparative Example 3. Each layer of the lens was polymerized by anUV irradiation at λ= 385 nm using a Light-Emitting Diode (LED) Lamp. When all layers hadpolymerized to make the printed lens, the lens was removed from the vat containing the resin. At this point, there was a layer of non-polymerized resin on the top surface of the lens.The lens with non-polymerized resin on its top surface was then spinned in order to spreadthe non-polymerized resin homogeneously on the surface. The lens was spinned at a first specific speed (300 RPM) and then, the speed was increased to a second specific speed (800 RPM). As a result, the non-polymerized resin spreads out to substantially cover the entire top surface of the lens. After the spinning, a post UV irradiation with a metal halide lamp was applied for 3 min to polymerize the resin.The surface roughness of the concave top surface of the lens obtained in Example 3 isshown in Table 3. Figure 6 represents the surface profile of the concave top surface of thelens of Example 3.The surface roughness of lens of Example 3 was compared with the surface roughness oflens Comparative Example 3 with the following formulae:^(Rz, µm)=(RzComp Ex 3-Rzn)÷RzComp Ex 2^(Ra, µm)=(RaComp Ex 3-Ran)÷RaComp Ex 2The effect of the manufacturing process conditions on the surface roughness / smoothnessof the resulting lens is illustrated in figures 7 and 8. As shown in these figures, the qualityof the image obtained through the lens of Example 3 is significantly superior to thatobtained through the lens obtained after a single cleaning step (Comparative Example 3).In particular, the surface of the lens in Example 3 is acceptable for optical application,unlike the surface of the lens in Comparative Example 3. Example 1 Example 2 Example 3Solvent(s) Yes (about 1 Yes (about 1 No supplied during mL, IPA) mL, IPA) the spinning with organic solvent(s) Spinning speed 1600 (10s) 400 (10s) 300 (10s)(RPM) Spinning speed 21200 (30s) 600 (30s) 800 (30s)(RPM) Post UVYes Yes Yesirradiation / polymerization step Table 1 Comparative Comparative Comparative Example 1 Example 2 Example 3 Cleaning step byNo Yes (PLM-403-Yes (PLM- dipping the lens SUB) 403-SUB) in organic solvent(s) Spinning step No (leave for No No1h30) Post UVYes No Noirradiation / polymerization step Table 2 Comp.Ex.1 Ex.2 Comp.Comp Ex.3 Ex.2 Ex.1 Ex.3 Rz (µm) 17.00 2.95 1.77 0.58 15.27 0.32σ(Rz) (%) / 83 90 97 / 98Ra (µm) 3.00 0.72 0.38 0.17 2.12 0.05σ(Ra) (%) / 76 87 94 / 98preservation of the notch246 284 201 12 280 145geometry (depth of the print)(µm) Relative deviation of the39 29 50 97 30 64depth (%) (original depth 400 µm) Width of the print (mm) 0.4 0.7 0.7 2 0.4 0.5Relative deviation of the0 75 75 400 0 25width (%) (original width 0.4mm) Smoothed top surface No Yes Yes Yes No YesInitial geometry Saved Saved Saved Lost Saved SavedTable 3The results of Examples 1 to 3 demonstrate that the surface roughness of the lens isefficiently reduced, without generating waste. These results also show that the range ofthe surface roughness of the lens can be controlled by the spin speed. Thus, the opticalfunction of the lenses of Examples 1 to 3 is improved. Moreover, the method according tothe invention allows preservation of the initial geometry of the lens. In comparison, the traditional method involving a dip-washing step (ComparativeExamples 2 and 3) results in lenses with higher surface roughness. Based on theseresults, it can be concluded that the use of centrifugal force to homogeneously spread thenon-polymerized resin on the surface of the lens before curing, leads to a smoother lenssurface and improved optical properties. Moreover, this method keeps the initial geometry printed during the additive manufacturing process.

Claims

CLAIMS 1. A method of manufacturing an optical article with smoothed top surface having an accurate optical surface including the steps of: ^step a) manufacturing an optical article by polymerizing a polymerizable materialduring an additive manufacturing process, said additive manufacturing process being performed such that the optical article comprises an unfinished top surface, said unfinished top surface having a relief pattern formed by traces of the additive manufacturing process, having at least one depression with regard to a mean curved surface of the surface and being constituted of non-polymerized material, ^step b) spinning the article with the unfinished top surface such that said non-polymerized material spreads out to substantially cover the entire top surface ofthe article and forms a layer of non-polymerized material, ^step c) polymerizing the layer of non-polymerized material.

2. The method according to claim 1, wherein no further polymerizable material is dispensed to the unfinished top surface after step a) or during step b).

3. The method according to claim 1 or 2, wherein the additive manufacturing process is a3D printing method, preferably inkjet printing, stereolithography (SLA) or digital lightprocessing stereolithography (DLP-SLA), more preferably stereolithography (SLA) or digital light processing stereolithography (DLP-SLA).

4. The method according to any one of the preceding claims, wherein the method further comprises at least one step of removing the article from the vat containing thepolymerizable material before the step b) of spinning the article.

5. The method according to any one of the preceding claims, wherein the spin speed isequal or less than 5000 RPM, preferably 4000 RPM, more preferably 3000 RPM, still morepreferably 2000 RPM, even more preferably equal or less than 1400 RPM.

6. The method according to any one of the preceding claims, wherein at least two differentspeeds can be applied during the spinning step c), the second speed being higher thanthe first speed.

7. The method according to any one of the preceding claims, wherein a solution of at least a first organic solvent is dispensed to the unfinished top surface during the spinning, notably to sufficiently fill up the at least one depression in said unfinished surface.

8. The method according to the preceding claim, wherein said first organic solvent shows an evaporation rate of equal to or greater than 0.8, and equal to or less than 5, preferably equal to or less than 3.0 and more preferably equal to or less than 1.5.

9. The method according to claim 7 or 8, wherein said solution comprises a second organicsolvent, said second organic solvent showing preferably an evaporation rate lower thanthe evaporation rate of the first organic solvent.

10. The method according to any one of claims 7 to 9, wherein said solution furthercomprises one or more additives selected in the group constituted of surface additives, surfactants, rheology additives, defoamers, air release additives and adhesion additives.

11. The method according to any one of claims 7 to 10, wherein said first and secondsolvents are independently selected in the group constituted of 1-methoxy-2-propanol,isopropyl alcohol, isobutyl alcohol, methanol, diethylene glycol monobutyl ether, 1- tetradecene, water, ethanol, acetone, butanone, toluene, xylene, chloroform, chlorobenzene, benzene, trichlorobenzene, diiodooctane, octanedithiol, glycerol, methylethyl ketone, hexane and naphta.

12. The method according to any one of claims 7 to 11, wherein the solution consists of asolvent or a mixture of solvents and optionally one or more additives.

13. The method according to claims 7 to 12, wherein the step of spinning includes twodistinct spinning phases:- a first spinning phase, during which the article with the unfinished top surface is rotatedat a first speed, while simultaneously dispensing the solution of solvents onto the unfinished top surface of the article;- a second spinning phase, during which the article is rotated at a second speed, such thatsaid non-polymerized material spreads out to substantially cover the entire top surface of the article and forms a layer of non-polymerized material;wherein the second speed is higher than the first speed and the solvent solution isdispensed only during the first spinning phase.

14. The method according to any one of the preceding claims, wherein the polymerizable material in step a) is a photocurable resin based on (meth)acrylate, urethane acrylate orthio-acrylate monomers / oligomers.

15. The method according to any one of the preceding claims, wherein the optical articleis an ophthalmic lens.

Images