TITLE

Method for printing a three-dimensional optical structure comprising a hard coating

BACKGROUND

The present invention relates to a method for printing a three-dimensional optical structure, in particular an ophthalmic lens, wherein the three-dimensional optical structure is built up from layers of printing ink deposited through targeted placement of droplets of printing ink at least partially side by side in consecutive printing steps.

Printing three-dimensional optical structures such as ophthalmic lenses is known from the prior art. In three-dimensional printing, a three-dimensional structure is built up from layers of printing ink. Each layer is deposited through a targeted placement of droplets of printing ink at least partially side by side by the ejection nozzles of a print head. The layers are printed at least partially above each other to form the intended three-dimensional structure.

The optical function of optical structures, in particular the optical function of lenses, is typically determined to a large extent by the geometrical design of the surface of the optical structure. Even minor influences on the surface, such as scratches, can have a significant impact on the optical function.

To prevent this, a hard coating can be applied to the surface of the optical structure to prevent mechanical damage to the surface. Typically, such hard coatings are applied to printed optical structures by dip-coating or spin-coating after the printing process and then cured. This is disadvantageous because the printing of the optical structure takes place in a printing apparatus and the optical structure must be transferred from the printing apparatus to a dip-coating or spin-coating device in order to apply the hard coating. This is work- and time-consuming. Furthermore, several machines must be kept ready for the production of an optical structure with a hard coating. SUMMARY

Hence, it is a purpose of the present invention to provide a method for printing three- dimensional optical structures, in particular ophthalmic lenses, which does not show the described disadvantages of the prior art, but allows a smooth and fast production process.

According to the present invention, this object is achieved by a method for printing a three- dimensional optical structure wherein a base structure is built up from layers of a first printing ink deposited through targeted placement of droplets of the first printing ink at least partially side by side in consecutive base structure printing steps, wherein a hard coating is built up from least one layer of a second printing ink deposited through targeted placement of droplets of the second printing ink at least partially side by side on top and/or the sides of the base structure in at least one hard coating printing step. The process according to the invention enables the rapid production of a printed optical structure with a hard coating without the need for spin-coating or dip-coating together with the corresponding transfer from the printing apparatus to a spin-coater or dip-coater.

The embodiments and advantages described in conjunction with this subject matter of the present invention also apply to the further subject matter of the present invention and vice versa.

The person skilled in the art knows that a hard coating is different from the base structure. A hard coating preferably provides the optical structure with increased durability and increased longevity. This relates to a so-called scratch resistance. Hence, the hard coating preferably comprises a higher mechanical resistance than the base structure. More preferably, the hard coating comprises a higher resistance against chemical and/or radiational influences.

The properties of the hard coating are preferably due to mechanical and/or chemical properties of the second printing ink, in particular compared to the first printing ink. For instance, the second printing ink preferably comprises a lower surface tension than the first printing ink. Consequently, after depositing the second printing ink, it will spread out easily and hence result in a smoother, wider and/or thinner layer compared to a similar layer printed with the first printing ink. More preferably, the second printing ink comprises a higher wettability than the first printing ink.

Additionally or alternatively, the second printing ink preferably comprises a higher amount of multifunctional and/or high functional monomers than the first printing ink. Multifunctional monomers are monomers with at least three polymerizable groups. In contrast, the first printing ink preferably comprises a higher amount of low functional monomers than the second printing ink. Low functional monomers have two or less polymerizable groups. More preferably, the first printing ink comprises at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90%, in particular at least 99% of low functional monomers.

The person skilled in the art understands that on the one hand, this yields a higher amount of branching and/or crosslinking of the second printing ink during curing. On the other hand, the higher amount of branching and/or crosslinking typically results in a higher amount of shrinkage during curing. Naturally, this is not desirable for the base structure.

Additionally or alternatively, the second printing ink preferably comprises inorganic nanoparticles, in particular silica nanoparticles, urethane-based oligomers and/or urethane- based monomers. Preferably, the first printing ink does not comprise such nanoparticles or comprises a lower amount of such nanoparticles than the second printing ink.

Additionally or alternatively, the second printing ink preferably comprises a higher amount of hydrogen bond compared to the first printing ink.

Additionally or alternatively, the second printing ink preferably comprises epoxysilane polymers. More preferably, the first printing ink is substantially free of epoxysilane polymers.

Taking into account the above considerations, the person skilled in the art acknowledges, that the second printing ink - and in consequence the hard coating - is substantially different from the first printing ink and in consequence the base structure.

Optical structures in the sense of the present invention comprise lenses. Lenses may comprise ophthalmic lenses. Ophthalmic lenses comprise concave, convex, biconcave, biconvex and meniscus lenses. Ophthalmic lenses in the sense of the present invention also comprise multifocal lenses as well as gradient-index lenses. Ophthalmic lenses comprise in particular spectacle lenses or other lenses that are not inserted into the eye.

In the sense of the present invention, printing of an optical structure comprises building up the optical structure from layers of printing ink. These are obtained through a targeted placement of droplets of printing ink at least partially side by side. The droplets of printing ink are ejected from the nozzles of a print head, typically towards a substrate. Droplets of layers constituting a second and following layers are at least partly ejected towards a previously deposited layer, such that the three-dimensional structure is built up layer by layer. The first printing ink and the second printing ink preferably comprise a translucent or transparent component. Preferably, the first printing ink and the second printing ink comprise at least one photo-polymerizable component. The at least one photo-polymerizable component is preferably a monomer that polymerizes upon exposure to radiation, e.g. ultra violet (UV) light. The deposited droplets of the base structure’s layers are preferably pin cured, i.e. partially cured, after deposition. Preferably, the viscosity of at least one component of the first printing ink is increased. Pin curing is preferably carried out after deposition of the respective droplet or after deposition of an entire or only part of a layer. Alternatively, pin curing is carried out at certain intervals, e.g. after printing of every second layer.

According to a preferred embodiment of the present invention, the base structure is pin cured before the at least one hard coating printing step. In this way, the surface of the base structure is smoothened to accommodate the hard coating.

For this it is preferably provided that the base structure is not completely cured before the at least one hard coating printing step. It has been shown that this improves the flow of the hard coating on the surface of the base structure. It has also been found that this improves the adhesion of the hard coating to the base structure.

It is conceivable that a further structure is arranged between the base structure and the hard coating. Preferably, the further structure is built up from layers of a third printing ink deposited through targeted placement of droplets of the third printing ink at least partially side by side in consecutive base structure printing steps. Preferably, the third ink comprises a UV-blocker. Preferably, the third ink is a photocromic ink. Preferably, further structure encloses the base structure. Preferably, the third printing ink comprises at least one photo- polymerizable component. The at least one photo-polymerizable component is preferably a monomer that polymerizes upon exposure to radiation, e.g. ultra-violet (UV) light. The deposited droplets of the further structure’s layers are preferably pin cured, i.e. partially cured, after deposition. Preferably, the viscosity of at least one component of the third printing ink is increased. Pin curing is preferably carried out after deposition of the respective droplet or after deposition of an entire or only part of a layer. Alternatively, pin curing is carried out at certain intervals, e.g. after printing of every second layer.

According to another preferred embodiment of the present invention, the base structure and the hard coating are completely cured, wherein the base structure is preferably completely cured after the at least one hard coating printing step. This makes it possible to completely cure the base structure and the hard coating simultaneously in a time-saving manner. The complete curing after the application of the hard coating improves the geometrical quality of the surface of the optical structure and the adhesion of the hard coating on the base structure.

According to another preferred embodiment of the present invention, the hard coating is left at rest during a waiting time before curing. This results in a significant improvement of the optical properties of the hard coating and thus of the optical structure.

According to another preferred embodiment of the present invention, the Bayer ratio of the hard coating is at least 0.5, preferably 0.9 higher than the Bayer ratio of the base structure after curing. Preferably, the Bayer ratio of the hard coating is above 1.2 , more preferably above 1.65 after curing. The Bayer ratio contains information about the scratch resistance of a surface. A Bayer ratio of 1 means that the layer has the same scratch resistance as standardized CR39 glass. The higher the Bayer ratio, the higher the scratch resistance. The Bayer test to determine the Bayer ratio was measured with a COLTS Bayer tester and the corresponding method. 500 grams of Alundum 10 was chosen as the abrasive. The test was performed at a temperature of 23°C. Before testing, the test samples are washed together with standardized CR39 glass samples and their initial haze is measured. The samples are arranged in a test-pan of the Bayer tester along the abrasive medium and are oscillated back and forth at a distance of 4 inches at 150 cycles per minute for 4 minutes. After testing, the samples are washed again and the post-testing haze is determined. From this, the Bayer ratio is calculated.

According to another preferred embodiment of the present invention, the base structure printing steps and the at least one hard coating printing step are performed without transferring the base structure from a first printing apparatus to a second printing apparatus. This significantly increases the efficiency of the optical structure manufacturing process. A transfer which is prone to errors is no longer necessary. The optical structure can remain encapsulated in the printing apparatus during the entire printing process.

According to another preferred embodiment of the present invention, the hard coating is selectively built up only on parts of the top and/or parts of the sides of the base structure.

This makes it possible that only certain areas are protected, e.g. areas that can be assumed to be particularly mechanically attacked. These can be exposed areas of the optical structure or areas where the optical structure is typically touched. It is conceivable that the hard coating is built up in such a way that an opening is created in hard coating. For this purpose, the hard coating is selectively not built up at one location, namely at the location of the opening. It is conceivable that the opening forms an access to an area under the hard coating, in particular to the base structure. It is conceivable that the opening is an entrance pupil of a waveguide.

According to another preferred embodiment of the present invention, the base structure comprises a first face with a first curvature and a second face with a second curvature and the optical structure comprises a further first face with a further first curvature and a further second face with a further second curvature, the hard coating being built up on the first face, the further first curvature following the first curvature, wherein preferably the hard coating is also built up on the second face, with the further second curvature following the second curvature. This is an advantageous way to ensure that the optical function of the optical structure remains guaranteed. The hard coating follows the curvature of the base structure with preferably constant thickness.

According to another preferred embodiment of the present invention, the optical structure is provided with a marker. Preferably, the optical structure is provided with the marker after the at least one hard coating printing step. Preferably, the optical structure is provided with the marker before the at least one hard coating printing step. Preferably, the optical structure is provided with the marker on top of the hard coating or in the hard coating or between the hard coating and the base structure or in the base structure. A marker can contain a permanent ophthalmic marker, a QR code, a bar code or a sequence of numbers and/or letters. The marker is used to identify the optical structure and can be printed. It is conceivable that the marker is printed or laser engraved. It is thinkable that the marker is printed with a third print of ink.

According to another preferred embodiment of the present invention, the hard coating is built up from least one layer of the first printing ink and the second printing ink deposited through targeted placement of droplets of the first printing ink and the second printing ink at least partially side by side on top and/or the sides of the base structure in the at least one hard coating printing step. This makes it possible to mix the first printing ink and the second printing ink in an advantageous way and thus individually adjust the properties of the hard coating.

According to another preferred embodiment of the present invention, a ratio of the first printing ink to the second printing ink in the at least one hard coating printing step is selected specifically by a location on the top and/or the sides of the base structure, where the corresponding droplets are placed. This results in the advantageous possibility of giving areas of the hard coating different properties than other areas. Thus an individually adapted hard coating is possible.

According to another preferred embodiment of the present invention, for printing the base structure a first layer is printed in a first layer printing step and a second layer is printed at least partially on top of the first layer in a second layer printing step, wherein the second layer encloses the first layer. Herewith it is advantageously possible to prevent the formation of deviations and defects on the comparatively small, final layers of the base structure. In particular, the layers are printed in reverse order, starting with the layer of smallest diameter and continuing with layers of larger diameter. Accordingly, the base structure’s final layer printed has a large surface area, reducing the probability of defect formation in that layer. Hence, an optical component of improved quality is obtained.

According to another preferred embodiment of the present invention, the hard coating encloses the base structure. Herewith it is advantageously possible to prevent the formation of deviations and defects on final layers of the optical structure. Hence, the optical structure of improved quality is obtained.

According to another preferred embodiment of the present invention, at least one of the layers of the base structure comprises an UV-blocker, wherein preferably second layer comprises an UV-blocker. The UV-blocker prevents the base structure from discoloring when exposed to UV light. Preferably, at least one of the layers of the base structure is tinted, wherein preferably second layer is tinted.

According to another preferred embodiment of the present invention, the hard coating comprises an UV-blocker. The UV-blocker prevents the optical structure from discoloring when exposed to UV light. Preferably, the hard coating is tinted.

According to another preferred embodiment of the present invention, the optical structure is an ophthalmic lens.

A further subject matter of the present invention is a three-dimensional optical structure, in particular ophthalmic lens, wherein the optical structure comprises a printed base structure and a hard coating printed on the base structure. It is preferable that the optical structure is printed utilizing a method according to an exemplary embodiment of the present invention. The embodiments and advantages described in conjunction with this subject matter of the present invention also apply to the further subject matter of the present invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 (a) and (b) schematically illustrate a printing method according to an exemplary embodiment of the present invention.

Figures 2 (a) and (b) schematically illustrate a three-dimensional optical structure according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

The present invention will be described with respect to particular embodiments and with target to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and for illustrative purposes may not be drawn to scale.

Where an indefinite or definite article is used when referring to a singular noun, e.g. “a”, “an”, “the”, this includes a plural of that noun unless something else is specifically stated.

Furthermore, the terms first, second, third and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

In Figure 1 (a) and (b), a printing method according to an exemplary embodiment of the present invention is schematically illustrated. Figure 1 (a) shows a base structure printing step. A base structure 2 is printed by targeted placement of droplets 6 of a first printing ink using a printing head 5. A first layer of droplets 6 is placed onto a substrate 4. One after the other, several layers of the first printing ink are printed on top of each other. The layers of the base structure 2 are pin cured during the base structure printing step, i.e. the deposited droplets 6 of the first printing ink are partially cured, which increases the viscosity of at least one component of the first printing ink.

The base structure 2 constitutes the bulk of the three-dimensional optical structure. For printing the base structure 2 a first layer is printed in a first layer printing step and a second layer is printed at least partially on top of the first layer in a second layer printing step, wherein the second layer preferably encloses the first layer. With this, the formation of deviations and defects is prevented. After the base structure printing step, a hard coating 3 is printed on the surface of the base structure 2 in a hard coating printing step, as shown in Fig. 2 (b). The hard coating 2 is printed by targeted placement of droplets 6 of a second printing ink in one or more layers using the same motion-platform and bracket to hold the optical structure 1 , as in the base structure printing step. Since the base structure 2 was only partially cured before printing the hard coating 3, the adhesion of the hard coating 3 on base structure 2 is significantly improved. , which leads to improved surface properties of the three-dimensional optical structure 1. Preferably, the hard coating 3 encloses the base structure. Herewith it is advantageously possible to prevent the formation of deviations and defects on final layers of the optical structure. Hence, the optical structure of improved quality is obtained. Preferably, the base structure 2 and/or the hard coating 3 comprise an UV- blocker. Preferably, the base structure 2 and/or the hard coating 3 are tinted.

After printing hard coating 3, Base Structure 2 and hard coating 3 are completely cured by irradiation with UV light. The first printing ink and the second printing ink are selected such that the Bayer ratio of the hard coating 3 after curing is at least 0.5 higher than the Bayer ratio of the base structure 2 after curing. The hard coating 3 is thus an effective protection against damage to the surface of the three-dimensional optical structure by mechanical influences.

The hard coating 3 can either be printed on the entire surface of the base structure 2 or only on parts of the surface, such as the side edges (see Fig. 2 (b)) or an omission for an entrance pupil of a waveguide.

After the hard coating printing step, a marker 7 (see Fig. 2 (a) and (b)) can be printed or laser engraved either on the hard coating 3 or on areas of the base structure 2 on which no hard coating 3 is printed. Marker 7 can be used to identify the three-dimensional optical structure 1 and may include a barcode or QR code, for example. In the hard coating printing step droplets 6 of the first printing ink and the second printing ink are preferably used to build up the hard coating 3. The ratio of the first printing ink and the second printing ink can be chosen depending on the position.

Figures 2 (a) and (b) schematically illustrate three-dimensional optical structures 1 according to an exemplary embodiment of the present invention. The three-dimensional optical structures

1 , preferably ophthalmic lenses, are manufactured by a process according to a preferred embodiment of the present invention.

Fig. 2(a) shows a three-dimensional optical structure 1 , which comprises a hard coating 3, which is arranged on the entire surface of the base structure 2. The optical structure 1 comprises a further first face T on top of the hard coating 3 on the front side with a further first curvature and a further second face 1” on top of the hard coating 3 on the back side with a further second curvature. The hard coating 3 is printed such, that it follows a first curvature of a first face 2’ of the base structure 2 (i.e. the surface of the base structure 2) on the front side and a second curvature of a first face 2’ of the base structure 2 (i.e. the surface of the base structure 2) on the back side. This means that the shape of the optical structure 1 , which is decisive for the optical function of the optical structure 1 , is determined by the base structure

2.

Furthermore, the optical structure 1 shown in Fig. 2 (a) exhibits a marker 7 printed on the hard coating 3 or laser engraved in the hard coating 3 to identify the optical structure 1.

Fig. 2(b) shows the three-dimensional optical structure 1 of Fig. 2(a) with the difference that the marker 7 is not applied to the hard coating 3 but to the base structure 2. The marker 7 can be printed on or engraved with a laser. Here, the marker 7 is enclosed by the hard coating 3, which protects the marker 7 from mechanical damage.

Fig. 2(c) shows a three-dimensional optical structure 1, which comprises a hard coating 3, which is arranged only on parts of the surface of the base structure 2. As with the embodiment shown in Fig 2 (a), the optical structure 1 comprises a further first face T on top of the hard coating 3 on the front side with a further first curvature and a further second face 1” on top of the hard coating 3 on the back side with a further second curvature. The hard coating 3 is printed such, that it follows a first curvature of a first face 2’ of the base structure 2 on the front side and a second curvature of a first face 2’ of the base structure 2 on the back side. This means that the shape of the optical structure 1, which is decisive for the optical function of the optical structure 1, is determined by the base structure 2. Moreover, the optical structure 1 comprises a marker 7 printed or laser engraved on the surface of the vase structure 2 to identify the optical structure 1. Fig. 2(d) shows the three-dimensional optical structure 1 from Fig. 2(a) with the difference that the hard coating 3 comprises an opening 8. The opening 8 is an area where selectively no hard coating 3 has been rinted. The opening 8 provides access to the base structure 2 and can, for example, serve as an entrance pupil of a (not shown) waveguide of the base structure 2.

KEY TO FIGURES

1 Optical structure

1’ Further first face 1” Further second face

2 Base structure

2’ First face

2” Second face

3 Hard coating 4 Substrate

5 Print head

6 Droplet

7 Marker

8 Opening