FAST FABRICATION OF CUSTOM PLASTIC LENSES

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for rapid fabrication of a custom plastic lens based on 3D printing technology providing improvements in both speed and cost over existing methods. In many embodiments, the methods of the invention can make use of existing 3D printing systems, however the invention also provides methods that provide excellent results using newly modified 3D printing systems, including the new systems provided herein. While explicit examples of the present methods provide for the 3D printing of lenses, specifically spectacle lenses, the methods may also be used in other, less highly demanding processes, as will be readily appreciated by one of ordinary skill in the art.

BACKGROUND OF THE INVENTION

Plastic eyeglass lenses rapidly grew in popularity since their introduction 1952 due to their light weight and greater resistance to breakage compared to conventional glass eyeglass lenses. More than 80 percent of eyeglasses worn today have plastic lenses. Surprisingly, plastic eyeglass lenses are still manufactured using the same processes used in the manufacture of glass eyeglass lenses, i.e., by the blocking, cutting, fine grinding or diamond turning, polishing, coating, and shaping of stock lens blanks, which pre-finished, stock lens blanks usually are produced by a molding process. The manufacture of both the finished lens and the blank rely on precision machining of the surface of the lenses or the molds, which requires expensive computer numerical control (CNC) equipment.

Projection micro-stereolithography (PpSL) utilizes 3D printing technology for microfabrication and stands out from other 3D printing technologies due to its high fabrication speed. The core component of this technology is a high resolution spatial light modulator which is either a liquid crystal display (LCD) panel or a digital light processing (DLP) panel similar to those found in micro-display industries. The high fabrication speed and good compatibility with liquid photo-sensitive resins make PpSL a good candidate for plastic lens fabrication.

However, the traditional layer-by-layer scheme of PpSL or other 3D printing techniques will cause the optical and mechanical properties to change periodically inside the finished parts, and on the external surfaces. This may lead to very poor optical performance of the 3D printed lenses due to light scattering. Therefore, there is a need for alternative methods and apparatus for rapid fabrication of custom plastic lenses. Existing methods for continuous 3D printing of objects can be found, e.g., in US 7,892,474, US 9,205,601 , US 9,21 1 ,678, US 9,216,546, and US 9,360,757.

SUMMARY OF THE INVENTION

The present invention provides 3D object fabrication techniques based on PpSL or other 3D printing technologies. Advantages of the techniques can be readily seen in the fabrication of lenses, but similar advantages can be seen in the 3-D fabrication of other objects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a projection micro-stereolithography system;

FIG. 2 is a scanning electron microscope image of layered features on a surface of PpSL printed object;

FIG.3 schematically illustrates the process of correcting surfaces of a dummy lens by a vacuum wrapping method;

FIG. 4 schematically illustrates the process of molding a lens by replicating a mold using a 3D printed dummy lens; FIG. 5 schematically illustrates the process of lens molding with a liner in 3D printed molds;

DETAILED DESCRIPTION OF THE INVENTION

The define-layer-then-exposure process scheme of traditional PpSL methods (FIG. 1 ) can cause undesired variations of refractive index in the printed layers of the final parts (FIG. 2). This is a significant drawback in using traditional 3D printing technology in the fabrication of plastic optical lenses. Although continuous PpSL technology has been developed to address this issue, it can only fabricate one product at a time and is limited to the use of photo sensitive resin lens materials. In order to further increase the manufacturing compatibility with other lens materials, for example, thermoplastics or reactive thermoplastics, and to facilitate the batch production of final objects with a smoother surface, alternative molding methods have been developed. The methods of present invention offer ways to overcome this problem faced in the 3D printing of lenses and other exacting applications.

Lens Molding by a Replicated Mold from a Dummy Lens

Undesirable layered features due to PpSL or other 3D printing methods for optical lens fabrication, can be avoided by preparing a lens using traditional layer-by-layer 3D printing methods and then using this 3D printed lens as a dummy lens in a subsequent molding process to replicate a lens mold. The dummy lens has exactly the same surface profile as the designed and intended lens, but the dummy lens surface will inevitably have the tiny physical steps as shown in FIG. 2. The dummy lens is fabricated by a traditional layer-by-layer 3D printing method, wherein each layer has a thickness of 10-20 microns. Therefore, the surface roughness of the printed dummy lens is around 10 microns. This roughness is not acceptable for a good optical performance, which usually tolerates only a few hundred nanometers depending on the working optical wavelengths. The vacuum wrapping method of FIG. 3 can be used to fix the surfaces of a dummy lens. Accordingly, the dummy lens is first soaked with UV/thermal curable resin such that all the steps and defects are covered by the resin.

Typically, the viscosity of the UV/thermal curable resin is over 50 cP. For example, in one aspect, the UV/thermal curable resin is Polyethylene glycol) diacrylate (molecular weight 575, viscosity 57 cP) mixed with 1wt% Ciba® IRGACURE® 819 as photo initiator.

The soaked dummy lens is then placed between two elastic membranes and a vacuum is applied. For example, the lens is placed between elastic membranes, typically having a thickness of about 100 microns, a rubber pumping pipe is inserted between the two membranes, and the pumping pipe is connected to a vacuum, about 0.1 to 0.5 Bar depending on the strength of the membrane, for about 5 minutes. As the air and residual resin is pumped away, the elastic membranes will perfectly sandwich the dummy lens. Care must be taken to make sure that no bubbles reside on the surface of the dummy lens. In this configuration, all the steps and defects on the surfaces of the dummy lens will be filled with UV/thermal curable resin.

The whole membrane and lens assembly, still connected to vacuum, or held under vacuum due to a vacuum shut-off valve, is then exposed to UV radiation or heat to solidify the resin on the dummy lens surfaces. In one aspect, the whole membrane and lens assembly is cured by exposure to 2 W/cm ² UV light, typically for about 1 minute on each surface.

In another aspect, the whole membrane and lens assembly is thermally cured at about 80 °C for about 2 hours. When following this method, the surface roughness of the dummy lens will be close to that of the elastic membrane, which makes the use of an elastic membrane with a smooth surface desirable. The dummy lens is then used to prepare a mold.

FIG. 4 schematically illustrates the process of molding a lens by replicating a mold using the modified 3D printed dummy lens. After the surfaces of the dummy lens have been improved by the vacuum wrapping method, the dummy lens is placed in a molding container. The empty space in the molding container is filled with curable silicone rubber solution. The dummy lens is fully submerged in the silicone rubber solution. The molding container with the dummy lens is placed in an oven for curing the silicone solution. When cured, the silicone rubber should have a hardness of over 30 shore A to minimize the deformation. After the silicone is fully cured, the molding container is cut open to remove the dummy lens, providing a silicone mold for the designed lens replicated from the dummy lens.

In one aspect, the mold can be made of other materials that undergoes phase change with temperature or UV radiation, for example, wax or photo curable resins.

In various aspects, the mold materials can be optically transparent or not, depending on the curing method. For thermally cured lens materials, optical transparency is not required, but when using photo curable lens materials, the mold needs to be transparent to the UV light used to cure the lens inside the molds.

The silicone mold then is closed and a thermally, chemically or photo curable resin for the final lens is injected to fill the space vacated by the dummy lens. For thermally curable resins, the replicated mold with the lens resin is place in a thermal oven set at an appropriate curing temperature, typically about 50 °C to 100 °C. For UV curable resins, the replicated mold with the lens resin is place in a UV oven. For a good optical performance, the resin curing speed is typically adjusted to be slow and spatially controlled. After the lens resin is cured, the surface of the cured lens resin is polished as needed, and the edge of the cured lens resin is trimmed for its particular application, for example, fitting to an eyeglass frame.

Lens Molding by a 3D Printed Negative Mold

One can also avoid internal layering of different refractive indices by directly preparing negative molds by 3D printing, and then applying an elastic liner membrane to reduce the surface roughness on the molds, see FIG. 5. The application of the liner membrane and the molding of the lens is conveniently accomplished in a single step by encapsulating the lens forming resin in the membrane material. Thus, lens resin encapsulated by the liner membrane, is injected into the space between the pairing molds. Then by compressing the molds or by casting, the injected liner membrane conforms to the inner surface of the molds and concurrently the lens resin conforms to the shape of the surfaces of the molds. The mold with the lens resin is placed in an oven set at a temperature to cure the lens resin, typically, about 50 °C to 100 °C. After the lens resin is cured, the molds are opened to demold the final cured lens.

In another aspect, UV curable lens resin and the mold with the lens resin is placed in a UV oven for curing.

In one aspect, the 3D printed negative molds have been designed to accommodate the membrane thickness, for example, if the membrane is 25 microns thick, then the negative molds inner surfaces will be recessed for around 20 microns. The difference is due to the membrane thickness reduction when it is under tension.

In one aspect, the injection of the lens resin can be further improved by directly inserting a membrane casing filled with lens resin in the molds, wherein the volume of the resin filled-casing is predefined to fit the molds. This method eliminates the silicone mold replicating step used saving a significant amount of time in custom lens fabrication, especially when time is critical.

In one aspect, the 3D-printed material forming the mold is optically transparent and the lenses are prepared from photo-curable resins.

In another aspect, the 3D-printed material forming the mold does not have to be transparent, the lenses are cured thermally, and the mold can be prepared from either a polymer or a 3D printed metal.

In another aspect, the transparent material forming the manufactured lens can be any familiar transparent material such as a thermoplastic, a thermoset, or a casting resin conventionally used for molding lenses.

In some embodiments, the inner surfaces of the molds are lubricated with, e.g., thin liquid lubricants, prior to applying the liner and the lens resin, to allow the liner membrane to deform and expand freely, so that the liner membrane can conformally follow the mold surface profile.

Again, the surface roughness of the cured lens is close to that of the liner membrane, so it is desirable to select a liner membrane with smooth surfaces.

The pressure used to push the liner against the mold surface should be monitored. For example, applying a high pressure to a thin and soft-liner membrane can force the liner membrane to conform to the details on the mold surfaces, such as the steps or other surface defects due to 3D printing, and compromise the optical performance of the manufactured lens. It has been determined that when a 25 microns thick silicone rubber membrane of elastic modules 0.085 GPa covers a 20 microns size defect, at one atmosphere pressure difference, the deformation of membrane due to the defect is less than 100 nanometers. Numerical Surface Design

The foregoing disclosures for manufacturing a spectacle lens all require a numerical definition of the shape that is to be produced. The mathematical design for a progressive-power optical surface needed for manufacturing a spectacle lens for an individual wearer can be provided easily by recalculating a public-domain design to provide any desired separation, or “corridor length”, between the lens regions intended for distance vision and for near vision. By fitting the original data with a continuous mathematical function, such as the sum of the appropriate number of the Zernike Polynomial terms, or with the bicubic splines, or with a polynomial expression relating the local height Z as a function of its position X and Y on the surface, the newly designed surface can have custom surface profile for each user. The surface can be adjusted to have a different“corridor length” by scaling it in the X, Y plane. That scaling will also change the amount of “add”, in proportion to the square of the scaling factor used, but that change is of no concern; after the desired corridor length has been achieved, the intended amount of “add” can be provided by scaling the equation to increase or decrease the calculated Z values as needed. Finally, the required custom lens surface can be calculated by adding the Z values of this progressive shape together with the calculated Z values for the spherical, cylindrical, and prism components of the wearer’s prescription. If desired, the spectacle lens surface can be further improved by adjusting its local effective optical power where it is viewed at oblique angles.

A modern spectacle lens, comprising progressive optical power for correction of presbyopia along with any prescriptive cylinder and prism power, can be constructed with one surface of the lens simply spherical, with all of the wearer’s individual correction structure on the opposite side, preferably on the concave side. The disclosed invention provides manufacturing capability for putting the non-spherical structure on either side, or dividing it between the two sides. It also can fabricate a lens with one curved surface, e.g., spherical surface or rotationally symmetric aspherical surface. In one embodiment the spherical side of a 3D printed lens may be controlled more accurately and fabricated more smoothly if it is produced in contact with a solid guiding surface made of glass, metal, or of another material that has been polished or fabricated in a conventional way. That guiding surface can then be removed after the printed lens has been formed. The removal can be facilitated by providing a thermally or chemically deposited layer of silver on the solid guiding surface, because a silver layer can be separated easily.

Particular embodiments of the invention include:

A method for fabrication of a custom plastic lens comprising:

(a) preparing a dummy lens by 3D printing;

(b) fixing surfaces of the dummy lens by a vacuum wrapping method comprising: i) preparing a soaked dummy lens by soaking the dummy lens in a UV curable or thermally curable resin to cover defects on one or more surfaces of the dummy lens;

ii) placing the soaked dummy lens between elastic membranes, typically between two elastic membranes;

iii) placing the dummy lens between the elastic membranes under vacuum to pump away air between the two membranes forming a wrapped dummy lens; iv) exposing the wrapped dummy lens to UV radiation or heat to solidify the UV curable or thermally curable resin and provide a final dummy product, c) preparing a lens mold from the final dummy product by immersing the final dummy product in a curable resin, curing the resin, and then removing the final dummy product from the cured resin, and

d) inserting a curable lens resin into the lens mold and curing the curable lens resin.

In one aspect the soaked dummy lens is prepared by soaking the dummy lens in a UV curable resin, and in another aspect the soaked dummy lens is prepared by soaking the dummy lens in a thermally curable resin. In one aspect the soaked dummy lens between the elastic membranes is placed under vacuum by inserting a pipe connected to a vacuum between the membranes, and in one aspect the soaked dummy lens between the elastic membranes is placed under a vacuum of 0.1 to 0.5 Bar.

In one aspect the curable resin in which the final dummy product is immersed in to prepare the mold is a silicone resin, and in another aspect the resin the final dummy product is immersed in to prepare the mold is a UV curable resin.

Another embodiment provides a method of rapid fabrication of a custom 3D object comprising:

(a) preparing a negative mold of the custom 3D object by conventional 3D printing;

(b) lubricating inner surfaces of the molds with thin liquid lubricant;

(c) injecting a liner membrane and a curable resin;

(d) spreading the liner membrane by monitored compression pressure;

(e) curing the curable resin;

(f) demolding the cured resin from the negative molds.

More particularly, a method according fabrication of a custom plastic lens comprising:

(a) preparing a negative mold of the custom plastic lens by conventional 3D printing;

(b) lubricating inner surfaces of the molds with thin liquid lubricant;

(c) injecting a liner membrane and a curable resin, wherein the resin is encapsulated by the liner membrane;

(d) spreading the liner membrane by monitored compression pressure;

(e) curing the curable resin;

(f) demolding the cured plastic lens from the negative molds.

In one aspect the curable resin is a UV curable resin, in another aspect the curable resin is a thermally curable resin. In another aspect, the negative mold is prepared from a 3-D printed polymer or 3-D printed metal, and in another aspect, the curable resin is a UV curable resin and the negative mold is prepared using a material that is transparent at wavelengths at which the curable resin is cured.

EXAMPLES

Example 1 : Vacuum Wrapping Method to Fix the Surface of a 3D Printed Lens

A 3D printed lens is soaked with a UV curable resin, Polyethylene glycol) diacrylate (molecular weight 575, viscosity 57 cP) mixed with 1wt% Ciba® IRGACURE® 819 as photo initiator, such that all the steps and defects are covered by the resin. .

The soaked lens is placed between two elastic membranes having a thickness of 100 microns. A 4 mm-diameter rubber pumping pipe is inserted between the two membranes. The pumping pipe then is connected to a vacuum (0.1 to 0.5 Bar, depending on the strength of the membrane) for 5 minutes. As the air and residual resin is pumped away, the elastic membranes will sandwich the lens. Care must be taken to make sure that no bubbles reside on the surface of the dummy lens. In this configuration, all the steps and defects on the surfaces of the dummy lens will be filled with UV curable resin.

The vacuum is maintained, either by connection to the vacuum source or by means if a shut off valve is used and the whole membrane and lens assembly is exposed to 2 W/cm ² UV light for curing for about 1 minutes to solidify the resin on the lens surfaces.

In a variant of Example 1 , a thermally curable resin is used in place of the polyethylene glycol) diacrylate /Ciba® IRGACURE® 819 mixture, and the whole membrane and lens assembly is cured at 80 °C for about 2 hours. Lens

The dummy lens produced in example 1 is placed in a molding container, which is filled with a two-component condensation cure silicone rubber solution to fully submerge the dummy lens. The molding container with the dummy lens is placed in an oven to cure the silicone rubber solution, about 1 hour at room temperature, followed by about 2 hours at 60 °C, and then about 2 hours and 80 °C. After curing, the silicone rubber has a hardness of over 30 shore A.

After the silicone is fully cured, the cured silicone is cut open to remove the dummy lens, providing a silicone mold for the designed final lens replicated from the dummy lens.

Example 3: Molding a Lens from a Mold

The replicated silicone mold from Example 2 is closed and a thermally curable lens resin is injected to fill the space. The replicated mold with the lens resin is place in a thermal oven set at 50 °C to 100 °C until the lens is cured in a slow and spatially controlled manner.

After the resin is cured, the surface of the lens is polished as needed, and the edge trimmed for its particular application.

Example 4: Molding a Lens from a Mold

The replicated silicone mold is closed and a photo curable lens resin is injected to fill the space. The replicated mold with lens resin is place in a UV oven until the resin is cured.

After the resin is cured, the surface of the lens is polished as needed, and the edge trimmed for its particular application.

In the specification, the singular forms also include the plural forms, unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In the case of conflict, the present specification will control.