Method for injection molding a negative power lens element free of

Abstract

A method for injection molding a solderless negative power lens element, the method comprising injecting a melt of a thermoplastic material at a temperature above a glass transition temperature (Tg) of the thermoplastic material into an initial mold cavity defined by two facing mold inserts, the melt of thermoplastic material comprises at least one UV absorber. During injection, the two opposing mold inserts move toward each other to define a final mold cavity having a volume smaller than that of the initial mold cavity. After the two facing mold inserts are cooled and detached, a negative power lens element without a weld line is obtained. One of the two facing mold inserts includes a flat surface facing the initial mold cavity, thereby forming a flat surface on a side of the solder-line-free negative power lens element. The other of the two facing mold inserts includes a convex surface facing the initial mold cavity, thereby forming a concave surface on opposite sides of the solder-line-free negative power lens element.

Description

[0001] This application is a divisional application of the patent application with the application number 201880006919.8, the application date of January 26, 2018, and the invention title of "Method for injection molding a negative focal length lens element without welding lines". BACKGROUND OF THE INVENTION 1. TECHNICAL FIELD

[0003] The present invention generally relates to a method for injection molding a negative focal length lens element without welding lines. 2. BACKGROUND ART

[0004] Prescription augmented reality glasses may include a lens assembly, where, for example, a light guiding optical element (LOE) as described in U.S. Patent No. 7,457,040 is sandwiched between a front plano-concave (positive) lens and a rear plano-convex (negative) lens, and the front lens and the rear lens correct the vision of the wearer.

[0005] However, the front lens and the rear lens of such an assembly are much thinner than a traditional single-piece lens blank, which poses significant challenges in their manufacture. It is highly desirable to produce such blanks and cut such positive and negative lenses from these blanks by an injection molding process, as is done for lens blanks for traditional spectacle lenses; however, the positive and negative lenses used in the three-part lens assembly of augmented reality glasses have such dimensions that they are not well-suited for manufacture using traditional injection molding techniques (such as those described in co-owned U.S. Patent No. 7,854,865). SUMMARY OF THE INVENTION

[0006] Thus, in one aspect, the present invention relates to a method for injection molding a negative focal length lens element without welding lines, the method comprising: injecting a melt of the thermoplastic material at a temperature above the glass transition temperature (Tg) of the thermoplastic material into an initial mold cavity defined by two facing mold inserts, wherein the melt of the thermoplastic material comprises at least one UV absorber. During the injection process, two opposing mold inserts move towards each other to define a final mold cavity, the volume of which is less than the volume of the initial mold cavity. After cooling and opening the mold cavity, the negative focal length lens element without welding lines is obtained. One of the two facing mold inserts includes a flat surface facing the initial mold cavity, thereby forming a flat surface on one side of the negative focal length lens element without welding lines. The other of the two facing mold inserts includes a convex surface facing the initial mold cavity, thereby forming a concave surface on the opposite side of the negative focal length lens element without welding lines.

[0007] In an exemplary embodiment of the method according to the present invention, each of the two facing mold inserts comprises a material having a coefficient of thermal expansion α satisfying 1 ≤ α / αg a material with a thermal diffusivity α of <11>, where α g is 6.20×10 -7 m 2 / s.

[0008] In an exemplary embodiment of the method according to the invention, each of the two facing mold inserts is at least partially formed of borosilicate crown glass.

[0009] In an exemplary embodiment of the method according to the invention, the injection is carried out at a speed not exceeding 25 mm / s.

[0010] In an exemplary embodiment of the method according to the invention, the injection is carried out at a speed not exceeding 15 mm / s.

[0011] In an exemplary embodiment of the method according to the invention, the center distance between the two facing mold inserts when defining the initial mold cavity does not exceed five times the center thickness of the weld-free negative power lens element.

[0012] In an exemplary embodiment of the method according to the invention, the center distance between the two facing mold inserts when defining the initial mold cavity is 1.5 to 4 times the center thickness of the weld-free negative power lens element.

[0013] In an exemplary embodiment of the method according to the invention, the movement of the two facing mold inserts towards each other is achieved at a speed less than 15 mm / s.

[0014] In an exemplary embodiment of the method according to the invention, the movement of the two facing mold inserts towards each other is achieved at a speed of 25 - 100 mm / s.

[0015] In an exemplary embodiment of the method according to the invention, the movement of the two facing mold inserts towards each other is achieved with a pressure not exceeding 50 US tons.

[0016] In an exemplary embodiment of the method according to the invention, the movement of the two facing mold inserts towards each other is achieved with a pressure less than 25 US tons.

[0017] In an exemplary embodiment of the method according to the present invention, the thermoplastic material comprises one or more of polycarbonate, polyacrylate, polyol, polyamine, polyamide, polyanhydride, polycarboxylic acid, polyepoxide, polyisocyanate, polynorbornene, polysiloxane, polysilazane, polystyrene, polyolefin, polyester, polyimide, polyurethane, polythiourethane, polyallyl, polysulfide, polyvinyl ester, polyvinyl ether, polyarylene, polyoxide, polysulfone, polycycloolefin, polyacrylonitrile, polyethylene terephthalate, polyetherimide, polypentene, and cellulose triacetate.

[0018] In an exemplary embodiment of the method according to the present invention, the thermoplastic material is a polycarbonate resin.

[0019] In an exemplary embodiment of the method according to the present invention, the polycarbonate resin has a melt flow rate of at least 15 cm 3 / 10 min at 300 °C / 1.2 kg.

[0020] In an exemplary embodiment of the method according to the present invention, the polycarbonate resin has a melt flow rate of at least 20 cm 3 / 10 min at 300 °C / 1.2 kg.

[0021] In an exemplary embodiment of the method according to the present invention, the polycarbonate resin has a weight average molecular weight of less than 26,000 g / mol.

[0022] In an exemplary embodiment of the method according to the present invention, the at least one UV absorber is a benzotriazole absorber.

[0023] In an exemplary embodiment of the method according to the present invention, the at least one UV absorber is 2,2'-methylenebis(6-(2H-benzotriazol-2-yl)-4-1,1,3,3-tetramethylbutyl)phenol).

[0024] In an exemplary embodiment of the method according to the present invention, the at least one UV absorber is 2-(5-chloro-2H-benzotriazol-2-yl)-6-(1,1-dimethylethyl)-4-methylphenol.

[0025] In an exemplary embodiment of the method according to the present invention, the flat surface of the mold insert in the two facing mold inserts has fewer than 20 fringes as indicated by the flatness measurement using monochromatic light interference fringes.

[0026] In an exemplary embodiment of the method according to the present invention, the flat surface of the mold insert in the two facing mold inserts has fewer than 10 fringes as indicated by the flatness measurement using monochromatic light interference fringes.

[0027] In an exemplary embodiment of the method according to the present invention, the solderless negative power lens element has a central thickness of at most 1.5 mm.

[0028] In an exemplary embodiment of the method according to the present invention, the solderless negative power lens element has an edge thickness in the range of 1.0 to 11.5 mm.

[0029] In an exemplary embodiment of the method according to the present invention, the flat side of the solderless negative power lens element is bonded to one side of the light guiding optical element.

[0030] In an exemplary embodiment of the method according to the present invention, a positive power lens element is bonded to the opposite side of the light guiding optical element.

[0031] In another aspect, the present invention relates to a solderless negative power lens element, comprising a sheet made of a thermoplastic material, the sheet comprising at least one UV absorber. The sheet has a flat first major surface and a concave second major surface. The sheet has a diameter in the range of 55 - 85 mm and a central thickness of at most 1.5 mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Other objects, features and advantages of the present invention will become more apparent after reading the following detailed description of the exemplary embodiments of the present invention given with reference to the accompanying drawings, in which:

[0033] Figure 1a is a schematic side view of a light guiding optical element suitable for a prescription augmented reality glasses lens assembly;

[0034] Figure 1b is a schematic side view of a plano - concave lens suitable for a prescription augmented reality glasses lens assembly;

[0035] Figure 1c is a schematic side view of the light guiding optical element to which the plano - concave lens has been bonded;

[0036] Figure 1d is a schematic side view of a plano - convex lens suitable for a prescription augmented reality glasses lens assembly produced by the method according to the present invention;

[0037] Figure 1e is Figure 1c a schematic side view of the sub - assembly to which the plano - convex lens produced by the method according to the present invention has been bonded;

[0038] Figure 1f is Figure 1e a schematic side view of the sub - assembly to which a microdisplay projector has been bonded; and

[0039] Figure 2 Yes Figure 1f Schematic side view of the components already installed in the spectacle frame. Detailed implementation

[0040] Now refer to Figure 1a Figure 1a , a light guiding optical element (LOE) 11 can be produced as described, for example, in U.S. Patent No. 7,457,040. To provide prescription augmented reality glasses, Figure 1b The front plano-concave (positive) lens 13 shown is combined with the LOE 11, as Figure 1c shown. Figure 1d The rear plano-convex (negative) lens 15 shown is then combined with the rear side of the LOE 11, as Figure 1e shown. Next, as Figure 1f shown, a microdisplay projector 17 is combined with the edge of the LOE 11, and then the lens assembly is installed in a pair of spectacle frames 19, as Figure 2 shown.

[0041] The sheet for manufacturing the negative lens 15 includes a flat side and a curved side, and preferably meets very strict geometric, optical, and performance requirements as shown below. In particular, such a lens should have a center thickness (CT) of at most 1.5 mm. Such a lens should also allow for obtaining a lens assembly with a dioptric power distribution (spherical, cylindrical, prism) where the spherical and cylindrical powers are within the range of ±0.06 diopters (D), and the prism is within ±(0.25 + 0.1 * dioptric power) / 2. The dioptric power uniformity determined by a DualLensMapper (DLM, manufactured by Automation & Robotics SA) for a 16×16 mm 2 square area at the center of the lens preferably has a spherical-cylindrical peak-to-valley (PtV) value of less than 0.15 D. The warpage of the flat surface of the negative lens 15 determined by Automation & Robotics SA using the Automation & Robotics Focovision SR-2 preferably results in spherical and cylindrical powers within ±0.06 D (whereas in typical lens products, warpage up to 0.50 D is allowed). Additionally, the evolution of the warpage of the coated flat surface measured by the SR-2 preferably results in changes in spherical and cylindrical powers within ±0.03 D. The assembly should also pass FDA / CEN, thermal shock, compressive strength, and UV aging tests.

[0042] Such requirements pose a great challenge to injection molding. Injection molding of negative thin sheets (or lenses) is accomplished by filling the mold cavity from one side to the other. However, for negative thin sheets, the center thickness is thin while the edge thickness is much thicker. Filling such thin sheets with a high aspect ratio is difficult because the flow resistance around the thin center is much higher than that around the thick edge. As a result, the melt front is prone to wrap around the center, causing the front edge to fold back onto itself, thus generating weld lines that are considered appearance defects.

[0043] A further difficulty is that the thinner center cools and solidifies much earlier than the rest of the thin sheet, which may lead to non-uniform shrinkage. Typically, for injection molding, part shrinkage is compensated by loading more material. However, the high flow resistance around the thin center of the negative thin sheet hinders the effectiveness of such loading. Therefore, non-uniform shrinkage still exists and forms small protrusions, resulting in high distortion measured from the flat side to the center of the thin sheet through SR2.

[0044] Furthermore, the high flow resistance at the center of the negative thin sheet requires a very high injection pressure to fill the cavity, which may generate high residual stresses in the molded part, and these high residual stresses ultimately lead to high warpage before and after coating.

[0045] All of the aforementioned difficulties have become obstacles to the use of existing polycarbonate lens injection process configurations (including machines and high-viscosity ophthalmic-grade PC resins) in producing thin sheets that meet the LOE assembly requirements.

[0046] The method according to the present invention provides an injection molding technique for making negative-powered thin sheets (preferably made of polycarbonate), which meet the above performance criteria, particularly the criteria regarding thickness and warpage.

[0047] In particular, it has been found that combining the injection compression process with a glass insert is effective in preventing the formation of weld lines. Additionally, it has been found that using a high-flow resin instead of a conventional ophthalmic-grade resin will reduce the flow resistance and thus reduce the residual stresses. A process for injection molding negative thin sheets (preferably made of polycarbonate) using an injection compression process with specific process parameters, a flat glass insert with specific flatness, and a high-flow preferred polycarbonate resin with a specific viscosity range is described herein.

[0048] Examples

[0049] Generally, the mold cavity for injection molding of negative thin sheets is formed by two relatively facing inserts, one being flat and the other convex (CX). To solve the weld line problem, inserts made of crown glass or other materials with a thermal diffusivity α satisfying 1 ≤ α / α g <11 are required to be used together with the injection compression process, where α gis the thermal diffusivity of borosilicate crown glass and is equal to 6.20×10 -7 m 2 / s.

[0050] The injection-compression process is typically characterized by forming a cavity with an opening greater than the target part thickness at the start of the injection cycle. This increased cavity thickness will significantly reduce the flow resistance and thus reduce the pressure requirements for filling the cavity. Subsequently, before the injection ends, the cavity opening will be closed by movement of the insert and / or the mold plate to the final part thickness. However, the injection speed, the initial opening of the cavity, the closing speed, and the closing pressure need to be carefully determined in order to not only control the movement of the flow front to avoid the formation of weld lines but also prevent the rupture of inserts made of brittle materials such as crown glass. It has been found that the injection speed should be ≤25 mm / s, preferably ≤15 mm / s. The center distance of the initial cavity opening should not exceed 5 times the target center thickness of the negative lens, preferably between 1.5 and 4 times. The closing of the mold cavity should be slow, with a speed ≤150 mm / s, preferably between 25 and 100 mm / s. The closing pressure is preferably not more than 50 US tons per cavity, preferably less than 25 US tons.

[0051] However, it has also been found that when using a high-viscosity ophthalmic-grade polycarbonate (PC) resin such as Sabic Lexan RL7220, the combination of the injection-compression process alone and the glass insert is generally not sufficient to produce negative lenses that meet the optical requirements. A high-flow PC resin having a weight-average molecular weight of less than 26,000 and a melt flow rate of at least 15 cm 3 / 10 min, preferably at least 20 cm 3 / 10 min according to ISO 1133 at 300 °C / 1.2 kg, or a viscosity of less than 400 Pa for shear rates below 1000 s-1 at 300 °C contributes to the effective filling of the thin center, reduces residual stress, and minimizes warping.

[0052] In addition, a very flat insert with fewer than 20 fringes, and preferably fewer than 10 fringes, as indicated by flatness measurements using monochromatic interference fringes should be used in order to ensure that the flat side of the lens meets the flatness requirements of the LOE assembly process.

[0053] Finally, since most commercial high-flow optical grade PC resins are used for indoor applications (such as the production of CD / DVD or Blu-ray discs), they typically do not contain UV absorbers (UVA), which are required to prevent UV aging of the LOE components and provide UV protection for the wearer's eyes. An internal formulation was developed consisting of 0.1 wt% of BASF Tinuvin 360 benzotriazole UV absorber blended with the high-flow PC resin. The formulation showed a UV cut-off at 380 nm, effectively preventing UV aging of the resulting sheet, as indicated by zero increase in the yellowness index after an 80-hour QSun test.

[0054] The sheets so produced will include gate marks as a post-molding phenomenon of the injection molding process.

[0055] Using the foregoing criteria, an injection-compression process with carefully selected process parameters in combination with a flat glass insert having a specific flatness and a high-flow PC resin containing UVA and within a specific viscosity range has been used to injection mold negative LOE PC sheets that meet very stringent geometric, optical, and performance requirements.

[0056] In the table below, the injection molding results of the negative LOE PC sheets produced according to the present invention shown in the right column are compared with the conventionally produced sheets shown in the left column:

[0057]

[0058]

[0059] As shown by the above data, -1.00 sheets meeting the optical and geometric requirements were successfully molded using the present method. On the other hand, using conventional ophthalmic grade PC and a direct injection process produced sheets with poor optics. The high positive SR-2 spherical lens value at the center of the flat side of the sheet indicates the presence of a bulge.

[0060] It should be understood that the present method allows the use of existing injection molding machines and convex glass inserts, such that there is no need to obtain new machines, while negative sheets meeting all performance requirements can be produced.

[0061] Although the present invention has been described in connection with various exemplary embodiments of the invention, it should be understood that these embodiments are provided merely to illustrate the invention and should not be used as a pretext for limiting the scope of protection conferred by the true scope and spirit of the appended claims.

Claims

1. A method for injection molding a solderless negative power lens element, the method comprising: Injecting a melt of the thermoplastic material at a temperature above the glass transition temperature (Tg) of the thermoplastic material into an initial cavity defined by two facing mold inserts, the melt of the thermoplastic material comprising at least one UV absorber; During the injection process, moving the two facing mold inserts towards each other to define a final cavity, the volume of the final cavity being smaller than the volume of the initial cavity; And After cooling and opening the cavity, obtaining the solderless negative power lens element; One of the two facing mold inserts includes a flat surface facing the initial cavity, thereby forming a flat surface on one side of the solderless negative power lens element; And The other of the two facing mold inserts includes a convex surface facing the initial cavity, thereby forming a concave surface on the opposite side of the solderless negative power lens element.

2. The method according to claim 1, wherein, The solderless negative power lens element has a central thickness of at most 1.5 mm.

3. The method according to claim 1, wherein, Each of the two facing mold inserts is at least partially formed of borosilicate crown glass.

4. The method according to claim 1, wherein The injection is performed at a speed not exceeding 25 mm / s.

5. The method according to claim 1, wherein The center distance between the two facing mold inserts when defining the initial cavity does not exceed five times the central thickness of the solderless negative power lens element.

6. The method according to claim 1, wherein, The movement of the two facing mold inserts towards each other is achieved at a speed less than 150 mm / s.

7. The method according to claim 1, wherein The movement of the two facing mold inserts towards each other is achieved with a pressure not exceeding 50 US tons.

8. The method according to claim 1, wherein The thermoplastic material is a polycarbonate resin.

9. The method according to claim 8, wherein, The polycarbonate resin has a weight average molecular weight of less than 26,000 g / mol.

10. The method according to claim 1, wherein The at least one UV absorber is a benzotriazole absorber.

11. The method according to claim 1, wherein, The at least one UV absorber is 2,2'-methylenebis(6-(2H-benzotriazol-2-yl)-4-1,1,3,3-tetramethylbutyl)phenol).

12. The method according to claim 1, wherein, The at least one UV absorber is 2-(5-chloro-2H-benzotriazol-2-yl)-6-(1,1-dimethylethyl)-4-methylphenol.

13. The method according to claim 1, wherein The flat surface of the mold insert among the two facing mold inserts has less than 20 fringes as indicated by flatness measurement using monochromatic light interference fringes.

14. The method according to claim 1, further comprising: Bonding the flat side of the solderless negative power lens element to one side of a light guiding optical element.

15. A solderless negative power lens element, comprising: A thermoplastic material sheet, the thermoplastic material comprising at least one UV absorber; The sheet has a flat first major surface and a concave second major surface; The sheet has a diameter in the range of 55 - 85 mm and a central thickness of at most 1.5 mm.

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