JP2022190808A - Optical device manufacturing method and optical device - Google Patents

Abstract

To provide an optical device in which a surface shape accuracy is improved and birefringence is reduced.SOLUTION: There is provided an optical device manufacturing method that comprises the steps of: injection molding a lens 13 integrally formed with a gate 12 and a runner 11; removing a portion of the runner 11; annealing the lens 13 in a state where the gate 12 and the runner 11 are maintained; and removing the gate 12 and the runner 11 from the lens 13.SELECTED DRAWING: Figure 4

Description

The present invention relates to an optical device manufacturing method and an optical device.

Optical devices such as lenses and prisms are required to be thick, to have high surface shape accuracy, and to have small birefringence, as the equipment in which they are mounted is becoming more sophisticated. Also, in order to reduce the manufacturing cost of optical devices, it is required to shorten the molding takt time.

By the way, in the injection molding of optical devices, if the molded product is taken out from the molding machine before the temperature of the central part of the molded product is sufficiently lowered, sink marks occur on the surface of the molded product and the accuracy of the surface shape deteriorates.

Here, in order to prevent sink marks from occurring, it is necessary to slowly cool in the mold, which poses the problem of a significant increase in molding takt time. In addition, there is also the problem that birefringence increases when molding is performed at a high holding pressure in order to suppress sink marks.

Patent Document 1 describes multi-layer molding using a core lens in order to shorten the molding tact, and molding with the gate direction of the core lens and the gate direction of the covered lens shifted by 90 to 180 degrees to suppress birefringence. A method is proposed.

Patent Document 2 proposes a method of defining a lens shape and using only a region with a small refractive index distribution (birefringence).

Patent Document 3 discloses a method of suppressing birefringence by annealing after molding.

JP 2012-111117 A JP-A-08-201717 JP-A-11-077842

However, in the invention of Patent Document 1, since the two-layer molding is performed in two steps, multiple sets of molds are required. As a result, the cost of the mold becomes high, and the effect of reducing the cost by shortening the molding tact becomes small. Moreover, although it is attempted to reduce the birefringence by changing the gate direction, the birefringence due to multiple molding operations remains.

In the invention of Patent Document 2, only the central region with a small birefringence distribution (birefringence) is used. Since a large molding machine is required, the manufacturing cost increases.

In the invention of Patent Document 3, the birefringence is removed by annealing, but when the stress strain (birefringence) is released, the shape of the lens surface changes, resulting in deterioration of the shape accuracy.

The present invention has been made in view of the above points, and an object thereof is to provide an optical device with improved surface shape accuracy and reduced birefringence.

The present invention includes steps of injection molding an optical device in which a gate and a runner are integrally molded, removing a part of the runner, and annealing the optical device with the gate and the runner left. and removing the gate and the runner from the optical device.

In the present invention, by annealing the optical device with the gates and runners left, it is possible to minimize the shape change due to stress strain (birefringence) release during annealing and to suppress the birefringence near the gate. can. This makes it possible to obtain an optical device that achieves both high surface shape precision and low birefringence.

According to the present invention, it is possible to provide a thick optical device that achieves both a highly accurate shape and low birefringence while efficiently shortening the takt time by molding with a high holding pressure.

FIG. 4 is a diagram showing a state in which gates and runners are integrally molded in the optical device according to the present embodiment; FIG. 4 is a schematic diagram showing a birefringent state of a lens viewed from the optical axis direction; It is a figure for demonstrating the conventional annealing treatment. It is a figure for demonstrating the annealing process of this embodiment. It is a figure which shows the measurement result of the surface shape precision and retardation amount of a lens at the time of changing holding pressure. FIG. 5 is a graph showing the relationship between the distance from the gate before annealing and the amount of retardation. FIG. 5 is a graph showing the relationship between the distance from the gate after annealing and the amount of retardation.

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. It should be noted that the following description of preferred embodiments is essentially merely an example, and is not intended to limit the present invention, its applications, or its uses.

As shown in FIG. 1, the optical device is a thick lens 13 . Lens 13 is manufactured by injection molding. Molten resin injected into a mold (not shown) passes through a runner 11 and a gate 12 and fills the transfer surface of the lens 13 in the mold.

After filling the mold with molten resin, the inside of the mold is kept in a high holding pressure state, and the shape of the mold is transferred with high precision in a short tact. After cooling the molten resin in the mold for a certain period of time, the lens 13 in which the gate 12 and the runner 11 are integrally formed is obtained by removing the molded resin product from the mold.

Here, by keeping the inside of the mold in a high holding pressure state, the resin in the lens mold cavity at holding pressure is in a state of being overfilled, and sink marks due to contraction during cooling can be suppressed. Therefore, even with short tact resin molding with a shortened cooling time, it is possible to obtain the lens 13 with high surface shape accuracy.

By the way, the lens 13 molded at a high holding pressure tends to have a large birefringence in the predetermined region 15 near the gate 12 (see FIG. 2). Specifically, in the high holding pressure state, the pressure difference between the gate 12 side, to which pressure is easily applied, and the opposite side, to which pressure is difficult to apply, tends to be larger than in the low holding pressure state. Further, in the vicinity of the gate 12 where the resin flow path rapidly expands, shear stress is generated and the stress tends to concentrate. Therefore, large birefringence occurs in a predetermined region near the gate 12 .

In particular, birefringence tends to increase as it approaches the gate 12 . Specifically, the birefringence tends to increase in a range where the distance from the gate 12 is 15% or less of the outer dimension of the lens 13 .

Therefore, in order to remove the birefringence of the lens 13 generated in the molding process, annealing treatment of the lens 13 was considered. In the conventional example shown in FIG. 3, the gate 12 and the runner 11 integrally formed with the lens 13 are removed before annealing.

In general, the gate cutting operation for removing the gate 12 from the lens 13 is performed using nippers, hot nippers, or the like. Therefore, the gate 12 can only be cut linearly, leaving traces of the gate cut in an optical device such as the lens 13 having a curved surface. At this time, depending on the gate cutting method, the gate 12 may remain about several millimeters, and the gate cutting does not completely remove the gate 12 .

The gate-cut lens 13 is put into the annealing furnace 30 . The annealing furnace 30 has a lens palette 31 and a heater 35 . The lens pallet 31 has a holding portion 32 recessed corresponding to the curved shape of the lens 13 . A plurality of holders 32 are provided on the lens pallet 31 . The lens 13 is held by the holding portion 32 with the gate 12 directed upward.

In the example shown in FIG. 3, the heaters 35 are arranged on both the left and right sides of the lens pallet 31 . The heater 35 heats the inside of the annealing furnace 30 .

By the way, in the conventional method of manufacturing an optical device, annealing treatment is performed to suppress birefringence. deteriorates greatly.

In particular, there is a tendency for the shape change to be large near the gate 12, which has large birefringence. In other words, in the conventional annealing treatment, there is a trade-off relationship between the shape accuracy of the surface of the lens 13 and the residual birefringence. Therefore, if the set temperature is lowered so as not to deteriorate the surface profile accuracy, the stress strain cannot be sufficiently suppressed, and birefringence remains.

Therefore, in the present embodiment, by devising the procedure before performing the annealing treatment, it is possible to provide the lens 13 with improved surface shape precision and reduced birefringence.

Specifically, as shown in FIG. 4, after the lens 13 integrally formed with the gate 12 and the runner 11 is injection molded, a part of the runner 11 is removed. Then, the lenses 13 with the gates 12 and the runners 11 left are arranged on the lens pallet 31 and put into the annealing furnace 30 . Shape deformation during annealing concentrates on gate 12 and runner 11 where stress strain is greater. In other words, the distortion of the lens 13 can be released to the gate 12 and runner 11 side, as indicated by the arrow lines in FIG. Since the gate 12 and part of the runner 11 remain in this way, it is possible to suppress deformation of the lens 13 during annealing.

By removing the gate 12 and the runner 11 from the lens 13 after annealing, a thick optical device having both high surface shape accuracy and low birefringence can be obtained.

Here, as the thickness of the lens increases, there is a tendency for the sink mark during molding to increase, and a higher holding pressure is required. Therefore, in order to firmly apply pressure to the molded lens surface, it is required to reduce the pressure loss at the gate 12, and the gate diameter tends to increase.

When the gate diameter becomes large, it becomes difficult to cut the gate with a normal nipper, and the gate 12 is cut after being softened by applying heat with a hot nipper or the like. However, if heat is applied to the stress-distorted lens 13 during gate cutting, the shape may be adversely affected.

Therefore, as in this embodiment, it is preferable to perform gate cutting in a state in which the stress strain is released after annealing.

(Lens before annealing treatment)
Hereinafter, as an example of a thick optical device according to the present embodiment, a case of manufacturing a lens 13 of cycloolefin polymer (COP) having an outer diameter of 40 mm and a thickness of 12 mm in the optical axis direction will be described.

Focusing on mold transferability, we set the resin temperature to 260°C and the mold temperature to 120°C, and changed the holding pressure to confirm the surface shape accuracy and birefringence (retardation amount) due to the holding pressure. rice field.

FIG. 5 is a diagram showing measurement results of lens surface shape accuracy and retardation amount when holding pressure is changed. The surface shape accuracy of the lens 13 was measured with an ultra-high-precision three-dimensional measuring machine (UA3P, manufactured by Panasonic Production Engineering Co., Ltd.). The retardation amount was measured with a birefringence meter (WPA-200 manufactured by Photonic Lattice Co., Ltd.).

As shown in FIG. 5, the surface shape accuracy of the lens 13 improved as the holding pressure increased. Above all, there was almost no change.

On the other hand, the retardation amount tended to increase as the holding pressure increased. In other words, it was found that, depending on the holding pressure conditions, the retardation amount tended to increase as the surface profile accuracy improved.

Based on the results described above, in this example, the holding pressure during molding was set to 140 MPa. A thick optical device is preferably molded under a high holding pressure of 100 MPa or more in order to suppress sink marks and obtain high surface shape accuracy. Moreover, the gate size was 4 mm wide×3 mm thick as a size that allows pressure to be sufficiently transmitted.

FIG. 6 is a graph showing the relationship between the distance from the gate before annealing and the amount of retardation. As shown in FIG. 6 , in the lens 13 , the birefringence of the predetermined region 15 near the gate 12 was large, and the maximum retardation value was 390 nm at the position directly below the gate 12 .

Here, the predetermined area 15 is in the range of 5% or more and 15% or less of the outer dimensions of the lens 13 . In this embodiment, the outer dimension of the lens 13 is 40 mm. Therefore, the retardation amount is high from the gate 12 to 6 mm (corresponding to 15% of the outer dimension). was getting smaller.

Further, when the surface shape accuracy of the molded lens 13 was measured, the peak-to-valley value (hereinafter referred to as PV), which is the shape error with respect to the design value of the lens 13, was 2.6 μm, indicating that the lens 13 was highly accurate.

Here, the larger the outer shape of the lens 13, the larger the shape accuracy error PV tends to be. The surface shape accuracy of the thick optical device varies depending on the type of equipment using the optical device, but is preferably 0.05% or less, more preferably 0.02% or less, relative to the external size. preferable.

In this embodiment, since the outer dimension of the lens 13 is 40 mm, the shape error PV is preferably 20 μm or less, more preferably 8 μm or less.

In this example, cycloolefin polymer was used as an example of the lens resin material. Other materials include cycloolefin copolymer (COC), acrylic (PMMA), polystyrene (PS), and polycarbonate (PC). ) may be used. Here, COP, COC, and PMMA with small photoelastic coefficients are more preferable in order to suppress birefringence.

Also, although the lens 13 having a thickness of 12 mm has been described as an example of a thick optical device, the present invention is not limited to this form. For example, the thick lens 13, which requires molding with high holding pressure depending on the outer shape, may have a thickness of 5 mm or more and 50 mm or less.

(Regarding the lens after annealing)
In performing the annealing treatment, a part of the runner 11 is cut from the lens 13 in which the gate 12 and the runner 11 are integrally molded. Then, the lens 13 with the gate 12 and the runner 11 left is put into the annealing furnace 30 (see FIG. 4). Here, the runner 11 is cut with 10 mm or more left.

The remaining amount of the runner 11 during annealing may be adjusted according to the size of the annealing furnace 30, the size of the lens pallet 31, and the surface shape accuracy required for the lens 13. FIG. For example, the runner 11 should be in a state of being left 2 mm or more and 80 mm or less. Here, the runner 11 may be 2 mm or more, preferably 5 mm or more, and more preferably 10 mm or more, separately from the gate 12 .

Then, the set temperature of the annealing furnace 30 is set to a temperature lower than the glass transition point of the material by a predetermined temperature. Here, the predetermined temperature is 10° C. or higher and 30° C. or lower. In this example, the furnace temperature was raised to 110° C., which is 10° C. or more lower than the glass transition temperature, and then slowly cooled over 3 hours to return to room temperature. After that, the lens 13 was taken out from the annealing furnace 30 and subjected to gate cutting.

FIG. 7 is a graph showing the relationship between the distance from the gate after annealing and the amount of retardation. As shown in FIG. 7, the annealing treatment suppressed birefringence over the entire surface of the lens 13, and the retardation amount was suppressed to 30 nm at maximum.

Here, the retardation amount was low over substantially the entire area of the lens 13, but in the predetermined region 15 near the gate 12 where the birefringence was large before annealing, the retardation amount was slightly large, with respect to about several tens of nanometers. In areas other than the vicinity of 12, the retardation amount was as small as about 10 nm.

In addition, when the surface shape accuracy of the molded lens after annealing was measured with an ultra-high-precision three-dimensional measuring machine (UA3P, manufactured by Panasonic Production Engineering Co., Ltd.), the shape error PV with respect to the design shape was 3.2 μm, indicating high accuracy. was kept

(Comparative example)
In the following, as a comparative example, a conventional method of putting the lens 13 into the annealing furnace 30 after gate cutting after molding was verified.

In the example described above, when the gate was cut after molding the lens 13, the remaining amount of the gate 12 was 1.1 mm. In the comparative example, the gate-cut lenses 13 were arranged on a lens pallet 31 and put into the annealing furnace 30 (see FIG. 3). Annealing conditions were the same as in the above-described example.

It was confirmed that the birefringent state after annealing was almost the same as that of the lens 13 of Example, and the maximum retardation value was 30 nm.

However, when the surface shape accuracy of the lens 13 was measured with an ultra-high-precision three-dimensional measuring machine (UA3P, manufactured by Panasonic Production Engineering Co., Ltd.), the shape error PV with respect to the designed shape was 32.8 μm, and the shape was greatly deformed. This is considered to be caused by shape deformation due to stress strain release during annealing.

In addition, in order to suppress shape deformation during annealing, when the set temperature of the annealing furnace 30 was lowered to 100° C., the shape error PV of the surface shape accuracy with respect to the design shape was 3.9 μm, and shape deformation was almost eliminated. However, the birefringence could not be suppressed sufficiently, and the retardation amount was 310 nm.

As described above, when the annealing treatment is performed after the gate cut of the lens 13, the maintenance of the surface shape accuracy of the lens 13 and the suppression of birefringence are in a trade-off relationship, and it is difficult to achieve both.

In contrast, according to the method for manufacturing an optical device according to the present embodiment, by annealing the lens 13 with the gate 12 and the runner 11 left, the shape change due to stress strain (birefringence) release due to annealing can be minimized and the birefringence near the gate 12 can be suppressed. This makes it possible to obtain an optical device that achieves both high surface shape precision and low birefringence.

The optical device of the present invention can be applied mainly to thick optical devices that require high precision. For example, lenses for head-up displays (HUD) and headlights for vehicles, lenses and prisms for head-mounted displays (HMD), lenses and prisms for projectors, and thick optical devices such as lenses for imaging systems. It is particularly suitable for

11 runner 12 gate 13 lens (optical device)
15 Predetermined region 30 Annealing furnace

Claims (8)

injection molding an optical device in which a gate and a runner are integrally molded;
removing a portion of the runner;
annealing the optical device with the gate and runners remaining;
and removing the gate and the runner from the optical device. In the method of manufacturing an optical device according to claim 1,
The method of manufacturing an optical device, wherein the annealing treatment is performed with the runner remaining 2 mm or more and 80 mm or less. In the method for manufacturing an optical device according to claim 1 or 2,
The annealing treatment is performed at a set temperature that is a predetermined temperature lower than the glass transition temperature of the optical device,
The method for manufacturing an optical device, wherein the predetermined temperature is 10° C. or higher and 30° C. or lower. In the method for manufacturing an optical device according to any one of claims 1 to 3,
The method for manufacturing an optical device, wherein the optical device has a thickness of 5 mm or more and 50 mm or less in the optical axis direction. An optical device, wherein the retardation amount of a predetermined region near the gate is 100 nm or less. 6. The optical device of claim 5,
The optical device, wherein the predetermined area is within a range of 5% or more and 15% or less of the outer dimensions of the optical device from the gate. The optical device according to claim 5 or 6,
An optical device, wherein a peak-to-valley value with respect to a design value of the optical device is 0.05% or less of an outer dimension of the optical device. In the optical device according to any one of claims 5 to 7,
An optical device having a thickness in the optical axis direction of 5 mm or more and 50 mm or less.

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