JP3728119B2 - Optical element mold and molding method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a molding die and a molding method for an optical element.
[0002]
[Prior art]
Conventionally, plastic elements have been used as optical elements used in scanning optical systems such as copying machines and scanning optical systems such as scanners, but for the purpose of improving optical performance, downsizing, and reducing costs. Designed based on diffractive optics, plastic mold diffractive optical elements (molded by injection molding, injection compression molding, compression molding) having a complex fine diffraction grating pattern formed on the surface have come to be used (for example, (See Japanese National Publication No. 9-505245 and Japanese Utility Model Publication No. 4-55282).
[0003]
The diffraction grating of the diffractive optical element is provided with a plurality of gratings having a height of 0.1 to 10 μm on one side or both sides of two optical function surfaces, or an elliptical grating. It is generally provided in multiple layers, and each lattice shape has various shapes such as a stepped plane or a sawtooth shape. In many cases, the diffraction grating has a lattice interval that varies in the longitudinal direction.
[0004]
In recent years, downsizing of the optical system and further improvement in scanning accuracy have been demanded, and the shape accuracy required for the diffractive optical element has become strict.
[0005]
Further, the shape of the diffractive optical element is required not only to form a diffraction grating on a flat plate shape, but also to have a thick shape having a distribution in thickness as in an ordinary optical lens.
[0006]
[Problems to be solved by the invention]
However, in the case of molding a long diffractive optical element, the holding pressure is set high in order to obtain good transferability from the mold even on the optical function surface away from the gate. Is too high, and the holding pressure is also set to be high in the entire diffractive optical element. As described above, as a problem when the holding pressure of the gate portion of the diffractive optical element is too high, the gate portion is overpacked and has a large internal stress when taken out from the mold, so that the releasability is deteriorated. Deformation of the diffraction grating occurs.
[0007]
For example, when the diffractive optical element (such as an fθ lens) of the scanning optical system has an elongated shape and the distance between the diffraction gratings at the end in the longitudinal direction is smaller than the central part in the longitudinal direction of the diffractive optical element, the vicinity of the gate is compressed by overpacking Since the amount is large and the diffractive optical element is released while being deformed at the moment when it is ejected from the mold, the optical functional surface is released while sliding with respect to the mold. As shown in FIG. The shape of the grating is deformed, the shape accuracy of the diffractive optical function surface is deteriorated, and the optical performance is lowered.
[0008]
Further, when the holding pressure is too high in the entire diffractive optical element, the entire optical element has a large amount of compression due to overpacking, and the diffractive optical element is released while being deformed mainly in the longitudinal direction at the moment when it is ejected from the mold. For this reason, the optical functional surface is released while sliding with respect to the mold, and the shape of the diffraction grating is deformed as shown in FIG. 15, and the shape accuracy of the diffractive optical functional surface is deteriorated and the optical performance is lowered. .
[0009]
The present invention has been made in view of the above-described problems, and an object thereof is to provide a molding die and a molding method for an optical element that can prevent deterioration of optical performance due to deformation of the optical element at the time of mold release.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problems and achieve the object, the optical element molding die of the present invention is a molding for molding a long optical element in which a lattice is formed at a non-uniform pitch in the longitudinal direction of the optical element. In the mold, the mold is composed of a fixed mold and a movable mold. A cavity is formed inside the mold when the mold is clamped, and a gate connected to the cavity is provided in the vicinity of a portion where the lattice pitch is long compared to other places. .
[0012]
The optical element molding method of the present invention is a method of molding a long optical element in which a lattice is formed at a non-uniform pitch in the longitudinal direction. Forming an optical material injected into the cavity through the gate in a molding die provided with a gate connected to the cavity in the vicinity of a portion where the lattice pitch is longer than other portions, and pressurizing the gate. When the temperature of the optical material is lower than the glass transition point of the material and is released, the transfer surface for transferring the shape of the lattice is released with priority over the other surface.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[First Embodiment]
FIG. 1 shows a mold used when injection-molding the diffractive optical element of the first embodiment, and is a view seen from the side of a gate 6 for injecting a plastic resin. FIG. 2 is a front view of the diffractive optical element injection-molded by the mold shown in FIG. 1 as viewed from the diffractive optical function surface.
[0014]
As shown in FIGS. 1 and 2, a runner 7 and a gate 6 for injecting a plastic resin material at the time of injection molding are formed on a parting line 5 provided for taking out a diffractive optical element from a mold. Has been. (In FIG. 1, it is shown in the center of the diffractive optical element by a dotted line). The mold 3 is formed with a diffractive transfer surface 1 which is mirror-finished in a desired shape in order to transfer the diffractive optical functional surface of the diffractive optical element.
[0015]
In addition, on the other side of the parting line 5, a mold 4 that constitutes a flat transfer surface 2 that is mirror-finished into a shape for transferring a flat optical functional surface of the diffractive optical element, and In this case, as shown in FIG. 2, the shape of the diffractive optical functional surface provided on the mold 3 has a long diffraction grating interval of 2000 μm at the center of the diffractive optical element, and the diffraction grating interval becomes shorter at the end in the longitudinal direction. is there. The diffraction grating has a substantially straight shape in the short direction of the diffractive optical element.
[0016]
The part 3 on the diffractive optical function surface side and the mold 4 on the plane side are combined with the parting line 5 as a boundary to form a cavity 8 for molding the diffractive optical element, and the gate 6 has a long interval between the diffractive elements. It is provided at the center in the longitudinal direction.
[0017]
An injection molding process using the mold having the above configuration will be described. A mold clamping force is applied to the mold 3 and the mold 4, and the cavity 8 is filled with the molten plastic resin material through the gate 6 to apply pressure to the plastic resin material. The plastic resin material in the cavity is cooled and contracted by heat radiation to the mold. At this time, even if the shrinkage of the plastic resin material occurs, the optical function surface of the diffractive optical element, particularly the portion of the diffractive optical function surface, is sufficient for the plastic resin material to transfer as a highly accurate mirror surface close to the shape of the mold 3. It is necessary to apply pressure.
[0018]
When the temperature of the plastic resin material becomes sufficiently lower than the glass transition point of the plastic resin material due to heat dissipation, the mold is opened and the diffractive optical element is taken out. At this time, since the diffractive optical element in the cavity is overpacked, high internal stress is applied. In particular, since the compression amount is large in the vicinity of the gate 6 (longitudinal direction center) of the diffractive optical element, the mold 3 and the mold 4 open with the parting line 5 as a boundary, and the optical function on the mold 3 side of the diffractive optical element. The shape accuracy deteriorates as shown in FIG. 15 at the moment when the surface including the surface is released, but the diffraction grating interval near the gate of the diffractive optical element of this embodiment is as long as 200 μm, and therefore the diffraction efficiency b as shown in FIG. When evaluated in terms of a decrease in / a, the diffraction grating interval a is as short as 2000 μm and the deformed portion b is 50 μm, so that an optical element that can sufficiently satisfy the optical performance can be obtained.
[Conventional example]
FIG. 3 is a view of a mold used for injection molding of a conventional diffractive optical element as viewed from the parting line 105 side. FIG. 4 is a front view of a conventional diffractive optical element as seen from the diffractive optical function surface.
[0019]
As shown in FIGS. 3 and 4, a diffractive transfer mirror-finished to a desired shape in order to transfer a diffractive optical functional surface of a diffractive optical element onto a mold 103 followed by a gate 106 for injecting resin into the mold. A surface 101 is formed.
[0020]
On the other side of the parting line 105, there is formed a mold 104 constituting the flat transfer surface 2 which is mirror-finished into a shape for transferring the flat optical functional surface of the diffractive optical element.
[0021]
In the conventional example, the shape of the diffractive optical functional surface provided on the mold 103 is such that the diffraction grating interval is as long as 2000 μm at the center of the diffractive optical element, and the diffraction grating interval is as short as 100 μm at the end in the longitudinal direction. The diffraction grating has a substantially straight shape in the short direction of the diffractive optical element. A cavity 108 for forming a diffractive optical element is formed by combining the mold 103 on the diffractive optical function surface side and the flat side mold 104 with the parting line 105 as a boundary, and the gate 106 has a short distance between the diffraction gratings of the diffractive optical element. It is provided on the side in the hand direction.
[0022]
In the injection molding process using the mold having the above-described configuration, a mold clamping force is applied to the mold 103 and the mold 104, the melted plastic resin material is filled into the cavity 108 through the gate 106, and pressure is applied to the plastic resin material. The plastic resin material in the cavity is cooled and contracted by heat radiation to the mold. At this time, even if the shrinkage of the plastic resin material occurs, the optical function surface of the diffractive optical element, particularly the portion of the diffractive optical function surface is sufficient for the plastic resin material to transfer as a highly accurate mirror surface close to the shape of the mold 103. It is necessary to apply pressure.
[0023]
When the temperature of the plastic resin material becomes sufficiently lower than the glass transition point of the plastic resin material due to heat dissipation, the mold is opened and the diffractive optical element is taken out. At this time, since the diffractive optical element in the cavity is overpacked, high internal stress is applied. In particular, the surface near the gate of the diffractive optical element (longitudinal end = short side surface) has a large amount of compression, so that the surfaces including the optical functional surface on the mold 103 side of the diffractive optical element are opened by opening the molds 103 and 104. The shape accuracy deteriorates as shown in FIG. 15 at the moment when the mold is released. The distance between the diffraction gratings near the gate of the conventional diffractive optical element is as short as 100 μm, and a = 100 μm when evaluated in terms of reduction in diffraction efficiency. , B = 50 μm, an optical element that can sufficiently satisfy the optical performance cannot be obtained, causing a problem in optical performance.
[Second Embodiment]
FIG. 5 is a cross-sectional view of a mold used when injection molding the diffractive optical element of the second embodiment. FIG. 6 is a front view of a diffractive optical element injection-molded by the mold shown in FIG.
[0024]
In the following description, only different parts are given new numbers, and the other parts are given the same numbers so that differences from the first embodiment become clear.
[0025]
As shown in FIGS. 5 and 6, a gate 6 for injecting resin is provided across a parting line 5 provided in the mold, and the mold 3 is used to transfer the diffractive optical function surface of the diffractive optical element. A diffraction transfer surface 31 having a mirror finish in a desired shape is formed.
[0026]
On the other side of the parting line 5, there is disposed a mold 4 that constitutes a mirror-finished planar transfer surface 2 having a shape for transferring the planar optical functional surface of the diffractive optical element.
[0027]
In the case of the present embodiment, the shape of the diffractive optical functional surface provided on the mold 3 has a diffraction grating interval as long as 1800 μm at both ends of the diffractive optical element, and a diffraction grating interval as short as 50 μm toward the center in the longitudinal direction. The grating has a substantially straight shape in the short direction of the diffractive optical element. A cavity 8 for forming the diffractive optical element is formed by combining the diffractive optical function surface side mold 3 and the planar side mold 4 with the parting line 5 as a boundary, and the gate 6 has a short distance between the diffraction gratings of the diffractive optical element. It is provided on the side of the hand.
[0028]
In the injection molding process using the mold having the above-described configuration, a clamping force is applied to the mold 3 and the mold 4, and the melted plastic resin material is filled into the cavity 8 through the gate 6 to apply pressure to the plastic resin material. The plastic resin material in the cavity is cooled and contracted by heat radiation to the mold. At this time, even if the shrinkage of the plastic resin material occurs, the optical function surface of the diffractive optical element, particularly the portion of the diffractive optical function surface, is sufficient for the plastic resin material to transfer as a highly accurate mirror surface close to the shape of the mold 3. It is necessary to apply pressure.
[0029]
When the temperature of the plastic resin material becomes sufficiently lower than the glass transition point of the plastic resin material due to heat dissipation, the mold is opened and the diffractive optical element is taken out. At this time, since the diffractive optical element in the cavity is overpacked, high internal stress is applied. In particular, since the amount of compression is large near the gate of the diffractive optical element (center in the longitudinal direction), the mold 3 and the mold 4 are opened and the surface including the optical functional surface on the mold 3 side of the diffractive optical element is released. Although the shape accuracy deteriorates as shown in FIG. 15, the diffraction performance near the gate of the diffractive optical element of the present embodiment is long, so that the optical performance can be sufficiently satisfied when evaluated in terms of a reduction in diffraction efficiency. An optical element can be obtained.
[Third Embodiment]
7 to 10 are cross sections of a mold used when molding the optical element of the third embodiment, and are diagrams showing from molding to mold opening. FIG. 11 is a view of the mold of FIG. 7 as viewed from the movable mold when the parting line 10 is opened.
[0030]
As shown in FIGS. 7 and 11, one side of the parting line 10 provided in the mold is fixed to the fixed side mold plate 13 constituting the fixed side mold and the fixed side mold plate 13 inserted and mounted. Side mold pieces 11 are arranged. The fixed side mold plate 13 and the fixed side mold piece 11 are formed with a shape for transferring the shape of the present optical element. In particular, the surface of the fixed side mold piece 11 is desired to transfer the optical function surface of the optical element. A flat transfer surface 11a having a mirror-finished shape is formed.
[0031]
Further, on the other side of the parting line 10, the movable side mold plate 14 constituting the movable side mold, the movable side mold piece 12 inserted and mounted on the movable side mold plate 14, and the movable side mold piece 12 are movable. A sliding device 21 and an ejector pin 18 that are moved with respect to the side mold plate 14 are disposed. A concave portion for transferring the shape of the present optical element is formed in the movable side mold plate 14 and the movable side mold piece 12, and in particular, the diffractive optical function surface of the optical element is transferred to the surface of the movable side mold piece 12. Therefore, a diffraction transfer surface 12a having a mirror finish in a desired shape is formed. Further, a spool 24, a runner 23, and a gate 22 for injecting a plastic resin material are formed on the movable side mold plate 14 as shown in FIG.
[0032]
In the case of this embodiment, a fixed mold and a movable mold are combined with the parting line 10 as a boundary to form a cavity 25 for molding an optical element, and the movable mold 12 is moved by the sliding device 21. The side mold plate 14 can be moved.
[0033]
An injection molding process using the mold having the above configuration will be described.
[0034]
As shown in FIGS. 7 to 10, a clamping force is applied to the fixed side mold and the movable side mold, and the melted plastic resin material is filled into the cavity 25 through the spool 24, the runner 23, and the gate 22 to obtain the plastic resin material. Apply pressure. The plastic resin material in the cavity is cooled and contracted by heat radiation to the mold. At this time, even if the plastic resin material contracts, the optical functional surface of the optical element, in particular, the portion of the diffractive optical functional surface where the lattice interval is narrow is transferred as a highly accurate mirror surface close to the shape of the movable side piece 12 It is necessary to apply sufficient pressure to the resin material.
[0035]
When the temperature of the plastic resin material becomes sufficiently lower than the glass transition point of the plastic resin material due to heat dissipation, the mold is opened and the optical element is taken out. However, since the optical element in the cavity is overpacked, high internal stress remains. In particular, since the compression amount is large in the longitudinal direction of the optical element, as shown in FIG. 8, the movable side mold is opened before the mold is opened and the surface including the optical functional surface on the fixed mold side of the optical element is released. The piece 12 is slid with respect to the movable side mold plate 14 by the sliding device 21, and the diffractive optical function surface is released in a state where the shape of the entire optical element is constrained by the fixed side mold piece 11 and the fixed side mold plate 13. Let Thereby, since the deformation of the optical element during the mold release step can be prevented, the deformation during the mold release of the diffractive optical function surface can be suppressed as shown in FIG.
[0036]
In this way, since no slip occurs with respect to the mold, the deformation of the diffraction grating is minimized, the deformation portion b is about 1 μm, and the minimum grating interval a is about 100 μm. .
[0037]
Subsequently, as shown in FIG. 9, the mold is opened with the parting line 10 as a boundary, the ejector rod of the injection molding machine pushes the ejector plates 19 and 20, and the ejector pin 18 connected thereto ejects the movable side mold of the optical element. The end surface except the optical function surface on the side is released, and the optical element is released from the mold as shown in FIG.
[0038]
Accordingly, it is possible to mold a diffractive optical element that satisfies the diffractive optical performance without causing a shape defect of the diffractive optical function surface at the time of mold release.
[Fourth Embodiment]
FIGS. 13 and 14 are cross sections of a mold used when molding the optical element of the fourth embodiment, and are diagrams showing from molding to mold opening. FIG. 12 is a view of the mold of FIG. 13 as viewed from the movable mold when the parting line 10 is opened.
[0039]
As shown in FIGS. 13 and 14, one side of the parting line 10 provided in the mold is fixed to the fixed side mold plate 13 constituting the fixed side mold and the fixed side mold plate 13. Side mold pieces 11 are arranged. The fixed-side mold plate 13 and the fixed-side mold piece 11 are formed with a transfer surface for transferring the outer shape of the optical element. In particular, the diffractive optical function surface of the optical element is transferred to the surface of the fixed-side mold piece 11. Thus, a diffraction transfer surface 11a having a mirror finish in a desired shape is formed.
[0040]
Further, on the other side of the parting line 10, there are arranged a movable mold plate 14 constituting a movable mold and a movable mold piece 12 inserted and attached to the movable mold plate 14. The movable side plate 14 and the movable side piece 12 are formed with a transfer surface for transferring the outer shape of the optical element, and in particular, the optical function surface of the optical element is transferred to the surface of the movable side die piece 12. A flat transfer surface 12a having a mirror finish in a desired shape is formed. In addition, a spool 24, a runner 23, and a gate 22 for injecting a plastic resin material are formed on the movable side template 14 as shown in FIG.
[0041]
In the case of this embodiment, the fixed side mold and the movable side mold are combined with the parting line 10 as a boundary to form a cavity 25 for molding the optical element. The diffractive optical function surface can be preferentially released.
[0042]
Clamping force is applied to the fixed side mold and the movable side mold, the molten plastic resin material is filled into the cavity 25 through the sprue 24, the runner 23, and the gate 22 to apply pressure to the plastic resin material, and the plastic resin material is cooled. When the temperature is sufficiently lower than the glass transition point, the mold is opened and the optical element is taken out. As shown in FIG. 14, the outer shape of the entire optical element other than the diffraction transfer surface 11a is changed by the movement of the mold. The fixed mold is released while being restrained by the movable mold 14. This makes it possible to suppress deformation of the diffractive optical function surface during mold release by suppressing deformation of the optical element during the mold release process as shown in FIG. 16, and the deformed portion b is about 10 μm and the minimum grating interval a Is about 100 μm, and a molded product having a level with no problem in diffraction efficiency can be obtained.
[0043]
At this time, the mold opening operation of the movable side mold plate and the fixed side mold plate needs to be performed without precise displacement, and depending on the mold opening accuracy of the injection molding machine, the movable side mold plate and the fixed side mold plate In some cases, a guide pin is required to make the positioning precise.
[0044]
Accordingly, it is possible to mold a diffractive optical element that satisfies the diffractive optical performance without causing a defective shape of the diffractive optical function surface during release.
[0045]
According to each of the embodiments described above, by providing the gate at a portion where the diffraction grating interval of the optical function surface is rough, it is possible to prevent deterioration in optical performance due to deformation of the diffractive optical function surface at the time of mold release.
[0046]
Deformation of the diffraction grating at the time of mold release largely occurs in a portion where the pressure is locally high. On the other hand, in the case of a scanning optical system such as a laser beam, although the diameter of the laser beam passing through the diffractive optical function surface of the long optical element is the same at each point in the longitudinal direction, the distance between the diffraction gratings Are often different. That is, the number of diffraction gratings existing within the beam diameter when one beam passes through the optical element differs depending on the position in the longitudinal direction.
[0047]
In this embodiment, paying attention to the fact that the amount of deformation of the diffraction grating is large in the vicinity of the gate, by providing a gate in a portion where the diffraction grating interval is coarse and the number of diffraction gratings per one beam diameter is small, Degradation of the optical performance of the optical element due to deformation can be prevented.
[0048]
In the molding of the optical element according to the present embodiment, when the optical element is taken out from the mold, the optical functional surface having the diffraction grating is released by preferentially releasing it from other optical functional surfaces or constituent surfaces. Deterioration of optical performance due to deformation of the diffractive optical function surface at the time can be prevented.
[0049]
Since the diffraction grating at the time of mold release is generally high in pressure, it may extend about 0.5 mm to 1.0 mm at the moment of mold release. In this embodiment, in order to minimize the extension of the optical element at the time of mold release, in the process of taking out the optical element from the mold, the release of the diffractive optical function surface is given priority over the other surface. By doing so, the other surface (especially the optical function surface opposite to the diffractive optical function surface) is constrained by the mold, and the overall deformation (elongation) of the optical element is suppressed, and the diffractive optical function surface is separated. It is possible to prevent the deterioration of the optical performance of the optical element due to the deformation of the mold.
[0050]
The present invention can be applied to a modified or modified embodiment without departing from the spirit of the present invention.
[0051]
【The invention's effect】
According to the present invention, a gate for injecting a resin of the diffractive optical element is provided in a portion where the distance between the diffraction gratings is long, and the diffractive optical functional surface is released preferentially from the other surfaces, so that diffraction at the time of release is performed. Deformation of the diffractive optical function surface of the element can be suppressed.
[0052]
[Brief description of the drawings]
FIG. 1 shows a mold used when injection-molding a diffractive optical element according to a first embodiment, and is a view seen from the side of a gate 6 for injecting resin.
2 is a front view of a diffractive optical element injection-molded by the mold shown in FIG. 1 as viewed from a diffractive optical function surface. FIG.
FIG. 3 is a view of a mold used for injection molding of a conventional diffractive optical element as viewed from the parting line 105 side.
FIG. 4 is a front view of a conventional diffractive optical element as viewed from a diffractive optical function surface.
FIG. 5 is a cross-sectional view of a mold used when injection-molding a diffractive optical element according to a second embodiment.
6 is a front view of a diffractive optical element injection-molded by the mold shown in FIG.
FIG. 7 is a cross section of a mold used when molding the optical element of the third embodiment, and is a diagram showing from molding to mold opening.
FIG. 8 is a cross section of a mold used when molding the optical element of the third embodiment, and is a diagram showing from molding to mold opening.
FIG. 9 is a cross-sectional view of a mold used when molding the optical element of the third embodiment, and shows from molding to mold opening.
FIG. 10 is a cross section of a mold used when molding the optical element of the third embodiment, and is a diagram showing from molding to mold opening.
11 is a view of the mold of FIG. 7 as viewed from a movable mold when the parting line 10 is opened.
12 is a view seen from a movable mold when the mold of FIG. 13 is opened by a parting line 10. FIG.
FIG. 13 is a cross section of a mold used when molding the optical element of the fourth embodiment, and is a diagram showing from molding to mold opening.
FIG. 14 is a cross section of a mold used when molding the optical element of the fourth embodiment, and is a diagram showing from molding to mold opening.
FIG. 15 is a diagram showing a deformation state of a diffraction grating having a problem in diffraction efficiency.
FIG. 16 is a diagram showing a deformation state of the diffraction grating to such an extent that the diffraction efficiency does not become a problem.
[Explanation of symbols]
1 diffractive optical functional surface 2 optical functional surface 3 type (diffractive optical functional surface)
Type 4 (optical functional surface)
5 Parting Line 6 Gate 7 Runner 8 Cavity 10 Parting Line 11 Fixed Side Piece 12 Movable Side Piece 13 Fixed Side Plate 14 Movable Side Plate 15 Fixed Side Mounting Plate 16 Movable Side Receiving Plate 17 Movable Side Mounting Plate 18 Ejector pin 19 Ejector plate 20 Ejector plate 21 Sliding mechanism 25 Cavity

Claims (5)

In a molding die for molding a long optical element in which a lattice is formed at a non-uniform pitch in the longitudinal direction of the optical element,
The mold is composed of a fixed mold and a movable mold, and a cavity is formed in the mold clamped state, and a gate communicating with the cavity is provided in the vicinity of a portion where the lattice pitch is longer than other portions. Molding mold.   2. The mold according to claim 1, wherein the optical element is a lens of a scanning optical system in which diffraction gratings are formed at a non-uniform pitch. In a method of forming a long optical element that forms a lattice with a non-uniform pitch in the longitudinal direction,
A fixed mold and a movable mold are formed, and a cavity is formed inside the mold in a clamped state, and the gate communicated with the cavity is formed through the gate in the molding mold provided near the portion where the lattice pitch is long compared to other places. Injection and pressurization of molten optical material into the cavity,
A molding method characterized in that when the temperature of the optical material is lower than the glass transition point of the material and the mold is released, the transfer surface on which the shape of the lattice is transferred is given priority over the other surface. 4. The method of molding an optical element according to claim 3 , wherein the optical element is a lens of a scanning optical system in which diffraction gratings are formed at a non-uniform pitch. The molding method according to claim 3 or 4 , wherein only the transfer surface for transferring the shape of the lattice is released, and then the other surface is released.

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