SURFACE TREATMENT METHOD FOR TRANSPARENT RESIN FORMING MOLD, TRANSPARENT RESIN FORMING MOLD, AND TRANSPARENT RESIN FORMED ARTICLE

Abstract

A method of treating a surface of a mold for transparent resin molding. The method includes ejecting substantially spherical ejection particles against a surface of a mold employed to mold a transparent resin so as to bombard the surface. The method includes forming dimples with an equivalent diameter in a range that satisfies a condition defined by the following formula:

1+3.3e.sup.H/230W1.5+8.9e.sup.H/630

wherein W is an equivalent diameter (m) of the dimples and H is a base metal hardness (Hv) of the mold.

Claims

1. A method of treating a surface of a mold for transparent resin molding, the method comprising: ejecting substantially spherical ejection particles against a surface of a mold employed to mold a transparent resin so as to bombard the surface; and forming dimples with an equivalent diameter in a range that satisfies a condition defined by the following formula:
1+3.3e.sup.H/230W1.5+8.9e.sup.H/630Formula (1) wherein W is an equivalent diameter (m) of the dimples and H is a base metal hardness (Hv) of the mold.

2. The surface treatment method of the mold for transparent resin molding according to claim 1, wherein the dimples are formed with a depth in a range satisfying a condition defined by the following formula:
0.01+0.2e.sup.H/230D0.05+0.4e.sup.H/320Formula (2) wherein D is a depth (m) of the dimples and H is a hardness of mold base metal (Hv).

3. The surface treatment method of the mold for transparent resin molding according to claim 1, wherein the dimples are formed by ejecting the ejection particles having a median diameter not greater than 20 m at an ejection pressure of from 0.01 MPa to 0.6 MPa such that a surface area formed with the dimples is not less than 50% of a surface area of the mold surface.

4. The surface treatment method of the mold for transparent resin molding according to claim 1, wherein the ejection particles are ejected against a surface of a mold having a surface roughness adjusted to an Ra of 0.3 m or less.

5. A mold for transparent resin molding that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 1.

6. A transparent resin molded article molded with a mold that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 1.

7. The surface treatment method of the mold for transparent resin molding according to claim 2, wherein the dimples are formed by ejecting the ejection particles having a median diameter not greater than 20 m at an ejection pressure of from 0.01 MPa to 0.6 MPa such that a surface area formed with the dimples is not less than 50% of a surface area of the mold surface.

8. The surface treatment method of the mold for transparent resin molding according to claim 2, wherein the ejection particles are ejected against a surface of a mold having a surface roughness adjusted to an Ra of 0.3 m or less.

9. The surface treatment method of the mold for transparent resin molding according to claim 3, wherein the ejection particles are ejected against a surface of a mold having a surface roughness adjusted to an Ra of 0.3 m or less.

10. A mold for transparent resin molding that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 2.

11. A mold for transparent resin molding that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 3.

12. A mold for transparent resin molding that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 7.

13. A transparent resin molded article molded with a mold that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 2.

14. A transparent resin molded article molded with a mold that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 3.

15. A transparent resin molded article molded with a mold that has been surface treated with the surface treatment method of the mold for transparent resin molding according to claim 7.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0047] FIG. 1 is a diagram to explain projections arising on a mold surface accompanying the formation of dimples.

[0048] FIG. 2 is a diagram correlating ejection pressure and dynamic hardness.

[0049] FIG. 3 is a scatter plot of dimple equivalent diameter against hardness of mold base metal for Samples 1 to 22.

[0050] FIG. 4 is a scatter plot of dimple depth against hardness of mold base metal for Samples 1 to 22.

DESCRIPTION OF EMBODIMENTS

[0051] Next, explanation follows regarding exemplary embodiments of the present invention, with reference to the accompanying drawings.

[0052] Object to be Treated

[0053] The surface treatment method of the present invention may be applied to molds for transparent resin molding. For such molds, the surface treatment method is applicable to various types of mold irrespective of the type of mold, such as molds for injection molding, molds for extrusion molding, and molds for blow molding. Moreover, as long as the material of the transparent resin molding matter to be subjected to molding by such a mold is a transparent resin, the surface treatment method is applicable to molds for molding various molding matter, such as acrylic, Nylon, vinyl chloride, polycarbonate, PET, and POM.

[0054] The surfaces of portions within such molds that make contact with the molding material may serve as a surface to be treated by the surface treatment method of the present invention. Both surfaces on a cavity (concave mold) side and a core (convex mold) side can be subjected to treatment by the method of the present invention when the mold is configured by a combination of both a cavity (concave mold) and a core (convex mold).

[0055] There are no particular limitations to the material of the mold, and various materials employed as materials for molds may be subjected to treatment. As well as ferrous metals, molds of non-ferrous metals such as aluminum alloys and the like may also be subjected to treatment.

[0056] Note that the surface roughness of the surface of a mold is preferably adjusted in advance to an arithmetic average roughness (Ra) of 0.3 m or less prior to ejecting spherical ejection particles as described later.

[0057] Dimple Forming

[0058] Dimples are formed on the surface of a mold as described above by ejecting substantially spherical ejection particles so as to bombard the surface of molding faces of the mold.

[0059] The following are examples of ejection particles, ejection apparatuses, and ejection conditions employed to form such dimples.

[0060] (1) Ejection Particles

[0061] For the substantially spherical ejection particles employed in the method of the present invention, substantially spherical means that they do not need to be strictly spherical, and ordinary shot may be employed therefor. Particles of any non-angular shape, such as an elliptical shape and a barrel shape, are included in substantially spherical ejection particles employed in the present invention.

[0062] Materials employable as the ejection particles include both metal-based and ceramic-based materials. Examples of materials for metal-based ejection particles include steel alloys, cast iron, high-speed tool steels (HSS) (SKH), tungsten (W), stainless steels (SUS), and the like. Examples of materials for ceramic-based ejection particles include alumina (Al.sub.2O.sub.3), zirconia (ZrO.sub.2), zircon (ZrSiO.sub.4), hard glass, glass, silicon carbide (SiC), and the like. The ejection particles employed are preferably ejection particles of a material having a hardness at least equivalent to that of the base metal of the mold to be treated.

[0063] Regarding the particle diameter of the ejection particles employed, particles having a median diameter (D.sub.50) in a range of from 1 m to 20 m may be employed. From among ejection particles of these particle diameters, the particles employed are selected so as to be able to form the dimples of the diameter and depth described below in accordance with the material and the like of the mold to be treated.

[0064] (2) Ejection Apparatus

[0065] A known blasting apparatus for ejecting compressed gas and abrasive may be employed as the ejection apparatus to eject the ejection particles described above against the surface of the mold.

[0066] Such blasting apparatuses are commercially available, such as a suction type blasting apparatus that ejects abrasive using a negative pressure generated by ejecting compressed gas, a gravity type blasting apparatus that causes abrasive falling from an abrasive tank to be carried and ejected by compressed gas, a direct pressure type blasting apparatus in which compressed gas is introduced into a tank filled with abrasive and the abrasive is ejected by merging the abrasive flow from the abrasive tank with a compressed gas flow from a separately provided compressed gas supply source, and a blower type blasting apparatus that carries and ejects the compressed gas flow from a direct pressure type blasting apparatus with a gas flow generated by a blower unit. Any one of the above may be employed to eject the ejection particles described above.

[0067] (3) Treatment Conditions

[0068] Ejection particles may be ejected using a blasting apparatus described above, for example, with an ejection pressure in the range of from 0.01 MPa to 0.6 MPa, and preferably from 0.05 MPa to 0.2 MPa, and performed such that the dimple-formed surface area (projected surface area) of the portion subjected to treatment is 50% or more of the surface area of the mold surface.

[0069] When the ejection particles are ejected, a combination of material and particle diameter for the ejection particles, and type, ejection pressure, and the like of the blasting apparatus employed, is selected in relation to the material, etc., of the mold to be treated so as to be able to form dimples of a equivalent diameter (W) found according to Formula (1), given below.

1+3.3e.sup.H/230W1.5+8.9e.sup.H/630Formula (1)

In Formula (1) above, W is the dimple equivalent diameter (m), and H is the base metal hardness (Hv).

[0070] When the ejection particles are ejected, in addition a combination of conditions is preferably employed that also enable dimples to be formed at a dimple depth (D) found according to Formula (2), given below.

0.01+0.2e.sup.H/230D0.05+0.4e.sup.H/320Formula (2)

In Formula (2), D is the dimple depth (m), and H is the base metal hardness (Hv).

[0071] (4) Operation Etc.

[0072] A mold subjected to surface treatment by the surface treatment method of the present invention as described above is confirmed to be able to impart transparency to transparent resin molded articles obtained. The examples described below confirm that such a mold is able to impart an equivalent degree of transparency to that of a mold (polished object) finished smooth by polishing, for example.

[0073] Such an improvement in transparency is thought to arise for the following reasons. Due to the dimples formed by the method of the present invention having the equivalent diameter and depth of the present invention, and being comparatively smaller scale to dimples formed by a conventional surface treatment method to form dimples on a mold surface, any irregularities formed on the surfaces of transparent resin molded articles by transfer from the dimples are also small and shallow. When forming such small and shallow dimples, the amount of the base metal of the mold pushed out by plastic flow arising during bombardment with the ejection particles is lessened, such that projections are not formed at peripheral edge portions of the dimples, or such that even if projections are formed, they do not have a raised shape. It is thought that transparency can be imparted to the transparent resin molded articles obtained even though dimples are formed on the mold surface due to the absence of irregularities formed by transfer from such projections, and due to the absence of scratches formed by such projections scratching the surface of transparent resin molded articles.

[0074] Moreover, a mold subjected to the surface treatment method of the present invention was confirmed to obtain a great improvement in demoldability and an improvement in durability compared to a polished object.

[0075] Reasons for such an improvement in demoldability are thought to be as follows. There is an improvement in demoldability obtained by release agent being retained, or air being retained, in the dimples, similarly to in a conventional surface treatment method in which dimples formed on a mold surface, thereby reducing the contact surface area between the molding material and the mold surface. However, in addition, the ability to retain release agent and to retain air in the dimples is improved by the larger reaction force resulting from the larger surface pressure acting at the dimples due to the dimples formed being small and shallow, thereby improving demoldability. Moreover, one cause of improved demoldability is thought to be a reduction in resistance to extraction when demolding due to not forming projections with a raised shape.

[0076] Moreover, ejection particles that have a comparatively small particle diameter, i.e. a median diameter from 1 m to 20 m, are employed as the spherical ejection particles to form the comparatively small dimples as described above. The surface-hardness after treatment is therefor raised compared to a conventional surface treatment method employing ejection particles having a larger particle diameter, and this is also thought to be a contributing factor to the greatly improved demoldability obtained.

[0077] It is known that when shot peening is performed by ejecting shot so as to bombard the surface of a metal product to be treated, hardness rises and the surface structure of the workpiece is miniatualized. The rise in the surface hardness of a mold according to this principle is thought to be something that is not only obtained by the surface treatment method of the present invention, but is also similarly obtained by a conventional method of treating a surface of a mold in which treatment is performed by ejecting spherical ejection particles at a mold surface.

[0078] However, tests performed to measure the surface hardness of treated objects after performing treatment in which ejection particles of different particle diameters are ejected against the surface of a mold confirm that, within a comparatively low ejection pressure range, a higher rise in hardness is obtained when ejection particles with a smaller particle diameters are employed.

[0079] FIG. 2 is a diagram illustrating the results of performing the above tests on a mold manufactured from NAK 80 (Hv 430). In a range of ejection pressures up to 0.5 MPa, the dynamic hardness of the mold surface was found to be raised more when ejection particles (material: steel alloy) with a median diameter of 20 m were ejected (see the solid line in FIG. 2) than when ejection particles (material: high-speed steel) with a median diameter of 40 m were ejected (see the dashed line in FIG. 2).

[0080] Different effects arise in this manner from the differences of particle diameters of the ejection particles employed. When ejection particles having a small particle diameter are used, the flight speed of the ejection particles is raised, raising the bombardment energy when the mold surface is bombarded, which raises the bombardment energy per unit surface area at the bombarded positions. This is thought to result in a higher forging effect being obtained even when ejecting with a low pressure compressed gas. Abrasion and deformation of the dimples formed on the mold surface is not liable to occur due to obtaining such an increase in hardness. As a result, ideal diameters and depths can be maintained over a long period of time, such that the advantageous effects achieved by the surface treatment method of the present invention, i.e. imparting transparency, improving demoldability, and the like, can be maintained over a long period of time.

[0081] Note that reference to dynamic hardness means a hardness obtained from an indentation depth at a test force in a process to indent a triangular pyramidal indenter, and the dynamic hardness can be found for a test force P (mN) and an indentation depth D of an indentor (m) by the following formula.

DH=P(D.sup.2)

[0082] Herein, is an indenter shape coefficient. In the measurements described above, a Shimadzu Dynamic Ultra Micro Hardness Tester DUH-W201 (manufactured by Shimadzu Corporation) was employed, and was measured at 3.8584 when a 115 triangular pyramidal indenter was employed.

Examples

[0083] A description follows regarding imparting transparency to resin molded articles, and to the content of tests performed to derive the formation conditions (diameter and depth) needed for dimples in order to improve the demoldability of a mold.

[0084] (1) Test Objective

[0085] The test is performed in order to find formation conditions (diameter and depth) of dimples capable of imparting transparency to resin molded articles and capable of improving demoldability of molds.

[0086] (2) Test Method

[0087] (2-1) Summary

[0088] Dimples were formed on plural types of molds made from base materials which are respectively different, while employing varying combinations of material and particle diameter of the ejection particles employed and the ejection method (ejection apparatus, ejection pressure, etc.). The diameter and depth of the dimples formed was then measured.

[0089] After forming the dimples, molding with transparent resin was performed using each of the molds. The transparency was then compared by visual inspection to that of transparent resin molded articles molded by molds whose surfaces had been finished smooth by polishing (referred to below as polished objects). Molds giving a transparency inferior to that from polished objects were evaluated as X, and molds giving a transparency equivalent to that from polished objects were evaluated O.

[0090] A comparison of demoldability was also performed. Molds having a demoldability equivalent or inferior to that of polished objects were evaluated as X, and molds having a demoldability surpassing that of polished objects were evaluated as O.

[0091] A range of diameters and depths of dimples capable of imparting transparency to resin molded articles obtained was derived from the results of the above tests.

[0092] (2-2) Types of Mold and Treatment Conditions

[0093] The materials of molds to be treated and the treatment conditions for the surface treatment performed on each of the molds are listed in Table 1 and Table 2, below.

TABLE-US-00001 TABLE 1 Type of Mold and Mold Treatment Conditions 1 Spherical Ejection Particles Particle Ejection Conditions Mold Diameter Hard- Ejection Nozzle Ejection Hard- Sample D.sup.50 ness Ejection Pressure Diameter Duration Base Metal ness No. (m) Material (Hv) Method (MPa) (mm) (sec) (Type) (Hv) 1 21 steel 870 FD 0.5 5 1800 STAVAX 630 alloy (cavity) 2 80 high- 840 FD 0.5 5 1800 speed steel 3 13 steel 870 FD 0.2 5 360 alloy 4 36 high- 840 FD 0.2 5 360 speed steel 5 17 steel 870 SF 0.5 7 30 S50C 194 alloy (core pin) 6 20 alumina 1800 FD 0.3 5 30 7 64 high- 840 SF 0.5 7 30 speed steel 8 15 steel 870 SF 0.1 7 30 alloy 9 20 zircon 700 LD 0.05 9 30 10 80 high- 840 SF 0.1 7 30 speed steel

TABLE-US-00002 TABLE 2 Type of Mold and Mold Treatment Conditions 2 Spherical Ejection Particles Particle Ejection Conditions Mold Diameter Hard- Ejection Nozzle Ejection Hard- Sample D.sup.50 ness Ejection Pressure Diameter Duration Base Metal ness No. (m) Material (Hv) Method (MPa) (mm) (sec) (Type) (Hv) 11 12 steel 870 SF 0.2 7 30 S55C 270 alloy (mold for 12 20 zircon 700 FD 0.3 5 30 rubber) 13 15 high- 840 LD 0.01 9 30 speed steel 14 63 alumina 1800 SF 0.2 7 30 15 4 zircon 700 SF 0.5 7 30 NAK80 420 16 20 steel 870 FD 0.3 5 30 (mold for alloy acrylic) 17 8 alumina 1800 LD 0.02 9 30 18 80 high- 840 SF 0.2 7 30 speed steel 19 8 high- 840 SF 0.2 7 30 A7075 183 speed (mold for steel plastic) 20 20 alumina 1800 SF 0.1 7 30 21 10 zirconia 1300 LD 0.05 9 30 22 80 zirconia 700 SF 0.3 7 30 *The ejection methods indicated in Table 1 and Table 2 employed the following blasting apparatuses: SF: Suction Type (SFK-2 manufactured by Fuji Manufacturing Co., Ltd.) FD: Direct Pressure Type (FDQ -2 manufactured by Fuji Manufacturing Co., Ltd.) LD: Blower Type (LDQ-2 manufactured by Fuji Manufacturing Co., Ltd.)

[0094] Polished objects were prepared for each of the molds for comparison. Note that the surface roughness Ra after polishing was 0.1 m or less for the STAVAX (cavity) and NAK80, 0.2 m or less for the S50C (core pin), S55C (mold for rubber), and 0.2 m or less for the A7075 (mold for plastic).

[0095] (2-3) Dimple Diameter and Depth Measurement Method

[0096] The diameter and depth of the dimples were measured using a profile analyzing laser microscope (VK-X250 manufactured by Keyence Corporation).

[0097] Measurements of the surface of the mold were made directly in cases in which direct measurement was possible. In cases in which direct measurement was not possible, methyl acetate was dripped onto a cellulose acetate film to cause the cellulose acetate film to follow the surface of the mold, and after drying and peeling off the cellulose acetate film, measurement was performed based on the inverted dimples transferred to the cellulose acetate film.

[0098] Surface image data imaged by the profile analyzing laser microscope (or, image data inverted from captured images measured by employing the cellulose acetate film) was analyzed using a Multi-File Analysis Application (VK-H1XM by Keyence Corporation) to perform the measurements.

[0099] The Multi-File Analysis Application is an application that uses data measured by a laser microscope to measure surface roughness, line roughness, height and width, etc. The application analyzes the equivalent circular diameter, depth, and the like, sets a reference plane, and is capable of performing image processing such as height inversion.

[0100] In measuring, first the image processing function was used to set the reference plane (however, in cases in which the surface shape is a curved plane, the reference plane is set after the curved plane has been corrected to a flat plane by using plane shape correction). Then, the measurement mode is set to indentation in the volume/area measurement function of the application, indentations were measured with respect to the set reference plane, and the average depth in the indentation measurement results and the average value of the results for equivalent circular diameter were taken as the depth and equivalent diameter of the dimples.

[0101] Note that the reference plane described above was computed from height data using a least squares method.

[0102] Moreover, the equivalent circular diameter and the equivalent diameter described above are measured as the diameter of a circle determined by converting the projected surface area measured for an indentation (dimple) into a circular projected surface area.

[0103] Note that the reference plane described above indicates a flat plane at the origin (reference) measurement for height data, and is employed mainly to measure depth, height, etc. in the vertical direction.

[0104] (3) Measurement Results

[0105] The measurement results for each of the Samples with regards to dimple diameter and dimple depth and the evaluation results for transparency and demoldability are listed in Table 3 and Table 4. FIG. 3 is a scatter plot illustrating dimple diameter against hardness of mold base metal for all Samples, and FIG. 4 is a scatter plot illustrating dimple depth against hardness of mold base metal for all Samples.

TABLE-US-00003 TABLE 3 Measurement Results for Equivalent Diameter and Depth of Dimples, and Transparency and Demoldability Evaluation Results 1 Dimple Equivalent Dimple Sample Molding Diameter Depth Transparency Demoldability Mold No. Material (m) (m) Evaluation Evaluation STAVAX 1 Poly- 6.94 0.30 X O (cavity) 2 carbonate 20.72 0.66 X X 3 2.92 0.08 O O 4 16.85 0.43 X X S50C 5 PVC 8.52 0.54 X O (core pin) 6 (polyvinyl- 13.72 0.66 X O 7 chloride) 15.76 0.99 X X 8 4.39 0.18 O O 9 5.30 0.16 O O 10 16.88 0.91 X X * Evaluations of transparency indicate: O: Molds giving a transparency equivalent to that of polished objects X: Molds giving a transparency inferior to that of polished objects * Evaluations of demoldability indicate: O: demoldability exceeding that of polished objects X: demoldability the same as or worse than that of polished objects

TABLE-US-00004 TABLE 4 Measurement Results for Equivalent Diameter and Depth of Dimples, and Transparency and Demoldability Evaluation Results 2 Dimple Equivalent Sample Molding Diameter Dimple Transparency Demoldability Mold No. Material (m) Depth (m) Evaluation Evaluation S55C 11 Silicone 2.91 0.11 O O (mold for 12 rubber 8.37 0.32 X O rubber) 13 2.50 0.06 O X 14 14.92 0.36 X X NAK80 15 Acrylic 1.98 0.06 O O (mold for 16 6.92 0.22 X O plastic) 17 1.20 0.01 O X 18 13.31 0.29 X X A7075 19 Poly- 2.70 0.16 O O (mold for 20 carbonate 7.53 0.22 O O plastic) 21 2.33 0.09 O X 22 15.58 0.82 X X * Evaluations of transparency indicate: O: Molds giving a transparency equivalent to that of polished objects X: Molds giving a transparency inferior to that of polished objects * Evaluations of demoldability indicate: O: demoldability exceeding that of polished objects X: demoldability the same as or worse than that of polished objects

[0106] (4) Interpretation

[0107] In the scatter plots illustrated in FIG. 3 and FIG. 4, the numbers appended to the plots indicate the respective sample numbers. In the plots, indicates that both transparency and an improvement in demoldability were obtained, O indicates that although transparency was obtained, demoldability was not improved, indicates that demoldability was improved, but transparency was not obtained, .circle-solid. indicates that neither transparency nor an improvement in demoldability was obtained.

[0108] As is apparent from the scatter plots illustrated in FIG. 3 and FIG. 4, for both the diameter and depth of the dimples, it was found that the Samples that were able to impart transparency were concentrated at the lower side of the scatter plots, and the Samples that were not able to impart transparency were concentrated at the upper side of the scatter plots. Thus, it was confirmed that making both the diameter and the depth smaller for the dimples formed resulted in transparency being obtained.

[0109] The curves labeled boundary (upper limit) in the scatter plots of FIG. 3 and FIG. 4 are curves fitted to the upper limit of the group of Samples for which transparency was obtained. These curves represent approximations to the manner in which the upper limit values of equivalent diameter and depth of dimples that obtained an improvement in transparency change relative to changes in base metal hardness of the mold.

[0110] Thus, transparency can be imparted to resin molded articles obtained by employing a mold formed with dimples of a diameter not exceeding an equivalent diameter (W) found from a formula representing the curve boundary (upper limit) (W=1.5+8.9e.sup.H/630) illustrated in FIG. 3, which is a scatter plot of dimple equivalent diameter (W) against hardness of mold base metal (H). Furthermore, more preferably, the dimples are formed with a depth not exceeding a depth (D) found from a formula representing the curve boundary (upper limit) (D=0.05+0.4e.sup.H/320) illustrated in FIG. 4, which is a scatter plot of dimple depth (D) against hardness of mold base metal (H).

[0111] Transparent resin molded articles can be manufactured with molds polished to a mirror finish. Hence, if the only consideration is transparency, then there are no lower limit values to the diameter and depth of dimples to impart transparency.

[0112] However, the forming of dimples contributes to improved demoldability, as discussed above, and no improvement in demoldability could be confirmed for dimples formed with small diameter and depth, such as those of Sample 21, Sample 13, and Sample 17, even though transparency was imparted to resin molded articles.

[0113] Such a phenomenon is thought to occur because, as the dimples formed get smaller, the surface state of the mold after dimple formation approaches that of a mirror finish.

[0114] The curves labeled boundary (lower limit) in the scatter plots of FIG. 3 and FIG. 4 are curves fitted to the boundary between the group of Samples for which an improvement in demoldability was confirmed, and the group of Samples for which no improvement in demoldability was confirmed. These curves represent approximations to the manner in which the lower limit values of diameter and depth of dimples that obtain an improvement in demoldability change relative to changes in base metal hardness of the mold.

[0115] Thus, an improvement in demoldability can be obtained by using a mold formed with dimples of a diameter of at least a equivalent diameter (W) found from a formula representing the fitted curve at the lower values (W1+3.3e.sup.H/230) illustrated in FIG. 3, which is a scatter plot of the dimple equivalent diameter (W) against hardness of mold base metal (H). Furthermore, more preferably, the dimples are formed with a depth of at least a depth (D) found from a formula representing the fitted curve at the lower values (D0.01+0.2e.sup.H/230) illustrated in FIG. 4 which is a scatter plot of dimple depth (D) against hardness of mold base metal (H).

[0116] Thus, at the same time as improving demoldability, which is sometimes reduced in molds polished to a mirror finish, transparency is also obtainable by setting a dimple equivalent diameter (W) within a range defined by the following formula:

1+3.3e.sup.H/230W1.5+8.9e.sup.H/630Formula (1)

[0117] and, more preferably, furthermore by setting a dimple depth (D) within a range defined by the following formula:

0.01+0.2e.sup.H/230D0.05+0.4e.sup.H/320Formula (2)