Method of producing a mold for forming resinous domed article

Abstract: A mold for forming a resinous domed article which may be employed as a cover of a monitor camera is provided. The mold consists of a female block and a male block. The female block has formed therein a domed mold cavity which includes a hemispherical wall that is just half a sphere and an annular wall extending from an end of the hemispherical wall. The male block has a domed protrusion mating with the domed mold cavity of the female block. The domed protrusion consists of two parts: a hemispherical portion and a cylindrical portion. The hemispherical portion is mechanically joined to the cylindrical portion. This structure allows the surface of the domed protrusion of the male block to be formed smoothly without any irregularities on an interface between the hemispherical portion and the cylindrical portion.

Description: BACKGROUND OF THE INVENTION 1 Technical Field of the Invention The present invention relates generally to a production method of a mold for molding resinous domed articles which may be employed as a cover for monitor cameras. 2 Background Art Typical monitor cameras employed in stores, banks, etc. are designed to monitor a preselected range across a fixed direction. Cameras of this type have a dead angle so great as not to cover a wide range. In order to avoid this problem, there have been developed monitor devices which turns or pans a single camera horizontally and tilts it vertically for monitoring a wider range. Such prior art monitor cameras are usually suspended from the ceiling and present an appearance persons perceive to be a camera easily, thus giving an unpleasant feeling to customers in the store. Recently, small-sized monitor cameras are, thus, proposed whose major part of the camera and tilt and pan mechanisms are covered with a resinous dome. Fig. 4 shows such a dome-shaped monitor camera which includes a camera body 1, a mount ring 2, a transparent domed cover 3, an inner cover 4, a camera lens 5, a power supply cable 6, and an output coaxial cable 7. The inner cover 4 is disposed within the domed cover 3 and made from an opaque or a translucent material. The inner cover 4 has formed therein an opening 4a through which the camera lens 5 faces the outside. The power supply cable 6 supplied the power to the camera body 1, a tilt mechanism, and a pan mechanism. The output cable 7 outputs image signals and receives control signals. The domed cover 3 is made from acrylate resin in injection molding and required to have smooth surfaces and a constant thickness for ensuring a given degree of quality of captured images. The domed cover 3 is also required to have substantially a hemispherical surface for vertical and horizontal movements of the camera body 5 over 90° and 360° ranges, respectively. Fig. 5 shows a conventional mold for forming the domed cover 3 shown in Fig. 4. The mold consists of an upper block 8 (i.e., a female molding block) and a lower block 9 (i.e., a male molding block). The upper block 8 has formed therein a cavity 10 which consists of a hemispherical wall 10a and an annular wall 10b extending straight from an end of the hemispherical wall 10a. The formation of the cavity 10 of the upper block 8 is achieved by rotating the upper block 8 about a vertical axis V passing through the center O of a semicircle having the same radius of curvature as that of the hemispherical wall 10 and rotating and moving a grindstone, which has a surface curved outward with the same radius of curvature as that of the hemispherical wall 10a, from the top to an end of the semicircle. The annular wall 10b of the upper block 8 is grounded by moving the grindstone parallel to the vertical axis V . A hemispherical portion 9a of the lower block 9 may be grounded in substantially the same manner as that of forming the cavity 10, but formation of an annular wall 9b extending straight from an end of the hemispherical portion 9a requires interruption of the grinding and replacement of the grindstone used in forming the hemispherical portion 9a with another one having a flat grinding surface. This is because a corner between the annular wall 9b and a flat upper wall 9c needs to be formed at right angles. The replacement of the grindstone may result in an error in forming a curved outer surface near an end of the hemispherical portion 9a , thereby causing irregularities to be formed on an interface between the hemispherical portion 9a and the annular wall 9b, which will lead to formation of similar irregularities on an inner surface of the domed cover 3 near its end, thereby contributing to distortion of part of images captured by the monitor camera. From US-A-1 294 486 a method of making reflectors is known by using a bowl-shaped drawing punch. This document is, however, silent about a method of producing the drawing punch. SUMMARY OF THE INVENTION It is therefore a principal object of the present invention to avoid the disadvantages of the prior art. It is another object of the present invention to provide a resinous dome-forming mold having smooth surfaces which do not form irregularities on the surfaces of molded articles. According to the invention, there is provided a method of producing a male block of a resinous dome-forming mold. The male block has a dome-shaped protrusion which includes a hemispherical section that is just half a sphere and a cylindrical section continuing from an end of the hemispherical section. The method comprises the steps of: (a) preparing a spherical block including a hemispherical portion that is just half a sphere and a base portion connected to the hemispherical portion to form a spherical surface ranging over more than 180° of the sphere; (b) rotating the spherical block around an axis passing through the center of the spherical block; (c) grinding an area of the spherical surface of the spherical block ranging over more than 180° from a top of the spherical surface through which the axis of rotation passes to a circumference thereof using a grindstone with a curved surface having substantially the same radius of curvature as that the hemispherical section of the male block; (d) separating the hemispherical portion from the spherical block to complete the hemispherical section of the male block; (e) preparing a block on which the cylindrical section of the male block is formed; and (f) joining the cylindrical section to the hemispherical section to complete the male block. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. In the drawings: Fig. 1 is a vertical cross sectional view which shows a mold for forming resinous domed articles which is produced according to the present invention; Fig. 2(a) is an illustration which shows grinding of a hemispherical portion of a male block of the mold shown in Fig. 1; Fig. 2(b) is an illustration which shows joining of the hemispherical portion grounded in Fig. 2(a) to a cylindrical protrusion to complete the male block shown in Fig. 1; Fig. 3(a) is an illustration which shows grinding of a hemispherical portion of a male block of the mold shown in Fig. 1 according to the invention; Fig. 3(b) is an illustration which shows joining of the hemispherical portion grounded in Fig. 3(a) to a cylindrical protrusion to complete the male block shown in Fig. 1; Fig. 4 is a perspective view which shows a typical monitor camera with a resinous domed cover; and Fig. 5 is a vertical cross sectional view which shows a conventional mold for forming the resinous domed cover as shown in Fig. 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, particularly to Fig. 1, there is shown a die or mold which may be employed in injection molding for forming a resinous domed cover, like the one shown in Fig. 4, having a hemispherical portion and a hollow cylindrical portion continuing from an end of the hemispherical portion. The mold consists of a female block 11 and a male block 12. The female block 11 has a plane surface in which a mold cavity 13 is formed. The mold cavity 13 consists of a hemispherical wall 13a that is just half a sphere and an annular wall 13b extending straight from an end of the hemispherical wall 13a. The male block 12 includes a protrusion mating with the mold cavity 13 of the female block 11. The protrusion includes a hemispherical portion 14 and a cylindrical portion 15. The cylindrical portion 15 has a spigot joint 16 for attachment of the cylindrical portion 15 to the hemispherical portion 14 in alignment thereof. In production of the male block 12, a spherical block 21 which has, as shown in Fig. 2(a), a spherical wall ranging over more than 180° is first made using a grinding machine. The spherical block 21 is held by a rotary chuck 22 in electromagnetic or vacuum attachment in alignment of the center 0 thereof with an axis of rotation of the chuck 22. Next, the chuck 22 is rotated. A grindstone 23 having a grinding surface curved inwardly with a radius of curvature equal to that of the hemispherical portion 14 is also rotated and moved, as indicated by an arrow in Fig. 2(a), from the top of the spherical block 21 through which the axis of rotation of the chuck 22 passes to the circumference thereof over a range of more than 180° in contact with the surface of the spherical block 21 to finish a hemispherical portion 21a having the same radius of curvature as that of a molded product. The thus grounded spherical block 21 is cut along a horizontal line H to cut off the hemispherical portion 21a as the hemispherical portion 14 of the male block 12. The bottom of the hemispherical portion 14 is machined to form a recess 17, as shown in Fig. 2(b). The formation of the cylindrical portion 15 and the spigot joint 16 of the male block 12 is achieved in a known manner by machining a workpiece using, for example, a lathe. Finally, the spigot joint 16 formed on the cylindrical portion 15 is fitted into the recess 17 in the bottom of the hemispherical portion 14 and bonded together to complete the male block 12. This allows the hemispherical portion 14 and the cylindrical portion 15 to be connected together without formation of irregularities on an interface therebetween, so that a domed product having a smooth surface can be molded. Figs. 3(a) and 3(b) show a modification of the production method of the male block 12 which is different from the above first embodiment in that the spherical block 21 consists of two parts: a hemispherical portion 21b and a base block 21c to be gripped by the rotary chuck 22. Other arrangements are identical, and explanation thereof in detail will be omitted here. The base block 21c has a flat surface on which the spigot joint 16 is formed. The spigot joint 16 is fitted into the mating recess 17 machined in a flat bottom of the hemispherical portion 21b to complete the spherical block 21. The spherical block 21 is held by the rotary chuck 22 and grounded in the same manner as that in the first embodiment to finish the hemispherical portion 21b. The hemispherical portion 21b is separated from the base block 21c as the hemispherical portion 14. The hemispherical portion 14 is connected to the cylindrical portion 15 in tight engagement of the spigot joint 16 with the recess 17 to complete the male block 12.

Claims: A method of producing a male block (12) of a resinous dome-forming mold, the male block (12) having a dome-shaped protrusion which includes a hemispherical section (14) that is just half a sphere and a cylindrical section (15) continuing from an end of the hemispherical section, comprising the steps of preparing a spherical block (21) including a hemispherical portion (21b) that is just half a sphere and a base portion (21c) connected to the hemispherical portion (21) to form a spherical surface ranging over more than 180° of the sphere; rotating said spherical block (21) around an axis passing through the center of said spherical block (21); grinding an area of the spherical surface of said spherical block (21) ranging over more than 180° from a top of the spherical surface through which the axis of rotation passes to a circumference thereof using a grindstone (23) with a curved surface having substantially the same radius of curvature as the hemispherical section of the male block (12); separating the hemispherical portion (21b) from said spherical block (21) to complete the hemispherical section (14) of the male block; preparing a block on which the cylindrical section (15) of the male block (12) is formed; and joining the cylindrical section (15) to the hemispherical section (14) to complete the male block (21).

Inventors: TAMURA KAZUSHIGE, WADA JYOJI, TAMURA, KAZUSHIGE, WADA, JYOJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: B29C33
IPC codes: B29C45, B24B1, B29C33, B24B11, B29L25, B24B19