JPS6218218A - Mold for preparing flenel lens - Google Patents

Abstract

PURPOSE:To make it possible to scrape and make a concentric circle pattern by means of a machine tool such as a small lathe having higher limiting danger speed, by constituting a mold with a central mold and an outer mold. CONSTITUTION:A mold 3 is formed by inserting a center mold 4 in the recess 6 of an outer mold 5. The center mold 4 has a center pattern surface 4a which is symmetric to the center 1 and on which a center pattern 2a around the center 1 of the pattern 2 is formed. An outer pattern 2b continuous to the center pattern 2a of the center mold 4 is engraved on the outer pattern surface 5a of the outer mold 5. When the center mold 4 is inserted in the outer mold 5, adjusting to fit the level of the center pattern surface 4a with that of the outer pattern surface 5a by setting shims or such on the bottom surface of the recess 6 is desirable for optimum continuity of the center pattern 2a and the outer pattern 2b to each other.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD The present invention relates to molds used in the manufacture of Fresnel lenses.

BACKGROUND ART Conventionally, Fresnel lenses have been manufactured using molding equipment using a compression method or an injection method. A predetermined pattern is engraved on the pattern surface, ie, the transfer surface, of the mold used in these molding devices. A concentric pattern is carved by a lathe on the transfer surface of a mold used to manufacture a Fresnel lens. In order to obtain a Fresnel lens with excellent optical performance, the surface roughness of the concentric patterns becomes a problem.

This surface roughness is closely related to the cutting speed of the lathe, and in order to obtain a Fresnel lens with excellent optical performance, a surface roughness of 0.2 micrometers or less is required.
In order to meet this demand, the cutting speed of the lathe must be set at a critical speed of, for example, 100 m/s, to prevent damage to the lathe.
Must be more than a minute. The cutting speed V by the lathe is as shown in the following equation.

Here, π is the circumference, r is the radius of the circular pattern being cut, and n
is the rotation speed of the lathe. From equation (1), cutting speed V is proportional to the product of radius r and rotational speed n, and since rotational speed n naturally has a limit, the cutting speed V near the center where radius r becomes smaller divides the critical speed. Sometimes. In particular, when a large Fresnel lens, for example one with a diameter of more than 1 meter, is used, there are a limited number of lathes that can handle molds of that size, and the rotational speed of the lathe must be approximately 200 rpm or less in relation to the critical speed. Therefore, the cutting speed V can be maintained above the critical speed only when the radius r is 80 mm or more. Therefore, a surface roughness of 0.2 micrometers or less cannot be obtained near the center of the concentric pattern of the mold, and the optical performance near the center of the Fresnel lens molded by the mold is also poor. Ta.

1.5.I Accordingly, an object of the present invention is to provide a mold that eliminates the above-mentioned drawbacks and makes it possible to obtain a large Fresnel lens having excellent optical performance.

A mold for producing a Fresnel lens according to the present invention includes a center mold in which a pattern near the center of a concentric pattern where it is difficult to obtain a desired surface roughness is processed to a desired surface roughness, and an outside of the pattern of the center mold. and an outer mold having a pattern that is continuous with the outer mold and processed to have a desired surface roughness.

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. As shown in the figure, a mold 3 for producing a Fresnel lens having a concentric pattern 2 from a center 1 is formed by fitting a center mold 4 into a recess 6 of an outer mold 5. The center mold 4 has a center pattern surface 4a symmetrical to the center 1, and a center pattern 2a near the center 1 of the pattern 2 is formed on the center pattern surface 4a. The center mold 4 is provided on the outer pattern surface 5a of the outer mold 5.
An outer pattern 2b is carved that is continuous to the center pattern 2a. When the center mold 4 and the outer pattern mold 5 are fitted together, a shim (not shown) or the like is provided on the bottom of the recess 6 so that the heights of the center pattern surface 4a and the outer pattern surface 5a match. To adjust, center pattern 2
It is preferable that the outer pattern a and the outer pattern 2b are well continuous. The surface roughness of pattern 2 of mold 3 for Fresnel lens production is preferably 0.2 micrometers or less, and in order to satisfy the surface roughness of 0.2 micrometers or less, pattern 2 must be processed into mold 3. It is desirable that the cutting speed at the time of cutting is a critical speed at which phenomena such as peeling do not occur, for example, 100 m/min or more. For example, in order to carve the pattern 2 into a mold 3 whose outermost diameter exceeds 1 meter, a vertical lathe capable of gripping a large material is used. The critical speed of a vertical lathe is approximately 20 Orpm+, and the rotational speed is 200 Orpm+.
It is limited to below rpm. In this case, as is clear from equation (1), if the radius is less than 80 mm, the cutting speed becomes F below the critical speed, and a surface roughness of less than 0.2 micrometers cannot be obtained. Therefore, as mentioned above, the radius is 8
The outer pattern 2b with a diameter of 0 mm or more was carved into the outer mold 5 using a vertical lathe, and the center pattern 2a with a radius of 80 mm or less was cut into the center mold with a small lathe capable of high rotational speed. Center pattern 2
A and outer pattern 2b form a large and single pattern 2
Then, Fresnel lenses are produced.

According to the present invention, a mold for producing a Fresnel lens is formed by a center mold and an outer mold, so that it is a mold for manufacturing a large 70-nel lens. Also a concentric pattern, i.e. a pattern with a desired surface roughness') j! It is now possible to cut the pattern near the thin center using a machine tool such as a small lathe that has a high critical speed, and the surface roughness near the center of the pattern is improved compared to conventional integrated molds. , a Fresnel lens with excellent optical performance can be obtained, and even better effects can be obtained especially when formed as a large mold where cutting speed is an issue.

[Brief explanation of drawings]

FIG. 1 is a plan view of a mold for producing a Fresnel lens according to the present invention. FIG. 2 is a sectional view taken along line A in FIG. 1. Explanation of symbols of main parts 4...Center mold 5...Outer mold

Claims (1)

[Claims] Consisting of a center mold having a center, a center pattern surface that includes the center and is symmetrical to the center, and an outer mold that has an outer pattern surface that is continuous with and surrounds the center pattern surface. A mold for producing a Fresnel lens, characterized in that a pattern is formed in which a plurality of annular convex portions having different diameters are continuously arranged concentrically from the center over the center pattern surface and the outer pattern surface. Mold for producing Fresnel lenses.