Plastic article or earthenware shaping or treating: apparatus – With apparatus assembly or dismantling means or with idle part – For press shaping surface
Patent
1998-01-28
2000-09-19
Smith, Duane S.
Plastic article or earthenware shaping or treating: apparatus
With apparatus assembly or dismantling means or with idle part
For press shaping surface
249104, 249140, 249155, 264 134, 264 19, 264 25, 425DIG30, B29D 1100
Patent
active
061202800
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to microprism master molds which can be used in the production of cube-corner type retroreflectors (i.e., articles reflecting most of the incident light back toward the light source). More particularly, it relates to a microprism master mold which can preferably be used in the production of retroreflective elements useful in signs such as road signs and construction signs, license plates for vehicles such as automobiles and motorcycles, safety goods such as safety clothing and life-saving devices, markings such as signboards, reflectors for visible light or laser light reflection sensors, and the like, as well as retroreflective sheeting composed of such retroreflective elements and having excellent wide-angle properties.
BACKGROUND ART
Conventionally, retroreflective sheeting capable of reflecting incident light back toward the light source have been well known, and such sheeting is widely used in the above-described fields of application owing to its retroreflectivity. Among others, retroreflective sheeting utilizing the retroreflection principle of prisms, such as cube-corner type retroreflective sheeting, has markedly higher optical retroreflection efficiency than conventional retroreflective sheeting using micro glass beads, and the range of its use is expanding year by year because of its excellent retroreflection performance.
On the basis of their principle of reflection, cube-corner type retroreflective elements exhibit good retroreflectivity, so long as the angle between the optical axis of a prism type reflective element (sometimes referred to simply as "prism element") (i.e., an axis lying at an equal distance from three mutually perpendicular faces constituting the prism element) and incident light (i.e., the angle of incidence) is small. However, cube-corner type retroreflective elements have the disadvantage that, as the angle of incidence increases, their retroreflection efficiency is reduced. Moreover, when rays of light are incident on a prism face at an angle greater than the critical angle satisfying the conditions for total internal reflection which are determined according to the ratio of the refractive index of the transparent medium constituting the retroreflective element to the refractive index of air, most of them do not undergo total reflection at the interfaces of the prism element but pass to the backside of the prism. Thus, they have the additional disadvantage that the range of the angle of incidence which permits retroreflection is limited.
In order to overcome these disadvantages, various attempts have been made to improve the method of making a mold used for the formation of prisms. Now, some typical methods of making a prism mold which have been proposed in the prior art are described below.
This is a method in which a large number of metallic pins having a prism formed at the tip thereof are bundled to form an array of prisms. This method is characterized in that the design of the prism formed at the tip of each pin may be arbitrarily modified and is suitable for the production of relatively large prisms. However, it is not practicable when the formation of, for example, more than 2,000 microprisms per square centimeter is required as dictated by the primary object of the present invention.
This is a method for forming a microprism mold of the hexagonal prism type which comprises stacking a plurality of flat sheets having two mutually parallel major surfaces, cutting therein V-shaped grooves in a direction perpendicular to the major surfaces and at a fixed pitch to form a series of successive roof-shaped projections having a vertical angle of about 90.degree., and then shifting the flat sheets so that the vertices of the roof-shaped projections formed on each flat sheet meet the bottoms of the V-shaped grooves formed on an adjacent flat sheet. This method is characterized by a relatively high degree of design freedom, though it is lower than that of the bundled pin method. This method can improve the poor productivity in the fab
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Adachi Keiji
Mimura Ikuo
Nippon Carbide Kogyo Kabushiki Kaisha
Smith Duane S.
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