Key top with an identification legend

A key top of a manually operated key, comprising a base member having a top face on which a legend identifying said key is printed, and a transparent protective layer covering at least an area of said top face of the base member in which the legend is disposed. The protective layer is made of a mixture of a transparent base material, and a transparent finely particulate material which is different in refractive index from said base material.

BACKGROUND OF THE INVENTION 
The present invention relates generally to keys disposed on typewriters, 
electronic desktop calculators and various keyboards provided on or for 
similar instruments, and more particularly to key tops of such keys having 
appropriate legends including characters such as symbols, letters and 
numerals to identify the keys, i.e., to idetify input signals 
corresponding to the keys. 
Key tops of the type indicated above are finger-depressed on their top 
faces on which the legends are provided. For this reason, the top faces of 
the key tops are required to provide a high resistance to wear, so that 
the legends may wear for a long period of service. To meet this 
requirement, a key top commonly used in the art and generally referred to 
as "two-color key top" is constructed of a base member which is molded of 
a plastic material such that the top face has a recessed portion 
corresponding to a figure of a key-identifying legend. The shaped recess 
is then filled with a plastic material different in color from the base 
member, so that the legend is integral with the base member. This 
two-color key top is excellent in wear resistance of its legend, that is, 
the legend on the top face may wear for a long time. This advantage, 
however, accompanies a problem of a high cost of manufacture due to 
requirements of different molds for forming key tops with different 
legends. The mold preparation is difficult and costly, particularly where 
the key top is provided with a complicated legend such as Chinese 
characters. 
Attempts have been made to reduce the manufacturing cost of a key top. For 
example, a key-identifying legend is provided in a printing process on the 
top face of a base member of a plastic-molded key top, and the top face 
with the legend impression is covered with a protective layer excellent in 
wear resistance and optical transmittance. Such a key top suffers an 
operational drawback which arises from its high light transmittance 
(transmission factor), which causes a reduced visibility of the legend and 
an increased eye fatigue of the key operators due to reflection of light 
from the top face of the base member. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention, which was developed 
in view of the above drawbacks apparent to those experienced in the art, 
to provide a key top of a key which is simple in construction, economical 
to manufacture, durable in use with a high wear resistance of its legend, 
and high in visibility of the legend with minimized eye fatigue of the key 
operator. 
According to the present invention, there is provided a key top of a key, 
comprising a base member having a top face on which a legend identifying 
the key is printed. The key top further comprises a transparent protective 
layer of substantially uniform thickness covering at least an area of the 
top face of the base member in which the legend is disposed. The 
protective layer is made of an intimate mixture of a transparent base 
material, and a mass of fine particles distributed substantially uniformly 
throughout the entire thickness of the protective layer, that is, a finely 
particulate material which is high in transparency or degree of clearness 
and which is different in refractive index from the base material. 
In a key top constructed as described above, the protective layer which may 
be formed at a comparatively low cost, prevents otherwise possible wear of 
the legend covered therewith, thus enabling the legend to serve for a long 
period. Further, the use of transparent two materials having different 
refractive indexes assures high visibility of the legend through the 
protective layer, and minimum eye fatigue due to otherwise high level of 
reflected light toward the key operator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be described in more detail referring to the 
accompanying drawings. 
There is shown in FIGS. 1-3 a key top comprising a base member 1 which is 
molded of a plastic material such as ABS (acrylonitrile-butadiene- and 
styrene) plastics, so that the base member 1 has a smooth, substantially 
flat top face 1a, and an internal hole 1b in which a key stem (not shown) 
is fitted for fixation to the base member 1. The top face 1a carries a 
legend 2 printed thereon for identifying a key which uses the key top. 
The legend 2, which is different in color from the base member 1, is an 
imprint transferred from a decalcomania sheet onto the top face 1a of the 
base member 1. 
The protective layer 3 is made of a base material 3a, and a mass of fine 
particles 3b, i.e., finely particulate material 3b intimately mixed with 
the base material 3a, as illustrated in FIG. 3. The base material 3a is an 
achromatic acrylic resin excellent in wear resistance and optical 
transmittance (light transmission factor). The particulate material 3b 
consists of at least one highly transparent, achromatic material which is 
different in refractive index from the base material 3a, and which is 
selected from a group consisting of: siliceous sand (silica or quartz 
sand; sand containing particles of quartz as a primary component); silicic 
acid (H.sub.4 SiO.sub.4, H.sub.2 SiO.sub.3, H.sub.2 Si.sub.2 O.sub.3, 
H.sub.2 Si.sub.2 O.sub.5, H.sub.6 Si.sub.4 O.sub.11); anhydrous silica 
(silicon dioxide, silicic acid anhydride); and calcium carbonate. Due to 
the difference in refractive index, the fine particles 3b serve to cause 
irregular reflection of an incident light beam, which in turn causes the 
reflected light beam to pass an increased distance through the protective 
layer 3 and to be dispersed from the surface of the layer 3 in different 
directions, thus attenuating the amount of reflected light reaching the 
eyes of the key operator. In order to give a sufficient wear resistance to 
the top face 1a on which the legend 2 is printed, the protective layer 3 
has a thickness of not less than about 50 microns. It is noted that the 
amount of reflected light beam which passes the protective layer 3 is 
affected by the content of the particulate material 3b and by the 
thickness of the protective layer 3. When the particulate material 3b is 
used in a relative large amount, and/or the protective layer 3 has a 
comparatively large thickness, there is a tendency that the length of 
light passage through the protective layer 3 is increased, whereby the 
light attenuation coefficiency of the protective layer 3 is increased. In 
other words, the particulate material 3 contributes to reduction in ratio 
in amount of an incident light beam to the light beam reflected by the top 
face 1a, and serves to disperse the light from the surface of the 
protective layer 3, thereby reducing an amount of the light which reaches 
the eyes of the key operator. This function of the fine particles 3b is 
considered to be equivalent to such results as if the surface of the 
protective layer 3 were delustered or flattened. In fact, the fine 
particles 3b adjacent to the surface of the layer 3 more or less act as a 
delustering agent to rough or flatten the surface of the protective layer 
3. 
The key top constructed as described hitherto may be fabricated, for 
example, in a process which comprises steps as shown in FIG. 4. Stated 
more specifically, the base member 1 which has been plastic-molded is 
first washed or degreased to remove an oily substance and other foreign 
matters attached to its surface. Then, the legend 2 is formed on the top 
face 1a from a decalcomania sheet. A typical example of such a 
decalcomania sheet or image transfer sheet is a laminated sheet which 
consists of: a releasable layer in the form of a tape comprising a 
substrate of paper or the like containing silicone or wax; a colored image 
layer containing a pigment as a primary component material and carrying 
multiple image formers which have a shape corresponding to the legend 2 
and are spaced from each other along the length of the substrate; and an 
adhesive layer made of synthetic rubber as a primary material. For 
example, the transfer of the legend 2 is effected in the following manner. 
At first, the laminated decalcomania sheet is placed over the top face 1a 
of the base member 1, and heated to about 150.degree.-250.degree. C. by a 
heating element (not shown) having a surface covered with silicone rubber 
or other suitable elastic member. Subsequently, the decalcomania sheet is 
pressed, on the surface of the substrate (releasable layer), against the 
top face 1a of the base member 1 by the heating element with an optimal 
force, so that the adhesive layer and the colored image layer are bonded 
to the top face 1a. Thus, the image former is transferred as the legend 2 
onto the top face 1a. 
Successively, the top face 1a on which the legend 2 is printed is coated 
with the protective layer 3 in the following manner. At first, a solution 
containing the above described materials 3a, 3b is sprayed over the top 
face 1a. Subsequently, the sprayed materials 3a, 3b are air-dried 
(cold-cured), and the air-dried layer of the materials is then cured by 
forced-drying for about 30 minutes at an elevated temperature of about 
70.degree. C. Thus, the protective layer 3 is obtained. In this specific 
example, the above indicated solution for the protective layer 3 is 
prepared with a relatively high viscosity, so that an amount of the 
solution necessary to form the protective layer 3 can be obtained by a 
single spray of the solution. 
According to the instant embodiment of a key top, the legend 2 is protected 
by the protective layer 3 which uses highly wear-resistant acrylic resin 
as the base material 3a, and consequently may wear for a long period of 
service. Further, the highly transparent properties of the base and 
particulate materials 3a and 3b assure high visibility of the legend 
through the protective layer 3. 
Another advantage of the key top of the invention is derived from the 
difference in refractive index between the base material 3a and the 
particulate material 3b, which difference results in tendencies of 
increasing the length of passage of the incident light through the 
protective layer 3, dispersing the light from the surface of the layer 3 
in various directions, and reducing the amount of the reflected light 
reaching the eyes of the key operator. Further, the fine particles 3b 
serve to provide the protective layer 3 with a delustering property. All 
of these effects are combined to minimize the eye fatigue of the key 
operator. 
This single spray method permits formation of the protective layer 3 with a 
constant thickness and without voids, and is preferred for assuring 
uniform distribution of the finely particulate material 3b in the 
direction of thickness of the protective layer 3.