GB2279915A

GB2279915A - Making illuminatable keytops

Info

Publication number: GB2279915A
Application number: GB9314032A
Authority: GB
Inventors: Satoshi Mieno; Fumio Kouno
Original assignee: Shin Etsu Polymer Co Ltd; Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Polymer Co Ltd; Shin Etsu Chemical Co Ltd
Priority date: 1992-02-14
Filing date: 1993-07-07
Publication date: 1995-01-18
Anticipated expiration: 2013-07-07
Also published as: GB9314032D0; JPH0644855A; GB2279915B; JP2657160B2; DE4324625C2; DE4324625A1

Description

2279915 "Method for the Preparation of a Key-top Covering Member for an

Internally-illuminated Push-button Switch"

BACKGROUND TO THE INVENT-LON

The present invention relates to a method for the preparation of a rubber-made key-top covering member for an internally-illuminated push-button switch, hereinafter referred to simply as a covering member. More particularly, the invention relates to a method for the preparation of a covering member having an indication of a letter or sign on the key-top portion made from a colored transparent rubbery material illuminated from below so as to be suitable for use as a key-top covering member for a push-button switch panel on an instrument used in a dark place, such as car-borne telephone receivers, with good recognizability even in a dark place.

Along with the rapid progress in recent years in the electronicspredominant society, the demand for internally-illuminated key-top covering members having an internally- illuminated light-transmitting area showing a letter or sign on the key- top portion is rapidly growing. Such a covering member, of which the key- top portion consists of a light-shielding area as the background and a light-transmitting colored area for the indication of letters or signs, can be prepared by various different methods, including a method in which each of the light-transmitting areas is formed on the rubber-made light-transmitting base of the covering member from a transparent colored ink or coating composition by printing and light-shielding areas by coating with a lightshielding coating composition as is disclosed in

Japanese Patent Kokai 4-67521 and 4-62730. This method is widely used in the preparation of such covering members although the method is not always quite satisfactory in terms of the relatively low yield of acceptable products because the process involves a number of steps unavoidably accompanied by deposition of dust particles or unevenness of coating or printing.

Beside the above-described method in which formed light- laser beam irradiation to remove the light-shielding layer in a pattern of a letter or sign. This method is also not problem-free in that uniformity in the removal of the light-shielding layer cannot colored the light-transmitting areas are by printing, a method is known in which the transmitting colored areas are formed by be ensured, giving incomplete removal of the layer or undue removal of even a part of the lighttransmitting colored layer below, resulting in an iregularly uneven effect of illumination of the letter or sign as a consequence of the unevenness in the thickness of the light-shielding layer and fluctuation in the laser beam output, so that the yield of acceptable products cannot be as high as desired. This problem can be solved only by exercising a very exact and troublesome control on the thickness of the light-shielding layer and the laser beam output.

SUMMARY OF THE INVENTION

The present invention accordingly has as an object to provide a novel and efficient method for the preparation of an internally-illuminated key-top covering member made from a rubbery material, by which the above-described problems and disadvantages of the prior art methods are solved or avoided.

Thus, he method of the present invention for the preparation of an internally-illuminated key-top covering member made from a rubbery material comprises the successive steps of:

(a) forming an outer light-shielding layer of the key-top covering member consisting of a key-top - 4 part, riser part and base part from a light-shielding rubbery material by compression molding in the female of a metal mold; (b) pouring a flowable transparent or translucent uncured rubbery material on to the key-top part of the outer light-shielding layer of the lightshielding rubbery material in the female of the metal mold to form a light-transmitting layer; (c) at least partially curing the flowable transparent or translucent uncured rubbery material so that the rubbery material is no longer flowable to form a light-transmitting key-top layer in contact with the key-top part of the light-shielding layer; (d) filling the female of the metal mold with an uncured stock of an uncolored or light-colored transparent rubbery material; (e) compression-molding the uncured rubber stock to form a base of the key-top covering member which is integral with the light-transmitting key- top layer and the outer light-shielding layer; and (f) removing the outer light-shielding layer on the key-top part pattern-wise to form an openwork in the form of a letter or sign.

BRIEF DESCRIPTION OF THE DRAWINGS Figures la to le of the accompanying drawings

0 each illustrate one of the above-described steps of the method according to the invention by a crosssectional view and Figure lf is a schematic crosssectional view of a key-top covering member prepared in this way.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As is described above, the essential steps of the method of the present invention include the steps (a) to (f) by using three kinds of rubbery materials and each of them is preferably a siliconerubber-based composition, although the method is applicable to any other types of rubber materials.

In the following, the method of the present invention is described in detail with reference to the accompanying drawing for each of the steps.

In step (a), as is illustrated in Figure la, a sheet of a light-shielding silicone rubber 10, which is a laminate of a cured silicone rubber sheet 8 and an uncured silicone rubber sheet 9, is mounted on a female 11 of a metal mold. The light-shielding rubber sheet 10 can be thin enough provided that a full light-shielding effect can be obtained thereby, for example in the range from 40 to 100 pm depending on the light-shielding power thereof. The rubber sheet 10 is colored in a desired 6 color tone which can be the same as or different from the color of the light-transmitting key-top layer subsequently formed. The color is usually a sober color as a background - of the illuminated areas such as black, grey, brown and the like.

The rubber sheet 10 is then molded and cured by closing the metal mold with a first male mold 12 at a temperature and under a compressive pressure sufficient to cure the uncured silicone rubber of the layer 9 depending on the types of the rubber materials to form an outer light-shielding rubber layer 3 (see Figure lb) consisting of the key-top part 3a, riser part 3b and base part 3c on the female 11 of the metal mold.

The light-shielding rubber sheets 8 and 9 are prepared from a silicone rubber composition admixed with a light-shielding coloring agent, which is shaped to have the form of a sheet by calendering and curing the sheet by compression molding at a temperature of 150 to 1800C under a pressure of 10 to 40 kgf/cm2. It is preferable that curing of this light-shielding rubber sheet 8 is not complete but it is in an intermediately cured state with a degree of curing of, for example, to 80% as compared with full curing, in respect - 7 of the adaptability of 11 of the metal mold. 10 can be a laminate of 8 and an uncured rubber sheet 10 is mounted on mold with the uncured female mold 11.

the sheet 9 to the female Alternatively, the sheet a fully cured rubber sheet sheet 9, and the laminated the female 11 of the metal rubber layer 9 facing the In step (b), which follows the above-described step - (a), the male mold 12 of the metal mold is removed and, instead, a fluid dispenser 14 having metering device for extrusion and equipped with syringe 13 filled with a flowable, colored, uncured silicone rubber composition capable of giving a colored transparent silicone rubber by curing is brought above the key-top part 3a of the lightshielding layer 3 in the female 11 of the metal mold as is illustrated in Figure lb and a small predetermined volume of the flowable silicone rubber composition is poured from the nozzle of the dispenser 14 on to the key-top part 3a of the light-shielding layer 3 to form a uniform layer 4 of the rubber composition having a thickness, for example, in the range from 0.3 to 2.0 mm, preferably from 0.5 to 1. 0 mm. The accuracy of the metering extrusion of the flowable silicone rubber composition out of the dispenser 14 is desirably within +0.01 ml for each of the key tops though dependent on the size and shape of the key top. The flowable silicone rubber 'composition put on the key-top part 3a of the light- shielding layer 3 spontaneously spreads and forms a layer 4 of a uniform thickness when the composition has a sufficient flowability but optionally the layer 4 may be forcibly formed by press-ing with a suitable mold, depending on the consistency of the composition. It is important that the flowable silicone rubber composition is completely free from bubbles of entrained air so that the composition is preferably deaerated before pouring. In this regard, the flowable silicone rubber composition has a viscosity not exceeding 300 poise at 25 0 C and it is preferably of the type curable by the mechanism of the so-called hydrosilation reaction taking place between silicon-bonded vinyl groups in an organopolysiloxane and silicon-bonded hydrogen atoms in an organohydrogenpolysiloxane in the presence of a platinum catalyst, since the silicone rubber composition of this type is free from evolution of gases formed by the curing reaction. When a plural number of push-button switches are integrated into a single panel of the key-top covering z 9 members, it is advantageous that a plural number of the f luid dispensers 14 are provided corresponding to the number of the push-button switches and this filling, work of the cavities of the female mold 11 is performed in one step. When it is desired that the push-button switches each have a lightilluminated indication of a letter or sign exhibiting a different color from the others, of course the syringes 13 of the fluid dispensers 14 are filled each with a different-colored flowable silicone rubber composition.

The next step to follow, i.e. step (c), is to effect at least partial curing of the above-formed uncured silicone rubber layer 4 into a nonflowable form. The means to effect curing of the layer 4 is not particularly limitative but the most conventional method is to heat the layer 4 together with the metal mold 11 by a suitable heating means (not shown in the figure). It is important that curing in this step' is effected to such an extent that the layer 4 is converted into a non-flowable form since otherwise the uniformity in the thickness of this colored, light- transmitting layer 4 may eventually be lost in the subsequent steps. The curing conditions including the heating temperature and heating time naturally depend on the particular type of the flowable silicone rubber composition.

In the next stage, i.e. in step (d), the cavity of the female 11 of the metal mold, in whichthe outer light-shielding rubber layer 3 and the colored light-transmitting rubber layer 4 on the key-top part 3a have been formed in the preceding steps, is filled with an uncured stock of a transparent siliGone rubber composition.

performed, conveniently but This step can be not limitatively, by mounting, as is illustrated in Figure 1c, an uncured slab 15 of the silicone rubber composition on the female 11 of the metal mold and pressing the same into the cavity with a second male 16 of the metal mold which is a kind of raise-mold.

The uncured silicone rubber stock filling the cavity of the female mold 11 in contact with the layers 3 and 4 is then, in step (e), compressionmolded and cured under heating and compression by closing the metal mold with the female mold 11 and male mold 16 to form the key-top base 17 consisting of a key-top part 17a, riser part l7b and base part 17c. The silicone rubber composition must give an uncolored or lightly colored, cured silicone rubber having transparency to the illuminating 1 light. The conditions for this compression molding, including the temperature, pressure and time, naturally depend on the particular types of the uncured silicone rubber composition. For exampe, the compression molding is performed usually at a temperature in the range from 150 to 180 0 C under a pressure of 10 to 40 kgf/cm 2 - When the compression molding step of the silicone rubber composition is performed under adequately controlled conditions, the cured key-top base 17 is integrally bonded with the layers 3 and 4.

The cured silicone rubber forming the key-top base 17 should preferably have a hardness IRHD not exceeding 80 and an elastic resilience of at least 40 in order to ensure smooth performance of the push-button switch. Although silicone rubbers based on an organopolysiloxane are preferred in view of their excellent transparency, heat resistance, weatherability and electrical properties, various types of synhetic rubbers can satisfy these requirements, including polybutadiene rubbers, polyisoprene rubbers, rubbery ternary copolymers of ethylene, propylene and a diene monomer, binary copolymeric rubbers of hexafluoropropylene and vinylidene fluoride, polychloroprene rubbers, polyurethane rubbers and - 12 the like curable by heating in the presence of an organic peroxide. optionally the rubber composition is admixed with various kinds of known additives, besides the peroxide-based curing or vulcanizing agent, including vulcanizing agents other than organic peroxides, fillers, coloring agents, heatresistance improvers, weatherability-improvers, flame-retardant agents, anti-fungal agents, electroconductivity-imparting agents and the like, each in such an amount that the cured rubber is not unduly decreased.

The last step, i.e. step (f), of the method according to the invention is for the pattern-wise removal of the light-shielding layer 3 to form an openwork in the layer 3 having the form of a letter or sign to facilitate recognition and identifi cation of the particular key-top in a panel member having a number of push-button switches by exposing the light-transmitting layer 4 illuminated from inside. This patterning work is performed conveniently, as is illustrated in Figure le, by using a laser-beam marking machine 18 having, for example, a neodymium doped yttrium-aluminium garnet laser. The laser is not limited to a specific type and laser beam marking machines having a carbon dioxide laser or transparency of the 0 an excimer laser can be used. The light-shielding rubber layer 3 is irradiated in the key-top part 3a with the laser beam 20 emitted from the machine 18 through a patterned mask 19 aving a transparent 1 area of the same pattern as the letter or sign desired on the key-top under illumination from inside, if necessary by focusing with a suitable lens system. It is important that the intensity of the laser beam and the length of time of the laser beam irradiation are that the light-shielding removed pattern-wise but layer 4 therebelow is not laser beam irradiation.

Figure lf illustrates, by a schematic crosssectional view, a rubbery keytop covering member prepared according to the method of the invention which consists of a light-shielding rubber layer 3 having an openwork 6 made by pattern-wise removal of the layei 3 in the key-top part 3a to exhibit a letter or sign, a light-transmitting rubber layer 4 below the key-top part 3a of the light- shielding layer 3 colored in a desired color having distinguishability, which can be seen from above through the pattern-wise openwork 6 in the light-shielding exactly controlled so layer 3 is completely the light-transmitting unduly removed by the - 14 layer 3 under illumination from below, and a key-top base 17 made from a light-transmitting rubber consisting of the key-top part 17a, riser part 17b and base part 17c. In order for the key-top covering member to work as a switching device, a movable contact point 7, which is made, for example, from an electroconductive silicone rubber, is provided by adhesive bonding on the lower surface of the key-Cop part 17a of the key- top base 17.

In the following, the method of the invention for the preparation of a rubber-made key-top covering member is illustrated in more detail by way of an example, in which the term "parts" refers to "parts by weight".

Example

A light-shielding silicone rubber composition was prepared by uniformly blending 100 parts of a silicone rubber compound (KE 971U, a product of Shin-Etsu Chemical Co.) with 2.5 parts and 3.0 parts of the curing agents recommended by the manufacturer (C-19A and C- 19B, respectively, each a product of the same company, supra) and 2.0 parts of a light-shielding coloring agent recommended by the manufacturer (KE-Color BB, a product of the same company, supra) and the composition was sheeted by calendering and cured into a cured silicone rubber sheet of 2 mm thickness to serve as a carrier sheet 8 (see Figure la).

In the next stage, the s.ame light-shielding silicone rubber composition as prepared above was sheeted in a thickness of 30 Fm on the aboveobtained cured silicone rubber sheet 8 so as to obtain a laminated sheet 10 consisting of a cured silicone rubber layer 8 and an uncured silicone rubber layer 9. This laminated rubber sheet 10 was cut in a size sufficient to serve as a light-shielding rubber layer 3 of a key-top covering member panel having a plurality of the switching units and mounted on a female 11 of a metal mold as is illustrated in Figure la with the uncured rubber layer 9 facing the metal mold 11, followed by compression molding with a male mold 12 at 140 to 1600C under a pressure of 10 to 20 kgf/cm 2, to form a light-shielding rubber layer 3 on the female mold 11.

Separateiy, two light-transmitting flowable silicone rubber compositions of different colors were prepared each by uniformly blending, using a blade-type stirrer rotating at 300 rpm, 100 parts each of rubber compositions (KE 1935A and KE 1935B, each a product of Shin-Etsu Chemical Co.) jointly to form a liquid low-temperature-curable silicone rubber composition with 2.0 parts of a transparent coloring agent for silicone rubbers (KE-Color W or KE-golor MB, each a product of the same company, supra).

Each of the thus-prepared 1 flowable silicone rubber compositions was poured into the respective cavities of the light-shielding rubber layer 3 on a female mold 11 by using respective dispensers 14 in such an amount that the pool of the flowable rubber composition as standing spontaneously spread to form a layer 4 having a thickness of 1 mm on the key-top part 3a of the light- shielding rubber layer 3 (see Figure lb). Thereafter, the female mold 11 was heated at 1300C for 30 seconds so that the flowable silicone rubber composition forming the layer 4 became partially cured to lose flowability.

Also separately, a sheet of a curable silicone rubber composition having a thickness of 6 mm was prepared from a silicone rubber composition (KE 1915, a product of the same company, supra) by injection molding. This rubber sheet 15 was mounted on the female mold or engraved mold 11 holding the light-shielding rubber layer 3 and the light- ght-transmitting transmitting rubber layer 4 (see Figure lc) and compression-molded with a male mold or raise-mold 16 under a pressure of 20 kgf/cm2 at 170 0 C, so that a, key-top base 17 consisting of the key-top part 17a, riser part 17b and base part 17c, which was integrally bonded to the underlying layers 3 and 4, was formed (see Figure ld). The metal mold was removed from the thus-obtained integral rubber body which was a key-top covering member panel having a plurality of key-tops for the switching operation.

The light-shielding layer removed to form an openwork 6 3 was pattern-wise on each of the keytop parts 3a by using a laser beam marker 18 (Model SL-478J, manufactured by NEC Corp.) with a neodymiumdoped yttrium-aluminium garnet having an output of 15 watts through a pattern-wise see-through glass mask 19 (see Figure le), of which the lightshielding areas w.e(re f ormed by a vapor-deposited pattern-wise layer of aluminium to leave a letterformed transparent area. The underlying lighttransmitting layers 4, which of different colors, could be thus-formed openworks 6.

seen through the

Claims

A method for the preparation of an internal illuminated key-top covering member made from rubbery material, which comprises the successive steps of:

(a)' forming an outer light-shielding layer of the key-top covering member consisting of a key-top part, riser part and base part from a light-shielding rubbery material by compression molding in the female of a metal mold; (b) pouring a flowable transparent or translucent uncured rubbery material on to the key-top part of the outer light-shielding layer of the light shielding rubbery material in the female of the metal mold to form a light-transmitting layer; (c) at least partially curing the flowable transparent or translucent uncured rubbery material so that the rubbery material is no longer flowable to form a light-transmitting key-top layer in contact with the key-top part of the light-shielding layer; (d) filling the female of the metal mold with an uncured stock of an uncolored or light-colored transparent rubbery material; ly- (e) compression-molding the uncured rubber stock to form a base of the key-top covering member which is integral with the light-transmitting key- top layer and the outer light-shelding layer; and (f) removing the outer light-shielding layer on the key-top part pattern-wise to form an openwork in the form of a letter or sign.
2. A method for the preparation of an internallyilluminated key-top covering member as claimed in claim 1, in which each of the light-shielding rubbery material used in step (a), flowable transparent or translucent uncured rubbery material used in step (b) and uncolored or light-colored transparent rubbery material used in step (d) is a silicone rubber.
3. A method for the preparation of an internallyilluminated key-top covering member as claimed in claim 2 in which the flowable transparent or translucent uncured rubbery material used in step (b) is a liquid silicone rubber composition curable by the mechanism of the hydrosilation reaction.
4. A method for the preparation of an internallyilluminated key-top covering member as claimed in any of the preceding claims, in which the f lowable transparent or translucent uncured rubbery material used in step (b) has a viscosity not exceeding 300 poipe at 25 0 C.
5. A method for the preparation of an internallyilluminated key-top covering member made from a rubbery material, which method is substantially as hereinbefore described with reference to the accompanying drawings.
6. A method for the preparation of an internallyilluminated key-top covering member made from a rubbery material, which method is substantially as hereinbefore described in the foregoing Example.