Device for transmitting electric current to disc elements in surface-coating thereof

A device for evenly distributed electrical current transmission to a disc element when electroplating the disc element includes a closed loop of an elongated, elastic, electrically conducting body contacting one side of the disc element at least at its outer peripheral area.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for electroplating of disc 
elements, such as matrices for audio and video discs, said device 
comprising a housing for an electrolyte bath, the disc element being 
clampable in the housing in such a manner that the disc side to be plated 
can be in contact with the electrolyte bath while its other side is kept 
sealed from the electrolyte bath, and a means arranged to abut against the 
disc element to transmit electrical current thereto during the plating 
process. 
In electroplating of matrices for audio and video discs it is of great 
importance that the current transmission to the matrix be as evenly 
distributed as possible around the periphery of the matrix to thereby 
achieve a plating layer, the thickness of which is as even and uniform as 
possible. 
A previously known device for this purpose comprises a central hub from 
which there extend essentially radially a plurality of electrically 
conducting elements, such as antennae, strips of sheet metal or sheet 
metal in the form of circle sectors, which, at their distal ends, are 
resiliently in contact with one side of the matrix at its outer peripheral 
area. Thus, a certain number of contact points are obtained distributed 
around the periphery of the matrix. One problem which can occur in such a 
design is that current concentrations can be formed at the contact points 
between the electrically conducting elements and the matrix disc, as a 
result of microstructural uneven areas in the matrix disc, which can 
create bums in the disc and thus unevenness in both the disc and the layer 
thickness in the surface coating obtained. 
JP-A-63-134 688 shows a device for producing a press matrix, with a plating 
carried out on a metal layer with signal information on a glass substrate. 
An elastic, electrically conducting central washer is placed on the 
electrolyte bath side and conducts current to the central portion of the 
metal layer without damage thereto when the center screw is tightened. 
SUMMARY OF THE INVENTION 
A primary purpose of the present invention is to suggest a solution which 
removes the above mentioned problems and which thus makes possible an even 
and dense current transmission to the matrix disc, regardless of any 
macro- or micro-unevennesses therein. 
A purpose of the present invention is also to achieve a current 
transmission device which makes possible both an even current transmission 
and thus even heat distribution, and a radially and axially resilient 
tensioning of the disc during the plating process, thus avoiding 
mechanical stresses in the disc due to thermal movements. 
In accordance with the present invention, the device described by way of 
introduction is characterized in that the current-transmitting means 
comprises at least one closed loop of an elongated, elastic, electrically 
conducting body arranged to be in contact with at least one side of the 
disc element at least at its outer peripheral area, said loop being placed 
so that it is sealingly separated from the electrolyte bath. There is 
thereby achieved in electroplating of disc matrices a very even, pliable 
and flexible clamping of the disc, thus assuring a favourable distribution 
of current and force on the disc. Both macro- and micro-unevennesses can 
be absorbed by the elastic, electrically conducting body, thereby 
resulting in a very even thickness of the disc-coating layer. By virtue of 
the fact that the electrically conducting body is also sealingly separated 
from the electrolyte bath, it is not itself subjected to any 
electroplating and therefore can be reused a great number of times. 
The elastic, electrically conducting body can be made in many different 
ways within the scope of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an apparatus, generally designated 10, for electroplating of 
matrix discs of metal for production of audio and video discs. The 
apparatus comprises a housing 12, which encloses an electrolyte bath. In 
the bath there is a perforated anode basket 14 containing balls 15 of the 
metal with which a matrix disc 16 is to be coated, e.g. nickel. The anode 
basket 14 is connected to a positive pole of an electrical current supply 
circuit. The numerals 18, 20 and 22 designate the inlet and outlets for 
the electrolyte. 
The matrix disc 16 is clamped in the apparatus 10 by means of a holder in 
such a manner that one side 24 thereof, which is to be plated, is in 
contact with the electrolyte bath, while its other side 25 is sealed from 
the electrolyte bath by means of an O-ring 26 in an electrically 
insulating detainer ring 28, which is fixed by means of electrically 
insulating screws 30, for example, to an underlying bottom plate 31. On 
the bottom plate 31 there is supported a current conductor plate 2 with a 
peripheral margin 34 on which there is supported a current-transmitting 
means in the form of an elastic, electrically conducting body 36 according 
to the invention. The elastic body 36 has the form of a closed, annular 
loop which is arranged to conduct electric current between the "dry" side 
25 of the matrix disc 16 and the current conductor plate 32 during the 
electroplating process. The matrix disc 16 and the current conductor 32, 
which are connected to the negative pole of the electrical current supply 
circuit, form a cathode in the electroplating process. 
According to a first embodiment of the elastic body 36 according to the 
invention, this consists of a core 38 of elastic material, for example an 
elastomer, such as silicon rubber, neoprene rubber or the like. The 
elastic core 38 can be hollow. i.e. it can be in the form of a hose, as is 
shown in FIG. 2a, or be solid, as is shown in FIG. 2b. Around the core 38 
in FIGS. 2a and 2b, there is applied one or more layers of a fine metal 
net 40 of electrically conducting material, e.g. stainless steel. The 
elastic core 38 provides the elastic body 36 with the desired resilience 
in the required directions to permit a certain compression of the elastic 
body 36 and thus an intimate contact surface between the metal net casing 
40 and the matrix disc 16, when the detainer ring 28 via the O-ring 26 
clamps the matrix disc 16 against the current-distributing elastic body 36 
(see in particular the encircled magnified portion in FIG. 1). The fine 
metal net 40 with its resilient elastic core 38 thus assures a very good 
and dense contact between the matrix disc 16 and the metal net 40, which 
means that both macro-unevenness, e.g. warping and non-planarness, and 
micro-unevenness, e.g. bumps and particle formations on the disc, can be 
compensated for by the electrically conducting metal net 40. The large 
number of small contact points between the metal net structure 40 and the 
matrix disc 16 also provides a more even heat distribution than what was 
previously known within this area of technology. 
Furthermore, this elastic, electrically conducting elastic body 36 provides 
a looser and more resilient clamping of the matrix disc than traditional 
technology in the field, and thus the matrix disc 16 is provided with a 
certain possibility of moving during the surface-coating process, as heat 
develops, which substantially reduces the mechanical stresses in the 
matrix disc 16. 
According to a second conceivable embodiment of the elastic body 36 
according to the invention, it can consist in its entirety of windings of 
fine metal netting of electrically conducting material, as is shown 
schematically in FIG. 2c, even though the elasticity is not as good in 
this case. 
FIGS. 2a-c show the cross section of the elastic body 36 in the unloaded 
state and is in this case circular. In a loaded, clamped state, the cross 
section is oval, as is shown in FIG. 1. Although not shown in the drawing, 
it is, however, conceivable that the cross sectional shape of the elastic 
body 36 can have another configuration in the unloaded state than 
circular, e.g. oval, square or the like. 
According to a third conceivable embodiment, the elastic, electrically 
conducting body can be made of an elastomer which has been made 
electrically conducting by the addition of conducting material, such as 
platinum, carbon or silver. Alternatively, the elastic electrically 
conducting body can consist of a so-called conducting polymer. 
According to a fourth conceivable embodiment of the elastic body (not 
shown), it can be formed of a helically wound spring wire of electrically 
conducting material and with an elliptical cross section, where the 
windings of the spring have a substantial angle of inclination towards the 
longitudinal central axis of the elastic spring, so that the spring can be 
resiliently compressed somewhat upon compression perpendicular to said 
longitudinal center axis to thus create many contact points between the 
matrix disc and the current-transmitting spring body. 
Even if the matrix disc is oriented horizontally in the embodiment 
according to FIG. 1, it can also have an inclined or vertical position 
during the electroplating process while retaining the above described 
advantages of the elastic body 36. The unit (a cathode) made up of the 
detainer ring 28, the matrix disc 16, the elastic body 36 and the current 
conductor plate 32 can be rotatable or fixed relative to the anode. Within 
the scope of the invention it is also possible to use more than one 
elastic body 36, for example a small centrally placed elastic body. It is 
also conceivable to arrange plural elastic bodies 36 abutting against the 
two opposite sides of the matrix disc at its outer peripheral area.