Method for the manufacture of stampers

It has been found that metal stampers, and more particularly nickel stampers, can be simultaneously hardened and passivated by cathodically treating the stampers in an aqueous bath of a passivation agent.

This invention relates to an improved method for the manufacture of record 
molding stampers and more particularly is concerned with an improved 
process for hardening and passivating the surface of the stampers. 
BACKGROUND OF THE INVENTION 
In the manufacture of molded records, it is conventional to mold a 
thermoplastic resin composition between a set of metal stampers to form 
the desired record. The metal record stampers have formed in their molding 
surfaces the information desired to be molded into the surface of the 
record. 
The stampers are the final product of a replication process referred to as 
matrixing. The initial steps in the matrixing process is to cut the 
desired recording into a suitable substrate that can thereafter be 
replicated. Recordings for audio records are generally cut into wax or 
lacquer substrates. Recording for records such as video disc, and the 
like, where the signal elements desired to be recorded are considerably 
smaller than for audio records, are cut in a much harder substrate such as 
an electroformed copper substrate. 
After the desired recording is made in the substrate, the substrate is then 
replicated by electroforming a metal, preferably nickel, onto the surface 
of the substrate. The electroformed part which is obtained is referred to 
as a master. The master is then replicated a number of times to produce a 
series of parts which are referred to as molds. The molds in turn are then 
replicated by electroforming a metal, preferably nickel, onto the surface 
of the molds to form stampers, which are negative copies of the original 
recorded substrate. 
The stampers, after they are separated from the molds, are trimmed and 
shaped. The stampers are then mounted on the molding platens of a record 
molding press. In the operation of the press, a thermoplastic molding 
composition is placed between a pair of the stampers and the stampers are 
pressed together to form the molded record. The quality and fidelity of 
the molded record to the original recording is directly related to the 
quality of the stamper with which it is pressed. 
There are a number of important properties which are required in stampers 
in order for the stampers to be satisfactory for the molding of records, 
especially in commercial production. The most important property is that 
the molding surface of the stamper must have sufficient release property 
so that the molding composition does not stick to the surface of the 
stamper. If the molding composition does adhere to the surface of the 
stamper a number of problems are encountered. Even slight sticking of the 
molding composition to the stamper results in distortion of the recorded 
signal element and as a result produces defective records. Furthermore, if 
the molding composition adheres strongly to the surface of the stamper, 
when the press is opened it can often cause distortion and destruction of 
the stampers as they are pulled apart by the opening action of the press. 
In addition to having non-sticking properties, the surface of the stamper 
must likewise be scratch resistant, as any scratches on the surface of the 
stamper result in undesired noise in the molded record. Scratching of the 
stampers is a long-standing problem in the manufacture of molded records, 
and is especially troublesome in the manufacture of a video disc, and the 
like, because of the small size of the recorded signal elements in 
comparison to even microsize scratches. Scratching problems are especially 
troublesome in the manufacture of video discs because the compositions 
which are used in the manufacture of the video disc, contain substantial 
amounts of conductive carbon particles which can cause microscratching of 
the surface of the stampers. 
A further property which is required in the stampers is that they be 
resistant to chemical attack. If the surfaces of the stamper are not 
relatively chemically inert, additives conventionally used in the molding 
compounds attack the surface of the stamper and cause staining of the 
surface of the stamper. The problem of staining is especially severe in 
the case of compositions used for the molding of video discs, and the 
like, because these compounds inherently must contain considerably greater 
amounts of various additives, such as mold releases, lubricants and the 
like. 
Untreated virgin nickel stampers, as formed in the matrixing processes, 
have been found to be generally unsuitable for use in the molding of 
records on a commercial scale. The plastic compositions used for the 
molding of records stick excessively to virgin nickel surfaces unless 
substantial amounts of mold releases are added to the compositions. The 
addition of the mold release compounds on the other hand causes a further 
undesirable effect of promoting development of stains on the surface of 
the stampers. Untreated nickel stampers also have a relatively soft 
surface so that they can be easily scratched. The virgin nickel surface of 
the stampers is likewise quite chemically active and numerous reactions 
occur between the nickel stampers and the molding compositions which can 
cause deterioration of the quality of the molding surface of the stamper. 
The method which has heretofore been widely used to provide additional 
protection for metal stampers, and especially for nickel stampers, is to 
electroplate the surface of the stampers with chromium. The chromium 
appears to give some additional protection to the surface of the stampers 
from mechanical damage, such as scratches, and also appears to passivate 
the surface of the stampers. 
Chrome plating, however, is not a completely satisfactory solution. The 
amount of protection provided to the stampers by the chrome plating 
appears to be directly related to the relative thickness of the chrome 
plating applied to the surface of the stampers. Increasing the thickness 
of the chrome plating however, results in a corresponding decrease in the 
quality of the records molded from the stamper in that the chrome plating 
tends to fill in and obliterate the recorded signal elements in the 
grooves of the stamper. With conventional audio records there has 
generally been an accepted tradeoff of fidelity of the final recorded 
records in order to obtain the protection afforded by chrome plating. 
However, with records having extremely small recorded features, such as 
the video disc, it was found that the stampers could not effectively be 
chrome plated in that chroming virtually obliterated the recorded signal 
information. 
A further problem of chrome plating is that even on audio stampers, and the 
like, chrome plating is at best a temporary to a semi-permanent treatment. 
On extended press runs, the chrome plating is gradually removed from the 
surface of the stampers. As the chrome plating is removed, there is a 
tendency for sticking and chemical reactions to occur at the stamper 
surfaces which eventually cause failure of the stampers, especially on 
extended press runs. 
A further related problem of chrome plating is that the plating solutions 
that are used in the plating process, once they are spent, are extremely 
difficult to dispose of because of environmental pollution problems. 
An additional problem encountered with chrome plating stampers, is that the 
chrome, when applied to the stampers, substantially reduces the scrap 
value of the nickel in the stampers as compared to pure nickel scrap. The 
reduction in cost is quite substantial, and results in increasing the 
overall production cost. 
It would be highly advantageous if a method could be provided to treat the 
molding surface of metal stampers, especially nickel stampers, to make 
them more scratch resistant and chemically passive but which would not 
have the inherent disadvantage of chrome plating. 
SUMMARY OF THE INVENTION 
It has been found that metal stampers, and more particularly nickel 
stampers, can be simultaneously hardened and passivated by cathodically 
treating the stampers in an aqueous bath of a passivation agent.

DETAILED DESCRIPTION OF THE INVENTION 
The record stampers which are treated in accordance with the present 
invention can be formed by conventional matrixing processes. Since nickel 
stampers are the most important class of stampers, specific reference will 
be made to the treatment of this type of stamper. However, it should be 
appreciated that other types of stampers made of other metals such as 
copper, iron or certain alloys can likewise be treated advantageously in 
accordance with the teachings of this invention, and that the treatment of 
such stampers is likewise included in the scope of the present invention. 
In the preferred method of the present invention, the molding surface of 
the stamper is not treated with any protective coatings, after it is 
separated from the mold in matrixing. Protective coatings, such as organic 
plastic film, tend to leave microresidues on the surface of the stamper 
which contaminate the surface of the stamper in further treatments unless 
completely removed. If, however, a protective coating is employed, it is 
important that as much of the coating as possible be stripped from the 
stamper before treatment. Furthermore, special care should be taken in the 
mechanical processing of the stampers to avoid contacting the molding 
surface of the stamper with oils, dirt or other contaminants. 
The treatment of this invention is conducted in an electrolytic bath. An 
apparatus 10 which is suitable for conducting the treatment is 
schematically illustrated in the figure. The treatment apparatus 10 
consists of a tank 11 in which there is mounted an insoluble anode 12. The 
insoluble anode can be made of various materials, such as stainless steel 
and the like. A cathode 13 is mounted at the opposite end of the tank 11 
in an opposing relationship to the insoluble anode 12. The cathode 13 has 
a holder (not shown) to which the stamper 14 is secured for treatment. The 
cathode 13 and the insoluble anode are electrically connected to a 
conventional power source (not shown). The tank 11 is filled with an 
aqueous mixture 15. The level of the aqueous mixture is maintained at a 
level such that the stamper 14 can be completely immersed in the aqueous 
mixture 15. 
The aqueous mixture contains a passivation agent and preferably an alkali 
material and an alkaline cleaning agent. The passivation agent which is 
advantageously employed includes, for example, potassium dichromate, 
sodium dichromate, sodium permanganate, potassium permanganate, and 
mixtures of these materials, with potassium dichromate being the most 
preferred material. The amount of the passivation agent added to the bath 
should be sufficient to effectively passivate the surface of the stamper 
14 when electric current is supplied to the treating apparatus 10. The 
amount of passivation agent will vary somewhat depending on the particular 
material selected; however, an amount of generally about 0.15 to 1.0 gm/l 
is generally quite effective for this purpose with about 0.25 gm/l of 
potassium dichromate giving optimum results. The amount of passivation 
agent should be maintained at a relatively low amount consistent with 
being effective as a passivation agent, rather than being present in 
relatively large amounts. Excessive amounts of passivation agent in the 
aqueous mixture tends to result in uneven passivation and staining of the 
treated stampers. 
The alkali materials which are advantageously added to the aqueous mixture 
are, for example, sodium hydroxide, sodium carbonate, potassium hydroxide, 
and mixtures thereof, with potassium hydroxide being the preferred 
material. The alkali material is added to the bath in an amount of from 
about 7.5 to 38.5 gm/l with about 15 gm/l being optimum. 
The alkaline cleaning agent which is preferably added to the aqueous 
treating mixture include materials, such as trisodium phosphate, 
tetrasodium pyrophosphate, tetrapotassium pyrophosphate, and mixtures of 
these materials, with tetrapotassium pyrophosphate being the most 
preferred material. The amount of the alkaline cleaning agent added to the 
bath should be about 7.5 to 38.5 gm/l with preferably about 15 gm/l being 
added to the bath. 
An aqueous bath composition, which has been found to be highly satisfactory 
for use in the method of this invention, was made up of 0.25 gm/l of 
potassium dichromate, 15 gm/l potassium hydroxide, and 15 gm/l of 
tetrapotassium pyrophosphate. 
In the practice of this invention, the stamper 14, which is to be treated, 
is mounted in the cathode 13 in the holder so as to be in electrical 
contact with the cathode 13. The stamper is then completely immersed into 
the bath 15. The cathode 13 and anode 12 are connected to a suitable power 
source. The bath 15 is preferably heated to and maintained at a 
temperature of 66.degree. to 77.degree. C.The current is then maintained 
in the bath at a sufficient level to provide a current density at the 
surface of the stamper which is from about 4 to 6.5 amp/dm.sup.2, 
preferably from about 5.5 amp/dm.sup.2. During treatment there will be a 
marked evolution of hydrogen in the form of bubbles 16 at the cathode. The 
hydrogen which is evolved will flow over the surface of the stamper 14. 
The treatment, once the evolution of the hydrogen commences, is completed 
quite rapidly with treatment times of one minute generally being quite 
adequate to achieve the desired results. 
After completion of the treatment of the stamper in the aqueous bath 15, 
the stamper is removed from the bath 15 and rinsed with clear water and 
dried. The stamper 14 is now ready for use. 
The stampers 14 treated, in accordance with the above method, have a unique 
combination of properties. The surface of the stampers are significantly 
hardened, as compared to the untreated surface, as is demonstrated by a 
substantially higher scratch resistance. The scratch resistance which is 
imparted by the treatment of this invention is at least equivalent to the 
scratch resistance imparted by chrome plating the stampers. The release 
properties of the stampers of this invention are also superior to the 
untreated stampers, and are at least equivalent to the chrome-plated 
stampers. Even more surprisingly, the surface of the treated stampers were 
found to be virtually inert to chemical attack, especially staining. In 
addition, the improvements imparted to the stampers by the treatment is 
longer lasting than chrome plating. The number of satisfactory records 
which can be pressed from a stamper treated in accordance with this 
invention is substantially greater in number than that which could be 
pressed with equivalent chrome-plated stampers. 
A further advantage of the treatment method of the present invention is 
that the stampers can be treated at a substantially lower cost. 
Furthermore, no substantial pollution problems are encountered with the 
treatment process of this invention unlike the chrome-plating processes 
used in the prior art. 
A very important advantage of the present invention is that the treatment 
is a surface reaction, not an additive process like chrome plating. 
Accordingly, there is no substantial change in the recorded signal so that 
the fidelity of playback of the records is improved. Because the treatment 
method of this invention does not obliterate the recorded signals, like 
chrome plating, it is now possible to use the method of the present 
invention for hardening and passivating stampers used in the manufacture 
of video disc and the like. The pressing runs for the video disc can be 
extended many times over as compared to the use of unplated nickel 
stampers which require an excessive amount of mold-release agents. 
The exact mechanism by which the hardening and passivation occurs is not 
known, and is not intended to limit the scope of protection of this 
invention by any specific theory of reaction. It is, however, believed 
that the excellent results obtained with the method of this invention are 
the results of a series of simultaneous reactions occurring during the 
electrolytic treatment of the stamper. It is believed that the alkali and 
the alkaline cleaner in the bath removes the oils and contaminants from 
the stamper surface insuring that the surface of the stamper is clean for 
treatment. The most important action which is believed to occur is a 
simultaneous and synergistic reaction of the liberated hydrogen and the 
passivation agent with the surface of the stamper. The hydrogen causes a 
hardening of the surface of the stamper, while the passivation agent 
passivates the surface of the stamper. Neither reaction by itself is 
sufficient to protect the stamper, and the simultaneous treatment is 
superior to sequential treatments. 
EXAMPLE 
A series of stampers for audio-type records were electroformed from nickel 
using conventional matrixing techniques. The stampers were divided into 
three groups. The first group was left in the virgin nickel state. The 
second group was chrome plated in the standard method. The third group was 
treated in accordance with this invention. 
Each stamper of the third group was mounted in a cathode holder. The 
stampers were then immersed in the electrolytic bath which contained: 
15 gm/l of potassium hydroxide 
15 gm/l of tetrapotassium pyrophosphate 
0.25 gm/l of potassium dichromate The bath was maintained at 70.degree. C. 
A direct current was applied across the bath in an amount sufficient to 
provide 5.5 amp/dm.sup.2 at the exposed surface of the stamper. Hydrogen 
gas was evolved at the cathode on the surface of the stamper. After one 
minute, the current was shut off and the stamper removed, rinsed with 
water and dried. 
Each type of stamper, that is the untreated stampers, the chromed stampers, 
and the stampers treated in accordance with this invention, were mounted 
on the recording molding press. Audio records were pressed from a 
polyvinylchloride molding composition to determine the molding properties 
of each type of stamper. 
The untreated stampers were almost immediately found to be unsatisfactory. 
The molding composition stuck to the surface of the stamper excessively. 
Many of the stampers split apart at the edges causing them to be scrapped. 
The test with the untreated stampers was discontinued because of the 
serious problems encountered in molding. The stampers which were not 
damaged were removed from the press and then treated electrolytically in a 
bath of a passivation agent as noted above. 
The stampers were used under production molding conditions to determine the 
average stamper life and ultimate cause of failure. The results of this 
life test are as follows: 
______________________________________ 
CHROME STAMPER 
PLATED OF THIS 
STAMPER INVENTION 
______________________________________ 
Average number 1,982 2,550 
of records 
per stamper 
Cause of failure: 
(a) Split edges (sticking) 
39.3% 35.0% 
(b) Scratches 20.3% 21.4% 
(c) Dents 5.4% 9.9% 
(d) Stamper damage 
4.2% 4.0% 
(e) Miscellaneous 
30.8% 29.7% 
______________________________________ 
The stampers treated in accordance with this invention had a significantly 
longer stamper life than that of the chrome-plated stampers. The causes 
for ultimate failure were essentially the same for both types of stampers. 
The stampers which were initially used in the untreated state and 
thereafter removed and treated in accordance with this invention, were 
found after treatment to be essentially equivalent to the stampers which 
were treated initially.