Method for the manufacture of a shaping mask for an electroerosion tool

A method of making a shaping mask from copper or copper alloy for electrical machining tools which comprises applying to a steel matrix or form, a thin conductive coating of a neutral material, especially a hard chromium, passivating this layer and applying into the copper or copper alloy the galvanic deposition until the electro deposited layer lifts autogenously from the layer intact.

FIELD OF THE INVENTION 
The invention relates to a method for the manufacture of a shaping mask for 
an electroerosion tool, said shaping mask corresponding to a matrix and 
consisting of copper or copper alloy,--whereby a mold having a surface 
corresponding to the shaping mask is provided with a conductive layer on 
the surface of which a metal layer of copper or copper alloy is 
galvanically built up and the metal layer is lifted from the mold as a 
shaping mask.--Shaping mask here designates the shape-defining tool 
component of an electroerosion tool which is connected to a mechanical 
support or carrier. 
BACKGROUND OF THE INVENTION 
In known methods of this kind (DT-OS 16 90577) the mold is an auxiliary 
element formed according to the matrix and is, for example, of artificial 
material, wax or an easily fusible alloy. One thus works with a 
reproduction of the matrix which necessarily results in inaccuracies. 
Therefore, the shaping mask obtained by the means of the auxiliary 
elements does not satisfy requirements for high. This applies also and 
particularly when, in addition, (DT-OS 19 12312) conductive lacquer and/or 
separating lacquer is used. On the other hand, modern tool manufacture 
sets increasingly high standards on precision, particularly with respect 
to precision-machining tools. These demands can no longer be met using the 
conventional methods. Consequently, electroerosion tools meeting the 
precision requirements can no longer be produced by hitherto known methods 
and therefore can no longer simply be reproduced. 
OBJECT OF THE INVENTION 
The object of the invention is to find a method for the manufacture of a 
shaping mask corresponding to the matrix for an electroerosion tool by 
which high-precision shaping masks having practically zero tolerance can 
be produced. 
SUMMARY OF THE INVENTION 
According to the invention the matrix itself can be used as a mold and is 
directly provided with a thin coating layer galvanically applied to it as 
a conductive layer, the coating layer having a compact surface and 
consisting of a neutral metal with respect to the material of which the 
shaping mask is to be formed. The coating layer is then cleansed and the 
metal layer for the shaping mask being built-up thereon until said metal 
layer can be lifted as a shaping mask. The term "neutral metal" designates 
in the scope of the invention all metals which, although they form a 
coating layer on the material of the matrix, do not react chemically 
and/or physically in such a way as to cause a chemical or physical 
amalgamation between this coating layer and the metal layer of which the 
shaping mask is produced. A chemical and/or physical amalgamation between 
the shaping mask and the coating layer preventing the shaping mask from 
being removed. In the scope of the invention, neutral metals are 
particularly the ones that do not form an alloy or an intermetallic 
amalgamation with the material of the shaping mask to be produced, at 
least not upon depositing on the material of the matrix. Suitable metals 
are easily found by testing. The desired high precision can be attained 
because, according to the method of the invention, there is no need to 
first reproduce a matrix which already would condition an inaccuracy. On 
the contrary, the matrix itself is used as the mold, whereby the matrix is 
not subject to any change by elastic deformation or other forces in the 
scope of this use. The coating layer to be provided onto the matrix can be 
extremely thinly applied and is extremely thin, as expressed by the term 
"coating layer". A preferred method of the invention in this connection is 
characterized by the fact that a coating layer having a layer thickness of 
the order of magnitude of one-thousandth of a millimeter and less is being 
applied onto the matrix. This means, in other words, that a tolerance of 
the same order of magnitude can be indicated and safely maintained for the 
tool manufacture. A metal layer having a layer thickness of the order of 
magnitude of one-tenth of a millimeter to one millimeter can then be 
applied onto the coating layer for the shaping mask, whereby the layer 
thickness of the shaping mask is so selected that the shaping mask, upon 
removal from the matrix and thereafter, can absorb all mechanical stresses 
without and permanent deformation. It is understood that the shaping mask 
has to be carefully handled when removing it from the matrix. A careful 
lifting of the shaping mask from the matrix is usually achieved by 
extraction, whereby the shaping mask is connected to an extraction or 
lifting device by means of a synthetic-resin adhesive. This allows a 
lifting or extraction with minimum surface use. 
A particularly successful method, according to the invention, is 
characterized by the fact that a chromium layer, preferably a 
hard-chromium layer (if possible with a high degree of hardness, for 
example, of 70 R.sub.C and higher) is applied as a coating layer onto the 
matrix. 
Surprisingly, and particularly with the method using the hard-chrome layer, 
it is possible to produce without any difficulty a removable shaping mask 
when the metal layer for the shaping mask is galvanically built-up onto 
the coating layer, thus particularly the hard-chromium layer, with a 
coating duration lasting from hours to days, and when in doing so that 
bath voltage of the copper bath or copper alloy bath is, to begin with, of 
the order of magnitude of one-tenth of a volt and less and increased up to 
one to several volts. In the scope of this general indication the rates 
can be so selected that the formation of either pulverized copper 
precipitation or clods or singular structures can be avoided during 
build-up of the copper or copper alloy layer for the shaping mask. Of 
course, the bath agitation and the bath temperature have to be adapted to 
the circumstances. Overlapping and undercutting are to be avoided during 
build-up of the coating layer, thus particularly the hard-chromium layer, 
as well as during build-up of the shaping mask. The usual bath current 
should always be directed, if possible, perpendicularly to the 
shape-defining surface of the matrix.

EXAMPLE OF OPERATION 
In the case of a shaping mask to be produced for an electroerosion tool 
which, for its part, serves to manufacture injection-molding dies for the 
production of precision-machining construction elements produced by 
injection molding, as usual to begin with, a so-called original matrix is 
manufactured with optimum precision, for example by engraving. The same 
may consist of steel. 
The matrix is to be used immediately as the mold for the production of the 
shaping mask. For this purpose, according to the present example, the 
matrix will be coated, entirely or solely on the shaping surface, in the 
usual way with a thin, preferably extremely thin hard-chromium layer 
having a thickness of the order of magnitude of one-thousandth of a 
millimeter and that, if possible, with a hard-chromium layer having a high 
degree of hardness according to Rcokwell C scale. This can be done 
according to Dettner/Elze "Handbuch der Galvanotechnik" (1966), Vol. II, 
17.05 chromium. The hard-chromium plating is usually carried out in such a 
way that a smooth and compact surface is formed. Polishing of the 
hard-chromium-plated surface is unnecessary. Nevertheless, in addition and 
after the hard-chromium plating has taken place, a passivation treatment 
by, for example, wetting or rinsing with a bichromate solution can be 
useful. In any event, the thin to extremely thin hard-chromium-plated 
surface of the matrix is cleansed by rinsing as is customary in 
electroplating in preparation for further galvanic process steps. Before 
the next machining all drops, air bubbles and the like have to be removed 
from the surface. 
To now galvanically build-up the shaping mask, a galvanic copper bath is 
prepared, for example, as described in the literature Dettner/Elze 1. c., 
17.02 copper. It can be a copper bath of pyrophosphate. One always works 
with extremely pure copper as is also the case, for example, in the 
manufacture of printed circuits. The matrix itself is suspended in the 
copper bath as an electrode. Moreover, the electrodes of the copper bath 
are connected to a precision rectifier. The copper bath indicated has a 
bath temperature of 61.degree. C. and is kept constant at this temperature 
by corresponding installations for the regulation of the bath temperature. 
The copper bath itself is usually agitated or pumped, if necessary by 
means of a group of filters. Overlappings and intersections are to be 
avoided and the hard-chromium shape-defining surface of the matrix is 
advantageously orthogonally struck by the current of the copper bath. 
After that the build-up of the shaping mask beings and that, to begin with, 
by applying an extremely low direct voltage to the electrodes, said 
voltage being of the order of magnitude of 0.1 volt or even lower. This 
galvanic treatment is carried out until a first filmy and thin copper 
deposit on the hard-chromium-plated mold surface of the matrix can be 
detected by the naked eye, for this, a duration of generally one to 
several hours is necessary. Now the bath voltage can be gradually 
increased from one to several hours up to 0.5 volt. After further hours it 
can be increased to 1 volt. After that a further gradual increase can take 
place until, at the end of the treatment duration of approximately 24 
hours, the bath voltage reaches 5 volts. By this time the shaping mask, in 
the form of a copper layer having a thickness ranging from tenths of 
millimeters to millimeters and able to absorb all mechanical stresses, has 
not only been built-up on the hard-chromium layer but surprisingly the 
so-produced shaping mask has sufficiently detached itself from the 
hard-chromium layer as to permit its removal by means of a synthetic resin 
sticker or soft plummet. Adapted die and synthetic-resin sticker are used 
in order to avoid, on removal, deformations caused by singular 
applications. The so-produced shaping mask has with respect to its own die 
surface, even for the equally filmy-hard-chromium coating layer deposited 
on the matrix, the identical shape of the matrix and thereby a precision 
of practically zero tolerance. The shaping mask is customarily combined 
with a support carrier.