Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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_ROCESS OF GALVANIC TREATMENT BY PULSED C~RRENTS
FIELD OF THE IN~ENTION
The present invention relates to galvanic treat-
ment by deposit of nickel alone or with other metals,
and more particularly to the production of rotatable
stencils for printing textiles and in general to
the galvanic treatment of pieces comprising openings
of reduced dimensions, particularly smaller than
120 micrometres.
BACKGROUND OF THE INVENTION
The engraved stencil is conventionally made
by D.C. galvanoplasty. Starting from the pattern
which it is desired to print, a half-tone negative
film is made. The screened pattern is transferred
by exposure of the film on a photosensitive coating
lS covering a roller which performs the role of matrix
of the stencil to be manufactured. After insolation
and development, the coating corresponding to the
non-exposed zones is dissolved. The matrix performing
the role of cathode is placed in the galvanic bath,
for example nickel sulfamate at the rate of 250 to
450 g/l. Nickel is regularly deposited over the outer
surface of the matrix; the zones corresponding to
the insulating coating form the screen dots in the
deposit. The cylinder consisting of the deposit of
nickel is then separated from the matrix and consti-
tutes the stencil. When the thickness of the deposit
of nickel is greater than that of the coating, it
is observed that the deposit takes a conical form
on the periphery of the screen dots, tending to obtu-
rate the orifice of the dot. The rate of obturationor coefficient of blocking is 35~ at minimum. Thus,
in order to obtain on his stencil a dot of 180 micro-
metres, the engraver must start from a theoretical
dot of 300 micrometres, which is difficult to produce
at screening. This high rate of obturation is a conside-
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rable limitation of the galvanic technique for makingpieces comprising openings of reduced dimensions,
of the stencil type.
It has been sought to overcome this drawback
by making stencils of small thickness, less than
70 micrometres, but demoulding is difficult and the
stencil has a shorter life duration. Stencils have
also been made in two steps. The first step consists
in obtaining a stencil of small thickness, as indicated
above; the second consists in continuing the galvanic
treatment on the demoulded stencil mounted on a rota-
ting shaft so that the deposit is effected on the
two faces of the stencil. Apart from the difficulty
of demoulding, this technique presents other drawbacks:
successive handlings, necessity of using two baths,
risks of delamination of the deposits particularly
if there is passivation of the first deposit.
The use of pulsed currents, with square pulses,
in galvanoplasty is known, forcontinuous deposi-ts, in
particular by the work of LANDOLT. It has made it
possible to employ higher densities of current than
in direct current and to improve the distribution
of the deposit.
Document EP.0079642 discloses a process of galva-
25 nic treatment using pulsed currents. In this document, .
the development of the metal, inter alia nickel,is effected on a screen. It aims at limiting blocking
of the openings of the screen, but without apparently
descending down to 120 micrometres. The process des-
cribed employs pulsed currents of which the time
of imposition (T) of the cathodic current is included
between 0 and 9900 ms. It is question of simple pulsed
current with rest times (T') of between 9 and 9900
ms. It may also be question of reverse pulse current.
It is an object of the present invention to
increase the quality of galvanic treatment and tobe able to limit blocking of openings of much smaller
dimensions, which may go as far as about ten micro-
metres.
SUMMARY OF THE INVENTION
This object is perfectly attained by the process
of the invention which is, in known manner, a process
of galvanic treatment by deposit of nickel, alone
or mixed with another metal, which employs reverse
pulsed currents.
According to the invention, the process is charac-
terized in that the reverse pulsed current has a
rest time (Tr) shorter than or equal to 10 ms, a
time of imposition of the cathodic current (Tc) inclu-
ded between 0.1 and 10 ms, a time of imposition of
the anodic current (Ta) included between 0.5 and
10 ms, a density of the cathodic peak of between
4 and 40A/dm2 and a density of the anodic peak of
between 1 and 2OA/dm 2 .
Thanks to the combination of these parameters
and in particular the employment of a rest time ofthe reverse pulsed current, it has been possible
to go down to openings of the order of about ten
micrometres. The rest time modifies the cathodic
polarization and makes it possible to orient the
crystallization along a vertical growth of the deposit.
The development of the deposit is preferably
effected from a matrix, locally coated with coating
dots, these dots being adapted to form the holes
in the deposit. In that case, and contrary to what
occurs when the development of the deposit is effected
from a screen as in document EP.007~642, the matter
constituting the coating dots participates in crystal-
lization.
The voltage at the terminals between the anode
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and the cathode is preferably included between O
and - 40volts. This particular arrangement gives
the pulsed current an orientation of the equipotentials
and improves the crystallization of the galvanic
deposit.
In the case of producing a rotating stencil
for printing on textiles, using reverse pulsed currents
defined hereinabove, the rate of obturation is at
the most 25~.
10The galvanic bath preferably comprises from
550 to 600 mg/l of nickel sulfamate.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more readily understood
on reading the following description with reference
15 to the accompanying drawings, in which:
Figures lA to lD are longitudinal sections illus-
trating the steps of producing an engraving by galvano-
plasty.
Figure 2 represents a current density/time curve
20 of a simple pulsed current.
Figure 3 represents the transitory curve of the
potential/time of response to a square current pulse.
Figure 4 represents the current density/time
curve of a reverse pulsed current.
25DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, in order to obtain
by galvanoplasty an engraved piece of which the engra-
ving reproduces a determined pattern, a half-tone
negative film 1 is made, in accordance with the well
30 known half-tone screen technique. The half-tone nega-
tive 1 shown in Figure lA comprises opaque zones
2 which delimit transparent zones 3; these are the
transparent zones 3 which correspond to the drawing
to be printed in the case of a stencil for printing
35 on textiles in particular.
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The half-tone negative 1 is placed above a matrix
4 of which the outer surface is coated with a photo-
sensitive coating 5. The matrix 4 is a roller in
the case of a rotating stencil.
After insolation, the non-exposed parts of the
photosensitive coating 5 are dissolved, whilst the
insoluble exposed parts form protuberances 6 on the
surface of the matrix 4 (Figure lB).
The matrix 4/protuberance 6 assembly is placed
in a galvanic bath based on nickel sulfamate. The
matrix 4 performs the role of cathode. A deposit
7 of nickel is formed uniformly at first on the surface
of the matrix, surrounding the protuberances 6 then,
when the thickness of the deposit increases beyond
the thickness of the protuberances, the deposit pre-
sents holes 8 at the level of said protuberances
6; these holes 8 constitute the screen dots (Figure
lC). When its thickness is sufficient, the deposit
7 is separated from the matrix 4 and forms the stencil
(Figure lD).
It is observed that the dots 8 do not present
vertical, regular walls 9, but these walls 9 are
conical in shape, which tends to partially block
the opening due to the presence of a protuberance
25 6. The rate of obturation or coefficient of blocking
corresponds to the part of the initial opening which
is occupied by the conical excroissances of the depo-
sit. It will be understood that, when the openings
are intended to contain a printing fluid, this phenome-
30 non of blocking is a handicap.
Figure 2 represents the current density/timecurve of a simple pulsed current. A current of this
type is characterized by its period T which is split
up into a time of imposition of the cathodic current
35 Tc and a rest time Tr, and by the current density
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of the cathodic peak Jc.
Figure 3 represents the transito;-y cu ve of the
potential/time in response to a square current pulse.
The variation of the potential during time Tc of
imposition of the cathodic current is produced in
several distinct steps. The first step is virtually
instantaneous: it corresponds to the charge of the
double electro-chemical layer in the vicinity of
the cathode, from potential Vo at initial time To
to potential Vl. The second step corresponds to the
faradic plateau, the potential remaining constant.
During the third step which expires with time Tc,
the potential progressively increases up to V2. The
fourth step, which begins with the rest time Tr,
corresponds to the discharge of the double electro-
chemical layer.
Applicants have observed that the electro-deposit
of nickel, during the galvanic treatment, is effected
under different conditions depending on whether one
is is the second, third or fourth step, and that
the phenomenon of blocking is produced for the deposits
effected preferentially during the third and fourth
steps. Thus, the choice of the time of imposition
of the cathodic current, included between 0.1 and
10 ms, has for its purpose to reduce the overall
time of diffusion whilst conserving the faradic plateau.
In one embodiment, the current used was a reverse
pulsed current, of the type represented in Figure
4, in which the time of imposition of the cathodic
current Tc is immediately followed by an anodic rever-
sal during a time Ta then a rest time Tr. The para-
meters were the following: Tc=lOms, Ta=3ms, Tr=O.lms,
Jc=13A/dm 2, Ja=5A/dm 2 .
A galvanic bath having the following approximate
composition was employed: nickel sulfamate from 550
2~
to 600 g/l, nickel chloride from 5 to 15 g/l, boric
acid from 30 to 40 g/l, the pH being included between
3.5 and 4.5; the temperature between 40 and 70C.
The anodes were made of electrolytic nickel or nickel
depolarized with sulfur. Benzoic O-sulfimide was
added as ductilizing agent and, in order to facilitate
de-moulding, 2-butyne 1-4 diol, at a rate of a few
mg/l. The voltage at the terminals between the anode
and the cathode was included between 0 and - 40 volts.
A stencil was produced, having a thickness of 90
micrometres with openings of much reduced dimensions,
about ten micrometres. The rate of obturation observed
was 20%.
The invention is not limited to the embodiments
which have been described by way of example but covers
all the variants thereof. In particular, the invention
is not limited to the deposit of nickel alone, but
also concerns deposits of nickel mixed with other
metals, for example cobalt or tungsten.
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