CA2039106A1

CA2039106A1 - Process of galvanic treatment by pulsed currents

Info

Publication number: CA2039106A1
Application number: CA002039106A
Authority: CA
Inventors: Michel Panza; Christine Girodie; Pascal Panza; Remi Willmotte; Paul Buricand; Guy Joly
Original assignee: Individual
Current assignee: Ets Michel S A
Priority date: 1990-03-27
Filing date: 1991-03-26
Publication date: 1991-09-28
Also published as: PT97141A; TNSN91018A1; EP0448888A1; MA22090A1

Abstract

ABSTRACT OF THE DISCLOSURE

This invention relates to a process of galvanic treatment by deposit of nickel, alone or mixed, which is effected by reverse pulsed currents of which the rest time is less than or equal to 10 ms. The time of imposition of the cathodic current is included between 0.1 and 10 ms, the time of imposition of the anodic current is included between 0.5 and 10 ms, the cathodic peak density is between 4 and 40A/dm2, the anodic peak density is between 1 and 20A/dm2.
The process makes it possible to limit the blocking of pieces comprising openings of much reduced dimen-sions, in particular less than 120 micrometres, in particular in the case of producing a rotating stencil for printing.

Description

~5~6 _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-:
' b ` : , ~, ~

~3~J ~6 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 2~3~Q~

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.

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.

..

2~3~0~

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 :

2~3~

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.
;

Claims

1. A process for galvanic treatment by deposit of nickel, alone or mixed with another metal, for limiting the blocking of pieces comprising openings of reduced dimensions, in particular less than 120 micrometres, and employing reverse pulsed currents, wherein the reverse pulsed current has a rest time Tr less than or equal to 10 ms, a time of imposition of the cathodic current Tc included between 0.1 and 10 ms, a time of imposition of the anodic current Ta included between 0.5 and 10 ms, a cathodic peak density of between 4 and 40A/dm2 and an anodic peak density of between 1 and 20A/dm2.

2. The process of galvanic treatment for making a rotating stencil for printing, of Claim 1, wherein the rate of obturation is at the most 25%.

3. The process of Claim 1, wherein the galvanic nickeling bath comprises from 550 to 600 mg/1 of nickel sulfamate.

4. The process of Claim 1, wherein the development of the deposit is effected from a matrix locally covered with coating dots intended to form holes in the deposit.

5. The process of Claim 1, wherein the voltage at the terminals between the anode and the cathode is included between 0 and ? 40 volts.