EP0962553A1 - Process and apparatus for coating a surface by electrophoresis - Google Patents

Abstract

The surface of a substrate is coated by electrophoresis in which, during the passage of electrophoretic current, the bath is subjected to some vibrations in to generate vaporous cavitations near the surface. The vibrations are applied only during a first phase at the start of the passage of current and/or only in a second phase at the end. AN Independent claim is also included for an installation for the continuous coating of the surface of components by electrophoresis.

Description

• faire passer un courant électrique entre ladite surface servant d'électrode et une contre-électrode également plongée dans le bain,
• pendant le passage du courant, soumettre le bain au voisinage de la surface à des mouvements de vibration, notamment à des fréquences sonores ou ultrasonores.

The invention relates to a method of electrophoresis coating the surface of a substrate immersed in an electrophoresis bath, comprising the steps of:

• passing an electric current between said surface serving as an electrode and a counter-electrode also immersed in the bath,
• during the flow of the current, subject the bath in the vicinity of the surface to vibrational movements, in particular to sound or ultrasonic frequencies.

Electrophoresis painting is widely used for automotive body parts.

The electrophoresis bath generally consists of an emulsion aqueous of a film-forming polymer; resins of the type are commonly used polyepoxides.

The electrophoresis electric current is used to entrain particles of the emulsion towards the part to be painted where they will constitute the layer of painting ; the electrical resistance between the part to be painted and the counter-electrode increases with the thickness of the deposit.

A body paint installation involves in a way classic a paint tray and a conveyor to immerse the part in the bath, move it along the bath and extract it, as described for example in patent application JP 87 268 321 A.

The length of the bin and the speed of travel of the part in the bin are adapted to the thickness of the paint layer to be deposited, depending on paint deposition speed.

The deposition rate is proportional to the electric field in the vicinity of the part to be painted, i.e. the potential difference applied between the electrode and the counter electrode; at constant polarization, this speed decreases as a function of time until almost vanishing when the thickness of the layer of paint deposited provides significant electrical resistance to passage of electrophoretic current.

The part extracted from the bath is steamed to ensure the cooking of the coating; for polyepoxide type resins, the steaming treatment lasts about 20 minutes at about 180 ° C.

As described in document JP 87 268 321 A, when applying thus a layer of paint on steel sheets coated with zinc or alloy zinc, especially galvanized alloy steel sheets, surface defects (“pinhole gas” in English) on the layer of paint, which would come from the formation, during the electrodeposition, of bubbles gas on the surface to be painted.

To avoid the formation of these faults, this document teaches subject the electrophoresis bath to vibrational movements at ultrasonic frequencies, during the passage of the electrophoretic current.

To vibrate the bath, we immerse in the bath ultrasonic emitting generators along the scroll path of the room, on each side of the room; these ultrasonic transmitters are distributed from each side of the running path along the two longitudinal walls of the paint tray (reference 7 in figures 1 and 2 of document JP 87 268 321 A) and are connected to a suitable supply device.

This ultrasonic plating process is expensive because it requires the installation of numerous transmitters along the scroll path pieces.

The object of the invention is a more economical method and installation.

• faire passer un courant électrique entre cette surface servant d'électrode et une contre-électrode également plongée dans le bain,
• pendant le passage du courant, soumettre le bain à des mouvements de vibration au voisinage de cette surface,

   caractérisé en ce que :

• on applique lesdits mouvements de vibration de manière à générer des cavitations vaporeuses au voisinage de cette surface,
• on applique lesdits mouvements de vibration seulement dans une première phase D de début du passage du courant et/ou seulement dans une deuxième phase F de fin du passage du courant,

la phase D commençant au début du passage du courant et se terminant avant l'instant correspondant à la moitié de la durée de passage du courant,

la phase F commençant après l'instant correspondant à la moitié de la durée de passage du courant et se terminant à la fin de passage du courant.

The subject of the invention is a method of electrophoresis coating the surface of a substrate immersed in an electrophoresis bath, comprising the steps consisting in:

• passing an electric current between this surface serving as an electrode and a counter-electrode also immersed in the bath,
• during the flow of the current, subject the bath to vibrational movements in the vicinity of this surface,

characterized in that:

• said vibration movements are applied so as to generate vaporous cavitations in the vicinity of this surface,
• said vibration movements are applied only in a first phase D of the start of the current flow and / or only in a second phase F of the end of the current flow,

phase D starting at the start of the current flow and ending before the instant corresponding to half the duration of the current flow,

phase F starting after the instant corresponding to half the duration of current flow and ending at the end of current flow.

Adapting the vibration movements to generate phenomena of vaporous cavitations and cavities in the vicinity of the surface is a particularly important element to achieve the goal of the invention; the diameter of these cavities can also influence the efficiency of the process.

The expression "starting at the beginning" does not mean that the application vibration movements cannot start slightly before start of current flow, for convenience; so the phase D begins "approximately" at the start time of passage of the current.

The expression "ending at the end" does not mean that the application of vibration movements cannot continue slightly after the end current flow, for convenience; thus, phase F is ends "approximately" at the time when the current ends.

As the duration of application of the vibration movements D + F is then strictly lower than that of the current flow (if we except the any brief application times preceding the start of the current and / or following the end of current flow), the method according to the invention is less expensive than the methods of the prior art which provided to apply these vibration movements during the entire passage time current.

Preferably, to achieve the optimal economy for phase D, this phase D ends approximately at the instant corresponding to the point singular of the curve R ° (t) of evolution, as a function of time, of the resistance electric measured between this surface and the counter electrode in the same conditions but in the absence of said vibration movements.

In practice, it may suffice that the duration of phase D is less than or equal to a quarter of the duration of current flow, which saves a quarter of the costs relating to "means of vibration" compared to art prior.

• les mouvements de vibration correspondent à des ondes sonores ou ultrasonores.
• le substrat est en acier galvanisé allié.
• on applique lesdits mouvements de vibration uniquement dans ladite première phase D de début du passage du courant.
• au cours de ladite première phase D, l'intensité dudit courant que l'on fait passer provoque une tension de polarisation supérieure à la tension de cratérisation de ladite surface.
• au début du passage du courant, la durée de montée de la tension de polarisation jusqu'à une valeur prédéterminée supérieure à ladite tension de cratérisation est inférieure à 1 seconde.
• on applique lesdits mouvements de vibration uniquement dans la deuxième phase F de fin du passage du courant.
• pendant ladite deuxième phase F, on applique les mouvements de vibration uniquement au voisinage de zones prédéterminées de ladite surface en vue d'y déposer un revêtement plus épais que sur les autres zones de ladite surface.

The invention may also have one or more of the following characteristics:

• the vibration movements correspond to sound or ultrasonic waves.
• the substrate is made of galvanized alloy steel.
• applying said vibration movements only in said first phase D of the start of current flow.
• during said first phase D, the intensity of said current which is passed causes a bias voltage greater than the craterization voltage of said surface.
• at the start of the current flow, the duration of rise of the bias voltage to a predetermined value greater than said craterization voltage is less than 1 second.
• said vibration movements are applied only in the second phase F of the end of the current flow.
• during said second phase F, the vibration movements are applied only in the vicinity of predetermined areas of said surface in order to deposit a thicker coating thereon than on the other areas of said surface.

• appliquer lesdits mouvements de vibration de manière à générer des cavitations vaporeuses au voisinage de ladite surface de pièces,
• appliquer lesdits mouvements de vibration seulement dans la « zone d'immersion » des pièces et/ou seulement dans la « zone d'extraction » des pièces,

la « zone d'immersion » des pièces le long du chemin de défilement commençant approximativement à l'endroit correspondant au début de passage du courant et se terminant en deçà de la moitié de la longueur dudit bac,

la « zone d'extraction » des pièces le long du chemin de défilement commençant au delà de la moitié de la longueur dudit bac et se terminant approximativement à l'endroit correspondant à la fin de passage du courant.

The invention also relates to an installation for continuously coating the surface of parts by electrophoresis, usable for implementing the method according to any one of the preceding claims, of the type comprising a tank for containing an electrophoresis bath, means for immersing the surface, for conveying the moving parts into the tank and then extracting them therefrom, at least one counter-electrode immersed in the bath, means for passing an electric current between said surface and the counter-electrode, and means for applying vibration movements in the bath in the vicinity of said moving surface,
characterized in that these vibration means are suitable for:

• applying said vibration movements so as to generate vaporous cavitations in the vicinity of said workpiece surface,
• apply said vibration movements only in the "immersion zone" of the parts and / or only in the "extraction zone" of the parts,

the “immersion zone” of the pieces along the travel path starting approximately at the point corresponding to the start of current flow and ending below half the length of said tank,

the “extraction zone” of the pieces along the travel path starting beyond half the length of said tank and ending approximately at the place corresponding to the end of current flow.

As the total length of the zones ("immersion" + "extraction") the along which the said vibration movements are applied is then strictly less than the length of the area along which we pass a electric current, means for applying vibration movements are less expensive than in the prior art.

Preferably, to achieve the optimal economy of these means, the "Immersion zone" ends approximately at the point corresponding to the instant corresponding to the singular point of the curve R ° (t) of evolution, in as a function of time, of the electrical resistance measured between this surface and the counter electrode under the same conditions but in the absence of said vibration movements.

In practice, it may suffice that the length of the immersion zone in the scrolling direction is less than or equal to a quarter of the length of said tray, which saves a quarter of the costs relating to "means of vibration "compared to the prior art.

• lesdits moyens de vibration sont adaptés pour générer des ondes sonores ou ultrasonores.
• lesdits moyens de vibration sont immergés dans ladite « zone d'immersion » et/ou lesdits moyens de vibration sont immergés dans ladite « zone d'extraction ».

The invention may also have one or more of the following characteristics:

• said vibration means are adapted to generate sound or ultrasonic waves.
• said vibration means are immersed in said "immersion zone" and / or said vibration means are immersed in said "extraction zone".

The bases of the invention lie in the observation of the phenomena following.

The surface defects of the paint layer have the form of craters, which, on steel sheets, are privileged places of initiation of the corrosion; moreover, despite the three additional layers of paint (say respectively "sealer", "base" and "varnish") that the visible parts of the body above the cataphoresis layer, the craters remain visible and greatly degrade the appearance of these parts.

These craters are in the form of small frustoconical holes emerging at the surface of the cataphoresis layer; they have a diameter generally between 100 and 500 µm at the base, between 5 and 20 µm at the Mountain peak.

These so-called “craterisation” defects would come from the formation of gas, especially hydrogen, near the surface of the workpiece coating.

The ultrasound application conditions must be adapted to generate vaporous cavitation phenomena in the neighboring bath the surface to be coated, at the point where it is desired to avoid defects; indeed, ultrasonic vibration movements in fluids are deemed generate cavitation phenomena which, depending on the power work, fall under gas cavitation (low power) or cavitation vaporous (higher power: a few W / liter); cavitation gaseous does not effectively prevent surface defects, unlike vaporous cavitation; it seems that the cavities vaporous vapors created near the surface cause coalescence of bubbles of hydrogen being formed, thus preventing the formation of area.

The energy required to generate the vapor cavitation is independent of the frequency up to at least 100 kHz; the minimum values required for ultrasonic energy of insonation are, according to the various theories known in the matter, between 0.1 and 1 W / cm ² ; beyond this energy threshold, the lifespan decreases with the increase in power, between ten periods to a period; on the other hand, the density does not necessarily change according to the power.

The size of the vaporous cavities generated in the bath is inversely proportional to the frequency.

According to a variant of the invention, it is possible to add, in the bath electrophoresis, cavitation aids; some wetting agents are for example well suited; they reduce the power required to cause cavitation.

In the absence of vibrations generating vaporous cavitations, the craterization defects appear beyond a predetermined level of tension, called “craterisation tension”, and / or a predetermined speed polarization of this substrate; thus, in the prior art, to avoid these faults, a relatively low voltage is applied between the substrate and the counter-electrode, and / or this voltage is applied very gradually, which has the drawback of reducing the average deposition speed and, therefore, the productivity of the paint line.

On continuous coating lines, the gradual application of the voltage requires the use of several voltage generators (each suitable for a line area), which is very expensive.

By measuring the polarization resistance R ° at the start of polarization of the substrate in the absence of vibrations generating vaporous cavitations, we would expect values to grow steadily with thickness filed; on the contrary, we observed that the evolution curve as a function of time R ° (t) presented a singular point reflecting the appearance of the phenomenon of craterization; we checked that it was enough, to completely avoid faults, to apply ultrasound only between the time of start of circulation of the electrophoresis current and the instant corresponding to this singular point; in industrial practice, the time between these two moments is generally less than 15 seconds.

Even by extending the application of ultrasound after the instant corresponding to this singular point, we move away from the optimal conditions of implementation of the invention without departing from the invention as long as the duration of application of the vibration movements D + F remains strictly less than that of the current flow so as to limit the application of ultrasound compared to the prior art.

In industrial conditions, we generally raise very quickly bias voltage to a higher voltage value at the craterization voltage; between the instant of start of current flow electrophoresis and the instant when the bias voltage exceeds the voltage of craterization, it generally takes less than a second; without application of ultrasound generating vaporous cavitations according to the invention, this rapid rise in voltage further increases the risk of craterization, that the invention makes it possible to avoid.

Thus, according to the invention, on a paint line 4 m long where the parts are conveyed at a speed of 4 m / min., to avoid defects in surface, it may suffice to subject the start of immersion area to ultrasound pieces over a length of approximately 1 m from the running path; the duration of phase D is then less than or equal to a quarter of the duration of passage of the current and the length of the immersion zone is then less than or equal to quarter of the length of the immersion tank.

The invention therefore makes it possible to avoid surface defects while performing deposition under high voltages, even when applied abruptly; the invention thus makes it possible to avoid surface defects under speed conditions of high deposit.

As mentioned above, at constant bias voltage, the paint deposition speed decreases as a function of time until it is canceled almost when the thickness of the paint layer deposited provides a significant electrical resistance to the passage of electrophoretic current; we thus reaches a given limiting thickness.

It was found that, in the presence of ultrasound, the limiting thickness could increase from 15 to 40%; it has also been found that the effect obtained is identical depending on whether ultrasound is applied throughout the duration of the polarization, or only at the end of the polarization period, for example for only the second half of the duration of current flow electrophoretic.

Referring to Figures 4 and 5, in an installation 7 of coating of surface of parts 9 continuously by electrophoresis, of the type comprising a tank 8 to contain an electrophoresis bath, means 10 for immersing the surface to be coated, to convey the pieces 9 running in the tray and then extract therefrom, at least one counter-electrode (not shown) immersed in the bath, means for passing an electric current between the surface to coating and the counter electrode, and ultrasonic generators 11, 12 adapted to subject the bath in the vicinity of the moving surface to vibration movements, as described in document JP 87 268 321 A, the invention makes it possible to limit the area of the bath to be subjected to vibrations at the "Immersion zone" of the parts and / or only in the "extraction zone" parts, while limiting craterization and / or significantly increasing the filing speed.

Thanks to the limitation of the area of the bath to be subjected to vibrations, installation is much more economical, since we have the ultrasonic generators 11 only in the “immersion zone” and / or has 12 ultrasonic generators only in the "area extraction ”.

In the sense of limiting the area of the bath to be subjected to vibrations, the "Immersion zone" of the pieces along the scroll path begins to the place corresponding to the moment when the current begins to flow electrophoretic, so generally at the point of beginning of immersion of the part or part portion, and ends at the point corresponding to the instant of the singular point that one would observe on the curve R ° (t) of evolution as a function of time of the polarization resistance in the absence of vibrations generating vaporous cavitations.

In the sense of limiting the area of the bath to be subjected to vibrations, the "Extraction zone" of the pieces along the scroll path begins at earlier to half the area along which the room or portion of room is submerged and ends at the place corresponding to the end time of circulation of electrophoretic current, so generally to the point extraction of the part or part of a part.

Thanks to the invention, it is no longer necessary to install transmitters of ultrasound in the bath all along the path of travel of the parts to paint, as in document JP 87 268 321 A; so we decrease appreciably the number and / or the useful power of the transmitters, which is very economic.

The invention thus makes it possible to avoid, if necessary, the defects of craterization and / or improve the average deposition speed while saving on installation and operating costs compared to the process described in document JP 87 268 321 A.

The possibility, offered by the invention, of using bias voltages higher than the craterization voltage without risk of craterization, as well as that of reaching high deposition rates makes it possible to improve the plant productivity.

The effect of vibrations generating vaporous cavitations on speed deposit can be used to obtain localized over-thicknesses of coating: thus for example, by adapting the position of the transmitters of ultrasound in the tank in relation to areas of the room surface where wish to apply an extra thickness, we can obtain, in a single operation, a coating having these suitably localized excess thicknesses; these zones can correspond to welded joints or to hollow parts of parts, which require reinforced corrosion protection.

When using sound or ultrasonic waves to induce in the bath of vibrations generating vaporous cavitations on the surface of the submerged part, preferably the wave propagation is approximately perpendicular to said surface; using devices conventional cavitation can be caused from several meters away and waves can be concentrated, even from several transmitters, on predetermined areas of the surface.

According to a variant of the invention, the installation does not include sonotrodes in the bath but means for vibrating at a frequency ultrasound the part to be painted; thus, by vibrating the part to be painted instead of bathing, we achieve the same benefits as before.

The method and the device according to the invention can advantageously be used to paint car bodies, or body parts such as hoods, wings, doors or underbody parts.

• la figure 1 est une représentation schématique d'une installation de peinture utilisée pour les exemples décrits ci-après ;
• les figures 2 et 3 illustrent la variation en fonction du temps de la résistance électrique R°(t) entre la contre-électrode et la surface en cours de revêtement, à partir de l'instant de début de circulation du courant d'électrophorèse.
• les figures 4 et 5 sont des schémas d'installation de revêtement selon l'invention, en vue latérale (fig. 4) et vue de dessus (fig.5).

The following examples illustrate the invention with reference to the appended figures in which:

• Figure 1 is a schematic representation of a painting installation used for the examples described below;
• FIGS. 2 and 3 illustrate the variation as a function of time of the electrical resistance R ° (t) between the counter-electrode and the surface being coated, from the instant of start of circulation of the electrophoresis current.
• Figures 4 and 5 are diagrams of coating installation according to the invention, in side view (fig. 4) and top view (fig.5).

Example 1:

The purpose of this example is to illustrate the absence of surface defects after a deposit carried out under a voltage greater than the craterization voltage and under ultrasound.

This example is also intended to illustrate the impact of the management of vibration of the bath in the vicinity of the surface to be painted.

Coating tests by cataphoresis according to the invention are carried out on steel samples.

We choose a galvanized alloy steel substrate to be placed in the conditions where the risk of craterization is high.

These samples are cut from a flat sheet in 90 x 140 format mm and bent at right angles in the middle of the long side.

As a cataphoresis bath, a used bath reference 718 960 is used. of the PPG Company at a temperature of 28 ° C.

We choose a used bath to be placed in the conditions where the risk of craterization is high.

In the tank containing the bath, a flat counter-electrode is immersed, or anode, and, facing the anode, the sample to be painted.

The sample to be painted then has a part parallel to the anode at a distance of 130 mm and a perpendicular part oriented towards the anode.

Sonotrodes are placed in the bath between the sample and the anode, at a distance of approximately 2 cm from the part of the sample parallel to the anode, to generate vibrations in a direction parallel to this part of the sample and therefore perpendicular to the other square part of the same sample.

The vibrations generated in the bath have a frequency of 21700 Hz and a power of around 300 W; these conditions allow cause vaporous cavitations in the bath, especially in the vicinity of the surface to be coated.

A potential difference of 220 V is maintained between the sheet metal to be painted and the anode until the total electric charge transferred reaches 18 Coulombs; this bias voltage is greater than the voltage of craterization, i.e. at the tension under which phenomena of craterization appear in the absence of ultrasound.

Under these conditions, the time required for the passage of the charge 18 Coulombs' electric is about 17 seconds; the deposit obtained then has a thickness of between 15 and 20 μm.

After the coating operation, the sample is extracted from the bath and the oven for 20 minutes at 180 ° C to bake the paint layer.

• nombre de défauts sur la partie parallèle : 110.
• nombre de défauts sur la partie perpendiculaire : 110.

We then observe the number of “crater” type defects on the two painted parts of the sample, the part parallel to the anode and the perpendicular part.

• number of faults on the parallel part: 110.
• number of defects on the perpendicular part: 110.

The direction of vibration of the bath in the vicinity of the surface to be painted does not therefore does not seem decisive with regard to the risk of craterisation.

Comparative example 1:

This example illustrates the results obtained under the same conditions as in Example 1, but in the absence of ultrasound.

The procedure is as in Example 1, but without sonotrodes.

For the same electrical charge of 18 Coulombs, and, approximately the same coating thickness, polarization should be maintained for 24 s.

• nombre de défauts sur la partie parallèle : 240.
• nombre de défauts sur la partie perpendiculaire : 225.

The following results are obtained:

• number of faults on the parallel part: 240.
• number of defects on the perpendicular part: 225.

• de diviser environ par deux les risques de défaut de type "cratère",
• d'améliorer de environ 30% la vitesse de dépôt.

By comparing to Example 1, we deduce that the use of ultrasound allows:

• to roughly halve the risk of "crater" type defect,
• improve the deposition speed by around 30%.

Example 2:

The purpose of this example is to illustrate the incidence of the electrophoresis bath.

We proceed under the same conditions as in Example 1, but on 100 x 200 mm flat samples in an unused bath.

We also measure the quantity of craters on the painted surface, and, in in addition, the time required to obtain a coating thickness predetermined.

• nombre de défauts de cratère : 0.
• temps de dépôt : 14 s.

The following results are obtained:

• number of crater faults: 0.
• deposit time: 14 s.

Comparative example 2 :

This example illustrates the results obtained under the same conditions as in Example 2, but in the absence of ultrasound.

We proceed as in Example 2, but without sonotrodes and therefore without subject the bath to ultrasound.

• nombre de défauts de cratère : 42.
• temps de dépôt : 20 s.

For a coating of the same predetermined thickness, the following results are obtained:

• number of crater faults: 42.
• deposit time: 20 s.

• de supprimer l'apparition de cratères.
• de gagner environ 30% sur la vitesse de dépôt.

By comparing to Example 2, we deduce that the use of ultrasound allows:

• to suppress the appearance of craters.
• to earn about 30% on deposit speed.

We therefore confirm the teachings of Example 1, on a bath of different electrophoresis and on samples of simple shapes.

Example 3:

The purpose of this example is to show that, to effectively limit the appearance of craterization defects using ultrasound, the conditions application methods must be adapted to generate vaporous cavitation phenomena in the bath near the surface to put on.

An acoustic wave propagating in a liquid medium is characterized by a succession of positive and negative pressures; the variation of pressure at a point in the liquid is called "sound pressure".

The sound pressure is linked to the ultrasonic power dissipated in the liquid ; high sound pressure can cause local rupture of the liquid and creating a cavity in a low pressure area; it's the acoustic cavitation phenomenon.

• la cavitation gazeuse, dans laquelle la cavité est remplie de gaz initialement dissous dans le liquide, ou provenant de matériaux immergés (parois, électrodes, ...).
• la cavitation vaporeuse, dans laquelle la cavité est remplie de vapeur du liquide, la basse pression (ou dépression) dans la cavité étant inférieure à la pression de vapeur saturante de ce liquide.

There are at least two types of cavitation:

• gas cavitation, in which the cavity is filled with gas initially dissolved in the liquid, or coming from submerged materials (walls, electrodes, ...).
• the vapor cavitation, in which the cavity is filled with vapor of the liquid, the low pressure (or depression) in the cavity being less than the saturation vapor pressure of this liquid.

Vapor cavitation requires more energy than cavitation carbonated.

When we generate cavities in the bath near the surface, two preponderant phenomena are important to avoid defects in craterization: shock waves and micro-jets, which only occur with the vaporous cavities.

To materialize the vapor cavitation in a cataphoresis bath, the following installation is used (with reference to FIG. 1): a tank 1 containing a bath 2; a sample 3 and a counter electrode 4 kept immersed in the bath 2.

We implant a sonotrode 5 in the bath, so that the ultrasonic vibrations it generates are perpendicular to the surface of sample 3 to be coated.

Two types of sonotrodes can be installed according to the desired frequency: 16.3 kHz or 38.9 kHz

The distance between the sonotrode 5 and the sample 3 can be varied.

The filling level of the bath is 110 mm in height.

To materialize the vapor cavitation, fill the tank 1 with water and uses, instead of the sample, aluminum foil held in two grids.

It is then observed that, under ultrasound and at sufficient power, it forms “impacts” on the aluminum sheet: the amount of impact obtained provides information on the density of cavitation.

A series of 30 second tests is carried out at 300 W; in terms of impact density, the results obtained are shown in Table 1: XXXX to designate a very high impact density, xxx for a high density, xx for a medium density, x for a low impact density. Influence of the Sonotrode-Sample distance Distance (cm) Sonotrode-Échant. 1 2 3 4 5 6 7 8 9 10 Sonotrode 16.3kHz xxxx xxxx xxx xx xx xx x x x x 38.9 kHz controde xx x x no no no no no no no (no: not observed)

The power value of the sonotrode (300W) relates to the sonotrode itself and not the ultrasonic power dissipated in the bath proximity to the surface of the sample.

We note that the low frequency 16.3 kHz seems more favorable to the vapor cavitation as the high frequency 38.9 kHz.

We then carried out painting tests to cross-check the conditions for applying ultrasound (vapor cavitation) and the anti-cratering effect.

For a “standard” painting test, we use samples of galvanized alloy steel sheet, degreased not phosphated and a unused PPG cataphoresis bath, referenced 718 960, maintained under mechanical stirring and at a constant temperature of approximately 28 ° C; the sample is gradually polarized until it reaches 10 about seconds, a voltage of 220 V which is then kept constant for the duration of the test; using the sonotrode, the bath is subjected to ultrasound during the entire period of circulation of the electrophoresis current; the test duration is 30 seconds.

After testing, the presence (“YES”) or the absence (“NO”) of crater on each face 6A, 6B of the sample; the results are reported to table It.

From these results, at 16.7 kHz and 300 W, it is therefore confirmed that it to avoid craterization, the sonotrode-sample distance should be less than or equal to 6 cm; this condition seems to correspond well to that of the vapor cavitation established in the previous test series (Table I).

At 16.7 kHz and 50 W, it can be seen that, to avoid craterization, the sonotrode-sample distance is less than or equal to 3 cm.

At 38.9 kHz and 500 W (high power), we cannot avoid the craterization; it is possible that the diameter of the cavities is, at this frequency, too low to be effective against craterization.

The diameter of the cavities is inversely proportional to the frequency: around 30 to 100 µm at 10 kHz, around 15 to 50 µm at 20 kHz. Influence of ultrasound on craterisation. Sample No. Frequency (kHz) Ultrasonic power (W) Distance (cm) Sonotr.-ÉCh. Craters Face 6A Face 6B Craters 1 Without 0 4 YES YES 12 16.7 50 2 NO NO 22 16.7 50 3 NO YES 3 16.7 50 4 YES YES 8 16.7 300 2 NO NO 2 16.7 300 4 NO NO 17 16.7 300 5 NO NO 18 16.7 300 6 NO NO 10 16.7 300 7 YES YES 4 16.7 500 4 NO NO 19 16.7 500 7 YES YES 13 38.9 300 1 YES YES 9 38.9 300 2 YES YES 15 38.9 300 4 YES YES 7 38.9 500 4 YES YES

• la puissance de la sonotrode augmente, ou la distance échantillon-sonotrode diminue,
• la fréquence des ultra-sons diminue.

We therefore see that the anti-craterisation effect increases when:

• the power of the sonotrode increases, or the sample-sonotrode distance decreases,
• the frequency of ultrasound decreases.

Example 4:

The purpose of this example is to illustrate the use of the electrical resistance of the sample being coated to know the instantaneous level of surface craterization.

We use the same painting installation as in Example 3, with reference to FIG. 1: a tank 1 containing a paint bath 2; a sample 3 and a counter electrode 4 kept immersed in the bath 2.

• pour fonctionner à la fréquence de 8 kHz,
• et pour délivrer la puissance ultra-sonore minimale constante de 50 W.

A sonotrode 5 is implanted in the bath, so that the ultrasonic vibrations it generates are perpendicular to the surface of the sample 3 to be coated; Sonotrode 5 is suitable:

• to operate at the frequency of 8 kHz,
• and to deliver the constant minimum ultrasonic power of 50 W.

We distinguish side 6A exposed to the sonotrode and the other opposite side 6B; the distance between the sonotrode and the sample is fixed at 11 cm.

For a “standard” painting test, we use non-phosphated degreased steel sheet samples and an unused bath of cataphoresis of the Company PPG, referenced 718 960, kept under stirring mechanical and at a constant temperature of about 28 ° C; we polarize gradually the sample until, in approximately 10 seconds, a 220 V voltage which is then kept constant for the duration of the test; the duration of the test is at least 30 seconds; according to a variant, the “ramp up voltage” is almost 0 seconds, instead of 10 seconds.

During the tests, the electrical resistance between sample 3 and counter electrode 4.

Under “standard” conditions and on samples of alloyed galvanized steel, in the absence of ultrasound during current flow, we then observe an evolution R ° (t) of the resistance values as a function of the conforming time. in the schematic representation of FIG. 2: the curve R ° (t) presents a singular point, which can for example be as here a maximum (resistance value R _max. ) or an inflection point of the curve.

The shape and magnitude of this peak (or singular point) depends on the applied bias voltage.

Conversely, under the same conditions but in the presence of ultrasound, we finds that this peak decreases or disappears completely.

At the same time, after baking the coated samples, it is found that samples coated in the absence of ultrasound have defects in craters (on both sides 6A and 6B) while the samples coated in presence of ultrasound does not present these defects.

The suppression of craterization at 8 kHz observed at a distance higher than in example 3 at 17 kHz confirms that the frequency of ultrasound affects the removal of craterization; it is possible that the diameter of the cavities intervenes in the phenomena in question.

Finally, we note above all that we obtain the same surface condition without faults if the ultrasound is applied for the entire duration of the current (case P2 - fig.2) as in the prior art or if they are applied only between the moment when the current begins to flow (time: 0 s.) and the instant corresponding to the peak (case P1) according to the invention; conversely, if we applies the ultrasound only after the peak (case P3), even during a long duration (case P4), no anti-cratering effect of ultrasound is observed.

It was therefore established that the resistance measurement made it possible to detect the appearance of the craterization phenomenon during the coating operation and that the application of ultrasound only during a first phase D (case P1 - fig.2) the start of current flow is sufficient to avoid these defaults ; it is likely that ultrasound lowers the amount of hydrogen present on surfaces 6A and 6B, which produces a decrease in the electrical resistance during this first phase.

Example 5:

The purpose of this example is to illustrate the impact of ultrasound on speed coating coating.

Sample coating operations are carried out for 2 minutes under the same conditions as in Example 4; we measure the weight of paint deposited.

We observe that the application of ultrasound during the passage of the current can significantly increase the thickness or weight deposited.

Referring to Figure 3, we then see that we get the same deposition speed improvement depending on whether the ultrasound is applied during the entire duration of current flow (case P1 - fig.3) or only, depending on the invention, at the end of the coating operation (case P2).

The application of ultrasound increases the coating speed electrophoretic on all substrates; the level of improvement achieved nevertheless depends on the nature of the substrate: this has resulted in a increase between 30 and 35% on galvanized steel, and at a about 40% increase on galvanized alloy steel; generally the mass gain obtained is between 15 and 40%.

It has therefore been established that the application of ultrasound only during one second phase F (case P2 - fig.3) end of current flow is sufficient to significantly increase the average deposition speed.

Finally, it was found that the improvement in the deposition rate increased when the frequency of ultrasound decreases.

Example 6:

The purpose of this example is to illustrate, in addition to Example 5, the impact of the ultrasound treatment period on the deposition rate of the coating.

We measure the gain in mass deposited (%) brought by the treatment to ultrasound with respect to the mass deposited on the same substrate in the same conditions but without ultrasound, depending on whether the ultrasound treatment is carried out during the first ten seconds of current flow ("0 to 10 s. "), During the first minute of current flow (" 0 to 60s. "), during the entire duration of current flow ("0 to 120 s."), or during the last minute of current flow ("60s. to 120s.").

The tests are carried out on two types of substrate: galvanized steel (GZ) and galvanized alloy steel (GA).

The results are reported in Table III as a function of the applied bias voltage.

It is therefore confirmed that the increase in the deposition rate remains very weak when applying ultrasound in the start phase of passage of the current and on the contrary reaches a maximum when applied during the end of current flow phase.

Example 7:

The purpose of this example is to compare the effect of ultrasound on a surface bare metal and on a phosphated metal surface; indeed, before bet When painting metal surfaces, it is common to carry out a treatment of phosphating; it is therefore important to check that this treatment does not harm the effectiveness of ultrasound.

It was thus found, by observation with an electron microscope at scanning, that the application of ultrasound did not appear to alter the appearance of the phosphating layer.

It was also found that the application of ultrasound provided the same benefits (anti-cratering effect - improved deposition speed) on phosphated surface than on bare surface.

In the case of phosphate layers, the application of ultrasound during shorter time periods than in the prior art, namely only in a first phase D of the start of current flow and / or only in a second phase F at the end of the current flow, allows to limit the risks of degradation of the phosphating layer.

Claims (15)

Method for electrophoretically coating the surface (6A, 6B) of a substrate (3; 9) immersed in an electrophoresis bath (2), comprising the steps consisting in: passing an electric current between this surface serving as an electrode and a counter-electrode also immersed in the bath (2), during the flow of the current, subject the bath to vibrational movements in the vicinity of this surface (6A, 6B), characterized in that: applying said vibration movements so as to generate vaporous cavitations in the vicinity of this surface (6A, 6B), said vibration movements are applied only in a first phase D of the start of the current flow and / or only in a second phase F of the end of the current flow, phase D starting at the start of the current flow and ending before the instant corresponding to half the duration of the current flow, phase F starting after the instant corresponding to half the duration of current flow and ending at the end of current flow. Method according to claim 1 characterized in that phase D is ends approximately at the instant corresponding to the singular point of the curve R ° (t) of evolution, as a function of time, of the electrical resistance measured between this surface and the counter-electrode under the same conditions but in the absence of said vibration movements. Process according to either of Claims 1 and 2, characterized in in that the duration of phase D is less than or equal to a quarter of the duration current flow. Method according to any one of the preceding claims characterized in that the vibration movements correspond to waves audible or ultrasonic. Method according to any one of the preceding claims, characterized in that said substrate is made of alloyed galvanized steel. Method according to any one of the preceding claims characterized in that said vibration movements are applied only in said first phase D of the start of current flow. Method according to any one of the preceding claims characterized in that, during said first phase D, the intensity of said current passed through causes a bias voltage greater than the craterization tension of said surface. Process according to Claim 7, characterized in that, at the start of the current flow, the duration of the bias voltage rise to a predetermined value greater than said craterization voltage is less than 1 second. Method according to any one of the preceding claims characterized in that said vibration movements are applied only in the second phase F of the end of the current flow. Method according to any one of the preceding claims characterized in that, during said second phase F, the vibration movements only in the vicinity of predetermined areas of said surface (3A, 3B) in order to deposit a thicker coating thereon than on the other zones of said surface. Installation (7) for continuously coating the surface of parts (9) by electrophoresis, usable for implementing the method according to any one of the preceding claims, of the type comprising a tank (8) for containing an electrophoresis bath , means (10) for immersing the surface, for conveying the parts (9) running in the tank and then extracting them therefrom, at least one counter-electrode immersed in the bath, means for passing an electric current between said surface and the counter electrode, and means (11, 12) for applying vibration movements in the bath in the vicinity of said moving surface,
characterized in that these vibration means (11, 12) are adapted for: applying said vibration movements so as to generate vaporous cavitations in the vicinity of said surface of parts (9), apply said vibration movements only in the "immersion zone" of the parts and / or only in the "extraction zone" of the parts, the “immersion zone” of the pieces along the travel path starting approximately at the place corresponding to the start of current flow and ending below half the length of said tank (8), the “extraction zone” of the pieces along the travel path starting beyond half the length of said tank (8) and ending approximately at the place corresponding to the end of current flow. Installation according to claim 11 characterized in that the "Immersion zone" ends approximately at the point corresponding to the instant corresponding to the singular point of the curve R ° (t) of evolution, in as a function of time, of the electrical resistance measured between this surface and the counter electrode under the same conditions but in the absence of said vibration movements. Installation according to any one of claims 11 to 12, characterized in that the length of the immersion zone in the direction of scrolling is less than or equal to a quarter of the length of said tray (8). Installation according to any one of claims 11 to 13, characterized in that said vibration means (11, 12) are adapted to generate sound or ultrasonic waves. Installation according to any one of claims 11 to 14, characterized in that said vibration means (11) are immersed in said "immersion zone" and / or in that said vibration means (12) are immersed in said "extraction zone".

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