EP0063992A1 - Electrostatic painting process for small objects carried by an omega-shaped conveyor - Google Patents

Abstract

The projector (1) is placed sensibly on the axis of the loop (20) with its axis of rotation oriented so that the area of intersection of the spray paint blade (14) and the section of cylinder described by the axes pieces is contained between the limit circles (22a, 22b) but reaches these limits. An annular nozzle (13) is connected to a source of pressurized gas and disposed behind the bowl so that an annular jet of gas affects the origin of the thin blade of spray paint. The supply pressure of this nozzle is periodically modulated so that the intersection zone periodically and alternately reaches one and the other of said limit circles (22a, 22b).

Description

The invention relates to a method of electrostatic painting of parts of small dimensions extended along a main axis, where the parts are passed, in rotation about their axis on a conveyor with an almost closed loop, a bowl projector. rotating being located substantially on the axis of the loop to distribute the paint on the parts.

The electrostatic projectors known as rotating bowls emit, from an edge of a member rotating at high speed, on the surface of which paint is deposited, a thin blade of sprayed paint, between two cones of angle at the top very neighbors. The distance between the projector and the parts to be painted is of the order of a few decimeters.

When it comes to coating parts with small dimensions, that is to say dimensions smaller than the distance of the parts from the projector, it is clear that the profitability of the operation supposes that the parts are presented scrolling so as to intercept a substantial fraction of the spray paint blade. Furthermore, it is preferable, if the parts are extended along a main axis, to rotate the parts around this axis to obtain a regular thickness on the periphery, and to present this main axis substantially perpendicularly to the paint blade to release the parts gripping members. The solution generally adopted consists in using a conveyor which describes an almost closed loop with a horizontal axis, and on which the parts are mounted, with their main axis parallel to that of the loop and so as to describe several turns on themselves, as they travel the loop. The projector is placed with its bowl on the axis of the loop, so that the distance to the parts is constant. Finally the parts describe around the projector the periphery of a cylinder section, limited by two coaxial circles to the loop, described by the two ends of the main axes of the parts. But the axis of rotation of the projector bowl makes an angle with the axis of the loop, such that the zone of intersection of the cylinder described by the axis of the parts and the blade of sprayed paint comes to tangent the two limit circles of the section described by the parts . Thus, during their passage on the loop, the parts can be coated with paint over their entire length. The rotation of the parts around their axis, and jointly the electrostatic forces resulting from the action on the paint particles electrically charged with the electrostatic field created by the potential difference between rotating bowl and parts ensure a regularity of the paint deposition along the periphery parts (view of the axis). On the other hand, the inclined arrangement of the axis of rotation of the bowl results in extra thicknesses at the ends of the pieces, since the speed with which the intersection zone runs along the piece cancels out at the ends, where the zone tangent to the limit circle, and is maximum in the center.

This is practically not changed when the projector is equipped with an annular nozzle at the rear of the bowl, supplied with gas under pressure and directed so that the gas jets affect the paint slide at its origin. The blade then takes a generally conical shape, and thickens somewhat under the effect of turbulence. This arrangement, sometimes called "shower", allows the projector to be moved back with respect to the center of the cylinder section. However, the unevenness of paint thickness in longitudinal distribution is not diminished, because in addition to the extra thickness at the ends there is an asymmetry compared to the center of the part.

Furthermore, some parts may have shapes such that a variation in the quantity of paint deposited along the length of the part may be desirable, but it is unfortunately rare that the desired variation corresponds to the variation actually created by the obliquity of the rotation axis.

On the other hand, due to the small dimensions of the parts, the coating time is limited. If we wanted to modulate the quantities of paint emitted by controlled movement of the projector, as is done for the coating of certain large parts, it would be necessary for the displacement period to be small compared to the coating time, to uniformly affect all the parts. The corresponding orders would be complex, heavy and difficult to adjust.

The subject of the invention is a process for electrostatic painting of small parts where the longitudinal distribution of paint is equalized.

The invention also relates to a method of electrostatic painting of small parts where the longitudinal distribution of paint can be modulated relatively simply.

For these purposes, the invention provides a method of electrostatic painting of parts, extended along a main axis and of small dimensions, according to which the parts are arranged on individual rotary supports of a conveyor comprising an almost closed circular loop of parallel axis. to the axes of rotation of the support so that the parts, while rotating around their axis, describe the lateral surface of a section of cylinder limited by two circles coaxial with the loop, and a paint projector with a bowl rotating at high speed and brought to a high voltage relative to the conveyor emits a thin blade of sprayed paint, blade substantially of revolution around the axis of rotation of the bowl, this bowl being placed substantially on the axis of the loop with its axis of rotation oriented in so that the area of intersection of the spray paint blade and the cylinder described by the axes of the parts is contained between the limit circles of the cylinder section but reaches these limits, process where the projector is equipped with an annular nozzle connected to a source of gas under pressure and arranged behind the bowl so that an annular jet of gas affects the origin of the thin blade of sprayed paint, characterized in that the supply pressure is modulated periodically so that the intersection zone periodically and alternately reaches one and the other of the said limit circles.

The supply pressure modulation results in a modulation of the cone at the top of the paint slide, which reacts with reduced inertia. Everything happens, on the periphery of the cylinder section, as if the headlamp was driven by an axial reciprocating movement. It will be noted that, in addition to the displacement of the zone of intersection of the cylinder section by the sprayed paint blade, the action of the air jet blown by the nozzle on the paint blade creates turbulences which widen the blade.

Preferably, the supply pressure is modulated by periodically connecting the nozzle to a pressure source. You get all-or-nothing modulation in a remarkably simple way.

As a variant, the nozzle is permanently connected to a pressure source, and periodically to a higher pressure source. We then obtain a modulation by all or little, which, on the one hand allows the projector to be moved back, and on the other hand gives at any time a painting blade enlarged by turbulence.

It is also possible to divide the nozzle into sector fractions, and to feed these fractions at separate pressures. Each sectoral fraction defines a fractional intersection zone where the scanning regime is independent of the regime of the other zones. This allows in particular to preferentially cover certain parts of the parts.

• la figure 1 est un schéma en perspective d'une disposition utilisable pour la mise en oeuvre de l'invention ;
• la figure 2 est un schéma en bout d'un transporteur à boucle en oméga ;
• la figure 3 est une vue plus détaillée d'un projecteur à bol tournant ;
• la figure 4 est un schéma en vue latérale d'utilisation selon l'état de la technique de la disposition de la figure 1 ;
• la figure 5 est un schéma en vue latérale d'utilisation selon l'invention de la disposition de la figure 1 ;
• la figure 6 est un schéma de disposition pour l'alimentation en gaz sous pression d'une buse de projecteur, en modulation tout ou peu ;
• la figure 7 est un diagramme de répartition de peinture sur une pièce, avec une pression de soufflage en tout ou peu ;
• la figure 8 est un diagramme analogue à celui de la figure 7, mais qui correspond à une buse à deux secteurs.

The characteristics and advantages of the invention will become apparent from the description which follows by way of example, with reference to the accompanying drawings in which:

• Figure 1 is a perspective diagram of an arrangement usable for the implementation of the invention;
• Figure 2 is an end diagram of an omega loop conveyor;
• Figure 3 is a more detailed view of a rotating bowl projector;
• Figure 4 is a diagram in side view of use according to the prior art of the arrangement of Figure 1;
• Figure 5 is a diagram in side view of use according to the invention of the arrangement of Figure 1;
• FIG. 6 is an arrangement diagram for the supply of gas under pressure to a projector nozzle, in all or little modulation;
• FIG. 7 is a diagram of the distribution of paint on a part, with all or little blowing pressure;
• Figure 8 is a diagram similar to that of Figure 7, but which corresponds to a nozzle with two sectors.

According to the provisions of the prior art shown in Figures 1 to 4, an electrostatic projector 1 as a whole, with a rotating bowl 12, is arranged substantially on the axis of a loop 20 of a conveyor 2 in its together. The headlamp 1 is brought to a high potential with respect to the mass of the conveyor 2, by a high voltage generator 11, and comprises a turbine enclosed in the body 10 for rapidly rotating the bowl 12 (up to 60,000 revolutions per minute). Paint injected onto the internal surface of the bowl 12, substantially frustoconical, is drawn towards the edge 12a of the bowl, from which it is projected tangentially in the form of a blade of sprayed paint 14. At the base of the bowl 12, between this and the turbine body 10, is arranged an annular nozzle 13, supplied with air under pressure, and having outlet orifices which direct air jets on the external surface of the bowl 12. These air jets, originally intended especially to avoid spray paint return to the projector body 10, cause a deformation of the blade 14 of spray paint, which changes from a planar shape to a cone shape at an angle with a very open top. In Figure 4, there is shown the blade 14, seen by the edge, in planar shape.

The transporter 2 has an almost closed circular loop 20, which takes the form of a capital omega. Rollers 21 and 21 'ensure the return of the conveyor from a horizontal path to the loop 20, and thus define the inlet and outlet ends of the loop 20. The conveyor comprises supports for parts, mounted door-to-door. -false parallel to the axis of the loop 20. These part supports, as shown in FIG. 2, have knobs 25 at the rear which come to bear on a rail 26 concentric with the loop 20, so that the parts, in their path along the loop, are rotated around the support axis. The arrangement shown is used for painting small parts compared to the normal distance at which the pieces of the bowl 12 are arranged, this distance corresponding practically to the radius of the loop 20; in addition the parts are extended along a main axis, which will coincide with the axis of the supports, this main axis being understood as the direction of the largest dimension of the part, while the dimensions along directions orthogonal to each other and with l 'main axis are significantly weaker than the large dimension.

The axes of the support therefore define a cylinder coaxial with the loop 20, which constitutes its director, and the main axes of the parts will describe a section 22 of this cylinder, limited by two circles 22a and 22b coaxial with the loop 20.

Under these conditions, the parts will receive the paint sprayed essentially in the zone 15 formed by the intersection of the cylinder section 22 by the blade 14. As shown in FIG. 1 and especially in FIG. 4, the zone 15 comes to tangent on one side the limit circle 22a and the other the limit circle 22b, so that the parts receive paint over their entire length, while their rotation around the main axis ensures the peripheral overlap.

To obtain that the zone 15 is tangent to the limit circles 22a and 22b, the axis of rotation of the bowl 12 is inclined on the axis of the loop 20, as clearly shown in FIG. 4. It is specified that, in the developed, the zone 15 has a sinusoidal shape with amplitudes tangent to the straight lines developed from boundary circles 22a and 22b. Furthermore, the shape of the zone 15 will be little affected by a deformation in the open cone of the blade 14 under the effect of an air blowing by the nozzle 13, insofar as, of course, the bowl 12 will have been moved back on the axis of the loop 20 to maintain the zone 15 between the limit circles 22a and 22b.

Since the conveyor is moving at constant speed, the geometric development of zone 15 corresponds to the time diagram of scanning of each of the parts by the sprayed paint blade, the amount of developed corresponding to the total time of exposure of the part to the painting. Since the press form is sinusoidal, at least approximately, the scanning speed will cancel out at the ends of the part, to be maximum in the center. Despite the corrections made both by the thickness of the blade 14 and by the returns due to the electrostatic field, the paint deposited on the parts has excess thickness at the ends.

Figure 5 illustrates the process by which the invention overcomes the above defect. In this FIG. 5 we find, diagrammatically, the headlight body 10, the rotating bowl 12 and the annular nozzle 13, the conveyor loop 20 and the cylinder section 22, described by the main axes of the parts, and included between the circles limits 22a and 22b. However, the angle made by the loop 20 and rotating bowl 12 axes has been reduced so that the intersection zone of the paint blade and the cylinder section 22 occupies only about half the length. cylinder section along the axis. In addition, a first blowing pressure at the nozzle 13 was determined, such that the zone 15 of intersection of the blade 14 with the cylinder section 22 is tangent to the limit circle 22a, then a second blowing pressure at the nozzle 13 , such that the intersection zone 17 of the blade 16 with the section 22 is tangent to the limit circle 22b. Of course, the second pressure is less than the first, so that the angle at the top of the conical blade 16 is smaller than that of the conical blade 14.

To coat the parts with paint, the nozzle 13 is supplied with pressure alternately equal to the first and second pressures determined as explained above, the succession period being short compared to the duration of passage of a part over the loop 20. The pressure diagram is shown in FIG. 7 at line 40, the levels 41 and 42 respectively representing the values of first and second pressures. The strip 122 represents the section of the section 22, the lines 122a and 122b being the respective sections of the boundary circles 22a and 22b. Zone 115 is the development of the intersection zone, alternately 15 and 17, as seen by a piece scrolling on the loop 20. Zone 115 results from the superposition of two harmonic movements, one at the periodicity of scrolling of a part, and the other at the periodicity of alternating succession of the first and second presses. We can clearly see, even without in-depth analysis of the distribution, that the end thicknesses are attenuated, and that the distribution along the length of the part is substantially regular.

To specify orders of magnitude, the diameter of the loop 20 is approximately 0.5 m and the linear speed of the conveyor 2 is 10 meters per minute, so that a part describes the loop in approximately 9 seconds; in addition the part undergoes nine to ten rotations during its passage on the loop, that is to say 1 revolution per second. The diameter of the bowl 12 is approximately 70 mm, and the high voltage supplied by the generator 11 is around 90 kV.

The first blowing pressure is approximately 3 bars, and the second of 1 bar, the first pressure pulses lasting 0.25 seconds and succeeding each half-second.

To obtain pressure successions as shown in diagram 40 of FIG. 7, the assembly shown in FIG. 6 can be used. A source of pressurized air is connected to the inlet pipe 30. A first reservoir 31 is supplied from the pipe 30, through a first expander 31a (set at the first press). From the reservoir 31, a second analogous reservoir 32 is supplied through a second pressure reducer 32a (set at the second pressure). The second tank 32 is connected to an outlet pipe 36, to which the nozzle 13 will be connected, through a non-return valve 35. The first tank 31 is connected to the outlet pipe 36 through a solenoid valve 33, the control winding 33a receives an appropriate signal via a line 34. It is immediately specified that the valve 33 can also be pneumatically controlled. It will be understood that, when the valve 33 is closed, the line 36 is at the pressure of the tank 32, while when the valve 33 is open, the pressure in the line 36 is that of the tank 31; then the valve 35 prevents the air under the first pressure from entering the reservoir 32.

It will be noted that a substantially equivalent result would be obtained if the second pressure was atmospheric pressure, in other words if one sent to the nozzle 13 a succession of pressure pulses separated by intervals where the relative pressure is zero, provided of course of advancing the rotating bowl projector 12. However, the rear part of the intersection zone would be narrower.

The process that we have just described and commented on makes it possible to regularize the distribution of paint along the main axis of the parts. In certain cases, certain regions of the part must receive a greater quantity of paint, for example a region where the average diameter of the part is greater. To solve this problem, pressure pulses at different levels can be applied with selected successions. It is also possible, in a way which facilitates adjustments, to use a nozzle divided into sectors covering a certain angle around the axis of the bowl 12, and to apply to each sector a distinct pressure modulation. The diagrams in FIG. 8 relate to a nozzle with two sectors 113a and 113b, arranged on either side of a vertical plane passing through the axis of the rotating bowl. On the sector 113a are applied pressure pulses evolving between the levels 52 and 53, while on the sector 113b the pulses evolve between the levels 51 and 53.

The development of the intersection zone, in the band 222 between the development 222a and 222b of the boundary circles, as it appears for a moving part, presents itself as the superposition, to a harmonic displacement with periodicity corresponding to the along the loop, two harmonic displacements, with periodicity of pulses, successive and of distinct amplitudes. According to the representation, the amount of paint deposited in the region close to the limit 222a is significantly less than the amount of paint distributed over the rest of the part.

It is understood that, while retaining rectangular pulse shapes, corresponding to "all or little" or "all or nothing" pressure controls, it is possible to vary the pressure levels and the working cycles of the pulses. , in order to adapt the distribution of paint along the parts to the configuration of these parts. Together with the obliquity of the axis of rotation of the bowl with respect to the axis of the loop provides an additional adjustment parameter. It is easy to see that experience and routine tests will make it possible to implement the processes of the invention as well as possible.

An important industrial advantage of the process remains the economy of any mechanical scanning means.

Claims (4)

1. A method of electrostatic painting of parts, extended along a main axis and of small dimensions, along which the parts are arranged on individual rotating supports of a conveyor comprising a circular loop (20) almost closed circular axis parallel to the axes of support rotation so that the parts, while rotating about their axis, describe the lateral surface of a section of cylinder bounded by two circles (22a, 22b) coaxial with the loop, and a spotlight (1) of bowl paint rotating (12) at high speed and brought to a high voltage with respect to the conveyor emits a thin blade (14) of sprayed paint, blade substantially of revolution around the axis of rotation of the bowl (12), this bowl being placed substantially on the axis of the loop (20) with its axis of rotation oriented so that the intersection zone (15) of the spray paint blade and the cylinder described by the axes of the parts is contained between the limit circles (22a , 22b) of the tron Lesson of cylinder but reaches these limits, process where the projector (1) is provided with an annular nozzle (13) connected to a source of pressurized gas and arranged behind the bowl so that an annular jet of gas affects the origin of the thin blade (14) of sprayed paint, characterized in that the supply pressure is periodically modulated so that the intersection zone (15) periodically and alternately reaches both of said circles limits (22a, 22b). 2. Method according to claim 1, characterized in that the annular nozzle (13) is periodically connected to a source of pressurized gas. 3. Method according to claim 1, characterized in that the annular nozzle is periodically connected to a first source (31) of pressurized gas, the nozzle being furthermore permanently connected to a second source (32) of gas under pressure, the pressure of the second source being lower than that of the first. 4. Method according to claim 1, characterized in that the nozzle being divided into sectoral fractions (113a, ll3b), these fractions are supplied at different modulated pressures (52,53).

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