WO2016156700A1

WO2016156700A1 - Method for painting a motor vehicle part comprising thickness control of the coating by photothermal radiometry

Info

Publication number: WO2016156700A1
Application number: PCT/FR2016/050635
Authority: WO
Inventors: Sophie Vidal
Original assignee: Compagnie Plastic Omnium
Priority date: 2015-03-30
Filing date: 2016-03-23
Publication date: 2016-10-06
Also published as: FR3034328B1; FR3034328A1

Abstract

Method for painting a motor vehicle part, in which the part is introduced into a first zone for applying a first coating, then introduced into a second zone for desolvation of the first coating, characterised in that the following steps are carried out: • a first coating (R) is applied (REV1) on at least one area of the part (P) in said first zone; then, on leaving the second zone and before any other treatment of the part • the thickness (E) of the first coating is measured (MES1) in at least one location of the area, by means of a photothermal radiometric measurement.

Description

METHOD FOR PAINTING A PIECE OF A MOTOR VEHICLE WITH CONTROL OF THICKNESS OF THE COATING BY PHOTOTHERMIC RADIOMETRY

The present invention relates to the field of painting processes of a part for a motor vehicle, such as a bodywork part, with thickness control of the deposited coating.

In particular, the invention applies to a piece of polymer, in particular thermoplastic or thermosetting polymer.

Part for a motor vehicle, including a body part, participating in the aesthetic appearance of a vehicle, is usually coated with paint.

The painting step of such a plastic part generally comprises a surface treatment step, followed by successive application steps of at least three coating layers (a primer, a base and a varnish). The primer layer improves the conductivity and adhesion of the color layer. The base layer gives color to the room. The varnish layer protects the base layer against external aggressions.

To meet the requirements of car manufacturers, it is necessary to comply with the technical specifications of the suppliers of these coatings, which in particular impose precise thicknesses for each coating, so that each of them performs its function correctly.

To do this, the installations are configured to deposit the correct thickness of coating on the entire surface of the part, by regulating the flow rate, the amount of coating, the speed of movement of the part, the sweeping path of the sprayer, the type sprayer ...

However, because of the inherent hazards of industrial processes, slight variations may occur over time, or when a device no longer functions properly.

This is why the thicknesses of these different layers are regularly measured, at different places in the room, to verify compliance with the technical specifications of the suppliers of these coatings.

To do this, it is known for polymer parts to regularly select a piece at the end of the paint line, then cut it to make measurements by the slice using a microscope. However, these measures are destructive.

To avoid the destruction of a part, methods of non-destructive testing using ultrasonic technology are also known. But this technology does not allow to measure thicknesses less than 10 μηι, while the primary layers can measure 3 to 4 μηι. Moreover, the application of coatings can be performed wet-on-wet, that is to say that a layer is applied to another layer before the first has finished drying: there is interpenetration between the layers and the measurements are then difficult to interpret . Finally, this solution requires contact of the sensor with the surface whose thickness must be measured: there is no destruction of the part but damage to the surface appearance.

It is also known to measure the thickness by means of a gauge based on the magnetic induction principle and the eddy currents. The coatings are then deposited on a metal support to carry out this measurement. This technique involves applying each layer to a different plate to measure each thickness. In addition, there are variations in the values measured due to variations in the support and the application between a plastic part and a metal plate.

Thus the currently known methods comprise at least one of the following disadvantages:

the measurement is destructive of the part and / or the coating;

an operator is necessary in order to cut the workpiece and measure;

the measurement of all the layers of paint requires the use of a support other than the plastic parts;

- It is necessary to wait until the end of the painting process (primary + base + varnish) to perform a measurement at the output of the paint chain. The response time to detect a problem is then too long (a complete painting process of a bodywork part usually lasts more than 2 hours);

the measurement time of the thickness is long (15 minutes for a measurement under the microscope and for each zone of the part to be measured);

- Some layers are difficult to observe / identifiable by the measuring means (same color for example);

the measurement requires contact with the surface to be tested.

The aim of the invention is to overcome these drawbacks by providing a method for painting a motor vehicle part with thickness control by photothermal radiometry, making it possible to know the thickness of several coating layers applied to a surface, without contact and without destruction of the part and / or the coating.

Thus, the subject of the invention relates to a method of painting a motor vehicle part, in which the part is introduced into a first application zone of a first coating, then introduced into a second desolvation zone of the first coating. The method comprises the following steps:

i. a first coating is applied to at least one area of the part in said first zone; then out of the second zone, and before any other treatment of the part,

ii. a measurement of the thickness of the first coating is carried out in at least one location of the zone, by means of a photothermal radiometry measurement. The method according to the invention thus comprises a step of controlling the thickness of coating applied to the surface of the part which is immediate, and made just after removal of the coating.

It is therefore possible to react immediately without waiting for the end of the painting process, or even the destruction of the piece. The painting process continues to work during these measurements.

Furthermore, photothermal radiometry measurement is a non-contact, non-destructive and rapid measurement (at the scale of the industrial automotive body part painting process), which makes it possible to carry out several possible measurements on the same part, and on a larger part sample, or even all the painted pieces, to precisely map the coating thicknesses on the surface of the piece.

Finally, the measurement by photothermal radiometry can be performed by means of a robot, and therefore does not require the intervention of an operator. It is thus possible to carry out these measurements directly in the paint line, without stopping it, thus making it possible to create control loops on the paint application parameters.

According to one embodiment, steps i) and ii) are repeated for different coatings, step i) being carried out in coating application zones, and step ii) being carried out before application of the following coating, in second zone exit. For example, three coatings can be applied: a primer, then a base, then a varnish.

According to the invention, the thickness of the first coating obtained can be compared by means of a photothermal radiometry measurement at a coating thickness.

According to one embodiment, the coating setpoint thickness can be defined by a calibration method depending on the photothermal radiometry measuring instrument used and the analysis method used by said instrument. Alternatively, the measuring instrument and its method of analysis can be calibrated against a setpoint value.

According to the invention, the second desolvation zone may also comprise means for drying and cooling the first coating. This is particularly advantageous when the coating comprises water. Advantageously, it is possible to adapt, as a function of said measurement by photothermal radiometry, the paint application parameters.

The invention also relates to an installation for painting a motor vehicle part comprising at least one workpiece support, and a movable means for scrolling the support in front of at least one means for applying a coating to at least one surface of the room. The installation comprises at least one photothermal radiometry measuring instrument for measuring the thickness of said coating.

According to one embodiment, the installation comprises means for applying three different coatings on at least one surface of the part, and a photothermal radiometry measuring instrument comprising three measuring heads, each head being adapted to measure the thickness of a different coating.

According to another embodiment, the installation comprises means for applying three different coatings on at least one surface of the part, and three photothermal radiometry measuring instruments comprising a measuring head, each instrument being adapted to measure the thickness of a different coating.

The invention will be better understood on reading the appended figures, which are provided by way of examples and are in no way limiting, in which:

FIG. 1 illustrates the various steps of the method according to the invention.

FIG. 2 illustrates an exemplary embodiment of the method according to the invention.

FIG. 3 is a diagram illustrating an instrument used to carry out the method according to the invention, as well as its principle of operation. Referring now to Figure 1 which illustrates the method according to the invention for painting a part (P) of a motor vehicle.

The invention applies in particular to a piece of polymer, in particular thermoplastic polymer, for example polypropylene (PP), polyamide (PA), acrylobutadiene styrene (ABS), PPE + PA (noryl) or a mixture of Acrylobutadiene styrene and polycarbonate (ABS / PC), or thermosetting polymer, for example SMC (in accordance with the acronym for Sheet Molding Compound) or epoxy (EP).

According to the method, the part is introduced into a first application zone of a first coating, then introduced into a second desolvation zone, and optionally drying and cooling, of the first coating.

The method comprises the following steps: i. applying (REV1) a first coating (R) on at least one area of the workpiece (P) in said first area; then at the exit of the second zone and before any other treatment of the part (application of a second coating for example), ii. a measurement (MES1) of the thickness (E) of the first coating (R) is carried out in at least one location of the zone, by means of a photothermal radiometry measurement.

The thickness (E) of the first coating obtained is then compared (CMP) by means of the photothermal radiometry measurement at a coating thickness (EC). When a thickness is not in accordance with the specifications of the coating suppliers (E ≠ EC), then a control (CTRL) of the paint application parameters is carried out, and if necessary, the painting application parameters such as as flow rates, speed, rotation, high voltage.

The time elapsed between the moment when the part is painted and the one where measures are available to adjust the paint parameters, is considerably reduced compared to the processes that only check the thickness at the end of the paint line. . Reducing this time is essential, because all the parts produced in this elapsed time, can be to throw, to retouch or to deliver but with a lower quality.

Steps i) and ii) can be repeated for different coatings. Step i) is carried out in coating application zones (application zone of a primer, zone of application of a base, zone of application of a varnish, etc.), and step ii) is carried out before application of the next coating, that is to say at the exit of the second zone.

Indeed, after each coating application, the piece is introduced into a desolvation zone. In this desolvation zone, the coating which has just been applied is left to distribute homogeneously over the surface of the part. This is called "tension" of the coating, which results from a balancing of the surface tensions in the applied coating layer still liquid. At the same time, in this zone of desolvation, the solvent (water or other solvent) evaporates, which makes the applied coating less liquid and freezes it while keeping the stretch of the layer. This combined phase of voltage and evaporation of the solvent is generally referred to as "flash off". It intervenes immediately after each of the application phases of a coating (primer, base and varnish) and before the next step.

If the coating comprises water, there is added to this desolvation chamber furnaces for drying. Cooling of the coating is then necessary before applying a new coating.

A major interest in carrying out the measurement after the desolvation zone (rather than before), is that the thickness of the coating is more stable. Some of the solvents evaporate when passing through the desolvation zone. So the chemical composition of the coating is changed, so the measurement that is related to the chemical composition is distorted. In addition, the thickness at the beginning of application (called wet thickness) will be different from the thickness at the end of desolvation zone (called dry thickness) since part of its composition is evaporated.

The method for controlling the thickness of the various coatings is therefore never destructive, and does not slow down the overall painting process of the part. After each application step of a coating, it is possible to determine, without slowing down the process, whether the thickness of the coating is correct.

According to an exemplary embodiment (FIG. 2), the following steps are carried out:

i. applying a first coating (REV1), called primary, on at least one area of the room; then before applying the base,

ii. a measurement (MES1) of the thickness of the primer is made in at least one location of the zone, by means of a photothermal radiometry measurement; this measurement is compared with a setpoint value, and a regulation loop of the primary application parameters is set up;

iii. we apply a second coating (REV2), called base, then before application of the varnish,

iv. measuring (MES2) the thickness of the base in at least one location of the zone, by means of a photothermal radiometry measurement; this measurement is compared with a setpoint value, and a loop for regulating the application parameters of the base is set up;

v. a third coating (REV3), called a varnish, is applied to at least one zone of the part; then before complete drying of the varnish;

vi. a measurement (MES3) of the thickness of the varnish is made in at least one location of the zone, by means of a photothermal radiometry measurement; this measurement is compared to a setpoint value, and a regulation loop of the varnish application parameters is set up.

Advantageously, the thickness of each coating is verified at various locations in the zone which has received this coating.

Photothermal radiometry measurement (Figure 3)

Photothermal radiometry is a non-destructive technique operating at a distance from the studied surface, and therefore, without contact, with this surface. This technique makes it possible to measure punctually the thickness of a coating deposited on a area.

The document WO 2015/001210 A1 describes various photothermal radiometry measurement methods.

As illustrated in FIG. 3, photothermal radiometry consists of subjecting the coating locally to a luminous flux (FL), for example a laser, the absorption of which produces a local elevation of temperature in the vicinity of the point of impact of the flux ( FL). As the heating provided by the flux is very low, neither the coating (R) nor the part (the surface) are damaged or modified during the measurement.

Then, the variations of emittances (EV), that is to say the variations of the heat flux re-emitted by the thus heated coating, are measured. To do this, it is possible to use infrared optical detection means (4).

Finally, the information relating to the re-emitted heat flux is processed by means of a processing algorithm and physical models. There are pulsed, random, correlative, and parametric analysis methods.

The measurement is usually done in less than a second.

To perform a photothermal radiometry measurement, different types of instruments can be used. One can for example use one of the instruments proposed by Enovasense® (France), and in particular the device described in WO 2015/001210 A1. This instrument and its analysis method make it possible to measure the thickness without any contact with the part, and on moving parts.

A photothermal radiometry measuring instrument (1) has one or more measuring heads (2). A measuring head (2) is composed of at least one light source (3), such as a laser, and at least one sensor (4) of the reemitted heat flux, such as an infrared sensor. The measuring head (2) is connected to a means of analysis (5) of the data coming from the infrared sensor (4).

To overcome the type of instrument and / or the type of analysis used, the desired thickness of coating is determined according to the instrument and the analysis method. To do this, prior to the control measures, a calibration step (self-learning) is carried out comprising the following steps:

a thickness setpoint is defined;

a coating layer is applied to the surface of a workpiece having, in principle, the target thickness; then after a time t, less than the time required for complete drying of this first coating,

photothermal radiometry is measured with an instrument and a given analysis method; then

we check the thickness of this layer by a destructive method for example, and if this thickness corresponds to the setpoint thickness (if it is not the case, the second step is repeated):

the setpoint thickness is replaced by this value resulting from the measurement by photothermal radiometry.

Thickness checks are then carried out by carrying out measurements by photothermal radiometry on the painted parts, by comparing the thickness measured with this new setpoint.

Note that for each coating layer, the thickness setpoint may be different. These setpoints also depend on the type of part, but also for the same part, the desired color and the location on the part.

Depending on the chemical compounds involved in the formulation of the deposited paints and on the basis of the thickness measurements collected, it is then possible to dispense with the calibration step (self-learning).

According to one embodiment, an instrument (1) is used comprising a measuring head (2) adapted (and possibly calibrated) to a given coating. So many instruments (1) are used as layers of coatings. These instruments (1) can be mounted on one or more articulated robots (5).

Preferably, a multi-head instrument (1) is used, the number of measuring heads (2) corresponding to the number of deposited coating layers.

According to one embodiment, the laser (3) and the infrared sensor (4) are placed at a distance of substantially between 5 and 20 cm from the coating.

The method according to the invention therefore uses an application of photothermal radiometry to measure the thickness of a coating on the surface of a part, in a non-destructive manner (preserving the integrity of the part and the coating), and without contact between the measuring device and the coating, so as not to damage the surface of the coating.

Preferably, a surface treatment is carried out before application of the first coating. This preparation treatment of the surface to be painted in the room consists essentially of a cleaning and activation of the surface, in order to facilitate the adhesion and the wettability of the surface to receive the primer.

There is known a method of treating a surface of a polymer object, in which a cleaning jet comprising carbon dioxide is sprayed onto the surface to be treated, and a flame is applied to the surface to be treated.

The carbon dioxide contained in the cleaning jet is usually in the form of particles, usually in the form of snow or flakes.

The application of the flame optimizes the surface tension and adhesion of the subsequently deposited paint to the surface. Indeed, the flame (which can reach 1800 ° C) allows the development of very reactive radicals and, thus, polar functions on the surface of the treated object promoting interactions with the coating applied to the surface.

According to an exemplary embodiment, the piece is loaded on a support, called swing or hanger, carried by a mast. The mast is placed on a moving means (conveyor), to move the piece at a given speed, and possibly stop, in front of means applying a surface treatment to the piece.

Thus, the piece is moved by the conveyor along the paint line, in front of the different equipment (sprayer, desolvation zone, drying ...).

In front of the different equipment, the room can be motionless and the equipment sweeps the room, or the room can be moving around the equipment. It should be noted that in order to carry out the measurements, the measuring apparatus must be stationary relative to the workpiece.

A first means for performing the step of preparing the surface to be painted (cleaning and activation). It is essentially a means of flaming.

The parts whose surface has been treated, are then successively arranged in front of an articulated robot to apply to them the three layers of coatings: for the application of the primer, for the application of the base, and finally for the application of the varnish. There may be a robot or several robots for each coating application, or a robot adapted to perform all applications.

These robots are generally six-axis articulated robots, so as to reach all the surfaces of the room, according to a programmed trajectory.

The robots and sprays are programmed to apply a defined thickness of coating at each point of the surface area to be treated.

The primer layer improves the conductivity and adhesion of the color layer. The base layer gives color to the room. The varnish layer is used to protect the base layer against external aggressions and to obtain the desired final appearance.

Finally, the piece passes in front of a heating means, so as to bake, that is to say to polymerize the varnish, before being cooled and then discharged.

According to another embodiment, the parts are mobile around robots capable of applying a treatment (flaming, application of a coating) which are immobile: the parts are carried by the arm of a mobile robot, and the area to be measured is then presented in front of the measuring head of the measuring instrument.

The invention also relates to an installation (6) for painting a motor vehicle part (P) comprising at least one part support (7), called a swing or hanger, carried by a mast (8), and a moving means (9). ) to scroll the mast in front of at least one application means (10) of a coating (R) on at least one surface of the workpiece (8). The installation (6) comprises at least one photothermal radiometry measuring instrument (1) for measuring the thickness (E) of said coating (R).

According to one embodiment, the installation (6) comprises an application means (10) for three different coatings on at least one surface of the part, and a photothermal radiometry measuring instrument (1) comprising three measurement heads. (2), each head (2) being adapted to measure the thickness of a different coating.

According to another embodiment, the installation (6) comprises a means of application (10) of three different coatings on at least one surface of the part (P), and three measuring instruments (1) by photothermal radiometry comprising a measuring head (2), each instrument (1) being adapted to measure the thickness of a different coating.

Claims

1. A method of painting a motor vehicle part, wherein the part is introduced into a first application zone of a first coating, then introduced into a second desolvation zone of the first coating, characterized in that one carries out the following steps:

i. applying a first coating to at least one area of the workpiece in said first area; then at the exit of the second zone and before any other treatment of the part

ii. a measurement of the thickness of the first coating is carried out in at least one location of the zone, by means of a photothermal radiometry measurement.

The method according to claim 1, wherein steps i) and ii) are repeated for different coatings, step i) being performed in coating application areas, and step ii) being performed prior to applying the coating. next coating, at the exit of the second zone.

3. Method according to claim 2, wherein three coatings are applied, a primer, then a base, then a varnish.

4. Method according to one of the preceding claims, wherein the thickness of the first coating obtained is compared by means of a photothermal radiometry measurement at a coating thickness.

5. Method according to claim 4, wherein the thickness of the coating setpoint is defined by a calibration method according to the photothermal radiometry measuring instrument used and the analysis method used by said instrument.

6. Method according to one of the preceding claims, wherein the second desolvation zone also comprises means for drying and cooling the first coating.

7. Method according to one of the preceding claims, wherein is adapted, according to said measurement by photothermal radiometry, the paint application parameters.

8. Installation for painting a motor vehicle part comprising at least one workpiece support, and a movable means for scrolling the support in front of at least one means for applying a coating on at least one surface of the workpiece, characterized in that the installation comprises at least one photothermal radiometry measuring instrument for measuring the thickness of said coating.

Plant according to claim 8, comprising means for applying three different coatings on at least one surface of the part, and a photothermal radiometry measuring instrument comprising three measuring heads, each head being adapted to measure the thickness of the a different coating.

Installation according to claim 8, comprising a means for applying three different coatings on at least one surface of the part, and three photothermal radiometry measuring instruments comprising a measuring head, each instrument being adapted to measure the thickness of the a different coating.