FR2732908A1 - METHOD AND DEVICE FOR CHANGING A PRODUCT IN A PLANT FOR SPRAYING COATING PRODUCTS - Google Patents

Abstract

Process for changing the product in an installation for spraying coating products comprising a duct (2) for supplying a projector (3) with said product, of the type comprising a phase of filling said duct with a new product characterized in that that the downstream end of said duct is kept closed (T, P) during said filling phase. The device comprises an arrangement of valves (B, B ', T, P) arranged so as to allow the filling of the pipe by isolating the downstream end of the pipe from the outside and a flowmeter (4) able to determine the quantity of product introduced into the duct during a distribution phase subsequent to the filling phase.

Description

The invention relates to a method and a device for changing a product.

  in an installation for spraying coating products. It considerably limits the loss of paint during color changes. In the coating product change devices of the prior art, when it is necessary to change the product, air and solvent trains are injected into or downstream from a coating product change block or an emulsion of air and solvent in the duct connecting this block to a projector. We pour the mixture

  resulting from this cleaning in a purge, that is

  say, most often a reprocessing system.

  then comes a second phase, called the filling phase of this duct with the new coating product to be used, during which the evacuation of the remains of the air / solvent mixture takes place and the filling of the duct to the sprayer with the new product. This phase lasts approximately 8 to 10 seconds. The purge valve which connects the connection conduit between the product change block and the projector on the one hand and the purge bypass on the other hand is then closed. The head valve which is located at the base of the projector is then opened for an instant, for example 500 ms, to initiate the discharge of new product by the projector. The projector is now ready to operate. The duration of the phase of filling the duct with the new product must be determined with precision: - if it is too short, that is to say if it is completed before the front of new product in the duct has reached the purge valve, air and solvent remain in the pipe and this mixture is sprayed at the start of sprayer operation, which affects the quality of the coating. - if it is too long, a quantity of new product unnecessarily flows into the drain while

the duct is already full.

  In practice, the latter solution is chosen to avoid any danger of bad spraying due to a lack of paint. In addition, the duration of this phase is difficult to determine because it depends on the product supply pressure, which varies from one supply circuit to another, that is to say a product of coating to another, and its viscosity which may itself depend on the ambient temperature. However, the higher the supply pressure, the greater the quantity of product lost. Likewise, this quantity increases with the fluidity of the product used. Furthermore, the filling rate of the duct must be high because a change of coating product must take place in the shortest possible time interval, this high flow rate is directly proportional to the amount of product lost during the change of product. A flow meter, of the type usually placed on the projector supply pipe and which allows the control of product flow during the spraying phases, cannot be used to regulate the quantity of product which fills the pipe because, during during the filling phase, this flow rate reaches several liters per minute while the operating range of such a flow meter is between a few tens and a few hundred cubic centimeters per minute. An uncontrolled quantity of coating product is thus discharged in purge each time the change of

coating product.

  To overcome these faults, it has been envisaged to introduce, near the purge valve, a sensor, for example an optical or capacitive sensor to detect the arrival of paint at the level of the purge. The sensor emits a signal which is interpreted to close the purge valve: this eliminates

  errors due to the variations stated above.

  However, the response times of these sensors and valve control systems are around 500 ms. Taking into account the product flow during the filling phase, between the moment when the sensor detects the front of new product and the closing of the purge valve, approximately 30 cc of paint has flowed into the purge. Indeed, the response time of the sensor and the activation time of the valve add up to give a reaction time of the system close to one second. In addition, these reaction times vary from one valve to another and from one installation to another, which implies uncontrolled variations in the quantities lost. It also turns out that these sensors are fragile and that

  their industrial use is not very reliable.

  The invention makes it possible to reduce and control the quantity of product lost during product changes without requiring fragile sensors and / or

unreliable.

  It relates to a process for changing a product in an installation for spraying coating products comprising a supply duct for a projector with said product, of the type comprising a phase of filling said duct with a new product, characterized in that the downstream end of said duct is kept isolated during said filling phase. Thanks to the invention, no amount of coating product is lost during the filling phase. The product flow during the filling phase can be significant and can exceed 3 to 4 liters per minute, which reduces the duration of this

  phase, without additional loss of product.

  The filling phase is followed by a separate phase, called the distribution phase, during which the flow rate of coating product can be controlled and reduced and which includes a step of evacuating the air accumulated in the downstream part of the duct and a stage of priming the projector. Thus, filling can take place quickly while a flow meter can be used to precisely control the purge and overhead valves during the next phase, when the flow is lower. The method of the invention can be implemented with known installations and installed at the cost of

small changes.

  According to a variant of the invention, the flow meter is not active during the distribution phase. The management of valve openings and closings is

performed by time delay.

  The invention also relates to a product change device in an installation for spraying coating products comprising a supply duct for a projector, characterized in that it comprises an arrangement of valves arranged so as to fill said duct, when changing the coating product, insulating

said duct of said projector.

  This device makes it possible to implement the method

of the invention.

  The invention will be better understood and other advantages thereof will appear more clearly in the light

  of the description which follows of two modes of

  production of a coating product projection device in accordance with its principle given solely by way of example and with reference to the appended drawings in which: - Figure 1 is a block diagram of a device for supplying a coating product spraying installation according to the invention, - Figure 2 is a diagram similar to Figure 1 of a variant of the invention, - Figure 3 is a diagram of the opening phases and

  for closing the valves of the device in FIG. 2.

  The device of FIG. 1 comprises a block for changing the coating product 1 connected by a supply duct 2 to a projector 3, for example electrostatic. Block 1 comprises several valves C1 to Cn each corresponding to a coating product, for example to a color, a solvent valve S and an air valve A. A valve T, called the head valve, makes it possible to supply or not the projector 3 with the coating product present in the duct 2. A flow meter 4 is connected to a control unit not shown and comprising a computer which controls the entire installation and, in particular, the valves A, S , T and C1 to Cn. The flowmeter can be gear, electromagnetic or any other suitable type. A pressure regulator 5 is inserted in the duct 2 near the head valve T. A valve B, controlled by the computer, is arranged at the upstream end of the

conduit 2.

  The operation is as follows: At the end of a projection phase, when it is necessary to change the product, the valves of colors C1 to Cn are closed and the valves of air A and of solvent S are alternately open. Trains of air and solvent are thus sent into the duct 2 to clean it. The head valve being open, these air and solvent trains pass through the projector 3 and are discharged into an appropriate place in the installation's coating cabin, for example a grating. When the duct 2 and the projector 3 are cleaned, that is to say in practice after a few seconds of cleaning, the valves A and S are closed. The head valve T is then closed to isolate the projector 3 from the duct 2. The valve Ci corresponding to the coating product chosen for the next spraying phase is then opened and a filling phase takes place during which the chosen coating product is poured into the conduit 2, the downstream end of which is isolated from the headlight since the valve T is closed. The volume of air present in the duct 2 at the start of the filling phase is compressed at the downstream end of the duct, the compressed air pressure being equal to the pressure for supplying the coating product, ie in an installation industrial between 6 and 12 bar. The filling flow can be high, of the order of several liters per minute, because the flow meter 4 is not active during this phase, the filling taking place until the air and product pressures s 'balance. No coating material was spilled into the cabin during the

filling phase.

  After the end of the filling phase, there is a distribution phase during which the compressed air in the duct 2 is evacuated through the projector 3 thanks to the opening of the head valve T. The solvent residues pushed by the front of new product are also evacuated in the same way. In practice, the volume of air leaves duct 2 almost instantaneously, then a mixture comprising the solvent residues and part of the new coating product is pushed out of duct 2 by the quantity of new product introduced into the duct during the allocation stage. The projector

  is then started with the new product, i.e.

  say that new coating product is introduced into the paint injector of the projector so that the projector is operational from the start of the next projection phase. During the distribution phase, the flow rate in the duct 2 is of the same order of magnitude as during the spraying phases, for example 200 cubic centimeters per minute, since the regulator 5 establishes a sufficient pressure drop. The flow meter 4 is thus able to provide a representative signal which can be integrated by the control unit to determine the volume of product entering the conduit. The control unit closes the head valve as soon as a quantity

  predetermined product has entered the conduit.

  The quantity of product which must enter the duct can be calculated as that equal to the volume Va of the compressed air in the duct, plus a safety margin determined by the operator, this safety margin also serving to evacuate the remains. any solvent. This volume Va is related to the total volume of the conduit Vb by the relationship Pa * Va = Pb * Vb where Pa is the pressure of the compressed air and Pb is the atmospheric pressure. Thus, the control unit closes the head valve T as soon as the volume Va has been detected by the flow meter 4 and the amount of product lost during this evacuation step is zero or almost zero, at the response time of l control unit, which is a few milliseconds, which, taking into account the flow rate considered during this step corresponds to a maximum loss of 2 or 3 cc, this loss including the remains

  solvent pushed by the front of new product.

  The same precision is obtained during the priming step which follows the evacuation step since the product flow rate is of the same order of magnitude, the

  flow meter capable of fulfilling its function as a meter.

  According to a variant of the invention, the valve B is closed during the priming step. No new coating product is allowed in the duct, but the internal volume of the duct decreases during this step. For example, a pressure pulse is applied to the control input of the pressure regulator 5 so that the coating product contained in the chamber of the regulator which is part of the conduit is expelled from the latter. The product does not rise upstream of the pipe which is closed by the valve B but flows downstream of the pipe where the valve T is open. By a judicious choice of the internal volume of the regulator 5, it is thus possible to start the projector with a small quantity, of the order of 4 to 5 cc, of product. This is particularly advantageous since it is difficult to give an opening order and a closing order to the valve T in order to allow a very small quantity of product to pass. In fact, for safety reasons, this valve is generally a pneumatic valve whose response time is longer than that of a solenoid valve. Alternatively, instead of using the regulator 5, one can choose to crush the conduit 2 on a

  length corresponding to the volume of air to be evacuated.

  According to a variant of the invention, it is not necessary to use the flow meter 4 as a meter during the distribution phase. In fact, the valves T and / or P can be controlled by time delay, that is to say kept open for a determined time interval corresponding to the estimated evacuation of the compressed air and of the solvent residues. At equivalent precision, the uncertainty on the quantities of product poured through the projector is divided in the ratio of the volumes Va and Vb which corresponds to the inverse ratio of the pressures. If the product is supplied under a pressure of 6 bars, the uncertainty is therefore divided by 6 (ratio 6 to 1), which represents a substantial saving. Although simpler, this solution does not overcome the uncertainties associated with variations in viscosity of

coating products.

  The device of FIG. 2 comprises certain elements identical to that of FIG. 1, the references of which are the same. In this device, a purge bypass 8 is provided in the direction of a recovery system 9. A purge valve P makes it possible to open or close the purge bypass

  according to the instructions given by the control unit.

  The step of evacuating the compressed air in the duct 2 takes place by opening the valve P. a small amount of product, of the order of a few

  cc, being discharged into the recovery system 9.

  The valve B 'corresponding to the valve B is a valve provided with a calibrated flow of small diameter which allows the flow of a low product flow

  when the valve is in the closed position.

  The operation is as follows: The cleaning phase is similar to that described with reference to FIG. 1. During the distribution phase, the valve B 'is closed, the purge valve P is open while the head valve T stays closed. Due to the calibrated flow of valve B ', the flow rate of coating product is limited to a lower value during the distribution phase than during the filling phase, which allows the flow meter 4

  to perform its counting function, in line 2.

  Compressed air, solvent residues and a small amount of product are discharged into the recovery system 9. The end of the evacuation step is calculated by the control unit according to the volume of product introduced into the conduit 2. The valve P is then closed and the priming step takes place during which, the valve T being open, the new coating product enters the projector 3. The flow meter operates as in the previous step and the same accuracy can be achieved. At the end of the distribution phase, the valve B 'is opened again to allow

  product flow without pressure drop.

  According to a variant of the invention, the function of the valve B 'can be achieved by means of a valve identical to the valve B in parallel with which is

  connected a calibrated branch of small diameter.

  Alternatively, the function of valve B ′ could be fulfilled by a pressure regulator similar to regulator 5, the pressure drop of which would be controlled to be greater during the distribution phase than during the filling phase, which would decrease the flow in the duct 2 and allows the flow meter 4 to fulfill its function of

  counting under acceptable conditions.

  It can be noted that the variations in viscosity and / or temperature of the products have little influence on the quantity of product poured into the purge during the changes. Indeed, the filling phase will be more or less long but no product will be lost during this phase. During the distribution phase, the introduction of the product is controlled in volume and

  variations in viscosity have no effect.

  If the product supply pressure varies, for example from 6 to 8 bars, the volume of compressed air in the duct 2 will be lower at the end of the filling phase. The distribution phase takes place as before and, if the computer receives the information of the pressure change, the volume corresponding to the closing of the valve P is adjusted by calculation. If the computer does not receive the information corresponding to the pressure increase, which will be the case, for example, in a simplified installation, the quantity of coating product discharged to the drain will be greater by a volume corresponding to the difference compressed air volumes, at 6 and 8 bars. this quantity remains much lower than that obtained with

prior art installations.

  The method and the device of the invention can be used regardless of the type of coating product, water-soluble or solvent-based and regardless of the type of projector, electrostatic or

  no, pneumatic or rotating bowl.

  The invention also applies to the case where the head valve and the purge valve are integrated in the headlight. The evacuation stage and the stage

  priming are then simultaneous.

Claims (9)

  1 - Method for changing a product in an installation for spraying coating products comprising a conduit (2) for supplying a projector (3) with said product, of the type comprising a phase of filling said conduit with a new characterized product in that the downstream end of said conduit is kept isolated (T, P) for said filling phase.   2 - Method according to claim 1 characterized in that a distribution phase takes place after said filling phase, said distribution phase comprising a step of evacuating the air contained in said conduit (2).   3 - Method according to claim 2, in an installation comprising a bypass (8) purge (9), characterized in that the evacuation of the air contained in said conduit (2) takes place through said purge bypass.   4 - Method according to claims 2 or 3   characterized in that said distribution phase further comprises a step of initiating said   coating product projector (3).   5 - Method according to claim 4 characterized in that the internal volume of said conduit is reduced during the priming step, in particular by action on the volume of the chamber of a pressure regulator   (5) inserted in said conduit (2).   6 - Method according to one of claims 2 to 5   characterized in that the flow rate of coating product is limited to a lower value during the distribution phase than during the filling phase.   7 - Method according to one of claims 2 to 6   characterized in that the duration of the stages of said distribution phase is controlled from a signal emitted by a flow sensor (4) located in said conduit. 8 - Device for changing the product in an installation for spraying coating products comprising a pipe (2) supplying a projector (3), characterized in that it comprises an arrangement of valves (B, B ', T, P) arranged so as to allow the filling of said duct, during changes of coating product, by insulating the downstream end of said conduit.   9 - Device according to claim 8 characterized in that it further comprises a bypass (8) purge (9) provided with an arrangement of valves (P) arranged so as to allow the filling of said conduit (2), during changes in coating product, by isolating said conduit from said bypass purge.    - Device according to one of claims 8 and 9   characterized in that it comprises a flow sensor (4) in said conduit (2), said sensor being able to supply a signal representative of the quantity of product introduced into said conduit, said signal being used to control said arrangement of valves (B, B ', T, P).   11 - Device according to one of claims 8 to 10   characterized in that it comprises a flow regulating means (B ') capable of reducing the flow rate of coating product when said valves (P, T) for insulating the duct with respect to said projector (3) or   said bleed bypass (8) (9) are open.