Patent Publication Number: US-10307773-B2

Title: Coating system with an ultrasonic head

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a coating system for coating a workpiece with a liquid coating product. 
     BACKGROUND OF THE INVENTION 
     Ultrasonic liquid atomization has been used so far for spraying a liquid coating product onto a workpiece to be coated. In “Ultrasonic Liquid Atomization Theory and Application” (Partridge Hill Publishers—1998 and 2006) Harvey L. Berger considers the application of electrostatics to ultrasonic liquid atomization and explains that implementing electrostatics onto ultrasonic atomization would theoretically bring a very high efficiency to the control of the spray application, even with voltage much lower than those used in conventional electrostatic spraying. 
     Since then, some applications of ultrasonic atomization with electrostatics have been considered. For instance, in a paper “Deposition of CuInS 2  films by electrostatic field assisted ultrasonic spray pyrolysis” (Solar Energy Materials and Solar Cells 95 (2011) 245-249), Dong-Yeup Li and JunHo Kim discloses a case where a liquid to be sprayed is excited by an ultrasonic probe in order to be converted into an aerosol which is conveyed by a carrying gas towards an outlet of a tube, in order to be used for spray pyrolysis deposition. Such a system does not allow precisely controlling the flow rate nor the density of the aerosol and, in its path between the bath where it is formed and the outlet of the tube, the aerosol might agglomerate, so that the size of the droplets used for coating an object cannot be accurately controlled because of coalescence generally observed between droplets. The coating might be rough or non homogeneous and some defects may be generated on the coated surface. 
     In “Deposition of Ni-CGO composite anodes by electrostatic assisted ultrasonic spray pyrolysis method” (Materials Research Bulletin 42 (2007) 1674-1682), Jing-Chiang Chen et al. consider creating an aerosol and conveying it in a glass tube toward an nozzle facing a copper plate which supports a substrate to be coated. Here again, there is a risk of agglomeration of the droplets of the aerosol. 
     These academic works would not be easy to use in an industrial environment because the coating deposition is highly dependent on many factors, such as the geometry and length of a tube conveying an aerosol. 
     On the other hand, in a known system, the electrostatic field is generally created between an electrode and the object to be coated, this electrostatic field having a field line distribution which is not optimized for the workpiece to be coated. Under such circumstances, a substantial part of the atomized product might end up outside of the surface to be coated, which is detrimental in terms of pollution and from an economic stand point. 
     SUMMARY OF THE INVENTION 
     The invention aims at solving these problems with a new coating system which makes use of the versatility of an ultrasonic head and which allows optimizing the distribution of electrostatic field lines, thus optimizing the coating efficiency. 
     To this end, the invention concerns a coating system for coating a workpiece with a liquid coating product, this coating system including:
         an ultrasonic spray head for generating droplets of coating product,   an electrode for generating an electrostatic field between the electrode and the ultrasonic spray head and   a high voltage generator connected to the electrode for supplying the electrode with high voltage.       

     Thanks to the invention, the electrode allows precisely controlling the electrostatic field line distribution towards the workpiece, thus the path of the coating product droplets towards the workpiece. More precisely, the electrode acts as a conformator for an electric field which guides the droplets towards the workpiece to be coated 
     According to further aspects of the invention which are advantageous but not compulsory, the coating system might incorporate one or several of the following features, taken in any technically admissible combination:
         The shape of the electrode is configured on the basis of the geometry of the workpiece. Thanks to this aspect of the invention, the shape of the electrode can be customized according to the actual geometry of the workpiece to be coated, which allows building the electrostatic field lines closely around the workpiece, thus avoiding, or limiting to a great extent, an overspray phenomenon.   An edge of the electrode is an image of the contour of the workpiece, as seen from the ultrasonic spray head, preferably an exact image of this contour.   The coating system includes a metering device for feeding the ultrasonic head with liquid coating product in controlled amount, in particular in the form of a pump with a movable piston. Alternatively, other types of pumps can be used as metering devices.   The electrode supports the workpiece and is in contact with the workpiece.   The electrode is shaped according to the workpiece geometry and supported by a base part which includes the high-voltage generator.   The base part insulates the workpiece from the ground potential, preventing any electrical leak from the electrode.   The base part defines an area configured for accommodating electrodes of different shapes.   An electrical contact member is located in the area for connecting the high voltage generator to an electrode accommodated in the area.   The electrical contact member is movable along an axis non parallel to an electrode accommodated in the area and biased towards this electrode.   The high voltage generator is integrated in a sub-module which includes also a discharge resistor and connection means, said sub-module defining two housings, where the high voltage generator and the discharge resistance are respectively received, and a connection housing, where a connector is received to connect the high voltage generator and the discharge resistor.   The electrical contact is supported by, and electrically connected to, the connector received in the connection housing.   The electrode is shaped to be at least partially surrounded by the workpiece.   The electrode is rotatable around its longitudinal axis.   The electrode is secured to the ultrasonic spray head and configured to charge the droplets by ionization.   The electrode is provided with spikes spread around a discharge nozzle of the ultrasonic spray head with a distribution based on the geometry of the workpiece.   An orientation angle of the spikes with respect to a central axis of the ultrasonic spray head is adjustable on the basis of the geometry of the workpiece and/or of a distance between the electrode and the workpiece.   The electrode is formed by the workpiece.   The ultrasonic head is provided with an air ejection unit configured to deliver a jet of shaping air around the droplets leaving the ultrasonic head.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on the basis of the following description which is given in correspondence with the annexed figures and as an illustrative example, without restricting the object of the invention. In the annexed figure: 
         FIG. 1  is a perspective view of a coating system according to the invention; 
         FIG. 2  is a perspective view, on a larger scale, of some components of the system of  FIG. 1 , in a first configuration of use; 
         FIG. 3  is a perspective view similar to  FIG. 2  when the system is in a second configuration of use; 
         FIG. 4  is a schematic representation of the wiring used for controlling an electrode module of the system of  FIGS. 1 to 3 ; 
         FIG. 5  is a perspective view of the electrode module in a first configuration of use corresponding to the configuration of the system on  FIG. 2 ; 
         FIG. 6  is a cut-away view of the electrode module in the configuration of  FIG. 5 ; 
         FIG. 7  is an enlarged partial cut view along line VII-VII on  FIG. 6 ; 
         FIG. 8  is an exploded view of the electrode module; 
         FIG. 9  is a cut-away view similar to  FIG. 6  when the electrode module is in a second configuration of use; 
         FIG. 10  is a cut away view similar to  FIG. 6  when the electrode module is in a third configuration of use corresponding to the configuration of the system on  FIG. 3 ; 
         FIG. 11  is a schematic representation of a coating system according to a second embodiment of the invention; 
         FIG. 12  is a front view of a coating system according to a third embodiment of the invention; 
         FIG. 13  is a view in the direction of arrow A 13  on  FIG. 12 ; 
         FIG. 14  is a view similar to  FIG. 13  when the coating system of this third embodiment is in another working configuration; 
         FIG. 15  is a partial perspective view of a coating system according to a fourth embodiment of the invention; and 
         FIG. 16  is a detailed cut-away view of the electrode module of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF SOME EMBODIMENTS 
     The coating system  2  represented in  FIGS. 1 to 7  includes an enclosure  4  which defines an internal volume V 4  closed by a door  6  provided with a transparent window  8  made of mineral or organic glass, which allows checking volume V 4  through door  6 . 
     Alternatively, door  6  is not transparent. 
     Enclosure  4  forms a cabin for a coating process to be implemented with coating system  2 . 
     Coating system  2  also includes a control cabinet  10  which comprises, amongst others, a programmable logic controller or PLC  12 . On  FIG. 1 , the front panel of control cabinet  10  has been removed in order to show PLC  12 . 
     Coating system  2  includes an ultrasonic spray head  14  which is supported by a 3 axis Cartesian robot and which has its longitudinal axis X 14  oriented vertically. Ultrasonic head is directed downwardly towards a workpiece W 1  to be coated which lies underneath ultrasonic spray head  14 . 
     In another embodiment, the spray head can be handled by a six axis robot allowing that axis X 14  presents an angle with the vertical for an optimal position of the spray head in front of the workpiece surface to be coated, especially for volumic/3D workpieces. 
     The ultrasonic head frequency can be chosen in the whole ultrasonic frequency range from 20 kHz to 10 MHz, and for instance at the following frequencies: 25 kHz, 35 kHz, 48 kHz, 60 kHz, 120 kHz, 180 kHz, 250 kHz. The size of the droplets generated by ultrasonic spray heads, expressed in Number Median Diameter (NMD or DN0.5) are typically given as follows: 69 μm (25 khz), 50 μm (35 kHZ), 38 μm (48 kHZ), 32 μm (60 kHZ), 18 μm (120 kHZ), 12 μm (180 kHZ), 8 μm (250 kHZ). 
     Ultrasonic spray head  14  may be of any commercially available type. 
     A pipe  18  feeds ultrasonic spray head  14  with a liquid to be atomized and a non-represented vibrating member integrated within ultrasonic spray head  14  is actuated to atomize this coating product when workpiece W 1  is actually located under ultrasonic spray head  14 . On  FIG. 2 , arrows F 2  represent the flow of droplets of atomized coating product coming out of ultrasonic spray head  14  and directed towards workpiece W 1 . 
     Pipe  18  is fed with coating product from a metering pump  19  visible on  FIG. 1  only and represented as a syringe. Preferably, metering pump  19  has a movable piston. Alternatively, other types of metering devices can be used to feed ultrasonic spray head  14  via pipe  18 , such as a gear pump. Using a metering device to feed ultrasonic spray head  14  allows a precise control of the amount of coating product, thus a precise control of flow F 2  of droplets. 
     In order to better control the shape of the spray of droplets exiting ultrasonic spray head  14 , a shaping gas unit  20  is mounted around the lower extremity of ultrasonic spray head  14 . This shaping gas unit  20  is fed with air via a pipe  22  and expels a flow of air, represented by arrows F 4  on  FIG. 2 , around the flow of droplets F 2 . This avoids that this flow diverges radially with respect to axis X 14 . Alternatively, a gas different from air can be used in shaping gas unit  20 . 
     Ultrasonic spray head  14  is movable in three directions of the space with a 3 axis Cartesian robot represented by a guide rail  16 . 
     In order to enhance the effectiveness of the coating of workpiece W 1 , an electrostatic field E is generated between ultrasonic spray head  14  and an electrode. 
     To this end, an electrode module  30  is located within volume V 4 , together with ultrasonic spray head  14  and the 3 axis Cartesian robot. The electrode module  30  supports workpiece W 1 . In other words, workpiece W 1  lies on electrode module  30 . Workpiece W 1  is electrostatically charged by contact with electrode  32 A. 
     This electrode module  30  has a flat upper surface constituted by an electrode  32 A made of a sheet of electrically conductive material, in particular a metal, such as a ferrous metal (steel, stainless steel . . . ) or a non-ferrous metal (aluminum, copper . . . ) and their alloys. 
     This electrode  32 A lies on a base part  34  of electrode module  30 , which is made of an electrically insulating material, such as a synthetic material, for instance polypropylene (PP), polyoxymethylene (POM), polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), polyvinyl chloride (PVC). Base part  34  insulates workpiece W 1  and electrode  32 A from the ground potential, preventing any electrical leak from the electrode. 
     As shown on  FIG. 4 , electrode module  30  is controlled by a high voltage controller  36  located in control cabinet  10 . More precisely, high voltage controller  36  is connected to a power grid via a first cable  38  and to electrode module  30  via a second cable  40 . A third ground cable  42  connects electrode module  32  to enclosure  4 , which is grounded. 
     On the other hand, a control cable  44  allows PLC  12  to control high voltage controller  36  according to a pre-established control sequence and in a way consistent with applicable safety rules. 
     Ultrasonic spray head  14  is grounded via a non represented grounding cable and a non represented ultrasonic connector, which can be considered as “double grounding”. Actually, this is important insofar as the vibrating member of ultrasonic spray head  14  is an electrical device whose operation could be disturbed if it were submitted to a high voltage. Thus, the general layout of coating system  2 , where high voltage is applied at the level of electrode module  30 , not at the level of ultrasonic spray head  14 , is advantageous. 
     When electrode module  30  is supplied with an electrical current via second cable  40 , electrostatic field E is generated between ultrasonic spray head  14  and electrode  32 . Electrostatic field lines L extend between ultrasonic spray head  14 , on the one hand, and electrode  32  and workpiece W 1 , on the other hand. 
     Electrode  32 A is charged, with a high voltage in the range between 0.1 kV and 100 kV, and preferably between 5 kV and 30 kV. Assuming the high voltage is negative, the droplets of atomized coating product exiting ultrasonic spray head  14  are charged positively by influence, so that they follow the field lines L towards electrode  32 , thus towards workpiece W 1  which lies on top of electrode  32 A. 
     In the example of  FIG. 2 , workpiece W 1  is a flat piece, for instance a solar cell glass cover. On  FIG. 5 , workpiece W 1  is represented in chain-dotted lines in order to fully show electrode  32 A. 
     In such a case, the shape of electrode  32 A is flat and rectangular, with an edge having substantially the same shape as the edge of workpiece W 1 , as shown on  FIG. 5 . 
     For instance, electrode  32 A can be a rectangle with the same proportions as the rectangle defined by workpiece W 1  and with a surface area S 32  comprised between 80% and 5000%, preferably between 100% and 125%, of the surface area SW of the workpiece W 1 . 
     More precisely, surface area S 32  can be smaller than or equal to surface area SW, with the ration S 32 /SW in the range between 0.8 and 1. Alternatively, surface areas S 32  and SW can be equal or substantially equal, with ratio S 32 /SW in the range between 0.95 and 1.05. According to another approach, surface area S 32  may be larger than or equal to surface area SW, with ratio S 32 /SW in the range between 1 and 50. 
     Under such circumstances, the portion of electrode  32 A which is “visible” from ultrasonic spray head  14  around workpiece W 1  can be reduced like in the configuration of  FIG. 2 , so that all the field lines L end up and close around workpiece W 1 . Thus, droplets coming out of ultrasonic spray head  14  are efficiently directed by electrostatic field E towards workpiece W 1 . 
     As can be derived from  FIGS. 5 to 10 , base part  34  defines a top cavity  46  which forms an area with a length L 46  and a width W 46  corresponding to the maximum dimensions of an electrode  32 A and any other electrode to be used in electrode module  30 . 
     In the configuration of  FIGS. 2, 5   6  and  8 , electrode  32 A has the maximum possible dimensions, so that it fills cavity  46  up. A notch  37  allows accessing laterally cavity  46  to exert a lifting effort on electrode  32 A when it must be separated from base part  34 , e.g. with a tool such as the tip of a screwdriver or even the finger tip. 
     Base part  34  also defines a recess  48  which accommodates a sub-module  50  adapted to convert the current received from high voltage controller  36  via second cable  40  into a high voltage to be applied to electrode  32 . 
     Sub-module  50  includes a high voltage generator  52  made by a cascade of diodes, according to a known technique in the field of electrostatic spraying, in particular in hand guns. Sub-module  50  includes an insulative body  54  which defines a first elongated housing  56  extending along a first axis X 56 . Body  54  also defines a second elongated housing  58  which extends along a second axis X 58  parallel to axis X 56 . Actually, axes X 56  and X 58  can be non-parallel with any orientation. In a preferred configuration, axes X 56  and X 58  are substantially parallel, that is converge or diverge with an angle between them of less than 30°, so that they open out on the same side of body  54 . This configuration makes a very neat connection since the second supply cable  40  may bring together the high voltage power and the grounded reference. A connection housing  60  connects housings  56  and  58  perpendicularly to axes X 56  and X 58 . Thus, housings  56 ,  58  and  60  together define a U shaped volume for accommodating the high voltage supply means of sub-module  50 . 
     A discharge resistor  62  is located within housing  58 , together with a connecting rod  64 . On the other hand, a connector  66  is accommodated within housing  60 . 
     A spring  68  is interposed between high voltage generator  52  and connector  66 . Another spring  70  is interposed between connector  66  and resistor  62 . Thus, connector  66  connects high voltage generator  52  and discharge resistor  62  with the help of springs  68  and  70 . 
     A ground plate  72  is located at one end of body  54  where housings  56  and  58  open out, opposite from housing  60 . This allows connecting high voltage generator  52  and connecting rod  64  to the ground via third ground cable  42 , whereas high voltage generator  52  is also connected to second cable  40  via individual conductors  40 A,  40 B and  40 C. For the sake of clarity, connectors  40 A,  40 B and  40 C are represented only on  FIG. 8 . 
     Connector  66  bears a contact member  74  which goes through an opening  76  of a wall  78  of base part  34  separating cavity  46  from recess  48 . Thus, when sub-module  50  is mounted within recess  48 , contact member  74  is flush with wall  78  or protrudes into cavity or area  46 , so that it can apply to any electrode installed within cavity or area  46  a high voltage originating from high voltage generator  52 . 
     As shown on  FIG. 7 , contact member  74  is formed by an electrically conductive stud housed in a vertical bore  67  of connector  66 . Connector  66  is immobilized within wall  78  by an O-ring  79 . The vertical position of contact member  74  within bore  67  can be adjusted along the central axis X 67  of bore  67 . A spring  69  is advantageously provided at the closed end of bore  67 , which biases stud against electrode  32 A. 
     As can be derived from  FIGS. 5, 6, 9 and 10 , electrodes with different geometries can be used with base part  34  and sub-module  50 . 
     For example, the rectangular electrode  32 A used in the configuration of  FIG. 2  for a large workpiece W 1  of a rectangular shape can be replaced with an electrode  32 B having a flat and rectangular shape adapted to a workpiece W 2  with smaller dimensions, such as a screen of a cellphone, as shown on  FIG. 9 . Provided that electrode  32 B is positioned in the center part of cavity  46 , so that it contacts contact member  74 , this electrode is also suitable for creating an electrostatic field with ultrasonic spray head  14 , so that the operation described with respect to  FIG. 2  can be obtained an electrostatic field lines close on electrode  32 B around workpiece W 2 . 
     An electrode  32 C can also be used in case a workpiece W 3 , in the form of a disc or a portion of a sphere, is used, as shown on  FIGS. 3 and 10 . For example, the workpiece W 3  can be an optical lens to be coated with different coating layers such as a hydrophobic layer, a top coat protective layer, a hard coating or even anti-reflective coating layers. If workpiece W 3  is a disc, electrode  32 C is flat. If workpiece W 3  is in the form of a portion of a sphere, electrode  32 C is preferably curved or warped to conform to the shape of workpiece W 3 . In this second case, electrode  32 C may protrude upwardly with respect to cavity  46 , which still forms an area for accommodating electrode  32 C. 
     On  FIGS. 9 and 10 , for easier understanding, the global shape of workpieces W 2  and W 3  and the global shape of electrodes  32 B and  32 C are shown partly in dotted lines. 
     As shown on  FIG. 3 , the electrostatic field E established between electrode  32 C and ultrasonic spray head  14 , which is grounded, gives rise to field lines L which extend between items  14  and  32 C, so that the droplets leaving ultrasonic spray head  14  in the form of flow F 2  are guided by these field lines towards workpiece W 3 . 
     In this example, the ratio of the respective diameters of workpiece W 3  and electrode  32 C can be chosen between 0.8 and 2. 
     Other shapes can be selected for the electrode placed on area  46 , depending on the actual geometry of the workpiece to be treated. For instance, a triangular electrode or a polygonal electrode, with more than four sides, can be manufactured in order to follow the contour of the workpiece to be treated which can be polygonal or rounded, in particular with an oval shape. Also, a tridimensional or curved shape design of the electrode can be chosen. For instance, a hemispherical or paraboloid electrode can be designed in order to support a hemispherical or paraboloid optical lens. By extension, any volumic regular or irregular shape electrode can be chosen depending on the geometry of the workpiece to be treated. 
     Actually, the edge of the electrode  32 A,  32 B,  32 C or equivalent is advantageously an image of the contour of the workpiece, as seen from ultrasonic spray head  14 . In other words, the edge of the electrode can be defined so as to evenly distribute the droplets around the contour of the workpiece, and increase the transfer efficiency of the product to the workpiece. 
     Advantageously, the edge of the electrode  32 A,  32 B or  32 C is an exact image of the contour of the workpiece, where “exact image” means that the difference between an area of a surface bordered by the edge of the electrode and an area bordered by the contour of the workpiece is less than 10% of the area bordered by the contour. 
     On the other hand, the geometry of the electrode  32 A,  32 B or  32 C can be adapted by increasing the exposed surface of the electrode in this region, the exposed surface of the electrode being the surface which is visible around the workpiece from ultrasonic spray head  14 , so as to precisely control the coating profile on the edge of the workpiece. 
     In the second, third and fourth embodiments of the invention represented on  FIGS. 11 to 15 , the parts similar to the ones of the first embodiment bears the same reference and are not explained in detail. Hereafter, mainly the differences with the first embodiment are explained. 
     In the second embodiment of  FIG. 11 , the workpiece W 4  to be coated is a vascular stent. In such a case, the electrode  32 D is shaped as a mandrel which extends along a longitudinal axis X 32  and which is held by an insulating base part  34 , and driven in rotation around this axis by a mechanism included in a support part  35 . Electrode  32 D is cylindrical and shaped to be totally or partially surrounded by stent W 4 . 
     The sub-module  50  which includes the high voltage generator  52  is located outside the base part  34  and connected to the electrode  32 D by a cable  80  ending with a conductive brush  82  in sliding contact with the outer peripheral surface of electrode  32 D. 
     As can be seen on the right of  FIG. 11 , some other mandrels  32 E,  32 F can be used with parts  34 ,  35  and sub-module  50  of electrode module  30 , depending on the geometry of the stents W 4  to be coated. 
     Alternatively, electrodes  32 D to  32 E can be non cylindrical, depending on the shape of the workpieces W 4  to be coated. 
     In the third embodiment of the invention represented on  FIGS. 12 to 14 , electrode  32 G is mounted around ultrasonic spray head  14  and supplied with high voltage via a sub-module  50  of electrode module  30  which includes a high-voltage generator  52 . In such a case, the flow F 2  of droplets coming out of a discharge nozzle  15  of ultrasonic spray head  14  is electrostatically loaded by Corona effect, that is by ionization of air around nozzle  15 , thanks to spikes  32 G 1  of electrode  32  which extends towards a workpiece W 5  to be coated which lies on a grounded plate  84 . 
     When workpiece W 5  is circular as shown in solid line on  FIG. 12 , electrode  32 G is also circular and its spikes  32 G 1  are regularly distributed around the longitudinal axis X 14  of ultrasonic spray head  14 , as shown on  FIG. 13 . 
     When workpiece W 5  has an oval shape as shown with reference W 5 ′ on  FIG. 12 , then the electrode can be also shaped as an oval, as shown on  FIG. 14  with electrode  32 H. Its spikes  32 H 1  can be regularly or non-regularly distributed around discharge nozzle  15  and central axis X 14  of ultrasonic head  14  in order to evenly or unevenly distribute the droplets on the surface of workpiece W 5 ′ to be coated. 
     As shown on  FIG. 15 , the spikes  32 I 1  of an electrode  32 I may converge towards the central axis X 14  and the nozzle  15  of the ultrasonic spray head  14  in the direction of the flow F 2  of droplets. 
     α 32  denotes an angle between the central axis X 14  of ultrasonic spray head  14  and a longitudinal axis X 32  of a spike  32   h . This angle is adjustable depending on the geometry of the workpiece to be coated and/or on a distance, measured along axis X 14 , between electrode  32 I and the workpiece. Angle α 32  is an orientation axis of a spike  32 I 1  with respect to axis X 14 . 
     According to a non represented alternative embodiment, the electrode may be formed by the workpiece itself. For instance, in the first and second embodiments, workpiece W 1  to W 4  may be in direct electrical contact with contact member  74  or with brush  82 . 
     The features of the embodiments and variants considered here-above can be combined in order to generate new embodiments of the invention. 
     In particular, an air ejection unit similar to shaping gas unit  20  can be used in the second to fourth embodiments.