Abstract:
An apparatus for applying a thin film of a liquid coating medium onto a curved surface of an article includes a receptacle for the article, a rotatably drivable turntable on which the receptacle is held and a nonrotatable process pot surrounding the receptacle and the turntable. A spray ring surrounds the surface to be coated and is retained at the open end of the process pot. It defines a central inflow opening. Several annular gaps are defined and the receptacle is provided with a suction cone. A laminar flow is aimed at the spray ring at the top of the process pot and the bottom of the process pot is joined to a suction flow.

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for applying a thin film of a liquid coating medium onto a preferably curved surface of an article, preferably a glass bulb of a picture screen, having a rotatably drivable turntable on which a receptacle for the article to be coated is held fixed against relative rotation, a process pot held nonrotatably and surrounding the receptacle and the turntable, and a spray ring surrounding the surface to be coated and retained on the open end of the process pot. 
     PRIOR ART 
     In a known apparatus of this type, the glass bulb of a picture screen, for instance, is introduced into the process pot from the underside and pressed against a mask that surrounds the surface to be coated; the mask is intended to prevent the coating medium from reaching undesired regions of the glass bulb. This way of delivering and removing the glass bulb of the screen is mechanically complicated and inconvenient. 
     In another known apparatus, the glass bulb of the screen is introduced into the process pot from the top, and an open, split mask is brought in its closed position around the boundary edge of the surface to be coated. Once again, because of the split mask that has to be moved, this is mechanically complicated. 
     Both of the known apparatuses described have the common feature that especially at the edges of the surface to be coated, streaks and other discontinuities in the coating develop, unless special provisions for improved uniformity of the coating are made. This is the purpose of air guide elements placed against the corners of the mask in the known apparatuses. Nevertheless, such provisions do not prevent the flow from separating at the circumferential edges and becoming turbulent, which in turn means that the article to be coated becomes contaminated in undesired regions with the coating medium. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an apparatus for applying a thin film to a surface of an article of the type described at the outset which is easier to handle as the article to be coated is being introduced and removed and which offers greatly improved uniformity of the film in the critical corner and edge regions of the surface to be coated. 
     To attain this object, in an apparatus for applying a thin film to an article as defined at the outset, the spray ring is provided with a central inflow opening and between the top part of the process pot and the spray ring, a first annular gap extending to beneath a fastening edge of the article to be coated on the receptacle is provided. The cross section of the annular gap is substantially less than an annular flow passage between the spray ring and the article to be coated retained in the receptacle. The receptacle is provided with a section cone which points toward a bottom of the process pot, and between which and an outer wall of the process pot there is a further annular gap and inside which a pressur relief chamber is formed. The pressure relief chamber is jointly defined by the bottom of the process pot. A laminar flow is aimed at the spray ring, or the top of the process pot, and the bottom of the process pot is joined to a suction flow. 
     By means of the provisions of the present invention, an air flow is produced which because of its different air velocities in various regions on the one hand makes the film to be applied uniform, even in the corner and edge regions of the preferably curved surface and on the other prevents contamination of other regions of the article to be coated. Because of the laminar flow in the region of the corners and edges of the preferably curved surface to be coated and because of the suction exerted on them toward the suction cone and the resultant downward-oriented flow component, on the one hand separation of the air flow at the edges and corners of the surface and thus turbulence are prevented, and on the other back spraying onto the surface to be coated is averted. In the further course of the air flow, more-effective suction of the atomized coating medium out of the critical region above the suction cone is achieved, after which the atomized coating medium can collect in the pressure relief chamber. In this region, the further gap prevents reverse turbulence in regions of the rotary bearings. On the other hand, the aspirated coating medium can be collected on the bottom of the process pot and removed by suction. 
     A vapor barrier from the pressure relief chamber to the bearing regions for the turntable is attained, without having to use vulnerable rotary seals and the pressure relief chamber is defined radially on the inside by an annular baffle between which and an inner wall of the process pot there is a third annular gap. It is expedient in this respect to secure the annular baffle to the underside of the turntable. 
     A process pot of low height is attained, which in its lower region surrounds the neck of, for instance, the glass bulb of the picture screen when the process pot is embodied cylindrically in its upper region and circularly-annularly in its lower region. 
     The outflow or removal by suction of the coating medium mist and/or coating medium droplets is advantageously achieved and the annular bottom of the process pot is curved in the circumferential direction and/or inclined in the radial direction and is joined to the suction flow via an opening in the region or regions of the lowest point or points. 
     To enable automatic or robot-controlled access to the article to be coated during its insertion and removal, the spray ring is retained in lowerable fashion on the process pot. Expedient features in this regard are obtained by the process pot having a removable top part, on which the spray ring is equipped retainers, which are distributed uniformly over the circumference and rest on one flange of the top part of the process pot, and is retained removably and which can be lowered into the bottom part of the process pot or can be lowered to below the upper end of the receptacle for the article to be coated. The result achieved is that the spray ring can be lowered down to or to below the height of the support of the article on the receptacle, so that the article to be coated can be grasped easily and moved through the spray ring. 
     In order, with the apparatus, to coat articles on their surface with the coating medium, such articles for instance being glass bulbs of pictures screens of various sizes or diagonal measurements, the receptacle for the article to be coated is retained interchangeably. Thus in an especially advantageous way, a receptacle suitable for a particular size of article to be coated can be retrieved from the process pot and replaced by a receptacle that can hold an article to be coated that is of different dimensions. Preferred features in this respect are obtained from the receptable being secured to the turntable and the turntable being interchangeably retained on a driven shaft which is hollow and is supported to rotate on a hollow stationary stand, which receives one end of the article to be coated and quick-action closures distributed uniformly over the circumference are provided between the turntable and one flange of the hollow shaft. 
     Thus in an especially advantageous way, a complete unit comprising the turntable, receptacle, suction cone and sealing cuff can be removed from the process pot and replaced by another one. To that end, the spray ring can be removed in a simple way from the top part of the process pot. 
     In a further feature of the present invention, the receptacle, on its support, is provided with a sealing cuff, which protrudes radially inward and on which a shoulder, remote from the surface to be coated, of the article to be coated rests sealingly. are provided, with which the surface to be coated of the article to be coated can be sealed off in a simple way from the other regions of the article to be coated. If the sealing cuff, in its radially inner region, is prestressed resiliently toward the article to be received a resiliently prestressed active contact of the sealing cuff with the shoulder of the article to be coated is achieved. The resilient prestressing can be achieved by means of concave curvature of the sealing cuff or by a kink provided along a circumferential line. To stabilize the sealing cuff, the sealing element rests on a resiliently prestressed lower part and is covered in the region of the fastening face by a retaining part so that the resilient prestressing is transmitted from a lower part onto the actual sealing element. In this respect, the sealing element protrudes radially inward past the resiliently prestressed lower part. 
     In a further feature of the present invention, provides the sealing cuff is fastened on the receptable by platelike components distributed over the circumference, and the platelike components are each provided with one protruding bolt, which bolts serve the purpose of detachable connection, in a manner fixed against relative rotation, to tabs disposed on a tension belt secured to the article on be coated which provides a fast and simple connection, in a manner fixed against relative rotation, attained between the article to be coated and the receptacle. Glass bulbs of picture screens, for instance, are provided on their circumference with a tension belt and retaining tabs secured to it, which are used for the aforementioned connection, in a manner fixed against relative rotation, with the bolts of the fastening plates. 
     Since the article to be coated on its surface is nonround, an example being a glass bulb of a picture screen which in plan view is rectangular with rounded corners, the corners located outside the inner circle meet the still air, so that in these corner regions severe turbulence and a shear flow arise, which carry the coating, already distributed outward by the spin coating process, still further away. To prevent this, and to improve the apparatus for the sake of uniformity of the film in the critical corner and edge region of the surface to be coated, in a variant the article to be coated on its surface, the surface being nonround in plan view, is at least partly surrounded by a guard baffle device is disposed sudstantially upright and extending to at least the plan of the surface to be coated. 
     Since essentially only the corner regions are threatened in this respect, is formed by individual guide baffles in the region of the corners of the articles the guide baffle device. It is expedient in this respect that the guide baffle, in terms of the direction of rotation, is disposed upstream in and downstream of the region of the corner, and the guide baffle portion upstream of the region of the corner is substantially longer than the guide baffle portion downstream of the region of the corner, because the side that is leading in the rotational direction relative to the corner region moves against the still air, while the trailing side is in the lee, as it were. 
     Thus with the aid of the guide baffle device, or the individual guide baffles, air turbulence can be kept away from the surface to be coated, for instance of the picture screen, to enable unhindered coating in the region of the corner. 
     To introduce and remove the article to be coated on its surface, retaining tabs are provided at the corners on the tension belt, as is realized for a glass bulb of a picture screen, for instance. To achieve the introduction and removal with the aid of a tool, such as a robot, it is possible that the guide baffle can be moved, for instant pivoted, out of the region of the corner of the article to bring the individual guide baffles out of this path of motion of the tool to the retaining tabs. 
     A simpler way of accomplishing access to the tool or robot to the individual retaining tabs in the corner region is obtained if the guide baffle device or guide baffle in the region of the corners or corner of the article has an outward-pointing recess surrounding a retaining tab of the article. With this baffle-like recess, constant access to the retaining tabs is assured, without requiring that the guide baffles be movable and without this recess having a negative effect on the guide baffle function. 
     A structurally advantageous embodiment is obtained by the guide baffle device or the individual guide baffles protruding past the surface to be coated. 
     Further details of the present invention can be learned from the ensuing description, in which the present invention is described and explained in further detail in terms of the exemplary embodiment shown in the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is in a schematic longitudinal section, showing an apparatus for applying a thin film of a liquid coating medium onto a curved surface of a glass bulb of a picture screen in a coating position, in accordance with a preferred exemplary embodiment of the present invention; 
     FIG. 2 is a view corresponding to FIG. 1, but in a position for changing the glass bulb of the picture screen to be coated; 
     FIG. 3 is a schematic plan view in the direction of the arrow III of FIG. 1; 
     FIG. 4 is a schematic longitudinal section of an interchangeable unit for glass bulbs of picture screens of different sizes; 
     FIG. 5 is on a larger scale, a detail outlined by the circle V in FIG.  4 . 
     FIG.  6 . on a larger scale, a detail outlined by the circle IV of FIG. 1, but in accordance with a variant; and 
     FIG. 7 is a view corresponding to FIG. 5 with the variant of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus  10  shown in the drawing in terms of a preferred exemplary embodiment is used to apply a thin film of a liquid coating medium onto what here is a curved surface  12  of an article represented here by a glass bulb  11  of a picture screen. The application of the coating medium is done in the usual way via a spray arm, not shown here, over the surface  12  and, by centrifugal force distribution of the coating medium, along the surface  12  to be coated, or by spinning off the remaining coating medium by a rotary motion of the glass bulb  11  of the screen. While the surface  12  to be coated is being sprayed, the glass bulb  11  of the screen is driven at a relatively low rpm, while for centrifugal force distribution or for spinning the coating medium off along the surface  12  or past it, the rpm at which the glass bulb  11  of the screen is rotated is increased substantially. 
     The glass bulbs  11  of the screen to be coated are provided on their top  13  with a tension belt  14 , on which circumferentially distributed retaining tabs  15  are provided that serve the purpose of retention both in the apparatus  10  and later in an equipment housing. Remote from the top  13  having the surface  12  to be coated, the glass bulb  11  of the screen has a neck  16 . 
     The apparatus  10  has a receptacle  30  for the glass bulb  11  of the screen and also has a turntable  20 , with which the receptacle  30  is connected in a manner fixed against relative rotation. The turntable  20  and the receptacle  30  are surrounded by a nonrotatably retained rotationally symmetrical process pot  50 , which holds an upper spray ring  60  on its axially lowerable top part  51 . 
     The turntable  20  can be connected to a flange  21  of a hollow shaft  22  in a manner that is fixed against relative rotation but is easily released. The hollow shaft  22  is rotatably held via roller bearings  24  on a concentric, stationary hollow shaft  23 , and on its end remote from the flange  21  it has a pulley  25 , which is fixed against relative rotation and is coupled via a drive belt  26  with the drive shaft of a drive motor  27 , not shown in further detail. The hollow shaft  23  protrudes at right angles from an equipment base  28 . The turntable  20  can be connected releasably to the flange  21  via quick-action closures  29 , not shown in further detail. 
     The process pot  50  has an upper cylindrical pot part  52  inside which the top part  51  can be received in lowerable fashion, and a lower annular pot part  53 , whose annular chamber is closed off by an annular bottom  54 . The lower annular pot part  53  begins below the turntable  20  and has an inner annular flange  55 , which in radially spaced-apart fashion surrounds the flange  21  of the hollow shaft  22  below the turntable  20 . The process pot  50  is retained via its annular bottom  54  on legs  56  distributed over the circumference, which protrude at right angles from the base  28 . At at least one point, the annular bottom  54  communicates via an opening with a suction neck  57 , to which in a manner not shown a suction device is connected directly or indirectly. The annular bottom  54  is radially inclined toward the suction neck  57  and from a highest point extends in both circumferential directions downward toward the suction neck  57 . The top part  51  of the process pot  50  can be lowered hydraulically, for instance, into the upper cylindrical pot part  52  and has an upper outer flange  58 . The top part  51  is also retained in a manner not shown in further detail such that it can be removed or pulled out of the upper cylindrical pot part  52 . 
     The spray ring  60  has a rotationally symmetrical hoodlike insert  61 , which is removably held on the top part  51  of the process pot  50  via retainers  62 , three of which, for instance, are distributed uniformly over the circumference. The retainers  62  are secured to a conical part  63 , which changes over toward the bottom into a cylindrical part  64 , along the circumference of a central opening  65  of the spray ring insert  61 , an opening defined by the conical part  63 . The retainers  62  are suspended by one arm  66  in a recess  59  of the outer flange  58  of the top part  51 . Between the cylindrical part  64  of the spray ring insert  61 , which is suspended from the top part  51 , and the inside circumference of the top part  51  of the process pot  50 , a first narrow annular gap  67  is formed. 
     The receptacle  30  has a boxlike receiving part  31 , whose rectangular shape in plan view is approximately equivalent to the external dimensions of the top  13  of the glass bulb  11  of the screen. The receiving part  31  is seated on the top side of the turntable  20  and is joined to the turntable in a manner fixed against relative rotation. A suction cone  32  is provided in a manner fixed against relative rotation with the receiving part  31 , whose external dimensions are markedly smaller than the inside diameter of the process pot  50  or its top part  51 . The suction cone  32  has a circumferential flange  33 , which is secured to the receiving part  31  and on which a hoodlike conical part  34  is firmly held that toward its end changes into a cylindrical part  35 . The outer diameter of the cylindrical part  35  extending toward the annular  54  of the process pot  30  is such that there is a second annular gap  37  between the outer circumference of the cylindrical part and the inner circumference of the lower annular part  53  of the process pot  50 . The cylindrical part  35  of the receptacle  30  ends at a certain distance from the annular bottom  54  of the process pot  50 . Inside the cylindrical part  35 , a pressure relief chamber  36  is formed, which is defined, away from the wall of the cylindrical part  35  in a radially inward direction, by a sheet-metal sleeve  38 , which is retained in a manner fixed against relative rotation on the underside of the turntable  20  and between which and the inner wall of the lower annular pot part  53  a third narrow annular gap  39  is formed. A central opening in the bottom  72  of the receptacle  30  is adjoined by a tube  73 , which is closed at the bottom and protrudes vertically downward, for receiving a thin end  18  of the glass bulb  11 . 
     The receptacle  30  or its receiving part  31  has an upper end face  41  extending all the way around, on which a sealing cuff  40  for the top  13  of the glass bulb  11  of the screen is secured. As shown in FIG. 5, the sealing cuff  40  has an elastically resiliently prestressed lower part  42 , a flat seal  43 , and a retaining part  44 . The elastically resiliently prestressed lower part  42  is either curved or kinked toward the top in a radially inner region. The lower part  42 , which reinforces the flat seal  43  and by its curvature lifts partway away from the end face  41 , protrudes outward somewhat past the inner edge of the receiving part  31 , while the flat seal  43  is drawn radially inward still farther. The retaining part  44 , conversely, is substantially narrower; that is, it is located on the far side of the curved region of the lower part  42  only in the fastening region, where the sealing cuff  40  is firmly held, among other means by platelike components  45  distributed over the rectangular circumference, by means of screws. The platelike component  45 , which is recessed in the curved inner region of the sealing cuff  40 , is equipped, remote from where it is fastened to the receiving part  31 , with a bolt  46  that protrudes upward and is provided with an annular collar  47 ; the glass bulb  11  of the screen is suspended from this bolt  46  by means of the retaining tabs  15  retained on the tension belt  14 . In this suspended arrangement, as shown in FIG. 5, the lower end  17  of the retaining tab  15  presses against the inner edge of the flat seal  43 , thereby sealing off the space  48  below the top  13  of the glass bulb  11  of the screen, or below the sealing cuff  40 , from the coating medium. 
     The mode of operation of the apparatus  10  is as follows: In FIG. 1, the apparatus  10  is acted upon by a laminar flow represented by the arrows A that extends over the entire surface of the process pot  50 ; the laminar flow has a velocity of approximately 0.3 to 0.4 m/s, for example. In addition, as already noted above, a suction flow represented by the arrow B is generated in the suction neck  57  of the process pot  50 . 
     During the spraying operation and in particular during the spinning operation, the laminar flow A passes through the central opening  65  to reach the surface  12  to be coated of the glass bulb  11  of the screen and moved past it into a passage  71  between the edges and corners of the glass bulb  11  of the screen and the conical part  63  of the spray ring  60 . The laminar flow A also enters the first annular gap  67  between the spray ring insert  61  and the process pot top part  51 . The partial flow through the first annular gap  67  is brought to a flow velocity between approximately 7 and 10 m/s as a result of this narrowed passage, thus exerting a pull, at the outlet of the annular gap  67 , a pull on the partial flow in the passage  71 . This has advantageous effects on the removal of the coating medium spun away from the peripheral and corner regions of the glass bulb  11 , in such a way that there is no separation of the flow and hence no turbulence at the corners and edges of the glass bulb  11 . This laminar flow that prevails overall leads to a very high degree of uniformity in the resultant film, including in the corner and peripheral regions of the glass bulb  11 , thus preventing streaks or the like. The area of both the upper spray ring  60  and the first annular gap  7  between the process pot  50  and the upper spray ring  60 , and also the area between the screen receptacle  30  and the upper spray ring  60 , are dimensioned such that a downward-oriented flow component with air velocities between 7 and 10 m/s is generated at the circumference of the process pot  50 . This also prevents back spraying onto the article  11 . The free volume between the screen receptacle  30  and the process pot  50  is selected such that at spin speeds up to 250 rpm, no substantial dynamic pressure occurs in the region between the upper edge of the screen  11  and the suction cone  32 . 
     The flow now enriched or atomized with coating medium, passes through the second annular gap  37  between the process pot  50  and the suction cone  32 . This second annular gap  37  is selected such that a flow velocity of 3 to 5 m/s, for instance, is attained there, resulting in effective remove by suction of the atomized coating medium from the upper, critical region. In the further course of the downward-oriented flow, the flow leaves the second annular gap  37  to enter the pressure relief chamber  36 , in which the in flowing coating medium mist is calmed by the reduction of the flow velocity to approximately 0.6 to 0.9 m/s. This allows a laminar inflow of the coating medium mist via the annular bottom  54  into the suction neck  57  on the one hand, and on the other, because of the third annular gap  39  which acts as a vapor barrier, penetration of this mist into the region of the turntable bearing is additionally prevented. This is attained both by the very slight cross section and by the considerable length of the third annular gap  39 . In addition, the pressure relief chamber  36  causes settling of the mist, which then precipitates onto the inclined bottom. 
     For replacing the glass bulb  11  to be coated, according to FIG. 2 the top part  51  of the process pot  50  is lowered together with the spray ring  60 , so that with the aid of gripper arms, not shown, the glass bulb  11  can be grasped, for instance by the retaining tabs  15 , and removed, and another glass bulb  11  can be inserted accordingly. 
     If the dimensions of glass bulbs  11  to be coated change, then the apparatus  10  can be adapted thereto in such a way that the unit  70  shown in FIG. 4, comprising the turntable  20  and receptacle  30  with the sealing cuff  40  is replaced wither another. This purpose is served by the quick-action closures  29 , with which the turntable  20  can be releasably secured to the flange  21  of the hollow shaft  22 . To remove such a unit  70 , the spray ring  60  is removed from the top part  51 , or depending on the diameter ratios between the top part  51  and the suction cone  32  of the receptacle  30 , the top part  51  can optionally be removed along with the spray ring  60  from the process pot  50 . 
     In the variant shown in FIGS. 6 and 7, the apparatus  10  has a guide baffle device  80 , which is disposed around the top region  13  of the glass bulb  11 . The guide baffle device  80  is disposed upright and substantially parallel to the pivot axis of the turntable and thus to the axis of the glass bulb  11  of the screen and extends from a region at the level of the lower edge of the tension belt  14  upward to a vertical range of a few centimeters, for instance 5 cm, above the zenith of the curve surface  12  of the glass bulb  11  (FIG.  7 ). 
     The guide baffle device  80  comprises a plurality of individual guide baffles  81 , in this case four of them, of which each guide baffle  81  is disposed in the region of one corner  82  of the top  13  of the glass bulb  11 . The guide baffle  81  is in one piece and has a plurality of partial regions  83 ,  84  and  85 . While the lateral partial regions  83  and  85  extend rectilinearly along and spaced slightly apart from the applicable sides of the top  13  or tension belt  14 , the middle partial region  84  is recessed in baffle-like fashion in such a way that it surrounds the retaining tab  15 , thereby creating a free space  86  inside the middle partial region  834  that allows free access to the retaining tab  15  by means of a tool or robot, for inserting and removing the article  11  into and from the apparatus  10  from above. In the exemplary embodiment shown, this recess  84  is rectangular, it is understood that it may also be in the form of a circular arc or the like. The particular partial region  83  of the guide baffle  81  that is leading in the rotational direction C of the turntable  20  is substantially longer than the partial region  85  that trails in that direction of rotation. For example, the leading partial region  83  extends over more than half the length of the applicable side of the top  13 , while the trailing partial region  85  extends over only a small fraction, such as one-fourth, of the length of the applicable side of the top  13 . With the aid of the guide baffles  81  mounted at the corners  82  of the glass bulb  11 , it is possible to keep air turbulence away from the surface  12  to be coated of the glass bulb  11 , and thus to attain an unhindered or uniform coating, even in the region of the corners  82 . 
     In a variant of the guide baffle device  80  that is not shown, the individual guide baffles are provided extending linearly in every partial region, including in the middle partial region, along the outer edge of the top  13 . In this variant, to gain access to the retaining tabs for inserting and removing the article  11 , the individual guide baffles are movable or pivotable in such a way that access to the retaining tabs  15  can be enabled. This can be done for instance in such a way that the leading partial region of the guide baffle is connected pivotably by its free end.