Abstract:
The invention relates to a method for the radiation curing of surfaces of workpieces, wherein the workpieces are held in a multiplicity of hermetically sealable and at least partly radiation-transparent holding fixtures and are fed consecutively in a cycle to an irradiation device ( 26 ), wherein the holding fixtures with the workpieces contained therein are purged with inert gas, and before irradiation or in an irradiation station inert gas is fed into a holding fixture and the holding fixture is purged with the inert gas and the gas displaced by purging is fed to a holding fixture at a previous stage of the operating cycle, for purging of the same, and an apparatus for implementation of the method.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to a method and an apparatus for radiation curing. 
     (2) Description of Related Art 
     WO 02/32641 A1 and the publication by Erich Beck et. al., “UV-curing in Carbon dioxide” disclose a method of radiation curing in which containers open on one side are used, with the opening at the top or bottom depending on the inert gas involved. The parts to be irradiated are inserted in these containers. In this process, the parts provided with a coating to be cured pass through an entry zone, a radiation chamber and an exit zone. The radiation chamber is provided with one or more irradiation sources for radiation curing, and a gas inlet for the entry of inert gas. 
     In WO 02/32641, the aim is to avoid the entry of oxygen into the radiation chamber, since reactions with oxygen during curing are undesired and lead to poor results. For this purpose the radiation chamber is designed as a tower, with a bottom section connected to the entry zone and the exit zone, and the irradiation source located in its top section. The parts are therefore fed in to the irradiation chamber from the bottom to the top, while the radiation chamber is open only at the bottom. This should prevent convective or turbulent gas flows which, in the prior art, are supposed to lead to the escape of inert gas and the inflow of oxygen. The inert gas is meant to be fed continuously into the top section of the radiation chamber, where it forms a displacement cushion against oxygen areas of the gas. It is also possible to provide gas curtains. 
     Known from DE 10 153 878 B4 is an installation for radiation curing with a radiation chamber and which has an irradiation zone with one or more irradiation sources for radiation curing, together with an entry zone and an exit zone on either side of the former, for the guiding in and out of parts provided with a coating to be cured. The entry zone and/or the exit zone are in the form of inert gas locks, with an outer jacket tube and an inner jacket tube at a distance from the outer jacket tube so as to form an annular gap. On that side of it facing away from the irradiation zone, the inner jacket tube is connected to the outer jacket tube so as to be substantially flow-tight, while the annular gap is connected to one or more gas input lines for inert gas. A radiation chamber according to this prior art has a device for feeding the inert gas into the radiation chamber, while the base area of the radiation chamber has a gas outlet line to take away gas which has become impure or mixed with incoming oxygen. To further reduce the oxygen concentration in the radiation chamber, an inert gas lock is provided in both the entry zone and the exit zone. 
     DE 34 27 315 A1 discloses in order to improve a continuous system for treating coated mouldings with high-energy radiation under an inert gas atmosphere, having a conveying zone, an irradiation device arranged in the course of the conveying zone and having a channel system which encloses the conveying zone in the region of the irradiation device and has an inlet channel extending from an inlet opening to the irradiation device and an outlet channel extending from the irradiation device to an outlet opening and into which channel system inert gas can be introduced in the vicinity of the irradiation device, in such a manner that a higher throughput of mouldings per unit time can be achieved. It is proposed to provide the inlet channel and the outlet channel with, in each case, an injection opening for inert gas and to provide, in each case, an extraction device in the vicinity of the inlet opening and of the outlet opening so that the inert gas introduced into the inlet channel and the outlet channel flows substantially towards the corresponding inlet opening and outlet opening respectively. 
     The disadvantage with the methods according to the prior art is that the locks provided, also the inflow and outflow systems, are unable to prevent the entry and subsequent inflow of atmospheric oxygen and a creeping contamination of the atmosphere. In addition, a large volume of inert gas must be supplied on a continuing basis, but is then able to escape for example at the gas locks. The methods are therefore relatively inefficient. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to create a method of radiation curing in which the inert gas requirement is reduced, and the inflow of atmospheric oxygen can be minimised. 
     The problem is solved by a method with the features of claim  1 , which recites a method for radiation curing of surfaces of workpieces, wherein the workpieces are held in a multiplicity of hermetically sealable and at least partly radiation-transparent holding fixtures and are fed consecutively in a cycle to an irradiation device wherein the holding fixtures with the workpieces contained therein are purged with inert gas, and before irradiation and/or during irradiation inert gas is fed into a holding fixture and the holding fixture is purged with inert gas and the gas displaced by purging is fed to one or more holding fixtures at a previous stage of the operating cycle, for purging of the same. 
     Advantageous developments are set out in the relevant dependent claims. For example, in a preferred embodiment of the invention, the steps of loading of a holding fixture with a workpiece, closing of the holding fixture, purging of the holding fixture with the purge gas of at least one holding fixture in a previous stage of the operating cycle, purging of the holding fixture with inert gas from an inert gas reservoir, irradiation of the workpiece and removal of the workpiece are carried out as consecutive steps. In a further embodiment, before purging of the holding fixture, the holding fixture is closed and evacuated. In a further preferred embodiment, before purging of the holding fixture, the holding fixture is closed and evacuated. In yet another embodiment, in the case of inert gases which are heavier than the air to be displaced, the inlet and the feed line to the holding fixture are provided in a bottom section and the corresponding vent opening in an upper section, and in the case of an inert gas which is lighter than the air to be displaced, the oxygen is provided in an upper section and the outlet line in a lower section. 
     A further problem is to create an apparatus for implementation of the method, which allows irradiation under a steady inert gas atmosphere, is economical, and gives better workpiece surface results. 
     The problem is solved by an apparatus with the features of claim  7 , which is a preferred embodiment in which the holding fixtures are moved in a cycle from work station to work station wherein, in an irradiation station or an irradiation station upstream in the operating cycle the holding fixture at that station is supplied with inert gas and in a work area in which the holding fixture, after insertion of the workpiece, is closed, the inert gas is exhausted, wherein the inert gas is fed in a cycle through all intermediate holding fixtures. 
     Advantageous developments are indicated in the relevant dependent claims. For example, a preferred embodiment of the invention is wherein the holding fixtures are moved in a cycle from work station to work station wherein, in an irradiation station or an irradiation station upstream in the operating cycle the holding fixture at that station is supplied with inert gas and in a work area in which the holding fixture, after insertion of the workpiece, is closed, the inert gas is exhausted, wherein the inert gas is fed in a cycle through all intermediate holding fixtures. In yet another embodiment, the holding fixtures are static and the consecutive working steps are performed in a cycle at the static holding fixtures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The method according to the invention provides for components which are to be subjected to radiation treatment to be placed in separate encapsulated housings which are transparent to radiation, and for the relevant housings to be guided to an irradiation station. 
     According to the invention, the housings have inert gas feed lines and inert gas outlet lines, wherein the outlet line of a first housing is or may be connected to the feed line of a second housing. In the area of an irradiation station or a work station ahead of an irradiation station, pure inert gas is fed to a first housing, which is purged by the inert gas. This inert gas atmosphere is withdrawn and simultaneously fed to the next housing, which is the next in line for irradiation. From this housing in turn, a line leads to the next housing in the process sequence, which it purges with the inert gas atmosphere of the preceding two housings. 
     This process may be continued in any way desired so that, after opening of the housing and the insertion of one or more components (depending on size), the housing is closed, after which the inert gas atmosphere of the preceding chambers flow through it. 
     The inert gas requirement in this connection is low since the housing matched to the size of the component, or the number of components matched to the size of the housing, ensure that only the slight difference in volume between the housing and the component must be purged. In addition, the apparatus according to the invention and the method according to the invention are economical due to the fact that the purge gas atmosphere becomes increasingly purer from the start to the finish of the process. Consequently, an inert gas with a relatively high level of impurities may be used to start with, since this is not important at the beginning of the process. Therefore for example ten chambers or housings connected in series may be purged simultaneously, with however only a tenth of the gas volume emerging at the end. Due to the movement of the components in the opposite direction to the inert gas flow, the atmospheric oxygen concentration at the irradiation station is at its lowest. 
     The method is implemented preferably as a circulation process in which the housings or chambers are first of all loaded, then closed, then for example moved on, with purging taking place at various work stations until a last purging with inert gas takes place at an irradiation station. After the irradiation station, the chamber is disconnected from the inert gas supply. It may then be opened and guided to the loading stage. The circulation process is in the form of an indexed circular movement. 
     In this connection the chambers or housings may for example be connected to an inlet and an outlet valve to which are connected lines which respectively feed in the gas from the preceding chamber or withdraw it for irradiation of the chamber. 
     In the invention it is of particular advantage that the purging process is carried out especially efficiently and, according to the invention, a pure inert gas atmosphere may be obtained with relatively little expense, so that components with better surface quality may be produced. 
    
    
     
       The invention is explained by way of example with the aid of a drawing which shows schematically in: 
         FIG. 1  a first embodiment of an apparatus according to the invention in a plan view; 
         FIG. 2  the apparatus according to  FIG. 1  in a cross-sectional view; 
         FIG. 3  a further embodiment of the apparatus according to the invention in plan view; 
         FIG. 4  the apparatus of  FIG. 3  in a side view; 
         FIG. 5  a third embodiment of an apparatus according to the invention in plan view; 
         FIG. 6  the apparatus of  FIG. 5  in a cross-sectional view. 
     
    
    
     An apparatus  1  according to the invention has for example an indexing rotary table  2 . The indexing rotary table  2  has a circular plate shape, with a drive allowing it to rotate continuously or in cycles by preset angular dimensions. 
     Provided on the indexing rotary table  2  are several holding fixtures  3  for workpieces  4  to be processed. 
     In  FIGS. 1 and 2 , the holding fixtures  3  are designed for elongated workpieces, for example automobile or truck axles. The holding fixtures  3  here are star-shaped with their longitudinal extent facing outwards around the periphery of the indexing rotary table  2 . 
     Each of the holding fixtures  3  has a tray area  5  on which a workpiece  4  is placed, and a hood  6  to cover the workpiece  4  from above. 
     The tray area  5  may have a contoured section, applied to or integrated in the indexing rotary table  2 , comprising for example bases or mounts (not shown) for the placing of the workpieces  4 , or comprising a shaped recess into which the workpiece  4  is placed. 
     The tray area  5  may also be shell- or trough-like, with high side wall sections inside which the workpieces  4  may be placed. In the case of such an embodiment, the area  5  is preferably made transparent or transmissive to radiation. 
     The hood  6  is so designed in respect of its height, width and length that it completely covers from above a workpiece  4  placed on the tray area  5 . 
     The hood  6  is transparent, or at least transmissive to radiation to be used in the irradiation process. Preferably the hood  6  has a continuous seal (not shown) on its bottom edge areas  7 , by which the hood lies on the tray area  5  and the indexing rotary table  2  respectively. 
     The indexing rotary table  2  has several work stations, fixed immovably to the indexing rotary table  2  and the apparatus  1  respectively. 
     A first work station  8  is the loading station  8 . In the loading station  8 , the hood  6  is raised or folded back from the tray area  5  ( FIG. 1 ), and a workpiece  4  is placed on the tray area  5  or pushed in so that the hood  6  may then be placed over the tray area  5  and closed. In this way the workpiece  4  is fitted into a quasi-sealed container. 
     When the hood  6  has been closed, the holding fixture  3  is hermetically sealed, and the indexing rotary table  2  cycles on to the next position, so that the workpieces  4  resting under the hood arrive at the next work station. 
     The containers for the workpieces  4  formed by the tray area  5  and the hood  6  each have a feed line and an outlet line for inert gas. 
     In principle, the term “inert gas” should be understood to mean pure inert gas, such as e.g. pure nitrogen or pure CO 2 . In certain circumstances, though, it may also be expedient to supply as inert gas a gas mixture containing an inert gas as a main constituent. 
     The feed lines and outlet lines may be arranged differently, depending on the inert gas and the remaining gas volume in the working space formed by the tray area  5  and the hood  6 . 
     In the case of inert gases which are heavier than the air (for example CO 2 ) to be displaced, the inflow and the feed line are provided at the bottom of the tray area  5 , or below the tray area  5  through the indexing rotary table  2 , at positions  9   a  or  11   b . The corresponding vent opening is provided preferably diagonally opposite at positions  9   b  or  11   a  at the top of the hood. 
     In the case of an inert gas which is lighter than the air (for example pure nitrogen) to be displaced, the feed line is provided in an upper area of the hood  6 , for example in positions  11   a  or  9   b , while the outlet line is located in a lower area in positions  9   a  or  11   b.    
     If volumes are small, the arrangement of these feed and outlet lines is not so important, so that the feed lines may also be provided on the opposite end face area of the hood  6  in positions  10   a  and  10   b  ( FIG. 2 ). 
     Through the cycling movement of the indexing rotary table  2 , the workpieces and the containers comprised of the tray area  5  and hood  6  which enclose them are moved from one work station to the next work station. 
     For example eight containers are provided at eight work stations  8 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17  and  18 , the containers each comprising a tray area  5  and a hood  6 . 
     The work stations  8 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17  and  18  are so connected to one another by lines provided in or on the indexing rotary table  2  or on the hoods  6 , that inert gas is fed from one container which has already advanced in the process, to the next container which follows. 
     This means that when the indexing rotary table  2  is rotated in the clockwise direction, the inert gas is locked through anti-clockwise, via all work stations, up to the outlet. 
     From a gas supply unit, not shown, such as e.g. a bottle, tank or air decomposition unit, gas is fed through a feed line  19  into a first holding fixture  3  at an irradiation station  17 . At an opposite end of the holding fixture  3  is an outlet line  20  which feeds the gas from the holding fixture  3  in work station  17  into the next holding fixture  3  in the clockwise direction. At the opposite end of this holding fixture  3  is an outlet line  21  which in turn feeds the gas into the next holding fixture in the clockwise direction. At the opposite end of the latter is another line  22  through which the gas is taken from this holding fixture  3  into that which follows, from which the gas is again fed via a further line  23  into the next holding fixture  3 . From there, a line  24  carries the gas to the last holding fixture  3  as far as the inert gas supply is concerned, from which either an exhaust line  25  locks the gas out of the system, or else the gas is able to escape the system via the opened hoods of the loading station and the removal station  18 . 
     The line system comprising the lines  19 ,  20 ,  21 ,  22 ,  23 ,  24  and  25  is fitted immovably to the indexing rotary table  2 , and therefore preferably does not rotate with it. Lines  19  to  25  have coupling elements by which they may be coupled to the relevant lines  19  to  25  on or in the indexing rotary table  2  and to the holding fixtures  3  respectively. This means that, before onwards rotation of the indexing rotary table  2 , decoupling takes place, followed by recoupling after the further rotation, and then the relevant purging. During the periods of rotation, when no gas is supplied, the feed lines of the holding fixture  3  concerned are closed with a gas-tight seal. For purging, inert gas is supplied only through feed line  19  and not only undertakes purging at work station  17  but also, due to the displacement of the gas already present in the holding fixture  3  at work station  17 , displaces the next following holding fixture  3  or container  3 . This process continues in the same manner until the first container or holding fixture  3  to be purged, from which the atmosphere it contains, which at this point in time does not yet contain any inert gas, is expelled to line  25 . 
     The work stations  12 ,  13 ,  14 ,  15  and  16  are used here for example solely for purging. In work station  17 , the workpieces  4  are irradiated through the hood  6  by an irradiation device  26 . 
     The irradiation device  26  may extend in the manner of a hood around the hood  6 . If the workpieces  4  are elevated, then the irradiation device  26  may be so designed as to irradiate the workpiece  4  from all sides. 
     After irradiation in the work area  17 , the holding fixture  3  with the suitably irradiated workpiece  4  moves on to a removal station  18  in which the hood  6  is opened by folding back, lifting up or removing, and the finished irradiated workpiece  4  may be removed. 
     With a further rotation, the empty tray area  5  moves into the loading station  8  described earlier, at which the cycle begins again. 
     The number of rotations and the angle of rotation  27  naturally depend on the number of holding fixtures  3  on the indexing rotary table  2 . 
     The holding fixtures  3  may have any shape desired, depending on the workpiece  4  to be irradiated. 
     In a further embodiment of the invention ( FIGS. 3 ,  4 ), once again several work stations  28  are provided on a first indexing rotary table  29 . 
     Parallel to the first indexing rotary table  29  is a second indexing rotary table  30 . 
     In this embodiment, the components  31  are suspended from the upper indexing rotary table  29  or set up on the lower indexing rotary table  30 . Arranged on the respectively opposite indexing rotary table  29 ,  30  are cup-like hoods  32 , which may be put over the workpieces or components  31  from above or below. Within the area of the hood  32 , each work station has a feed port  33  for inert gas and an outlet port  34  for inert gas. 
     The respective ports  33 ,  34  may be similarly coupled for fluid flow by feed lines  35  and outlet lines  36  to the respectively preceding or following hood or the preceding or following work station, so that through-flow and purging similarly take place against the direction of rotation. 
     In the case of this apparatus according to the invention too, the workpieces  31  may be handed over at a special station and removed at another station, while irradiation may also take place at a third station. The general arrangement then corresponds to that of the first embodiment. 
     With small workpieces  31  and small hoods  32 , irradiation devices (not shown) may be provided at each work station, so that only suitable removal and insertion devices must be provided on the apparatus  1 . Here it is not necessary to make a full rotation for insertion and removal of the workpieces  31 , but instead removal may be effected when on the one hand adequate purging and subsequently adequate irradiation have taken place. With a large indexing rotary table  29 , this may already be the case for example after a quarter of the full rotation. 
     When the components  31  are suspended, the hoods  32  are moved upwards against the upper indexing rotary table  29  for example by a pneumatic or hydraulic punch  33 . In the case of a static arrangement of the components  31 , the hoods  32  may be lowered from above by a pneumatic or hydraulic punch, a servo or linear motor or similar drives. 
     Common to all the embodiments described above is that the direction of flow of the inert gas is opposite to the direction of movement of the components  31 , so that the residual oxygen concentration steadily declines in the direction of an exposure position. 
       FIGS. 5 and 6  show a third embodiment of an apparatus according to the invention for the radiation curing of surfaces of workpieces  4 . The basic design of this embodiment corresponds in principle to that of the apparatus of  FIGS. 1 and 2 , for which reason identical parts have the same reference numbers. 
     This third embodiment also has an indexing rotary table  2  with several holding fixtures  3  for workpieces to be processed. Each holding fixture  3  has a tray area  5  on which the workpieces are placed, and a hood  6  for covering the respective workpiece  4  from above. The adjacent holding fixtures or work stations  3  are again connected to one another by communicating lines. In the present embodiment, these connecting lines  37  are each connected by their ends to the hoods  6 . In each of the connecting lines  37  a non-return valve  38  is provided in such a way that a flow between two adjacent holding fixtures  3  is possible only against the rotary movement of the indexing rotary table  2 , i.e. against the direction of movement of the holding areas  3 . In addition, each hood  6  is provided with a line coupling  39  to which is coupled in each case a feed line  40  for inert gas. The feed lines  40  terminate with their ends which face away from the holding fixtures  3  at a carousel (not illustrated) which has a passage to a gas supply unit in the position of work station  16 , and which isolates the ends of the feed lines at the other positions. 
     The supply of inert gas may also as an alternative be controlled by automatically switching valves. These valves are then located in the feed lines  40 . 
     The line couplings  39  are each provided with a non-return valve in such a way that inert gas may be fed into the holding fixtures  3  through these line couplings  39 , but is then unable to escape from the holding fixtures  3 . 
     This apparatus is operated in the same way as the first embodiment but, as already explained above, the irradiation station  17  is separated from the work station  16  into which the inert gas is fed, and here in principle during the feeding-in of inert gas at least one of the hoods  6  of the loading station  8  or the removal station  18  is opened, so that inert gas present in work station  12  may be discharged into the surrounding environment, and no air/inert gas mixture is fed to the irradiation station  17 . 
     In this embodiment, there is no gas flow at the irradiation station  17 . It has been shown that a settled atmosphere is of benefit to the curing process. 
     For all the embodiments described above, control of the inert gas may be effected via the residual oxygen content in the holding fixtures  3 , i.e. through a suitable sensor in the outlet line  25  or beneath each hood  6 . 
     Due to the inert gas routing according to the invention via the individual holding fixtures  3  and containers or housings respectively, the displacement of the original atmosphere and of the oxygen is effected substantially in the form of displacement purging and not as dilution purging. This also leads to a reduction in the inert gas requirement. 
     In particular in the embodiments according to  FIGS. 1 ,  2  and  3 , the components  4  may be mounted rotatably or pivotably within the transparent hood  6  and holding fixture  3  or container, so that an even dose of radiation is given to the component surface during the UV irradiation. 
     It is also possible to move the UV radiation devices  26  during the drying process. Any desired shadows within the holding fixture  3  may be obtained by inbuilt reflector panels, as also improved illumination. In this connection the hoods  6  may be completely transparent or provided with windows. 
     For further improvement of efficiency, the closed holding fixtures  3  may be subject to evacuation before purging with inert gas. 
     REFERENCE NUMBERS 
     
         
           1  apparatus according to the invention 
           2  indexing rotary table 
           3  holding fixtures 
           4  workpieces 
           5  tray area 
           6  hood 
           7  bottom edge areas of  6   
           8  first work station/loading station 
           9   a  position of the feed line 
           9   b  position of the vent opening 
           10   a,b  position of the feed line 
           11   a  a position of the vent opening 
           11   b  position of the feed line 
           12 - 18  work station 
           19  feed line 
           20  outlet line 
           21  outlet line 
           22  line 
           23  line 
           24  line 
           25  outlet line 
           26  irradiation device 
           27  angle of rotation 
           28  work stations 
           29  indexing rotary table 
           30  indexing rotary table 
           31  components 
           32  hoods 
           33  feed port 
           34  outlet port 
           35  feed lines 
           36  outlet lines 
           37  connecting line 
           38  non-return valve 
           39  line coupling 
           40  feed line