GB2091859A

GB2091859A - Drying paint

Info

Publication number: GB2091859A
Application number: GB8136474A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-12-19
Filing date: 1981-12-03
Publication date: 1982-08-04
Also published as: DE3048005A1; DE3048005C2; GB2091859B; IT1142107B; IT8125638A0

Abstract

A layer (2) of paint on a carrier (1) is supplied with heat from a source (3) of radiant energy to that side of the layer (2) remote from the carrier (1) whilst a cooling medium is applied to the side (5) of the carrier (1) remote from the layer (2). <IMAGE>

Description

SPECIFICATION Process and apparatus for drying a layer of paint or the like on a carrier This invention relates to a process for drying paints, lacquers, varnishes, and the like capable of being heat dried which form a coating on one side of a layer carrier in the form of a sheet, web or hollow body made of metal, in which the energy required for drying is supplied to the layer from the side remote from the layer carrier. The invention also relates to an apparatus for carrying out such a process.

Known such processes are used mainly for drying metal sheets for the manufacture of cans. After the application of print and/or lacquer, the sheets are passed through an oven in which the layer is dried by a stream of hot air flowing over it. Since this hot air drying in the oven is relatively slow but the production speed of the sheet printing and lacquering machine is relatively high, the sheets are placed in a substantially vertical position and closely spaced apart as they pass through the over. The tunnel oven is required to be of a considerable length inspite of this space saving arrangement of the sheets.The attempt to reduce the length of the oven by shortening the drying time has not been successful because, as has been shown, drying air streaming over the sheets from below upwards cannot be used at a temperature of more than about 270 C, otherwise the tin would melt at the soldering edges of the sheets, apart from other trouble. In addition to the disadvantage of the great space requirement for the oven, the known drying processes have the disadvantages of the effort required for turning the sheets and the excessive proportion of heat and solvent contained in the exhaust air corresponding to the quantity of hot air supplied.

It is an object of the present invention to provide a process of the type indicated above which enables drying to be carried out much more economically and with an apparatus taking up less space.

In a process of the type indicated above, this problem is solved according to the invention by the fact that during the supply of energy to the layer, the layer carrier is directly and intensively cooled by a cooling medium supplied to its surface remote from the layer. Due to this intensive cooling of the back, of the layer carrier, (hereinafter briefly referred to as carrier) while the energy for drying is supplied to the layer, this supply of energy may safely be arranged in such a manner that the layer is heated by a type of heat shock so that very rapid drying is achieved. The drastic reduction in drying time achieved has the advantage of reducing the length of oven required for a given production speed.If a liquid cooling medium is used, for example softened water, the drying energy can be supplied to the layer at such high intensity that the cooling water evaporates on the back surface of the carrier. This evaporation process taking place at atmospheric pressure ensures that the temperature on the back surface of the carrier does not rise above the evaporation temperature, in other words that the surface temperature of the back of the carrier is kept constant.

The drying energy is preferably supplied in the form of radiant energy by moving the carrier with its layer carrying surface facing upwards on a substantially horizontal moving conveyor under one or more sources of radiant heat while the downwardly facing back of the carrier is sprayed or otherwise supplied with liquid cooling medium. Examples of suitable sources of radiant heat include inter alia infrared radiators, UV radiators, microwave radiators, IRAS ERS (infrared emitting radiators) mercury vapour lamps and the like. Depending on the physical conditions of the drying process for the particular layer and the thickness and nature of the layer carrier, the intensity of supply of energy may also be adjusted by suitable choice of the distance between the layer and the radiant source.By using this adjustment in combination with suitable adjustment of the intensity of cooling at the back of the carrier it is possible to obtain optimal rapid drying of the particular material with a drying temperature on the layer of, for example, the order of 800"C. This type of rapid drying provides the further advantage of low solvent consumption. Since the layer is dried abruptly so that there is no risk of formation of a surface skin on the layer during the drying process, a problem which in conventional hot air drying has to be avoided by using a very high proportion of solvent in the layer being dried, application of the process according to the invention enables the proportion of solvent in the layer to be kept comparatively low without entailing the risk of skin formation.This has the further advantage that the exhaust air from the oven has a lower solvent content.

In one exemplary embodiment of the invention, in which cooling of the carrier is continued beyond the moment when the supply of drying energy is stopped, the additional advantage is obtained that the cooling medium can remain on the back of the carrier in the form of a liquid layer and evaporate after the drying process. If the carriers in the form of sheets are subsequently stacked, this layer of fluid prevents the accumulation of heat and hence the risk of the sheets caking together.

It is also an object of the present invention to provide an apparatus which enables the process to be carried out economically.

In an apparatus comprising an oven in which the interior forms a dring zone containing a mounting device on which the carrier is supported on its surface facing away from the layer and at least one heating device supplying the energy for drying, this problem is solved according to the invention in that the mounting device is arranged between the heating device and a cooling device supplying cooling medium to the back of the carrier facing away from the layer, and in that the mounting device is designed to allow the cooling medium access to the back of the carrier. The mounting device which enables the cooling medium to have access to the carrier may be a conveyor having a conveyor belt perforated in the form of a screen or grid so that the cooling medium reaches the carrier through the openings in the conveyor belt.Alternatively, the mounting device may be in the form of a roller train and the cooling medium may reach the carrier through the spaces between the rollers. The mounting device may also form part of the cooling device if, for example, a transport roller on which the carrier is placed with its back downwards serves as application roller for the liquid cooling medium.

Although the application of the drying process according to the invention enables the length of the drying zone and hence also the length of the oven to be considerably reduced, an even greater reduction in the length of the oven may be obtained by constructing the oven in two or more passages, i.e.

by providing two or more pathways which pass through the oven or its drying zone spaced apart side by side or above one another so that the layer carriers introduced into the oven can be distributed over these pathways. If a second pathway is provided, the length of the oven or its drying zone is halved while a third pathway reduces the length of the zone to one third.

Since, as a result of the horizontal arrangement of the layer carriers, the oven according to the invention is substantially lower than the known ovens in which the layer carrier are placed vertically, it is particularly advantageous to arrange the pathways one above the other. Uniform distribution of the layer carriers over the individual pathways may advantageously be achieved by means of switch points, for example.

The invention will now be described in detail with reference to exemplary embodiments illustrated in the drawings, in which: Figure 1 is a schematic and broken section of a carrier in the form of a sheet having on one side thereof a layer which is to be dried. The carrier, the thickness of which is exaggerated in the drawing but with the correct proportions of carrier to layer, is situated between a source of radiant heat supplying the energy for drying and a cooling device; Figures 2 to 4 are schematic simplified cross sections through three exemplary embodiments of the apparatus for carrying out the drying process according to the invention; Figure 5 is a schematic, incomplete longitudinal section through an exemplary embodiment having two pathways in the oven.

Figure 1 shows a section through the end portion of a layer carrier (hereinafter briefly referred to as carrier) in the form of a metal sheet 1 in which the wall thickness has been greatly exaggerated. The sheet 1, which is held in a horizontal plane by a mounting device not shown in Figure 1, carries on its upper surface a layer 2 which is required to be dried.

The mounting device is arranged so that it holds the sheet 1 under a heating device generally indicated by the reference 3, which directs radiant energy to the sheet from above for drying the layer 2, and above a cooling device generally indicated by the reference 4 which intensively cools the sheet 1 on its surface 5 remote from the layer 2 by supplying it with cooling medium while the sheet is exposed to the radiant energy for drying the layer 2.

In the example illustrated in Figure 1, the cooling device 4 comprises several nozzles 6 (only one of which is shown in Figure 1) which spray cooling medium in the form of softened water against the back 5 of the sheet 1. The intensity with which layer 2 is irradiated with drying energy may be adjusted so that the cooling medium at the back 5 begins to evaporate. The temperature of the back 5 of the sheet 1 is kept constant at ca. 1 000C by the evaporation process of the softened water while in the region of the layer 2 the temperature may rise to ca.

800"C by the intensive heating by radiation. The intensive cooling of the back 5 thus enables the layer 2 to be subjected to very intensive heat so that it dries rapidly. The length of time during which the layer 2 must be kept under the heating device is therefore correspondingiy short. If the sheet 1 is moved under the heating device 3 at a speed corresponding to the production speed of a preceding continuous sheet printing and lacquering machine, the short dwell time required for drying has the advantage of requiring only a short length of oven in which the process according to the invention is carried out.

Figures 2 to 4 illustrate three examples selected from the large number of possible embodiments of the apparatus for carrying out the process.

Figure 2 shows a tunnel oven 10 in which the interior 11 serving as drying zone contains the heating device 3 in the form of a plurality of infrared radiators 12 which can be switched on individually.

These radiators are provided with reflector hoods 13 which are arranged so that the radiant energy is thrown mainly downwards. The cooling device 4 which is provided in the lower part of the oven 10 comprises a bath 14 of softened cooling water in which the lower part of a driven applicator roller 16 rotatably mounted on a horizontal axis of rotation 15 is immersed. The applicator roller 16 serves a double function. It constitutes a major part of the cooling device 4 in that it collects a film of cooling water by immersion in the bath 14 and transmits it to the back 5 of the sheet 1 whose layer 2 is to be dried, and in addition it forms part of the mounting device which is also constructed as conveyor device supporting the metal sheet 1 as it is moved horizontally through the interior 11 of the oven 10 during the drying process, with its layer 2 facing upwards.Lateral discharge channels 17 are provided to discharge the exhaust air from the interior 11 of the oven 10. This exhaust air contains a proportion of steam derived from the evaporated cooling water and a proportion of evaporated solvent.

The exemplary embodiment shown in Figure 3 differs from that of Figure 2 by a different arrange ment of the individually controlled radiant heaters 12, a different construction of cooling device 4 which corresponds in its arrangement to that of Figure 1, and a different construction of the conveyor device serving as mounting device for the sheets 1 which are to be transported through the oven 10. In the example of Figure 3, the infrared radiators 12 are arranged so that each radiator 12 with its hood 13 extends over the whole width of the sheet 1 which is passed with its layer 2 upwards under the radiators 12 which are arranged one behind the other in the direction of transport of the sheet. The conveyor device consists of a roller train comprising a plurality of transport rollers 18, at least some of which are adapted to be driven.In the gaps between the transport rollers 18, the nozzles 6 of the cooling device 4 spray softened water against the back 5 of the sheet 1.

In the example of Figure 4, the heating device 3 is similar in arrangement to that of Figure 2, i.e. the radiators 12 do not extend transversely to the direction of transport of the sheet 1 as in Figure 3 but parallel to the direction of transport. In the example of Figure 4, not only can the radiators 12 be switched off individually as in the example of Figure 2 to limit the heating zone laterally and adapt it to the format of the layer 2 to be dried on the sheets 1 but in addition pivotally mounted movable lateral shields 19 are provided for adjusting the lateral boundary of the heating zone. The shields 19 may be cooled from the back by additional devices not shown.

The cooling device 4 of the example of Figure 4 is similar to that of the examples of Figures 1 and 3 and comprises a plurality of nozzles 6 for spraying the back 5 of the sheets 1 with softened water. The conveyor device serving as mounting device for the sheets lisa band conveyor with conveyor belt 20 in the form of a screen enabling the nozzles 6 to spray cooling water to the back 5 through the apertures of the screen. The conveyor belt 20 is adapted to be driven and is carried over guide rollers 21, only one of which is shown in Figure 4.

The rapid drying of layer 2 achieved according to the invention enables a shorter oven 10 to be used for a given production rate and consequent transport speed of the sheets 1, as already mentioned above.

Compared to hitherto conventional hot air drying, the additional advantage of increased efficiency is obtained since the quantity of exhaust air and hence the quantity of heat removed from the oven 10 is much less than in hot air drying. The volume to be discharged through the channels 17 according to the invention is substantially only equal to the volume of evaporated cooling water and of the solvent evaporated from layer 2.

The proportion of solvent is also less in the process according to the invention than in conventional processes, for the reasons indicated above.

Since the volume of vapour-solvent mixture discharged through the channels 17 is so small, the mixture may easily be separated in the condenser.

The cooling water supplied to the cooling device 4 may pass through the condenser prior to the vapoursolvent mixture. The separated solvent may be returned to the heating device 3 and burnt there if the heating device shows signs of scarfing.

In the course of the process, the cooling device 4 may continue to have a cooling effect on the back 5 of a sheet 1 after the sheet has moved out of the range of the heating device 3. The sheet 1 is thereby cooled to the temperature for storage or further processing while it is still within the same system. In the example of Figure 2, the lateral boundary of the heat radiating zone is limited by switching off the radiators 12 individually so that the zone can be adapted to the format, as already mentioned above.

In addition to this possibility, the adjustable shields 19 for limiting the zone of radiation in the lateral direction are provided in the example of Figure 4. In the example of Figure 3, the radiant zone can be limited selectively in the longitudinal direction (direction of transport) by switching off individual radiators 12. In all the examples, switching the individual radiators 12 on and off may also be used for adjusting the intensity of radiant heating. In addition, a device may be provided for varying the distance of the radiators 12 from the layer 2 in order to adjust the intensity of heating. Suitable devices may be provided in the cooling device 4 to adjust the intensity of cooling, for example, the nozzles 6 may be designed to be adjustable and/or adapted to be individually switched on and off.Whereas the examples described above have only a single pathway along which the carriers move through the drying zone of the oven during the drying process, the example shown in Figure 5 has a tunnel oven 110 containing two pathways. Both pathways extend over the whole length of the oven, i.e. both through the drying zone which is shown incompletely in Figure 5 and through the adjacent cooling zone (not shown in the figure). The two pathways may have the same form as the pathways described in the previous examples. They are preferably the same so as to provide the same conditions for the drying process.

As shown in Figure 5, a first pathway is formed in this example by transport roller 118, infrared radiators 112 arranged above said rollers and a cooling device 104 below the rollers. As in the example shown in Figure 3, the cooling device has nozzles 106 which are directed upwards to spray softened water against the undersurface of the sheets 101 moving over the transport rollers 118.

A second pathway is arranged above and parallel to the first pathway. It has the same form as the first pathway and is thus formed by transport rollers 118', radiators 112' above said rollers and a cooling device 104' below the rollers. The partition 122 between the radiators 112 of the lower pathway and the cooling device 104' of the upper pathway sub-divides the drying zone of the oven 110 into two chambers arranged one above the other so that the two pathways cannot influence each other.

Both the transport rollers 118 of the lower pathway and the transport rollers 118' of the upper pathway are driven by way of a transmission 123 illustrated schematically so that the speed of transport will be the same on both pathways. The transmission 123 is driven by the machine delivering the sheets 101 to the oven 110, for example by way of a drive shaft 124. In the example given, this machine may be, for example, a printing or lacquering machine. The drive shaft 124 ensures that the movement of the transport rollers 118 and 118' is synchronized with that of the printing or lacquering machine.

Following on the extension 125 at the exit of the printing and lacquering machine, which carries the sheets 101 towards the oven 110 on a conveyor belt 126, is a feed device 127 serving as switch rail comprising a conveyor belt 128 adapted to pivot about a horizontal axis. The pivotal axis coincides with the axis of rotation of the deflecting roller which is adjacent to the conveyor belt 126. The pivot drive in this example consists of a crank drive 129 which is also driven by the drive shaft 124. The pivotal axis of the conveyor belt 128 is positioned so that the end of the belt 128 adjacent to the conveyor belt 126 is at the same level as the conveyor belt 126.When the crank drive 129 has tilted the conveyor belt into its upper position shown in full lines in Figure 5, the front end of the belt is situated at the level of the transport rollers 118' of the upper pathway. Asheet 101 delivered by the conveyor belt 126 is therefore carried to the upper pathway of the oven 110. The crank drive 129 then tilts the conveyor belt 127 downwards into the position shown in dash-dot lines in the figure. In this lowermost position, that end of the conveyor belt 127 which is adjacent to the oven 110 is at the level of the transport rollers 118 of the lower pathway of the oven 110. The following sheet is therefore delivered to the lower pathway.

The crank drive 129 then again tilts the conveyor belt 127 into its upper end position, and subsequently into its lower position and so forth.

In the exemplary embodiment, a switch rail similar to the delivery device 127 is arranged at the end of the two pathways of the oven 110 to transfer the sheets coming from the two pathways to a common conveyor path.

Claims

1. Process for drying paint, lacquer, varnish or the like which forms a layer on one face of a carrier, in which energy for drying the layer is supplied to the side thereof remote from the carrier, and in which, during supply of the energy, the carrier is cooled by a medium supplied to the side thereof remote from the layer.

2. Process for drying paint, lacquer, varnish or the like and capable of being heat dried and which forms a layer on one side of a carrier in the form of a sheet, web or hollow body of metal, in which the energy required for drying is supplied to the layer from the side thereof remote from the carrier, and in which during supply of energy to the layer, the carrier is intensively cooled by a cooling medium supplied to it on the side remote from the layer.

3. Process according to claim 1 or claim 2, wherein the cooling medium is brought to the carrier in liquid form and the temperature of that side of the carrier contacted by the cooling medium is maintained in the region of the evaporation temperature of the cooling medium.

4. Process according to any preceding claim wherein the cooling medium is softened water.

5. Process according to any preceding claim wherein cooling of the carrier is continued beyond the time when the supply of energy is terminated.

6. Process according to any preceding claim wherein the cooling medium is sprayed against the carrier.

7. Process according to any of claims 1 to 5, wherein the cooling medium is applied to the carrier by rollers which dip into the cooling medium.

8. Process according to any preceding claim wherein the energy is supplied in the form of radiant energy.

9. Process according to claim 8, wherein, during the supply of energy to the layer, the carrier is moved relatively to the source of radiant energy.

10. Process according to claim 9, wherein, during drying of the layer on carriers in the form of sheets, the carriers are moved along a substantially horizontal path each with its layer upwards below the source of radiant energy which transmits the energy to the layer from above.

11. Process according to any preceding claim wherein successively delivered carriers are distributed among at least two paths, all the carriers moving along their respective paths undergoing the same heat treatment.

12. Apparatus for carrying out a process according to any preceding claim comprising an oven whose interior constitutes a drying zone within which is a mounting device supporting a carrier on the back surface thereof remote from the layer and at least one heating device supplying energy, wherein the mounting device is disposed between the heating device and a cooling device which supplies cooling medium to the back of the carrier remote from the layer, the mounting device being such as to enable the cooling medium to have access to the back of the carrier.

13. Apparatus according to claim 12, wherein the mounting device forms a mounting surface which holds a carrier sheet in a substantially horizontal plane with the layer facing upwards.

14. Apparatus according to claim 13, wherein the mounting device is a transport device by which carriers can be moved under the heating device along the mounting surface.

15. Apparatus according to claim 14, wherein the mounting device forms part of the cooling device.

16. Apparatus according to claim 15, wherein the mounting device has at least one transport roller mounted with its axis of rotation horizontal, which roller dips in a bath of cooling liquid with its lower circumferential section for applying liquid cooling medium to the back of the carrier lying on it.

17. Apparatus according to claim 15, wherein the mounting surface is formed by a cooling device producing a supporting flood of liquid cooling medium.

18. Apparatus according to claim 14, wherein the cooling device comprises nozzles for spraying liquid cooling medium against the back of the carrier, the mounting surface of the mounting device being permeable to liquid at least in parts thereof so that the cooling medium can have access to the back of the carrier.

19. Apparatus according to claim 18, wherein the mounting surface is formed by a train of transport rollers or transport cylinders, at least some rollers or cylinders being drivable to transport the carrier, and the nozzles being directed towards those regions of the back of the carrier which are situated between the rollers or cylinders.

20. Apparatus according to claim 18, wherein the mounting device is in the form of a conveyor having a perforated conveyor belt in the form of a screen or a grid, the nozzles of the cooling device being arranged between the upper and lower section of the conveyor belt.

21. Apparatus according to any of claims 12 to 20, wherein the heating device has at least one source of radiation supplying energy to the layer in the form of radiant energy.

22. Apparatus according to claim 14 and 21, having a plurality of radiant sources distributed transversely to the path of movement of the layer carrier, which sources are individually switchable on and off to suit the format according to the width of layer to be dried.

23. Apparatus according to claim 21 or claim 22, having adjustable shields for selectively limiting the range of radiant heat in the lateral direction to suit the format of the layer to be dried.

24. Apparatus according to any of claims 12 to 23, wherein, in addition to the path formed by the mounting device, the heating device and the cooling device along which the layer carriers are moved through the oven, the latter contains at least one additional path.

25. Apparatus according to claim 24, wherein the or each additional path extends parallel to the first path.

26. Apparatus according to claim 24 or claim 25, wherein the individual paths are spaced apart one above another.

27. Apparatus according to any of claims 24 to 26, having a switch point for distributing successive layer carriers, preferably uniformly, over the individual paths.

28. Apparatus according to any of claims 24 to 27, having a transfer device by which the layer carriers arriving from the various paths are transferred to a common path.

29. Process for drying a layer of paint, varnish, lacquer or the like, the process being substantially as herein described with reference to the drawings.

30. Apparatus for drying a layer of paint, varnish, lacquer or the like, the apparatus being constructed and arranged substantially as herein described and shown in the drawings.