Prior Art From US-A-5,086,570 a drying apparatus for screen printing is previously known, said drying apparatus comprising high frequency means in combination with hot air means in order to effect drying of the screen printed products. Due to the essential drying effect that contributes from the high frequency means the size of the drying apparatus may be reduced simultaneously as it is working at a comparatively low temperature. In this connection it should however be notified that the high frequency means consumes a great deal of energy.

From SE-B-455 479 it is previously known to use IR- radiating units to effect drying of print on a material. In order to avoid heating of a large part of the material and in order to reduce the power consumption only those IR-units are used that are oriented in such a way that they are able to dry only the applied print.

From SE-B-455 480 it is previously known to use lamps that generate ultraviolet radiation as drying or curing means.

In order to reduce the power consumption the invention starts out with the print drying/print curing means arranged for intermittent operation and the control device for intermittent operation being arranged to activate the means for drying/curing during time periods when a wet print layer is adjacent to the drying/curing means.

Objects and Features of the Invention A primary object of the present invention is to disclose a drying device of the type defined above, the air velocity being considerably higher than for known drying devices.

A further object of the present invention is to reduce the energy consumption since the high air velocity generates fiction losses that implies a higher temperature to the air used for the drying process.

Still an object of the present invention is to arrange for an effective evacuation of the drying air.

At least the primary object of the present invention is realised by means of a drying device that has been given the features defined in the appending independent claim 1.

Preferred embodiments of the drying device according to the present invention are defined in the dependent claims.

Brief Description of the Drawings Below an embodiment of the drying device according to the present invention will be described, reference being made to the accompanying drawings where: Fig 1 shows a schematic side view of a drying device according to the present invention; and Fig 2 shows a schematic end view of the drying device according to Fig 1.

Detailed description of a Preferred Embodiment of the Drying Device according to the Present Invention The drying device according to the present invention shown in Fig 1 and 2 comprises an outer framework 1 that is equipped with insulation and plates, said framework 1 in its turn supporting a vacuum box 3, a driven roll 5, a turning roll 6 and an endless conveyor belt 7 that runs around the driven roll 5 and the turning roll 6. Preferably the conveyor belt 7 comprises a metal net that is covered by teflon. An upper part of the endless conveyor belt 7 abutting the upper side of the vacuum box 3 while a lower part of the endless conveyor belt 7 running inside the vacuum box 3 adjacent its lower limiting surface.

Preferably the vacuum box 3 is in the shape of a parallelepiped and is preferably manufactured from metal sheet. As is evident from both Fig 1 and 2 a crossbar structure is provided inside the vacuum box 3, said crossbar structure according to the shown embodiment comprising a number of first plate girders 9 and a number of second plate girders 10 that have C-profile. The first plate girders 9 extend in the direction of movement of the conveyor belt 7 while the second plate girders 10 extend transverse to the direction of movement of the conveyor belt 7. Said girders 9 are equipped with holes 11 and traversely arranged inside the vacuum box 3, said girders 9,10 in the mutual intersection points being connected to each other, preferably by means of welding. On its upper side, i. e. where an upper part of the conveyor belt 7 is running, the vacuum box being equipped with a number of perforations/holes (not shown) having the object of sucking the sheet shaped material to be dried against the upper part of the endless conveyor belt 7, said upper part in its turn being sucked to abutment against the vacuum box 3.

As is evident from Fig 2 and 3 the lower part of the conveyer belt 7 being located between the lower limiting surface of the vacuum box 3 and the crossbar structure.

On the lower side of the vacuum box 3 a number of first fans 12 are provided, in the shown embodiment three fans, that are connected to the vacuum box 3 by means of sleeves 8 of flexible material, e. g. rubber. These first fans 12 are connected to the vacuum box 3 in such a way that they may create a negative pressure in said vacuum box 3. Each one of the first fans 12 comprises a fan motor 13, a fan casing 14 and an impeller 15 provided in the fan casing 14, said impeller 15 being provided on an output shaft of the fan motor 13. The first fans 12 are preferably of radial fan type. A distribution box 16 belongs to each first fan 12, the fan outlet of each fan 12 being connected to the distribution box 16. Nozzle tubes 17 are attached to the distribution box 16, said nozzle tubes 17 being only schematically shown in Fig 1 and 2. The nozzle tubes 17 are arranged above the vacuum box 3, each nozzle tube 17 being in the shape of an elongated element that extends transverse to the direction of movement

of the conveyor belt 7. In this connection it should be pointed out that an important feature of the device according to the present invention is that the nozzle tubes 17 are arranged at a distance from each other, i. e. the drying air that is ejected from the nozzle tubes 17 may circulate freely around said nozzle tubes 17 subsequent to said drying air having hit the sheet shaped product and become solvent saturated. The distance between the nozzle tubes 17 may of course vary, e. g. due to the size of the device according to the present invention. In exemplifying and non-restricting purpose it may be mentioned that said distance is in the magnitude of 25 mm.

A second fan 18 is provided above the nozzle tubes 17, said second fan 18 comprising a fan motor 19, a fan casing 20 and an impeller 21 located in the fan casing 20, said impeller 21 being provided on an output shaft of the fan motor 19. The second fan 18 is preferably a radial type fan. An evacuation pipe 22 is connected to the fan casing 20, the drying air being evacuated through said evacuation pipe 22. This will be explained more in detail below.

The drying device according to the present invention comprises a pressure equalisation means 24 that is connected to the vacuum box 3, via a pipe 23, said pressure equalisation means 24 guaranteeing that the negative pressure in the vacuum box 3 is not increasing too much. Said pressure equalisation means 24 comprises preferably a valve that opens when the negative pressure in the vacuum box increases too much, whereby air may flow into the vacuum box 3 and reduce the existing negative pressure.

The drying device according to the present invention functions in the following way. The sheet shaped product that is about to be dried, said product being equipped with screen print, is fed on the upper part of the endless conveyor belt 7. Said sheet shaped product is then brought to bypass the nozzle tubes 17 and drying air is ejected through said nozzle tubes 17. In order to bring the sheet shaped product to remain on the endless conveyor belt 7 a negative pressure is created in the vacuum box 3 by means of the first fans 12.

The air that is evacuated from the vacuum box 3 by the first

fans 12 is symbolised by the first arrows 25. It is important for the drying process that the hole configuration of the crossbar structure of the vacuum box 3 is such that the air flow in the vacuum box 3 is not disturbed to a too high degree by said crossbar structure. Simultaneously it must also be observed that the crossbar structure should give sufficient rigidity to the vacuum box 3, i. e. it must not be deformed by the negative pressure generated by the first fans 12.

The air that has been evacuated from the vacuum box 3 by the first fans 12 is via the fan casing 14 forced out into the respective distribution box 16 that belongs to each fan 12.

This is symbolised by the second arrows 26. The air 26 that bypasses the distribution box 16 then continues into the different nozzle tubes 17 that are distributed along the direction of movement of the endless conveyor belt 7. The air that is ejected through the nozzle tubes serves as drying air for the screen print that is present on the sheet shaped products that are located on the upper part of the endless conveyor belt 7. A significant feature of the drying process according to the present invention is that the drying air is ejected from the nozzle tubes 17 at an extremely high velocity, more precisely up to about 90 m/s. In exemplifying and non-restricting purposes it may be mentioned that the air velocity in connection with the method according to the present invention preferably is in the interval of 60-90 m/s.

It is of course realised that the first fans 12 of the drying device according to the present invention must be designed in such a way that they manage to generate such an air velocity.

This high air velocity brings about that the drying air is heated due to the friction in the nozzle tubes 17, said drying air normally being heated to about 55°C. This means that no further heating of the drying air is needed, this being frequent in connection with prior art drying. Since the drying air both has a relatively seen high velocity and a relatively seen high temperature, said drying air is capable of disposing large amounts of solvent from the surface of the sheet shaped article that is to be dried. A pre-requisite for the drying process to continue in an acceptable way is however that the air ejected through the nozzle tubes may be disposed

in an effective way subsequent to its saturation by solvent.

For this purpose the second fan 18 is provided, said second fan 18 creating a negative pressure around and above the nozzle tubes 17. This means that the drying air at first flows out of the nozzle tubes 17 in a direction downwards, towards the sheet shaped product that is to be dried, and then, when the drying air is saturated by solvent, the air is sucked upwards past the nozzle tubes 17 and then into the fan casing 20. In order to provide an effective evacuation of the saturated drying air over the entire surface of the sheet shaped product it is important, as pointed out above, that the nozzle tubes 17 are arranged at a certain distance from each other in order to make it possible for the drying air, saturated by solvent, to freely circulate between the nozzle tubes 17. From there the drying air is discharged via the evacuating pipe 22. Due to the fact that the second fan 18 creates a negative pressure on the side of and above the nozzle tubes 17 the drying air that is saturated by solvent will permanently be evacuated from the area close to the sheet shaped product that is to be dried. By this arrangement it is guaranteed that an effective drying of said she shaped product takes place.

If it should occur that the negative pressure in the vacuum box 3 should increase to such an extent that the sheet shaped product tends to get stuck to the upper side of the vacuum box 3, i. e. that the feeding of the endless conveyor belt 7 is retarded or stopped, the pressure equalisation means 24 starts to operate. This means that a valve is opened and the negative pressure in the vacuum box 3 is equalised. This happens automatically and the feeding of the conveyor belt 7 continues and without any disturbances.

By the method described above and the device for drying described above the process is made more effective to such a high degree, compared to prior art, that the supplied electrical energy may be decreased by about 60% and the length of the drying device reduced to about 1/3 of the length of a prior art drying device.

Feasible Modifications of the Invention In the embodiment described above the vacuum box 3 comprises a crossbar structure that consists of plate girders 9,10 that intersect each other. However, within the scope of the present invention it is feasible that the crossbar structure is designed in several different ways. The demands that the crossbar structure should prevent deformation of the vacuum box 3 and not obstruct the air flow to any degree worth mentioning must however be fulfilled.