AID OF MICROWAVES

This invention concerns a method for drying, with the aid of microwaves, a humid layer which is applied on a nonmetallic carrier. The invention further relates to an apparatus for implementing this method, said apparatus comprising a compartment which is adapted to receive at least part of the carrier and in which a microwave-emit¬ ting means is arranged.

In today's industry, microwaves are used for many different purposes, such as the vulcanisation of rubber, the pasteurisation of milk products and the drying of pasta. As compared with conventional heating techniques, the microwave technique here primarily has the advantage of enabling a great saving of time and energy.

In view of these advantages, it may seem surprising that the conventional heating techniques have not been superseded by the microwave technique in a great many more industrial fields of application. The reason for this is that it has proved difficult to exactly antici¬ pate and control the propagation and the effect of the microwaves, owing to various focalisation and reflection phenomena. Accordingly, it is very difficult to dry a humid layer on a subjacent carrier without giving rise to cracks or surface irregularities in the layer as a result of too high a temperature in the carrier material. The object of this invention is to obviate these inconveniences by providing a method and an apparatus which are of the type mentioned by way of introduction and which enable even drying of humid layers.

According to the invention, this object is achieved by a method which is characterised in that at least part of the carrier is introduced into a restricted compart¬ ment, in which are arranged a microwave-emitting means and a microwave-absorbing means, in order to be irra-

diated with microwaves, said microwave-absorbing means having a greater capacity to absorb microwaves than do, when taken together, the carrier part introduced into said compartment and the humid layer applied on said carrier part.

According to the invention, this object is also achieved by an apparatus which is characterised in that said compartment accommodates a microwave-absorbing means which has a greater capacity to absorb microwaves than do, when taken together, the carrier part intended to be introduced into said compartment and the humid layer applied on said carrier part.

By the expression "capacity to absorb microwaves" is here meant the capacity to absorb an electromagnetic field generated by the microwaves emitted by the micro¬ wave-emitting means. In the absorption proper, the elec¬ tromagnetic energy is converted to e.g. heat energy.

In the method and the apparatus according to the invention, the conversion of energy seems to occur chief- ly in the fairly large microwave-absorbing means, proba¬ bly owing to the focalisation phenomena referred to above. Temperature measurements have at least indicated that it is in this means that most of the energy is con¬ verted, while temperature measurements involving the car- rier part introduced into the compartment and the layer applied on this carrier part have shown that but a very limited increase in temperature, or even a reduction in temperature, takes place in the carrier and the layer thereon. Surprisingly enough, the humid layer on the carrier nevertheless dries very quickly as well as even¬ ly, the energy contribution required for the microwave- absorbing means being so limited that the inventive solu¬ tion is advantageous also from the point of view of ener¬ gy consumption, for instance as compared with the hot-air ovens conventionally used for drying paint or varnish. Preferred embodiments of the invention are recited in the appended dependent claims.

The invention will now be described in more detail with reference to three embodiments, which are schemati¬ cally illustrated in the accompanying drawings, in which

Fig. 1 is a sectional view showing a microwave oven intended to be used for drying a varnish layer on a piece of wood;

Fig. 2 is a sectional view showing a microwave oven intended to be used for drying a paper layer on a wire;

Fig. 3 is a side view showing a varnish-drying appa- ratus; and

Fig. 4 is a cross-sectional view of the varnish-dry¬ ing apparatus taken along line A-A in Fig. 3.

In the drawings, like elements are designated by like reference numerals. Thus, Fig. 1 is a sectional view showing a micro¬ wave oven 1 from one short side. The microwave oven 1 comprises an oven chamber 2, in the upper part of which is arranged a magnetron 3, which is connected to a wave¬ guide 4 adapted to emit microwaves 5 into the oven cham- ber 2.

At the bottom of the oven chamber 2, there is pro¬ vided a microwave absorber 6, here shown in its simplest embodiment in the form of a humid piece of wood 7.

Further, a carrier 8 is arranged between the wave- guide 4 and the piece of wood 7. In the embodiment shown in Fig. 1, the carrier 8 is a piece of wood 9 on which is applied a humid layer 10 (here a varnish layer 11). For clarity purposes, the varnish layer 11 is in the Figure shown at a distance from the piece of wood 9, but it will be appreciated that this layer is, of course, joined with the piece of wood 9 in actual practice.

As appears from the Figure, the absorbing piece of wood 7 has a much larger volume than the varnished piece of wood 9, the two pieces of wood being disposed at a distance d from each other. The larger volume of the absorbing piece of wood 7 contributes to concentrating the microwave energy and, hence, the heat generation in

this piece of wood, whereas the varnished piece of wood 9 is but to a fairly small extent affected by this energy.

In a test involving a microwave oven of the type described above, a layer 11 of water-based varnish was applied on a room-dry board measuring 1500 x 350 x 22 mm. The board was then immediately introduced into a micro¬ wave oven 1, in which a plurality of magnetrons 3 having a total output of 5 kW were arranged in a row. Apart from the varnished board 9, a microwave absorber 6 was placed at the bottom of the oven chamber 2, to be more specific a humid piece of wood 7 having a total volume of 30 1 ( to be compared with the volume of the varnished board of barely 12 1 ) . There was a distance d of 30 mm between the two pieces of wood 7, 8. The test was carried out at room temperature, i.e. at approximately 20°C; a temperature at which the normal drying time of the varnish employed is 1.5-2.0 h. The varnished board 9 placed in the microwave oven 1 was, for 30 s, exposed to the microwaves. As a result, the tempe- rature of the board 9 increased to barely 25°C, while the temperature of the absorbing piece of wood 7 increased to above 50°C. When the varnished board 9 was removed from the microwave oven 1 immediately after the microwave treatment, the varnish layer 11 applied on the board was already dry and could be top-coated, despite the fact that the limited temperature increase that could be mea¬ sured should only have had a marginal effect on the dry¬ ing time (1.5-2 h at room temperature).

Fig. 2 shows another embodiment of the apparatus according to the invention. Like Fig. 1, Fig. 2 shows a microwave oven 1 having an oven chamber 2 in which are arranged a magnetron 3 and a waveguide 4. The waveguide 4 emits microwaves 5 towards the bottom of the oven chamber 2, where a microwave absorber 6 is arranged. Here, the microwave absorber 6 is a water bath 12 which is contained in a tub 13 and which, via conduits

14, can be circulated through a cooler 16 with the aid of a pump 15.

Part of a carrier 8, which here is in the form of a wire 17, extends through the oven chamber 2. A humid layer 10 in the form of a paper web 18 is arranged on the wire 17. Naturally, the paper web 18 rests on the wire 17, even though it is here, for clarity purposes, shown to be separated from the wire. The wire 17 and the paper web 18 can be continuously fed through the oven chamber 2 with the aid of pairs of rollers 19 and 20.

Also in the embodiment of the inventive apparatus shown in Fig. 2, the same restricted and even heating has been observed as in the above-mentioned embodiment of the carrier 8 and the humid layer 10 applied thereon, as has the concentration of the microwave energy (evident heat¬ ing of the water bath 12) in the microwave absorber 6. As appears from Fig. 2, the microwave absorber 6 (the water bath 12) comprises also in this embodiment a much larger volume of liquid than does the wire 17 arranged at a dis- tance above the absorber.

Of course, there is nothing preventing a microwave absorber of the type used in the embodiment illustrated in Fig. 2 to be employed also when drying varnish layers or for that matter other similar layers. This type of absorber has the advantage that the cooler 16 which is connectible thereto enables continuous operation for a considerable period of time and that the absorber enables the drying operation to be optimised by a simple adjust¬ ment of the volume of the water bath 12. An embodiment reflecting this fact is illustrated in Figs 3 and 4 and will be briefly described below as regards a few component part differing from the embodi¬ ments described above.

Thus, Figs 3 and 4 illustrate a varnish-drying appa- ratus 21, which is intended to be integrated in a var¬ nishing plant (not shown). This apparatus 21 comprises a floor stand 22 supporting a belt conveyor 23, which is

adapted to feed nonmetallic objects, such as wood-par¬ ticle boards, on which a still humid varnish layer 11 is applied, through an oven chamber 2, preferably from the left to the right as seen in Fig. 3. The oven chamber 2 forms part of a microwave oven 1 which by and large corresponds to the microwave ovens described above. As appears from the cross-sectional view of Fig. 4, several (four to be more specific) magnetrons 3 with associated waveguides 4 are, however, arranged in a row transversely of the belt conveyor 23 so as to produce an even more uniform distribution of the effect of the microwaves across the varnish layer 11 than is already achieved by the inventive absorber 6. Naturally, this solution or similar solutions may also be applied to any of the embodiments described above.

At the bottom of the oven chamber 2, there is pro¬ vided the absorber 6 which, like in the solution of Fig. 2, is a water bath 12 contained in a tub 13, which is arranged between the upper and lower runs 24, 25 of the belt conveyor 23. It will be appreciated that the conveying belt of the belt conveyor 23 must, in the illu¬ strated embodiment, be made of a material allowing micro¬ waves to pass through it. It should, however, be empha¬ sised that the preferred position described here of the absorber 6 opposite to the magnetrons and below the upper run 24 is not the only conceivable position, but that the absorber 6 may also be disposed elsewhere in the oven chamber 2, provided that the capacity of the absorber 6 to absorb microwaves is used to the full. Also, the con- veying belt might then be made of a material not allowing microwaves to pass through it.

In order to get rid of excess heat generated in the water bath 12 in continuous operation, the water bath of the apparatus 21 is, like the water bath 12 shown in Fig. 2, connected to a cooler (not shown), and the oven chamber in the apparatus 21 is further in communication with ventilating ducts 26, 27. In the oven chamber 2,

the air effectively takes up all condensation and is then discharged through the duct 27.

In practice, the varnish-drying apparatus 21 has been found to confer other advantages than extremely rapid drying of the varnish. Owing to the brief period of time elapsing, due to the integration in the varnish¬ ing plant, between the application of a varnish layer 11 and the departure of water or solvent from this layer, but a minor amount of the varnish mixture has time to penetrate into the varnished object. Especially in the case of varnished wooden products, the grain raising will be much less pronounced than before. As a result, e.g. the polishing of the primer is facilitated. Also, there is a much smaller risk that erected fibres in the varnish layer should be polished off, such that the primed sur¬ face becomes more dense and the subsequent varnish layer thus becomes more even and can be made thinner.

Finally, it should be mentioned that it has been found that varnish layers 11 that have been dried in accordance with the inventive method obtain a surface which is highly resistant to blackening when coming into contact with metal objects, and that varnish specialists have, in view of the extremely short drying times made possible by the present invention, held out the prospect of more environment-friendly varnishes.