WO2004040191A1

WO2004040191A1 - Cooling of devices for uv hardening

Info

Publication number: WO2004040191A1
Application number: PCT/DK2003/000730
Authority: WO
Inventors: Knud Andreasen
Original assignee: Knud Andreasen
Priority date: 2002-10-29
Filing date: 2003-10-29
Publication date: 2004-05-13
Also published as: AU2003273789A1

Abstract

The invention relates to a method for the supplementary cooling of e.g. UV lamps for the UV-hardening of photosensitive substrates such as ink, varnish and glue in the graphical field, where some other kind of cooling in the form of e.g. liquid cooling has been established to avoid undesired heating, by collecting at least a part of the radiation emitted by the UV light source not leaving the UV lamp directly or by reflection from any of the parts of the UV lamp by supplementary, selective cooling of the quartz cover of the UV light source and at least a part of the optical units in close vicinity to said UV light source of said UV lamp, with recirculating air.

Description

COOLING OF DEVICES FOR UV HARDENING

The invention relates to a method for the supplementary cooling of e.g. UV lamps for the UV-hardening of photosensitive substrates such as ink, varnish and glue in the graphical field, where some other kind of cooling in the form of e.g. liquid cooling has been established to avoid undesired heating of at least parts of said UV lamp, by collecting at least a part of the radiation emitted by the UV light source not leaving the UV lamp directly or by reflection from any of the parts of the UV lamp by supplementary, selective cooling of the quartz cover of said UV light source and at least a part of the optical units in close vicinity to said UV light source of said UV lamp, with recirculating air.

Apparatuses of the present type as e.g. disclosed in international patent application PCT/DK96/00102 usually require cooling as it is necessary to generate large amounts of UV light within the frequency range from about 180 to about 420 nanometers [nm] in order to achieve the desired hardening of the photosensitive substrates. This involves a certain loss of energy, as only about one third of the overall energy from the generated electric arc will be UV light, even if the currently preferred method of generating UV light for said purpose, making use of discharge lamps, is utilised in full. The remaining two third are radiation in the form of visible light and heat. In addition, the generation of the electric arc also involves a current leakage.

As used in the following, the term UV light source is to be construed including the complete light-emitting device, the vapour lamp, having a quartz bulb including electrodes, wherein an electric arc can be ignited. The radiation from said light source originates from an electric arc generated in the light source when it is switched on. Consequently, said electric arc constitutes the radiation source.

In order to utilise the optical units of a UV lamp, said units reflecting, refracting or in some other way effecting the radiation and being used to lead the radiation in desired directions in a desired manner, it is important that the radiation source is positioned in the focus of the system, which determines the specific position in e.g. an UV lamp where the radiation from the radiation source can be better utilised than would be the case with the radiation source positioned in any other position in the UV lamp. Accordingly, in order to define the course of radiation in the optical system of a UV lamp, it is important to have a well-defined starting point for the radiation, the radiation source.

In a system wherein the light source contains an electric arc, the radiation source is the electric arc generated in the light source when it is switched on, and one problem with an electric arc as radiation source is the fact that the position of an electric arc within the diameter of a vapour lamp's quartz bulb is not constant. During perfect, normal, stable operation, the entire surface of the vapour lamp's quartz bulb has a mainly uniform temperature. This is achieved by establishing cooling of the vapour lamp's quartz bulb. The electric arc can be expected to be approximately cylindrical with a primarily constant cross-sectional area throughout the length of the electric arc, apart from the ends of the electric arc at the electrodes where the cylindrical form tapers towards the electrodes. In vapour lamps of the type used for the present purpose, these phenomena at the ends of the electric arc seldom exceed a distance from the electrodes of the order of 15 mm. In the area of the arc positioned between these end phenomena the cross section is mainly constant and circular seen in a cross section perpendicular to the longitudinal axis of the electric arc, and the axis of the electric arc will be parallel or almost parallel to the axis in the quartz bulb of the light source. The precise position of the electrodes is not of great importance, as they are not decisive for the course of the electric arc in the quartz bulb apart from the mentioned phenomena at the ends. The actual diameter of the electric arc and the actual position of the electric arc in the quartz bulb of the light source will vary according to the voltage keeping up the electric arc, the kind of electricity powering the electric arc and thus the power generated in the electric arc as well as the temperature in the quartz bulb and at the ends of the bulb.

The crux of the matter is that the electric arc only exceptionally is in the centre of the quartz bulb. Thermal currents in the vapour lamp resulting from significant differences in temperature between the interior of the electric arc and the interior of the quartz bulb will cause the longitudinal axis of the electric arc to be at respectively higher or lower levels in the vapour lamp. With a virtually symmetrical cooling of the quartz bulb around the vertical level through the longitudinal axis of the vapour lamp, the thermal movements will be virtually symmetrical around said level. This means that the longitudinal axis of the electric arc is elevated to or close to said vertical level. This elevation can move the longitudinal axis of the electric arc close to the upper quarter of the vapour lamp's diameter, or higher in UV light sources having a quartz bulb with a diameter within the range of the sizes generally used. If the quartz bulb has a large diameter in proportion to the diameter of the electric arc, which will be the case with lamps of high voltage and low power, said elevation of the electric arc means that said electric arc can be so far away from the axial area of the quartz bulb that not a single part of the cross section of the electric arc is in the axial area of the quartz bulb.

An efficient cooling of the quartz bulb will moderate, but not prevent the thermal movements in the quartz bulb and thus reduce the displacement of the electric arc in the quartz bulb. Ideally, the temperature of the quartz bulb should be in the range of 800°C to 1000°C.

It is obvious that such large movements of the electric arc must lead to contemplations regarding the vapour lamp's position in the lamp casing if it is desired to create a well- defined starting point for the radiation. The optimal position of the vapour lamp in the optical system thus depending on the thermal elevation of the electric arc at any time, the vapour lamp can not be given a fixed position in an optical system, but must be movable in such a way that the electric arc at all times will be in the optical centre. Hereby a dynamic focusing of the optical system is obtained.

Given the fact that the better the focusing of the optical system, the lower the power requirements of the light source for a given hardening task, and the lower the power requirements of the light source, the lower the cooling requirements, dynamic focusing will reduce cooling requirements.

In accordance with known methods, the radiation source is placed in one or more holders which can be moved individually or jointly, but maintain the radiation source in a position defined by the position of said holders after fixation.

A radiation source, be it a light source with glow wire or a light source emitting the light from an electric arc, can be positioned approximately so that at least a part of the radiation source, at least periodically, is in said focus. However, due to construction tolerances and changes in the light source during operation, particularly the displacement of the electric arc, this approximate focusing will not be sufficiently precise for the highly developed optical systems used in this type of lamps. In order to ensure that the light-emitting part, be it a glow wire or a type of electric arc, always is in focus, the light source must be movable during the operation of the lamp. This is particularly useful when the radiation source is an electric arc, the form and position of the electric arc in the quartz bulb varying considerably as a result of temperature variations in the quartz bulb, the power of the light source, the temperature of the electric arc and other variables. An increase in the power of the electric arc leads to an increase in the light-emitting diameter of the electric arc, and due to the thermal conditions in the quartz bulb the axis of the electric arc is displaced vertically upward.

The position of the electric arc in the quartz bulb is defined by the overall effect of a number of factors such as the heating of the quartz bulb during passage of the emitted radiation, the heat generated due to convection from the electric arc, the electric voltage above the electric arc, the power of the electric arc, the temperature of the quartz bulb and temperature variations in the longitudinal direction of the quartz bulb and along the circumference etc.

When the lamp is cold and has just been started, the electric arc is thin, about 1 mm in diameter. The voltage and power of said electric arc are quite low in proportion to nominal voltage and nominal effect. Shortly after ignition, said electric arc gives the impression of being unstable as it is very susceptible to thermal conditions in the quartz bulb and these are changing rapidly when the lamp is heated. Simultaneously with increasing temperatures in the electric arc and thus in the quartz bulb, the conditions stabilise and the electric arc in the quartz bulb will be increasingly stationary. The luminous diameter of the electric arc is expanded and the power increased. Hereafter the light can be utilised.

When the lamp is hot after shining with high power for some time and then is adjusted to a lower power, the electric arc will be thin in accordance with the new power, and it will be situated rather high in the quartz bulb due to the high temperature in the quartz bulb. As a decrease in the light source's power lowers the heat in the quartz bulb, the temperature of the quartz bulb will fall, the electric arc thereby descending to a position in the quartz bulb closer to the centre line of the quartz bulb. The changes in the diameter of the electric arc and the position of the electric arc in the quartz bulb are on such a large scale that for a certain period after decrease of the power, which happens quickly, the light source is entirely out of focus. After 10 to 20 seconds, depending on the cooling system, the electric arc will again be close to the centre line of the quartz bulb.

The aim of the invention is to develop a system constantly ensuring that the cover of the light source is moved in such a way that the electric arc is in the focus of the optical system.

According to the invention this is achieved by fixing the ends of the quartz bulb to light source holders, each of which by means of a moving mechanism can be moved to any position within a predetermined area, e.g. a circle having a diameter that is smaller than the diameter of the quartz cover of the light source. Furthermore, said light source holders contain devices for connecting the light source electrically to its electricity supply.

An integrated part of the optical system is a cylindrical part with interior reflection and concentric with the focus line of the system. Said cylindrical part should surround the light source at an angle a least large enough to increase the light intensity of the electric arc measurably when increased by the light reflected from said reflecting cylindric part.

The moving mechanism is controlled by information on the light intensity of the radiation source. One or more pilot cells which are sensitive to at least a part of the characteristic radiation emitted by the radiation source and aimed at the focus line of the optical system, measure the radiation intensity from said light source. In the following, said pilot cells will be called light meters. When the light source or at least one of its ends is moved in an arbitrary direction, the radiation intensity from the electric arc will be changed. If the movement leads the radiation source closer to the focus line, the radiation intensity is increased, and if the movement leads the radiation source away from the focus line, the radiation intensity of the light source is reduced.

The light source is moved continuously in a movement pattern comprising movements longitudinal to and transverse of the direction leading out of the lamp, starting on the system's focus line. The lamp can be positioned to emit light in a vertical direction downwards, and in this case the movement of the light source will be on a vertical level and on horizontal levels. If the lamp is moved from this position by rotation, the movements of the light source are changed accordingly. Fig. 1 of the drawings shows the light source in a lamp emitting the light vertically downwards.

When the electric arc is on the focus line, the optical centre line of the system, the electric arc is in the middle of the central measuring area of the light meter, where the radiation sensitivity is highest. The main part of the radiation emitted from the electric arc in directions to the reflecting cylindrical part, is reflected back in directions passing through the electric arc. The passage of this radiation implies an increase in the overall energy radiation of the electric arc in all directions. This increases the radiation intensity registered by the light meter, and a maximum is reached when the electric arc is on the optical centre line of the system.

The electric arc is on the optical centre line of the system when an arbitrary movement of the electric arc in any direction results in a reduction of the measured electric arc radiation intensity value.

By means of the optical system 6 the sensitivity of the light meter is adjusted in such a way that the display in the central area of its window shows only a slight difference, irrespective of the light entering the light meter on the centre line or slightly diverted therefrom. Fig. 6 shows the sensitivity curve of the light meter. Between the angles V1 and -V1 the variation is depicted by S, which is only a small variation.

The variation in direction to the angles V2 and -V2 is considerably higher. This solves the problem that due to reflections in the reflecting cylindrical part, radiation energy variations of the electric arc can only be measured when at least one section of the electric arc is on the optical centre line of the system. Until the electric arc is in this position, it is indicated by variations of the measured light value after movement of the electric arc if the electric arc is moved away from the optical centre line of the system or if the electric arc is moved closer to the optical centre line of the system.

As the sensitivity of the light meter decreases towards the verge of the measured angle area, a given radiation intensity will result in a higher measured value the closer the radiation source is to the optical centre line of the system. The measured radiation intensity value obtained when the electric arc is on the optical centre line of the system is the highest value obtainable, and this maximum value is used as a feedback for controlling the power of the light source.

Furthermore, this maximum value is used for calibrating the light measuring system after insertion of a new light source. The element measuring the UV light will be aged by the UV light it is subjected to. The ageing is slow, and the precision of the element will not deviate from the required tolerance within the life of a light source.

In practice, it will only be required to displace the light source on one level, viz. the vertical level, but the turning range of the lamp in proportion to this level can only be determined when the lamp has been mounted. The autofocus system is provided with a simple neural circuit which following a test period of the lamp quickly will determine the position of this level in proportion to the normals of the lamp casing and indicate in which direction on this level the electric arc may be expected to be moved by light sources of varying power. This results in adjustments being made at least twice as fast as would be the case if each adjustment was to be carried out on two mutually perpendicular levels.

The vapour lamp will have to be suspended in such a way that the movements of the electric arc in the quartz bulb of the vapour lamp are compensated for by moving the vapour lamp in the opposite direction and the same distance the electric arc has been moved away from the axis of the vapour lamp.

However, it is possible to compensate somewhat for the varying positions of the electric arc by positioning the vapour lamp in such a way that the electric arc only rarely is entirely outside the optical centre. This means that the longitudinal axis of the vapour lamp will have to be positioned in a slight displacement vertically downwards from the optical centre of the system. As the UV lamps can be mounted at an arbitrary angle in proportion to vertical in the printing machines, a prerequisite for the vertical displacement of the vapour lamp is a knowledge of how the UV lamp will be mounted on the printing machine later on. Alternatively, the position of the vapour lamp will have to be adjusted when the UV lamp has been mounted. This ensures that the adjustment is carried out in accordance with the actual turning movement of the UV lamp. An automatic adjustment for the same purpose can be carried out by not installing the vapour lamp in a fixed position in the UV lamp, but adapting the UV lamp in such a way that the vapour lamp like a free pendulum in equilibrium at all times is positioned corresponding to the longitudinal axis of the vapour lamp at a predetermined distance vertically below the optical centre line of the UV lamp when the UV lamp is turned. Hereafter the vapour lamp can be fixed in this position, if desired. This can be achieved by applying a brake which blocks said free pendulum movement of the vapour lamp.

In order to reduce the overall cooling requirement, the entire apparatus has to be constructed for obtaining maximum efficiency. There are several ways of doing it. Most importantly, it must be ensured that the UV light is delivered to the photosensitive substrates in such a way that a maximum reaction is achieved in said photosensitive substrates. Experience shows that a certain amount of UV energy is utilised more efficiently at a high intensity within in a shorter period of time than at a lower intensity within a longer period of time. A certain concentration of the UV light increases the hardening efficiency of a certain amount of UV energy. Therefore, the aim is to increase the flash effect obtained when the photosensitive substrates are transported past the UV light. This is done by concentrating the UV light in a band transverse to the printing medium's direction of travel, thereby obtaining a high UV intensity top value.

However, it appeared that said top intensity can be so high that energy is lost during transillumination of the photosensitive substrates. The maximum hardening efficiency is observed when the top intensity of the UV irradiation is adapted so that the UV radiation is of sufficient intensity to reach all layers of the photosensitive substrate and that said intensity is then retained long enough to give the photo sensitivity time to act, as shown in fig. 4. In order to reach all layers of the photosensitive substrate efficiently, the UV radiation will have to hit the substrate from as many directions as possible. As a result, the UV radiation can pass down into the substrate even if the pigments in the substrate are relatively large and therefore will shade a part of the UV radiation if it is coming from parallel or virtually parallel directions.

According to the invention, an optical system aimed at providing UV radiation of sufficient intensity to reach all layers of the photosensitive substrate and then retaining said intensity long enough for the photo sensitivity to act is achieved by positioning the mainly cylindrical light source within a system of mirrors efficiently reflecting the UV light in said frequency ranges and primarily transmitting radiation at all other frequencies. In a preferred embodiment of the invention, said mirrors are constructed in the form of glass tube sections provided with UV reflective coating.

The inside of the outer cover of the apparatus is provided with a system of liquid- cooled surfaces absorbing the radiation, primarily in the form of visible light and heat radiation, hitting said cooling surfaces either directly or after passage through said UV reflecting mirrors. In a preferred embodiment of the invention, said UV reflecting mirrors are positioned as shown in fig. 3, furthermore showing a number of radiation passages 22, 23 and 24 from the light source, 22 being a ray from the light source containing energy in all the frequencies emitted by the light source, 23 being a ray from the light source which has passed the UV mirror 8 where a part of the energy has been absorbed and a part, particularly UV light 24, is reflected, primarily towards the substrate 10. The ray 23 continues to the cooling device 2, where most of it is absorbed.

The cooling of this type of apparatus is generally achieved by blowing or sucking in fresh air through the apparatus. In a common embodiment, the air is led in through filters collecting dust and other impurities from the air before leading it to the apparatus. The air is led past the parts of the apparatus to be cooled viz. the light source, reflectors and possible other parts which without cooling would be heated to an undesired high temperature. The air thus heated is then discharged from the apparatus, generally through ventilation tubes and by means of air blowers.

On its way to the air ventilation, the air can be passed through filters reducing the amount of ozone formed when the oxygen contained in the cooling air was illuminated by short wave UV light in the wave length range of 180 to 190 nanometers. Ozone is a poisonous gas having a limit value of 0.01 ppm.

The formation of ozone can be avoided if the air used as cooling air does not contain oxygen. If the atmospheric air conventionally used is replaced by e.g. nitrogen, cooling can be achieved without the formation of ozone. As this cooling method is very expensive, it is preferred to filter out the ozone from the discharge air when common atmospheric air is used for the ventilation, or to lead the discharge air to places where ozone is not considered a problem. By using internally recirculating air cooling within a closed system this problem can be avoided. The oxygen present in the apparatus can be converted to ozone, and hereafter the same amount of air is recirculated. If the systems are completely sealed, ozone is not discharged to the environment. In practice, there may be discharged very small and therefore absolutely harmless amounts of ozone, but if it is desired to eliminate the ozone discharge completely, nitrogen or another oxygen-free cooling air may be used as recirculating air, which is possible because the system is a closed system not requiring large supplies of air. Nitrogen is a particularly advantageous choice because it is comparatively inexpensive, does not generate ozone, is not foreign to the environment and has a slightly increased specific heat capacity compared to atmospheric air, which will increase the cooling efficiency of the recirculating air.

Said recirculating air cooling which is a supplement to the regular cooling, the radiation cooling, is adapted so that the amount of air is adjusted in accordance with the requirements and so that the air streams are aimed directly at the parts to be cooled, particularly the quartz bulb of the light source. These measures will contribute considerably to keeping said quartz bulb at a constant temperature within a predetermined temperature range.

In certain types of apparatus such as the present apparatus, cooling is carried out in the form of liquid cooling. In a general embodiment, liquid cooling is based the fact that heat from light sources and hot parts is transmitted by radiation from the hot surfaces to the cooling surfaces according to Stefan-Boltzmann Law. According to this law, the energy transportation follows the difference between the temperature in Kelvin degrees to the 3rd power, but also depends on the emission and absorption factors for hot and cold surfaces, respectively.

The cooling surfaces can be adapted to very high absorption values, but liquid cooling of light sources has the disadvantage that the glass or quartz bulb of the light source has a low emission factor. For this reason the cooling of high-powered light sources cannot be based on radiation cooling alone. From known embodiments it appears that an efficient radiation cooling can be achieved for light sources having a light source power up to about 130 W/cm. Accordingly, the supplementary cooling is mainly required because the emission factor of the light source's quartz cover is not very high. This also applies to the cooling of the other parts of the apparatus if consisting of transmitting materials such as glass or quartz. The other interior parts of the lamp can be cooled sufficiently when they are provided with suitable, emitting surfaces and when the cooling water system is dimensioned accordingly.

Accordingly, the purpose of the supplementary cooling is primarily to counteract an undesired increase in temperature in the quartz cover of the light source and in other transparent materials, and therefore the cooling capacity of the cooling air does not have to be very high.

The present invention relates to methods for supplementing said radiation cooling with a recirculating air cooling.

As said supplementary cooling is not designed to remove large amounts of energy, it can be carried out with quite small amounts of air and therefore in tubes etc. of quite small dimensions.

The apparatus is cooled by integrated liquid and air cooling, said liquid cooling being used for the final removal of the undesired heat energy from the apparatus. Said undesired heat energy is transmitted to the cooling system either directly by radiation or by means of air cooling comprising recirculation of cooling air in a closed circuit, i.a. comprising the unit containing the UV light source.

Hereby a part of the generated and undesired heat energy which is not radiated directly to cooled surfaces, is transmitted to the cooling water by convection and conduction.

Particularly in small apparatuses, the recirculating device can be inserted in the apparatus containing the UV light source and therefore requiring cooling, or it may be positioned outside the apparatus requiring cooling, and in this case it is provided with connections leading to and from said recirculating cooling device which preferably is positioned in close proximity to the apparatus. In the following, the system according to the invention will be described with reference to fig. 1, where an UV lamp 16, wherein the recirculating air is heated by passing hot parts of the UV lamp such as the quartz bulb of the UV light source, optical units and other parts, by means of hollow connection units, e.g. heat insulating tubes 17, 18 and 19, is connected to a unit circulating the air forcibly within the system, e.g. a blower and an air cooling unit 15, wherein the temperature of said recirculating air is lowered to sufficiently so that it during its next passage through the UV lamp will cool said hot parts of the UV lamp so that said hot parts are prevented from reaching detrimental temperatures.

Fig. 2 shows a part of the interior of the UV lamp in which spaces 21 between the UV reflecting mirrors 7 and 8 are provided, between which said recirculating air 6 is directed to the UV light source and continues to the opposite side of the UV lamp, as shown in fig. 3. The UV lamp is closed for air passage in the area where the UV radiation leaves the UV lamp at a UV-transmitting quartz glass 9.

In fig. 3 it is shown schematically how said recirculating air passes preferably transverse to the longitudinal symmetry level of the UV lamp, and how said recirculating air enters and leaves the UV lamp through connections 4 and 5.

In fig. 4 it is shown how the axis 13 of the electric arc 11 ignited in the light source 3 of the UV lamp is displaced perpendicular to the axis in the quartz bulb 12 of the UV lamp. The displacement 20 is to be counteracted by an equal displacement of the UV lamp 3, but in the opposite direction.

In a light source up to about 130 W/cm, the cooling of the vapour lamp can be accomplished exclusively by radiation to radiation absorbing cooling surfaces. If the lamp power is increased beyond this value, the cooling is supplemented by an internal circulation of cooling air according to the invention.

An increase of the vapour lamp power from 130 to 200 W/cm light source requires an estimated amount of air of the following order to remove heat energy from the quartz of the quartz bulb:

The cooling of a light source having a length of 35 cm requires the circulation of about 9 m³ air/h. Correspondingly, a light source having a length of 68 cm requires the circulation of about 18 m³ air/h.

These amounts of cooling air are quite moderate and can be delivered by a multiple step centrifugal blower.

In order to achieve an efficient air cooling of the light source the cooling air will have to touch the light source directly and there will have to be an efficient ventilation from the light source which can return the heated cooling air back to cooling and into a new circulation.

In the present invention this is achieved by splitting the reflector system into several parts which by their mutual distances leave room for air streams, as shown in fig. 2.

In order to enable the cooling of the quartz bulb of the light source and the other parts of the apparatus in their entire or virtually entire length, the air must enter the apparatus in such a way that the amount of air is distributed in a suitable manner.

This can be achieved by allowing the air to enter transverse to the length of the apparatus through a number of nozzles distributing the cooling air uniformly throughout the length of the apparatus, or by allowing the air to enter from one end of the apparatus and compressing it longitudinally in the apparatus with a certain velocity in a rather thin stream. By adapting the apparatus so that the deceleration of the side of the air stream turning towards the light source is accomplished with a suitable strength, as shown in fig. 2, it is possible to establish an air distribution where the air passes transverse to the apparatus with a uniform distribution of the air longitudinally in the apparatus.

The establishment of an air reflux at the opposite side of the apparatus and preferably at the same end of the apparatus will ensure a uniform cooling of both the quartz bulb of the light source and the rest of the interior equipment of the apparatus.

Said cooling air is returned to the blower through said reflux. In this closed circuit the blower will be able to reach a high yield, as the pressure loss in the circuit can be distributed to both the pressure and the vacuum side of said blower. It is generally known that UV lamps of the type described herein have shutter systems shutting off the emitted UV light when the printing medium is at a standstill or moves so slowly that without this function there would be a risk of excessively heating the printing medium and the substrate. In systems only cooled by air, the air streams continue and will also cool the shutter systems when closed. This is indispensable, as the UV radiation in the apparatus aimed at the substrate is shut off by said shutter system when closed and therefore will heat it. Without the air stream the shutter system could be overheated. In a system according to the invention, where the cooling is mainly based on radiation cooling, it will be necessary to supply the shutter system with channels containing cooling liquid, at least when the shutter system is in function, the reason being that the amounts of energy to be collected and carried away from the closed shutter system are too large for the recirculating air cooling system. Furthermore, said shutter system can advantageously be positioned outside the UV lamp cover, and therefore it can not be cooled by the recirculating air circulating inside the lamp cover.

Claims

1. A method for supplementary, selective cooling in a UV lamp unit, from where UV radiation activates photo initiators in photosensitive substrates such as e.g. printing ink, printing varnish and printing glue which are hardened by radiation with said UV rays, said UV lamp unit furthermore being cooled by liquid flowing in one or more mutually connected cavities that with or without connecting heat-conducting materials are mainly positioned along one or more of the outer surfaces of the UV lamp unit not to be penetrable for UV light aimed at the photosensitive substrates, and that during radiation collect parts of the radiation from said UV light source, primarily with wavelengths outside the UV range, an air stream being circulated in a closed circuit comprising at least the following three main parts: said UV lamp unit, an air blower and one or more supply nozzles and tube connections between said main parts.

2. A method according to claim 1 , wherein said air stream in the UV lamp unit is distributed to pass transversely through the UV unit, thereby passing around or at least at a close distance past said UV light source and the parts of said UV lamp in close vicinity to said UV light source and primarily perpendicular to said UV light source, at least in virtually the entire length of said UV light source.

3. A method according to claims 1 or 2, wherein said air stream enters said UV lamp unit through a passage on one side of one of the gable ends of said UV lamp and leaves said UV lamp unit through a passage on the other side of same gable end.

4. A method according to claims , 2 or 3, wherein the velocity of said air stream is adjusted in the range of 0 to a particular, predetermined maximum value depending on the temperature on the surface of the quartz bulb of said UV light source so that the surface temperature of said UV light source is kept at least approximately on a predetermined temperature.

5. A method for supplementary, selective cooling in a UV lamp unit, from where UV radiation activates photo initiators in photosensitive substrates such as e.g. printing ink, printing varnish and printing glue which are hardened by radiation with said UV rays, said UV lamp unit furthermore being cooled by liquid flowing in one or more mutually connected cavities that with or without connecting heat-conducting materials are mainly positioned along one or more of the outer surfaces of the UV lamp unit not to be penetrable for UV light aimed at the photosensitive substrates, and that during radiation collect parts of the radiation from said UV light source, primarily with wavelengths outside the UV range, an air stream being circulated in a closed circuit comprising at least the following four main parts: said UV lamp unit, an air blower, a cooling unit for said air stream and one or more supply nozzles and tube connections between said main parts.

6. A method according to claim 5, wherein said air stream in the UV lamp unit is distributed to pass transversely through the UV unit, thereby passing around or at least at a close distance past said UV light source and the parts of said UV lamp in close vicinity to said UV light source and primarily perpendicular to said UV light source, at least in virtually the entire length of said UV light source.

7. A method according to claims 1 or 2, wherein said air stream enters said

UV lamp unit through a passage on one side of one of the gable ends and leaves said UV lamp unit through a passage on the other side of same gable end.

8. A method according to claims 6, 7 or 8, wherein the velocity of said air stream is adjusted in the range of 0 to a particular, predetermined maximum value depending on the temperature on the surface of the quartz bulb of said UV light source so that the surface temperature of said UV light source is kept at least approximately on a predetermined temperature.

9. An apparatus for supplementary, selective cooling in a UV lamp unit, from which UV radiation activates photo initiators in photosensitive substrates such as e.g. printing ink, printing varnish and printing glue which are hardened by radiation with said UV rays, said UV lamp unit furthermore being cooled by liquid flowing in one or more mutually connected cavities that with or without connecting heat-conducting materials are mainly positioned along one or more of the outer surfaces of the UV lamp unit not to be penetrable for UV light aimed at the photosensitive substrates, and that during radiation collect parts of the radiation from said UV light source, primarily with wavelengths outside the UV range, the apparatus including an air stream generator for generating an air stream in a closed circuit comprising at least the following three main parts: said UV lamp unit, an air blower and one or more supply nozzles and tube connections between said main parts, and optionally designed for performing its operation according to the method according to any of the claims 2 to 8.

10. A UV lamp unit including an apparatus for supplementary, selective cooling in the UV lamp unit, from which UV radiation activates photo initiators in photosensitive substrates such as e.g. printing ink, printing varnish and printing glue which are hardened by radiation with said UV rays, said UV lamp unit furthermore being cooled by liquid flowing in one or more mutually connected cavities that with or without connecting heat-conducting materials are mainly positioned along one or more of the outer surfaces of the UV lamp unit not to be penetrable for UV light aimed at the photosensitive substrates, and that during radiation collect parts of the radiation from said UV light source, primarily with wavelengths outside the UV range, the apparatus including an air stream generator for generating an air stream in a closed circuit comprising at least the following three main parts: said UV lamp unit, an air blower and one or more supply nozzles and tube connections between said main parts, and optionally designed for performing its operation according to the method according to any of the claims 2 to 8.