This invention relates to UV dryers and, in particular, provides a printing machine having a UV dryer fitted therein.

UV dryers are frequently used in the printing industry for drying photopolymerisable inks. The UV lamp is disposed transversely to the direction of feed ofthe freshly printed sheets or web, and the web or sheets are passed alongside the dryer at a speed such that the exposure time is sufficient to dry the ink. Heat is also emitted from UV dryers and they are sometimes fitted with shutters in order to protect the web from over-heating when the printing machine stops. This is because excessive heat may cause the web to shrink or be damaged. One commercial UV dryer fitted with pivoting shutter blades is described in PCT Apphcation WO 93/02329.

Most UV lamps seek to overcome this problem by reducing the power output ofthe lamp as the web speed reduces. However this solution does not provide a complete answer because below a power output of about 40% of maximum, a UV lamp is not stable and the heat output can therefore not be further reduced. As a result, the printed web or sheets become very hot when the web is moving at low speeds, such as about 10% of normal speed. If this causes shrinkage of the web, registration becomes very difficult. The present invention seeks to overcome this problem

According to one aspect of the present invention, there is provided a printing machine which comprises at least one inking unit and at least one UV dryer positioned downstream from said inking unit, said dryer having a UV lamp mounted in a reflector housing, said housing having an open mouth directed towards a path for printed sheets or a web, wherein said open mouth is

closeable by a shutter and wherein the movement ofthe shutter is linked to the speed of travel ofthe printed sheets or web through the rrachine, such that as the speed of travel ofthe web is reduced, the degree of opening ofthe shutter is correspondingly reduced. Preferably, the degree of opening ofthe shutter is substantially proportional to the speed of travel ofthe web.

It will be appreciated that the shutter is controlled so that the higher the web speed, the greater the opening of the shutter. In practice, as described in more detail below, the lamp output will be reduced initially as the web speed falls, while maintaining the shutter position at or close to its maximum opening. Normally, only after the lamp output has been reduced to about 40% of maximum power will the shutter opening be reduced to decrease the radiation received by the more slowing moving web.

In one embodiment, the mouth of the reflector housing is closeable by one or more pivoted shutter blades, movement of the blades being controllable by the speed ofthe web or sheets.

Most printing machines incorporate a tachometer or other speed indicator which generates an electrical signal, such as a voltage or series of pulses, the value or frequency of which is indicative ofthe speed of he web.

In accordance with this invention, the electrical signal from the web tachometer can be used directly to control the opening of the shutter. The output signal from the tachometer may also simultaneously be used to control the output from the lamp so that as the speed reduces, the output ofthe lamp is simultaneously reduced to a minimum setting of 40% of the maximum output ofthe lamp. Reduction ofthe operating output ofthe lamp can be controlled,

e.g. by thyristor control ofthe electrical supply to the lamp in accordance with a corresponding reduction in the speed ofthe web.

During the phase from 100% lamp output down to 40% output, the shutter may be controlled to remain fully open, but as the speed of the web drops below a speed at which a 40% output would cause excessive heating of the web, the shutter blades begin to close. The closing of the shutter blades can be arranged to be stepwise in one or more steps down to full closure or, altematively, arranged to be a continuous movement.

One embodiment of the present invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 shows a schematic view of a continuous web being drawn through a printing machine;

Figure 2 is a section through a UV dryer fitted with pivoting shutter blades;

Figures 3A and 3B are respectively an end view and a partial horizontal elevation ofthe shutter mechanism ofthe dryer shown in Figure 2;

Figure 3C is a view similar to Figure 3 A but with the shutters in a closed position; and

Figure 4 is a block diagram ofthe control system.

The printing machine may be fitted with one or more UV dryers 1, corresponding to a series of colour printing units 2, 3, 4 and 5. The web is drawn through the machine on take-up reels 6, 6A or 7 and a speed indicating device or tachometer can be fitted to the take-up reels or a drive thereto. Preferably, the tachometer 9 directly senses the speed of travel ofthe web. The tachometer generates a voltage which is proportional to the speed of the web 8

through the machine and this voltage is applied to a control device arranged to control the power supply to the UV lamps. This control device is also arranged to control movement of a shutter blade or blades movable between an open and closed position across the lamp.

The tachometer may be mechanical or optical (visible or I.R.). A probe 9 may be located adjacent the web and generate a signal or series of pulses by detecting movement of a series of identifying marks on the web. Alternatively, the speed of the web may be sensed by direct contact with a roller or sprocket which is linked to a reed switch or other device to convert rotary movement into an electrical output.

The lamp is shown in more detail in Figure 2, which is a view partly in section of a UV dryer fitted with shutter blades. The dryer comprises a UV lamp 10 which is mounted within a reflector housing 11, and has a mouth 12 directed towards the web 8 passing the open mouth of the lamp. The open mouth of the lamp can be closed by a pair of pivoting shutter blades 13 whose construction and operation is described in the above-mentioned PCT Application WO 93/02329.

The UV lamp may be cooled by air or water or a combination of both. In the construction illustrated, the housing 11 is formed as a water jacket for passage of cooling water and the lamp itself is cooled directly by an air curtain supplied through tube 14.

In operation, the shutter blades 13 are linked to a rotary disc 15 in the manner described in the above-mentioned PCT Apphcation and the disc may be driven electrically, mechanically or hydraulically by an appropriate motor.

Figures 3A 3B and 3C show the mechanism for operating the shutters. Mounted on one end of the lamp housing 2 is the operating mechanism for a pair of shutters 31, which consist in a pair of shutter blades. The shutter blades are each pivotably mounted on an associated shaft 32 via a pivot plate 33. In the operational condition ofthe lamp, the shutter blades he parallel with the longitodinal side members of the lamp housing so as not to impede the passage of light from the lamp. This condition is illustrated in Figure 3A. Also, in Figure 3C, the position ofthe blades in the closed position is indicated. As can be seen, one blade closes just before the other and the second blade closes onto the first.

Pivoting ofthe shutter blades is effected by a rotary actuator 34, (which may be driven pneumatically, hydrauhcally or by electric power). The rotary actuator is connected by a drive shaft 35 to a driven disc 36. The shutter blades 31 are linked to the disc 36 by lever arms 37. The arrangement is such that on effecting rotation of the actuator in the direction of the arrow X in Figure 3A, the shutter blades are pivoted towards each other as indicated by the arrow Y until they touch or overlap.

In normal operation, the shutter blades are normally opened fully as soon as the output from the lamp exceeds 40%. However, when the web speed drops below a level where even a lamp operating at 40% output would cause excessive heating ofthe web, the control device referred to above causes the shutters to partially close. The control device is linked to the motor driving the shutters either directly in the case of an electric motor or to a valve controlling supply of air or hydraulic fluid to the motor. Because the shutters are not fully closed at slow web speeds, the dryer will continue to apply a

drying effect to the web, but at a reduced level and thus the danger of excessive heating ofthe web is avoided.

Figure 4 shows the control system. Reference numeral 40 indicates the tachometer which is connected to the sensor 9 and generates a signal which is fed to the CPC (central processing unit). Control of the lamp output is effected by means of the lamp power supply 41 which applies a voltage to the lamp 42, depending on the signal received from the CPC. Lamp 42 has a feed back loop 43 connected to a photocell or other detector so that the operating level of the lamp (or even total lamp failure) is monitored by the CPC. A pneumatic control valve 44 controls the operation of the pneumatic actuator 34, which actuates the shutters 31. A feedback line 45, connected to a detector at the shutters, enables the CPC to monitor the position ofthe shutter blades.