GB2134449A

GB2134449A - Laser printing apparatus

Info

Publication number: GB2134449A
Application number: GB08302724A
Authority: GB
Inventors: William Henry Stevens
Original assignee: LASERPRINT
Current assignee: LASERPRINT
Priority date: 1983-02-01
Filing date: 1983-02-01
Publication date: 1984-08-15
Also published as: GB8302724D0

Abstract

The laser printing apparatus comprises a pulsed laser (1) and a beam optical system (5) to direct the beam from the laser through a selected one of a plurality of printing masks arranged in units (11, 13) in succession laterally of the axis of the beam as it emerges from the laser, the optical system comprising in respect of each mask except the last in the succession a beam-reflecting member (7) which can be moved between a position in which it directs the beam through the printing mask and a position in which it leaves the laser beam unobstructed, and in respect of the last of the masks, a beam deflector (15) to direct the beam through the mask (13) when the beam is not obstructed by an upstream beam deflector (eg. 7). <IMAGE>

Description

SPECIFICATION Laser printing apparartus The present invention relates to laser printing apparatus.

Laser printing apparatus is known and is being increasingly used for the purpose of printing codes and the like on product containers e.g. cartons and plastic bottles. Limitation, however, of this technique of printing is that it permits only a small area to be imprinted on any one operation of the laser and a need exists to be able to increase the area of print that can be achieved but without increasing the power requirement of the laser or alternatively of decreasing the power dissipation of the beam per unit area of the matter to be printed.

It is an object of the present invention to provide laser printing apparatus by which the above-described needs can be satisfied.

There is provided by the present invention laser printing apparatus comprising a pulsed laser and a beam optical system therefor to direct the beam from the laser through a selected one of a plurality of printing masks arranged in succession laterally of the axis of the beam as it emerges from the laser; said optical system comprising in respect of each mask except the last in the succession of beam-reflecting member which can be moved between a position in which it directs the beam through the printing mask and a position in which it leaves the laser beam unobstructed.

In use, the succession of printing masks will be arranged parallel wih a products conveyor so that the laser beam passing through a mask is directed onto a product on the conveyor to effect printing thereon. Because it takes the product moving on the conveyor a period of time to pass from one mask to another, and since it is possible to operate a laser at a greater frequency than a product can be conveyed from one mask to another, the present invention makes it possible to use the beam from a single laser without any power dimunition to print the material of a plurality of masks on the product.

Because of the nature of the material which is printed by means of a laser, usually it will be sufficientto have two masks although it will be understood that in principle the present invention could employ any number greater than two provided that the system does not give rise to the requirment that two masks have got to be printed at the same time by the same beam.

For simplicity of description hereinbelow, it will be assumed that the apparatus employs two masks and, therefore, one displaceable beam reflecting member.

The displaceable beam reflecting member, hereinafter called the beam deflector for convenience, is conveniently made displaceable by bodily rotation thereof and preferably at a constant speed. For this purpose, it is conveniently used in plurality in the form of a disc bearing slots generally radially arranged so that the portion between each two adjacent slots constitutes a beam deflector. The rotation is conveniently effected by mounting the beam deflector or the disc comprising the plurality thereof on a rotatable shaft, which may be rotated by means of an electric motor.

In one presently preferred embodiment of the present invention, the beam deflector is continuously rotated in use of the apparatus and this makes it necessary to employ means for sensing the angular orientation of the beam deflector at any given time. This means may conveniently employ a beam obstructor, or, where the beam deflector disc is employed, a plurality of beam obstructors, mounted on the shaft with the or each beam obstructor being mounted in the same orientation as the respective beam deflector.The means further comprises a source of light to produce a light beam and a photo-detector to sense the light beam; the light beam being arranged so that on rotation of the beam obstructor or the plurality thereof in the form of the disc the beam obstructor or each of them in turn obstructs a beam of light to the photo-detector at the same time that the beam deflector or a respective one thereof is in a position to obstruct the beam emerging from the laser. The photo-detector thus provides a signal indicating the angular orientation of the beam deflector and thereby indicates which of the two masks can be used to effect printing.

The preferred embodiment also comprises, for each mask to be used, a sensor to sense when a product has reached a position just before the position of the mask to provide a signal to indicate the imminence of the arrival of the product at that position. The means for sensing the angular orientation of the beam deflector also comprises a control circuit such that, if at the time a product sensor senses a product, the beam deflector is positioned to prevent the beam being directed at that mask, the control circuit delays operation of the laser until the beam deflector is positioned to permit the beam to be deflected through that particular mask. For this purpose, the control circuit may comprise a gate arrangemnt whereby only on the coincidence of a signal from a product sensor and an appropiate signal from the photo-detector is an output signal provided to fire the laser.The arrangement may comprise for each mask to be used a coincidence gate comprising two AND gates one receiving input from the downstream sensor and from the photo-detector and the other receiving an input from the upstream product sensor and from the photo-detector.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure lisa diagrammatic plan view of a laser printing apparatus according to the present invention; Figure 2 is an end elevation of a device comprising a displaceable beam reflecting member; Figure 3 is a front elevation of the device of Figure 2; Figure 4 is an end elevation of the device of Figure 2 from the opposite end showing a slot sensing arrangement; and Figure 5 is a circuit diagram of a control circuit employed in the apparatus of Figure 1.

Referring now to the drawings, the apparatus of the embodiment comprises a pulsed, molecular gas (CO2) laser operating at substantially atmospheric pressure. The laser delivers its beam through an outlet 3 into an optical system generally indicated at 5. The optical system comprises a first mirror7 (see especially Figure 2) and a second mirror 9 both angled with respect to the beam so as to be capable of deflecting the beam through 90" to respective units 11, 13 containing a printing mask (not shown). Since printing with a laser beam using a printing mask is already well known, this technique will not be further described in the present specification. The mirror 9 is fixed in a position in a housing 15, but the mirror 7 is carried on the shaft 17 of an electric motor 19 (see especially Figure 3) so as to rotate with the shaft of the electric motor.The mirror 7 is in the form of a segmented disc the adjacent segments 21 of which define between them generally radial slots 23. It will be apparent that on rotation of the disc alternately a segment of the disc and an intervening radial slot will be presented to the pathway of the beam from the laser. Each segment of the disc is itself capable of deflecting the laser beam and on rotation of the disc each segment in turn is presented in the pathway of the laser beam.It will be evident from this that, if the laser is fired with the frequency with which any adjacent segment and slot of the disc 7 successively lie in the pathway of the beam, the beam will be alternately deflected by a segment of the mirror 7 and by the mirror 9 so that it alternately prints the mask in the two prints 11, 13. To permit the firing of the laser to be synchronised with the rotation of the mirror 7, the shaft of the electric motor 19 at the end thereof remote from that carrying the mirror 7, carries an orientation sensing disc 25. This disc comprises two component discs 27,29 each formed with generally radial slots 31 with the two component discs being rotatable in relation to one another so that the two discs can form slots 33 which can be varied in width.The arrival of a slot 33 at a given orientation is sensed by a photo-detector device 35 comprising a lamp 37 positioned at the periphery of the disc 25 at one face thereof and a photo-detector 39 positioned at the periphery of the disc at the other face thereof so that whenever a slot leaves the beam from the lamp 37 unobstructed, the beam is received by the photo-detectorto provide a signal. By varying the width of a slot 33, the length of the signal emitted by the photo-detector can be varied according to need.

In use of the illustrated embodiment, the apparatus is positioned laterally of a conveyor 41 on which are carried articles, in this instance imagined to be rectangular boxes such as box 43, to be printed on. The print units are directed to face a lateral side of the article to be printed on so that the imprint is formed on that lateral side ofthe article. The arrival of an article at a position immediately before a print unit is sensed by a photo-detector, namely, photo-detector 45 in the case of print unit 13 and photo-detector 47 in the case of print unit lithe lamps of the photo-detectors not being shown.

It will be understood that the disc 25 is mounted on the shaft of the electric motor with the radial axis of a slot therein aligned with the radial axis of a slot in the mirror 7 so that, whenever the photo-detector 39 detects a slot of the disc 25, a radial slot of the mirror 7 must be positioned in the same orientation. By fixing that orientation in relation to the position which the beam occupies in relation to the axis of the mirror 7, it can be further deduced whether or not a segment of the mirror or a slot of the mirror is positioned in the pathway of the beam.

The illustrated embodiment also comprises a control circuit, which will be further described hereinbelow in connection with Figure 5, to ensure that the laser is fired only when there is a coincidence of a signal from photo-detector 45 and from photo-detector 39 indicating that a radial slot of the mirror 7 is positioned in the pathway of the laser beam or when there is a coincidence of a signal from photo-detector 47 and a signal from the photo-detector 39 indicating that a radial slot of the mirror 7 does not lie in the pathway of the beam.The motor is operated at a constant speed and since this speed is chosen with regard to the speed of the articles on the conveyor and since the laser is capable of being fired at any rate required, it follows that it is not necessary to know positively that a segment of the mirror 7 is positioned in the pathway of the laser beam i.e. the absence of a signal from photo-detector 39 indicating that a slot is present in the pathway of the beam can be taken as indicating that a segment 7 is properly positioned in the pathway of the beam so as to be capable of deflecting it into printing unit 11. It will be apparent from this that the signal from photo-detector 39 that does not satisfy the required coincidence in respect of printing unit 13 is the signal that does satisfy the requirement of the coincidence required in respect of printing unit 11.

Turning now to Figure 5, the control unit CC comprises an arrangemant of four 2-NAND gates 51,53,55 and 57 arranged to form two separate coincidence gates, namely, one to sense the coincidence relevant to printing unit 13 and the other to sense the coincidence relevant to printing unit 11. In this instance, the two-stage gate combination of gates 51 and 53 may be taken as being that sensing the former and the two-stage gate combination of gates 55 and 57 as sensing the latter. In each combination, the output of the first gate is connected to one of the inputs of the second gate with the output of the second gate being connected to a switch arrangement comprising transistors 59 and 61 which on receiving a signal from either of the gates acts to fire the laser. Gate 51 receives an input from photo-detector 39 and its second input by means of a feed-back line from the output of gate 53 whereas gate 53 receives its second input from photo-detector 45. Similarly, gate 55 receives one input from photo-detector 39 and the second input by means of a feed-back line from the output of gate 57 whereas gate 57 receives its second input from photo-detector 47.

The Truth Table illustrating the gates logic is shown below. In respect of the coincidence gate for the print unit 13, the 1 input to the first stage of the gate from photo-detector 39 means that the photo-detector indicates that the laser beam is unobstructed by mirror 7 and the 0 signal provided by photo-detector 45 at the second stage of the gate means that the photo-detector 45 has sensed an article on the conveyor.

Similarly in respect of coincidence gate for print unit 11, the 1 signal from photo-detector 39 supplied to the first stage of the gate means that the photo-detector has sensed the absence of the slot of the disc 25 and the 0 signal from sensor 47 is supplied to the second stage of the gate again indicates that the sensor has sensed the article on the conveyor.

The output signal from either coincidence gate which is capable of operating the switch arrangement triggering the laser is taken as being a 1 signal and it will be evident from the Truth Table that each gate only responds to the respective coincidence to provide an output signal to fire the laser.

Thus, in operation of the illustrated embodiment, when the leading leadge of the article 43 has passed the printing mask of unit 13 by a predetermined amount the laser is fired at that instant at which its pathway to the mirror 9 is unobstructed and the material on the mask is imprinted on the side of the article. When the article then progresses to a position in which its leading edge has passed the printing mask inprint unit 11 by a predetermined amount, the laser is again fired at the instant the pathway of the beam is obstructed by a segment of mirror 7 thereby causing the material on the printing mask to be imprinted on the side of the container adjacent to the imprint from the mask of print unit 13.Usually it is required that the areas printed by the two masks should be adjacent and this can be achieved by means of the present invention within acceptable tolerances at the conveyor speeds now commonly employed in production lines. It may be, however, that no requirement arises for the imprints of the respective printing masks to be positioned adjacently to one another. In this case, even higher conveyor speeds can be achieved for any given speed of the electric motor. The rate of printing can also be increased by increasing the speed of the motor to the limit imposed by the pulse duration of the laser.

TRUTH TABLE INPUTS TO INPUTS TO INPUTS TO GATE GATE 1STSTAGE 2NDSTAGE OUTPUT RESULT (1) 0 0 1 o (2) 0 0 0 (1) 0 0 1 o (2) 1 0 1 (1) 1 1 0 LASER 1 (21 0 1 0 FIRES (1) 1 1 1 o (2) 1 0 1

Claims

1. Laser printing apparatus comprising a pulsed laser and a beam optical system therefor to direct the beam from the laser through a selected one of a plurality of printing masks arranged in succession laterally of the axis of the beam as it emerges from the laser; said optical system comprising in respect of each mask except the last in the succession, a beam-reflecting member which can be moved between a position in which it directs the beam through the printing mask and a position in which it leaves the laser beam unobstructed.

2. Apparatus according to Claim 1, wherein the beam optical system is such as to be able to direct the laser beam through a selected one of two masks.

3. Apparatus according to Claim 1 or 2, wherein the displaceable beam reflecting member, hereinafter called the beam deflector, is made displaceable by being arranged for bodily rotation thereof.

4. Apparatus according to Claim 3, wherein the beam deflector is arranged for bodily rotation thereof at a constant speed.

5. Apparatus according to Claim 3 or 4, wherein the beam deflector is used in plurality in the form of a disc bearing slots generally radially arranged so that the portion between each two adjacent slots constitutes a beam deflector.

6. Apparatus according to Claim 3,4 or 5, wherein the rotation is effected by mounting the beam deflector or the disc comprising the plurality thereof on a rotatable shaft.

7. Apparatus according to Claim 6, wherein the shaft is rotated by means of an electric motor.

8. Apparatus according to any of Claims 3 to 7, wherein the beam deflector is continuously rotated in use of the apparatus, and the apparatus, and the apparatus comprises means for sensing the angular orientation of the beam deflector at any given time.

9. Apparatus according to Claim 8, wherein said sensing means comprises a beam obstructor or, where the beam deflector disc is employed, a plurality of beam obstructors, mounted on the shaft with the or each beam obstructor being mounted in the same orientation as the or the respective beam deflector, and a source of light to produce a light beam and a photo-detector to sense the light beam; the light beam being arranged so that on rotation of the beam obstructor or the plurality thereof in the form of the disc the beam obstructor or each of them in turn obstructs a beam of light to the photo-detector at the same time that the beam deflector or a representive one thereof is in a position to obstruct the beam emerging from the laser.

10. Apparatus according to any of the preceding Claims, including the masks.

11. Apparatus according to Claim 10 as dependent on Claim 8 or Claim 9, wherein, for each mask to be employed situate a product line and parallel thereto, a sensor is employed to sense when a product has reached a position on the line just before the position of the mask to provide a signal to indicate the imminence of the arrival of the product at that position.

12. Apparatus according to Claim 11 as dependent on Claim 9 wherein said means for sensing the angular orientation of the beam deflector also comprises a control circuit such that, if at the time a product sensor senses a product, the beam deflector is positioned to prevent the beam being directed at that mask, the control circuit delays operation of the laser until the beam deflector is positioned to permit the beam to be deflected through that particular mask.

13. Apparatus according to Claim 12, wherein the control circuit comprises a gate arrangement whereby only on the coincidence of a signal from a product sensor and an appropiate signal from the photo-detector is an output signal provided to fire the laser.

14. Apparatus according to Claim 13, wherein the gate arrangement comprises for each mask to be used, a coincidence gate comprising two AND gates one receiving an input from the downstream sensor and from the photo-detector and the other receiving an input from the upstream product sensor and from the photo-detector.

15. Laser printing apparatus substantially as hereinbefore described with reference to the accompanying drawings.