Marking surfaces

This invention relates to marking surfaces such as with paints or inks.

Our particular interest in marking surfaces arises from the requirement from time to time to mark electrical accessories, for example switch plates, according to their intended use, for example controlling or supplying a "water heater", "cooker", "fan" etc. We have used transfers and been able to meet British Standards applying to resistance to rubbing off. However, cleaning activities over the (often long) life of an electrical accessory inevitably does involve a succeptibility to loss of markings applied by transfers. Also, though only as is to be expected, there is a lesser resistance to scratching than applies to markings applied by line-engraving and filling of engraved lines with marking material. Accordingly, we have also used engraving and filling techniques at least where requirements included scratch resistance.

We find that engraving is readily and efficiently automated by application to engraving tools of computer-controlled X-Y plotter technique, also as would be expected nowadays. However, we found that there does not yet appear to be as efficient and reliable a technique for filling and engraving.

A common technique involves applying a filler material of waxy type and consistency satisfactorily to give a flush finish after filling. However, excess material generally does have to be cleaned off and, whilst that is not difficult in itself, we have noted that some materials, including some plastics used for electrical accessory plates, are left with at least slight staining. Another investigated tehnique involves applying paint or other liquid filler material by way of an hypodermic needle manually tracked round the engraving. However, controlling rate of flow and movement of the hypodermic needle so as properly to regulate liquid filler flow and to avoid all excess proved to be so difficult in practice that a cleaning stage was regularly necessary. Moreover, cleaning off excess paint in conventional manner, say using white spirit, frequently leads to its unwanted incursion into the engraving with consequent risk of thinning and loss of the desired marking.

It is an object of this invention to mitigate the impact of these problems in affording a marking system that is capable of efficient implementation.

According to this invention, marking by filling of an engraved surface by application of flowable marking material is done by driving a dispenser to traverse the engraving with associated means operative to control the amount of material delivered, particularly as to its rate of delivery relative to the engravure and to the rate of traverse.

For a liquid filler material, such as paint, at least relative to line engravings, such as letters or numerals, appropriate control of filler rate of delivery, say continuous for each character or part thereof and interrupted between, can achieve a highly satisfactory resulting effect. For a filler material, such as paint, that shrinks as it dries, it is found possible to fill effectively proud of engraved lines, relying on surface tension effects and subsequent shrinkage, all without risk of flooding over sides of lines of engraved characters, and using computer-controlled X-Y plotter techniques for moving the filler applicator, thus reducing satisfactory filling to devising a suitable computer program for movement and pulsing control then advantageously shaping some basic program material with control of an engraving tool.

Controlled applications of flowable material automatically through dispensers is, in fact, known in itself, at least for application of lubricants, or glue spots or strips. In the latter case, say to packaging materials etc, but not, of course, for the purposes of filling an engravure to serve for marking purposes. Our first trials used a proprietary glue applicator, but substituting paint for glue and allying to computer control for an engraving tool, and were successful beyond our expectations.

A particular advantageous aspect of this invention arises in relation to implementing engraving and filling steps within the same computer-controlled X-Y-Z plotter type equipment, at least by way of an engraving tool and related computer program followed by a marking material applicator and related program, say with the engraving tool and pulse applicator being paired physically relative to a single operating head system.

The marking of workpieces of large surfaces area and/or the surfaces of several workpieces at one loading of a fixture highlighted variations in surface levels on larger surfaces and the even greater variations in level from one surface to another with multiple workpieces and gave rise to problems in accurate filling. We now find that such variations can be accommodated thus eliminating the problems by introducing a "floating head" for the filler, rather than being positively fixed to the movement of the X-Y-Z coordinate drive in the Z direction. By means of a foot or follower which senses the surface level of the workpiece to be filled such as by gliding over the surface of that workpiece, the height of the floating head is automatically adjusted to give the correct positioning of the filler.

Having succeeded in controlling the filling head to compensate for variations in surface levels, the engraving and filling of several workpieces at one loading become a practical possibility providing loading, locating and clamping of the workpieces could be achieved in an economical manner. This has been achieved by providing a mounting frame or jig having abutments for locating each of the workpieces to be engraved and/or marked which, in the case of electrical accessory face plates, is conveniently by way of pegs engaging in the fixing holes of the plates, and means for holding the plates preventing movement during engraving and/or marking. That holding means conveniently comprises a movable frame part cooperable with the mounting frame and engagable with the workpieces to be marked conveniently by way of resilient means. That resilient means preferably comprises flexible tubes which can be selectively pressurised to provide clamping during engraving and/or marking and holding the movable frame in a latching position and then depressurised for unlatching of the movable frame and removal of the marked workpieces.

A further practical development is concerned with the purging and cleaning of the market fluid flow channels such as at the end of a production period or for a change in colour. Previously that involved tedious hand cleaning of the flow lines. By means of a controlled pressure/vacuum cycle, pressure can be used to purge the marker fluid from the flow lines via the dispenser head, and vacuum used to draw solvent into the flow lines via the dispenser head, then purged using pressure, with the last two cycles repeated until all traces of colour are removed.

The present invention will now be described further hereinafter, by way of example only, with reference to the accompanying drawings; in which:-

• Figure 1 is a perspective view of engraving and marking apparatus and associated control means according to one embodiment of the invention,
• Figure 2 is a part sectional side view of a marking head of floating head type,
• Figure 3 is a perspective view of a workpiece holding device,
• Figure 4 is a fragmentary part sectional end view of the workpiece holding clamping device showing latching means.

In the drawing part of X-Y-Z plotter/coordinate machine is shown generally at 1.A first beam 2 is mounted for movement on a frame in the X direction such as per conventional X/Y coordinate machine design or any other convenient method, and therefore not described further. A slide 3 is mounted for movement along the beam 2 in the Y direction. Again conventional drive systems are employed. The motors for controlling movement in the X-Y directions are controlled either manually from a keyboard or alternatively from prerecorded computer data. A keyboard 5, V.D.U. 6 and computer control unit 7 for the apparatus are shown diagrammatically, for illustrative purposes.

The slide 3 carries an engraving head 9 and a marking head 11. The engraving head 9 is rotatably mounted in a housing 13. It can be directly driven by an electric or pneumatic motor or indirectly by way of drive band 15 as illustrated. The housing 13 is mounted for movement in the Z direction, again in a conventional manner, for movement towards and away from a workpiece 17 clamped to a table/bed 19 of the coordinating machine.

As illustrate in Figure 1 the marking head 11 is secured to the housing 13 so as to be movable in the Z direction along with the engraving head. We have found this to be satisfactory for present applications, although separate mounting for movement of the marker head in the Z direction is also envisaged or at least limited movement relative to the engraving head as described hereinafter.

The shaft 21 of the engraving head can in fact be adjusted axially by means of locking collar 23.

The marking head 11 comprises a reservoir 27 for flowable material and is dispensed by way of a hollow needle-like member 25. The dispenser is fed with flowable material by way of pipe 29 from reservoir 31. A control passage 33 connects with the upper side of the reservoir and leads from a material controller 35. Pressure, (either constant or pulsed) generated by the controller 35, is applied by way of passage 33 to the upper surface of the flowable material to act as a feed pump and thus control flow rate to the dispenser head when the marking head is lowered into its desired dispensing position. In its raised position, flow of material is prevented by application of an under pressure to the reservoir. A micro switch controls negative or positive pressure generation of the controller in relation to the position of the marking head, i.e. whether raised or lowered.

A pneumatic actuator serves to raise and lower the marking head relative to the engraving head with air lines 37, 39 being provided for this purpose. Actuation may be manual from a suitable switch or computer controlled as part of the program. This enables the same program used for moving the engraving head to move the marking head after allowing for the offset.

Referring now to Figure 2, here there is illustrated an alternative embodiment of marker head. It comprises a support frame 51 for fixing it to the engraving head by way of suitable fixing screws and a housing 53 movably mounted in an axially slidable manner on the framework by way of two parallel shafts 55. The housing is raisable against the biasing of springs 57 surrounding the shafts, in response to movement of a foot or follower 59 secured to the housing. A piston and cylinder assembly shown at 61, 63 provides for controlled movement of the dispensing head also against the spring biasing and actuated by air pressure through ports 65,67.

The dispensing head is shown at 69 fitted with a hollow needle like dispenser shown dotted at 71 and fed from a manifold 73 which in turn is fed with the flexible marking material by way of nipple 75. It will be noted that the end of the dispenser 71 is aligned horizontally with the bottom of foot 59 which is preferred although it may be slightly higher, but not lower than the foot.

Referring now to Figures 3 and 4, here there is illustrated a holding device especially for multiple workpieces. It comprises a mounting frame 91 having location means for the workpieces which in the illustration is for electrical accessory face plates 92 and comprises a pair of upstanding pegs 93 to engage in the securing screw holes of the respective workpieces. The illustrated device accommodates 9 workpieces in a 3 x 3 matrix which is a convenient number although it will be appreciated that the actual number of workpieces accommodated is optional. The mounting frame is adapted to be located on the table 19 of the X-Y-Z coordinate machine.

Cooperating with the mounting frame 91 is a movable frame 95, actually hinged thereto in the illustration about axis 97. A spring biased holding device 99 is provided for holding the movable frame in an open position and held open by lever 101. Closure involves peg and buttress 100A,B. The movable frame incorporates resilient means which are cooperable with the workpieces. As will be seen the movable frame comprises four longitudinals 103 and two laterals 105 connecting with the opposite ends of the longitudinals. Resilient means are disposed inwardly of the longitudinals and take the form of three pairs of flexible tubes 107, one to each row of workpieces of which only one is visible in the illustration. The tubes are cooperable with the workpieces and are connected to a manifold by which air pressure is admitted to the tubes to expand them to hold the workpieces in position. That air pressure is easily controlled to set on latching of the movable frame in its closed position.

The purpose of the apparatus is to engrave desired charaters into one or more workpieces according to the selected program and then to fill said engraving with a coloured material contrasting to that of the one or more workpieces so that the engraving is clearly visible. Such is particularly advantageous for engraving electrical accessories made of plastics or otherwise and to be marked as to appropriate function, such as water heater or whatever, and for example with such lettering highlighted in red. The flowable material we have in mind is a paint although other flowable coloured materials are possible.

In operation a computer program is produced and used to control movement of the engraving head as to X-Y direction and workpiece contact as required to engrave desired characters in desired position and with movement in the Z direction between separate characters, where interconnection is not required. The program preferably includes positioning checks for determining orientation of workpiece and correct start positions for commencing engraving.

Running of the program thus executes engraving of the desired characters on the or each workpiece whereafter the material dispenser traces the position of these characters and material is dispensed from the nozzle in relation to speed of travel.

In a prototype, the material dispenser follows exactly the path traced by the engraving head using that part of the program after suitable positioning of the dispenser at the start point allowing for offset of the dispenser head from the engraving head and lowering of the marking head by manual or automatic switching.

For practical purposes multiple workpieces are engraved and marked in one loading although it will be understood that the method is equally applicable to a single workpiece, such as for one off items, and that the reference to multiple loading of nine workpieces is merely an example of one suitable number.

The workpieces, such as electrical assessory face plates, are positioned on the mounting frame in positions determined by the aforemention abutments, i.e. pegs 93 in the example. The movable frame 95 is then lowered over them and into a latched position, where-upon a valve is actuated to pressurise the resilient means, i.e. tubes 107, causing them to be urged forcibly into contact with the workpieces to hold them positively. The programmed operation of the apparatus is then commenced, with the engraving head moving to successive workpieces by appropriate controlled movements in the X, Y and Z directions and making the programmed engraving.

Thereafter, under program control the movement sequence is repeated with the dispensing head following the programmed sequence and the flow of ink controlled to dispense the fillable material into the engraving, shadowing the movements of the engraving head to ensure correct filling of the engraved characters. Pressure applied through line 33 gives dispensing when required and vacuum prevents dispensing when moving between workpieces. This is controlled by the program.

The floating head embodiment can be used, for a flat workpiece, to override the basic positioning. The foot rests on the surface of the workpiece actually taking up movement in the Z direction against the resilient biasing so that the dispensing head is at an appropriate height which ensures that the ink enters the engraving.

On completion of the marking by filling, the resilient means is depressurised and latch released to enable the movable frame to be raised and the completed workpieces removed to be replaced by another set.

Alternatively separate specific program material may be used to control movement of the dispenser head since all detailed movements may not be essential to ensure adequate filling of the engraving where an easily flowable material is involved which may spread into areas of the engraving not otherwise traversed by the dispensing head. The amount of material dispensed is conveniently controlled in relation to size of engravure.

The tip of the dispensing head can be at a higher level than the tip of the engraving tool as it is not required to actually project into the engraving where sufficiently small oriface size is used and atmospheric disturbances allow vertical depositing of material into the engravure from above. However, where the tip of the dispensing head can be arranged to project below that of the engraving head, the former may advantageously trace the engravure with accurate filling thereof and no overflow inconjunction with control of amounts delivered.

At the end of a production run or where a change of marker colour is required, the pressure controller is actuated to purge out the marker liquid from the reservoir and lines, and the vacuum employed to draw in solvent via the dispensing head. This in turn is purged by application of pressure. This drawing in of solvent and purging is repeated until there is no trace of solvent.