WO2020070480A1

WO2020070480A1 - Label marking apparatus

Info

Publication number: WO2020070480A1
Application number: PCT/GB2019/052761
Authority: WO
Inventors: John Cridland; Tristan Phillips
Original assignee: Datalase Limited
Priority date: 2018-10-01
Filing date: 2019-10-01
Publication date: 2020-04-09
Also published as: GB2577690A

Abstract

A label marking apparatus suitable for use with a label substrate comprising colour change material operable to change colour in response to controlled exposure to laser light, the apparatus comprising a label supply for retaining and supplying the label substrate, transport means for transporting the label substrate from the supply to a marking zone, a marking head operable to emit laser light so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone and at least one roller to cool the label substrate, where the roller surface is adapted to absorb, diffuse or deflect incident light. In an alternate embodiment, a cooling block is provided, which comprises a facing surface provided with one or more slits in the marking zone, the slits aligned with the light emitted from the marking head.

Description

LABEL MARKING APPARATUS

Technical Field of the Invention

The present invention relates to label marking. In particular, the invention relates to an apparatus and method for label marking using one or more lasers to mark said labels.

Background to the Invention

Traditionally, labels for products and/or packaging have been pre-printed in bulk and applied to the products/packaging as required. The drawback to such actions is that the information on pre-printed labels is fixed. This does not provide flexibility to customise labels or to adapt labels readily at short notice.

One solution to this issue is to digitally print labels on demand using inkjet printers. This requires the supply of liquid inks, solvents and the like at the point of printing. This can be undesirable in many sectors, such as food and beverages, particularly if the inks/solvents provide a contamination risk.

In such cases, labels may be produced using laser marking. Various forms of laser marking apparatus are known and are used in conjunction with label substrates that comprise colour change material. Upon controlled exposure to laser light from the marking apparatus, portions of the substrate change colour forming a desired image. The image may be monotone or coloured depending on the material and/or the nature of the exposure. The image may comprise text, numbers, codes or the like as well as pictographic elements.

In a typical apparatus, the substrate is provided in the form of an extended tape on a supply reel. The tape is unwound from the supply reel via one or more rollers and passes a laser marking head. This enables successive like or custom labels to be marked on the substrate. The laser making head may comprise a single laser or a multisource laser array. Subsequently, the tape can be wound on to a storage reel via one or more further rollers for later application to products/packaging. Alternatively, individual labels may be cut from the tape and applied directly to products/packaging. To induce colour change the substrate must be exposed to a threshold energy density. The bulk of the laser energy required for colour change is absorbed in the colour change material but a portion will be absorbed by the other components of the substrate, or will be conducted away from the colour change material following the colour change reaction by thermal diffusion. In many cases, waste heat generated during operation of the apparatus may also be absorbed by the substrate. This results in the substrate heating up. In the case of substrates formed from thin tapes this may increase the temperature of the tape to a level where the mechanical properties of the tape change. In particular the tape may become softer and more pliable.

This issue is exacerbated when a multisource laser array is used for marking. In such circumstances, a large number of adjacent sources may be activated at the same time. This produces a continuous or intermittent line that heats the substrate. This can result in a band of tape that has reached sufficient temperature to significantly soften that region of the tape. In some cases, this can result in a ripple forming in the tape which may lead to folding over on itself after marking, rendering the label unusable.

It is possible to provide a reel control system operable to monitor the tension in the tape and measure the diameter of the reels. This allows automated adjustment of the supply reel rotation and storage reel rotation to maintain constant linear speed, thereby minimising the tension applied to the substrate. Nevertheless, even the use of sophisticated reel control system does not eliminate the prospect of there being sufficient tension in the tape to cause the ripple and folding.

It is therefore an object of the present invention to provide a label marking apparatus and method that at least partially alleviates the above problems.

Summary of the Invention

According to a first aspect of the present invention there is provided a label marking apparatus suitable for use with a label substrate comprising colour change material operable to change colour in response to controlled exposure to laser light, the apparatus comprising: a label supply for retaining and supplying the label substrate; transport means for transporting the label substrate from the supply to a marking zone; a marking head operable to emit laser light so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein at least one roller is provided to cool the label substrate, where the roller surface is adapted to absorb, diffuse or deflect incident light..

The provision of the cooling unit in the above apparatus allows for the heating effect of marking to be overcome or at least mitigated. This therefore reduces the prospect of the label substrate forming a ripple and folding over during the marking process and/or during subsequent transport.

The cooling unit may be provided in the marking zone or adjacent to the marking zone. In instances where the cooling unit is provided adjacent to the marking zone, it may be provided immediately prior to the marking zone such that the substrate is cooled before being subjected to laser illumination. Alternatively, and preferentially in instances where the cooling unit is provided adjacent to the marking zone, it may be provided immediately after the marking zone such that the substrate is cooled after being subjected to laser illumination.

Beneficially, the interaction between the substrate and the roller is relatively low in friction. Use of a roller also reduces potential problems in relation to charge build or abrasion between the cooling unit and the substrate.

. In some embodiments, the active cooling system may comprise a network of internal conduits through which a cooling fluid may be circulated. The cooling fluid may be water, air or a refrigerant. In further embodiments, the active cooling system may comprise one or more thermoelectric cooling devices.

The roller may be formed from any suitable metal. In this context, suitable metals include, but are not limited to: steel, nickel, zinc, copper, aluminium, aluminium alloys or the like.

The roller surface may be adapted to reduce the fraction of incident light reflected directly back along the incident ray path. This can reduce the fraction of emitted light being reflected back into the marking head and thus reduce the prospect of potential damage to the marking head if the substrate supply to the marking zone fails. In some embodiments, the roller surface may be adapted to absorb incident light. This may be achieved by the provision of an absorbing layer. The absorbing layer may comprise a chemically blackened layer or an anodised layer. The absorbing layer may be formed by plasma deposition, flame spraying or the like. If required, the absorbing layer may be further treated to have a smooth matt surface. In other embodiments, the roller surface may be adapted to diffuse or deflect incident light. This may be achieved by any one or more of a roughened layer.

In some embodiments, the roller surface may additionally or alternatively comprise a transparent coating. The transparent coating may have a substantially smooth outer surface. If the transparent coating is sufficiently thick, light emitted from the marking head will not be in focus when incident upon the roller surface and hence will not be efficiently reflected back into the marking head. The transparent coating may comprise any suitable transparent material including but not limited to glass, fused silica, enamel, glass ceramic enamel, glass ceramic, diamond, sapphire or the like.

The thickness of the transparent coating may be, say, greater than lmm, or, say, greater than 3mm, or, say, greater than 5mm. The thickness of the transparent coating may be selected by considering both the reduction in reflection of incident light and the thermal conductivity of the coating material.

In some embodiments, the roller may be provided with a fluid layer between an outer transparent shell and the roller surface. The transparent shell may comprise any suitable transparent material including but not limited to glass, fused silica, enamel, glass ceramic enamel, glass ceramic, diamond, sapphire or the like. The thickness of the transparent shell may be, say, lmm to 2mm. The thickness of the fluid layer may be, say, greater than 3 mm, or, say, greater than 5 mm.

The fluid comprising the fluid layer may be indefinitely retained between the roller surface and transparent shell. In other embodiments, the fluid comprising the fluid layer may be circulated through an active cooling system. The cooling system may be integrated with an active cooling system for the roller. The fluid may be transparent. The fluid may comprise a solution or suspension containing light absorbing materials. Suitable light absorbing materials include dyes and/or NIR (near infrared) absorber material.

According to a second aspect of the present invention, there is provided a label marking apparatus suitable for use with a label substrate comprising colour change material operable to change colour in response to controlled exposure to laser light, the apparatus comprising: a label supply for retaining and supplying the label substrate; transport means for transporting the label substrate from the supply to a marking zone; a marking head operable to emit laser light so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein at least one cooling block is provided to cool the label substrate, the or each cooling block comprising a facing surface over the label substrate, wherein the facing surface is provided with one or more slits in the marking zone, the slits aligned with the light emitted from the marking head.

The facing surface of the cooling block may be arranged such that the label substrate runs over the facing surface of the cooling block. In this manner, the cooling block may provide a heat sink for the label substrate.

The facing surface may have a curved shape. The curved shape may be convex with respect to the direction of transport of the substrate. Within the marking zone, the curved shape is preferably only convex in the direction of transport of the substrate. This helps to simplify focussing of the light emitted by the marking head on the substrate. At either side of the marking zone, the curved shape may be convex in a direction transverse to the direction of transport of the substrate.

In some embodiments, the substrate may run directly over the facing surface of the block. This facilitates heat conduction between the substrate and the block. If the block is provided in the marking zone, this also has the benefit of precisely defining the separation between the substrate and the marking head. This can help to ensure that the light emitted from the marking head is in focus at the substrate. This is particularly beneficial if the marking head has a limited depth of focus. The facing surface of the cooling block may be substantially smooth. The facing surface may be polished to enhance smoothness. This can minimise the friction between the facing surface and the substrate. This can thereby reduce the frictional heating effect of contact between the substrate and the facing surface. Additionally, minimising friction reduces the build-up of charge between the substrate and block and/or the build-up of material abraded from the substrate on the facing surface. In such embodiments, the cooling block may be formed from any suitable metal. In this context, suitable metals include, but are not limited to: steel, nickel, zinc, copper, aluminium, aluminium alloys or the like.

The slits in the marking zone may extend inwardly from the facing surface of the cooling block. The inward end of the slits may terminate in an opposing side of the cooling block. A beam absorber may be provided at or aligned with the inward end of the slits. In this manner light emitted by the marking head passes through the slit and is absorbed in the case that the substrate supply to the marking zone fails. This prevents emitted light being reflected back into the marking head and potentially damaging the marking head.

In alterative embodiments, the substrate may run over the facing surface of the block at a small separation from the facing surface. In such embodiments, the block may be adapted to enable the emission of a flow of air from the facing surface. This therefore generates an air cushion upon which the substrate travels over the facing surface. In some embodiments, the emission of a flow of air may be facilitated by dedicated air flow conduits provide in the block. The air channels may be a suitable pattern of discrete holes or the facing surface may be formed from a porous material such as a porous metal matrix or porous carbon.

The contact distance L during which the label substrate is in contact with the cooling unit may be selected so as to ensure that sufficient cooling is achieved. In some embodiments, the contact distance may be determined in response to the expected transport velocity V of the label substrate. In particular, the contact distance L may satisfy:

L > 2_X10 ⁵ . V. t_f/K Where L is in m, V is the product speed in m/s, t_f is the label substrate thickness in m and k is the thermal diffusivity in m²/s.

In some embodiments, the cooling unit may comprise one or more fluid blowers. Such fluid blowers may be operable to output a flow of cooled fluid, which is subsequently incident upon the substrate. The cooled fluid may be air, water or the like. The fluid blower may be provided with a supply of cooled fluid. Alternatively, the fluid blowers may be provided with an active cooling system for cooling supplied fluid. The active cooling system may comprise a heat exchanger supplied with coolant and/or a heat exchanger powered by thermoelectric devices.

The apparatus may comprise a convex roller subsequent to the marking zone.

The apparatus may comprise a position sensor. The position sensor may be operable to monitor the transporting of the label substrate so as to ensure that images are marked at the required spatial pitch of the pixels. The position sensor may comprise an encoder that outputs pulses in response to the motion of the substrate. The encoder may be an optical encoder. The encoder may be connected to a roller that is in contact with the substrate as it is transported. The encoder roller may be adjacent to the marking area the position sensor may additionally or alternatively comprise a sensor operable to detect one or more registration marks and or the edges of previously marked images. Such a sensor may be an opto-sensor. This can allow the apparatus to ensure that images are marked at the correct location and/or such that successive images are marked at the correct separation. This may be particularly relevant where the label tape has previously printed with logos and the like by standard printing techniques. For an initially blank substrate the position of the image may be determined from the encoder counts.

The label supply may comprise a reel upon which label substrate is wound. Subsequent to the marking zone, the label substrate may be transported to a label store. The label store may comprise a reel upon which marked label substrate is wound.

In alternative embodiments, subsequent to the marking zone, the marked label substrate may be transported to a label cutter and/or a label applicator. The label cutter may be operable to cut individual marked labels from the substrate for application to objects. The cutter may comprise a blade or a laser beam. The applicator may be operable to apply a cut labels to an object. The applicator may comprise a roller or brush operable to press the label on to the object.

The transport means may comprise one or more belts or rollers. Any of or all of said one or more belts or rollers may be powered.

The transport means may be operable to transport the substrate through the marking zone substantially continuously or in indexed steps. Additionally, or alternatively, the transport means may be operable to halt travel of the substrate during marking illumination. This can enable the marking of higher definition images or higher definition sections within images. This is particularly advantageous for marking barcodes within images.

The marking head may comprise a single laser emitter or multiple emitters. The marking head may further comprise a lens arrangement operable to focus the light emitted by each emitter. In some embodiments with multiple emitters, the emitter may each be lasers mounted in the marking head. In other such embodiments, the emitters may comprise the emitting ends of optical fibres, wherein the input ends of said fibres are coupled to one or more lasers mounted outside the marking head. The multiple emitters in the marking head may be arranged in an array. The array may be a one- dimensional or two-dimensional array. Where the array is a one-dimensional array, it may be a simple linear array or it may be a staggered array. Provision of the emitters in the head in an array enable the generation of a corresponding array of emitted laser beams for marking.

The multiple laser emitters may be individually addressable. In particular, the individual laser emitter may comprise an individually addressable laser array (IALA) or an individually addressable laser diode array (IALDA).

In some embodiments, multiple marking heads may be provided. This can enable a multi-stage marking process. In such embodiments, successive marking heads may be operable to emit laser light in different wavelength ranges. In such embodiments, each marking stage may be provided with a dedicated cooling unit. The label substrate may comprise an elongate tape. The colour change material may be provided in a colour change layer. The colour change layer may be a surface layer or coating.

The label substrate may comprise a linerless label substrate.

The base layer may comprise paper or a polymeric film. Suitable polymeric films include but are not limited to polypropylene or polyethylene. More particularly, suitable polymeric films may include high and low density polyethylene, polypropylene homopolymer and copolymers, orientated polypropylene (opp) and bi-axially orientated polypropylene (bopp), Polyethyleneterapthalate (PET) and the like. Where the base layer is paper, the colour change layer may be omitted and the paper may be impregnated with a colour change material.

The substrate thickness may be in the range 20pm to lOOpm. In some embodiments, the substrate thickness may be in the range 25pm to 60pm.

The colour change material may comprise substances including but not limited to any of: a metal oxyanion, a leuco dye, a diacetylene, a charge transfer agent or the like. The metal oxyanion may be a molybdate. In particular, the molybdate may be ammonium octamolybdate (AOM). The colour change material may further comprise an acid generating agent. The acid generating agent may comprise thermal acid generators (TAG) or photo-acid generators (PAG). In one embodiment, the acid generating agent may be an amine salt of an organoboron or an organosilicon complex. In particular, the amine salt of an organoboron or an organosilicon complex may be tributyl ammonium borodisalicylate.

The substrate may comprise an NIR (near infrared) absorber material. The NIR absorber material may be provided in a common layer with the colour change material or in a separate layer adjacent to the colour change material. The NIR absorber material may be operable to facilitate the transfer of energy from an NIR laser illumination means to the colour change material. The NIR absorber material may comprise substances including but not limited to any of: Indium Tin Oxide (ITO), non- stoichiometric reduced ITO, Copper Hydroxy Phosphate (CHP), Tungsten Oxides (WOx), doped WOx, non-stochiometric doped WOx and organic NIR absorbing molecules such as copper pthalocyanines or the like.

The emitters may be operable to emit light with any suitable wavelength, including but not limited to visible or near infrared (NIR) wavelengths. Generally, for marking applications, wavelengths in the range 200nm to 20000nm might be suitable. In some embodiments, the emitters are operable to emit light with wavelengths in the ranges: 390 to 460nm, 500 to 550nm, 620 to 660nm, 900nm to l lOOnm and 1400 to l600nm.

According to a third aspect of the present invention there is provided a method of marking labels comprising colour change material operable to change colour in response to controlled exposure to laser light, the method comprising: supplying label substrate; transporting label substrate to a marking zone; emitting laser light from a marking head so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein the method includes the additional step of cooling the label substrate using the apparatus of claims 1 to 10.

The method of the third aspect of the present invention may incorporate any or all features of the first aspect of the present invention as desired or required.

According to a fourth aspect of the present invention there is provided a method of marking labels comprising colour change material operable to change colour in response to controlled exposure to laser light, the method comprising: supplying label substrate; transporting label substrate to a marking zone; emitting laser light from a marking head so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein the method includes the additional step of cooling the label substrate using the apparatus of claims 11 to 15.

The method of the fourth aspect of the present invention may incorporate any or all features of the second aspect of the present invention as desired or required.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic illustration of a label marking apparatus according to the prior art;

Figure 2 is a schematic illustration of a label marking apparatus according to a first embodiment of the present invention, wherein the cooling unit comprises a roller;

Figure 3 provides schematic illustrations of different embodiments of a cooling unit comprising a roller according to the present invention; and

Figure 4 is a schematic illustration of a label marking apparatus according to a second embodiment of the present invention, wherein the cooling unit comprises a block.

Turning now to figure 1, a schematic illustration of a known apparatus 1 for marking a label substrate 4 comprising colour change material is shown. The label substrate 4 is in the form of an elongate tape. Unmarked substrate 4 can be supplied from a label supply, typically in the form of a reel (not shown). The substrate 4 moves in the direction 9 from the supply to the marking zone 5. The movement of the substrate 4 from the supply to the marking zone 5 is achieved by use of transport means, typically comprising powered supply and storage reels and one or more change of direction rollers, such as roller 7. In some implementations one or more of the change of direction rollers may also be powered.

In the marking zone 5, laser light is emitted from a laser emitter 2 and is focussed by lens arrangement 3 on the substrate 4. Together, the laser emitter 2 and lens arrangement 3 comprise a marking head 2, 3. As the substrate 4 continues to move past the marking head 2, 3 in direction 9, a marked area 6 along the substrate 4 is defined. By modulating the light emitted from the marking head 2, 3, colour change in the marked area 6 can be controlled resulting in the marking of an image or series of images in the marked area 6. The images in the marked area 6 may comprise a series of like or individualised labels.

Whilst the examples shown incorporate a single marking head 2, 3 the skilled man will appreciate that multiple marking heads 2, 3 may be used in alternative embodiments. Such multiple marking heads 2, 3 may be staggered to mark different areas of the substrate 6. Additionally or alternatively, multiple marking heads 2, 3 may be adapted to emit light in different wavebands enabling colour images to be marked using a multistep process. The skilled man will also appreciate that in different embodiments, the laser emitter 2 may comprise a single emitter, an array of emitters or multiple staggered arrays of emitters.

Past the marking zone 5, the substrate 4 may travel on to a label store typically comprising a reel (not shown). Alternatively, the substrate 4 may travel on to a label cutter, typically a mechanical blade or laser cutter (not shown) and/or to a label applicator. The onward travel may be facilitated by the label applicator, one or more powered rollers or the powered label store of the transport means.

As a result of the incident marking radiation, the marked area 6 of the substrate 4 experiences a rise in temperature. The marked area 6 is thus softened and is susceptible to forming a ripple and folding 8 as it passes around roller 7. This can result in a folded or distorted portion 14 where part of the marked image is obscured. Such a folded portion 14 is unsuitable for use as a label.

In view of the above, the present invention proposes the use of a cooling unit operable to cool the label substrate 4 in the marking zone and/or adjacent to the marking zone. By cooling the substrate 4, the rise in temperature of the marked area 6 can be mitigated such that the marked area 6 is no longer susceptible to forming a ripple and folding or distorting or is at least less susceptible to folding or distorting.

Turning now to figure 2, one embodiment of an apparatus 100 for marking a substrate 4 comprising colour change material the present invention is illustrated schematically. This embodiment 100 of the present invention differs from the prior art apparatus 1 in the provision of a cooling unit comprising a roller 10. The substrate 4 runs over the surface 15 of the roller 10 as it travels from the supply to the store or cutter/applicator. The roller 10 is typically formed from metal such as steel, nickel, zinc, copper, aluminium, aluminium alloy or the like.

The roller 10 is provided with an active cooling system (not shown). The active cooling system can typically comprise one or more thermoelectric elements or a network of conduits through which a cooling fluid can be circulated. By virtue of the active cooling of the roller 10, it is maintained at a lower temperature than the substrate 4. Accordingly, contact between the substrate 4 and the roller surface 15 results in the substrate being cooled. If the roller 10 and particularly the roller surface 15 is maintained at a suitably low temperature the substrate 4 is cooled sufficiently to mitigate or eliminate the prospect of the marked area 6 folding as it passes around roller 7.

Beneficially, the roller surface 15 provides precise definition of the separation between the marking head 2, 3 and the substrate 4 within the marking zone 5. This can help ensure that light emitted from the marking head is focussed appropriately in the marking zone 5. In the event that the substrate 4 breaks or runs out, it is possible for light emitted from the marking head 2, 3 to be reflected off the roller surface and back into the marking head. This can cause damage to the marking head 2. Accordingly, the roller surface 15 can be adapted to mitigate this risk.

For the case of a single emitter 2 (or single emitter array 2), this emitter 2 could be aligned directly with a radial axis 11 between the marking head 2, 3 and the centre of the roller 10. Alternatively, the single emitter 2 may be displaced to one side of said axis. The displaced implementation has the benefit that any light reflected from the substrate 4 (or roller 10 in the absence of substrate 4) surface would occur at a small angle to this axis and hence would not be reflected directly back to the emitter 2.

In the case of two emitters (or two emitter arrays 2) staggered in the direction of motion of the substrate 4, the marking head 2, 3 may be positioned such that the image of the staggered emitters 2 is displaced to either side of a radial axis 11 between the marking head 2, 3 and the centre of the roller 10. This provides an equal distance between the lens 3 and substrate 4 for each emitter 2. It also has the benefit of avoiding reflection directly back to the emitters 2.

Turning now to figure 3 a, one adaptation of the roller surface 15 is illustrated. In this example, the surface 15 comprises a chemically blackened, plasma coated, flame sprayed or anodised layer 16 which absorbs radiation. In this manner reflections from the roller surface 15 are mitigated. An alternative possibility is shown in figure 3b. In this example, the roller surface 15 is provided with a roughened layer 17 which acts to diffuse or deflect incident light away from the marking head 2, 3. The skilled man will appreciate that other diffusing features may also be effective.

Turning to figure 3c, a further alternative is shown. In this instance, the roller surface 15 has a transparent coating 18. If the transparent coating 18 is sufficiently thick, light emitted from the marking head 2, 3 will not be in focus when incident upon the roller surface 15 and hence will not be efficiently reflected back into the marking head 2, 3.

The thickness of the transparent coating 18 may be determined by considering both the reduction in reflection efficiency which occurs as the transparent coating gets thicker and the effective level of thermal insulation provided by the coating 18. Transparent coatings 18 with lower thermal conductivity can sustain greater equilibrium temperature differentials for a given coating thickness and thus reduce the effective cooling provided by the roller 10. In this manner, transparent coatings 18 formed from materials with higher thermal conductivity may be preferred, at least where these materials do not unduly add to the expense of manufacture. For instance, with a transparent coating of 5mm thickness, the back reflection power density at the marking head 2, 3 is reduced by a factor of greater than 10. Nevertheless, if the transparent coating is formed from a material with relatively low thermal conductivity such as fused silica, the equilibrium temperature differential between the roller 10 and the exterior of a 5 mm thick coating 18 may be around 17C. This will therefore require the roller 10 to be cooled to a low temperature say 0C in order for the equilibrium surface temperature to be 17C.

Depending upon cooling requirements and cost, the skilled man may choose to use a surface coating l8formed from a material with a higher thermal conductivity, for example, diamond or sapphire or accept a relatively higher back reflection power density at the marking head.

A further option is to provide a thinner transparent coating 18 over a chemically blackened or anodised layer 16 or over a diffusing surface 17 as is shown in figures 3d and 3e respectively. In such cases, a fused silica transparent coating 18 of say 3mm may have similar or better performance in reducing back reflection power density at the marking head 2, 3 to a 5mm thick transparent coating over a smooth surfaced roller 10. In such a case, a lower equilibrium surface temperature of 15C could be achieved with a higher roller 10 temperature of 5C. This therefore reduces the energy input and hence cost required for a given level of cooling.

Turning now to figure 3f, another possibility is for the roller 10 to be provided with a fluid layer 19 provided between the roller surface 15 and a transparent outer shell

20. This could enable the use of a relatively thin shell 20, of thickness, say, lmm-2mm or so, and a thicker fluid layer 19, of thickness, greater than say 3 mm or 5mm. The use of a relatively thin shell 20 reduces the heat insulation effect of it being formed from a material with relatively low thermal conductivity.

Whilst the fluid 19 may be transparent in simple embodiments, in other embodiments, it can be selected to further reduce the back reflection power density at the marking head 2. This could be achieved by introducing light absorbing materials or dyes into the fluid layer 19.

To further improve the cooling performance of this embodiment, the fluid layer

19 may be actively cooled. This may be achieved by use of captive fluid body in cooled by contact with the surface 15 of the cooled roller 10. In alternative embodiments, the fluid 19 may be circulated away from the space between the roller surface 15 and shell

20 for cooling. This can help achieve a lower temperature on the inner surface of the shell 20 and hence a greater cooling effect on the substrate 4.

Turning now to figure 4, a schematic illustration of a further alternative embodiment of the present invention is shown. In this embodiment, in place of a cooling unit comprising a roller 10 there is provided a cooling unit comprising a block

21. The block 21 is typically formed from metal such as steel, nickel, zinc, copper, aluminium, aluminium alloy or the like.

As in the roller embodiments, the substrate 4 is transported to the marking zone 5 by powered label supply and storage reels and directed by roller 7. In the marking zone 5, light emitted from a marking head 2, 3 is focussed onto the substrate 4 by a suitable lens arrangement, resulting in the marking of the substrate 4.

Within and adjacent to the marking zone 5, the substrate 4 runs over the curved facing surface 25 of the block 21. The block 21 is provided with an active cooling system (not shown) such as a network of internal conduits through which a cooling fluid may be circulated and/or one or more thermoelectric cooling devices. In this manner, the block 21 may provide a heat sink, cooling the substrate 4.

Typically, as is shown in figure 4, the substrate 4 runs directly over the facing surface 25. This facilitates heat conduction between the substrate 4 and the block 21. The contact with the block 21 also has the benefit of precisely defining the separation between the substrate 4 and the marking head 2, 3, ensuring that the light emitted from the marking head 2, 3 is in focus at the substrate 4.

To minimise friction and the adverse effects of friction (heating, abrasion), the facing surface 25 is typically smooth and may be polished to enhance smoothness and additionally improve thermal contact. The facing surface 25 typically has a convex curved shape with respect to the travel direction 9, as is shown in figure 4. At either side of the marking zone 5, the facing surface 25 can also have a convex curve perpendicular to the travel direction 9. This can help further by introducing some tension in the tape in an outward direction perpendicular to travel direction. This may reduce the tendency to create a ripple and fold and in addition may concentrate cooling effects on the marking area 6. Within the marking zone 5, there is preferably only the convex curve along the travel direction 9, so as simplify accurate focussing of the light emitted by the marking head 2, 3 on the substrate 4.

As is shown in figure 4, the facing surface is provided with one or more slits 22 within the marking zone 5. The slits 22 extend inwardly from the facing surface of the block 21 and are aligned with the image produced by lens 3 of the emitter 2. The provision of the slits 22 mean that the focussed light emitted by the marking head 2, 3 enters the slits 22 in the absence of a substrate 4 rather than being reflected directly back at the marking head 2, 3 from the facing surface 25. In examples such as that shown in figure 4, the inward end 23 of the slits 22 may terminate in the opposing side of the block 21 to the facing surface 25. Optionally, a beam absorber (not shown) may be provided at or aligned with the inward end 23 of the slits 22 so as to avoid any other issues that may result from stray reflections of the light that passes through the slits 22.

In alterative embodiments, the substrate 4 may run over the facing surface 25 of the block 21 at a small separation from the facing surface 25. This can be achieved by providing a flow of air from the facing surface 25 so as to generate an air cushion upon which the substrate 4 travels over the facing surface 25. In order to provide such an air cushion, a network of dedicated air flow conduits (not shown) are provided in the block 21, the conduits terminating in outlets (not shown) on the facing surface 25. The air channels may be a suitable pattern of discrete holes or the facing surface 25 or may be formed from a porous material such as a porous metal matrix or porous carbon.

If, in addition to the various cooling units described above, further cooling is required, the skilled man will appreciate that the apparatus 1 could be provided with additional cooling units. In this context, this may involve substituting one or more rollers 7 forming the transport means with additional cooled rollers 10 or cooled blocks 20 as described above. Additionally, the apparatus 1 could be provided with one or more blowers operable to direct a flow of cooled fluid, for instance air or water onto the substrate 4.

Further to the above, the skilled man will appreciate that the present invention could be adapted for use with a multi-stage marking process. In such cases, it may be necessary or desirable to provide a suitable cooling unit such as roller 10 or block 21 for each stage of the marking process.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1 A label marking apparatus suitable for use with a label substrate comprising colour change material operable to change colour in response to controlled exposure to laser light, the apparatus comprising: a label supply for retaining and supplying the label substrate; transport means for transporting the label substrate from the supply to a marking zone; a marking head operable to emit laser light so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein at least one roller is provided to cool the label substrate, where the roller surface is adapted to absorb, diffuse or deflect incident light.

2 An apparatus as claimed in claim 1 wherein the cooling unit is in the marking zone or adjacent to the marking zone.

3. An apparatus as claimed in claim 1 or 2 wherein the roller surface is adapted to absorb incident light by the provision of a chemically blackened layer or an anodised layer.

4. An apparatus as claimed in any preceding claim wherein the roller surface is adapted to diffuse or deflect incident light by provision of a roughened layer.

An apparatus as claimed in any preceding claim wherein the roller surface comprises a transparent coating.

6 An apparatus as claimed in any preceding claim wherein the roller is provided with a fluid layer between an outer transparent shell and the roller surface.

An apparatus as claimed in claim 6 wherein the fluid comprising the fluid layer is indefinitely retained between the roller surface and transparent shell.

An apparatus as claimed in claim 6 or claim 7 wherein the fluid comprising the fluid layer is circulated through an active cooling system.

An apparatus as claimed in any one of claims 6 to 8 wherein the fluid is transparent.

10 An apparatus as claimed in any one of claims 6 to 9 wherein the fluid comprises a solution or suspension containing light absorbing materials.

11. A label marking apparatus suitable for use with a label substrate comprising colour change material operable to change colour in response to controlled exposure to laser light, the apparatus comprising: a label supply for retaining and supplying the label substrate; transport means for transporting the label substrate from the supply to a marking zone; a marking head operable to emit laser light so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein at least one cooling block is provided to cool the label substrate, the or each cooling block comprising a facing surface, wherein the facing surface is provided with one or more slits in the marking zone, the slits aligned with the light emitted from the marking head.

12. An apparatus as claimed in claim 11 wherein the substrate runs directly over the facing surface of the cooling block and the facing surface of the cooling block is substantially smooth.

13. An apparatus as claimed in claim 11 or claim 12 wherein the substrate runs over the facing surface of the block at a small separation from the facing surface, the block adapted to enable the emission of a flow of air from the facing surface.

14. An apparatus as claimed in any one of claims 11 to 13 wherein the facing surface is convex with respect to the direction of transport of the substrate.

15. An apparatus as claimed in any one of claims 11 to 14 wherein at either side of the marking zone, the facing surface is convex in a direction transverse to the direction of transport of the substrate.

16. An apparatus as claimed in any preceding claim wherein the cooling unit comprises one or more fluid blowers.

17. An apparatus as claimed in any preceding claim wherein the apparatus comprises a convex roller subsequent to the marking zone.

18. An apparatus as claimed in any preceding claim wherein the transport means are operable to transport the substrate through the marking zone substantially continuously or in indexed steps.

19. An apparatus as claimed in any preceding claim wherein the transport means are operable to halt travel of the substrate during marking illumination.

20 An apparatus as claimed in any preceding claim wherein multiple marking heads are provided, each marking stage provided with a dedicated cooling unit.

21. An apparatus as claimed in any preceding claim where the contact cooling length‘L’ after the marking zone satisfies the inequality L > 2_X10^-5 . V. tf /k

22 An apparatus as claimed in any preceding claim wherein the label substrate comprises an elongate tape.

23. An apparatus as claimed in any preceding claim wherein the colour change material comprises any of: a metal oxyanion, a leuco dye, a diacetylene, or a charge transfer agent.

24. An apparatus as claimed in claim 28 wherein the colour change material further comprises an acid generating agent.

25. An apparatus as claimed in any preceding claim wherein the substrate comprises an NIR (near infrared) absorber material.

26. A method of marking labels comprising colour change material operable to change colour in response to controlled exposure to laser light, the method comprising: supplying label substrate; transporting label substrate to a marking zone; emitting laser light from a marking head so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein the method includes the additional step of cooling the label substrate using the apparatus of claim 1 to 10.

27. A method of marking labels comprising colour change material operable to change colour in response to controlled exposure to laser light, the method comprising: supplying label substrate; transporting label substrate to a marking zone; emitting laser light from a marking head so as mark a desired image in the colour change material of the label substrate as it is transported through the marking zone; wherein the method includes the additional step of cooling the label substrate using the apparatus of claim 11 to 15.