WO1998027471A1

WO1998027471A1 - Powder transfer marking device and method for sign plates

Info

Publication number: WO1998027471A1
Application number: PCT/US1997/020587
Authority: WO
Inventors: Pauline E. Ludwig; Friedhelm Kuhlmann; Harald W. Bielak
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1996-12-14
Filing date: 1997-11-12
Publication date: 1998-06-25
Also published as: ZA9710350B; AR009660A1; AU5200598A

Abstract

A marking device and marking method are described for sign plates in which toner is transferred from a toner transfer surface (2) to the raised portions (9) of an embossed sign plate (8) which is moved forwards by means of a conveyor (12). To prevent compact areas (52) of toner developing in the toner layer (24), a decompacting device (20) is provided which may include a roller (22) and a structured surface (23). Surface (23) may be provided by a textile material with a velvety nap. Further, areas of toner (51) which have been transferred to raised areas (9) may be partially removed by the action of the decompacting device (20). Toner (6) is lightly adhered to the toner transfer surface by the tribo-charge effect of a doctor roller (7) which meters toner (6) from a hopper (5).

Description

POWDER TRANSFER MARKING DEVICE AND METHOD FOR SIGN

PLATES

Technical Field The present invention relates to a marking device and method for sign plates, particularly traffic signs and durable embossed automobile license plates, using a dry toner system as well as a printer for sign plates and a method of printing sign plates.

Background of the Invention Electrographic printing of license plates is known from U.S. Patent No.

5,378,575. A conventional electrographic printer process has been described in the article by Dr. Trevor Martin, entitled "Tutorial. Dry toner fundamentals" from the Seventh Annual Toner & Developer Industry Conference, 15 September 1990. As shown in Fig. 1 of the present application, a typical electrographic process consists of 6 steps: charging the photographic drum, exposure, developing, transfer, fusing and cleaning. For cleaning, a brush may be used to completely remove the toner from the drum before it is again charged and ready for the next print. In Figure 1, components include photoconductive drum 100, mirrors 101, magnetic brush development system 102, paper path 104, transfer corona 106, fuser 108, clean corona 110, clean lamp 112, cleaning brush 114, charge corona 116, and light source 118.

U.S. Patent No. 5,378,575 also describes a transport system for license plates, a hard rubber roller for applying dry toner onto embossed regions of a license plate followed by a fusing station for fusing the toner to the license plate. Preferably, the fused toner is covered by dip coating or lamination in order to protect the plate from weathering. A disadvantage of this system is that the toner tends to form compacted areas on the drum which can lead to imperfect printing.

A device for marking embossed license plates is known from DE-40 06 376 which includes a printing roller, a container for a liquid colorant, an adjustable submerged roller in the container for determining a precise thickness of liquid colorant on the printing roller and a transport system for the embossed license plates to bring the plates into contact with the coated roller and thus to mark the raised portions of the embossed areas with colorant. Water based dispersed lacquers using a vinyl-acrylic copolymer as binding agent are preferred.

A dry printing system for marking embossed sign plates would be preferred which is simple, high speed, robust, does not have long start up times or warm up times, uses little energy and is economical and efficient in the use of consumable materials, is small in size, light in weight, is reliable, does not require complex and expensive imaging components, does not use complex control circuits, does not use dangerous or combustible materials, avoids use and/or drying of solvents, can be stopped and started and re-started at will and provides a license plate with high quality lettering having resistance to scratching and other forms of mechanical damage, resistance to weathering including UV radiation as well as resistance to aggressive materials such as fuels, oils and greases commonly associated with internal combustion engines. None of the known prior art printers and printing methods achieve all of these desired properties. It is an object of the present invention to provide a marking device and a printing method for sign plates which overcomes one or more of the above mentioned disadvantages.

Summary of the Invention The present invention may provide a device for marking sign plates, comprising an endless moveable toner transfer surface, a transport system for transporting the sign plates close to the endless moveable toner transfer surface, a toner hopper for applying toner to the endless moveable toner transfer surface and decompacting means for decompacting toner on the endless moveable toner transfer surface.

The present invention may also provide a printer comprising an endless moveable toner transfer surface, a toner hopper for applying toner to the endless moveable toner transfer surface, decompacting means for decompacting toner on the endless moveable toner transfer surface, a moveable printing surface for receiving toner from said endless moveable toner transfer surface and a transport system for transporting the sign plates close to said moveable printing surface. The present invention may also provide a method of marking sign plates comprising: applying toner to an endless moveable toner transfer surface, transferring toner from the endless moveable toner transfer surface to at least a portion of the sign plate leaving unused toner on the endless moveable toner transfer surface, and decompacting the unused toner on the endless moveable toner transfer surface before further toner is applied to the endless moveable toner transfer surface.

The decompactor and decompacting step in accordance with the present invention differ in principle, in construction and action from the known cleaning brush shown in Fig. 1. In accordance with the present invention the decompactor should loosen up compacted areas of toner. The cleaning brush of Fig. 1 would be less suitable in the marking device of the present invention as complete cleaning of the toner transfer surface requires that a new layer of toner be formed with a single pass through the toner hopper. In accordance with the present invention a toner without charge carrier may be used and the toner transfer surface may be a simple rubber surface and not a special light sensitive semiconductor surface. Hence, the toner adheres less well to the toner transfer surface and it requires several passes to build up a final layer. In accordance with the present invention there is no substantial cleaning of the toner transfer surface by the decompactor. It is therefore only necessary to replace toner used in the toner/printing operation and not the complete toner layer.

The dependent claims define further embodiments of the invention.

The invention, its embodiments and advantages will be described in the following with reference to the drawings.

Brief Description of the Drawings

Fig. 1 is a schematic diagram of a conventional electrostatic printing process.

Fig. 2 is a schematic side view of a marking device in accordance with a first embodiment of the present invention.

Fig. 3 is a schematic representation of a radiation fusing and/or toner curing device in accordance with the present invention. Fig. 4 is a schematic representation of a fusing roller in accordance with the present invention.

Fig. 5 shows a schematic representation of a decompacting roller in accordance with the present invention. Fig. 6 is a schematic representation of a detail of a marking device in accordance with a second embodiment of the present invention.

Illustrative Embodiments of the Invention

The present invention will be described with reference to specific embodiments and to the drawings but the invention is not limited thereto but only by the claims.

Fig. 2 shows a schematic representation of a marking device 1 in accordance with a first embodiment of the present invention from the side. The marking device 1 includes an endless moveable toner transfer surface 2 which may be attached to, or be part of a circular drum 3 or may be an endless conveyor belt as, for instance, the intermediate toner carrying medium known from U.S. Patent No.4, 187,774, or any other similar device for delivering a layer of toner to a printing station 10. The endless moveable toner transfer surface 2 preferably consists of a rubber layer, for instance, a smooth, electrically conductive silicone rubber with a nominal thickness of 5 mm and a Shore A hardness of 65 to 75, available from Praezisionswalzen Produktions GmbH,

Herbrechtingen, Germany. The rubber layer may be adhered to the drum 3. The endless moveable surface 2 may be driven by any suitable means (not shown), e.g. an AC or DC electric motor, stepper motor, or a servo-controlled DC motor. Preferably, the moveable surface 2 moves at a constant speed. The drum 3 and moveable surface 2 is typically as wide or wider than the material to be marked, for instance 250 mm wide when it is to be used for marking automobile license plates. It is not anticipated that the size of drum 3 is a particular limitation on the present invention.

Optionally, the endless moveable surface 2 and if necessary drum 3 may be insulated from electrical grounding and may be charged to an appropriate voltage. A voltage generator 4 may be provided which may be a DC voltage generator. One pole of generator 4 may electrically connected to the endless moveable surface 2 either directly or via drum 3 in order to charge the endless moveable surface 2 to a negative or a positive potential, for example ±1500 Volts or less, preferably between ±500 and ±1500 Volts.

Optionally, more than one moveable surface 2 may be used, e.g. when more than one color is to be printed. Optionally, more than one moveable surface 2 may be used in tandem, for instance tandem drums 3 for transferring toner in accordance with U.S. Patent No. 4,571,059.

Located adjacent to the moveable surface 2 is a toner hopper 5. Toner hopper 5 contains toner 6. Preferred solid toners 6 have the following characteristics: resistance to degradation by weathering; and good adhesion to the substrate to which they are applied. A dry toner powder suitable for practicing the present invention requires a colorant and a binding agent. Optionally, a charge carrier may also be included within the toner 6.

A suitable binding agent may be an alkyl substituted acrylate or methacrylate polymer, with alkyl groups having from 1 to 9 carbon atoms, or mixtures of such acrylates, and especially a copolymer of methyl and butyl methacrylates (such as for example, Acryloid B-66, available from Rohm & Haas Company). Other suitable binding agents are polyvinyl acetals, for example, polyvinyl butryal (such as BUTVAR brand polyvinyl butryals B-90 or B-72 available from the Monsanto Chemical Company); polyolefins; polyesters (such as VITEL brand PE-200D from the Goodyear Tire & Rubber Company or ARAKOTE 3000 brand carboxyl terminated polyester optionally in mixture with ARALDITE PT810 brand polyfunctional epoxy resin (tri- glycidyl isocyanurate) both available from the Ciba-Geigy Chemical Company); and vinyl resins (such as VTNYLITE brand vinyl resin, VAGH copolymer of vinyl chloride and vinyl acetate available from the Union Carbide Corporation). The preferred binding agents are characterized by relatively high transparency and clarity. Additionally, preferred binding agents have glass transition temperatures (Tg) from about -15°C to about 150°C, preferably from about 35°C to about 110°C, and most preferably about 50°C. The most preferred binding agents are chosen based upon their potential strong chemical interactions with the surface to be printed. Specifically envisioned as factors to be considered as providing the potential for strong chemical interactions are the likelihood of formation of bonds such as ionic or covalent bonds, donor-acceptor bonds, as well as secondary bonds such as hydrogen bonds and van der Waals bonds between the binding agent and the surface to be printed. In evaluating the potential, the relevant bond energies may be obtained from textbooks such as "Adhesion and Adhesive: Science and Technology" by A. J. Kinloch; 1987, University Press Cambridge, Great Britain. Most preferred are binding agents which may be used at temperatures of less than 150°C. Preferred binding agents are also resistant to ultraviolet (UV) light degradation and are adhesive to the surface upon which the toner is printed.

Suitable optional charge carriers may be positive charge control agents designed for use as additives in dry toner formulations such as, for example, copolymers of butyl and methyl methacrylate (such as TRIBLOX PC- 100 brand acrylic polymer

(available from E. 1. DuPont de Nemours Company)). Polyesters and vinyl resins may also be used as charge carriers. A preferred acrylic copolymer charge carrier has the following characteristics: molecular weight of 2000 to 5000; glass transition temperature (Tg) of 53°C to 59°C, onset at about 46°C, nitrogen content of about 1% as measured by NMR. Preferred charge carriers are also relatively light transmissive or transparent materials and are resistant to UV light degradation. For a black toner a transparent charge carrier is not essential. For example, an azine dye (Nigrosine Solvent Black 7, CI#50415:1) available from the Orient Chemical Company, Newark, N.J., may be used as a charge carrier for such a toner. The most preferred charge carriers are acrylic polymers (i.e. alkyl acrylates or alkyl methacrylates) having amine functionality (i.e. functional groups including amine nitrogen or quaternary ammonium nitrogen).

Suitable colorants may be pigments such as Pigment Red 179 or 224 available from the Harmon-Mobay Chemical Company, Pigment Yellow 1010 or Pigment Violet 37 available from the Ciba-Geigy Company; Pigment Green 7 or 36 available from the Sun Chemical Company; Pigment Blue 15;1 or Blue 15;6 available from BASF; and Regal 500R carbon black available from the Cabot Corporation. Suitable colorants may also be dyes such as Amaplast Yellow available from the Color- Chem International Corporation or LATYL Brilliant Blue BGA available from the DuPont Company. Generally, pigments or dyes should be resistant to environmental pollutant chemical degradation and UV light degradation. Preferably, pigments are dispersed in a dispersing resin, for example Red 229 dispersed in Vinyiite VAGH resin in a 1 : 1 weight ratio. Such dispersion helps to maintain the small pigment particle size that is desired for obtaining a light transmitting image.

The binding agent, colorant and optional charge carrier agent may be mechanically mixed (and the binding agent as well as the optional charge carrier melted) using a twin screw extruder such as a variable speed twin screw extruder, for example a

Baker Perkins gear drive model having a Haake rheocord torque theometer. Preferably, the twin screw extruder generates a temperature of approximately 150°C to approximately 225°C during extrusion. The extruded product may be hammer milled and then jet milled to generate a mixture having particle sizes ranging from about 5 to about 100 micrometers, preferably from about 5 to about 50 micrometers and most preferably from about 5 to about 20 micrometers. A suitable jet mill is a NPA Supersonic Jetmill model PJM IDS-2 available from the Nippon Pneumatic Manufacturing Company.

Attached to the toner hopper 5 may be a vibrator (not shown) to assist in distribution of the toner 6 in the hopper 5 and to prevent bridging. Toner hopper 5 is preferably mounted at such a position that toner 6 is temporarily adhered to the surface of moveable surface 2 and is carried downwards towards a printing location 10. Toner

6 preferably adheres to moveable surface 2 by electrostatic charge to form a uniform layer 24 of toner 6. Toner hopper 5 preferably has a doctor blade or doctor roller 7 for determining the thickness and uniformity of the layer 24 of toner 6 applied to the moveable surface 2. Doctor roller 7 is preferably a driven roller which is adjustably mounted outside hopper 5 at the position where toner 6 exits from hopper 5. The hopper adjustment is preferably at least sufficient to regulate the thickness of the toner layer 24. Roller 7 is preferably driven in the same direction as drum 3 so that the surface of drum 3 and surface of roller 7 move in opposite directions. Preferably, a plate or scraper 5 A is located at the bottom of hopper 5 and in contact with the surface of roller

7 to prevent toner 6 escaping between the hopper 5 and the roller 7 in the direction of rotation of roller 7. Preferably, the roller 7 tribo-charges toner 6 due to the light milling action of the roller 7 with respect to drum 3 so that the toner 6 is charged and hence adheres lightly to the endless surface 2. Doctor roller 7 is preferably of a different size from the roller drum 3 to prevent shadowing effects in the toner 6 adhered to the moveable surface 2. Preferably, the gear ratio between the drum 3 and roller 7 is about 1:2 to 3: 1. When the drum 3 is connected to voltage generator 4 it is preferable if doctor roller 7 is isolated from electrical grounding or is made of a non-conductive material such as plastic.

In a less preferred embodiment of the present invention, the doctor device 7 may also be an adjustable plate or scraper, known as a doctor blade (not shown). Doctor blade 7 may be adjustably mounted on the bottom edge of hopper 5 so as meter the amount of toner 6 to the toner layer 24.

At printing position 10, an individual embossed sign plate 8 or an embossed endless strip 11 (not shown) is brought sufficiently close to the moveable surface 2 that toner 6 transfers from the moveable surface 2 to embossed areas 9 of sign plate 8 or strip 11. In the following, the invention will be described with respect to individual sign plates 8, however, the skilled person will appreciate that in accordance with the present invention, embossed endless strip 11 could also be used and then cut into lengths after marking. Embossed sign plate 8 or strip 11 may be made of any suitable material such as metal, e.g. aluminum or steel, enameled metal, plastics such as acrylic plates or any other suitable relatively flat and rigid sheet material. The embossed areas 9 may be produced in the sign 11 by any suitable means such as deep drawing, hot or cold embossing, casting, adhering letters and numbers to a flat surface, injection molding, etc. In accordance with the present invention, signs, figures, letters, designs, legends etc. may be embossed into the sign plate 8. The embossing 9 is preferably 0.5 to 5 mm, more preferably 1 to 3 mm high with respect to the rest of sign 8. In accordance with the present invention the embossed areas 9 may be embossed or may be the areas left when the remaining parts of the sign 8 are embossed in the reverse direction. Preferably, the sign plate 8 is a thin piece of embossable material, for example, a web from a roll of aluminum. A thickness of about 0.8 mm to 1.5 mm, preferably 1 mm is suitable for aluminum license plates. In a most preferred embodiment, the aluminum web may include a facing layer of retroreflective sheeting. Such retroreflective sheeting is known in the sign art. Enclosed lens retroreflective sheeting and the use of glass beads to provide for reflexed light reflectors are described in Palmquist, et al., U.S. Patent No. 2,407,680; May, U.S. Patent No. 4,626,127; Tung et al., U.S. Patent No. 4,367,920, Tung et al., U.S. Patent No. 4,511,210; and Tung et al. U.S. Patent No. 4,569,920; Bailey et al., U.S. Patent No. 4,767,659; Bailey et al. U.S. Patent No. 4,664,966; and Bailey, U.S. Patent No. 4,648,932.

Generally, enclosed lens retroreflective sheetings include, in order, an adhesive layer for application to a support such as a license plate blank, a specular reflective surface, a light transmitting spacing layer, and a monolayer of glass beads within a light transmitting resin layer. Often, a protective outer layer or top layer is also present. Retroreflective sheeting typically functions in the following manner: Light from an external source, is transmitted to the beads, which serve as lenses and direct the light toward the specular reflective surface, through the spacing. The reflective surface, preferably cupped about each of the glass beads, returns the light to the glass beads which in turn return the light toward the source.

Suitable surfaces of sign plates 8 to be printed may be made from materials including polymers selected from the group consisting of polyalkylacrylates, polyalkylmethacrylates, polyesters, vinyl polymers, polyurethanes, cellulose esters, fluoropolymers, polycarbonates, polyolefins, ionomeric copolymers and copolymers of ethylene or propylene with acrylic acid, methacrylic acid, or vinyl acetate. Suitable retroreflective sheetings include SCOTCH-LITE brand HIGH INTENSITY retroreflective sheeting and REFLECTO-LITE brand retroreflective sheeting. The surface layers may be made of polyalkylacrylates or polyalkylmethacrylates (especially polymethyl methacrylate (PMMA)), polyesters, vinyl polymers and polyvinyl acetals such as, for example, polyvinyl butryals. The SCOTCH-LITE brand and REFLECTO- LITE brand retroreflective sheetings are available from the Minnesota Mining and Manufacturing Company of St. Paul, Minnesota. Particularly suitable are Scotchlite sheeting types 4780, 4770, and 4750. Sign plate 8 is transported to the printing position 10 by means of a conveyor 12. Conveyor 12 preferably consists of two parallel endless chains which go around sprockets at each end. The chains are preferably separated by at least the width of a sign plate 8. Corresponding chain links of the two parallel chains may be joined together at regular intervals by rigid plates or trays which may be of metal, plastic or similar. The plates or trays are carried forward when the endless chains are driven and the plates form smooth, stable surfaces for supporting the sign plates 8. The plates or trays are designed to travel around the sprockets at the ends of the chains without damage thereto. Such a conveyor 12 is available from Stolzenberg, Germany. Conveyor 12 typically moves at 0.5 to 3 m/minute. Conveyor 12 may be adapted to provide support for the sign plate 8 at printing position 10. For instance, as explained above, conveyor 12 may include a sequence of trays or plates, each flexibly linked to its neighbors and each tray or plate being of the right size and rigid enough to support a sign plate 8. Alternatively, one or more support rollers 25 may be placed under the belt of conveyor 12 in order to support the sign plate 8 across its width at the printing position 10. Conveyor 12 is normally electrically grounded but may be optionally insulated from earth and connected to a second DC generator (not shown) and may be charged to any suitable positive or negative voltage. Upstream of printing position 10, heating means 13 may be arranged for preheating the surface of sign plate 8. The preheating means 13 may be provided by infra-red lamps or similar. The preheating means 13 preferably raises the temperature of the raised portions 9 of the embossed sign plate 8 to such a temperature that the sign plate 8 is not damaged but the surface of the raised portions 9 become slightly tacky. Preferably, heating means 13 should be adapted to raise the temperature of embossed areas 9 to between about 80 to 120°C. Also upstream of printing position 10, corona discharge apparatus (not shown) may be provided to corona treat the surface of sign plate 8. Further, flame brushing means (not shown) may be provided upstream of position 10 to flame brush the surface of sign plate 8. Still further, other devices may be placed upstream of printing position 10 to improve the adherence of toner 6 to the embossed areas 9 of sign plate 8 at the printing position 10, such as devices to apply a layer of an adhesive or size or lacquer or primer, which may be applied by spraying, gravure printing, offset printing, flood coating, sponging with a sponge roller, laminating (all not shown) or similar methods. Preferably, the embossed areas 9 of sign plate 8 after heating are sufficiently tacky that toner 6 transfers from endless surface 2 when the toner 6 comes in contact with the embossed area 9. Preferably, the rubber layer 2 of drum 3 is sufficiently flexible and the pressure applied between the endless surface 2 and the embossed area 9 of plate 8 in the printing position 10 is sufficient to transfer tone 6 reliably and uniformly to only the embossed areas 9 of plates 8. Preferably the mechanical properties of rubber layer 2 are such any differences in height of the embossed areas 9 may be absorbed by deformation of the layer 2. A fusing station 14 may be provided downstream of printing position 10. Depending upon the toner type, fusing station 14 may be any suitable fusing device such as a non-contact fuser or a glossed contact fusing station. Examples of a non-contact fusing station may be provided by a pulsed laser device as described in DE 44 43 129, infra-red laser device as described in U.S. Patent No. 3,743,777, light pulsed heater as known from DE 39 36 716, laser fuser as known from DE 2129926 or focused halogen heater known from DE 43 09 400. Particularly preferred are focused pulsed radiation means as shown schematically in Fig. 3. The fusing station 14 may include a radiating device 26 producing a beam of radiation 27. This beam 27 may be focused by a short focal length lens such as a glass rod 28 to a focused beam 29. By using pulsed radiating devices, with which the pulse frequency is determined by the through speed of a plate 8, no energy is consumed when no plates 8 are being marked so that the idle energy consumption is low. Further, pulsed radiation provides the advantage of applying heat energy to the toner 6 on the embossed areas 9 without generally warming the sign plate 8.

Retro-reflective sheeting often contains microspheres for retro-reflecting incident light. A collimated beam of radiation can be focused by the microspheres onto the back of the microspheres. The intense concentration of radiation by the microspheres can cause damage to the underlying layers. To prevent this the radiation 27 shown in Fig. 3 may be focused onto the embossed area 9 by a short focal length lens

28 so that the converging beam 29 is de-focused on the main part of the sign plate 8. By defocusing, a concentration of radiation may be avoided and the microspheres will reflect the radiation without damage. On the other hand the toner 6 on the top of the embossed areas 9 preferentially absorbs the radiation and the rise in temperature can be rapid with little total heat input. Thus toner 6 can be fused without damage to the layers underneath and without consumption of much energy.

Alternatively or additionally, the fuser station 14 it is particularly preferred if fusing station 14 is provided by a hot fuser roller 15 as shown schematically in Fig. 4. Fusing roller 15 includes a roller 16 which may be externally or internally heated to a temperature suitable for fusing the toner 6 without damaging sign plate 8.

The roller 16 may be heated by an external heater 41, 42 or alternatively or additionally by an internal heater 40, e.g. infra-red strip lamps. The roller diameter is preferably, 20 to 120 mm, typically 60 mm. The roller 16 is preferably configured as a blanket fuser roller as shown in principle in U.S. Patent No. 3,967,042. A rubber layer or blanket 17 may be located on the outer surface of roller 16. The rubber layer 17 may be made from vulcanized silicon rubber, Shore A hardness of 35 to 60, preferably 45 to 50, having a nominal thickness of 5 mm and may be adhered to roller 16. The surface of rubber layer

17 may be made non-adhesive by a low energy coating 41 such as PTFE. For instance, the low energy coating 41 may be provided by a PFA tube from KB-Roller Tech. Co., Bergheim, Germany having a thickness of 50 to 500 micron. The fusing temperature on the surface of coating 41 is preferably in the range 140°C to 220°C. Fusing roller 15 may fuse the toner 6 on the embossed areas 9 by hot offset fusing. It is preferred, however, if fuser roller 15 contacts and fuses the toner 6 on the embossed areas 9 by exerting pressure and heat on the embossed areas 9. The rubber layer 17 preferably chosen so that it may absorb differences in height of the embossed areas 9 by deformation. It is preferable that sign plate 8 is supported underneath roller 16 by support means 18 which may be provided by a specific design of conveyor 12 or by an additional roller or rollers 18 to prevent deformation of the sign plate 8 under pressure of the fusing roller 15. The support means 18 may also be the dual roller support as disclosed in DE 195 39 410. A toner curing station 19 may optionally be located downstream of the fuser station 14. The toner curing station 19 initiates cross-linking or cross-links the toner 6 on the embossed areas 9 when toner 6 contains a hardening agent. Where toner 6 contains a blocked hardening agent, toner curing station 19 may only need to raise the temperature of toner 6 to a temperature suitable to unblock the hardening agent. The cross-linking action may then proceed by itself. The toner curing station 19 may be any of the radiation devices described with reference to the fuser station 14 and shown in Fig. 3. In addition, the toner curing station 19 may also include a UV light source when the toner is UV cross-linkable.

Cooling station 21, e.g. fans, may be located downstream of the fusing station 14 to cool the sign to handling temperatures. Also downstream of fusing station

14 and/or curing station 19, laminating or clear top-coaters may be located for applying a protective layer to the sign as explained in U.S. Patent No. 5,378,575. After transfer of toner 6 from the moveable surface 2 to the embossed regions 9 of sign 8, the unused toner travels back with the movement of drum 3 and the endless surface 2 is re-coated by the toner hopper 5 and doctor roller 7. It has been found that toner layer 24 can compact itself on the moveable surface 2 which can result in shadowing effects on the printed embossed areas 9 and other undesirable printing errors. To prevent these printing blemishes a decompacting device 20 may be located in contact with the moveable surface 2 and located at a position around the circumference of the drum 3 between the printing position 10 and the hopper 5. Decompacting device 20 has the following functional requirements. The decompactor 20 does not clean the moveable surface 2 but rather breaks up the surface of any compacted areas of toner 6 and also may tend to re-distribute at least partially the toner 6 on endless surface 2 from areas with a lot of toner 6 to areas with less. The decompacting device 20 should not remove substantial quantities of toner 6 from endless surface 2, i.e. it should not clean toner layer 24 off the endless surface 2. At start up some toner 6 may transfer from layer 24 to the decompacting device 20 but this should cease once equilibrium conditions have been achieved. Decompacting device 20 should also not have a substantial damming action so that amounts of the toner build up on the upstream side of decompacting means 20.

The decompacting device 20 is preferably a freely rotating roller 22 which rotates by being driven by endless surface 2. The present invention includes that the surface speed of roller 22 is driven slightly faster or slower, e.g. ± 10% than the speed of the endless surface 2. Roller 22 preferably has a structured surface 23. The structured surface 23 should be adapted to decompact the toner layer 24, i.e. break up any compacted area of layer 24 on the endless surface 2. For instance, surface 23 may be a profiled plastic or rubber layer or may be made of a foam, soft textile, soft fur- or soft brush-like material adhered to the surface of roller 22. For example, soft hairs of a suitable brush surface 23 may be 5 to 10 mm long having a compressed thickness of about 1 mm and may form a velvety nap. By velvety nap is meant a hair surface which is smooth in the direction in which the hairs lie down and rough in the opposite direction when the hairs bristle up as may be found with many animal pelts and with the textile velvet. It is preferred if the surface 23 with the velvety nap is arranged on the roller 22 so that the nap lays down rather than bristling up, i.e. the roller 22 rotates in such a way that the surface 23 moves in the direction of the orientation of the nap. A decompacting roller 22 of this type is shown schematically in Fig. 5. Fig. 5 shows a drum 3, a toner layer 24, a toner hopper 5, a scraper blade 5a, a doctor roller 7, a conveyor 12, a sign plate 8 with embossed areas 9 of the first embodiment. The decompactor 20 includes a freely rotating roller 22 and a structured surface layer 23 which is a textile material with a velvety nap. The length of the hairs of the nap has been exaggerated for clarity purposes. The roller 22 is driven by contact between the toner transfer surface 2 and the layer 23. The hair orientation is such that the hairs are pulled as they accelerate the roller 22 and lay flat by doing so. By rotating in this direction the surface 23 has a decompacting and not a brushing, sweeping or cleaning action. Toner layer 24 may include areas 51 where the toner has been removed by transfer to the raised areas 9 of a sign plate 8 as well as areas 52 of compacted toner. The decompacting roller 22, 23 decompacts areas 52 and may distribute some toner 6 to partially fill up spaces 51. Further, the hairs are pressed against each other on the roller 22 and can therefore absorb little toner. Plush materials may also be used for surface 23 but there is a disadvantage with long-haired materials that they may trap too much toner in the long hair structure.

It is preferable if decompacting device 20 is so constructed that it assists in tribo-charging the toner 6. To this purpose the material of the soft brush may be made of plastic material which has the appropriate tribo-charging properties. A suitable material for surface 23 may be obtained from Girmes GmbH, Germany under the product code 15053 and this material may be wrapped spirally around the roller 22 to form surface 23.

Decompacting device 20 is not limited to a roller 22, 23. Any suitable device which meets the functional decompacting requirements mentioned above may be used. A suitable alternative to a brush roller may be an ultrasonic transducer which beams ultrasonic waves at the layer of toner 24 through the air gap between the transducer and layer 24. By choosing the correct frequency and power, the toner particles in compacted areas of layer 24 may be made to loosen. Further, a plurality of small air jets directed at the layer 24 may decompact the layer 24. The air pressure and volume should be chosen so that the toner layer 24 is disturbed but not blown away. Other mechanical decompactors 20 may be suitable. For instance, the decompactor 20 may be a flexible net or rake across the width of drum 3 which rakes up layer 24 but does not remove it. Further, electrostatic or magnetic decompactors are included within the present invention. For instance, a wire or wires may be located close to layer 24 and a high frequency AC voltage applied thereto. If the frequency is sufficiently high, the particles may be disturbed by being rapidly attracted and repelled but not detached from the surface of drum 3. Similarly, where a magnetic toner is used, the present invention includes a magnetic decompactor. This may consist of a roller including an array of north and south poles which is rotated rapidly close to layer 24. The rotation frequency should be high enough to disturb layer 24 but to prevent toner 6 being transferred to the rotating magnets.

Fig. 6 shows a detail of a second embodiment of a marking device 30 in accordance with the present invention. Items with the same reference numerals are the same as described for the first embodiment. In accordance with the second embodiment, marking device 30 is configured to print an image or sequence of images, characters, legends or similar on to sign plates or strip. A moveable printing surface 32 is provided with raised characters, e.g. by engraving, in reverse which are supplied with toner 6 from a counter-rotating drum 3. The printing surface may be provided by the surface of a drum 31 as shown in Fig. 6 or by any other form of intermediate image carrying medium such as the flexible printing medium of U.S. Patent No. 4,187,774. Marking device 30 may include the endless moveable surface 2, toner hopper 5, toner 6, scraper

5A, toner layer 24, doctor roller 7, and decompacting device 20 including preferably, roller 22 and structured surface layer 23 of the first embodiment.

Toner may be transferred from surface 2 to drum 31 by electrostatic transfer as described in U.S. Patent No. 4,571,059. For this purpose an additional or alternative DC voltage supply 34 may be provided and one pole thereof connected to drum 31. The voltage difference between drums 3 and 31 may be set at ± 300 to ± 1200 volts, preferably about ± 500 volts.

When roller 31 comes close to, or touches unembossed sign material 33 in the printing area 10, the toner 6 on the raised characters of the printing surface 32 is transferred to the sign material 33. Unembossed sign material 33 is conveyed by conveyor 12 as in the first embodiment. Unembossed material 33 may also be strip material. Fusing, curing, laminating, coating or cooling stations 14, 19, 21 etc. may be provided downstream of the printing position 10 as for the first embodiment. Heating, coating, priming, corona discharge, or flame brushing stations 13 etc. may be provided upstream of the printing position 10 as in the first embodiment.

Claims

We claim:

1. A device for marking sign plates, comprising an endless moveable toner transfer surface, a transport system for transporting the sign plates close to the endless moveable toner transfer surface, a toner hopper for applying toner to the endless moveable toner transfer surface and decompacting means for decompacting toner on the endless moveable toner transfer surface.

2. A marking device according to claim 1, wherein said decompacting means includes a roller with a structured surface in contact with the toner on the endless moveable toner transfer surface.

3. A marking device according to claim 1, wherein said structured surface of said roller includes a soft brush material.

4. A marking device according to claim 1, further comprising a fusing station for fusing toner on said sign plate.

5. A marking device according to claim 3, wherein said fusing station includes a hot contact fusing roller.

6. A marking device according to claim 1, further comprising a preheater for heating said sign plates.

7. A marking device according to claim 1, further including a toner curing station.

8. A printer comprising an endless moveable toner transfer surface, a toner hopper for applying toner to the endless moveable toner transfer surface, decompacting means for decompacting toner on the endless moveable toner transfer surface, a moveable printing surface for receiving toner from said endless moveable toner transfer surface and a transport system for transporting the sign plates close to said moveable printing surface.

9. A printer in accordance with claim 8, wherein said decompacting means includes a roller with a structured surface in contact with the toner on the endless moveable toner transfer surface.

10. A printer according to claim 8, wherein said structured surface of said roller includes a soft brush material.

11. A printer according to claim 8, further comprising a fusing station for fusing toner on said sign plate.

12. A printer according to claim 10, wherein said fusing station includes a hot contact fusing roller.

13. A printer according to claim 8, further comprising a preheater for heating said sign plates.

14. A printer according to claim 8, further including a toner curing station.

15. A method of marking sign plates comprising: applying toner to an endless moveable toner transfer surface, transferring toner from the endless moveable toner transfer surface to at least a portion of the sign plate leaving unused toner on the endless moveable toner transfer surface, and decompacting the unused toner on the endless moveable toner transfer surface before further toner is applied to the endless moveable toner transfer surface.

16. A method according to claim 15, further including the step of preheating the sign plate before transferring toner from the endless moveable toner transfer surface to the portion of the sign plate.