WO2004086150A1

WO2004086150A1 - Printing on metal by selective electrostatic powder coating

Info

Publication number: WO2004086150A1
Application number: PCT/DK2004/000213
Authority: WO
Inventors: Henrik Givskov Larsen; Henning Schou
Original assignee: Multi Sign A/S
Priority date: 2003-03-26
Filing date: 2004-03-26
Publication date: 2004-10-07

Abstract

The present invention relates to a method and a system for printing predefined prints on a metal plate. The prints are received as instructions by the system e.g. after having been generated using a computer program, and, based on the instruction, a print transfer unit is electrically charged in specific areas corresponding to the instructions. Further, the metal plate is electrically biased, and by using positive and negative bias it is possible to electrostatically transfer powder via the print transfer unit to the metal plate in a pattern similar to the predefined print. The metal plate is placed on a printing table being adjustable to ensure a contact between the print transfer unit and the metal plate. Further, the printing table comprises means for generating a vacuum between the surface of the printing table and the metal plate to ensure that the metal plate lies plane on the printing table.

Description

Printing on Metal by Selective Electrostatic Powder Coating

FIELD OF THE INVENTION

The present invention relates to a system for printing predefined prints on a substantially stiff metal plate. The invention further relates to a method of printing predefined prints on a substantially stiff metal plate.

BACKGROUND OF THE INVENTION

Today when making prints on metal plates, e.g. when making road signs or advertising signs, some labour intensive techniques are used. One common printing method is attaching coloured foils to the surface of the metal plates. When making multicoloured signs, foils with different colours and shapes are placed in layers on the metal plate. Different types of foil can also be used, such as a foil having retro reflective properties. The problem when using this technique is that the process requires a very precise positioning of the foils, and further only very limited prints can be made both with regard to the shape of the print and the colours used in the print. A further disadvantage is that gluing foils to a metal plate only lasts for a limited period, after which the glue looses its effect, where after the foils will no longer stick to the surface of the metal plate. This is especially a big problem with regard to road signs, since a number of countries have requirements of a minimum periods during which the road signs should be useable, and the before mentioned foil layer technique is typically not even close to fulfilling these requirements.

Another known method of creating prints on metal is by using silk pressure. When making silk pressure, a first step is generating a film permitting light to penetrate some areas of the film, whereas other areas cannot be penetrated by light. A light sensitive fabric is then placed on a frame and exposed with light through the film, whereby some areas of the fabric is exposed to light, and other areas are not exposed to light. The exposed fabric is then placed on the surface of the metal plate, and the fabric is covered with paint. The paint then passes through the fabric to the surface of the metal plate in the areas where the fabric has been exposed to light, and in the other areas the paint stays on top of the fabric. Finally, the metal plate with the paint is passed through a drying oven for drying the paint. Specific films and fabrics can then be made for different colours, whereby a multicoloured print can be made on the metal plate. This method can be good when making a large number of similar prints, but when a print is being changed often, the process of making a film for each colour, making a fabric for each colour and finally adding and drying each paint colour make the process very time-consuming and cumbersome. Further, the use of the large number of materials (film, fabrics) for making one print makes it very expensive to generate a print on a metal plate.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a solution to the above- mentioned problems.

This is obtained by a system for printing predefined prints on a substantially stiff metal plate, the system comprising:

means for receiving instructions describing a predefined print,

- means for obtaining a print transfer unit being electrically charged in predefined areas corresponding to said received instructions, said means ensuring that the predefined areas are electrically charged either negatively or positively,

- means for providing a surface of the print transfer unit with a powder by electrostatically attracting the powder to the surface in a pattern de- termined by said charged areas, said powder being electrically charged either negatively or positively,

- means for generating an electrical potential at the surface of said metal plate, said potential being either negative or positive opposite the charging of powder on said print transfer unit or intermediate belt or drum,

- means for electrostatically moving at least some of the powder from the surface of the print transfer unit to the surface of the metal plate, - means for heating the surface of said metal plate and the powder on said surface to ensure that said powder is fused to said surface.

Thereby a print can easily be generated on a metal plate, and by using the 5 system the print to be made on the metal plate can easily be changed by transmitting instructions to the printing system describing another predefined print. In one embodiment the predefined print could be defined in a drawing program on a computer, and then the instruction could be transmitted via a communication network to the printing system. Alternatively, the instruction 0 could be stored on a storage medium, and then the system could comprise means for reading the instructions on the storage medium. The print transfer unit could be a drum shape, where the pattern of charged areas is obtained using a laser beam either for discharging or charging areas depending on the received instructions. The temperature to be used in the final heating process 5 depends on both the type of powder and the metal type of the metal plate. The essential part of the printing principle is that the powder is moved by electrostatically attracting the powder from one position to another position. This can be done by:

0 1. Generating a positive electrical field for attracting negatively charged powder.

2. Using neutral field for attracting negatively charged powder.

5 3. Generating a negative electrical field for attracting neutral powder.

4. Generating a positive electrical field for attracting neutral powder.

5. Generating a negative electrical field for attracting positively charged o powder.

6. Using a neutral field for attracting positively charged powder.

The metal plate could be coated to improve the print properties of the metal 5 plate surface. Such a coating could e.g. be a layer of polymer, such as a polymer foil, which is added before starting to print on the metal plate. Alter- native layers could be a dielectric layer, a layer of white or another pigmented color. Further the layer could be a retroreflective layer.

In an embodiment said means for electrostatically moving at least some of the powder from the surface of the print transfer unit to the surface of the metal plate comprises means for ensuring a contact between the surface of the metal plate and said print transfer unit or an intermediate transfer belt or drum, said means being a printing table having a surface on which the metal plate can be placed, the printing table comprising an elevation mechanism making the distance between said surface and said print transfer unit or intermediate transfer belt or drum adjustable. Thereby e.g. a contact can be obtained between said surface and said print transfer unit or intermediate transfer belt or drum, ensuring that powder can be electrostatically transferred from the print transfer unit to the surface of the metal plate. Further, metal plates of varying thickness can be printed on without damaging the print transfer unit.

In a specific embodiment the means for ensuring said printing table comprises a surface, which further comprises means for creating a vacuum be- tween the metal plate and the surface of the printing table. Thereby the metal plate to be printed on lies plane on the printing surface ensuring a uniform distance between the metal plate and the print transfer unit, and further ensuring that the metal plate does not damage the print transfer unit.

In a further embodiment the means for generating an electrical potential on the surface of the metal plate is comprised in a printing table, said printing table having a surface on which the metal plate can be placed and said printing table being adapted for generating an electrical potential on said surface. Thereby the electrical potential can easily be generated.

In an embodiment said means for obtaining a print transfer unit being electrically charged in predefined areas is based on Electron Beam Imaging (EBI). Electron Beam Imaging (EBI) technology offers many advantages over comparable print engine technologies including: faster print speeds, more consis- tent drive pulsing and improved dot uniformity and control. The technology is ideal for high-volume print runs of sequential numbers and/or variable infor- mation. The technology also lends itself to providing high quality imaging on a wide variety of substrates such as polymer, which could be coated on the metal plate.

In an alternative embodiment the means for electrostatically moving at least some of the powder from the surface of the print transfer unit to the surface of the metal plate comprises means for ensuring a contact between the surface of the metal plate and said print transfer unit or an intermediate transfer belt or drum, said means being two parallel rows of transport drums, between which the metal plate can be transported, the two parallel rows comprising a top row and a bottom row, said top row being statically mounted in a predefined distance from said print transfer unit, and said bottom row comprising an elevation mechanism making the distance between said bottom row and said top row adjustable. This method has the advantage that e.g. contact be- tween the surface of the metal plate and the print transfer unit can be ensured by the statically mounted top row of the transport drums.

In a specific embodiment the bottom row comprises means for generating an electrical potential at the surface of said metal plate. This is an easy way of generating the potential, since the transport drums are already in contact with the surface when transporting the metal plate.

In a specific embodiment the powder being used is made from glass. By using glass powder a print having a retro reflective effect can be made. In a specific embodiment the glass powder could comprise a fluid membrane to ensure that the glass powder can be sufficiently electrically charged. A fluid membrane on glass powder could e.g. be made by first placing the powder in a fluid and then drying the powder, whereby a thin fluid membrane will remain on the powder corns.

In another specific embodiment the system further comprises means for removing the potential from the metal plate. Thereby the metal plate can be removed from the printing system by an operator without the operator getting an electric shock. The means for removing the potential from the metal plate could in one embodiment be means for ensuring that an electrical contact between the metal plate and the power source is broken before the operator gets access to the printed metal plate.

The invention further relates to a method for printing predefined prints on a substantially stiff metal plate, where the method comprises the steps of:

- receiving instructions describing a predefined print,

- obtaining a print transfer unit being electrically charged in predefined areas corresponding to said received instructions, ensuring that the predefined areas are electrically charged either negatively or positively,

- providing a surface of the print transfer unit with a powder by electro- statically attracting the powder to the surface in a pattern determined by said charged areas, said powder being electrically charged either negatively or positively,

- generating an electrical potential at the surface of said metal plate, said potential being either negative or positive opposite the charging of powder on said print transfer unit or intermediate belt or drum,

- electrostatically moving at least some of the powder from the surface of the print transfer unit to the surface of the metal plate,

- heating the surface of said metal plate and the powder on said surface to ensure that said powder is fused to said surface.

In a specific embodiment the step of electrostatically moving at least some of the powder from the surface of the print transfer unit or intermediate transfer belt or drum to the surface of the metal plate comprises the step of adjusting the distance between the print transfer unit or intermediate transfer belt or drum and the surface of said metal plate by adjusting a printing table adapted for placing said metal plate. BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will be described referring to the figures, where

figure 1 illustrates a system for printing on metal plates according to the present invention,

figure 2 illustrates a printing table being a part of the system according to the present invention,

figure 3 illustrates another embodiment of a printing table according to the present invention,

figure 4 illustrates a specific embodiment of a printer according to the present invention adapted for making retro reflective coating of a metal plate,

figure 5 illustrates another embodiment of a method of transferring the powder to a metal plate based on an intermediate transfer belt,

figure 6 illustrates yet another embodiment of a method of transferring the powder to a metal plate based on e-beam,

figure 7 illustrates an embodiment of a method of controlling a system according to the present invention,

figure 8 illustrates another embodiment of a method of controlling a system according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 illustrates a system for printing on metal plates according to the present invention. The system comprises a printing table 101 for placing the metal plate to be printed on and a printing unit 103. The printing unit comprises a laser 105, a controllable scanning mirror 106 and a drum 111 coated with photoconductive material and placed next to a charging device 108. The inside surface of the photoconductive drum 111 is connected to electrical neutral or ground. The charging device 108 places negative electrical charge on the surface of the photoconductive drum. The laser 105 selectively emits infra red light to the scanning mirror 106. The surface of the negatively electrically charged metal drum 111 , charged by the charging device 108, is thereby selectively discharged in accordance with the laser beam. This results in a pattern on the drum of charged areas, the pattern corresponding to the print which is to be made on the metal plate. Further, the printing unit comprises a powder roller 109 where negatively charged powder is brought from a powder compartment 107 toward the drum 111 via the powder roller 109. The powder is attracted to the parts of the drum that are exposed to light where it is no longer negatively charged. The toner adheres to the areas of the drum that have been exposed by light and the process results in a pattern of powder being the inverse of the pattern of charged areas. The printing system further comprises a printing table 101 on which the metal plate 116 to be printed on is placed. A positive potential is generated on the surface of the metal plate 116 by a power supply 113 connected to the plate or table, and when the metal plate 116 passes the drum 111 the negatively charged powder on the drum 111 is electrostatically attracted to the positive potential on the surface of the metal plate 116. Now the pattern of powder has been re- moved from the surface of the drum 111 and lies on the surface of the metal plate 116, and as the next step the surface of the metal plate 116 with the powder is heated by a heating system, e.g. a heater drum 115 (or another heating system, using e.g. convection, radiant or IR-heating) until the powder melts on the surface of the metal plate 116, whereby, after cooling, the pow- der is fused to the surface. The printing system further comprises a cleaning system 117 to remove powder that has not been transferred to the metal plate.

In another embodiment the laser beam may write the image areas, and the negative toner will be attracted to the discharged image areas.

In another embodiment the photoconductor drum may be charged positively and a positively charged powder may be used to develop the image. In this embodiment the electrical potential placed on the plate is negative. In another embodiment the photoconductor drum may be charged positively, and the laser beam may write the image areas, and a positively charged powder may be used to develop the image. In this embodiment the electrical potential placed or generated on the plate is negative.

In another embodiment the laser beam may be replaced with an array of light emitting diodes.

In one embodiment the colour powder could be a special UV resistant pow- der with good surface properties, alternatively it could be standard toner powder similar to the powder used in the toner cartridge of a paper laser printer. Further, the colour of the print to be made can be changed by changing the colour of the powder, and further multicoloured print can be made by adding different powder colours e.g. by using colour specific printing units.

Figure 2 illustrates a printing table 101 being a part of the system according to the present invention. The table comprises a printing surface 201 for placing a metal plate 203 to be printed on. The printing surface comprises a number of air holes 205 and is placed on an elevation surface 209, which can be moved in a direction illustrated by the arrow 211 via transport elements 207, such as transport drums or wheels. The elevation surface can be lowered or raised in the direction illustrated by the arrow 213. This process is performed via elevation elements 215, which e.g. could be controlled electronically.

When the powder is to be electrostatically attracted from the drum to the surface of the metal plate, it is very important that the plate makes contact with the drum. Therefore, based on the total thickness of the metal plate and the thickness of the powder layer, it is necessary to adjust the distance by lower- ing or raising the elevation surface 209. Further, the air holes 205 are used for generating a vacuum between the metal plate and the print surface 201. Since there must be contact between the drum 111 and the metal plate 203, the system is very sensitive to uneven surfaces being a result of the metal plate not being plane. The vacuum generated by sucking air out of the air holes ensures that the metal plate lays plane on the printing surface thereby also ensuring that the metal plate does not damage the drum 111. Figure 3 illustrates another embodiment of a printing table 301. The table comprises two rows of parallel transport drums, a bottom row 303 and a top row 305. The transport drums rotate in a direction pushing the metal plate through the printer via the printing unit 315. The bottom row 303 can be lowered or raised in the direction illustrated by the arrow 307. This is performed via elevation elements 309, which e.g. could be controlled electronically. The top row 305 is mounted statically, whereby the distance between the two rows 305, 303 of transport drums can be adjusted by lowering or raising the bottom row 303. The distance can then be adjusted based on the thickness of the metal plate 313. An advantage of this embodiment of the printing table 301 is that the contact between the surface of the metal plate 313 and the printing unit 3 5 is easy to control. Further, the position of the top surface of the metal plate 313 is defined by the diameter of transport drums placed in the top row 305.

The metal plate receives an electrical potential or bias from its contact with the lower metal rolls 303. The powder is fused to the surface of the metal plate before the plate exits the printing unit 315.

Fig. 2 and 3 illustrate a printing table having means for adjusting the distance between the metal plate surface and the printing unit, whereby it is possible to print on metal plates of different thickness. In a specific embodiment the printer could comprise a photo detection system for automatically detecting the thickness of the metal plate, and then afterwards automatically adjusting the distance based on the detected metal plate thickness. The distance is typically adjusted to ensure contact between plate and printing unit but also to avoid collision between the plate and printing transfer unit.

Figure 4 illustrates another embodiment of a printer according to the present invention being adapted for making retro reflective coating of a metal plate. The system comprises a printing table 401 and a printing unit 403, where the printing unit comprises two drums 405 and 407. The first drum 407 is for adding powder as described in connection with figure 1 , the second drum 405 is for adding powder with specific properties, e.g. material having retro reflective properties. In figure 4 a retro reflective powder (e.g. glass or acrylic) is dispensed via the powder compartment 415, via a powder roller 417, where the powder is electrically charged by the charge bar 419. In the figure a laser 409 is illustrated, and by using the laser 409 and a controllable mirror 421 for discharging areas on the drum, it is possible to add retro reflective powder only on specific areas of the metal plate 413. Alternatively, a simpler system would be obtained by not using a laser. In this case a non negatively electrically charged drum could be used, which would result in the drum 405 being electrostatically covered by powder on the total area, which again would result in the total surface of the metal plate 413 being covered by retro reflec- tive powder. The printing system further comprises a cleaning system 418 to remove powder that has not been transferred to the metal plate.

In an embodiment the charge bar 419 could be a voltage source ionizing the surface of the powder corns; in one embodiment the voltage source could be embodied as a high frequency high voltage source, and in a specific embodiment the voltage source establishes a voltage between 10kV - 25kV in a frequency between 1 MHz - 3MHz.

In one embodiment the powder having retro reflective properties is glass powder, which has been electrically charged, and in one embodiment a thin liquid membrane could be added to the glass powder corns in order to enhance the degree of electrically charging the powder. In one embodiment the mentioned membrane could be added by placing the powder in a liquid and then drying the powder. As a result, a thin membrane will remain on the glass powder corns. When glass powder is used for generating the retro reflective print, the glass powder can be added on top of the colour powder and both powders can be fused to the surface of the metal plate 413 by heating in one process. In fig. 4 a heated roller 411 warms up the metal plate, which has been covered with both colour powder and glass powder in order to melt both colour powder and glass powder to the surface of the metal plate 413.

As an alternative to glass powder, powder made of acryl could be used, whereby it is not necessary to add a thin liquid membrane in order to make it electrically chargeable. When acryl is used for generating the retro reflective print, it is necessary to first melt the colour powder and then the acryl powder in a two step process because of the different melting points of respectively the powder and the acryl.

In figure 5 another embodiment is illustrated where the powder is electro- statically charged in a conventional developer unit such as those being used in commercial laser printers. The system comprises a printing table 501 for placing the metal plate to be printed on and a printing unit 503. The printing unit comprises a laser 505, a controllable scanning mirror 506 and a drum 511 coated with photoconductive material and placed next to a charging de- vice 508. The inside surface of the photoconductive drum 511 is connected to electrical neutral or ground. The charging device 508 places negative electrical charge on the surface of the photoconductive drum. The laser 505 selectively emits infra red light to the scanning mirror 506. The surface of the negatively electrically charged metal drum 511 , charged by the charging device 508, is thereby selectively discharged in accordance with the laser beam. This results in a pattern on the drum of charged areas, the pattern corresponding to the print, which is to be made on the metal plate. Further, the printing unit comprises a powder roller 509 where negatively charged powder is brought from a powder compartment 507 toward the drum 511 via the powder roller 509. The powder is attracted to the parts of the drum that are exposed to light where it is no longer negatively charged. The toner adheres to the areas of the drum that have been exposed by light, and the process results in a pattern of powder being the inverse of the pattern of charged areas. The printing system further comprises an isolative transfer belt 518 rid- ing on conductive rollers. An electric field is provided between the drum and the transfer belt by placing an electrical potential on the rollers. As the belt 518 is rotated in synchronous motion with the drum 511 , the powder image is transferred to the belt under the influence of the electric field and is carried down to the lower roller location. The printing system further comprises a cleaning system 519 to remove powder that has not been transferred to the belt. The printing system further comprises a printing table 501 on which the metal plate 516 to be printed on is placed. A positive potential is generated on the surface of the metal plate 516 by a power supply 513 connected to the plate or table, and when the metal plate 516 passes the transfer belt 518 the negatively charged powder on the belt 518 is electrostatically attracted to the positive potential on the surface of the metal plate 516. Now the pattern of powder has been removed from the surface of the drum 511 and lies on the surface of the metal plate 516, and as the next step the surface of the metal plate 516 with the powder is heated by a heating system, e.g. a heater drum 515 (or another heating system, using e.g. convection, radiant or IR-heating) until the powder melts on the surface of the metal plate 516, whereby after cooling the powder is fused to the surface

In one embodiment the rollers that transport the intermediate transfer belt are constructed with a compliant or conformable conductive or partially conduc- tive material to optimise contact with the metal plate when the metal plate is not perfectly flat

In one embodiment the intermediate transfer belt is constructed with a compliant or conformable insulative material to optimise contact with the metal plate when the metal plate is not perfectly flat

In figure 6 a further embodiment is illustrated, which is similar to the above but where the powder is provided using electronic beam imaging (EBI). The system comprises a dielectric drum 601 , which is charged in predefined ar- eas using an electron cartridge 603 providing the surface of the dielectric drum with electron. The electron cartridge 603 selectively places negative electrical charge on the surface of the photoconductive drum. This results in a pattern on the drum of charged areas, the pattern corresponding to the print, which is to be made on the metal plate. Further, the printing unit com- prises a powder roller 609, where negatively charged powder is brought from a powder compartment 607 toward the drum 601 via the powder roller 609.

As described above, the print being generated on the metal plate only depends on the areas in which the drum is being exposed by infra red light from the laser. Therefore, by controlling the laser light directly and via the controllable mirror, the print to be printed on the metal plate can easily be changed and new prints can easily be defined. There are different methods of electrostatically moving the powder via the drum to specified areas on the metal surface. In general, the process of electrostatically attracting the powder to a surface could be by; 1. Generating a positive electrical field on the surface by using a voltage source and negatively charging the powder.

2. Negatively charging the powder and then using a surface being elec- trically neutral.

3. Generating a negative electrical field on the surface and using a powder being electrically neutral.

0 4. Generating a positive electrical field for attracting neutral powder.

5. Generating a negative electrical field for attracting positively charged powder.

5 6. Using a neutral field for attracting positively charged powder.

Which of the above methods to be used can e.g. be determined by the needed properties of the powder.

0 Figure 7 illustrates an embodiment of a method of controlling a system according to the present invention. In this example the printer 701 is controlled by a computer 703. First, in 705 a print is being defined using the computer 703. This could e.g. be via a dedicated drawing program. Then in 707 the printer settings are determined. These settings comprise information about 5 the thickness and the shape of the metal plate to be printed on. In 709 the defined print and the printer settings are communicated to the printer, this could e.g. be by using a common network or by a direct connection. In 711 the printer receives the printer settings and information about the defined print, and in 713 the printer is set up according to the received settings. Set- o ting up the printer comprises raising or lowering the elevation surface of the printing table according to the received information about the metal plate thickness. In 715 the print is generated on the metal plate as described with reference to fig. 1 , where the laser is controlled to electrically discharging the drum in predefined areas corresponding to the received print definition. 5 Figure 8 illustrates another embodiment of a method of controlling a system according to the present invention. In this example the printer 801 is controlled by a computer 803. Not until in 805 a print is being defined using the computer 803, this could e.g. be via a dedicated drawing program. In 807 the defined print is communicated to the printer. This could e.g. be by using a common network or by a direct connection. In 809 the printer receives information about the defined print, and in 811 the printer is set up by raising or lowering the elevation surface of the printing table according to the information about the metal plate thickness. In 813 the print is generated on the metal plate as described with reference to fig. 1 , where the laser is controlled such that it discharges the drum electrically in predefined areas corresponding to the received print definition.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'comprising' does not exclude the presence of other elements or steps than those listed in a claim. In claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A system for printing predefined prints on a substantially stiff metal plate, the system comprising:

- means for receiving instructions describing a predefined print,

- means for obtaining a print transfer unit being electrically charged in predefined areas corresponding to said received instructions, said means ensuring that the predefined areas are electrically charged ei- ther negatively or positively,

- means for providing a surface of the print transfer unit with a powder by electrostatically attracting the powder to the surface in a pattern determined by said charged areas, said powder being electrically charged either negatively or positively,

- means for electrostatically moving at least some of the powder from the surface of the print transfer unit to the surface of the metal plate,

- means for heating the surface of said metal plate and the powder on said surface to ensure that said powder is fused to said surface.

2. A system according to claim 1 , wherein said means for electrostatically moving at least some of the powder from the surface of the print transfer unit to the surface of the metal plate comprises, means for ensuring a contact between the surface of the metal plate and said print transfer unit or an intermediate transfer belt or drum, said means being a printing table having a surface on which the metal plate can be placed, the printing table comprising an elevation mechanism making the distance between said surface and said print transfer unit or intermediate transfer belt or drum adjustable.

3. A system according to claim 2, wherein said printing table comprises a surface, which further comprises means for creating a vacuum between the metal plate and the surface of the printing table.

4. A system according to claim 2-3, wherein said means for generating an electrical potential on the surface of the metal plate is comprised in said printing table, said printing table having a surface on which the metal plate can be placed, and said printing table being adapted for generating a potential on said surface.

5 A system according to claim 1-4, wherein said means for obtaining a print transfer unit being electrically charged in predefined areas is based on Electron Beam Imaging (EBI).

6. A system according to claim 1-5, wherein said means for electrostatically moving at least some of the powder from the surface of the print transfer unit to the surface of the metal plate comprises means for ensuring a contact between the surface of the metal plate and said print transfer unit or an intermediate transfer belt or drum, said means being two parallel rows of trans- port drums, between which the metal plate can be transported, the two parallel rows comprising a top row and a bottom row, said top row being statically mounted in a predefined distance from said print transfer unit, and said bottom row comprising an elevation mechanism making the distance between said bottom row and said top row adjustable.

7. A system according to claim 6, wherein said bottom row further comprises means for generating said electrical potential at the surface of said metal plate.

8. A system according to claim 1-7, wherein said powder is made from glass.

9. A system according to claim 1-7, wherein said powder is made from glass with a fluid membrane.

10. A system according to claim 1-9, wherein the system further comprises means for removing the electrical potential from the metal plate.

11. A method for printing predefined prints on a substantially stiff metal plate, the method comprising the steps of:

- receiving instructions describing a predefined print,

- obtaining a print transfer unit being electrically charged in predefined areas corresponding to said received instructions, ensuring that the predefined areas are electrically charged either negatively or posi- tively,

- providing a surface of the print transfer unit with a powder by electrostatically attracting the powder to the surface in a pattern determined by said charged areas, said powder being electrically charged either negatively or positively,

12. A method according to claim 11 , wherein the step of electrostatically moving at least some of the powder from the surface of the print transfer unit or intermediate transfer belt or drum to the surface of the metal plate com- prises the step of adjusting the distance between the print transfer unit or intermediate transfer belt or drum and the surface of said metal plate by adjusting a printing table adapted for placing said metal plate.

13. A method according to claim 10-12, wherein the method further com- prises the step of discharging the metal plate.