Powder purging apparatus and method

Abstract

Systems, methods, and apparatus are provided for purging charged powder particles from a mixture of ferromagnetic carrier particles and said powder particles. One example embodiment may include a powder purging apparatus comprising a support surface comprising a first side for supporting said carrier particles; a plurality of magnets, arranged on a second side of said support surface for generating magnetic fields at said first side to attract said carrier particles for forming a magnetic brush of said carrier particles, said magnetic fields and said support surface moveable relative to each other; an attracting surface facing said first side and spaced apart therefrom; and a field generator adapted for generating an electrical field between said attracting surface and said support surface for attracting said powder particles away from the carrier particles and towards said attracting surface, wherein said magnetic brush of carrier particles is spaced apart from said attracting surface.

Claims

1. The powder purging apparatus for purging charged powder particles from a mixture of ferromagnetic carrier particles and said powder particles, comprising: a support surface comprising a first side for supporting said carrier particles; a plurality of magnets, arranged on a second side of said support surface opposite from said first side, for generating magnetic fields at said first side to attract said carrier particles to said first side for forming a magnetic brush of said carrier particles, wherein said magnetic fields and said support surface are moveable relative to each other; a driving element adapted for driving movement of said magnetic fields relative to said support surface; an attracting surface facing said first side and spaced apart there from; and a field generator adapted for generating an electrical field between said attracting surface and said support surface for attracting said powder particles located at the magnetic brush away from the carrier particles and towards said attracting surface, wherein said magnetic brush of carrier particles is spaced apart from said attracting surface.

2. The powder purging apparatus according to claim 1, further comprising a mixture supply device for supplying said mixture to said first side.

3. The powder purging apparatus according to claim 2, wherein said mixture supply device comprises a supply outlet spaced apart from said first side by a distance substantially equal to a predetermined maximum height of said brush of carrier particles.

4. The powder purging apparatus according to claim 2, wherein said mixture supply device comprises fluidizing means for fluidizing said mixture.

5. The powder purging apparatus according to claim 1, wherein said plurality of magnets and said support surface are moveable relative to each other, and wherein the driving element is adapted for driving movement of said support surface relative to said plurality of magnets.

6. The powder purging apparatus according to claim 1, wherein said support surface comprises a sleeve having a longitudinal axis, wherein said first side is substantially cylindrical and arranged around said plurality of magnets and wherein said plurality of magnets extend radially from said longitudinal axis.

7. The powder purging apparatus according to claim 6, wherein said first side is provided with a spiral for guiding movement of the carrier particles along a path having a starting point and an end point, wherein said starting point and said end point are spaced apart along said longitudinal axis.

8. The powder purging apparatus according to claim 7, wherein said spiral is a spiral strip comprising a first layer comprising a ferro-magnetic material, and a second layer, preferably arranged on top of the first layer, comprising or made of a substantially electrically non-conductive material.

9. The powder purging apparatus according to claim 7, further comprising a carrier particle container arranged at or downstream of said end point of said path.

10. The powder purging apparatus according to claim 9, further comprising a rotating brush close to the end point and the carrier container, for brushing carrier particles away from the end point and into the carrier particle container or an inlet thereof.

11. The powder purging apparatus according to claim 1, wherein, at least during use, said attracting surface is substantially rotationally fixed relative to said support surface, or wherein, at least during use, said attracting surface is substantially rotationally fixed relative to said plurality of magnets.

12. The powder purging apparatus according to claim 1, wherein said attracting surface comprises a plurality of openings for letting through powder particles from a first side of said attracting surface facing said support surface to a second side of said attracting surface facing away from said support surface.

13. The powder purging apparatus according to claim 12, further comprising an air manifold connected to a vacuum source, wherein said air manifold is arranged adjacent to said attracting surface for removing powder particles from said first side of the attracting surface.

14. The powder purging apparatus according to claim 1, wherein said attracting surface substantially envelops said first side of the support surface.

15. The powder purging apparatus according to claim 1, wherein the support surface is moveable from a first position in which said plurality of magnets is arranged for generating said magnetic field at said first side for attracting said carrier particles to said first side, to a second position in which said first side is substantially outside the magnetic field generated by said plurality of magnets, and wherein the support surface is moveable from the second position to the first position.

16. A method of purging charged powder particles from a mixture of ferromagnetic carrier particles and said powder particles, comprising: supplying said mixture to a first side of a support surface; attracting said carrier particles of said mixture to said first side using a plurality of magnets arranged on a second side of said support surface opposite to said first side to generate magnetic fields to form a magnetic brush of said carrier particles on said first side; moving said magnetic fields relative to said support surface; generating an electrical field between said support surface and an attracting surface for attracting said charged powder particles located at the magnetic brush away from the carrier particles and spaced apart from and facing said first side, wherein said magnetic brush of carrier particles is spaced apart from said attracting surface.

17. The method according to claim 16, wherein said attracting surface is substantially rotationally fixed relative to said support surface while movement of said plurality of magnets is driven relative to the support surface.

18. The method according to claim 16, further comprising a step of fluidizing said mixture prior to supplying said mixture to said first side of the support surface.

19. The method according to claim 16, wherein said first side has a cylindrical shape and wherein said carrier particles are moved relative to said first side by rotating first side relative to said plurality of magnets, wherein said magnets of said plurality of magnets are arranged substantially along a spiraling path having a starting point and an end point, wherein the starting point and the end point are spaced apart along a longitudinal axis of said cylindrical shape, wherein said mixture is substantially continuously supplied to said first side at said starting point, and wherein carrier particles on said first side are substantially continuously moved over said first side along said spiraling path towards said end point.

20. The method according to claim 16, further comprising a step of placing a substrate to be provided with said powder particles between the attracting surface and the first side, spaced apart from said first side.

21. The method according to claim 16, wherein substantially all of said mixture is supplied to said first side, and wherein said mixture comprises a substantially predetermined amount of powder particles.

22. The powder purging apparatus according to claim 1, wherein the magnets face the first side of the support surface with alternating north-south polarities.

23. The powder purging apparatus according to claim 1, wherein the attracting surface extends along a portion the first side of the support surface.

24. The powder purging apparatus according to claim 23, wherein the attracting surface extends along the entire first side of the support surface.

25. The powder purging apparatus according to claim 7, wherein the strength of the magnets of the plurality of magnets decreases towards the end point of the spiral path.

26. The powder purging apparatus according to claim 1, wherein the plurality of magnets is placed, such that the magnets are arranged to form the magnetic brush at a portion of the first side to which the attracting surface is facing.

27. The powder purging apparatus according to claim 26, wherein the plurality of magnets is placed, such that the magnets are arranged to form the magnetic brush at substantially the entire circumference of the first side.

28. The powder purging apparatus according to claim 26, wherein a plurality of magnets is arranged in a spiraling path around the longitudinal axis of the sleeve.

29. The powder purging apparatus according to claim 27, wherein a plurality of magnets is arranged in a spiraling path around the longitudinal axis of the sleeve.

30. The powder purging apparatus according to claim 6, wherein the attracting surface and the support surface are concentric.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a cross-sectional view of a powder purging apparatus according to a first embodiment of the invention,

(2) FIG. 2A schematically shows a partial side view and cross-sectional view of a powder purging apparatus according to a second embodiment of the invention,

(3) FIG. 2B schematically shows a cut-open section of the apparatus of FIG. 2A,

(4) FIG. 3 schematically shows a path followed by carrier particles in an apparatus according to the invention,

(5) FIG. 4A schematically shows a cross-sectional side view of a further embodiment of the invention,

(6) FIGS. 4B and 4C schematically show a cross-sectional front view of the embodiment of FIG. 4A,

(7) FIG. 5A shows an exploded isometric view of a powder purging apparatus according to the invention, adapted for applying a predetermined amount of powder to a substrate,

(8) FIG. 5B shows a cross-sectional view of an apparatus of FIG. 5A during use,

(9) FIG. 6 shows a flow chart of a method for purging powder particles from a mixture of and ferromagnetic carrier particles and charged powder particles according to the present invention,

(10) FIGS. 7A and 7B schematically show cross-sectional views of a preferred embodiment of powder purging apparatus according to the present invention,

(11) FIG. 7C schematically shows an isometric exploded view of a portion of the powder purging apparatus of FIG. 7A, including a cylindrical support surface provided with a spiral on its first side and a plurality of magnets is arranged on a roll which is rotatable relative to and arranged within the cylindrical support surface.

DETAILED DESCRIPTION OF THE INVENTION

(12) In laser printers or coating apparatuses, in particular powder coating apparatuses, ferromagnetic carrier particles are often used to transport smaller non-magnetic powder particles to a surface to be provided with said powder particles for the development of an image on said surface and/or providing a layer of powder particles on said surface. However, even after such an image or layer has been formed on the surface, typically a residue of powder particles remains which sticks or adheres to said carrier particles. In order to be able to reuse the carrier particles with a different kind of powder particle, e.g. when using the same carrier particles with powder particles of a different color, the powder particles must be purged from the carrier particles.

(13) FIG. 1 schematically shows a cross-sectional view of an apparatus 1 for purging powder from charged particles according to the present invention. The powder purging apparatus 1 comprises a mixture supply device 20 containing a mixture 30 of ferromagnetic carrier 31 particles and charged powder particles 32, wherein the powder particles typically adhere to the carrier particles.

(14) The mixture supply device 20 supplies the mixture through a mixture supply outlet 21 having an open end 22, towards a cylindrical support surface, or sleeve 2. The cylindrical support surface comprises a first side 3 for supporting said carrier particles 31 thereon. Any powder particles 32 adhering to the carrier particles are supported on the first side 3 as well. On a second side 4 of the support surface 2, which is a side of the support surface opposite to the first side 3, a plurality of magnets 50 is arranged in a spiraling path around longitudinal axis L of the cylindrical support surface 2. The magnets are arranged along said spiraling path, facing the first side 3 with alternating north-south polarities, i.e. any two neighboring magnets 53, 54 along said path have opposite polarities facing the first side. The magnets are arranged proximate to and spaced apart from the first side 3 and generate a magnetic field for attracting the ferromagnetic carrier particles 31 to the first side 3 for forming a magnetic brush of carrier particles in a manner known in the art. A maximum height of the brush is determined by the distance h between the open end 22 of the supply outlet 21 to the first side 3; because a brush can be formed of a height at most equal to said distance no doctor blade is required for limiting the maximum height of the brush.

(15) The cylindrical support surface 2 is arranged as a sleeve around the plurality of magnets 50. An electromotor 40 is adapted for driving rotation of the support surface 2 around the plurality of magnets 50. The electromotor comprises a stator 41, which supports the plurality of magnets 50 at a first and, and a rotor 42, which supports the support surface 2 at a first end. The plurality of magnets is supported at a second, opposite end by support 44, and the cylindrical support surface 2 is supported at a second opposite end by bearings 45.

(16) When the support surface 2 rotates around the plurality of magnets 50, the magnets continue to attract the carrier particles 31 to the first side, which, in combination with friction between the carrier particles 31 and the first side 3 due to rotation of the support surface 2 around the plurality of magnets 50, drives movement of the carrier particles over the first side 3. The movement of the carrier particles 31 and any powder particles 31 adhering thereto over the first side 3 of the support surface substantially follows the spiraling path along which the magnets of the plurality of magnets 50 are arranged from a starting point 51 of said path to and end point 52 of said path. During purging, the plurality of magnets 50 remains stationary relative to the supply outlet 22. The supply outlet may thus be arranged to supply carrier particles to said sleeve 2 at the position of the starting point of the path. During said movement of the carrier particles 31 along the spiraling path of the plurality of magnets 50, the ordering between the carrier particles 31 in the magnetic brush of changes as well, such that carrier particles that were first at the interior of the of the brush may move to the exterior of the brush and vice versa, and the orientation of the carrier particles relative to the first side may change as well. As a consequence, powder particles adhering 32 to the carrier particles 31 will eventually be located at the outer side of the brush where they may relatively easily be separated from the carrier particles 31.

(17) For separating charged powder particles from carrier particles, the purging apparatus further comprises a stationary attracting surface 60, spaced apart from said first side. The purging apparatus further comprises a field generator 70 for generating an electrical field 71 between the support surface 2 and the attracting surface 60. In the embodiment shown, both the support surface 2 and the attracting surface 60 comprise a conductive metal. The field generator 70 generates the electrical field 71 between the attracting surface and the support surface by providing a potential difference between the attracting surface 60 and the support surface 2 of 1500 to 3000 Volts. Preferably, the support surface is at a voltage of 2000 Volts while the attracting surface 60 is grounded. Charged powder particles 32 at the exterior of the brush are attracted towards the attracting surface 60 and are thus purged from the carrier particles 31.

(18) In the embodiment shown, the charged powder particles 31 are positively charged powder particles, and the electrical field 71, which is normal to the support surface 2, attracts positively charged powder particles in the direction of the field 71 away from the support surface 2 and towards the attracting surface 60. If the mixture 30 would have comprised negatively charged powder particles instead of positively charged powder particles, the direction of the electrical field would have been reversed. Because the field generator 70 is adapted for providing an electrical field in a first direction 71, and in an opposite second direction, the apparatus 1 may also be used to purge a mixture comprising ferromagnetic carrier particles and both positively and negatively charged powder particles. In such a case the field generator 70 would be configured for reversing the direction of the electrical field after half the time required for a carrier particle to traverse the path from the starting point 51 to the end point 53 has passed, such that during half the traversal of the path by the carrier particles the positively charged powder particles are purged from the carrier particles, and such that during traversal of the remaining half of the path by the carrier particles the negatively charged powder particles are purged from the carrier particles.

(19) To reduce the build-up of powder particles 32 at the attracting surface 60, which would influence the electrical field, the attracting surface 60 is provided with a plurality of apertures 61 through which the powder particles 32 can pass from a first side 63 of the attraction surface facing the first side 3 of the support surface 2, to an opposite second side 64 of the attracting surface. At the second side 64, the attracting surface 60 is connected to an air manifold 80 which in turn is connected to a vacuum pump 82. The air manifold 80 supplies the air containing the powder particles to an air filter 81 placed between the vacuum pump and the air manifold 80. To improve the flow of air for transporting the powder particles away from the first side of the attracting surface, a stream of fresh air is supplied between the first side 3 of the support surface 2 and the first side of the attracting surface. The vacuum pump 82 causes an under pressure at the second side 64 of the attracting surface 60, which causes air and the powder particles at the first side to move through the apertures 61 towards the second side and then towards the air filter 81. The filtered powder particles 32 are deposited in a powder container 82.

(20) When carrier particles 31 have been purged once they traveled along the spiraling path from the starting point 51 to the end point 52, they are removed from the first side of the support surface and collected in a carrier particle container 90. To remove the carrier particles from the first side at the end of the path a skive may for instance be used. In the embodiment shown however, the magnetic strengths of the magnets of the plurality of magnets decreases along the path towards the end 52 of the path. At end point 52 the force exerted by the magnets on the carrier particles attracting the carrier particles towards the first side 3 is less than the force of gravity and/or friction caused by rotation of the circumferential surface directed away from the first side 3. As a result, purged carrier particles which have reached the end 52 of the path fall into carrier particle container 90.

(21) FIG. 2A shows a side view of a cylinder 200 on which a plurality of magnets 250 as used in an embodiment of the invention is arranged. The plurality of magnets 250 is arranged on a cylindrical body 257, along a spiraling path having a starting point 251 and an end point 252. The magnets of the plurality of magnets 50 are arranged such that neighboring magnets along the path have opposite polarities, at a side of the plurality of magnets facing away from the cylinder 257, i.e. the magnets depicted in black, such as magnet 253 are facing outward with their south poles, and their neighboring magnets depicted in white, such as magnet 254, are facing outward with their north poles. Neighboring magnets are spaced apart along the path using spacing elements 255. For comparison, a schematic cross-section of a cylindrical support surface 202 or sleeve for supporting a magnetic brush of carrier particles on a first side 203 thereof is shown as well, together with a supply device 220 containing a mixture of carrier particles 231 and charged powder particles 232. The supply device comprises a supply outlet 221 arranged with its end 222 at a distance h2 from the first side of the support surface 202. The first side 263 of the attracting surface 260, is spaced apart from the first side 203 of the support surface 202 by a distance h3 which is greater than the distance h2, such that the magnetic brush of carrier particles formed on the first side of the support surface when the carrier particles are supplied thereto cannot not contact the first side 263 of the attracting surface 260. The attracting 260 surface comprises apertures or through holes 261, allowing air and powder particles to pass from the first side 263 of the attracting surface facing the first side of the support surface 202 to the oppositely facing second side 264 of the attracting surface. The outer diameter d1 of the spiral in which the plurality of magnets 50 is arranged is smaller than an inner diameter d2 of the support surface 202, such that the support surface 202 may rotate substantially freely relative to the plurality of magnets 50. Preferably during purging, the plurality of magnets 50 remains substantially stationary, while an electromotor drives rotation of the support surface 202 around the plurality of magnets 250. In this manner the position of the start 251 of the path relative to the end 222 of the supply outlet 221 remains substantially constant, and the position of the end 252 of the path relative to a carrier particle container 290 remains substantially constant as well. However, in an alternative embodiment, an electromotor may be used to drive rotation of the plurality of magnets within the circumferential surface 202 while the circumferential surface remains substantially stationary, or one or more motors may be provided for driving rotation of both the circumferential surface 202 and the plurality of magnets 250.

(22) FIG. 2B shows a cut-out side view of the sleeve 202 of FIG. 2A. The first side 203 of the sleeve is provided with a number of grooves 203a extending substantially parallel to the longitudinal axis L of the sleeve, and a number of elliptical grooves 203b substantially circumscribing the first side 203. The grooves 203a and 203b on the first side provide additional friction with the carrier particles when the support surface 202 is rotated relative to the plurality of magnets 250. The additional friction helps in driving displacement of the carrier particles over the first side when the support surface rotates around the plurality of magnets. Moreover, scraping of the carrier particles against each other and the first side, in particular against the grooves 203a,203b of the first side, helps in loosening powder particles from carrier particles to which they are attached.

(23) FIG. 3 schematically shows a path P followed by carrier particles over a cylindrical support surface 302. A mixture of ferromagnetic carrier particles and charged powder particles is supplied onto a first side 303 of the cylindrical support surface 302 at or close to a starting point 351 a plurality of magnets 350 which are arranged spiraling over the outer side of cylinder 357. The spiraling path P spirals around the longitudinal axis L of the cylinder from said starting point to an end point spaced apart from the starting 351 point along the longitudinal axis L. Neighboring magnets of the plurality of magnets 350 along said path having opposite polarities at the first side. When a mixture of ferromagnetic carrier particles and substantially non-magnetic powder particles is supplied to the first side and the support surface 302 rotates relative to the plurality of magnets 350, the carrier particles are driven along the path in a direction from the starting point to the end point of the path.

(24) FIG. 4A shows a cross-sectional side view of an embodiment of the present invention. Powder purging apparatus 400 comprises a support surface or sleeve 402 having a first side 403 for supporting a mixture 430 of ferromagnetic carrier particles and charged powder particles supplied from an end 422 of a supply outlet 421 of a supply device 420. When the mixture is supplied to the support surface it forms a magnetic brush of carrier particles thereon under the influence of the magnetic field exerted by plurality of magnets 450. The apparatus further comprises a powder container 491 and carrier particle container 490 here shown in a position these containers would be in during purging of powder from the carrier particles. The plurality of magnets 450 of the apparatus is rotatable relative to the support surface 402 around an axis of rotation L. The rotation causes carrier particles in the magnetic brush to change position and orientation within the brush, such that different carrier particles and different parts thereof are presented at the exterior of the brush. Electrical field generator 470 applies a potential to the support surface 402, such than an electrical field is generated between the support surface 402 and an attracting surface 460 which is a ground potential. The electrical field attracts charged powder particles located at the exterior of the magnetic brush away from the carrier particles, thus purging the carrier particles from the powder particles. Charged powder particles that have been attracted away from the magnetic brush may fall along with gravity into powder container 491, while the carrier particles remain on the support surface during purging.

(25) FIG. 4B shows a cross-sectional front view of the apparatus of FIG. 4A. A shaft 441 connects the plurality of magnets 450 to a motor 440 for driving rotation of the plurality of magnets around its axis of rotation L.

(26) When purging is complete, substantially only the carrier particles are left on the sleeve 402. During purging it may be determined that purging is complete by determining whether or not there is a transfer of powder particles from the sleeve into the powder container 491, for instance using a scale for weighing the mass of the powder sump, using an optical detector for optically determining whether particles are transferred into the powder sump, or by detecting whether there is a change in potential of the attracting surface 460 due to charged powder particles contacting said attracting surface, and/or by setting a time limit for purging.

(27) When it is established that purging is complete, the support surface 2 is moved from a first position, in which said magnets of said plurality of magnets are proximate to said first side as shown in FIG. 4BA, to a second position, in which said first side is substantially outside the magnetic field generated by said plurality of magnets as shown in FIG. 4C. In this latter case, when the sleeve or receiving surface 2 is slid in a direction away from the plurality of magnets 450 along the axis of rotation L, the plurality of magnets 450 no longer attract the carrier particles towards the sleeve 402, as a result of which the carrier particles are free to fall into carrier particle container 490 located below the sleeve when the sleeve is in the second position. Preferably the powder container 491 is removed from the purging apparatus prior to moving the support surface to the second position.

(28) FIGS. 5A and 5B schematically show an exploded view and a cross-sectional view of an alternative embodiment

(29) of the present invention. During use, a mixture of carrier particles and powder particles is placed in a container 507, which comprises a support surface 502 having a first side 503 for supporting said mixture thereon. A disc 511 comprising a plurality of magnets 553,554 arranged radially around the center of the disc is provided on a side 504 of the support surface 102 facing away from said first side 503. The disc is 511 connected via a shaft 508 to a motor 509, for rotating the disc around an axis of rotation R. The apparatus further comprises an attracting surface 506, which is connected to a shaft 510, and moveable parallel to the axis of rotation R, for positioning the attracting surface closer to or further away from the mixture in the container 507. The attracting surface 506 is at a first potential V0 and the first side 503 is at a different, second potential V1. The electrical field thus generated between the attracting surface and the first side causes the powder particles to be attracted towards the attracting surface 506. The ferromagnetic carrier particles remain attracted to the first side 503 by the magnetic force exerted on the carrier particles by of the plurality of magnets of the disc 511. The attracting surface preferably comprises a surface to be coated with a layer of powder particles. In use, the disc 511 is rotated while a potential difference is applied between the first side 503 and the attracting surface 506, until substantially all powder particles have been purged from the carrier particles and attracted onto the attracting surface 506. The embodiment thus allows coating of a surface to be coated with a precisely predetermined amount of powder particles.

(30) FIG. 6 shows a flow chart of steps of the method according to the present invention. At step 601 a mixture of ferromagnetic carrier particles and powder particles adhering thereto is supplied to a first side of a support surface. At step 602 the ferromagnetic carrier particles are attracted to the first side using a plurality of magnets arranged on a second side of said support surface opposite to the first side, such that a magnetic brush of carrier particles is formed on said first side. The support surface is rotated relative to the plurality of magnets at step 603, such that the position and orientation of carrier particles within the magnetic brush changes, and different carrier particles and parts thereof are arranged at the outer surface of the brush. At step 604 an electrical field is applied between the support surface and an attracting surface which is space apart from the support surface, such that powder particles are attracted away from the outer surface of the brush towards the attracting surface. As a result, the carrier particles are purged from powder particles. Though steps 601-604 are here shown in sequential order, it will be appreciated that they may be performed substantially at the same time.

(31) FIG. 7A schematically show a cross-sectional view of a powder purging apparatus 700 according to a preferred embodiment, and FIG. 7B schematically shows a cross-sectional view along line VIIB-VIIB of FIG. 7A.

(32) The apparatus 700 comprises a hollow support cylinder 702 for supporting on a first side 703 (or outer surface) thereof a magnetic brush (not shown) of ferromagnetic carrier particles. During purging, the support cylinder 702 remains substantially rotationally fixed to attracting surface 760 and housing 775,776,777. Because the attracting surface substantially entirely envelopes the first side 703 of the support surface 702 (see also FIG. 7B), a relatively large and homogeneous electric field can be generated between the support surface 702 and the attracting surface 760, so that substantially independent on the position of a powder particle in the magnetic brush the electric field exerts a force on the powder particle for moving the powder particle towards the attracting force.

(33) The support cylinder 702 is a metal support cylinder, and the attracting surface 760 is a metal attracting surface which also is substantially cylinder-shaped and substantially surrounds the metal support cylinder and is spaced apart therefrom by distance h3 greater than the maximum height of the magnetic brush. Field generator 770 is adapted for generating an electrical field between the attracting surface 760 and the support surface 702, which field attracts the powder particles towards the attracting surface 760. An air supply 785 injects air through an air outlet 786 to a point between the attracting surface 760 and the first side 703 which is upstream of the location of the end point 752 of the path. At an opposite side of the path, air outlet 784 which extends through housing wall 777 is connected to an air pump 783 for removing air from the housing. Thus the air pressure between inner side 763 of the attracting surface 760 and the first side 703 of the support surface 702, e.g. at point P1, is less than the air pressure between outer side 764 of the attracting surface 760 and the substantially air-tight housing 775,776,777, e.g. at point P2, such that the powder particles are urged from the inner side 763 of the attracting surface, through openings 761 in the attracting surface to the outer side 764 of the attracting surface 760 after which they can be filtered from the air in a manner similar as described for FIG. 1. The phrase air-tight herein does not necessarily mean completely air tight, but sufficiently air tight to substantially prevent air carrying powder particles from flowing through the walls 775,776,777 of the housing.

(34) For keeping the carrier particles attracted to the first side 703 of the support surface 702, the apparatus 700 is provided with a plurality of magnets 753,754 arranged on a roll 757 which is rotatable relative to the support surface 702 and arranged on a second side 704 of the support surface which is opposite to said first side 703, i.e. arranged on the inner side of the support cylinder 702. The plurality of magnets 753,754 is partitioned into a number of parallel and distinct sections 756,757, with the magnets within each section 756,757 arranged with a same magnetic pole proximate to the first side 703 of the support cylinder 702. Neighboring sections 756,757 of magnets are spaced apart from each other and held in place by aluminum strips 755, though any kind of other substantially non-ferromagnetic material may be used for this purpose.

(35) The first side 703 of the support cylinder 702 is provided with a spiral strip 780 which defines a spiraling path around said first side for carrier particles of the magnetic brush to follow. The spiral strip 780 comprises a first layer 781 comprising a ferromagnetic material, and a second layer 782 comprising an electrically insulating material, e.g. rubber tape, which is attached to the first layer using an adhesive and has a substantially greater height than the first layer. In the embodiment shown the metallic first layer has a height of between 0.2 and 0.6 mm, and the insulating second layer has a height of at least 4 mm.

(36) The different sections 756,757 of magnets together exert a magnetic force on the carrier particles of the magnetic brush, over substantially the entire outer surface 703 of the cylindrical support 702 between at least between starting point 751 and end point 752 of the path defined by spiral 780. Thus when the carrier particles travel along the spiral 780 from the starting point 751 towards the end point 752, the magnets keep the carrier particles attracted to the first side 703. To ensure that there is sufficient magnetic force at the entire first side 703 of the cylindrical support 702, the different sections 756,757 are arranged radially around the longitudinal axis of the roll 757, with neighboring sections arranged close enough to each other for attracting carrier particles arranged on or between the neighboring sections to the first side 703.

(37) The maximum height of the magnetic brush is substantially defined by the distance h5 of the supply outlet 222 of the mixture supply device 230 (see also FIG. 2A) to the first side 703 of the support cylinder 702. In the embodiment shown the height of the spiral strip 780 is greater than the maximum height of the magnetic brush, so that the carrier particles are prevented from traveling over and across the spiral 780 along the longitudinal direction of the support cylinder. As a result carrier particles which travel over the first side 703 from starting point 751 to end point 752 follow the entire spiral. Though not shown, the spiral strip 780 could also be arranged to contact both the attracting surface 760 and the first side 702 of the support surface 703.

(38) In an alternative embodiment, not shown, the height of the spiral strip 780 may be substantially less than the maximum height of the magnetic brush of carrier particles to be supported by the first side 703 of the support cylinder 702. Because the height of the spiral strip 780 is substantially less than the maximum height of the magnetic brush, e.g. less than the distance h5, at least some of the carrier particles which travel across the first side of the support cylinder 702 from the staring point 751 towards the end point 752 may travel over and across the spiral strip 780 along the longitudinal direction of the cylinder L. Thus in case a mass of carrier particles on the first side clogs a portion of the path defined by the spiral 780, carrier particles in the magnetic brush which are arranged at a greater distance from the first side 703 than the height of the spiral may still move along the first side when the magnet roll 757 is rotated relative to the stationary first side 703. In such a case the total length of the path traveled by the carrier particles remains substantially larger than the length of the cylinder 702 along its longitudinal axis L.

(39) FIG. 7B shows a cross-section along line VIIB-VIIB of FIG. 7A. When the roll 757 with the plurality of magnets 755,756 rotates in a first direction of rotation R1 relative to the first side 703 of the support cylinder 702, this causes carrier particles in the magnetic brush to travel along the spiral 780 along an opposite second direction of rotation R2. During rotation of the magnets the support surface 702 and the attracting surface 760 remain substantially stationary to the substantially air-tight housing 775,776. At the end point 752 of the path defined by the spiral 780 (see FIG. 7A) the support surface 703 transitions into a ramped surface, or thickened portion 795. When the carrier particles are near the end point of the path, they are driven onto this ramped surface, increasing the distance between the magnets and the carrier particles, until the magnetic force on the carrier particles is insufficient to attract them to the first side or ramped surface thereof, and the carrier particles fall into carrier particle container 790. To further facilitate movement of carrier particles away from the plurality of magnets and into the carrier particle container 790, a rotating brush 797 is provided, which rotates in a direction R3 counter to direction R2 in which the carrier particles move over the first side 703.

(40) As mentioned before the air pressure P1 between the inner side 763 of the attracting surface 760 and the first side 703 is higher than the air pressure P2 between the outer side 764 of the attracting surface 760 and the cylindrical housing portion 775 to facilitate transportation of the powder particles away the attracting surface 760. In order to prevent powder particles from entering the particle container 790, the particle container is substantially air-tight except for at an inlet for receiving carrier particles. Thus at point P3 there is substantially no air flow for blowing carrier or powder particles into or out of the carrier particle container. The inlet is arranged an area close to the ramped portion 795 which is located substantially outside of the airstream generated by the air supply 785 and air manifold 783. FIG. 7C schematically shows an exploded view of a cylindrical support surface 703, magnet roll 757 and spiral 780 of FIGS. 7A and 7B. The cylindrical first side 703, or outer surface, of the support is provided with the spiral 780 which extends over substantially the same length as the sections 757,756 of magnets 753,754. Within each section the magnets are arranged with a same magnetic pole proximate to the first side 703. Magnets of neighboring sections are arranged with opposite poles proximate to the first side 703. The plurality of magnets in the distinct sections are held in place by means of the aluminum strips 755.

(41) It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention.

(42) For example, although in the present exemplary embodiments the driving element is adapted for driving a mechanical movement of the support surface relative to said plurality of magnets, a person skilled in the art would understand that, in the case that the magnets are electromagnets, the driving element may be adapted for switching subsequent electromagnets for driving movement of the magnetic fields generated by the electromagnets relative to said support surface.