WO2007073206A1

WO2007073206A1 - Method and device for manufacturing a powder layer for in layer production of objects

Info

Publication number: WO2007073206A1
Application number: PCT/NO2006/000492
Authority: WO
Inventors: Roald Karlsen; Olav ÅSEBØ
Original assignee: Sinvent As
Priority date: 2005-12-20
Filing date: 2006-12-20
Publication date: 2007-06-28
Also published as: EP1963038A1; WO2007073205A1; EP1963038A4

Abstract

A method for manufacturing a layer of a workpiece is described, wherein the work-piece includes an object, the method comprising: forming at least a first pattern on a printing face of at least a first part of the layer of the workpiece to be manufactured, the at least first pattern being constituted by an adhesive material, and developing the first pattern by arranging/depositing a first powder material onto the adhesive material. An apparatus for implementing the method is also described.

Description

METHOD AND DEVICE FOR MANUFACTURING A POWDER LAYER FOR IN

LAYER PRODUCTION OF OBJECTS

INTRODUCTION

The present invention relates to a method and device for manufacturing powder layers to be used for the layer-wise production of objects, in particular objects of a metallic material, or ceramics in which metal serves as a binding agent.

BACKGROUND

In 1988, the first machine for the layer-wise production of plastic models was introduced to the market by 3D-Systems Corporation. The machine uses a building material in the form of a liquid that hardens when subject to UV light. The individual layers are manufactured in that a UV laser hatches the plane geometry of the layers. When a layer is completed, new liquid corresponding to the thickness of the next layer is added. This process is repeated until the object is finished. The process is called stereolithography, and is described in [Hull, 1986, US 4,575,330].

The manufacture of layers for the layer-wise production of components has been described earlier, both in technical books and in the patent literature. However, what seems to be lacking is a description of a concept of fabrication of metallic powder materials that may be integrated with a subsequent consolidation process suitable for industrial applications. It is particularly the high temperatures of the consolidation process that cause problems relative to the choice of concept and materials of manufacture for the layers. The previously described approaches (xerography and ionography, [Bynum, 1989, US 5,088,047], [Kumar, 1998, US 6,066,285], [Grenda, 1994, US 6,206,672]) use a latent electrostatic picture for developing powder layers having a desired shape, in the same manner as copy machines and laser printers. The advantage of ionography over xerography is that the fragile photoreceptor is replaced with a more robust dielectric material. This is especially important when the soft toner particles are replaced with hard and irregular powder particles of metal and ceramics. However, a problem associated with the use of ionography is to find a dielectric material that allows for the production of an ionic picture having a voltage that is sufficient to retain the heavy partic- les, and that at the same time is able to resist the repeated temperature shocks it is subject to in the consolidation process. The generation of high charge density inherently causes poor accuracy due to the Blooming effect [Bares, 1992, Color Hard Copy and Graphic Arts, SPIE volume 1670]. The advantage of the xerographic method is the accuracy and the voltage that may be achieved in the latent ionic picture.

NO 317085 B and WO 2004/037469 disclose layer-wise production of objects of metal and ceramics. Powder layers having a desired shape are manufactured using xerography and carried to a conveyor belt. The conveyor belt is made of a plastic material that evaporates at the sintering temperature, and its purpose is to bring the powder layers in between the compacting piston and the object. The powder layer is consolidated in that the piston is pressed against the object with the powder layer located thereinbetween, and the temperature of the powder layer is increased to the sintering temperature.

However, the burning of the conveyor belt, which is located between the object and the powder layer, may cause the formation of pores and fouling in the micro- structure of the object. Sintering is the diffusion of atoms through the lattice structure. Sintering is time-consuming (63 μm copper powder is sintered for up to two hours). As the device is described in NO 317085, with an individual heating of each powder layer, the sintering time will be too short to provide an adequate binding between the powder particles.

WO 2004/037469 also describes a method wherein the finished powder layer (produced using xerography on a powder receptor) is transported to the compacting device by way of a discrete transport element. A potential is applied between the transport element and the powder receptor, and the powder layer is moved over to the transport element. The powder layer adheres to a wax layer on the underside of the transport element. The transport element is carried to the compacting device, in which it is bound to a piston. The piston is then pushed down onto the workpiece/object, and a desired pressure is maintained until sintering has taken place. The complete sintering of each powder layer before the piston returns is a time consuming technique. Again, this process uses xerography for manufacturing powder layers. Xerography utilizes fragile photo receptors that have a short functional life and therefore are not suited for the production of metallic objects having a ceramic support structure. I the method, the powder layer is manufactured on a powder receptor and subsequently transferred to a transport element. In this transfer the powder layer loses geometrical accuracy. In order to achieve a satisfactory geometrical accuracy of the powder layer, it is important that the number of transfers of the powder layer between medias in the process is kept as low as possible.

SUMMARY OF THE INVENTION

The present invention solves the problems described above and provides an approach for the manufacture of powder layers that may be implemented in the production of functional objects.

In a first aspect, the invention provides a method for manufacturing a layer of a workpiece, the workpiece including an object, wherein the method comprises forming at least a first pattern on a printing face of at least a first part of the layer of the workpiece to be produced, the at least first pattern being constituted by an adhesive material, and developing the first pattern by arranging/depositing a first powder material onto the adhesive material.

In a further embodiment, the method further includes forming a second pattern on the printing face of at least a second part of the layer of the workpiece to be produced, the second pattern being constituted by a dielectric solid material, and developing the second pattern by arranging/depositing a second powder material onto the dielectric solid material. The method may further comprise forming a second pattern of at least a second part of the layer of the workpiece to be produced, the second pattern being constituted by an adhesive material, and providing the second pattern by arranging/depositing a second powder material onto the adhesive material. At least a third pattern may also be formed on the printing face of at least a third part of the layer of the object to be produced, the third pattern being constituted by an adhesive material, and the at least third pattern being de- veloped by arranging/depositing at least a third powder material onto the adhesive material.

Hence, a powder layer may consist of at least one building powder and/or a support powder. Many building powders may exist in a layer, depending on the design/shape of the final object. A layer of the workpiece could conceivably consist of building powder only, but support powder will usually be present. It is the building powder that is subsequently sintered/consolidated to the finished object.

The method may further include charging the deposited dielectric solid material by spraying negative ions onto the surface of the coating before arranging/depositing the second powder material. Arranged/deposited powder material having an incorrect polarity may be removed before the application of a next pattern. The adjustment of the polarity and charge level of the formed dielectric material and the second arranged/deposited powder, as well as the melting of the second powder into the dielectric solid material after powder removal, may also be carried out before applying the next pattern. Undesired powder material may also be removed before depositing the layer on a workpiece/building table.

The printing face may further transport said patterns between developing stations for the powder materials of the layer, and deposit the layer onto the workpiece/ building table. Said pattern may be formed using an ink-jet print head.

In a second aspect, the invention provides a device for manufacturing a layer of a workpiece, the workpiece including an object, wherein the device includes: an application device for applying an adhesive material onto a printing face in order to form a first pattern of at least a first part of a layer of the workpiece to be produced, and a first developing device for arranging/depositing a first powder material onto the adhesive material.

In a further embodiment, the device may comprise a second application device for applying a dielectric solid material onto the printing face to form a second pattern of at least a second part of the layer of the workpiece to be produced, and a second developing device for arranging/depositing the second powder material onto the dielectric solid material. Also included may be a second application device for applying an adhesive material onto the printing face in order to form at least a second pattern of at least a second part of the layer of the workpiece to be produced, and a second developing device for arranging/depositing a second powder material onto the at least second pattern of an adhesive material.

The first and second application devices may be an ink-jet print head. Said first and second developing devices may be a developing station, in which the powder material is arranged/deposited by electrostatic attraction between the powder material and the printing face, so that the powder material is transferred to the printing face. The developing device could also be a developing station of the powder bed, conductive magnetic mono-component, or tonerhopper type. A developing station in which the powder material is arranged using a conductive magnetic brush, aerosol, cascade, isolating magnetic brush, or isolating non-magnetic mono-component, may also be used.

The printing face may be a piston face, and the printing face may also transport said pattern between developing stations for powder materials of the layer and deposit the layer onto the workpiece. The transport device may be constituted by a piston, wherein the piston applies a pressure pulse on the deposition of the powder layer onto the workpiece/building table.

The device may further include a correction device for removing undesired powder particles from the printing face after the development of the powder material. The correction device may be a polyester band outstretched between to drums.

The adhesive material may be wax, in particular viscous paraffin wax, or another viscous material. The first powder material may be a metal building powder, a metal alloy, an electrically conductive ceramics, or a metal mixture containing ceramics. The dielectric solid material may be a thermopolymer, or a material selected from the group consisting of animal wax (e.g. beeswax, stearine), vegetable wax (e.g. candelilla wax, castor wax), mineral wax (e.g. ozocerite wax), petroleum wax (e.g. paraffin wax), synthetic wax (e.g. polyethylene wax), photo- polymer, or be a mix of several wax types, such as sealing wax, for example. The second powder material may be a support powder of ceramics, in which case the support powder may be aluminum oxide, ^®Wollastonite, ^®Molokite, or a mixture thereof. The building powder has a particle size that is less than 200 μm, preferably 6-200 μm.

In order to make the metallic powder particles adhere to the printing face, an adhesive material is used that does not interfere with the sintering process. Such material may be the wax material used in powder metallurgy. The adhesive material is located between the printing face and the powder layer, and therefore does not have the same prone to affect the sintering process as the arrangement described in NO 317085, wherein the conveyor belt is located in the sintering zone (between the powder layers and the object).

The present invention manufactures powder layers in a manner that allows the manufacturing process to be integrated with a subsequent consolidation process for the layer-wise production of metal and ceramics objects.

The method for manufacturing powder layers according to the present invention is capable of providing functional objects of metal, for example, or ceramics mixed with metal. By functional is meant that the object satisfies the requirements that are set for the finished product with respect to tolerances and mechanical properties. The object may be purely metallic, or be comprised of ceramics in which metal serves as binding agent. The object is built in that powder layers are stacked onto each other and transformed into a solid material. This production method allows for new material compositions, improved control of the microstructure, and reduced production costs for objects having a complicated geometric shape or objects having a complicated material composition.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the invention will be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view showing a value chain from the very idea to the finished product in the case of a production method implementing layer-wise production of the product;

Figure 2 shows a core process for the manufacture of metal powder layers according to an embodiment of the invention;

Figure 3 shows a process for manufacturing a metal powder layer containing several metal types in the same layer, according to an embodiment of the invention;

Figure 4 shows a core process for the manufacture of ceramics powder layers according to an embodiment of the invention;

Figure 5 shows a core process for the production of metal objects of arbitrary geometry through the development of building and support powder according to an embodiment of the invention; and

Figure 6 shows a process for the production of arbitrary geometry metallic objects according to an embodiment of the invention.

DETAILED DESCRIPTION

The value chain for the layer-wise production of an object, from the very idea to the finished product, is shown in figure 1. In the layer-wise production of an object, initially a computer model is created by drawing or scanning the physical object. Then the computer model is divided into thin layers, providing a data file containing information on each layer (thickness, shape, materials, etc.) and the relative location of the layers. The fabrication of the object is initiated in that information on the first layer is sent to a manufacturing unit. In this unit, a physical powder layer is constructed based on the digital information on the layer. When the powder layer is complete (it may consist of several materials), it is transported to the compaction unit and transformed to a solid material. While the compaction process is proceeding, the manufacturing unit receives information on the next layer and starts to recreate this layer with powder. The manufacture and consolidation of powder layers are repeated until the object is finished. Optional post-processing could be the removal of support powder, heat treatment, or processing using subtractive techniques. Two main groups of powder are used for the manufacture of powder layers; building powder and support powder. The building powder of each layer forms a thin slice of the product being constructed (transformed to a solid material). In order to be able to build objects having an arbitrary shape (overhang, inner geometries, etc.), it is necessary to support the object during the building process. To this end, a support powder is used that does not sinter in the consolidation process, but serves to support during the building process. The support powder is compacted, whereas the building powder is compacted and sintered. In the construction of metallic objects, the support powder is typically a ceramic material, or a mixture of ceramic materials. The sintering temperature of the support powder must be substantially higher than the sintering temperature of the building powder, so that the support powder is not sintered but may be easily removed when the entire object is finished. The powder particles of the support powder typically have an irregular shape in order for the support powder to obtain a high green strength on compaction.

A powder layer may contain several kinds of building powder, both with respect to material and particle structure. This allows for the production of objects with custom properties for given applications. An example may be hip bone (based on a sequence of x-ray pictures of the patient) having a titan base structure, cobalt- chromium friction surfaces, and areas having a porosity that allows the body tissue to grow into and retain the new hip bone. A gradual transition between materials (graded materials) can be obtained by increasing the portion of a new material for each new layer being compacted. In this manner, problems associated with differing properties of the two materials are avoided, e.g. the coefficient of thermal expansion.

Figure 2 shows a process for the manufacture of metallic powder layers; i.e. using metal as building powder. The process mainly consists of the following steps:

1. Drawing a pattern with an adhesive material on a printing face (station 7);

2. Developing the pattern by transferring a metal powder to the printing face to as to adhere to the adhesive material (station 8); and 3. Transferring the powder layer to a compaction unit in which the layer is sintered to an object being built (station 10a).

The adhesive material may be a thin coating of a viscous paraffin wax applied to the underside of the piston (the printing face). The printing face consists of an electrically conductive material. By applying an electric potential between the printing face and the developer, the printing face attracts a metal powder located in the developing station 8. The powder adheres to the wax coating on the printing face. In station 10, the powder layer is consolidated. Details of the consolidation process are not described in this document (being described in a separate patent application). The process including the steps 1-3, as illustrated by stations 7, 8, and 10a of figure 2, is repeated until the object is finished. The same printing face transports the complete layers to the next stage of the process (consolidation). The printing face may be constituted by the underside of a piston that also acts to impart a pressure pulse on deposition of the layer in the sintering station 10.

The nature of the object is influenced by the powder and by the operation of the developing unit 8. In its simplest form the developer is a powder bed containing a uniform layer of powder facing the printing face. The object being built may be compacted to a full density object as illustrated in station 10a of figure 2. When developing a monodisperse powder, objects having a controlled porosity may be built, as illustrated in station 10b. By replacing the material type in the developer (or alternatively replace the developer) during the process, objects consisting of several materials (material A and material B) or having varying porosity may be built, as illustrated in station 10c. If a developer is used having an integrated powder mixer, objects having a gradual transition between materials A and B may be built, as illustrated in station 10d.

Figure 3 shows the manufacture of metal powder layers comprising several types of metal powder in each layer. In the station 7a, a pattern of viscous paraffin wax is sprayed onto the printing face (the piston underside). Building powder of type A is transferred to the printing face and adheres to the wax pattern, as shown in station 8a. In the station 7b, viscous paraffin wax is sprayed onto the areas of the printing face that have not already been developed. Building powder of type B is transferred to the printing face and adheres to the latest sprayed-on wax pattern (station 8b). The powder layer, now comprised of building powder of both type A and type B, is consolidated in the compaction station 10. By multiplying stations 7 and 8, several types of building powder may be included in the same layer.

Figure 4a shows a process for the manufacture of ceramics powder layers (support powder). The process mainly consists of the following steps:

1) A dielectric coating of a solid material is sprayed onto a printing face (station 1).

2) The dielectric coating is charged by spraying ions onto the coating (station 2).

3) The dielectric coating is developed using a ceramic powder (station 3).

4) The powder layer is compacted in a sintering station (station 10).

The printing face may be the surface of an electrically conductive piston that also acts to provide a pressure pulse on deposition of the layer in sintering station 10. The dielectric coating is comprised of a material that is viscous when sprayed on and that solidifies when contacted with the piston. As shown in figure 4b, negative ions are sprayed onto the coating by means of a scorotron. The ceramic powder developing station 3 produces a positively charged powder and brings the powder to a location nearby the printing face of the piston. The negatively charged coating attracts positively charged powder from the developer.

The following describes the stations and materials involved in the manufacture of the powder layer, as shown in the embodiments of figs. 2-6. Details of the actual manufacturing process will be described in a later section.

Print heads (stations 1 and 7)

The print heads operate according to the same principle as an ink-jet printer (ink- jet technology), however printing by way of either a dielectric material or an adhesive material disposed on a printing face (e.g. the underside of a piston). The materials are detailed in a later section. The materials must be sintering-friendly, meaning that the dielectric material and the adhesive material (e.g. wax) evaporate at the sintering temperature, without leaving pores or contaminations in the product. Adhesive materials in the form of wax materials are common in the modern powder metallurgy. By adhesive materials are meant materials that are capable of retaining powder particles that are subsequently deposited onto the pattern. The print heads, using a dielectric material and/or the adhesive material, print a pattern of the layer to be formed. The actual powder layer is then formed using powder from the developing stations, as will be explained below.

The print heads are controlled by a computer based on the information from the computer model.

Charging unit (stations 2 and 6)

The charging unit is a scorotron, i.e. basically a thin wolfram wire (corona wire) connected to a high voltage source. The electric field surrounding the wire is sufficiently strong to ionize atoms or atom groups in the ambient air. The most important aspect is the voltage level applied to the lattice. The lattice is the control element of the scorotron. Provided that the medium receiving the ions from the scorotron maintains the sprayed-on charge, the same voltage is achieved in the medium positioned on the lattice. The voltage applied to the corona wire influences the number of ions that are produced. If the piston is moved too fast across the scorotron relative to the ion production, the voltage applied to the lattice will not be achieved in the medium. The ions are used for imparting a negative charge to the dielectric material on the underside of the piston. In this manner, it is possible to control the attraction of ceramic powder in station 3. A scorotron may also be included as part of the correction station 6 for adjusting the polarity of the dielectric material and ceramics powder before the formation of a pattern in station 7 and the development of metal powder in station 8. For example, one does not desire that building particles (metals) adhere onto previously attracted support powder (ceramics). The support powder is then given a charge that repels the building powder. Charging units generating a corona discharge are used in copy machines and laser printers, and are available in mange many variants; corotron, scorotron, dicorotron, detach, and ion heads. Corotrons/scorotros are e.g. described in [Schein, 1992, Electrophotography and Development Physics].

Figure 4b shows an embodiment of a scorotron for charging a dielectric coating. The scorotron includes a corona wire arranged in a shield, having a lattice facing the pressure face of the piston. The dielectric coating material must be able to retain a charge density corresponding to -800 volts relative to earth. The charge density must be maintained until development has taken place in station 3 of figure 4a. The time interval between charging and development is less than 0,5 second.

A high voltage (+/-7000 volts) is applied to the corona wire while at the same time the shield and piston are grounded, giving rise to a strong electrostatic field between these parts. In the vicinity of the corona wire, the field strength becomes so large that the air is ionized and a corona arises. Negative ions follow the field toward the piston and adhere to the dielectric surface. Positive ions follow the field toward the corona wire and recombine in the collision. When the charge density at the surface increases, the field strength decreases, and the ions are guided toward the lattice and recombine thereon. The coating reaches the voltage applied to the lattice (-800 volt). Ions that hit the piston (the metal) are neutralized in the collision.

By way of a scorotron of a correction unit, a high voltage (e.g. +7000 volt) is applied to the corona wire while the voltage of the lattice is varied in order to charge the support powder and the dielectric coating to a desired level and polarity. Before correction is performed the voltage of the dielectric coating typically will be in the range of -700 to -800 volt, whereas the voltage of the support powder typically will be in the range of +100 to +200 volt. During the correction process the voltage of the dielectric coating and of the support powder is changed to a common voltage of +200 volt, for example. The voltage is changed to a level at which the support powder repels building powder in the development of building powder, and to the level that otherwise provides an advantageous field configuration for develop- ing building powder. What is the best field configuration in a particular case depends on the building powder to be developed.

Developing stations (3, 8)

The developing stations 3, 8 (as shown in figs. 2-6) transfer support powder and/or building powder to the underside of the piston/printing face, which has been coated with wax and/or a dielectric solid coating, respectively, in the desired pattern by the print heads at station 1 and 7. The selection of development method is based primarily on the electric and magnetic properties of the material (powder). Four groups may be formed:

1. electrically conductive powder

2. electrically isolating powder

3. electrically conductive magnetic powder

4. electrically isolating magnetic powder.

In the case of an electrically conductive powder, induction is used as the physical charging mechanism, whereas for an electrically isolating powder, charge injection and tribo-electrification is used as the physical charging mechanism.

For irregular particles (typically support powder consisting of ceramics or a mixture of ceramics), that easily lump, a conductive magnetic brush (Conductive magnetic brush development, described in [Schein, 1992, Electrophotography and Development Physics, chapter 7], and in US 4,076,857) may be used. This technique also gives the highest development rate. Developing station 3 is of this type. The support powder is mixed with larger magnetic particles (carriers) in a container using a blender. The Triboelectrical effect [Shaffert, 1980, Electrophotography, pp. 557] causes the small irregular particles to stick to the carrier particles. The feeding means is a rotating cylinder having internal stationary magnets. The magnetic forces cause carrier particles to stick to the feeding means. The negatively charged coating on the piston underside attracts the positively charged particles in the feeding means, as shown in figure 4a. The electrical forces hence pull the tribo- electricaily charged particles off the carrier particles when they are positioned nearby the piston. In this manner, a consistent transfer of irregular powder is ensured. New support particles are added to the container in order to maintain the mixture ratio of the carrier particles to the support powder as constant as possible. The developer is responsible for charging the powder with a positive charge, and to dispose the powder in the vicinity of the piston. The use of conductive magnetic brush as the development method is illustrated in station 3.

For developing a non-conductive, non-magnetic ceramic powder, the developer may be of the type; aerosol, cascade, isolating magnetic brush, conductive magnetic brush, and non-magnetic monocomponent. All these developing methods are widely used in conventional copy machines and printers. Common for the methods is that the powder is tribp-electrically charged. The transfer of powder from the developer to the piston takes place when negatively charged coating on the piston underside attracts positively charged powder. In addition to the attraction forces caused by the charges, bias and high frequency AC power may be used between the piston and the developer in order to increase and control the attraction forces. When bias and AC power are used, the need for a powder remover is eliminated. The removal of powder having incorrect polarity then becomes an integrated function of the developer.

For electrically conductive powder particles (typically building powder) a developing station of the powder bed type is used, as shown in station 8 of figure 5. The powder in the vessel may be kept uniform by way of a combined replenishment and scraper mechanism. Excess powder may then go to a powder collector and be carried back to the scraper mechanism. The powder vessel contains building powder, or a mixture of several building powders. If the building powder is a mixture of several materials, the powder may be premixed and fed using a feed screw (such as a plastic tube having an internal flexible screw), for example. A potential differential between the powder in the powder bed and the piston generates electrical forces (Induced charges [Sears, Zemansky, Young, 1987, University Physics, pp. 533]) lifting powder particles to the underside of the piston. For iron, steel, nickel, and cobalt, that are both magnetic and electrically conductive, and for other materials having corresponding properties, it will also be poss- ible to use a developing station based on conductive magnetic mono-component, as described in [Kotz, 1975, US 3,909,258]. In this case, magnetic forces make chains of powder particles adhere to a cylinder. The cylinder will then rotate in a given distance from the charged wax coated printing face. A charge displacement occurs in the powder chain as it passes the printing face. The electrical forces between the outermost particle and the printing face become so large that the particle is picked off and becomes part of the powder layer.

The developer is responsible for charging the powder, and the solution of station 8 shows a powder bed. For metals, induction may be used as charging method for the metal powder. Tonerhopper may be used for developing magnetic powder (iron). The building powder is attracted to the piston by way of an electric felt. The field is composed by means of the charge of the ions on the dielectric coating (from station 2), the charge of the support powder (from station 3), and the potential differential between the piston and powder bed. A process step 6 may also be implemented, as shown in figure 6, in connection with the development of the support powder, which makes it possible to control the polarity and charge density of the support powder and of the dielectric coating.

The charge of the support powder and of the dielectric coating will act to weaken the field between the piston and the developer, so that at these places, the field does not become sufficient to attract the building powder. The building powder is carried to the places at which support powder is not already present.

Correction units (stations 4, 5, 6, and 9)

Several stations enter the system as correction units, but not all of the stations need to be included in a deployed system. Which correction units are used depends on the particular support and building powders that are used and the purity requirements of the support and building powder. A summary of the potential correction units is given below. Removal of powder having incorrect polarity

The developing stations are to charge the powder to a given polarity and to a given charge density. However, no two powder particles are identical, and the charging achieved in the individual powder grains may vary. A portion of the powder particles may admit a negative charge, even though the developing station has been designed to produce positive powder particles. Support powder having a negative charge may stick to the edge of the latent electrostatic picture and impair the picture quality. Negatively charged support powder should be removed before the powder layer is deposited. Negative building powder may stick to the positively charged support powder. Negative building powder may result as one powder particle transfers its charge to another. If the support powder adhering to the printing face holds a too high positive charge, a negative charge may be induced in the powder bed that is sufficient to lift some powder grains, causing them to adhere to the support powder.

Powder having an undesired polarity may be removed using a powder remover. An electric field is applied between the piston and the powder remover, so that negative powder is pulled away from the surface and attach to the powder remover. One embodiment of a powder remover, as shown in station 4 and 9, is explained below.

The powder remover of station 4 and 9 consists of a polyester band (e.g. Mylar®) being outstretched between two drums. The drums and the band rotate. The powder remover is used for removing undesired powder from the underside of the piston/printing face. The undesired powder is powder having a negative polarity. A positive voltage is applied to the drum being closest to the piston. Any negative powder grains are drawn from the underside of the piston to the band. The voltage applied to the drum is varied according to the ability of the powder to polarize a charge. A scraper device removes the powder particles from the dielectric band. In this manner, any incorrectness in the individual developing processes may be corrected. Powder remover 4 is advantageous when using aerosol development, cascade development, and isolating magnetic brush development. In developing using electrically conductive magnetic brush with AC power and bias, the powder remover 4 will be superfluous, as powder removal in this case will be an integrated function of the developer.

Powder removers are used in conventional electrographical/ electrophotographical devices.

Melting support powder onto the piston (station 5)

In order to increase the ability of composing a favorable result field by the attraction of building powder (metal), it is desirable to be able to manipulate the charge of the support powder (ceramics) and that of the dielectric coating. Basically, the support powder is held on to the coating by the forces caused by the charges (the charge of the dielectric coating and the charge of the support powder). If this charge is manipulated, the result could be that the powder falls down or changes position. Before manipulating the charge, it is therefore required to provide a new force that retains the support powder on the coating. One alternative is to melt the support powder into the coating. In this process step the support powder is melted onto the coating through the application of heat using a heating element, as shown in station 5 of figure 6. The heating element may be an infrared light bulb.

Adjusting the polarity and charge level (station 6 of figure 6) In order to be able to design the result field to the desired size and direction, a process step 6 is implemented. By means of a scorotron, the support powder and dielectric coating are charged to the desired polarity and charge density. The configuration and manner of operation of the scorotron has been discussed earlier and is described in the section describing the charging unit. Briefly summarized, the manner of operation is as follows:

The field used for the attraction of building powder is established by applying a potential between the piston and the developer. The field is influenced by the ions of the coating and by the charge of the support powder. The sum of the potential, ions, and charge determines the direction and strength of the field. Cleaning the piston surface (station 11)

Cleaning of the printing face in order to make sure the face remains clean during the process may be implemented in the form of a mechanical brush or mechanical scraper.

Adhesive material

The adhesive material that is printed/sprayed onto the printing face in a pattern makes sure the building powder remains adhered to the printing face after development. The adhesive material may be comprised of wax, e.g. paraffin wax. The wax is sintering-friendly in the sense that it evaporates due to the heat within the sintering zone. The paraffin wax does not leave any contaminations in the building powder. The thickness of the coating may vary from 2 to 10 μm. A wax that can be used has a dynamic viscosity (at 20 ⁰C) in the range of 25-80 mPa-s. Such a wax is available from Merck KGaA of Darmstadt, Germany, type 1.07174. Paraffin wax is also used in traditional powder manufacture, but then to reduce the friction during compaction and to increase the green strength of the object in the time period between compaction and sintering.

Dielectric material

The dielectric material is printed/sprayed onto the piston surface and hardens thereon. The material is viscous during spraying. The dielectric material may be a thermopolymer, and then has a temperature in the range of 65 to 75 ⁰C during spraying. The printing face typically has a temperature of 30 to 45 ⁰C during spraying. The temperature ranges for the piston can also be 20-60 ⁰C and 25-55⁰C. After the pattern has hardened, ions are sprayed onto the printing face and deposit on the dielectric pattern.

The solid dielectricum could also be animal wax (beeswax, stearin), vegetable wax (candelilla wax, castor wax), mineral wax (ozocerite wax), petroleum wax (paraffin wax), synthetic wax (polyethylene wax), photopolymer, or be comprised of a mixture of several wax types, such as sealing wax, for example. The requirement is that the wax is liquid during spraying and that it hardens on contact with the piston. Some wax types require a higher temperature than 75⁰C to be liquid during spraying. In such cases, high temperature ink-jet technology must be used. The wax must transition from a liquid state to a solid state when the wax contacts the piston. The solid coating must prevent the ions being sprayed thereon from recombin- ing with the piston before the piston has passed through the developing stations. In any case, the temperature of the piston/ printing face must be lower than 200 ⁰C to prevent that the ions lose their binding to the printing face.

Support powder

In most cases, the support powder will be of ceramic materials having a sintering temperature that is significantly higher than the building material. The particle shape is typically irregular. The support powder may consist of a mixture of aluminum oxide, ^®Wollastonite, ^®Molocite. The particle size and packability of the support powder must be adapted so that the support powder supports the building powder during both compaction and sintering (station 10). When the product is finished, the support powder is removed by brushing or sandblasting. The support powder is typically comprised of aluminum oxide of a given size (diameter 100 μm, for example), Wollastonite of 1/7 of this size (diameter approx. 14 μm), and Molocite of 1/7 of the Wollastonite size (diameter approx. 2.5 μm). The support powder is an agglomerate, kept together by a solid paraffin wax, for example.

Building powder

The building material consists of a metallic powder. The metals used are metals commonly used in the metallurgy for the production of metallic products, such as Iron, Copper, Nickel, Aluminum, Cobalt, Chromium, Magnesium, Manganese; Molybdenum; Silicon; Sink, Titan. The building material may be any metal that conducts electric power, and that has a typical sintering temperature below 1200⁰C. Preferred sintering temperatures are in the range of 400 to 1200⁰C, and in particular the range of 600 to 1000⁰C. The powder material does not need to be a pure element, but may be an alloy. The metal powder may be spherical, irregular, or spongy, and have a particle size ranging from 6 to 200 μm, preferably from 10 to 50 μm, and most preferably from 15 to 30 μm. Materials having a high melting temperature, such as ceramics, may be used, provided that they are used as building powder in combination with metals, and that the powder is electrically conductive. Non-conductive ceramics may be used if the ceramics has been pre-blended into the metal powder, as long as the metal/ceramics mixture is electrically conductive.

Description of a manufacturing method

The following section details embodiments of the actual manufacturing of the powder layers. The thickness of the powder layers being formed by the method and apparatus according to the present invention are typically less than 0.1 mm after consolidation in subsequent process steps (not described in detail here). The consolidated thickness is determined by the particle size of the powder materials used and the density of the layer. The thickness of the layer while bound to the piston equals the diameter of the powder being used.

The manufacture of one complete powder layer using the machine concept of figure 6 will now be explained. The bracketed reference numerals in the description below refer to the stations as shown in figure 6. In general, in the method as shown in figure 6, the support powder is developed first and then the building powder. In most cases, a manufacturing module is comprised of at least one print head (stations 1 and 7), at least one developing station (stations 3 and 8), at least one correction unit (stations 4, 9), and at least one charging unit. Additional components are a sintering station 10 for consolidating powder layers to form the object, and a cleaning station 11 for cleaning the printing face. In figure 6, the printing face (piston) is shown as it is moved through all the stations, and the printing face is therefore given varying reference numbers that identify the station. If the printing face is the end face of a piston, the piston may be suspended from a fork being attached to a linear guide. In this manner, the piston may be run forward and backward between the stations of the manufacturing unit. The fork allows the piston to be delivered for consolidation when the powder layer is complete. The following description assumes that the printing face is the end face of a piston, that the support powder is a ceramics powder, and that the building materials are a metal powder. The process cycle is repeated until the object is finished. If a powder layer is comprised of several building powders, the process steps 7, 8, and 9 are repeated until all the individual building powders have been deposited in the desired pattern. The process steps 1-11 are repeated until all layers have been completed and the object is finished.

The manufacture of a layer is initiated by running the piston across the print head (station 1). A dielectric pattern of a solid material is sprayed onto the piston surface. The pattern being printed on the underside of the piston by the print head of station 1 corresponds to the plane geometry of the support powder that is to be picked from the developer in station 3. In figure 6, this is a ceramics support powder. Figure 6 shows a manufacturing unit including manufacturing stations; one for a ceramics support powder and one for a metal building powder, as explained above. In such a case the support powder is deposited first. However, in the simplest case it is also possible to have only one manufacturing module to form a powder layer having a pattern that is only comprised of one building material.

The pattern is imparted a negative charge by spraying negative ions onto the coating using a scorotron, as shown in station 2. The manner of operation of a scoro- tron is explained above. Thereafter the pattern is developed using support powder in station 3. Positive powder particles being attracted to the negatively charged pattern on the piston underside are retained. In addition to the attraction forces created by the ions, bias and high frequency AC power may be applied between the piston and developer in order to increase and control the attraction forces.

The piston is then carried on to a powder remover of station 4, in which any negative support powder particles are removed. Negative support powder is removed by drawing the powder from the printing face to the powder remover by means of an electrostatic field.

The ceramic support powder is then melted into the dielectric coating by moving the piston through the heating element of station 5. Thereafter, the piston is moved on to station 6, in which the support powder and coating are charged to the desired polarity and strength by means of a scorotron. A pattern of viscous paraffin wax is sprayed/printed onto the printing face (the underside of the piston) by an ink-jet printer in station 7. A powder layer must always cover the entire face to which it is bound. If the layer is comprised of one building powder (and one support powder), paraffin wax is sprayed onto the part that has not already attracted support powder. If the layer is comprised of several building powders (and one support powder), paraffin wax is first sprayed onto the part of the surface to receive building powder A and the pattern is developed using building powder A, thereafter paraffin wax is sprayed onto the part of the surface to receive building powder B and the pattern is developed using building powder B. The process is repeated until all powder types have been developed. Building powder is transferred to the printing face from a powder bed and adheres to the pattern of paraffin wax in station 8. The building powder is attracted to the piston by means of an electric field generated by a potential differential between the piston and the powder bed. As explained earlier, the charge of the support powder and of the dielectric coating act to weaken the field between the piston and the powder bed, so that the field at these places does not become sufficiently strong to attract building powder. The building powder is therefore guided to the spots at which support powder is not already present. Powder particles hitting the wax pattern adhere to the wax. Powder particles hitting the piston (i.e. the spots in lack of a wax pattern) are repelled and carried back to the powder bed. This is because the charge of the powder recombines in the collision with the piston and the powder adopts the polarity of the piston. If the powder particles hit already developed building powder, the powder particles are repelled and carried back to the powder bed. This is because the charge of the powder is mobile. New powder delivers charge to already developed powder and adopts the opposite polarity. In both cases, a powder cloud is formed. Any negative building powder is removed from the support powder using the powder remover of station 9 by the powder remover applying a positive voltage relative to the piston.

If there is to be several building powders in the same layer, the piston is run back to station 7 for being sprayed with paraffin wax in the desired pattern onto the piston underside. In most cases, however, one developing station is used for each building powder, so that the number of developing stations 8 will depend on the number of building powders. The developing stations 8 may be positioned consecutively one after the other. It is also possible to have a separate spraying station for printing a pattern of viscous paraffin wax for each building powder, as shown by stations 7a and 7b of figure 3, and with subsequent developing stations 8a, 8b. A powder remover may be provided following each developing station 8a, 8b. The preferred configuration with respect to the positioning and number of adhesive material spraying stations, developing stations, and powder removal stations depends on whether it is desirable to only run the piston stepwise linearly forward in the production line in the manufacture of a layer, or the piston is allowed to be run back to a previous station. This depends, inter alia, on the desired process speed, but also on the desired compactness of the production equipment.

When the entire powder layer has been developed, the powder layer is deposited in the sintering station 10 by forcing the piston, having the powder layer on the underside, into the working template under high pressure and temperature. The support powder is compacted and the building powder is compacted and sintered. The pressure controls the movement of the compression cylinder. When a predetermined pressure has been reached, the piston turns and retracts. The powder layer and object are both subject to a pressure pulse. As the mould holds the sintering temperature, the sintering (the increase of the density of the object) continues after the piston has pressed the powder layer against the object. The sintering temperature depends on the particular building material being used. The temperature used is in the range of 60 to 80 percent of the melting temperature of the building powder, as measured on the Celsius scale. The sintering rate also depends on the size of the building powder, as small powder grains are sintered more rapidly than large powder grains do. A portion of low melting temperature powder may be incorporated in the building material in order to increase the sintering rate. The low-melt phase may then be in a molten state while the building powder is at the sintering temperature.

In order to avoid contaminations, the printing face is cleaned before the piston is returned to station 1 for reapplication of dielectric material in a desired pattern for the next layer. The overall process is repeated the number of times necessary to finish the object. With an adequate cooling of the piston, the manufacture of the next layer may start immediately when the piston returns. A typical temperature for the piston while being sprayed is in the range of 30 to 45 °C. An object may be comprised of several thousand layers and hence it is important that the process is fast.

In the production of a functional object, it is also conceivable that some layers contain building powder only. In building such a layer the transport unit will transport the piston directly to the developing module for the building powder and start printing using wax from the print head in station 7. As mentioned above, the manufacturing process is fully computerized. Also, the number of manufacturing modules is not limited to two, as shown in figure 3, but depends on the number of materials necessary to build the functional object.

In the preceding sections, embodiments of the invention have been described. However, further embodiments of the invention can be constructed by a man skilled in the art. The scope of the invention is defined in the accompanying claims.

Claims

1. Method for manufacturing a layer of a workpiece, wherein the workpiece includes an object, the method further comprising:

- forming at least a first pattern on a printing face of at least a first part of the layer of the workpiece to be manufactured, the at least first pattern being constituted by an adhesive material, and

- developing the first pattern by arranging/depositing a first powder material onto the adhesive material.

2. The method of claim 1 , further ch a racte rize d i n :

- forming a second pattern on the printing face of at least a second part of the layer of the workpiece to be manufactured, the second pattern being constituted by a dielectric solid material,

- developing the second pattern by arranging/depositing a second powder material onto the dielectric solid material.

3. The method of claim 1 , further ch a racte rize d i n :

- forming a second pattern of at least a second part of the layer of the workpiece to be manufactured, the second pattern being constituted by an adhesive material,

- developing the second pattern by arranging/depositing a second powder material onto the adhesive material.

4. The method of claim 2 or 3, further ch a racte rized i n that the method comprises:

- forming at least a third pattern on the printing face of at least a third part of the layer of the object to be manufactured, the third pattern being constituted by an adhesive material,

- developing the at least third pattern by arranging/depositing at least a third powder material onto the adhesive material.

5. The method of claim 2, further ch aracte rized i n that the method comprises charging the deposited dielectric solid material by spraying negative ions onto the surface of the coating before arranging/depositing the second powder material.

6. The method of claim 5, further ch aracterized i n that the method comprises removing arranged/deposited powder material having incorrect polarity before applying an additional pattern.

7. The method of claim 2, further ch aracte rized i n that the method further comprises:

- adjusting the polarity and charge level of the formed dielectric material and the arranged/deposited second powder, and

- melting the second powder onto the dielectric solid material after the removal of powder having incorrect polarity before the application of the next pattern.

8. The method of claim 1, characterized i n removing powder having incorrect polarity before depositing the layer onto a workpiece/ building table before consolidation.

9. The method according to one of claims 1-4, ch aracte rized in that the printing face transports said patterns between developing stations for the powder materials of the layer, and deposits the layer onto the workpiece/ building table.

10. The method according to one of claims 1-4, characte rized in forming said patterns using an ink-jet print head.

11. A device for manufacturing a layer of a workpiece, wherein the workpiece includes an object, the device comprising:

- an application device for applying an adhesive material onto a printing face in order to form a first pattern of at least a first part of a layer of the workpiece to be manufactured, and - a first developing device for arranging/depositing a first powder material onto the adhesive material.

12. The device of claim 11 , comprising:

- a second application device for applying a dielectric solid material onto the printing face in order to form a second pattern of at least a second part of the layer of the workpiece to be manufactured, and

- a second developing device for arranging/depositing the second powder material onto the dielectric solid material.

13. The device of claim 11 , comprising:

- a second application device for applying an adhesive material onto the printing face in order to form an at least second pattern on at least a second part of the layer of the workpiece to be manufactured, and

- a second developing device for arranging/depositing a second powder material onto the at least second pattern of an adhesive material.

14. The device according to one of claims 11-13, c h a racte rized i n that the first and second application devices are ink-jet print heads.

15. The device according to one of claims 11-13, ch a racte rized i n that said first and second developing devices are developing stations, in which the powder material is arranged/deposited by an electrostatic attraction between the powder material and the printing face, transferring the powder material to the printing face.

16. The device according to one of claims 11-13, ch a racte rized i n that the developing device is a developing station of the powder bed, conductive magnetic mono-component, or tonerhopper type.

17. The device according to one of claims 11-13, cha racte rized in that the developing device is a developing station in which the powder material is arranged using conductive magnetic brush, aerosol, cascade, isolating magnetic brush, or isolating non-magnetic mono-component.

18. The device of claim 11, cha racte rized i n that the printing face is a piston face.

19. The device of claim 11, ch a racte rized in that the printing face transports said pattern between developing stations for powder material of the layer and deposits the layer onto the workpiece.

20. The device of claim 19, cha racte rized in that the transport device is comprised of a piston, the piston applying a pressure pulse when depositing the powder layer onto the workpiece/ building table.

21. The device according to one of claims 11-13, cha racte rized in a correction device for removing undesired powder particles from the printing face subsequent to the development of powder material.

22. The device of claim 11, ch a racte rized i n that the correction device is a polyester band extended between to drums.

23. The device of claim 11 or 13, cha racte rized i n that the adhesive material is wax, in particular viscous paraffin wax, or another viscous material.

24. The device of claim 11 , c h a r a c t e r i z e d i n that the first powder material is a metal building powder, a metal alloy, an electrically conductive keram, or a metal intermixed with keram.

25. The device of claim 12, characte rized in that the dielectric solid material is a thermopolymer, or a material selected from the group consisting of animal wax (e.g. beeswax, stearin), vegetable wax (e.g. candelilla wax, castor wax), mineral wax (e.g. ozocerite wax), petroleum wax (e.g. paraffin wax), synthetic wax (e.g. polyethylene wax), photopolymer, or a mixture of several wax types, such as sealing wax, for example.

26. The device of claim 12, cha racte rized in that the second powder material is a keram support powder, wherein the support powder may be aluminum oxide, ^®Wollastonite, ^®Molokite, or a mixture thereof.

27. The device according to one of claims 11-13, characte rized in that the building powder has a particle size of less than 200 μm, preferably 6-200 μm.