CN114761156A

CN114761156A - Method for manufacturing powder metallurgy parts comprising drying with a gas flow before sintering

Info

Publication number: CN114761156A
Application number: CN202080083993.7A
Authority: CN
Inventors: A·索雷亚; P·瓦莱; 阿兰·伊沃·瑟高
Original assignee: Grundfos Holdings
Current assignee: Grundfos Holdings
Priority date: 2019-12-04
Filing date: 2020-12-03
Publication date: 2022-07-15
Also published as: WO2021110830A1; EP4069451A1

Abstract

The invention relates to a method for manufacturing a powder metallurgical component (21) comprising at least one longitudinally extending channel (22). A powder mixture comprising a metal powder (11) and a binder (12) is prepared and transferred to a processing apparatus (31) comprising a die (32). And subsequently formed into a green body (20) via the die. Drying the green body (20) by directing a gas flow (G) through the at least one longitudinally extending channel (22), and sintering or oxidizing the dried green body (20) to obtain a final component. The invention also relates to a drying tool (40) having a plurality of nozzles (44) for directing the gas stream into the longitudinal channels of the green bodies during drying.

Description

Method for manufacturing powder metallurgy parts comprising drying with a gas flow before sintering

Technical Field

The invention relates to a method for producing a powder-metallurgical component having at least one longitudinally extending channel. In particular, the invention relates to a method for drying a green body by directing a gas flow along a channel before sintering or oxidizing the green body.

Background

When wet binders are used to manufacture powder metallurgical components, it has been found that slow drying due to water evaporation is not an option because it is difficult to ensure that the green body retains the desired shape during drying. In particular, uneven drying of the green body can induce tension in the part, resulting in cracking or deformation of the part. This is particularly the case for asymmetric geometries, large components, and thin-walled structures.

Therefore, controlled acceleration or sometimes deceleration of the in-depth drying is required. This is typically done by placing the green body to be dried in a space having a controlled temperature, such as in an oven. However, the use of heat only accelerates evaporation on the outer surface of the green body, i.e. leads to uneven drying. In ceramic manufacture, drying is typically accomplished using microwaves, but this is not suitable for drying parts made of metal powder.

Accordingly, an improved method for manufacturing powder metallurgical components would be advantageous.

Object of the Invention

It is therefore an object of the present invention to provide a method for manufacturing a powder metallurgical component, by using which the risk of deformation and damage occurring during drying of the green body prior to sintering or oxidation can be minimized.

It is a further object of the invention to provide a method for manufacturing powder metallurgical components which facilitates the manufacture of more complex component geometries than known methods. This is relevant for the purpose of providing a method with a lower risk of damage caused by drying, as such damage may be caused by uneven drying of complex geometries (e.g. due to large variations in the wall thickness of the part to be dried).

It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide a method for manufacturing a powder metallurgical component that solves the above mentioned problems of the prior art.

Disclosure of Invention

Accordingly, the above object and several other objects are intended to be achieved by a first aspect of the present invention, which provides a method for manufacturing a powder metallurgical component, the component comprising at least one longitudinally extending channel, the method comprising the steps of:

-preparing a powder mixture comprising a metal powder and a binder,

-transferring the powder mixture to a processing device comprising a mould,

-forming the powder mixture into a green body by forcing the powder mixture through the die, the die being adapted to form the component into a shape having the at least one longitudinally extending channel,

-drying the green body by directing a gas flow through the at least one longitudinally extending channel, and

-sintering or oxidising the dried green body to bond the powders together to form the powder metallurgical component.

The at least one longitudinally extending channel may be closed along all of its side walls. It may also be open along one of the sides, in which case the open side may need to be closed to achieve the necessary guidance of the gas flow for providing drying.

The processing apparatus may be, for example, an extruder, such as a piston extruder.

The powder mixture may be in the form of a paste. "paste" refers to a thick, soft, viscous substance made by mixing a liquid with a powder. In other words, pastes are generally composed of a suspension of particulate material in a background fluid. In the context of the present invention, the viscosity of the paste should be such as to allow the necessary handling of the paste during its transfer from the device for mixing to the handling equipment. It is also contemplated that the subsequent processing steps, i.e., the viscosity of the paste, should be low enough to allow shaping and high enough to ensure that the green body retains the desired geometry. The viscosity of a given paste can be determined by apparatus and methods designed for this purpose, such as by using a capillary rheometer (capillary rheometer) which is commonly used to measure shear viscosity (shear viscosity) as well as other rheological properties. However, since viscosity is related to the hardness of the material, this parameter can also be used to determine whether a given paste is suitable for use in the manufacturing process. One relevant measurement method that may be used is Shore Hardness (Shore Hardness) which may be determined according to ISO 868/ASTM D2240. Another option is to use a special tool designed for clay; this has been used during the development of the present invention. The tool is similar to the Shore tester (Shore tester), but it has been adapted to suit the characteristics of the clay; such instruments may also be referred to as clay durometers. The operating principle is based on the force of a calibrated spring penetrating the instrument, exerted by the sample material when the pin of the tool is pressed into the material to be tested until the pin reaches the support. In this way, a steady force is always applied to the instrument during a steady stroke. It has a scale of 0 to 20, which serves as a reference parameter for relative hardness, and a gram scale of the applied force. By means of the tool, the point of penetration is pressed into the paste as it comes out of the kneader (kneader). Subsequently, the maximum value indicated at the moment when the point of penetration is inside the paste is measured. Instead of waiting for it to settle, a maximum point (max) is used, because it eventually shows a much lower value, possibly close to 0, because the point of penetration will be forced through the paste. By this method it has been found that values higher than 12 Shore (Shore) are required in order to obtain satisfactory results, at least for the geometries tested.

The metal may be any metal that can be used as a powder. A non-exhaustive list of possible metals includes: 316L (316L stainless steel), FeCrAl (iron chromium aluminum alloy), Inconel 625 (Inconel 625), Hastalloy X (Hastelloy X), 17-4PH (17-4PH stainless steel), 430L (430L stainless steel), and 304L (304L stainless steel).

A binder or adhesive is any material or substance that mechanically, chemically holds or attracts other materials together by adhesion or cohesion (cohesion) to form an adhesive unit. The binder is preferably organic, such as cellulose ether, agarose or polyoxymethylene. Examples of binders are: methylcellulose, 25 polyethylene oxide, polyvinyl alcohol, sodium carboxymethylcellulose (cellulose gum), alginates, ethylcellulose, and pitch.

The powder mixture may also include other ingredients, such as ceramic powders or lubricants. A non-exhaustive list of possible ceramics includes: AlO, SiO, ZiO, alumina, Zirconia, boron nitride, Cordierite and silicon nitride.

The effect of the described drying step is that a more uniform drying of the entire component can be achieved. Studies conducted as part of the development leading to the present invention have shown that such a drying step makes it easier to ensure that the component retains its intended shape without deforming or cracking. This is particularly true for complex geometries or smaller wall thicknesses, such as for components having a large number of longitudinally extending internal passages (which may be separated by thin walls).

In some embodiments of the invention, the drying step further comprises directing a stream of gas along the outer surface of the green body such that drying from the outside also occurs as a result of the stream of gas.

In an alternative embodiment, the drying step further comprises covering the outer surface of the green body, for example with a plate, such that said drying is only performed as a result of the gas flow through the at least one longitudinally extending channel. Therefore, the liquid can be prevented from evaporating from the outer surface. It has been found that this provides more uniform drying, at least for some geometries of the part being manufactured. The choice of whether to cover the outer surface can be used to control the drying process, for example to avoid undesired deformations or cracks. Which may depend, for example, on the geometry of the part being manufactured, including the thickness of the walls surrounding the longitudinally extending channel. In addition to the effect on drying, the plates or other elements used to cover the outer surface may provide structural support to the green body during drying. Thus, the supporting effect may be used to ensure that the green body retains the desired shape during drying.

In some embodiments of the invention, the drying step is preceded by the steps of:

-providing a drying tool comprising:

-a first end comprising or connectable to a gas flow generating device, an

-an opposite second end comprising a plurality of nozzles, each nozzle being in fluid communication with the first end such that gas can flow through each of the nozzles under the action of the gas flow generating means during use of the drying tool,

-arranging the drying tool relative to the green body such that a nozzle of the drying tool extends into an end region of each of the at least one longitudinally extending channels of the green body,

-actuating the gas flow generating device such that gas flows into each of the at least one longitudinally extending channels.

Examples of possible designs for such drying tools will be described in connection with the accompanying drawings. Such drying tools would be particularly advantageous for drying green bodies having a plurality of longitudinally extending channels, as such geometries may otherwise be more difficult to dry uniformly.

In an alternative embodiment of the invention in which the green body has a plurality of longitudinally extending channels, the method may comprise using a drying tool as described above, but the step of arranging the drying tool relative to the green body is performed by: i.e. such that the nozzles of the drying tool extend into the end regions of at least some (such as most) of the plurality of longitudinally extending channels of the green body.

By "most" is preferably meant more than 50%, such as more than 70%, such as more than 90%.

By using the described drying means and having the nozzles extending into each or most of the plurality of longitudinally extending channels, uniform drying of the entire volume can be ensured. Further, the nozzle may be shaped and dimensioned such that the nozzle provides structural support to the portion of the wall of the at least one longitudinally extending channel that comes into contact with the tool, thereby preventing deformation thereof. The advantage is that the green body remains undeformed and the gas flow is not impeded as in a deformed (such as collapsed) longitudinally extending channel.

In some embodiments of the invention, the plurality of nozzles of the drying tool are arranged over the entire cross-section of the green body to be dried. Uniform drying of the entire component can thereby be ensured.

The nozzle may be shaped and dimensioned such that the nozzle provides structural support to the portion of the wall of the at least one longitudinally extending channel that comes into contact with the tool, thereby preventing deformation thereof.

An alternative to using such a drying tool comprising a nozzle may be to use a drying tool having a connecting end for directing a gas flow into at least one longitudinally extending channel. Such drying tools should preferably have at least one seal or gasket placed in engagement with the green body or the open end of the at least one longitudinally extending channel such that gas is thus directed into the channel. However, the scope of the claims covers any suitable method of directing gas through the at least one longitudinally extending channel. Which may be provided by a blowing or suction action. The flow generating device may be any type of device suitable for providing a flow of gas. It may be, for example, a blower (gas fan), a vacuum pump, a reversing fan or a compressor. The gas flow generating means may be an integral part of the drying means, or it may be an external device connected to the drying means.

An auxiliary tool may be arranged at an end of the green body opposite to the end where the drying tool is arranged, the auxiliary tool being adapted to support the at least one longitudinally extending channel during drying, thereby preventing undesired deformation of the green body.

By "support" is preferably meant that the auxiliary tool supports a portion of the inner surface of the at least one channel and thereby prevents it from deforming (such as collapsing). Support should preferably be done without significantly restricting the gas flow.

The length of the drying step may be a predetermined period of time or determined by measuring the humidity of the gas that has passed through the green body. It will depend on parameters including the material, geometry and dimensions of the green body. Which measures to use can be determined experimentally, possibly with the aid of computer simulations.

In some embodiments of the invention, the part is manufactured with a plurality of longitudinally extending internal channels, such as with a honeycomb structure.

The gas used for drying may have a higher or lower temperature than the ambient air and/or the gas may have a higher or lower humidity than the ambient air. The gas may be, for example, air. Drying can also be controlled by varying the velocity of the gas stream.

In any of the embodiments described above, a debinding step may be performed prior to the sintering or oxidation step, which typically includes heating the green body to a temperature at which at least some (such as all) of the binder burns off. The debinding step is typically performed after the drying step. Debinding refers to the process of removing the binder from the green body to ensure that there is no residual carbon in the part during sintering. This debonding is typically accomplished by heating the green body to a temperature between 200 degrees celsius and 750 degrees celsius and allowing the binder to burn off. Different binders require different de-binding temperatures. In embodiments using methylcellulose, debonding is typically accomplished in an oxidizing atmosphere (typically air), but may also be partially accomplished in the same atmosphere as the sintering atmosphere if the final part is not damaged by additional carbon content. To ensure that the debinded green body can still be handled, it may be necessary to slightly oxidize the powders together; these oxides will be removed during sintering.

A second aspect of the invention relates to a drying tool for drying a green body prior to sintering or oxidation during the manufacture of a powder metallurgical component, the drying tool comprising:

-a first end comprising or connectable to a gas flow generating device, an

-an opposite second end comprising a plurality of nozzles, each nozzle being in fluid communication with the first end such that gas can flow through each of the nozzles under the action of the gas flow generating means during use of the drying tool, and

-said drying means are adapted to dry the part obtained by the method according to the first aspect of the invention.

In such drying tools, the plurality of nozzles may be arranged in a predetermined pattern, such as a regular pattern of aligned rows and columns. They may for example be arranged to match the pattern formed by the mutual positions of the longitudinally extending channels of the component manufactured by the method according to the first aspect of the invention as described above. In some embodiments, there are at least three rows of nozzles, each row including at least three nozzles.

Drying tools according to the present invention may comprise a plurality of fluid channels, each extending between a first end and a nozzle. Thus, when using a drying tool in the method according to the first aspect of the invention, it may be helpful to achieve a uniform gas flow through all longitudinally extending channels. Thus, uniform drying of the component can be more easily ensured, so that the risk of deformation and cracking due to drying can be avoided or minimized.

At least some of the nozzles may comprise a closure mechanism for closing the respective nozzle such that no gas flow passes through the respective nozzle during use of the drying tool. The mutual position of at least some of the nozzles may be adjustable. By a drying tool having one or both of these two features, it is achieved that a given drying tool may be adapted to dry components having different geometries and dimensions and comprising a different number of longitudinally extending channels.

The first and second aspects of the present invention may be combined. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Drawings

The method for manufacturing a powder metallurgical component according to the invention will now be described in more detail with reference to the accompanying drawings. These drawings illustrate one way of implementing the invention and should not be construed as limiting to other possible embodiments that fall within the scope of the appended claims.

Fig. 1 schematically shows a method for manufacturing a powder metallurgical component according to a first aspect of the invention. Fig. 1a shows the following steps: preparing a powder mixture; transferring it to a processing device; and forming a green body. Fig. 1b shows how a gas flow is directed through the internal channels of a green body, wherein the green body has an outer surface covered by a plate. Fig. 1c shows sintering.

Fig. 2 schematically illustrates an example of a component having a plurality of longitudinally extending internal channels arranged in a regular pattern.

Fig. 3 schematically shows an embodiment of a drying tool according to a second aspect of the invention. Fig. 3a is a side view and fig. 3b is a three-dimensional partial view of a second end comprising a nozzle.

Fig. 4 shows how the drying tool in fig. 3 can be arranged such that the nozzle engages with the end section of the channel of the green body during drying.

Fig. 5 is a cross-sectional view of the drying tool of fig. 4.

Fig. 6 schematically shows the drying tool of fig. 3 to 5, some of the nozzles being closed by plugs.

Fig. 7 schematically shows a drying tool in which the mutual position of the nozzles can be adjusted.

Fig. 8 schematically shows a drying step in which auxiliary tools are used to support the green body.

Fig. 9 shows experimental results of tests performed during the development of the present invention.

Detailed Description

Fig. 1 schematically shows a method for manufacturing a powder metallurgical component according to a first aspect of the invention. As shown in fig. 1a, a powder mixture is prepared by mixing at least a metal powder 11 and a binder 12. The powder mixture may include other ingredients, such as ceramic powders or lubricants. The powder mixture is then transferred to a processing apparatus 31 comprising a mould 32; it may for example be an extruder, such as a piston extruder. The powder mixture is formed into a green body 20 by forcing the powder mixture through a die 32. This is done by applying a pressure P (as schematically indicated by the arrows in the figure). The die 32 is designed such that it is adapted to form the component 21 into a shape having at least one longitudinally extending channel 22. In fig. 1, the part has only one channel, but a part with multiple channels can be produced by a similar method using another die.

Fig. 1b schematically shows the step of drying the green body 20 by directing a gas flow G through the longitudinally extending channels 22. In the illustrated embodiment, the outer surface of the green body 20 is covered by a plate 39 such that drying occurs solely as a result of the gas flow G through the longitudinally extending channels 22. The gas may have a temperature above or below ambient air and/or the gas may have a humidity above or below ambient air. The gas is typically air, but other gases may be used. As can be seen from fig. 1b, in addition to ensuring that no evaporation of water from the outer surface occurs, the cover plate may also provide structural support to the green body during drying.

The length of the drying step may be a predetermined period of time, e.g. determined experimentally. It can also be determined from measurements of the humidity of the gas that has flowed through the green body 20 during the drying process.

After drying, the final part 21 is obtained by sintering the dried green body, as schematically shown in fig. 1 c. This can be done, for example, in a reducing atmosphere, in a vacuum or in an inert atmosphere. Sintering is typically performed in a furnace 34 at a temperature of 950 to 1430 degrees celsius. As explained in more detail above, a debinding step may occur prior to the sintering or oxidation step, which typically includes heating the green body 20 to a temperature at which at least a portion (such as all) of the binder burns off.

Fig. 2 schematically shows an example of a component 21 having a plurality of longitudinally extending internal channels 22 arranged in a regular pattern and separated by walls 23. Such components may be manufactured by the method described in connection with fig. 1, if a suitably designed mold 32 is used.

Fig. 3 schematically illustrates an embodiment of a drying tool 40 for drying the green body 20 prior to sintering or oxidation. Fig. 3a is a side view showing that the drying means 40 has a first end 41 comprising or connectable to a gas flow generating device 43 and an opposite second end 42 comprising a plurality of nozzles 44. The nozzles 44 are in fluid communication with the first end 41 such that gas can flow through each nozzle 44 under the action of the gas flow generating means 43 during use of the drying tool 40. Fig. 3b is a three-dimensional partial view of second end 42 including nozzle 44. In the illustrated embodiment, the nozzles 44 are arranged in a regular pattern of aligned rows and columns. In the illustrated embodiment, the nozzles 44 have two different shapes, but the nozzles may all be the same, or there may be more nozzles of different shapes and sizes.

Fig. 4 shows how the drying tool 40 in fig. 3 can be arranged such that the nozzle 44 engages with, such as extends into, an end section of the longitudinally extending channel 22 of the green body 20 during drying. By comparing fig. 2 and 3, it can be seen that the arrangement of the nozzles 44 of the drying tool 40 of fig. 3 matches the arrangement of the internal passages 22 of the components in fig. 2. However, when this is not the case, the drying tool 40 may still be used, as will be shown below. It must be ensured that the nozzle 44 does not damage the green body 20. When the nozzles 44 have been arranged, the gas flow generating means 43 are activated so that a gas flow enters each longitudinally extending channel 22. At the same time, the extension of the nozzles 44 into each internal passage 22 means that the nozzles both provide uniform drying and support for the wall 23. Both measures minimize possible deformation and damage to the green body 20.

Fig. 5 is a cross-sectional view of the drying tool 40 in fig. 4. It is shown that the drying means 40 comprises a plurality of fluid channels 45, wherein each fluid channel extends between the first end 41 and the nozzle 44. Thus, a more uniform distribution of gas in all nozzles 44 may be achieved compared to embodiments in which the middle section of the drying means 40 is one open space or has a smaller number of fluid channels 45. However, such embodiments are also to be covered by this scope of protection.

Some of the nozzles 44 may include a closure mechanism 46 for closing the respective nozzle 44 so that no gas stream passes through the respective nozzle during use of the tool. An example of such an embodiment is schematically illustrated in fig. 6, where nozzles 44 located in an upper row and a lower row of the drying tool 40 are shown as being closed using a closing mechanism 46, such as a removable plug. This may for example be suitable if the drying tool 40 is used for drying green bodies 20 having a smaller cross section or green bodies 20 having an outer region without internal channels.

Fig. 7 schematically shows a drying tool 40, wherein the nozzles 44 are in the form of flexible tubes, so that the mutual position of the nozzles 44 can be adjusted. Thus, the drying tool 40 may be used with green bodies 20 of different geometries. In some embodiments of drying tools 40 having such adjustable nozzles 44, at least some of the nozzles 44 may include a rigid end section (not shown) adapted to support the wall 23 of the longitudinally extending channel 22 and thereby prevent it from deforming during drying as described above.

Fig. 8 schematically shows how an auxiliary tool 47 can be arranged at the end of the green body 20 opposite to the end where the drying tool 40 is arranged. The auxiliary means 47 serves to support the longitudinally extending channels 22 during drying. In fig. 8, this is schematically shown as small pins 48 protruding from the end surface of the auxiliary tool 47, so that these small pins 48 can extend into the longitudinally extending channels 22 of the green body 20 being dried.

Fig. 9 shows the results of some tests carried out in order to study the effectiveness of the use of the drying tool according to the invention. All six components are made of the same material and extruded with multiple internal channels as shown in fig. 2. These experiments were repeated three times as shown in fig. 9a, 9b, and 9c, respectively. The lower part of the figure is dried without any forced air flow (left to dry) and the upper part is dried by guiding air through the inner channel using the drying tool according to the invention. The results clearly show how the use of the drying tool and the method according to the invention can be used to stabilize the component during drying and thus prevent undesired deformation thereof.

While the invention has been described in connection with specific embodiments, it should not be construed as being limited to the examples presented in any way. The scope of the invention is defined by the set of appended claims. In the context of the claims, the term "comprising" or "comprises" does not exclude other possible elements or steps. Furthermore, references to references such as "a" or "an" should not be construed as excluding the plural. The use of reference signs in the claims with respect to elements shown in the figures shall not be construed as limiting the scope of the invention either. Furthermore, individual features mentioned in different claims may advantageously be combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. Method for manufacturing a powder metallurgical component (21), the component (21) comprising at least one longitudinally extending channel (22), the method comprising the steps of:

-preparing a powder mixture comprising a metal powder (11) and a binder (12),

-transferring the powder mixture to a processing device (31) comprising a mould (32),

-forming the powder mixture into a green body (20) by forcing the powder mixture through the die (32) adapted to form the component (21) into a shape having the at least one longitudinally extending channel (22),

-drying the green body (20) by directing a gas flow (G) through the at least one longitudinally extending channel (22), and

-sintering or oxidizing the dried green body (20) to bond the powders (11) together to form the powder metallurgical component (21).

2. The method of claim 1, wherein the drying step further comprises directing a gas stream (G) along the outer surface of the green body (20) such that the drying is also performed externally due to the gas stream (G).

3. The method of claim 1, wherein the drying step further comprises covering an outer surface of the green body (20) such that the drying occurs solely as a result of the gas flow (G) through the at least one longitudinally extending channel (22).

4. A method according to any one of claims 1 to 3, wherein the drying step is preceded by the steps of:

-providing a drying tool (40) comprising:

-a first end (41) comprising or connectable to a gas flow generating device (43), and

-an opposite second end (42) comprising a plurality of nozzles (44), each of said nozzles being in fluid communication with said first end (41) such that gas can flow through each of said nozzles (44) under the action of said gas flow generating means (43) during use of said drying tool (40),

-arranging the drying tool (40) relative to the green body (20) such that a nozzle (44) of the drying tool (40) extends into an end region of each of the at least one longitudinally extending channel (22) of the green body (20), and

-actuating the gas flow generating means (43) such that gas flows into each of the at least one longitudinally extending channels (22).

5. The method of any one of claims 1 to 3, wherein the green body has a plurality of longitudinally extending channels, and wherein the drying step is preceded by the steps of:

-providing a drying tool (40) comprising:

-arranging the drying tool (40) relative to the green body (20) such that a nozzle (44) of the drying tool (40) extends into at least some, such as a majority, of end regions of the plurality of longitudinally extending channels (22) of the green body (20), and

-actuating the gas flow generating means (43) such that gas flows into a majority of the plurality of longitudinally extending channels (22).

6. The method according to claim 4 or 5, wherein the plurality of nozzles of the drying tool (40) are arranged over the entire cross-section of the green body (20) to be dried.

7. A method according to any one of claims 4 to 6, wherein the nozzle is shaped and dimensioned such that it provides structural support to the portion of the wall of the at least one longitudinally extending channel which is in contact with the tool, thereby preventing deformation thereof.

8. The method according to any one of claims 4 to 7, wherein an auxiliary tool (47) is arranged at an end of the green body (20) opposite to the end where the drying tool (40) is arranged, the auxiliary tool (47) being adapted to support the at least one longitudinally extending channel (22) during drying.

9. The method according to any of the preceding claims, wherein the length of the drying step is a predetermined period of time or is determined by measuring the humidity of the gas that has passed through the green body (20).

10. Method according to any of the preceding claims, wherein the manufactured part (21) has a plurality of longitudinally extending internal channels (22), such as having a honeycomb structure.

11. The method according to any of the preceding claims, wherein the gas has a higher or lower temperature than ambient air and/or wherein the gas has a higher or lower humidity than ambient air.

12. The method according to any of the preceding claims, wherein a debinding step is performed before the sintering or oxidation step, preferably comprising heating the green body (20) to a temperature at which some, such as all, of the binder (12) burns off.

13. Drying tool (40) for drying a green body (20) prior to sintering or oxidation during manufacture of a powder metallurgical component (21), the drying tool (40) comprising:

-an opposite second end (42) comprising a plurality of nozzles (44), each of said nozzles being in fluid communication with said first end (41) such that, under the action of said gas flow generating means (43), gas can flow through each of said nozzles (44) during use of said drying tool (40), and

-said drying means (40) being suitable for drying a part (21) obtained by a method according to any one of the preceding claims.

14. The drying tool (40) according to claim 13, wherein the plurality of nozzles (44) are arranged in a predetermined pattern, such as a regular pattern of aligned rows and columns.

15. Drying tool (40) according to claim 13 or 14, the drying tool (40) comprising a plurality of fluid channels (22), each extending between the first end (41) and a nozzle (44).

16. The drying tool (40) according to any one of claims 13-15, wherein at least some of the nozzles (44) comprise a closing mechanism (46) for closing the respective nozzles (44) such that no gas flow passes through the respective nozzles during use of the drying tool (44).

17. Drying tool (44) according to any of claims 13-16, wherein the mutual position of at least some of the nozzles (44) is adjustable.