WO2001098007A1

WO2001098007A1 - Device for preheating of powder

Info

Publication number: WO2001098007A1
Application number: PCT/SE2001/001428
Authority: WO
Inventors: Mikael Dahlberg; Björn Johansson; Flemming Kaad
Original assignee: Höganäs Ab
Priority date: 2000-06-21
Filing date: 2001-06-20
Publication date: 2001-12-27
Also published as: SE0002318D0

Abstract

A device (1) for preheating of powder (12), especially metal powder, in immediate connection with the filling of a mould. The device comprises an inlet (7), an outlet (8), an outer tubular means (2) and, arranged in the outer tubular means (2), an inner tubular means (3). The tubular means (2, 3) are arranged relative to each other in such manner that a vertically throughgoing gap (4) is formed between the inner circumferential surface of the outer means (2) and the outer circumferential surface of the inner means (3). The circumferential surfaces constitute heating surfaces (5) so that a flow of powder (12) falling through the gap (4) is heated to a predetermined outlet temperature over essentially the entire cross-section of the gap.

Description

DEVICE FOR PREHEATING OF POWDER

FIELD OF THE INVENTION

The present invention relates to a device for preheating metal powder, which after preheating is supplied to and compacted in a mould to form a powder compact .

BACKGROUND ART

There are several techniques of heating metal powder, which to some extent differ from each other depending on what type of powder is to be heated as well as the purpose of the heating.

When heating substantially loose or unpacked metal powder there is the problem of the powder acting as an insulating material since it consists of powder particles whose total contact surface with each other is relatively small. Thus the powder may comprise a large amount of air, which makes the transmission of heat between the powder particles difficult. Thus, an unpacked powder has a relatively low thermal conductivity compared with a compacted powder compact . When preheating metal powder in view of a subsequent compacting step, it is important that the powder attains an even temperature before being compacted, since an uneven temperature gives the compact an uneven density and, consequently, deteriorated strength properties. Furthermore, it is important that the powder is not overheated when preheated, since this may cause the powder to oxidise, giving an inhomogeneous powder and a powder compact of uneven density. The used heating equipment must not be too complicated or too bulky, but should be easily combined with existing compacting equipment.

A method of preheating metal powder in view of a subsequent compacting step is described in EP-A2- 0 516 467. This publication discloses a system which is intended for preheating and feeding a polymer-coated powder in a compacting process. The powder is heated while being fed through a heated duct by a horizontal feeding screw equipped with spiral heating elements disposed on the inside. The system is complicated in con- struction having many movable components, involving the risk of operational disturbance. In addition the system requires energy for rotating the feeding screw.

US-4,223,874 discloses a blast furnace for producing iron-base metal powder. The furnace is vertically arrang- ed and comprises, inter alia, a preheating zone through which the powder passes for heating. The preheating zone consists of a tube which holds a powder volume. The outside of the tube is heated by a number of gas jets which are directed towards the circumferential surface, after which the enclosed powder volume is heated by the penetrating heat . By the heating taking place by means of the radiant heat from, for example, an open flame, the heating process and the temperature level are difficult to regulate . A further method and a device for preheating of powder are disclosed in US-5 , 574 , 955. The device comprises a powder storage container and a powder heating unit for receiving and heating of powder. The powder heating unit comprises a plurality of spaced-apart heat- ing surfaces which between them define a plurality of flow channels. Each flow channel has an upper inlet for receiving powder from the storage container and a lower outlet for discharging a partial flow of heated powder from the storage container. Moreover there is a valve means for bringing the partial flows together into a common outlet flow of heated powder to an insulated outlet means which acts as a storage container for heated powder. The heating of the partial flows is controlled in such manner that the predetermined outlet temperature is attained over essentially the entire cross-sectional area of each of the partial flows before they are brought together. The method suffers from several drawbacks. The device is large and requires much space and functions best at a high production rate or alternatively with a great component weight and, thus, a great volume of consumed powder. It is time-consuming to obtain the correct temperature in the powder volume and this temperature is then to be retained in the outlet means before the powder is finally fed to the feed shoe. In terms of maintenance, the device is complicated since each flow channel forms a narrow space which is difficult to reach and the flow of heated powder is regulated by a valve means in the form of a movable slide valve. Since each heating surface may comprise a number of heating zones of different power supply, an advanced regulating system is necessary, which renders maintenance work still more difficult. US-5, 858,415 discloses a system for delivering heated powder to the feed shoe. The system comprises roughly three parts, which are stated in the order in which they are connected between the powder container and the feed shoe: a preheater, a dosing device and a magazine for heated powder. The preheater is an inner tube which is surrounded by an outer tube. The powder is conveyed in the inner tube while the gap between the two tubes is used for circulation of heated oil . The thus heated powder is fed via the dosing device into a magazine consisting of two parts. The first part is vertical and rigidly arranged in the dosing device. The second part is telescopically arranged relative to the first part. The second part besides makes an angle to the first part and the feed shoe respectively. The magazine consists, like the preheater, of an inner powder-receiving tube and an outer tube. The gap between the two tubes is used for circulation of oil. The system requires much space, which makes it difficult to fit into existing presses since the flow from the power container up to the feed shoe is based on gravity and thus should be practically vertically arranged. The telescoping motion of the magazine places additional demands on the space available in the press. The angling of the magazine relative to the feed shoe jeopardises the flow of powder. The powder is contained in the inner tube in the preheater and the magazine respectively. The cross-section of the powder volume will thus be great, which on the one hand delays the heating process and, on the other hand, makes an even distribution of heat in the cross-section difficult. Finally, heating takes place by means of oil, which may be less favourable from the viewpoint of working environ- ment .

OBJECTS OF THE INVENTION

An object of the present invention is to provide, using a simple technique which does not require much investment for implementation in an existing press, preheating of metal powder and preferably iron powder.

A second object of the invention is that it should be possible to mount the equipment directly against the feed shoe or as close as possible thereto. A third object is that the technique should ensure that the heating process and the distribution of heat will be even through the entire powder cross-section while at the same time the heating time is minimised.

A fourth object of the invention is that it should be easy to maintain the equipment, i.e. it should not contain movable parts and should contain a minimum of monitoring parameters. Thus the invention should offer the advantages that make it interesting also when producing small components/volumes in existing equipment.

DESCRIPTION OF THE INVENTION

The device for preheating of metal powder according to the invention comprises two opposite heating surfaces which in their simplest embodiment consist of an inner tube which is arranged in an outer, surrounding tube so that their centre axes coincide . The tubes should preferably be circular in cross-section, but other cross-sec- tions are conceivable. By the inner diameter of the outer tube being slightly greater than the outer diameter of the inner tube, a throughgoing gap is formed between the circumferential surfaces of the tubes. For the powder flow through the gap to be stable without interference, the gap should be vertical. The cross-section of the gap should be rotationally symmetrical for the heating to be even through the entire cross-section.

The heating surfaces are fixed relative to each other in a manner that affects the flow through the gap to a minimum extent, for instance by a stay being mounted over the inner and the outer tube transversely of the direction of flow. The stay must be narrow so as not to affect the flow of powder and its height must be so small that the stay does not act to divide the gap into compartments. The stay can be mounted, for instance, by being welded at the upper and lower edge of the device and thus contributes not only to mutual locking of the tubes but also to the total torsional rigidity of the device. Moreover, the stay secures the inner heat source in the longitudinal direction.

On the outer circumferential surface of the outer heating surface and on the inner circumferential surface of the inner heating surface, one or more heat sources are arranged along the flow direction of the gap, so that essentially the entire heating surface of the gap is covered by a heat source. Thus each heat source forms a heating zone along the gap. The heat sources for the outer encompassing heating surface most conveniently con- sist of a heating foil which is arranged on the side of the surface facing away from the gap, i.e. on the outside of the outer tube . The heat sources for the inner heating surface consist most simply of immersion heaters which are placed in the inner tube . By using a plurality of separate heat sources, there will be increased possibilities of controlling the heating process as desired by varying the power supply. Each

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The device operates by the powder falling by gravity through the gap from a powder container arranged above the device. While passing through the gap, the powder is heated by the radiant heat from the heating surfaces and is collected in the subjacent feed shoe. As the feed shoe discharges heated powder to the mould, the corresponding amount of powder is supplied at the upper edge of the device by gravity from the powder container. The flow from the powder container to the feed shoe will thus be self-regulating, and no further regulation is necessary. Thus the gap constitutes both the heating zone for unheated powder just added and the storage space for already heated powder.

The size of the device, i.e. length of gap and width of gap, is adjusted to the type of heat source, the desired rate of heating and the desired consumption volume per unit of time. The width of the gap should be between 5 and 20 mm and preferably between 10 and 15 mm. The length of the gap can be between 250 and 1000 mm and preferably is 500 mm.

The present invention will now be described in more detail with reference to the accompanying drawings.

DESCRIPTION OF DRAWINGS Fig. 1 is a schematic view of a preferred embodiment of the device connected to compacting equipment.

Fig. 2 illustrates an example of a mutual fixing of the heating surfaces for the device according to Fig. 1.

Fig. 3 shows winding of heating foil round the outer tube .

TECHNICAL DESCRIPTION

With reference to Fig. 1, a preferred embodiment of the device 1 comprises an inner circular tube 3 which is surrounded by an outer circular tube 2, so that the tubes will have a common centre axis. The tubes 2, 3 have such a difference in diameter as to form an intermediate, rotationally symmetrical gap 4 between the opposite circumferential surfaces which act as heating surfaces 5.

The tubes 2 , 3 are fixed to each other by means of throughgoing narrow stays 6 transversely of the direction of flow at the upper edge 7 and the lower edge 8 of the device, see Fig. 2. The stays 6 should be so thin as not to have a detrimental effect on the flow of powder and they must be not so high as to divide the gap into compartments. The stays 6 are conveniently mounted at the upper edge 7 and the lower edge 8 of the device 1 and thus also cause torsional rigidity of the device.

The heating surfaces 5 of the device are equipped with separate heat sources 9, 10 which preferably are freestanding relative to the heating surfaces 5. The heat sources 9 of the outer tuber 2 consist of one or more foil elements which are successively wound in the direction of flow of the gap so that essentially the entire circumferential surface is covered (see Fig. 3) . The heat source 10 of the inner tube 3 suitably consists of at least one immersion heater which is arranged in the tube

3 so that essentially its entire circumferential surface is covered.

Thermocouples 11 which regulate the temperature of each heat source 9, 10 are connected to all heat sources 9, 10 on the inner tube 3 and the outer tube 2. Moreover, one or more thermocouples 11 are to be arranged in such manner that the temperature in the gap 4 and, thus, of the powder 12 can be registered.

The upper end 7 of the device 1 is connected to a powder container 13 which is arranged above the device

1. The lower end 8 of the device is connected directly, or as closely as possible, to a feed shoe 14 arranged in the press. The connection at both ends 7, 8 conveniently takes place by means of sockets 15 and, when needed, with supplementary flexible tubes 16.

The material of the inner and the outer heating surface 5 should be copper or some other material with good thermal conductivity. Moreover the tubular walls should have such a thickness that the heating-through proceeds quickly. In order to minimise the heat losses, the entire device is protected by an outer insulating shell. However, this shell is not shown in the Figures. To illustrate what can be achieved by means of a device 1 according to the present invention, the following example is given. An inner copper tube 3 having a wall thickness of 2 mm surrounded by an outer copper tube 2 having a wall thickness of 4 mm forms an intermediate gap 4 having a width of 13 mm. The length of the device is 500 mm. Three foil elements 9 are wound along the direction of flow of the device, along the circumferential surface of the outer tube 2. Two immersion heaters 10 are mounted in the inner tube 3. All heat sources 9, 10 as used have the same power supply and have a temperature corresponding to the desired value of the powder, 130°C. This type of equipment gives a capacity of 500 g powder per minute and a temperature of 130°C. This is a capacity which is quite realistic for a manufacturer of small components.

Claims

1. A device (1) for preheating of powder (12) , espe- cially metal powder, in immediate connection with the filling of a mould, the device comprising an inlet (7) , an outlet (8) , an outer tubular means (2) and, arranged in the outer tubular means (2) , an inner tubular means (3), the means (2, 3) being arranged relative to each other in such manner that a vertically throughgoing gap (4) is formed between the inner circumferential surface of the outer means (2) and the outer circumferential surface of the inner means (3) , the circumferential surfaces constituting heating surfaces (5) so that a flow of powder (12) falling through the gap (4) is heated to a predetermined outlet temperature over essentially the entire cross-section of the gap.

2. A device (1) as claimed in claim 1, wherein the heating surfaces (5) form a rotationally symmetrical gap (4) .

3. A device (1) as claimed in claim 1, wherein at least one of the heating surfaces (5) is vertical.

4. A device (1) as claimed in any one of the preceding claims, wherein the heating surfaces (5) each comprise at least one heat source (9, 10) freestanding relative to the heating surfaces (5) , the heat sources being arranged along essentially the entire extent of the heating surfaces (5) .

5. A device (1) as claimed in any one of the pre- ceding claims, wherein each heating surface (5) is divided into a plurality of heating zones, seen in the direction of flow, with mutually different supply of power.

6. A device (1) as claimed in any one of the preceding claims, wherein a plurality of thermocouples (11) are connected to the heating surfaces (5) and the intermediate gap (4) .

7. A device (1) as claimed in any one of the preceding claims, wherein the heat sources (9) of the outer heating surface are arranged as encompassing foil elements .

8. A device (1) as claimed in any one of the preceding claims, wherein the powder (12) falls through the gap (4) by gravity.

9. A device (1) as claimed in any one of the preceding claims, which is arranged between a powder con- tainer (13) and a feed shoe (14) .

10. A method of preheating powder (12) and preferably metal powder before hot compacting, comprising the steps of making the powder (12) pass from a powder container (13) through a vertical, elongate gap (4) which is annular in cross-section, the walls of the gap (4) consisting of heating surfaces (5) having a preselected temperature, the powder (12) while passing through the gap (4) attaining the desired temperature, the powder (12) after passing through the gap (4) being collected in a feed shoe (14) , and the powder (12) being transferred from the feed shoe (14) to a press mould (17) .