GB2353585A

GB2353585A - A device for dewaxing sintered materials

Info

Publication number: GB2353585A
Application number: GB0016760A
Authority: GB
Inventors: Andreas Aust; Gerhard Dannehl; Peter Felski
Original assignee: ALD Vacuum Technologies GmbH
Current assignee: ALD Vacuum Technologies GmbH
Priority date: 1999-08-21
Filing date: 2000-07-10
Publication date: 2001-02-28
Also published as: DE19939765C1; GB0016760D0; LU90601B1

Abstract

A device for dewaxing sintered material comprises a wax separator 16 connected between a sintering oven 1 and a device for moving fluids 22, preferably a pump, wherein the amount of fluid removed from the oven 1 is determined by a control system (26, Fig 2) in response to a interior oven pressure reading from a pressure measuring device 12. The wax separator 16 has an outlet 19 for wax condensate and may have a heater (31, Fig 6) controlled by a unit acting in response to readings from a temperature sensor. The separator 16 may be connected by a fluid line to a gas burn off device (29, Fig 3), the gas flow controlled by a valve (27, Fig 3) adjustable by the control system (26, Fig 2). Preferably, valves 14 and 23 are provided between the separator 16 and the sintering oven 1 and the pump 22 each being controlled by the control unit (26, Fig 2). The sintering oven 1 comprises a gas supply 10 to diffuse gas to its interior. A temperature measuring device 15 is provided to measure the temperature between the oven 1 and separator 16.

Description

2353585 A DEVICE FOR DEWAXING SINTERED MATERIAL

Description

The invention relates to a device for dewaxing sintered material.

Tools which are particularly stressed, for example drills or saw blades, are often manufactured from sintered hard metals. Sintering is understood to be the compression, fritting together or agglomeration of pulverulent or granular materials under the action of pressure and/or temperature.

Compact materials manufactured both from metal powders and from a mixture of metal powders and non-metallic components are used as sintering materials. The particular properties of the workpieces manufactured from sintered material such is as high resistance to wear and hardness with a sufficient degree of toughness are obtained by a high proportion of carbides of the metals chromium, niobium, tantalum, titanium, tungsten and by a low proportion of cobalt. Instead of carbides, oxide ceramic materials contain aluminium oxide; they are used as cutting materials in chip-forming manufacture. Sintered magnetic materials such as aluminium nickel cobalt (A1NiCo) and barium ferrite are used in the manufacture of permanent magnets.

So that the metal powder which is to be sintered can be given a specific shape, a binding agent is added. In this manner, the shape of a workpiece can be f ormed by means of cold pressing or the like.

This binding agent is often a wax, e.g. paraffin. However, polyethylene glycol or hydrocarbonate containing metal or another plastic agent can also be used.

2 Prior to the actual sintering process, the cold-pressed workpiece is subjected to dewaxing, in which the workpiece, which is disposed in a vacuum oven, is slowly heated, so that the binding agent escapes as a gas or f luid. The released binding agent is then removed by wax gas, vacuum pumps or other methods.

The workpiece is then subjected to the actual sintering process. In this respect, it is brought under vacuum to a temperature at which the powder granules are partially fused. In this manner, the powder particles bake together and effect a compression of the workpiece.

In order to attain a higher degree of compression, it has been proposed to heat the workpieces, which have been sintered in the fluid phase, to a given temperature in the vacuum and to subject them to a given pressure (US-PS 4 591 481).

In addition to vacuum sintering there is therefore also sintering under pressure. The pressure sintering is used in particular for the manufacture of WC-Co hard metals. It is increasingly used to replace conventional vacuum sintering (VS) and the subsequent hot isostatic treatment.

In respect of the dewaxing, a distinction is made between low pressure dewaxing and high pressure dewaxing. In the low pressure dewaxing of paraffin, a pressure of the carrier gas (Ar, N2) of < 50 mbar abs is selected, whilst in the high pressure dewaxing of polyethylene glycol (PEG) the gas f low occurs at a slight overpressure 6... 10 mbar rel.

Modern sintering devices are therefore required to master both low pressure dewaxing and high pressure dewaxing as well as vacuum sintering and pressure sintering.

3 It is therefore the object of the invention to provide a sintering device by means of which it is possible to carry out a perfect dewaxing process both in the case of vacuum sintering and high pressure sintering. 5 This object is attained according to the features of claim 1.

Embodiments of the invention are illustrated in the drawings and will be described in further detail in the following. In the drawings:

Fig. 1 is a schematic diagram of a sintering oven for the low pressure dewaxing; 15 Fig. 2 is a circuit diagram for the control of the internal oven pressure during low pressure dewaxing; 20 Fig. 3 is a schematic diagram of a sintering oven for the high pressure dewaxing; Fig. 4 is a circuit diagram for the control of the internal oven pressure during high pressure 25 dewaxing; Fig. 5 is a graph showing the course of the temperature and pressure during vacuum or pressure sintering with upstream high pressure dewaxing; Fig. 6 is a schematic diagram of a sintering oven with a wax separator, whose temperature is controlled during low pressure dewaxing; Fig. 7 is a schematic diagram of a sintering oven with a wax separator, whose temperature is controlled during high pressure dewaxing; 4 Fig. 8 is a circuit diagram for the control of the temperature of a wax separator during low pressure and/or high pressure dewaxing.

Fig. 1 is a schematic illustration of a sintering oven 1, in which low pressure dewaxing is used. This sintering oven 1 comprises a plurality of concentric layers, of which an inner annular layer 2 is a muffle made of pure graphite or the like. Adjoining this muffle 2 is a thermal insulation 3, which can also be made of graphite, hard graphite felt or soft graphite felt or a combination thereof. The outer casing forms a metal housing 4, which represents the actual oven boiler. Steel is preferably used as the metal.

is The sintering oven 1, which therefore has a cylindrical shape, stands with legs 5, 6 on solid ground. Indicated in the interior 7 of the sintering oven 1 is a charge 8 of product to be sintered. Gas can be supplied to the sintering oven 1 via a valve 9 of a gas line 10 from a gas reser-voir, not illustrated. This gas passes between the outer casing 4 and the thermal insulation 3 and from there diffuses via the muf f le 2 into the interior 7 of the sintering oven 1. The pressure in the sintering oven 1 is usually measured by means of a pressure measuring device 11, which measures the gas pressure between the layers 3 and 4. The pressure measuring device 11 is therefore connected via a tube line to the space between layer 3 and layer 4.

According to the invention, a pressure measuring device 12 is provided, which is connected via a fluid line 13 directly to the interior 7 of the sintering oven 1. Also connected to this f luid line 13, in which a valve 14 is disposed, is a temperature measuring device 15.

The f luid line 13 leads to a wax separator 16, which is essentially formed by a condenser 17 and a collecting basin 18. Condensate can be released via an outflow line 19 of the collecting basin 18 by opening a valve 20. 5 The condenser 17 is connected via a further fluid line 21 to a vacuum pump 22, a control valve 23 being disposed in the fluid line 21.

During low pressure dewaxing, a vacuum exists in the interior of the sintering oven 1. If a heating element, which is disposed for example between the muffle 2 and the insulation 3, is turned up to a temperature value where the binding agent in the sintered material melts and evaporates, then said binding agent is extracted from the sintered material as a result of the vacuum. The binding agent firstly enters the interior 7, whence it passes as a result of the drawing capacity of the pump 22 via the valve 14 into the wax separator 16. In the condenser 17, which contains steel wool for example, the binding agent condenses and drips onto the floor of the collecting basin 18, whence it flows outwards as condensate via the outflow line 19 and the valve 20.

The exhaust air freed of wax is conveyed via the fluid line 21 and the control valve 23 to the vacuum pump 22 and from here to the outside.

As already mentioned, the respective pressure in the interior 7 of the sintering oven 1 is important for the quality of the manufactured sintered product. This pressure is therefore continuously monitored by the pressure measuring device 12 and used for controlling the control valve 23.

Fig. 2 shows the control circuit for a dewaxing process during vacuum operation in the oven 1. The pressure in the 6 interior 7 of the oven 1 determined by means of the measuring device 12 is supplied as an actual value both to a comparator 44 and to a threshold value circuit 24. A nominal pressure value is also supplied to the comparator 44 by a nominal value transmitter 25. The nominal value transmitter contains a recipe, which means data - as is usual in processing and method technology -, which controls the sequence of a process. In this manner, it is determined, for example, how high the sintering temperature or sintering pressure is, which dewaxing type is carried out and how the other parameters which control the process sequence are to be selected. The actual and nominal pressure values are compared with one another in the comparator 44 and the difference between the two is is supplied to a pressure control 26, which is preferably constructed as a PID control. The control valve 23 is controlled by this pressure control 26.

The internal pressure in the sintering oven 1 can be controlled by varying the degree of opening and closing of the control valve 23. If this internal pressure reaches an excessively high value, i. e. exceeds the threshold value of the threshold value circuit 24, then the gas inlet via the fluid line 10 and the valve 9 is blocked. The internal pressure in the oven 1 therefore drops.

Another variant of the invention is illustrated in Fig. 3. Those components having the same function as corresponding components in Fig. 1 are provided with like reference numerals. In contrast to the device of Fig. 1, the device of Fig. 3 does not comprise a pump; rather the binding agent evaporating as a result of heating is forced by the internal pressure existing in the interior 7 of the sintering oven 1 into the fluid line 13, whence it passes to the condenser 17 of the wax separator 16.

7 From the condenser 17, the cleaned gas is supplied via a control valve 27 and an open shut-off valve 28 to a burning off device 29, where it burns.

In order to control the pressure in the interior 7 of the sintering oven 1, the control valve 27 is opened to a greater or lesser degree as a function of pressure which has been measured in the pressure measuring device 12.

The driving agent in this case is the pressure difference. Whilst atmospheric pressure is present at the burning off device 29, a slight overpressure exists in the gas line and gas valve, so that the term high pressure dewaxing is also used here. The resulting pressure difference allows for the diffusion or gas flow.

The control circuit diagram for the device according to Fig. 3 is shown in further detail in Fig. 4. As can be seen, this control circuit diagram is almost the same as that for the device according to Fig. 1. Most blocks of the circuit diagram are therefore provided with the same reference numerals as the control circuit diagram according to Fig. 2. The only difference consists in that the control valve 27 is actuated instead of the control valve 23.

Fig. 5 serves merely to provide a general explanation of the sintering process. It shows, amongst other things, how the temperature path, power consumption and the pressure path look during vacuum and pressure sintering. The region marked "hydrogen dewaxing" is simultaneously the region of the high pressure dewaxing, whilst the region marked "high pressure sintering" is simultaneously the cooling region. It has proved particularly advantageous for the dewaxing process to control the temperature of the wax separator 16 or its condenser 17.

8 Fig. 6 is a schematic illustration of this temperature control during low pressure dewaxing. Most of the components shown in Fig. 5 correspond to those of Fig. 1 and therefore have the same reference numerals. In addition, a temperature measuring device 30 is provided, which measures the temperature in the condenser 17. An electrical heating element 31 provided in the condenser 17 is controlled as a function of the measured temperature.

Whilst Fig. 6 is a schematic illustration of the control of the condenser temperature during low pressure dewaxing, Fig. 7 shows an arrangement for the control of the condenser temperature during high pressure dewaxing. In respect of functionally corresponding components, there is is no difference between the arrangement of Fig. 6 and the arrangement of Fig. 7, so that Fig. 7 does not contain any reference numerals which are not already contained in Fig. 6.

The control circuit diagram associated with the device according to Figs. 6 and 7 is illustrated in Fig. 8. The temperature measured in the condenser 17 is supplied to a comparator 33, to which one of two nominal values 34, 35 is also supplied. With the aid of a selector switch 36, it is possible to switch from one value to the other. A temperature control 37 is actuated with the difference between the nominal and actual value and as a function of this difference turns the heating element 31 up or down. Cooling means can also be provided instead of or in addition to a heating element 31.

Which of the two nominal values 34, 35 is supplied to the comparator 33 is determined by the respective pressure which is measured by the pressure measuring device 12. If a given threshold value, which is determined by the threshold value switch 38, is exceeded, then instead of the 9 one nominal value the other nominal value is supplied to the comparator 33.

If a further threshold value, which is determined by the threshold value switch 39, is exceeded, then a signal to supplied to an OR member 40, at whose second input a signal is generated by a block 41. This signal is either the signal coming from the threshold value switch 38, which is advanced with a time delay to the OR member 40, or a signal which is produced by the differences in the pressure signal over time. If the signal from the block 41 or the threshold value switch 39 reaches the OR member 40, then the gas inlet is closed by means of the valve 9. If the measured pressure in the interior of the oven 1 exceeds a is third threshold value, then a threshold value switch 42 transmits a signal which stops the process sequence, as indicated by the block 43. There is then no further temperature increase during the dewaxing. Of the threshold values, that of the threshold value switch 39 is the highest; the threshold value of the threshold value switch 38 is lower than that of the threshold value switch 39, but substantially higher than that of the threshold value switch 42. The threshold value switch 42 therefore responds first, followed by the threshold value switch 38 and finally the threshold value switch 39. The nominal temperature values 34 are higher than the nominal values 35.

With the invention described above, it is therefore possible to control the internal oven pressure, i.e. at the charge 8 and not - as formerly the case - the pressure between the layers 3 and 4. In addition, the internal condenser temperature 16, 17, 31 can be controlled in order to optimise the degree of separation in the wax separator.

Claims

1 A device f or dewaxing sintered material, with 1.1 a sintering oven; 1.

2 a device (22) for moving fluids; 1.3 a wax separator (16), which is connected both to the 5 sintering oven (1) and to the device (22, 29) for moving fluids via a fluid line (13; 21); 1.4 a pressure measuring device (12), which measures the pressure in the interior (7) of the sintering oven (1); and 1.5 a control device (26), which controls the quantity of the fluid, which is moved by the device (22, 29) for moving fluids out of the interior (7) of the sintering oven (1), taking into account the pressure measured by the pressure measuring device (12).

is 2. A device according to claim 1, characterised in that the device for moving fluids is a pump (22).

3. A device according to claim 1, characterised in that the device for moving fluids is a device (29) influencing the pressure in the interior of the sintering oven.

4. A device according to claim 1, characterised in that the wax separator (16) comprises an outflow (20) for wax condensate.

5. A device according to claim 1, characterised in that the wax separator (16) is connected via a fluid line to a device (29) for burning off gases.

6. A device according to claim 2, characterised in that the pump (22) is connected via a fluid line (21) to the wax separator (16), a valve (23) being provided in the fluid line (21) which is adjustable by a control device (26).

7. A device according to claim 5, characterised in that a valve (27) is provided in the fluid line which is adjustable by the control device (26).

8. A device according to claim 1, characterised in that a temperature measuring device (11) is provided, which measures the external temperature of the sintering oven (1).

9. A device according to claim 1, characterised in that the fluid line, which is provided between the sintering oven (1) and the wax separator (16), comprises a valve (14).

10. A device according to claim 1, characterised in that a temperature measuring device (15) is provided, which measures the temperature of the fluid in the fluid line which is provided between the sintering oven (1) and the wax separator (16).

11. A device according to claim 1, characterised in that the sintering oven (1) comprises a gas supply (10), from which gas diffuses into the interior of the sintering oven (1).

12. A device according to claim 1, characterised in that the wax separator (16) comprises a heating element (31).

13. A device according to claim 12, characterised in that a temperature sensor is provided, which measures the 12 temperature in the interior (7) of the wax separator (16), and a control device (37) is provided, which controls the heating as a function of the measured temperature.