EP1631393B1 - Method for sinter coating - Google Patents

Abstract

A method for sinter coating a work-piece is disclosed, the work-piece having at least two sections of different surface-related heat capacities. The method includes a first step of pre-heating the work-piece to a first temperature that is higher than a fusion temperature of a sinter coating material. The method also includes a step of rapidly heating of the work-piece to a second temperature that is higher than the first temperature. However, the rapid heating step is halted before the temperature of the work-piece section with the greater surface-related heat capacity matches the second temperature. The work-piece then has a subsequent step of application of the sinter material to the work-piece. The step of shock heating of the work-piece is preceded by a step of pre-heating the work-piece under conditions which, with continuing effect on the work-piece, bring the work-piece under conditions which, with continuing effect on the work-piece, bring the work-piece to a second temperature between the fusion temperature of the sinter material and the first temperature.

Description

The present invention relates to a method for sintering a workpiece and a device suitable for carrying out the method.

Methods for producing protective layers on metal surfaces, in particular of wire products and small metal parts, by sintering plastic powder have long been known and used. Plastic powders suitable for carrying out such processes are e.g. from DEGUSSA AG, Marl, under the trade name VESTOSINT.

The sintering coating of a workpiece conventionally proceeds in such a way that the workpiece is first heated to a temperature above the melting temperature of the material to be sintered and then brought into contact with the material, which is generally pulverulent. The contact takes place at ambient temperatures, which must necessarily be below the melting temperature of the sintered material, so that the workpiece loses heat during contact with the sintered material and finally falls below the melting temperature of the sintered material, whereby the sintering process comes to a standstill. The thickness of the layer previously deposited on the workpiece is proportional to the time between the beginning of the contact with the sintering material and the time at which the melting temperature thereof is undershot. If the workpiece to be coated has a low material thickness, the cooling proceeds faster than with a workpiece with higher material thickness, so that in order to achieve equal layer thicknesses on workpieces with different material thicknesses, the temperatures must be different, to which the workpieces are heated before they are brought into contact with the sintered material. In the case of simply shaped workpieces with a homogeneous material composition and constant wall thickness, sintered coatings having a desired coating thickness can thus be achieved by suitably selecting the temperature at which workpieces are brought into contact with the sintered material.

For workpieces having uneven wall thicknesses or inhomogeneous material composition, more generally workpieces having sections with different surface-related heat capacity, this results in the problem that the sintered layers that deposit on a high surface-area heat capacity section cools below the melting temperature of the sintered material , are larger than a section with low surface-related heat capacity. It is therefore difficult to provide such workpieces with a coating of uniform thickness. If a minimum layer thickness has to be achieved on the sections with low surface-related heat capacity, then it must be accepted that the resulting layer becomes thicker at other sections. This not only leads to undesirable additional costs due to unnecessary consumption of sintered material, but the different layer thicknesses also increase the likelihood of defects of the sintered layer, which affect their protective effect on the underlying workpiece.

To solve this problem, shock-heating methods have been proposed in which the heating of the workpiece is stopped before it has reached a homogeneous temperature distribution. As a result, when contacted with the sintered material, portions of the workpiece having a low surface-related heat capacity have a higher temperature than those having a low surface-related heat capacity, so that the time periods until cooling below the melting temperature and thus the resulting layer thicknesses become approximately the same for both sections. In principle, it should be assumed that by such a method, by suitable choice of the heating conditions, ie the final temperature which would be set on a workpiece, if it were constantly exposed to the conditions of shock heating, and the time period in which the workpiece is exposed to shock heating , within certain upper limits, allow temperature differences between sections of different heat capacity to be set and optimized for identical deposition layer thicknesses. However, it has been found in experiments that satisfactory layer qualities could not be achieved in this way, and that in particular in transition areas between sections with different surface-related heat capacities, the tendency for layer defects was great.

The object of the invention is therefore to provide a method and a device which allow the production of sintered layers of high quality and homogeneous thickness on workpieces having sections with different surface-related heat capacities.

Surprisingly, it has been found that this object can be attained by precedent to conventional preheating of a step of preheating the workpiece, wherein the preheating conditions are selected so as to bring it to a temperature between the melting temperature of the workpiece as it continues to act on the workpiece Coating material and the temperature that would reach the workpiece if it were constantly exposed to the conditions of shock heating.

It is believed that the effectiveness of the method is based on reducing the high temperature gradient between the surface and the inside of a high surface heat capacity portion present in the conventional shock heating by the preheating step, and thereby the importance of internal temperature compensation within the workpiece is reduced for the cooling of its surface. While in the simple shock heating without preheating depressed surface regions of the workpiece, especially at a boundary between sections of different surface heat capacity take relatively little heat due to their protected position and accordingly cool quickly during coating, such areas retain in the inventive method by preheating suitable for sintering Temperature longer, so that even in these problem areas, a layer of good quality arises.

Both the preheating and the Schockerhitzen done preferably by introducing the workpiece in each case a heat bath, in particular in the form of a furnace. The residence time of the workpiece in the second heat bath, ie the preheating step, should preferably take longer than the stay in the first heat bath, ie the shock heating. In a coating system, these different residence times are preferably realized in that the expansion of the preheating furnace along a conveyor line for workpieces to be coated is greater than that of the oven for Schockerhitzen.

If the workpiece slowly cools down in the course of sintering, a rough surface may result in a final phase due to incomplete melting of the sintered material. In order to improve the quality of the surface, it is expedient after the application of the sintered material to reheat the workpiece at least superficially, at least to the melting temperature of the coating material, so as to achieve a smoothing of the surface.

The application of the sintered material to the workpiece is preferably carried out by introducing the heated workpiece into the sintered material in a fluidized state.

As a sintered material, a polyamide powder such as the already mentioned VESTOSINT powder is suitable. This has a melting point of 176 ° C; therefore, a temperature of the second heat bath between 240 and 340 ° C is suitable for preheating; For shock heating, a temperature of the first heat bath between 390 and 420 ° C is preferred.

Shock heating is conveniently stopped when the higher surface heat capacity section has reached an average temperature selected in a range between 300 and 370 ° C. The specific temperature selected depends on the ratio of the surface-related heat capacities; the more different they are, the lower the quenching temperature must be selected in order to ensure equal layer thicknesses on the different sections of the workpiece.

A preferred application of the method according to the invention is the coating of a heat exchanger, in particular a condenser for a refrigerator, wherein the portion with high surface-related heat capacity is a pipe for a heat transfer fluid and the portion with low surface-related heat capacity is a wire attached to the pipe.

Fig. 1

einen Wärmetauscher als Beispiel für ein Werkstück, an dem das Verfahren ausführbar ist;

Fig. 2

ein Blockdiagramm einer Anlage zur Durchführung des Verfahrens; und

Fig. 3

Oberflächentemperaturen des Verflüssigers als Funktion der Zeit beim Erhitzen gemäß dem erfindungsgemäßen Verfahren.

Further features and advantages of the method according to the invention will become apparent from the following description of an embodiment with reference to the accompanying figures. Show it:

Fig. 1

a heat exchanger as an example of a workpiece on which the method is executable;

Fig. 2

a block diagram of an installation for carrying out the method; and

Fig. 3

Surface temperatures of the condenser as a function of time when heated according to the inventive method.

Fig. 1 shows a perspective view of a section of a known condenser in wire-tube construction for a refrigerator, to which the coating method of the invention is advantageously applicable. Such an evaporator is basically composed of two different types of elements, a zigzag-shaped bent steel pipe 1 and a plurality of wires 2, which are respectively arranged transversely to rectilinear portions of the steel pipe 1 and connect them together. The wires 2 thus serve simultaneously to stiffen the evaporator and to increase its heat-exchanging surface.

The steel pipe 1 typically has an outer diameter of 8 mm and a wall thickness of 1 mm. The wires 2 are solid with a typical diameter of 1.6 mm. The wires 2 are fixed to the steel pipe 1 by spot welding, soldering or other suitable techniques, wherein in the contact region 3 between pipe 1 and wire 2 narrow, difficult to reach angle 4 arise.

As can easily be seen, the amount of material per unit of surface area of tube 1 is significantly greater than that of the wires 2, namely by a factor of about 2.5 in the dimensions chosen here. Accordingly, the heat capacity per surface unit in the wires 2 is significantly lower than in the tube 1, so that the former heat up much faster in a heat bath than the latter.

In the Fig. 2 A highly schematically illustrated coating device comprises a conveying device 5, on which groups of a plurality of heat exchangers 6 can be fastened. The groups of heat exchangers 6 are conveyed by stepwise movements of the conveyor 5 through the coating apparatus, wherein the time intervals between successive conveyor steps may be, for example, 20 to 40 seconds.

On their way through the coating apparatus, the heat exchangers 6 first pass through a preheating oven 7, which is held by a preheating burner 8 at a fixed temperature between 200 and 340 ° C., in this case at 240 ° C. The length of the preheating furnace 7 is chosen so that two groups of heat exchangers fit in or two conveying steps are required to convey a group through the preheating furnace 7.

The preheating furnace 7 is immediately followed by a shock-heating furnace 9, which is held by a further burner 10 at a temperature between 390 and 420 ° C. The two furnaces 7, 9 may be delimited from one another by a lock 15 indicated in the figure as a dashed line; however, this is not mandatory. The shock-heating furnace 9 accommodates a group of heat exchangers 6; Their residence time in the furnace 9 therefore corresponds to the time span between two conveying steps of the conveyor 5.

Following the shock-heating furnace 9, a fluidized bed 11 containing fluidized polyamide powder is provided. The conveyor 5 has actuators (not shown) for lowering a group of heat exchangers 6 into the fluidized bed 11 and for re-lifting the group. The fluidized bed 11 provides space for a group of heat exchangers 6, so that the maximum residence time of the heat exchangers therein corresponds to the time interval between two conveying steps of the conveyor 5. The actual residence time in the fluidized bed 11, however, can be arbitrarily shortened by the heat exchangers 6 are lifted out of the fluidized bed 11 at an arbitrarily selectable time between two delivery steps of the conveyor 5.

The provided in the fluidized bed 11 with a polyamide coating heat exchanger 6 finally reach a Nachheizofen 12, in which they are heated again to a temperature above the melting temperature of the polyamide powder. The Nachheizofen 12 is held for this purpose by a burner 13 at a temperature of 240 ° C. This Nachheizofen 12 serves to improve the quality of the deposited on the heat exchangers 6 polyamide layers. These may in fact have a certain roughness at their exit from the fluidized bed 11, which is due to the fact that towards the end of the deposition of the sintered material on the heat exchangers, the temperature may have fallen so far that it is no longer sufficient for complete melting of the sintered grains. The post-heating oven 12 accommodates two groups of heat exchangers 6, so that two steps of the conveyor 3 are required to convey the heat exchangers 6 through the post-heating oven 12.

Subsequent to the Nachheizofen 12 a plunge pool 14 is still provided, in which the finished coated heat exchanger 6 are quenched.

Fig. 3 shows the time course of the surface temperatures of wires and tube of a heat exchanger 6 on its way through the furnaces 7 and 9. The heating begins at time t = 0 with the entry of the heat exchanger in the preheating furnace 7. The temperature in the interior is 240 ° C. ; the temperature of the wires 2, represented by a curve 16, approaches this value faster than the temperature of the pipe 1 represented by a curve 17. During the residence time of the heat exchanger 6 in the preheating furnace 7, neither the wires nor the pipe reach the air temperature of the preheating furnace; the temperature of the wires is nearly equalized after 60 s with about 220 ° C; the tube is at about 170 ° C, much lower.

At time t = 60 s, the heat exchanger 6 is placed in the shock-heating furnace 9, where it is exposed to a temperature of 420 ° C. If at time t = 90 s, the heat exchanger is removed from the shock-heating furnace 9 and transported to the fluidized bed 11, the wires have reached a temperature of just over 400 ° C; the surface temperature of the pipe is approx. 330 ° C. There is a temperature difference between 10 and 15 ° C between the surface of the pipe and its interior. This means that even surface areas of the pipe, the immediately one Junction 3 are adjacent to a wire 2, and therefore heated by contact with hot gas in the ovens 5 and 7 only comparatively little efficient, have reached a temperature of the same order. Therefore, they do not largely cool, as in the conventional case of shock heating in a single step, by heat dissipation into the interior of the pipe, but essentially only by the pipe giving off heat to the fluidized bed in which it is immersed. This cooling does not occur at the points of contact 3 between wire 2 and tube 1 faster than at other areas of the tube. Rather, at the problematic coating areas such as the narrow gaps 4 in the contact area between wire and pipe, the heat transfer to the fluidized bed due to the protected position of these bodies is slower than exposed surface areas of the tube, so that it is expected that at these points sufficient temperature remains for melting the coating material longer than other location, whereby the difficult access of the coating material is compensated for these locations and a layer of uniform thickness and high quality is also obtained at these problem areas.

Claims (14)

1. A method for sinter coating a workpiece (6) formed from at least two sections (1, 2) of different surface-related heat capacity, comprising a step of shock heating the workpiece (6) under conditions which, with continuing effect on the workpiece (6), bring this to a first temperature and which is stopped before the temperature of the section (1) with the greater surface-related heat capacity matches said first temperature, and the subsequent step of application of the sinter material to the workpiece (6), characterised in that the shock heating is preceded by a step of pre-heating the workpiece (6) under conditions which, with continuing effect on the workpiece (6), bring this to a second temperature between the fusion temperature of the coating material and the first temperature.
2. The method according to claim 1, characterised in that the shock heating step involves inserting the workpiece (6) into a first thermal bath (9) at the first temperature.
3. The method according to claim 1 or claim 2, characterised in that the pre-heating step involves inserting the workpiece (6) into a second thermal bath (7) at the second temperature.
4. The method according to any one of the preceding claims, characterised in that the residence time of the workpiece (6) in the second thermal (9) bath is longer than that in the first thermal bath (9).
5. The method according to any one of the preceding claims, characterised in that the application of the sinter material is followed by a step of after-heating at least the surface of the workpiece (6) to at least the fusion temperature of the coating material.
6. The method according to any one of the preceding claims, characterised in that the application of the sinter material is accomplished by introducing the heated workpiece (6) into the fluidised sinter material.
7. The method according to any one of the preceding claims, characterised in that the sinter material is a polyamide powder.
8. The method according to any one of the preceding claims, characterised in that the temperature of the second thermal bath (7) is between 200 and 340 °C.
9. The method according to any one of the preceding claims, characterised in that the temperature of the first thermal bath (9) is between 390 and 420 °C.
10. The method according to claim 8 and claim 9, characterised in that the shock heating is interrupted when the section (1) having the higher surface-related heat capacity has reached an average temperature selected in a range between 300 and 370 °C.
11. The method according to any one of the preceding claims, characterised in that the workpiece is a heat exchanger, wherein the section having the higher surface-related heat capacity is a pipe (1) and the section having the lower surface-related heat capacity is a wire (2) affixed to the pipe.
12. The method according to claim 10, characterised in that the heat exchanger is a condenser for a refrigerator.
13. A device for implementing the method according to any one of the preceding claims, comprising respectively one furnace (7, 9) for carrying out the pre-heating and the shock heating and a fluidised bed (11) for carrying out the coating which is arranged on a conveying section (5) for the workpieces (6) to be coated to the furnaces (7, 9).
14. The device according to claim 13, characterised in that the extension of the furnace for the pre-heating (7) along the conveying section (5) is greater than the extension of the furnace for the shock heating (9).

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