EP1894648B1 - Low-pressure casting method and device therefore - Google Patents

Abstract

The low-pressure casting method for metals, comprises filling liquid metal in a form cavity (13) under pressure in a filling phase by a pump from a melt supply (1) over a pump pipe (15), solidifying the metals under balance vibration in cavity and applying low pressure on the metal by a piston in a low-pressure phase. In the melt supply a movable piston (8) is set in a holding pressure cylinder (7) connected to the pump pipe. The pump is arranged in a furnace chamber in a distance from the base and from upper melt level. The low-pressure casting method for metals, comprises filling liquid metal in a form cavity (13) under pressure in a filling phase by a pump from a melt supply (1) over a pump pipe (15), solidifying the metals under balance vibration in cavity and applying low pressure on the metal by a piston in a low-pressure phase. In the melt supply, a movable piston (8) is arranged in a holding pressure cylinder (7) connected to the pump pipe. The pump is arranged in a furnace chamber in a distance from the base and from upper melt level. A valve function is inserted within the melt supply for agitating the melt and blocks the connection between the pump pipe and the holding pressure cylinder. An independent claim is included for a device for die casting.

Description

The present invention relates to a casting method, in particular a low-pressure casting method in which liquid metal filled under pressure in a filling phase by means of a pump from a melt supply via a pump tube in a mold cavity and then, before complete solidification, to compensate for shrinkage in Mold cavity is applied in a Nachdruckphase a reprint by means of a piston on the metal.

When referring to a low-pressure casting method in the course of this description, it should be noted that a low-pressure IPC casting method is defined as a method in which the metal is filled into the mold cavity with a few bars. This is in contrast to normal die casting, for the E. Brunhuber, "Practice of Druckgußfertigung", publisher Schiele & Schön GmbH, Berlin, 1980, p. 16 , Pressures up to 2000 bar indicates.

If one looks at the casting cell of a die casting machine, in which a holding furnace as a melt supply next to a cold chamber die casting machine - usually not inconsiderable size - is, then you can see that the space requirements (and thus the cost of the claimed space) are quite significant. The space requirement of a diecasting cell is therefore cited here because, as already mentioned above, the prior art according to said DE-A-28 46 519 strikingly resembles a die casting machine, so that for such a low-pressure casting machine probably a similarly large space is needed. This large space requirement is due not least to the casting piston and the casting chamber comprehensive "shot" of the device.

• zum einen ist das Zuführrohr der Abkühlung unterworfen, d.h. es wird teilweise sich Metall an den Wänden absetzen, und das Zuführrohr muss - wenn überhaupt möglich - in gewissen Zeitabständen gereinigt oder ersetzt oder auch mit einer Heizung versehen werden;
• das so sich abkühlende Metall hat die Tendenz, an der oberen Frontseite eine halb erstarrte Oxydhaut zu bilden, die dann in das zu gießende Werkstück gelangt und unerwünschte "Sollbruchstellen" bewirkt, d.h. die Qualität des gegossenen Teils wird beeinträchtigt;
• da der Gießkolben - wie bei Kaltkammer-Druckgießmaschinen üblich - in einer separaten Gießbüchse außerhalb des Schmelzenvorrats untergebracht ist, können unerwünschte Leckagen auftreten, welche, insbesondere nach dem Absetzen in der vom Gießkolben durchlaufenen Gießbüchse, zu Störungen führen können, teilweise aber von diesem auch in den Formhohlraum geschoben werden und dort wieder Qualitätsprobleme verursachen.

Such a machine is for example from the DE-A-35 28 691 become known, which includes those features from which the present invention proceeds. There, a pump is immersed in a melt supply and promotes melt from this melt supply via an exposed feed tube melt to a type die casting machine and there into the mold cavity, wherein a casting piston then the reprint worried. This has, as has been recognized by the inventors of the present patent application, various disadvantages:

• on the one hand, the feed tube is subject to cooling, ie it will partially settle metal on the walls, and the feed pipe must - if at all possible - be cleaned or replaced at certain intervals or provided with a heater;
• the metal which has cooled down tends to form a semi-solidified oxide skin on the upper front side, which then passes into the workpiece to be cast and causes undesired "predetermined breaking points", ie the quality of the cast part is impaired;
• Since the casting piston - as usual in cold chamber die casting machines - is housed in a separate Gießbüchse outside the melt supply, undesirable leakage may occur, which can lead to disturbances, especially after discontinuation in the continuous casting of the casting piston, but partially from this also in pushed the mold cavity and there again cause quality problems.

The invention has for its object to reduce the space required for performing a low-pressure casting process - even in comparison to a die casting - or to ensure a good quality of the castings. Although devices can be after the US 6,422,294 quite space-saving to be built, only these are not able to apply a reprint and so improve the quality of the casting. But this should be possible according to the invention.

The aim of the present invention can be achieved according to the invention by pressurizing a piston acting on the metal in the mold cavity, moving in the melt reservoir and pressurizing it in a pressure cylinder connected to the pump tube for the holding pressure phase.

• gegenüber einem Füllen unter einem auf das Schmelzenniveau wirkenden Gasdruck besteht der Vorteil, dass das Beschicken des Schmelzenvorrates (im allgemeinen einer Ofenkammer) erleichtert wird und nicht bei jedem Beschicken, erst der Druck abgelassen und dann neu aufgebaut werden muss;
• Leckagen sind nicht so gefährlich, wie beim Füllen mittels eines relativ hohen Gas-Überdrucks;
• gegenüber dem Füllen mittels Unterdruck (vom Formhohlraum aus) besteht der Vorteil, dass die Füllung des Formhohlraumes rascher erfolgen kann, so dass auch größere Formhohlräume ohne die Gefahr des zwischenzeitlichen ganzen oder teilweisen Erstarrens der Schmelze gefüllt werden können;
• ein Pumpenrohr kann einen relativ geringen Durchmesser besitzen, so dass sich die Baugröße insgesamt verringert.

Because using a pump to fill results in the following benefits:

• compared to filling under a pressure acting on the melt level gas pressure has the advantage that the loading of the melt stock (generally a furnace chamber) is facilitated and not with each loading, first the pressure must be drained and then rebuilt;
• Leaks are not as dangerous as filling with a relatively high gas overpressure;
• compared to the filling by means of negative pressure (from the mold cavity) has the advantage that the filling of the mold cavity can be done faster, so that even larger mold cavities can be filled without the risk of intermittent total or partial solidification of the melt;
• a pump tube may have a relatively small diameter, so that the overall size is reduced.

The fact that the piston is now assigned only the task of reprint, the piston needs only a small volume of melt to compensate for shrinkage of the metal move in the mold cavity, so that the impression cylinder is no longer the size of the above DE-A-35 28 691 must have known casting chamber.

However, the fact that this Nachdruckkolben moves within the melt stock, there is the advantage that the freely projecting into the room supply pipe can be kept short, so that the risk of cooling is not so great, which so far led to an impairment of quality , In addition, this also reduces the height. Inevitable leaks are not a problem because they can only lead to the return of the leak material in the melt stock. In addition, the risk of contact of the metal with reactive gases (air, inert gas) - even when changing the shape - no longer exists.

In order to avoid the introduction of contaminants into the mold, it is advantageous if the pump is arranged in a furnace chamber at a distance from the bottom and from the upper melt level. Thus, space can also be saved by this arrangement again.

An inventive device for die casting with a mold cavity enclosing form, to which a pumped supply channel for melt from a melt supply is connected, wherein the device comprises means for applying a reprint, is characterized in that for the Nachdruckphase on the metal acting in the mold cavity, in the melt supply moving Nachdruckkolben is provided in a pressure cylinder connected to the pump tube. Essentially, the advantages mentioned above apply to this device.

Further details of the invention will become apparent from embodiments which are discussed below with reference to the single, schematic figure of the drawing, which shows a vertical section through a device according to the invention for low-pressure casting.

A crucible 1 is with its bottom 2 in a furnace housing 3 shown broken away per se known type and thus forms a furnace chamber for receiving a Schmelzenvorrates, for example up to a melt level N. The furnace can be designed to be arbitrary and more than one furnace chamber have, for example, as a known two-chamber furnace, whose first chamber is used for melting pigs, whereas the second, connected to the first, chamber serves as a removal chamber. The oven is expediently covered with a lid 4 which has, for example, a filling opening 5. A septum 6 may be provided for holding floating materials, but in the present case mainly serves to mount a cylinder 7, in which a pressure piston 8 is displaceable, which is drivable via a piston rod 9 of a drive unit 10 of known type. For example, for control and / or control purposes, the piston stroke can be measured via a displacement transducer of known design.

The cylinder 7 forms part of a supply channel 7, 11, of which the branch 11 vertically upwards to a mounted above the furnace chamber 1 formed by the crucible 1 low pressure mold 12 of two mold halves and leads one of these enclosed mold cavity 13, advantageously - as can be seen branches and so metal from the furnace chamber 1 enters the mold cavity 13. The upper, protruding from the furnace part of this branch 11 may, if desired, be heated, whereas, if necessary, a cooling arrangement is provided for the mold. The mold 12 and the devices for opening, closing or clamping are known per se and therefore will not be described here in detail.

The ratio of the inner volumes of cylinder 7 and mold 12 should be designed according to the material shrinkage in the mold cavity 13, which may be for example 5%. For this purpose, any gap losses between piston 8 and cylinder 7 - if such occur - should be taken into account. The size of the latter can be controlled, for example, by means of a displacement transducer on the piston rod 9: A piston travel that exceeds the material shrinkage would be a reason for replacing the piston 8 or piston rings.

However, this metal so supplied is not supplied from the piston 8, which here only has the function of a Nachdruckkolbens, but by a pump 14 to which a pump tube 15 is connected. The pump tube 15 carries the metal supplied by the pump 14 to an inlet opening 16 at the top of the cylinder 7. Thus, the pump 14 driven by a motor 18 via a pump shaft 17 can in itself be any volume of metal, even under a certain low pressure a few bar, promote the pump tube 16, the cylinder 7 and the riser channel 11 in the mold cavity 13, especially when - as shown in the preferred embodiment - the piston 8 is in the rearmost position and the mouth opening 16 of the pump tube 15 in the cylinder. 7 has released. It can be seen that while the cylinder 7, which only serves more to exert a reprint, is kept relatively narrow, and it may also be shorter than shown.

It should be noted here that the cylinder 7, which is no longer able to supply the main mass of metal to the mold cavity 13, can also sit directly on the mold 12 as a booster cylinder, as is known per se from pressure booster cylinders in the die-casting. It is important above all that the tasks of supplying the main mass of metal is assigned to the pump 14, whereas the piston 8 only acts as a post-pressure or Nachverdichterkolben for pressing metal for shrinkage compensation in the mold cavity 13. However, the illustrated arrangement is preferred, in which the pressure cylinder 7 opens into the pressure cylinder 7 in a pump tube 15 which forms at least part of the feed channel 7, 11, 15.

The pump 14, as can be seen, expediently arranged both at a distance from the bottom 2 of the crucible 1 and from the upper melt level N, so as to avoid that either floating matter or soils deposited on the bottom 2 are pumped into the pump tube 15.

In order to be able to supply 12 different volume flows via the pump 14 depending on the shape or even during the filling of the mold cavity 13 changing volume flows, for example, to avoid the spraying of the metal or turbulence of the same during passage of a narrow molding cross-section to another, it is advantageous if an adjusting device 19 is provided for the volume flow of melt delivered by the pump 14, so that the filling can be faster or slower. In the case of thick molding sites or sections with a large molding chamber volume, this area may then be filled up more rapidly if desired. For example, with this adjusting device 19, the rotational speed of the shaft 17 can be adjusted. In this case, the adjusting device 19 is connected to a control 20 for the motor 18 and can change the speed of the latter, for example with a frequency converter.

Once the pump 14 has pumped the required amount of metal into the mold cavity and thus it is completely filled, it can be useful, after forming a weak solidification crust on the walls of the mold cavity 13, a pressure built up, whereby two things is achieved: On the one hand is the Metal solidified in the mold 12, on the other hand is avoided by the supply of a small amount of metal balance, that is due to shrinkage Form voids, which affect the quality of the casting. Advantageously, the transition from the mold filling phase to the post-printing phase is automated.

This can be done in different ways. The representation of the figure shows three possibilities, each of which can be realized individually, two or all three of them. One possibility is that a pressure sensor 21 is provided in the riser 11 and measures the pressure exerted by the weight of the metal pumped up into the mold cavity 13. When this pressure indicates the complete filling of the mold cavity 13, the sensor 21 outputs a signal to a control device 22, which sets the drive 10 for the Nachdruckkolben 8 in motion. The further control of the Nachdruckkolbens via the step 22 may be conventional and time and / or away-dependent.

Depending on the design of the mold cavity, however, a more precise control can be used, which provides a metal front sensor (or a pressure sensor) 23 on the wall of the mold cavity (or in a channel associated therewith). Such a sensor 23 is advantageously located at the highest point of the mold cavity (or in an associated upstream channel). As can be seen, the sensor 23 is optionally connected to the controller 22.

A third possibility, for instance when using a volumetric pump 14, is that the duration of its operation as an output signal of the drive stage 20 for the motor 18 is supplied to a timer 24 which after the elapse of a predetermined pumping time applies the hold pressure via the controller 22 and the drive 10 triggers. In any case, it is advantageous to also output a signal from the controller 22 to the control stage 20, so that the operation of the pump 14 can be switched off. However, this is not absolutely necessary because the pump 14 may continue to pump without impact. It has been shown that with the switching off of the pump 14, this can be spared very.

The connections of the sensors or encoders 21, 23, 24 to the controller 22 are shown here as having a change-over switch 25 at the input of this control, but this can certainly be modified within the scope of the invention by making such a connection either directly to the control or, for example, a weighting stage which signals from two or three of the transmitters 21, 23, 24 receives and the controller 22 triggers accordingly.

The illustrated embodiment, in which within the furnace chamber 1 of the impression cylinder 7 via the opening 16 into the, at least a portion of the supply channel forming pump tube 15 and this opens into the pressure cylinder 7, is also of particular advantage, because the piston 8 optionally as Valve can be advanced just enough so that it covers the opening 16. If then the pump 14 is held in light movement, it acts only as a stirring member, which prevents temperature differences within the furnace chamber 1. Of course, it would also be possible to install a separate check valve, such as in the pump tube 15. But if, as preferred, the piston 8 - together with a correspondingly programmed controller 10, 22 - takes over the valve function, then play these parts 8, 10, 22 then of course the role of the valve.

Claims (13)

1. Casting method for metals, during which liquid metal under pressure, in a filling phase, is filled by means of a pump (14) from a melt stock (1) via a pump tube (15) into a die cavity (13), and subsequently, prior to complete solidification, an after-pressure is applied onto the metal in an after-pressure phase by means of a piston (8) for compensating shrinking in said die cavity (13), characterised in that for said after-pressure phase of the method an after-pressure piston (8), which acts onto the metal in said die cavity (13) and moves within said melt stock (1) in an after-pressure cylinder (7) that is connected to said pump tube (15) is set under pressure as the piston (8).
2. Method according to any of the preceding claims, characterised in that the pump (14) is arranged within said melt stock, which reaches from a bottom (2) up to an upper melt level (N), namely in a respective distance from said bottom (2) and from the upper melt level (N).
3. Method according to claim 1 or 2, characterised in that the connection between said pump tube (15) and said after-pressure cylinder (7) is locked for agitating the melt in the interior of said melt stock (1)
4. Method according to any of the preceding claims, characterised in that it is operated as a low-pressure casting method.
5. Device for casting under pressure comprising a die (12), which surrounds a die cavity (13), to which a supply channel (7, 11, 15), provided with a pump (14), for melt from a melt stock (1) is connected, said device exhibiting a device (7, 8, 10) for exerting an after-pressure, characterised in that for the after-pressure phase, an after-pressure piston (8), which acts onto the metal in said die cavity (13) and moves within said melt stock (1) in an after-pressure cylinder (7) that is connected to said pump tube (15) is provided.
6. Device according to claim 5, characterised in that said after-pressure cylinder (7) leads into a pump tube (15), which forms at least part of said supply channel (7, 11, 15) or said pump tube leads into said after-pressure cylinder (7).
7. Device according to claim 5 or 6, characterised in that at least one pump tube (15) , which forms at least part of said supply channel (7, 11, 15), is arranged within a furnace chamber (1) that contains said melt stock below the melt level (N) of this chamber (1).
8. Device according to claim 7, characterised in that said after-pressure cylinder (7) too is arranged within a furnace chamber (1) that contains said melt stock below the melt level (N) of this chamber (1).
9. Device according to any of claims 5 to 8, characterised in that a control unit (21-24) is provided which releases the after-pressure phase, and which receives at its input a signal from a sensor (21, 23, 24), which indicates replenishing of said die cavity (13), while its output is connected to at least one drive (10) for said after-pressure piston (8) for switching it on.
10. Device according to claim 9, characterised in that said output of said control unit (21-24), which releases said after-pressure phase, is also connected to said pump (14) for switching it off.
11. Device according to any of claims 5 to 10, characterised in that an adjusting device is provided for the volume flow of melt supplied by said pump (14).
12. Device according to any of claims 5 to 11, characterised in that a valve arrangement (8, 10, 22) is provided in the course of said pump tube (15) and said after-pressure cylinder (7), and that preferably said valve arrangement (8, 10, 22) includes the piston, which in the course of said after-pressure phase advances over the mouth opening (16) of said pump tube (15) in said cylinder (7), as well as a control (22) which releases this limited advancement.
13. Device according to claim 12, characterised in that said valve arrangement (8, 10, 22) is formed to separate said pump tube (15) and said after-pressure cylinder (7).

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