EP0956916B1 - Low pressure bottom casting process for casting metals, especially light metals - Google Patents

Abstract

An overwhelming proportion of the melt volume required for filling of the mold cavity is forced into the latter at a maximum velocity. The melt column in the riser pipe (7, 8) is sheared off, and the residual melt volume between the shearing-off point and the mold inlet (11) is forced into the mold cavity at a velocity with a negative gradient. An Independent claim is also included for an apparatus for carrying out the low pressure casting method.

Description

The invention relates to a method for increasing Low-pressure casting of metal, in particular light metal, by a casting mold provided with at least one feeder with her sprue on a riser to a Melting container connected and contained therein Melt under pressure through the riser into the mold is displaced.

Light metals, especially aluminum, have in the Past as construction material increasing Gained importance. This also applies to the motor vehicle industry, especially engine construction. So be in the newer Time engine blocks made of aluminum. Due to the Large-scale production in the automotive industry must also Casting and casting machines with high performance for Will be provided. At the same time, especially at highly loaded components a high quality standard to be guaranteed. Because aluminum is mostly in the molten state spontaneously oxidized with atmospheric oxygen forms open melt surfaces in the presence of oxygen very quickly an oxide skin.

To prevent this as much as possible, has become for example, both for the casting, as well as Chill casting Low pressure casting, especially in the Execution of the rising cast, proven, since here the Melt not swirled, but the mold or mold is filled with a calmed melt front. Thereby In particular, oxide inclusions in the casting can largely be avoided.

With these typical measures for low pressure casting However, the disadvantage is that the performance is relatively low. This is among other things on it attributed to the fact that after each casting the pressure in the melt vessel must be lowered, which in turn with the sinking the melt column is connected in the riser. there atmospheric oxygen penetrates from the environment into the riser on. It forms on the albeit small surface the melt column an oxide skin, which is renewed Increase the melt column at the next casting cycle to the Wall of the riser applies and on the melt front always imitates. As a result, the riser is growing gradually closed. At high power this requires one regular replacement of the riser in relatively short Time intervals, which in turn leads to a reduction in performance leads. Another decisive disadvantage is that the oxide film forming on the surface of the melt column is introduced into the mold or mold and itself later found in the cast structure.

In low-pressure casting plants it is known (WO 95/20449), in the area of the transition from the riser to the form a Provide closure. This one has the first Task, turbulence in the melting vessel, in particular in the gas cushion located above the melt level to avoid. The shutter consists of one at the transition between the riser and sprue to be used melting plate with lower melting point than the aluminum melt. This shutter plate is at increasing the Melt liquefied. These liquid foreign components are introduced into the mold and lead to the highest unwanted inclusions in the casting. The shutter must after each casting to be replaced, so that the performance is correspondingly low.

In another known method (WO 97/37797) between the mouth of the riser and the mold a make-up arranged in the form of a container through which through the melt from the riser into the mold is displaced. The container has a bottom side Slide lock on, after filling the mold closes. After the casting, the mold is combined with the Make-up decoupled from the riser and the casting shrinkage by feeding melt from the make-up in balanced the shape. This also allows the Performance compared to conventional low-pressure casting machines Do not increase, especially as the dessert for the next Drained pouring and returned to the casting station must become.

After an older, not pre-published proposal the applicant (DE 198 07 623) is at the end of each casting cycle reaching from the riser into the sprue of the mold Melting column near the mouth of the riser below simultaneous closure of the same sheared off. By the Shearing the melt column near the mouth of the riser forms no free above the melt column Volume and thus no free surface on the could form an oxide skin. It can therefore also no oxide layer on the riser attach and can do not grow this. By the closure is further ensured that in the subsequent casting cycle no Oxidhaut is introduced into the mold. Further, from Advantage, if after shearing the melt column in the Melting vessel at least one the melt column against the Closure retaining overpressure is maintained. By this measure, the performance of the casting plant increase.

The production of light metal casting in high quantities and high Gußtonnagen requires significantly higher specific Casting achievements, which so far in the range of 1 kg / s lie. To improve the efficiency of the plants ever shorter cycle times are desired. In order to The demand for shorter casting times ever arises Clock, or for equal proportions the demand for higher melt throughput during mold filling. The with increasing pouring higher specific pouring performances significantly more than 1 kg / sec, e.g. B. 3 - 10 kg / sec, lead to high local casting speeds. These are but due to the lack of erosion resistance of the molding materials, through the geometry of the castings and gates in narrow cross sections, but most strongly by the unwanted dynamic Füllstoß limited at the end of casting. The dynamic charge at the end of the pour depends on the given Part geometry primarily of the venting behavior the form (gas permeability in sand molds, vent holes etc.), but in any case from the casting speed at the end of the pouring shortly before the complete Filling the mold.

In Sandgußverfahren, especially in Naßgußverfahren are Surface defects on the castings known, predominantly caused by the filling impact at the end of the casting. In The last phase of the mold filling becomes the kinetic Energy of the melt on the elevated mold surfaces abruptly converted into impact energy. This dynamic thrust drives the melt into the pores of the sand surface and There generates undesirably rough surfaces on the casting contour. Here are inhomogeneously compressed form parts affected in different ways. In many cases so damaged castings but with reworked considerable cleaning or as a committee be discarded.

Another effect of this filling stroke leads to increased Burr formation in the division zones of the mold halves and the Cores. But burr formation means expensive cleaning work, depending on the type of part over 30% of the cost price of Casting can make out. Burr eggs or burr-free Casting is therefore of great economic importance.

A third type of defect is the blistering due to Air and gas inclusions that occur during casting, but mainly in the phase shortly before the end of the casting. Preventive, the casting gases via exhaust ducts, by air whistling, by using open feeders or use of molded materials with high gas permeability dissipated. Errors of the aforementioned type already occur the usual today low casting power from 1 to 2 kg / sec for aluminum alloys. Will the casting performance increased significantly, it is expected that the Gas bubble formation increases to a considerable extent. For the Rising pouring methods have been known to date. So z. B. the use of a gas pressure pad in Melting tank or furnace, which the melt over the fills the riser connected to the mold. Instead of Gas pressure in the melting tank will also be electromagnetic pumps used at the foot of the riser, which the Take on the filling task. In both cases, a so-called active filling for use, d. H. the filling process is in the form of a speed-time profile or volume-time profile depending on the level change controlled and regulated in the melting tank (DE 33 17 474 = EP 0 128 280, EP 0 215 153). With these Methods, it is possible in principle, the undesirable to counteract dynamic Füllstoß towards the end of the mold filling. The increase of Gußtonnage and thus the Increasing the casting capacity per casting, however, represents new Requirements for the known casting methods. larger Melt throughputs inevitably lead to larger Melt or Ofenchargen and furnace volume, because the Refill cycles because of the high demands on the Melt quality must not be neglected. Size Furnace volumes complicate the gas pressure filling process the controllability of the filling process. These greater mass inertia of the melt leads to rapid Changes the furnace pressures to swing the metal mass in the oven. The leadership of the melt-filling profile is made difficult or impossible. Especially at the end of the pouring towards the previously accelerated mass of the melt can not Delayed fast enough and vibration free.

The invention is based on the object in the rising Pouring of light metal a higher casting speed and thus higher cycle times with high casting quality to reach.

This object is achieved in that the vast majority of for filling the mold necessary melt volume with maximum casting speed displaced into the casting mold, then the melt column sheared in the riser and a between the Abscherstelle and the remaining remaining volume of up to about 10% of the volume of the casting with a negative Velocity gradients displaced into the mold becomes.

In the method according to the invention, the filling process divided into two sections, the first section following the usual process of rising casting, by at the given free cross-section of the riser the melt column alone by the propulsion on Melting vessel or furnace up to the maximum casting speed accelerated and this until just before the end of pouring is maintained. Subsequently, the melt column sheared off near the gate so that the maximum Casting speed is maintained for a short time until the melt column is completely sheared off is. The remaining between the Abscherstelle and the sprue Residual volume, which amounts to about 10% of the volume of Casting is then with constantly decreasing speed displaced into the mold, so that the Melt front within the mold rises more slowly and on the one hand controls the residual air remaining in the mold can escape, on the other hand, a dynamic safe Füllstoß on the overhead molding surfaces avoided becomes. This is equally prevented that in the Melt volume in the form of too high a hydrostatic Build up pressure.

The inventive method is in particular in the case of sand molds, the penetration of the melt into the mold surface and further eroding the shape by impulse forces avoided, so that despite possible increase the casting performance flawless Gußteiloberflächen received become. Further, by avoiding the construction Too high isostatic pressure avoided that the Melt penetrates into mold separation zones and burrs leads. By shearing off the remaining volume of the Melting column can be from the melt container ago touching Vibrations are not transferred to the residual volume. The residual volume in turn is due to its relative low mass and the negative speed gradient vibrationally displaced into the mold.

Preferably, the residual volume is discontinuous Velocity gradient displaced into the mold, for example, first with the maximum casting speed, then with a flat and then steep negative velocity gradient.

The inventive method is in particular the Possibility of depending on the size of the residual volume from the volume and / or geometry of the casting choose. The larger the casting volume and the more involved the geometry of the casting in the upper area is so bigger you will choose the remaining volume.

Furthermore, the velocity gradient and / or its time course depending on the volume and / or the geometry of the casting can be selected. there comes with an intricate geometry, especially in the upper area of the form, a faster degradation of the velocity gradient in consideration, as in large-volume and little involved forms.

The residual volume is preferably 3-7% by volume of the Casting.

For carrying out the method, the invention proceeds from a device with a transport path for intermittent Movement of the molds with at least one feeder and a sprue, a casting station at the transport track, at which the molds can be positioned one after the other are, a positionable at the casting station melt container, in particular melt furnace, one in the melt submerged riser, to which at the casting station the Mold is connectable with its sprue, and a Pressure generator for displacing the melt from the melt container over the riser and the sprue in the Mold.

Such a device is characterized according to the invention characterized in that at the transition between riser and Gutter a residual volume forming, tubular portion is arranged, in which a controlled displacer between one the mouth of the riser in this section releasing casting position over a the Mouth closing intermediate position with a negative Speed gradients in a sprue closing position is movable.

The displaced in the riser column passes with the maximum casting speed over the tubular Section and the pouring into the mold. Towards the end of the Gießtaktes the displacer is controlled and reduced the free cross section of the mouth in a given Time at the end of which the melt column shears off completely is, so that the furnace-side acting on them Driving forces no longer on the tubular section enclosed residual volume can act. This residual volume is then by means of the displacer with the predetermined, negative velocity gradient in the mold displaces until the displacer reaches its final position reached, in which he closes the sprue.

Preferably, the tubular portion is aligned with the Injection, so that the residual volume without translational translational and thus well controllable displaced into the mold becomes. This can be done with vertical sprue at the bottom of the mold equally beneficial, as in a horizontal lateral sprue.

The speed of the displacer is preferably in Dependence on the geometry and / or the volume of the Castable controllable, possibly also by means of a Program specifiable.

Furthermore, at least the tubular portion against a section with a different volume interchangeable or to be extendable to another section the residual volume of the geometry or the volume of Optimally adapt casting.

In a preferred embodiment, the tubular portion and the displacer of a cylinder-piston unit formed and opens the riser laterally into the cylinder near the gate far end position of the piston in the Cylinder on.

Optionally, the complete cylinder-piston unit exchanged for one with another cylinder volume become.

Fig. 1

eine schematische Ansicht einer Niederdruck-Gießanlage für den kastenlosen Sandguß in einer ersten Betriebsstellung;

Fig. 2

die Gießanlage gemäß Fig. 1 in einer weiteren Betriebsstellung;

Fig. 3

eine schematische Ansicht einer Niederdruck-Gießanlage für den Kastenguß in einer ersten Betriebsstellung;

Fig. 4

die Gießanlage gemäß Fig. 3 in einer weiteren Betriebsstellung und

Fig. 5

ein Zeitdiagramm zum Anlauf des Gießvorgangs.

The invention will be described with reference to exemplary embodiments shown in the drawing. In the drawing show:

Fig. 1

a schematic view of a low-pressure casting plant for casteless sand casting in a first operating position;

Fig. 2

the casting plant of Figure 1 in a further operating position.

Fig. 3

a schematic view of a low-pressure casting for the Kastenguß in a first operating position;

Fig. 4

the casting plant of FIG. 3 in a further operating position and

Fig. 5

a timing diagram to start the casting process.

The casting system designed according to the invention is both suitable for Sandformenguß, as well as chill casting. The reproduced in the drawings embodiments exclusively concern casting in sand molds (with or without box).

In the embodiment of FIGS. 1 and 2 are the Molds as boxless molded bales 1 on a die Transport path forming conveyor 2 sequentially arranged. The molded bales 1 are in the direction of Arrow 3 promoted to a casting station, at the one Melting vessel 4, optionally in the form of a furnace, hebund can be lowered and moved. In that Melt vessel 4 is the light metal melt 5, over whose melt surface a gas cushion 6 is, which is acted upon by a controllable pressure generator. In the melt vessel 4 emerges a designated overall with 7 Riser, which in this embodiment over a sloping section 8 and a short rectilinear, tubular portion 9 extends upward and ends at a mouth 10. Above this mouth the molded bales 1 are thus clocked by means of the conveyor 2, that at every bar a bale in the casting position comes. Then the device with the short vertical section 9 of the riser adjusted so that the latter in flight position with the sprue 11 of the form of bale arrives. In the tubular portion 9 is a displacer arranged in the form of a piston 12, by means of a pressure medium cylinder 13 is actuated.

From the reproduced in Fig. 1 close to the final position, in the gate 11 is closed, the piston 12 is in moves the other end position shown in Fig. 2, in which he the short vertical portion of the riser 7 completely releases, so that under the action of the melt vessel 4 prevailing overpressure the melt into the mold arrives. Before the mold is completely filled, the Piston 12 again in the other end position (Fig. 1) proceed, in which he the located in the section 9 residual volume 14 displaced the melt in the mold. The cast off Molded bales leave the casting station in the direction of of the arrow 3.

In the closed position of the piston 12 (FIG Melting vessel maintained a positive pressure, however lower than the casting pressure. Herewith ensures that not by lowering the melt column in the riser 7 in the upper part of a negative pressure arises, which is about leaks to the ingress of atmospheric oxygen could lead.

In the embodiment of FIGS. 1 and 2 is further just below the mold bales and in contact with arranged a cooling plate 16, over which the bales run with the sprue 11, so that the melt in the Gully 11 is quickly solidified.

The embodiment according to FIGS. 3 and 4 differs from that shown in FIG. 1 essentially only in that Box molds 17 are poured off.

In the casting shown in FIGS. 2 and 3, the Melting column from the melt container 4 to the mold. In this case, the melt column by means of the form in Melt container with maximum casting speed in the Form displaced. Towards the end of the casting cycle, the piston 12 driven, the first of the mouth 18 (Fig. 2 and 3) of the inclined portion 8 of the riser in the vertical section 9 passes while the melt column shears off and at the same time the residual volume 14 in the vertical section 9 displaced into the mold.

In Fig. 5 is the time course of the casting speed and the change of the riser cross-section against Played at the end of the casting process. Is a mold in brought the casting position, the piston 12 is down drove so that the mouth 18 is released and the Melt from the melt container 4 via the riser 7, 8 in the vertical section 9 and finally this is displaced over the sprue 11 in the mold. there increases the speed of the melt column in the Curve section 20 corresponding to the form in the furnace very fast to the maximum casting speed 21 on. The riser cross-section or the mouth 18 is fully released, as with the curve section 30 is indicated. Towards the end of the casting process, in which still up to about 10% of the melt volume in the mold are missing, the piston 12 is driven. this happens in the diagram shown in Fig. 5 at 22. The riser cross-section then takes over by increasingly driving over the mouth 18 through the piston 12 quickly, like this the curve section 31 shows, until finally the piston 12 the melt column at the mouth 18 completely sheared off has, as indicated in the diagram at 32. The casting speed remains on the curve section 24 initially still at the maximum value and falls then with negative velocity gradient, and Although initially over a short period of time with a flat Decrease in the curve section 25, then in the Curve section 26 to drop steeply and finally turn flat again in the curve section 27 until at 28 of the gate 11 is closed by the piston 12. The course of the casting speed in the curve sections 24, 25, 26 and 27 are shown by way of example only and otherwise dependent on the casting geometry these adaptable.

Claims (13)

1. Process for uphill low pressure casting of metal, particularly light metal, in that a casting mould provided with at least one feeder is connected by its gate, via a riser to a melting container and the melt contained therein is displaced under pressure through the riser into the casting mould, characterized in that most of the melt volume necessary for filling the mould is displaced into the mould at a maximum casting speed, then the melting column in the riser is sheared and a residual volume of up to approximately 10% of the casting volume remaining between the shearing point and the gate is displaced with a negative speed gradient into the mould.
2. Process according to claim 1, characterized in that the residual volume is displaced into the mould with a non-constant speed gradient.
3. Process according to claim 1 or 2, characterized in that the residual volume is displaced into the mould initially at the maximum speed, subsequently with a flat and then with a steep negative speed gradient.
4. Process according to one of the claims 1 to 3, characterized in that the size of the residual volume is selected as a function of the volume and/or geometry of the casting.
5. Process according to one of the claims 1 to 4, characterized in that the speed gradient and/or its time course is selected as a function of the volume and/or geometry of the casting.
6. Process according to one of the claims 1 to 5, characterized in that 3 to 7 vol.% of the casting form the residual volume.
7. Apparatus for performing the process according to claim 1 with a conveyor for the cyclic movement of casting moulds with at least one feeder and a gate, a casting station at the conveyor on which the moulds are successively positionable, a melting container, particularly a melting furnace, postionable at the casting station, a riser immersed in the melt and to which the mould gate can be connected at the casting station and a pressure generator for the displacement of the melt from the melting container, via the riser and the gate into the mould, characterized in that at the transition between the riser (7, 8) and the gate (11) is located a tubular portion (9) forming the residual volume of up to approximately 10% of the casting volume, in which a controlled displacer (12) is movable between a casting position freeing the opening (18) of the riser (7, 8) in said portion (9), via an intermediate position closing the opening and with a negative speed gradient into an end position closing the gate (11).
8. Apparatus according to claim 7, characterized in that the tubular portion (9) is aligned with the gate (11).
9. Apparatus according to claim 7 or 8, characterized in that the speed of the displacer (12) in controllable.
10. Apparatus according to one of the claims 7 to 9, characterized in that the speed of the displacer (12) is controllable as a function of the geometry and/or volume of the casting.
11. Apparatus according to one of the claims 7 to 10, characterized in that at least the tubular portion (8) can be replaced by a portion with a different volume.
12. Apparatus according to one of the claims 7 to 10, characterized in that the tubular portion (9) can be extended by a further tubular portion.
13. Apparatus according to one of the claims 7 to 11, characterized in that the tubular portion (9) and displacer (12) are formed by a cylinder-piston unit and the riser (7, 8) issues laterally into the cylinder (9) close to the end position of the piston (12) remote from the gate.

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