EP0956916A1 - 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, especially light metal, by a mold provided with at least one feeder with her sprue on a riser pipe to one Melting container connected and the contained therein Melt under pressure through the riser pipe into the mold is ousted.

Light metals, especially aluminum, have in the Increasing past as a construction material Gained meaning. This also applies to motor vehicle construction, especially engine construction. So be in newer Time engine blocks made of aluminum. Due to the Large series production in the automotive industry must also Casting processes and casting plants with high performance for Will be provided. At the same time, in particular highly stressed components a high quality standard to be guaranteed. Because aluminum is mainly in the molten state spontaneously oxidized with atmospheric oxygen, forms open melt surfaces in the presence of oxygen an oxide skin very quickly.

To prevent this from happening as much as possible for example, both for mold casting and for Gravity die casting, especially in the Execution of the rising casting, proven, since this is the Melt not swirled, but the shape or mold is filled with a calm melt front. Thereby can largely include oxide inclusions in the casting be avoided.

With these typical measures for low pressure casting however, there is the disadvantage that performance is relatively small. This is one of the reasons attributable to the pressure in the melt vessel after each casting must be lowered, which in turn with the decline the melt column is connected in the riser. Here atmospheric oxygen from the environment enters the riser pipe on. It forms on the surface, albeit a small one an oxide skin on the melt column, which is The melt column rises to the next pour cycle Wall of the riser creates and is on the melt front always replicates. This increases the riser pipe gradually to. At high performance, this requires one regular replacement of the riser pipe in relatively short Time intervals, which in turn leads to a reduction in performance leads. Another crucial disadvantage is that the oxide skin forming on the surface of the melt column is brought 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 shape Provide closure. This has primarily that Task, swirls in the melting vessel, especially in the gas cushion above the melting level to avoid. The closure consists of one at the transition between the riser and the cast-in melting plate with a lower melting point than the aluminum melt. This closure plate is when the Melt liquefied. These liquid foreign components are introduced into the form and lead to the highest unwanted inclusions in the cast. The closure has to each cast can be replaced so that the performance is correspondingly low.

In another known method (WO 97/37797) a replenisher between the mouth of the riser pipe and the mold arranged in the form of a container through which through the melt from the riser into the mold is ousted. The container has a bottom side Slider lock on after filling the mold closes. After the casting, the mold is made together with the Make-up unit uncoupled from the riser pipe and the pouring shrinkage by feeding the melt from the replenisher in the shape balanced. This also allows the Performance compared to conventional low pressure casting machines not increase, especially since the replenisher for the next Casting cycle emptied and returned to the casting station must become.

Based on an older, unpublished proposal the applicant (DE 198 07 623) is at the end of each casting cycle the one reaching from the riser pipe into the mold Melting column near the mouth of the riser pipe below sheared at the same time. By the Shear the melt column near the mouth of the riser no free is formed above the melt column Volume and therefore no free surface on which an oxide skin could form. As a result, it can also do not attach an oxide layer to the riser and can not overgrow this. Through the closure is also ensured that none in the subsequent pouring cycle Oxide skin is introduced into the mold. Furthermore, from Advantage if after shearing the melt column in the Melting vessel at least one against the melt column Closure overpressure is maintained. This measure can reduce the performance of the casting machine increase.

The production of light metal casting in large numbers and high cast tonnage requires significantly higher specific Casting performance, previously in the range of 1 kg / s lie. To improve the profitability of the systems, shorter and shorter cycle times are aimed for. In order to there is also a demand for shorter casting times each Clock, or for equal partial masses the demand for higher melt throughput during mold filling. The with increasing pouring higher specific casting performances of 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 molded materials, due to the geometry of the castings and gates in narrow cross sections, but most strongly through the Unwanted dynamic filling surge at the end of the pour is limited. The dynamic surge at the end of the pour depends on the given Part geometry primarily from the venting behavior the shape (gas permeability in sand molds, ventilation holes etc.), but in any case on the casting speed at the end of the casting shortly before the complete Filling the form.

In the sand casting process, especially in the wet casting process Known surface defects on the castings, the predominant caused by the filling impact at the end of the casting. In The last phase of the mold filling is the kinetic Energy of the melt on the high-lying mold surfaces abruptly converted into impact energy. This dynamic push drives the melt into the pores of the sand surface and creates undesirable rough surfaces on the casting contour. Here, inhomogeneously compacted parts of the mold become affected differently. In In many cases, castings that are damaged in this way have to be taken along considerable cleaning effort reworked or as a committee be discarded.

Another effect of this surge leads to increased Burr formation in the division zones of the mold halves and Cores. Burr formation means elaborate cleaning work, which, depending on the type of part, accounts for over 30% of the production costs of the Can make cast part. Burr free or low burr Casting is therefore of great economic importance.

A third type of error is blistering due to Air and gas inclusions that occur during casting, predominantly, however, arise in the phase shortly before the end of casting. The casting gases are preventive via exhaust air ducts, by blowing a whistle, by using open feeders or use of molded materials with high gas permeability dissipated. Errors of the aforementioned kind already occur today's low casting rates of 1 to 2 kg / sec for aluminum alloys. Will the casting performance it is expected that the Gas bubble formation increases significantly. For the so far pouring various methods are known. So z. B. the use of a gas pressure cushion in Melting container or furnace, which the melt over the filled riser pipe. Instead of Gas pressure in the melting tank are also electromagnetic pumps used at the foot of the riser pipe, which the Take over the filling task. In both cases there is 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 In principle, methods are possible, the undesirable counteract dynamic filling surge towards the end of the mold filling. The increase in casting tonnage and thus the However, increasing the casting performance per casting cycle represents new ones Requirements for the known casting processes. Larger Melt throughputs inevitably lead to larger ones Melt or furnace batches and furnace volume because the Refill cycles because of the high demands on the Melt quality must not be too short. Size Furnace volumes make it more difficult especially with the gas pressure filling process the controllability of the filling process. These greater inertia of the melt leads to rapid Changes in furnace pressures to vibrate the metal mass in the oven. The guidance of the melt filling profile is thereby making it difficult or impossible. Especially for the end of casting the previously accelerated mass of the melt cannot be decelerated quickly enough and without vibrations.

The invention is based, with increasing the task Casting light metal has 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 that for filling the mold necessary melting volume with maximum casting speed displaced into the mold, then the melt column repelled in the riser and one between the shear point and the remaining volume with a negative velocity gradients in the mold is ousted.

In the method according to the invention, the filling process divided into two sections, the first section follows the usual process of increasing pouring, by the given free cross-section of the riser the melt column solely by driving on Melt container or furnace up to the maximum casting speed accelerated and this until shortly before the end of casting is maintained. Then the melt column sheared near the gate so that the maximum Maintain pouring speed for a short time until the melt column is completely sheared off is. The one remaining between the shear point and the gate The remaining volume will then decrease continuously Speed displaced into the mold, so that the Melt front rises and slows within the mold on the one hand, the remaining air in the mold is checked can escape, on the other hand a dynamic Avoid filling impact on the upper mold surfaces becomes. This also prevents that in Melt volume in the mold is too high a hydrostatic Pressure builds up.

The method according to the invention in particular in the case of sand molds, 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 get perfect casting surfaces become. Furthermore, by avoiding construction too high an isostatic pressure prevents the Melt penetrates into mold separation zones and to form burrs leads. By shearing off the remaining volume from the Melting column can be stirred from the melt container Vibrations cannot be transferred to the remaining volume. The remaining volume in turn becomes relative because of it low mass and the negative velocity gradient displaced into the shape without vibrations.

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

The method according to the invention gives in particular the Possibility of depending on the size of the remaining volume on the volume and / or the geometry of the casting choose. The larger the casting volume and the more involved the geometry of the casting is in the upper area, so the remaining volume will be larger.

Furthermore, the speed gradient and / or its temporal course depending on the volume and / or the geometry of the casting can be selected. Here comes with an intricate geometry, especially in upper area of the shape, faster degradation of the speed gradient considered than for large-volume and little intricate shapes.

The remaining volume should be about 10% of the volume of the casting make up, preferably it is 3 - 7 vol% of the casting.

The invention is based on the implementation of the method from a device with a transport track to cyclical Movement of the molds with at least one feeder and a gate, a pouring station on the conveyor, on which the molds can be positioned one after the other are a melt container that can be positioned at the casting station, especially melt furnace, one in the melt immersing riser pipe to which the Casting mold can be connected with its sprue, and one Pressure generator for displacing the melt from the melt container over the riser pipe and the pouring into the Mold.

Such a device is distinguished according to the invention characterized in that at the transition between the riser and Infeed into a tubular section forming the residual volume is arranged in which a controlled displacer between a the mouth of the riser in this section releasing pouring position over a die Intermediate position closing the mouth in a Sealing position is movable.

The melt column displaced in the riser pipe reaches with the maximum casting speed over the tubular Section and pouring into the mold. Towards the end of Pouring cycle, 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 is completely sheared off is so that the furnace side acting on them Driving forces no longer on that in the tubular section included residual volumes can act. This residual volume is then removed using the displacer the given negative speed gradient in the shape displaces until the displacer reaches its end position reached in which he closes the gate.

The tubular section is preferably aligned with the Sprue so that the residual volume translationally without any deflection and thus squeezed into the form in a way that is easy to control becomes. This can be done with vertical pouring at the bottom of the mold done just as favorably as with a horizontal one side gate.

The speed of the displacer is preferably in Dependence on the geometry and / or the volume of the Cast part controllable, possibly also by means of a Program can be specified.

Furthermore, at least the tubular section can counter a section interchangeable with another volume or be extendable by another section to the remaining volume to the geometry or the volume of the Optimally adapt the casting.

In a preferred embodiment, the tubular section and the displacer from a cylinder-piston unit formed and the riser pipe opens laterally into the cylinder close to the end position of the piston in the Cylinder.

If necessary, the complete cylinder-piston unit exchanged for one with a different 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 is described below with reference to exemplary embodiments shown in the drawing. The drawing shows:

Fig. 1

is a schematic view of a low-pressure casting system for the castless sand casting in a first operating position;

Fig. 2

1 in a further operating position;

Fig. 3

a schematic view of a low-pressure casting system for box casting in a first operating position;

Fig. 4

3 in a further operating position and

Fig. 5

a timing diagram for starting the casting process.

The casting plant designed according to the invention is both suitable for sand mold casting as well as permanent mold casting. The embodiments shown in the drawings concern only casting in sand molds (with or without molded box).

1 and 2 are the Molds as boxless bales 1 on one Conveyor path forming conveyor 2 in succession arranged. The shaped bales 1 are in the direction of Arrow 3 promoted to a casting station at the Melting vessel 4, optionally in the form of an oven, lifting and is arranged lowerable and movable. By doing The light metal melt 5 is located in the melt vessel 4, a gas cushion 6 stands over the melt surface thereof, which is acted upon by a controllable pressure generator. A total of 7 is dipped into the melting vessel 4 Riser pipe, which in this embodiment over a sloping section 8 and a short one rectilinear, tubular section 9 extends upwards and ends at a mouth 10. Above this mouth the form bales 1 are clocked by means of the conveyor 2, that a bale of mold in the pouring position at every cycle is coming. Then the device with the short vertical section 9 of the riser is adjusted so that the latter in alignment with the sprue 11 of the bale reached. In the tubular section 9 is a displacer arranged in the form of a piston 12 by means of a pressure medium cylinder 13 is actuated.

From the near-end position shown in Fig. 1, in the gate 11 is closed, the piston 12 in the other end position shown in Fig. 2 moves, in which it the short vertical section of the riser 7 completely releases so that under the effect of in the melt vessel 4 prevailing overpressure the melt into the mold reached. Before the mold is completely filled, the Move piston 12 back to the other end position (Fig. 1), in which he has the remaining volume in section 9 14 displaced the melt into the mold. The cast Mold bales leave the pouring station in the direction arrow 3.

In the closed position of the piston 12 (Fig. 1) Maintain an overpressure in the melt vessel, however can be lower than the casting pressure. Herewith it is ensured that not by lowering the melt column a negative pressure in the riser 7 in the upper part thereof that arises from leaks to break in atmospheric oxygen could lead.

1 and 2 is also in the embodiment immediately below the shaped bales and in contact with a cooling plate 16 is arranged over which the shaped bales run with the gate 11 so that the melt in Gate 11 is quickly solidified.

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

In the casting process shown in FIGS. 2 and 3, that is sufficient Melting column from the melt container 4 to the casting mold. Here, the melt column by means of the form in the Melt container with maximum casting speed in the Form displaced. At the end of the casting cycle, the piston 12 driven, the first of the mouth 18 (Fig. 2 and 3) of the inclined section 8 of the riser in the vertical section 9 runs over and thereby the melt column shears and at the same time the remaining volume 14 in the vertical section 9 displaced into the mold.

5 shows the course of the casting speed over time and the change in the riser cross-section against Played at the end of the casting process. Is a mold in brought the pouring position, the piston 12 is down driven so that the mouth 18 is released and the Melt from the melt container 4 via the riser pipe 7, 8 in the vertical section 9 and finally this is displaced into the mold via the gate 11. Here increases the speed of the melt column in the Curve section 20 corresponding to the form in the melting furnace very quickly to the maximum casting speed 21 on. The riser tube cross section or the mouth 18 is fully released like this with the curve section 30 is indicated. Towards the end of the casting process where still up to about 10% of the melt volume in the mold missing, the piston 12 is driven. this happens in the diagram shown in FIG. 5 at 22. The riser cross section then decreases by increasing overrun the mouth 18 through the piston 12 quickly like this curve section 31 shows until finally the piston 12 completely melted the melt column at the mouth 18 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 a negative speed gradient, and initially over a short period with a flat Decrease in curve section 25 to then in Curve section 26 to drop steeply and finally turn out flat in the curve section 27 until at 28 the gate 11 is closed by the piston 12. The course of the casting speed in the curve sections 24, 25, 26 and 27 is only shown as an example and otherwise dependent on the casting geometry these customizable.

Claims (14)

Process for increasing low pressure casting of Metal, especially light metal, by using a casting mold provided with at least one feeder their pouring over a riser pipe to a melt container connected and the contained therein Melt under pressure through the riser pipe into the Casting mold is displaced, characterized in that the vast majority of that for filling the mold necessary melt volume with maximum Casting speed pushed into the mold, then the melt column in the riser pipe sheared off and on remaining between the shear point and the gate Residual volume with a negative speed gradient is displaced into the mold. A method according to claim 1, characterized in that the residual volume with an inconsistent rate gradient is displaced into the mold. A method according to claim 1 or 2, characterized in that that the remaining volume initially with the maximum speed, then with a flat and then steeply negative speed gradients is displaced into the mold. Method according to one of claims 1 to 3, characterized characterized in that the size of the residual volume in Dependence on the volume and / or the geometry of the casting is chosen. Method according to one of claims 1 to 4, characterized characterized in that the speed gradient and / or its course over time depending the volume and / or the geometry of the casting is chosen. Method according to one of claims 1 to 5, characterized characterized in that up to about 10% of the volume of the Cast the remaining volume. Method according to one of claims 1 to 6, characterized characterized that 3 to 7 vol% of the casting Form residual volume. Device for performing the method according to Claim 1 with a transport track for cyclical Movement of molds with at least one feeder and a gate, a pouring station on the conveyor, on which the molds can be positioned one after the other are positioned at the casting station Melt containers, in particular melt furnaces, an ascending pipe immersed in the melt, at the casting station with the mold Infeed can be connected, and a pressure generator for Displacing the melt from the melt container via the riser pipe and the pouring into the mold, characterized in that at the transition between Riser pipe (7, 8) and sprue (11) the remaining volume forming tubular section (9) arranged in which a controlled displacer (12) between a mouth (18) of the riser pipe (7, 8) in this section (9) releasing casting position via an intermediate position closing the mouth in a final position closing the gate (11) is movable. Device according to one of claims 1 to 8, characterized characterized in that the tubular section (9) aligned with the gate (11). Device according to claim 8 or 9, characterized in that that the speed of the displacer (12) is controllable. Device according to one of claims 8 to 10, characterized in that the speed of the Displacer (12) depending on the geometry and / or the volume of the casting is controllable. Device according to one of claims 8 to 11, characterized in that at least the tubular Section (9) against a section with another Volume is interchangeable. Device according to one of claims 8 to 11, characterized in that the tubular portion (9) through another tubular section is renewable. Device according to one of claims 8 to 12, characterized in that the tubular portion (9) and the displacer (12) from a cylinder-piston unit are formed and the riser (7, 8) laterally in the cylinder (9) near the Pour far end position of the piston (12) in the cylinder (9) flows into.

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