Patent Application
20050000676
The present invention relates to an apparatus for forming metal castings, particularly with hydraulic connection between the holding furnace and the mold and to a method for forming metal castings performed by the apparatus.
As is known, castings made of metal or light alloys are molded by using pouring processes, such as for example the gravity feeding of the molten metal or alloy into closed molds, and the generation of pressure inside the mold by way of feedheads, which feed the molten casting, increasing the hydrostatic load on the metal or alloy, and prevent the formation of cavities or porosities inside the casting during shrinkage in the solidification step.
Another method for forming castings by pouring metal or alloys is so-called low-pressure casting, which substantially consists in placing a mold over a closed furnace whose cavity is fed through a riser tube connected to the furnace, inside which pressure is applied to the free surface of the metal so that part of said metal rises through the riser tube and feeds the mold.
However, in practice, known casting techniques are not always suitable to ensure the formation of castings that are free from structural defects, for example internal tensions and microporosities, especially in the case of the formation of castings having a complex shape, such as wheels of road vehicles made of light alloy and the like.
In particular, for the low-pressure casting technique the problems are linked, most of all, to poor feeding of the cavity of the mold, especially in the case of castings that have a complex geometry, which generates porosities that can be detected by X-rays as a consequence of the turbulent motions of the metal and of overheating.
After the solidification step, moreover, the furnace below the mold is depressurized and because of this operation the column of molten metal inside the riser tube, initially at the level of the mold, returns to the level of the metal that is present inside the furnace.
During this descent, a fraction of the metal remains stuck along the inner wall of the riser tube, forming films of solidified metal and oxides.
This problem gradually increases, since during production the level of the metal inside the furnace decreases, the path followed by the aluminum during filling increases, and accordingly the exposed surface inside the riser tube increases, generating a greater quantity of films of solidified metal and oxides.
Films and oxides are of course entrained by the metal into the mold, forming a non-uniform alloy and detracting from the final quality of the provided casting.
In order to generate the pressure that allows the rise of the metal through the riser tube, a gas is injected into the furnace; said gas is usually air, which generates oxides owing to the presence of oxygen.
Furthermore, when the metal inside the furnace has been used up, the furnace must be opened to allow to fill it, exposing it fully to contact with air.
Moreover, the low-pressure casting process is a substantially discontinuous process, since it entails downtimes in the production step due to the time required to fill the riser tube and to the steps for filling said furnace.
In particular, the problems linked to the furnace are due substantially to the holding times for filling it and to the variation of the internal pressure during said filling.
Moreover, it is currently difficult to control the quantity of metal that is injected into the cavity of the mold.
The aim of the present invention is to solve or substantially reduce the problems of known types of apparatus for forming metal castings.
Within this aim, an object of the invention is to provide an apparatus that comprises a constantly loaded hydraulic connection between the holding furnace and the mold and in which the molten metal never makes contact with the air.
Another object is to provide an apparatus that eliminates the upward and downward flows of the metal along the riser tube, eliminating the formation of oxides and films of solidified metal.
Another object is to provide an apparatus that allows safe control of the level of metal that is present under the mold.
Another object is to provide an apparatus by way of which it is possible to provide a continuous casting cycle, eliminating the downtimes for loading the furnace connected to the riser tube.
Another object is to provide an apparatus by means of which the molten metal is exposed to air as little as possible, consequently avoiding the formation of oxides.
Another object is to provide an apparatus that allows to provide metal castings without limitations to the operating pressure except for those dictated by the technological limitations of the apparatus itself.
Another object is to provide an apparatus that offers high productivity.
Another object is to provide an apparatus that allows better feeding of the cavity of the mold.
Another object is to provide an apparatus that allows to obtain improved mechanical characteristics of the solidified casting.
Another object is to provide an apparatus that does not use the pressure of air or gas to move the metal and feed the mold.
This aim and these and other objects that will become better apparent hereinafter are achieved by an apparatus for forming metal castings, particularly with hydraulic connection between the holding furnace and the mold, characterized in that said hydraulic connection comprises:
The apparatus performs a method for forming metal castings that consists in:
Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein:
With reference to the figures, an apparatus for forming metal castings, particularly with hydraulic connection between the holding furnace and the mold, according to the invention, is generally designated by the reference numeral 10.
The apparatus 10 comprises a hydraulic connection between a holding furnace 11, which contains the molten metal ready for forming, and a mold 14, which in turn comprises a duct 36 for connecting the holding furnace 11 to one or more inlets 12 for introducing the metal into a the cavity 13 of the mold 14, said one or more inlets 12 being arranged, in this case, under said mold 14, and pumping means 15, which are interposed between portions of said duct 36, for pumping a controlled quantity of metal through one or more inlets 12 into said cavity 13, said means being connected to and controlled by an electronic control unit 16.
As shown in
The inlet 12 is formed by a distribution unit 17, which is arranged below the mold 14 and is connected to the upper end of a riser tube 18.
Induction-type measurement devices 19 for measuring the level of the metal present inside the distribution unit 17 are arranged adjacent and below the inlet 12, at the distribution unit 17, and are connected and managed by the control unit 16, as better described hereinafter.
Advantageously, the inlets 12 can also be arranged laterally to the mold 14.
The duct 36 comprises a first portion 20, which extends from the holding furnace 11 and is connected to the pumping means 15, from which a second portion 21 protrudes and leads into a reservoir 22.
The riser tube 18 rises from the reservoir 22, and the distribution unit 17 is arranged above it.
The apparatus 10 further comprises devices 23 for heating the molten metal and temperature sensors 24 for sensing the temperature of said metal, which are arranged along the hydraulic connection and are connected to the control unit 16.
The heating devices 23 are constituted by heating resistors, designated by the same reference numeral, which are arranged inside longitudinally elongated ceramic containers arranged at the pumping means 15, at the first portion 20 and second portion 21 of the duct 36, at the reservoir 22 and at the distribution unit 16.
The temperature sensors 24 are instead constituted by thermocouples, designated by the same reference numeral, which are also arranged on the apparatus 10, like the heating devices 23.
The pumping means 15 are constituted by a piston pump 25, upstream and downstream of which there are a first valve 26 and a second valve 27 for controlling the flow of metal that flows respectively within said first and second portions 20 and 21 of the duct 36.
The first and second valves 26 and 27 are designed to provide controlled one-way flow of the metal in the direction from the mold 14 to the holding furnace 11.
The piston pump 25 comprises a first piston 28, which can slide hermetically in a corresponding first cylinder 29, which is fixed to the head of a first stem 30 of a hydraulic actuator 31, whose stroke is controlled by the control unit 16.
The first and second valves 26 and 27 are fully identical, and each valve comprises a second piston 32 that can slide hermetically within a corresponding second cylinder 33, which is fixed to the head of a second stem 34 of a corresponding pneumatic actuator 35.
As shown in
In particular, the first valve 26 (like the second valve 27) is arranged between a first part 20a and a second part 20b of the first portion 20 of the duct 36, said parts being arranged with a longitudinal axis at different heights.
The second piston 32, once lowered, affects the inlet of the second part 20b, whose diameter is smaller than that of the second cylinder 33, and the outlet of the first part 20a, which lies substantially on a horizontal plane and is coaxial to the second piston 32.
As regards operation, when the piston pump 25 is fully loaded, with the first valve 26 closed and the second valve 27 open, the mold 14 is closed so as to connect the cavity 13 to the inlet 12.
The induction measurement devices 19 can sense temperatures and sense that the level of the metal is close below the inlet 12.
At this point, the control unit 16 actuates the advancement of the first cylinder 29 of the piston pump, which allows the injection of the metal into the cavity 13.
The cycle for forming the casting, which in this case is a road vehicle wheel, begins and consists in the steps of filling, feeding, cooling and solidification; said cycle is allowed by the injection of a preset quantity of material into the cavity 13 following the movement of the piston pump 25 actuated by the control unit 16.
During the solidification step, the piston pump 25 continues to feed the cavity 13, keeping the metal under pressure.
At the end of the solidification step, the control unit 16 actuates the piston pump 25 so that it retracts with the first cylinder 29, so as to lower the level of the metal with respect to the inlet 12 by a preset value, for example 5-6 mm, making it separate from the casting.
At the end of the formation of the casting, the mold 14 is opened and the casting is removed, while the level of the aluminum is kept always directly below the inlets 12.
The piston pump 25 must now be reloaded, and this occurs by drawing back the first stem 30 when the second valve 27 is closed and the first valve 26 is open, drawing metal from the holding furnace 11.
Once the piston pump 25 has drawn a sufficient quantity of metal, the cycle can resume.
Advantageously, a plurality of ducts 36 for corresponding molds can be associated with the holding furnace 11 in order to increase the productivity of said apparatus.
Since the apparatus 10 is provided with a plurality of heating devices 23, this ensures that the metal always remains liquid, at a temperature that is controlled by the temperature sensors 24.
The reservoir 22, moreover, is designed to keep the metal at a chosen temperature, which also is determined by heating devices 23 and is measured by the temperature sensors 24.
The quantity of metal contained in the reservoir is in fact such as to ensure the dissipation of any higher temperatures acquired by the metal in the path from the piston pump 25 to said reservoir 22.
In practice, the purpose of the reservoir 22 is to make the metal or alloy more homogeneous before entering the mold 14.
In practice it has been found that the present invention has achieved the intended aim and objects.
An apparatus has in fact been provided which allows to use a hydraulic system that is always loaded, a level of the metal that is always present at the lower level of the mold, with a closed hydraulic circuit that avoids the exposure of the metal to the air, avoiding the formation of oxides.
The metal injected into the cavity is therefore more homogeneous and allows to obtain a solidified casting whose mechanical characteristics are improved with respect to known apparatuses, using even very high operating pressures.
Moreover, the times for moving the material are reduced, the idle times for loading the furnace arranged below the mold are eliminated, and there is a considerable increase in production, since said production occurs in a continuous cycle.
The present invention is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.
The technical details may be replaced with other technically equivalent elements.
The materials, so long as they are compatible with the contingent use, as well as the dimensions, may be any according to requirements.
The disclosures in Italian Patent Application No. PD2001A000301 from which this application claims priority are incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| PD2001A000301 | Dec 2001 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP02/14496 | 12/18/2002 | WO |
