Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-09-22
2001-07-03
Dunn, Tom (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S066100, C164S258000, C164S259000
Reexamination Certificate
active
06253828
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a method for casting metallic objects in a casting device for precision casting with at least one casting mold, one device for pouring liquid metal into the casting mold and with devices for generating vacuum and pressure and a casting device for carrying out this method.
By the term precision casting is understood the casting of jewelry, objects of art or decoration, as well as of precision hardware for the industry, in particular comprising gold, silver, platinum, bronze and other metals. Casting devices are known in which a melting crucible and a casting mold are disposed in a receptacle. The melting crucible is equipped with an inner cavity for receiving raw material and with a heating device, for example an electric induction device. As pouring means can be used the known possibilities with the known example comprising a bottom drainage with a stopper. Underneath the melting crucible is positioned the casting mold which comprises a gas-permeable porous material. The casting mold has a mold cavity which most often makes possible the casting of a multiplicity of discrete parts in the same casting step, i.e. the mold cavity has a tree structure with an inlet funnel. The production of the casting mold is most often carried out with the aid of a model comprising a synthetic material or wax and the mold can only be used once. The method for casting metallic objects in this known casting device is comprised of several steps. First, with the receptacle open the melting crucible is filled with raw material and the casting mold in set into the lower part of the receptacle. The lower part of the receptacle is separated from the upper part.
The upper receptacle part is connected to a device for generating excess pressure and the lower receptacle part with a device for generating a vacuum. To initiate the casting process the receptacle is closed gas-tight and the melting process is initiated in the melting crucible. By opening the stopper the liquid melt flows by casting from the top into the mold cavity of the casting mold until it is completely filled. Before and/or during the casting process the lower receptacle part in which the casting mold is disposed, is exposed to a negative pressure which also extends into the mold cavity due to the porosity of the casting mold material. At the end of the casting process, i.e. when the mold cavity is filled, an excess pressure is generated in the upper receptacle chamber such that this pressure acts also onto the surface level of the melt in the inlet gate of the casting mold. This combination of negative pressure acting onto the bottom and the shell of the casting mold and of excess pressure acting onto the melt in the mold cavity, compared to casting methods with bilaterally equal pressure, leads to better filling of the mold cavity and better molding of fine detail.
In spite of the casting results, which per se were good, with this known device, problems occur repeatedly, in particular in the case of complicated and delicate casting objects. The liquid melt can, for example, be distributed too slowly in the mold cavity. The consequence is that to some extent fine complicated branchings are not filled out or differing structures of the solidified metal occur since the cooling rate and the of solidification time are different in differing regions of the mold cavity.
SUMMARY OF THE INVENTION
It is therefore the task of the present invention to create a method and a device with which the molding accuracy and the degree of filling of the mold cavity can still be further increased and with which the structures of the cast objects are also improved.
This task is solved with a method according to the invention through the characterizing features of this and with a device according to the characterizing features of the invention. Advantageous further developments of the invention are evident based on the characteristics of the dependent patent claims.
In accordance with the method according to the invention, before the start of the casting process, i.e. before liquid melt is poured into the mold cavity, the mold cavity of the casting mold and the ambient space of the casting mold is exposed to negative pressure. Thereby the openings and passages in the most often partially porous and gas-permeable walls of the casting mold are evacuated and air or other gas residues are aspirated from these porous openings. The mold cavity and the ambient space of the casting mold is subsequently flushed with a light gas of low density which yields the advantage that this gas penetrates into the pores in the walls of the casting mold and fills it. As the light gas is chosen which in the periodic table of elements has an atomic number between 1 and 10 and which causes a throughflow rate of maximum possible magnitude of this gas through the pores in the wall of the casting mold. Especially suitable gas in this group is helium. After flushing the mold cavity and the ambient space of the casting mold with this light gas, a negative pressure is again generated at least in the mold cavity and subsequently the liquid melt is filled into the mold cavity. This filling process now takes place extraordinarily rapidly since the light gas, for example helium, is readily and rapidly displaced through the pores in the wall of the casting mold and can flow off toward the outside. This advantage is caused by the high throughflow rate of the selected light gas through pores and capillary openings. The advantage with respect to the casting process comprises that in the individual regions of the mold cavity and between the inflowing liquid metal no partial excess pressure is being built up so that the liquid metal can flow rapidly and unhindered into extremely fine branchings of the mold cavity. Thereby alone an improved accuracy of shape and increased casting rate is attained. One consequence is also the fact that in all regions in the mold cavity a better structure for the cast object is produced. As soon as the mold cavity is completely filled with liquid metal, the surface level of the melt in the inlet region of the mold cavity is acted upon with another heavy gas of greater density. Relative to the ambient space of the casting mold this gas has an excess pressure. As the heavy gas with greater density therein a gas is selected which in the periodic table of elements has an atomic number of at least 7 and in any event a higher atomic number than the light gas with which the flushing had been carried out in the preceding process step. The heavy gas can also be a gas mixture with the same properties. An especially suitable gas in this group is argon since it has the property of flowing only at a relatively low throughflow rate through the pores of the wall of the casting mold. Experiments have shown that the pressure equilibration between the the inner wall of the casting mold and the outer wall of the casting in the event one side is acted upon with argon, takes place 8-10 times slower than in the case in which helium is used. This yields the advantage that the liquid melt in the mold cavity of the casting mold can be exposed to an increased pressure without the negative pressure in the ambient space of the casting mold being markedly reduced. This leads to an even better filling of the mold cavities and and improved structure of the cast objects.
These advantages of the described method according to the invention are attained through a casting device which comprises two sources each for a different gas with different density. Further advantages can be obtained if the casting mold is disposed in a first gastight receptacle and the melting crucible and the pouring means are disposed in a second receptacle separate from the first. The two receptacles are connected via connection lines and control valves with the first, or second, gas source and pumps are available for generating a partial negative pressure or excess pressure, as well as corresponding control devices. If in the region between the pouring opening on the m
Dunn Tom
Notaro & Michalos P.C.
Tram Lan
Yasui Shouzui
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