Metal founding – Control means responsive to or actuated by means sensing or... – Control of feed material enroute to shaping area
Reexamination Certificate
1999-10-08
2001-06-26
Dunn, Tom (Department: 1722)
Metal founding
Control means responsive to or actuated by means sensing or...
Control of feed material enroute to shaping area
C164S004100, C164S457000
Reexamination Certificate
active
06250367
ABSTRACT:
This invention relates to a casting machine for casting small articles, such as dental articles and personal ornaments, and also to an apparatus for timing the pouring of molten metal into a die of a casting machine.
BACKGROUND OF THE INVENTION
Molten metals to be cast have their own proper timings when they should be poured into a die. If molten metal is poured in a die at a time earlier than its proper pouring timing, its viscosity is too high to spread over the entire cavity in the die, so that articles cannot be cast with precision. On the other hand, if the metal is poured later than the proper pouring timing, the casting temperature is so high that the metal may be evaporated, oxidized or degraded in composition. In addition, when the metal is poured into the die, it may stick to the die because of its high temperature. Like this, the timing of pouring molten metal into the die is critical to the quality of cast articles.
Conventionally, the time at which a molten metal should be poured into a die is determined by artisans, who monitors, by eyes, the metal being melted for minute vibrations, flow, deformation, glow, color etc. of the metal, to determine when the viscosity of the entire molten metal has decreased to a viscosity suitable for pouring the metal into the die.
The proper timing of the pouring of a metal into a die is correlated to the surface temperature of the molten metal. Therefore, it has been proposed to use an infrared radiation thermometer for measuring the surface temperature of a mass of molten metal to time the pouring of the metal. It is, however, very hard to detect an accurate surface temperature of a molten metal mass with an infrared radiation thermometer because of various reasons including the following ones. First, the amount of infrared radiation emitted differs from metal to metal. In addition, for a particular metal, the surface state of the molten metal mass changes from time to time, so that the amount of infrared radiation varies from time to time, too. Furthermore, from the time at which the metal starts melting and its viscosity starts decreasing, metal films, such as an oxide film, are formed to partly cover the surface of the molten metal mass and move on the surface, which causes the amount of emission of infrared radiation detected by the thermometer to randomly vary. Also, some metals may evaporate, and the evaporated metal gas and other gas may absorb or attenuate the emitted infrared light.
Fresh metal is not always used in casting, but metal obtained by cutting off unnecessary portions of a completed cast article may be recycled. Such recycled metal has a thick oxide film on its surface, which prevents detection of correct surface temperature of the molten metal. In addition, since an infrared radiation thermometer measures the temperature only at a small point on the surface of the molten metal mass, it is not possible to know the temperature of the molten metal as a whole. In other words, it is difficult to determine when the whole molten metal attains its proper pouring temperature, with the viscosity decreased to an appropriate value.
For the reasons as above stated, when an infrared radiation thermometer is used to determine the surface temperature of molten metal, a large error may result in measured temperature, which, in turn, may result in erroneous determination of the timing of pouring of the metal into a die. Thus, an infrared radiation thermometer is not always useable to precisely time the pouring of various metals under various melting conditions.
Another possible method to determine the optimum time for pouring may be to compare the shape of a mass of metal exhibited when it is heated and melted to flow with the shape of the mass of the metal when it is solid. However, this method is not applicable to some metals and recycled metals since they have a thick or hard oxide film on their surfaces, and, therefore, the shape or appearance changes only little even when the interior has melted and liquefied enough. This may cause the metals to be heated more than necessary, leading to defective casting.
Another problem in prior art is that when a plurality of solid lumps of metal are placed in a vessel for melting, they may melt in different times and in different ways, and, therefore, it is not possible or difficult to determine when all the metal lumps have melted into a uniform molten mass only from shape or appearance changes.
Because of the problems described above, it was very difficult to realize a reliable automated casting machine which can properly operate for different melting conditions.
An object of the present invention, therefore, is to provide an apparatus for determining a proper timing of pouring molten metal and a casting machine with such pouring timing determination apparatus.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, an apparatus for timing the pouring of metal into a die is provided, which includes a melting vessel for receiving a metal material therein. Heating means heats the melting vessel by RF (radio frequency) induction heating with a RF (radio frequency) signal amplitude-modulated with a low frequency signal. A light receiver receives light emitted by the metal material in the melting vessel and develops a received-light-representative signal. Frequency component extracting means extracts a frequency component having a frequency above the modulating low frequency. A comparator develops an output signal when the output of the frequency-component extracting means exceeds a reference signal.
According to the first aspect of this invention, a RF signal for RF induction heating metal in a melting vessel is amplitude-modulated with a low frequency signal. The low frequency signal may have a sinusoidal waveform. It may have any of other waveforms, such as rectangular and triangular waveforms. In melting a lump or mass of metal by RF induction heating with an induction heating current above a predetermined value, when the metal begins to melt, the molten metal mass has a spherical shape due to the electromagnetic force acting thereon. The molten metal within the spherical metal mass is stirred, so that the entire metal mass can uniformly melt. On the other hand, if the induction heating current decreases below the predetermined value, the spherical shape of the molten metal mass collapses to have a flat surface, so that it appears as if water were in a crucible. In other words, by varying the magnitude of the RF induction heating current, the shape of the mass of molten metal with a reduced viscosity can be changed. When metal starts melting and its viscosity in a surface portion starts decreasing, the shape of the molten metal mass starts changing slightly in synchronization with the low frequency signal. As the melting advances inward of the melt, the change in shape of the molten metal mass becomes larger.
As the temperature of the metal in the melting vessel rises, the amount of light emitted by the molten metal increases in proportion to the temperature. An optical detector receives the emitted light and develops a received-light-representative signal representing the amount of light received. Since the metal in the melting vessel is heated with the RF signal amplitude-modulated with the low frequency signal, the temperature of the molten metal exhibits minute changes in accordance with the low frequency signal, which results in a small change in the amount of light emitted by the molten metal. The change is extracted by the frequency component extracting means. As the metal is melted and liquefied, the shape of the molten metal mass changes largely in synchronism with the low frequency signal, which causes the distance between the optical detector and the surface of the metal mass to change. The amount of light received by the optical detector changes largely due to the effect of the combination of the variations in amount of emitted light with the variations in distance, and, therefore, the output signal of the frequency component extracting means largely v
Komuro Yoshiaki
Tachibana Hidehisa
Duane Morris & Heckscher LLP
Dunn Tom
Sansha Electric Manufacturing Company Limited
Tran Len
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