Abstract:
A melt filling pressure difference control method controls a pressure difference used to supply melt from a holding furnace to a cavity of a casting machine by generating a pressure difference between an interior space of holding furnace and the cavity formed inside the mold. The method includes the steps of setting up a program pattern comprising time-varying characteristics of pressure difference target values, controlling the pressure difference so as to follow the program pattern that was set up, detecting whether the melt surface has risen to a predetermined level inside the cavity, compensating the program pattern based on the melt surface level when the melt surface has risen to a predetermined level inside cavity, and controlling the pressure difference between space inside the holding furnace and the cavity so as to follow the compensated program pattern.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a casting technique. In particular, it relates to a technique for filling a cavity with melt (i.e. molten metal). A casting machine employing this invention is equipped with a holding furnace that stores the melt, a mold with a cavity formed in its interior, a melt duct that interconnects the holding furnace and the cavity, and a device that generates a pressure difference between the pressure inside the holding furnace and the pressure inside the cavity, and is characterized in that said cavity is filled by the melt inside said holding furnace by way of said melt duct due to this pressure difference. 
     2. Prior Art 
     Prior art that relates to this is disclosed in Japanese Laid-Open Patent Publication JP-A-59-10461, and FIG. 4 shows a schematic view of a casting machine that employs this method. 
     This low-pressure casting machine  1  is equipped with holding furnace  6  that stores the melt, and mold  3  positioned directly above this holding furnace  6  by fixing plate  2 . Cavity  4  is formed in the interior of mold  3 . A tubular melt duct  5  is connected to mouth piece  4   h  of said mold  3 , and interconnects cavity  4  formed inside mold  3  with the interior of holding furnace  6 . Here, said cavity  4  is released to atmospheric pressure via exhaust ducts (not illustrated), while on the other hand said holding furnace  6  is sealed and compressed air is supplied to the interior thereof by compressor  7 . It is thus possible to generate a pressure difference between the pressure inside cavity  4  and the pressure inside holding furnace  6 . 
     Said compressor  7  is made able to vary (increase) the pressure in said holding furnace  6  according to a prescribed pattern, and this variation of pressure causes the melt inside holding furnace  6  to be filled into cavity  4  through melt duct  5 . Here, the difference in level between the surface of the melt filled in cavity  4  through said melt duct  5  and the surface of the melt inside holding furnace  6  is proportional to the pressure difference between the pressure inside the cavity and the pressure inside the holding furnace. It is thus possible to control the surface level of the melt filled into cavity  4  by controlling the pressure inside holding furnace  6 . It is also possible to control the rate at which the melt rises by raising the pressure inside holding furnace  6  according to a prescribed pattern. 
     The pressure control method in this low-pressure casting machine  1  establishes a three-tier pressure pattern that is divided between the period during which the melt rises through melt duct  5  to the entrance of cavity  4 , the period during which the melt is filled into cavity  4 , and a feeder head pressurizing stage. 
     That is, in the stage during which the melt is supplied as far as the entrance of cavity  4 , solenoid valves  8   a  and  8   b  of compressor  7  are opened and a large amount of compressed air flows into holding furnace  6  through pipelines  9   a  and  9   b . Accordingly, the pressure inside holding furnace  6  rises quickly and the melt rises up at high speed inside melt duct  5  to arrive at the entrance of cavity  4 . Next, when the pressure in said holding furnace reaches a first prescribed pressure, the melt surface is considered to have risen to the entrance of cavity  4  and solenoid valve  8   b  is closed. Consequently, compressed air is only supplied to holding furnace  6  through pipeline  9   b , and the rate of pressure increase inside holding furnace  6  is relaxed by the drop in the compressed air supply rate. As a result, the melt is slowly filled into cavity  4 . Then, when the pressure inside holding furnace  6  reaches a second prescribed pressure, cavity  4  is deemed to have been filled with melt, and solenoid valve  8   c  is opened. Consequently, compressed air is supplied to holding furnace  6  through pipelines  9   b  and  9   c , and the pressure rises quickly again so that the melt inside cavity  4  is subjected to feeder head pressurizing. 
     As mentioned above, in low-pressure casting machine  1 , the melt surface is considered to have arrived at the entrance of cavity  4  when the pressure inside holding furnace  6  has reached a first prescribed pressure, whereupon the pattern of pressure increase is changed into a relaxed pattern. That is, the increase in pressure per unit time is reduced when it has reached the first prescribed pressure. Also, cavity  4  is considered to have filled up with melt when it has reached a second prescribed pressure, whereupon the pattern of pressure increase is changed into a steep pattern. That is, the increase in pressure per unit time is increased when it has reached the second prescribed pressure. 
     However, the occurrence of phenomena such as back pressure in cavity  4  and variation in the melt surface level in holding furnace  6  arising from a variation in the amount of melt stored in holding furnace  6  can result in the melt surface not actually reaching the prescribed positions when the pressure in holding furnace  6  has reached the first or second prescribed pressure. Conversely, it is also possible that the actual melt surface will rise above the prescribed positions. 
     In such situations, if operations are continued according to the pattern of pressure increase set initially, it will become impossible to change the pattern of pressure increase at the point where the melt surface has actually reached the entrance of cavity  4  and at the point where cavity  4  has actually been filled with the melt. Therefore, this can give rise to defects whereby, for example, air is mixed in with the melt by filling cavity  4  at high speed when it should be filled slowly, or conversely whereby the melt temperature drops due to the melt surface being brought up slowly inside melt duct  5  when it should be brought up at high speed. Also, if the feeder head pressure after filling is insufficient, problems such as pipes in the moldings can Occur. 
     SUMMARY OF THE INVENTION 
     The present invention addresses itself to the technical problem of actually measuring the melt surface inside the cavity and compensating a preset pattern of pressure increase inside the holding furnace based on the result of this measurement, thereby filling the cavity with melt at an appropriate speed and achieving a satisfactory feeder head pressure after filling with melt by applying a suitable pattern of pressure increase inside the holding furnace. 
     Note that in the above-mentioned example, the interior of the holding furnace is pressurized to generate a pressure difference between the interior of the cavity and said holding furnace. However, it is also possible to fill the cavity with melt by reducing the pressure inside the cavity instead. Alternatively, the cavity can be filled by reducing the pressure inside the cavity and increasing the pressure inside the holding furnace. In any case, the surface level of the melt filled into the cavity is controlled according to the pressure difference between the pressure in said cavity and the pressure in said holding furnace. 
     The present invention is used in a casting machine. A casting machine employing this invention is equipped with a holding furnace that stores the melt, a mold with a cavity formed in its interior, a melt duct that inter connects the holding furnace and the cavity, and a device that generates a pressure difference between the pressure inside the holding furnace and the pressure inside the cavitey, and is characterized in that said cavity is filled by the melt inside said holding furnace by way of said melt duct due to this pressure difference. 
     The method of this invention is a method for controling said pressure difference in order to fill said melt into said cavity, and includes the steps of setting up a pressure difference control program defining target values for the rate of pressure difference increase after corresponding times have elapsed; based on this pressure difference control program and the actuale lapsed time, adjusting the actual rate at which the pressure difference rises to the target value of the rate at which the pressure difference rises at that time; detecting the time at which the melt surface reaches a predetermined level inside said cavity; and correcting the elapsed times in said control program based on the time detected in this time detection step. 
     The pressure difference control program, for instance, has a target rate of pressure difference increase of 1 kg/cm 2 •min until 3 minutes have elapsed after the filling operation is started, and the target rate of pressure difference increase after 3 minutes have elapsed and before 5 minutes have elapsed is defined as 0.5 kg/cm 2 •min. In general, the target rate of pressure difference increase is controlled according to the elapsed time by the pressure difference control program. This pressure difference control program it set up according to the relationship whereby, under normal conditions, the melt is satisfactorily filled into the cavity. For example, in the example mentioned above, if the pressure is made to rise by 1 kg/cm 2  each minute for the first 3 minutes after the filling operation is started, then it is presumed that under normal conditions the melt will have reached the bottom level of the cavity after 3 minutes have elapsed, whereafter the rate of pressure increase per unit time is reduced to a rate of 0.5 kg/cm 2 •min. 
     As mentioned above, this presumption may not hold true in actual operations. For example, after 3 minutes the melt may have already begun to fill the cavity before a pressure difference of 3 kg/cm 2  is reached, or conversely the melt may not yet have reached the bottom level of the cavity. In the present invention, the time at which the melt surface has risen to a predetermined level is detected. The time at which this is detected may be, for example, 2.5 minutes, which is ahead of schedule (Example 1), 3.0 minutes, which is on-schedule (Example 2), or 3.5 minutes, which is behind schedule (Example 3). Therefore, in this invention the lapsed times in the pressure difference control program are corrected according to the timing detected in this way. For example, in the case of Example 1 above, the elapsed time of 3 minutes in the pressure difference control program is corrected to 2.5 minutes. On the other hand, in the case of Example 3, the elapsed time of 3 minutes is corrected to 3.5 minutes. Note that the same results can be achieved by, in Example 1, correcting the actual elapsed time of 2.5 minutes to the value of 3 minutes in the control program and, in Example 3, correcting the actual elapsed time of 3.5 minutes to the value of 3 minutes in the control program, since this approach is mathematically identical. 
     One embodiment of the method of the invention includes the steps of: detecting a first timing at which the melt has risen to said bottom end of said cavity; reducing the rate of said pressure difference increase when this first timing is detected; detecting a second timing at which the melt has risen to said top end of said cavity; and increasing the state of said pressure difference increase when this second timing is detected. 
     With this method, the melt is made to rise quickly during the period when the surface of the melt rises up to the bottom of the cavity and slowly during the period when the surface of the melt is at a position inside the cavity, and the pressure difference is quickly increased after the cavity has been filled. 
     The invention can be understood in greater detail by reading the text of the following embodiments and claims with reference to the figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall cross-section of a low-pressure casting machine used to implement a melt-filling pressure-difference control method relating to an embodiment of the present invention. 
     FIG. 2 shows an example of the pattern of a pressure difference control program along with that of a corrected pressure difference control program in a melt-filling pressure-difference control method relating to an embodiment of the present invention. 
     FIG. 3 shows another example of the pattern of a pressure difference control program along with that of a corrected pressure difference control program in a melt-filling pressure-difference control method relating to an embodiment of the present invention. 
     FIG. 4 is an overall cross-section of a low-pressure casting machine used to implement a conventional melt-filling pressure-difference control method. 
     FIG. 5 is a detailed cross-section of the installation of a melt surface detection sensor used in the present invention. 
     FIG. 6 is a circuit diagram of a melt surface detector device used in the present invention. 
     FIG. 7 is a cross-section showing the overall mold of a casting device. 
     FIG. 8 is another example of the Circuit diagram of a melt surface detector device used in the present invention. 
     FIG. 9 is a graph showing the change in electrical resistance between the electrode and the mold in the interval between the start and finish of casting. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A pressure-difference control method for melt filling relating to an embodiment of the present invention is now described based on FIGS. 1 to  3 . FIG. 1 is an overall cross-section of a low-pressure casting machine  10  used to implement a melt-filling pressure-difference control method relating to the present embodiment. 
     Said low-pressure casting machine  10  is provided with holding furnace  16  which stores a molten metal such as aluminum (referred to as the melt hereinafter), and mold  13  positioned directly above this holding furnace  16  by fixing plate  12 , and a tubular stalk  15  (melt duct) is connected to mouth part  14   h  of said mold  13 . Said stalk  15  passes through opening  12   k  formed in the center of said fixing plate  12 , and is supported hanging down from fixing plate  12  with its lower end immersed in the melt stored in said holding furnace  16 . 
     Said holding furnace  16  comprises crucible  16   r  which stores the melt, and casing  16   c  which houses this crucible  16   r  and keeps it hot by means of a heater (not illustrated), and the top opening of said crucible  16   r  is closed off by said fixing plate  12 . Also, a melt inlet  18 , through which melt is supplied into said crucible  16   r , is provided at an inclined angle at the end of said fixing plate  12  (left of center in the figure), and a pressure sensor  18   p  for detecting the pressure inside crucible  16  is fitted at the position of this melt inlet  18 . The pressure signal from said pressure sensor  18   p  is input to control device  20 , which comprises a microprocessor. Note that said melt inlet  18  is closed off by cover  18   h  after supplying melt into crucible  16   r , and thus said pressure sensor  18   p  is able to accurately measure the pressure inside holding furnace  16 . 
     Also, a pressurizing pipeline  19  for pressurizing the interior of holding furnace  16  is connected to said melt inlet  18 . Said pressurizing pipeline  19  is a pipeline for guiding compressed air from a compressor (not illustrated) to the inside of holding furnace  16 , and is fitted along the way with reducing valve  19   r  and flow control valve  19   c  situated downstream thereof. Here, said flow control valve  19   c  is remotely operated by means of operating signals from said control device  20  to control the pressure inside holding furnace  16 , as mentioned below. Also, an exhaust valve  19   b  for exhausting the air inside holding furnace  16  is attached downstream of said flow control valve  19   c . Note that exhaust valve  19   b  is normally closed. 
     Said mold  13  comprises cope  13   u  and drag  13   d , which form cavity  14  when fastened together. Cavity  14  is interconnected with the atmosphere via exhaust ducts (not illustrated). Also, an upper melt level detection sensor  14   a  is fitted to cope  13   u  of said mold  13  at the top level of cavity  14 , and a lower melt level detection sensor  14   b  is fitted to drag  13   d  at the bottom level of cavity  14  (the top level Kb of mouth piece  14   h ). The melt level detection signals from upper melt level detection sensor  14   a  and lower melt level detection sensor  14   b  are input to said control device  20 . 
     Said control device  20  stores a pressure control program that determines the time-varying characteristics of the target rate of pressure increase in order to vary the pressure inside holding furnace  16  with time. This program determines target values for the rate of pressure increase with respect to the elapsed time; an example of a pattern produced by this program is shown by the solid lines (pattern P 0 ) in FIGS. 2 and 3. Note that said pressure control program can be inputted to the control device  20  from an input device (not illustrated), and can be revised. The orifice size of flow control valve  19   c  is controlled so that the pressure inside holding furnace  16  follows pattern P 0  of said pressure control program, That is, the pressure control program defines a rate of pressure increase per unit time corresponding to each elapsed time. 
     In pattern P 0  shown in FIG. 2, point S is the time at which the pressurizing of holding furnace  16  begins, and point a 0  is the time at which the pressure inside holding furnace  16  reaches pressure A 0 , at which it should be possible to bring the melt surface up to the entrance (bottom level) Ka of mouth part  14   h  of mold  13  (See FIG.  1 ). Also, point b 0  is the time at which said pressure reaches pressure B 0 , at which it should be possible to bring the melt surface up to the bottom level Kb of cavity  14  inside mold  13 . Furthermore, point c 0  is the time at which said pressure reaches pressure C 0 , at which it should be possible to bring the melt surface up to the top level inside cavity  14 , point d 0  is the time at which it reaches pressure D 0  on completion of feeder head pressurizing, and point e 0  is the time at which the pressure is dropped prior to opening the mold. 
     The pressure control program defines a rate of pressure increase for the slope of the straight line S-a 0  during elapsed time 0-t a , a rate of pressure increase for the slope of the straight line a 0 -b 0  during elapsed time t a -t b , a rate of pressure increase for the slope of the straight line b 0 -c 0  during elapsed time t b -t c , and a rate of pressure increase for the slope of the straight line c 0 -d 0  during elapsed time t c -t d , while the rate of pressure increase during elapsed time t d -t e  is set to zero, and the pressure at elapsed time t e  is set to zero. In this specification, elapsed time t b  is defined as the first elapsed time, t b -t c  is defined as the second elapsed time, t c -t d  is defined as the third elapsed time, and t d -t e  is defined as the fourth elapsed time. 
     In this basic pattern P 0 , since the rate of pressure increase from point S to point a 0  is large, the melt surface quickly rises to the bottom level Ka of mouth part  14   h . In this way, the drop in melt temperature due to stalk  15  is improved to some extent. Also, since the pressure increase from point ao to point bo is slightly smaller, the Tate at which the melt Surface rises between the bottom level Ka of mouth part  14   h  to the bottom level Kb of cavity  14  is slightly slower. Furthermore, since the rate of pressure increase is gentler from point b 0  to point c 0 , the rate at which the melt surface rises between the bottom level Kb of cavity  14  to the top level of cavity  14  is even gentler. In this way, the mixing of air in with the melt filled into the cavity is prevented. The rate of pressure increase between points c 0  and d 0  is set large so that the pressure quickly rises, and feeder head pressure is quickly applied to the melt filled into said cavity  14 . In this way, the occurrence of pipes and the like is diminished. That is, the rate of pressure increase is made large from the time (t c ) at which the melt surface rises up to the top level of the cavity until the third elapsed time has elapsed, the rate of pressure increase is made zero after the third elapsed time has elapsed, and the pressure is made zero after the fourth elapsed time has elapsed. Also, the rate of pressure increase is made smaller after time t b  has elapsed from the start of the filling operation, and subsequently the rate of pressure increase is made larger again after the second elapsed time has elapsed (time t c ). 
     First corrected pattern P 1  shown by the dashed line in FIG. 2 is the pattern used instead of basic pattern P 0  to control the pressure in holding furnace  16  when lower melt surface detection sensor  14   b  detects that the actual melt surface has risen to the bottom level Kb of the cavity ahead of schedule. That is, in basic pattern P 0 , the melt surface should rise up to the height of said bottom level Kb at time bo. However, when lower melt surface detection sensor  14   b  has judged that the actual melt surface has risen to the height of bottom level Kb of the cavity in a shorter period (while pressure control is being performed between points a 0  and b 0 ), the control switches from basic pattern P 0  to first corrected pattern P 1  at this time, and the pressure inside holding furnace  16  is thereafter controlled based on this first corrected pattern P 1 . Here, point b 1  of first corrected pattern P 1  is the time at which lower melt level detection sensor  14   b  detects that the actual melt surface has risen to the height of bottom level Kb of the cavity. Also, the slope from point b 1  to point cl is set equal to the slope from point bo to point c 0  in said basic pattern P 0 , and the slope from point c 1  to point d 1  in first corrected pattern P 1  is set equal to the slope from point c 0  to point d 0  in said basic pattern P 0 . That is, if point b 1  is superimposed on point b o , pattern P 1  will map exactly to pattern P 0 . This pattern correction is achieved by correcting the elapsed time in the pressure control program by the difference in elapsed time between point b 0  and point b 0 . 
     Also, second corrected pattern P 2  shown by the dotted line in FIG. 2 is the pattern used instead of basic pattern P 0  to control the pressure in holding furnace  16  when upper melt surface detection sensor  14   a  detects that the actual melt surface has risen to the top level of the cavity ahead of schedule. That is, in first corrected pattern P 1 , the melt surface should rise up to the height of the top of cavity  14  at time c 1 . However, when upper melt surface detection sensor  14   a  has judged that the actual melt surface has risen to the height of the top of cavity  14  in a shorter period (while pressure control is being performed between points b 1  and c 1 ), the control switches from first corrected pattern P 1  to second corrected pattern P 2  at this time, and the pressure inside holding furnace  16  is thereafter controlled based on this second corrected pattern P 2 . Here, point c 2  of second corrected pattern P 2  is the time at which upper melt level detection sensor  14   a  detects that the actual melt surface has risen to the height of the top of cavity  14 . Also, the slope from point c 2  to point d 2  is set equal to the slope from point c 1  to point d 1  in the first corrected pattern P 1 . If points c 2 , c 1  and c 0  are all superimposed, patterns P 0 , P 1  and P 2  will all map exactly to each other. The above pattern correction process is implemented by correcting the elapsed time in the pressure control program by the difference in elapsed time between points c 1  and c 2 . 
     Third corrected pattern P 3  shown by the dashed line in FIG. 3 is the pattern used instead of basic pattern P 0  to control the pressure in holding furnace  16  when lower melt surface detection sensor  14   b  detects that the actual melt surface has risen to the bottom level Kb of the cavity behind schedule. That is, in basic pattern P 0 , the melt surface should rise up to the height of said bottom level Kb at time b 0  as mentioned above. However, when the actual melt surface rises slowly and lower melt surface detection sensor  14   b  judges that the melt surface has risen to the height of bottom level Kb of the cavity while pressure control is being performed between points b 0  and c 0 , the c 0  ntrol switches from basic pattern P 0  to third Corrected pattern P 3  at this time, and the pressure inside holding furnace  16  is thereafter controlled based on this third corrected pattern P 3 . Here, point b 3  of third corrected pattern P 3  is the time at which lower melt level detection Sensor  14   b  detects that the melt surface has risen to the height of bottom level Kb of the cavity, and the line from point b 3  to point c 3  is made by duplicating the line from point b 0  to point c 0  in basic pattern P 0 . Also, the slope from point c 3  to point d 3  is set equal to the slope from point c 0  to point d 0  in basic pattern P 0 . As before, if point b 3  is superimposed on point b 0 , pattern P 3 . Will map exactly to pattern P 0 . This process is also performed by Correcting the elapsed time in the pressure control program. Note that in FIG. 3, the rate of pressure increase is reduced at point bo. That is, when first elapsed time (t b ) has elapsed from the start of the filling operation before the melt surface reaches bottom level Kb of the cavity, the rate of pressure increase is reduced even if the melt surface has not reached bottom level Kb of the cavity. Note that in this specification, the time at which the melt surface reaches level Kb is defined as the first timing. 
     Fourth corrected pattern P 4  shown by the dotted line in FIG. 3 is the pattern used instead of third corrected pattern P 3  to control the pressure in holding furnace  16  when upper melt surface detection sensor  14   a  detects that the actual melt surface has risen to the top level of cavity  14  behind schedule. That is, in third corrected pattern P 3 , the melt surface should rise up to the height of the top of cavity  14  at time C 3 . However, when the actual melt surface rises slowly and upper melt surface detection sensor  14   a  judges that the melt surface has risen to the height of the top level of cavity  14  while pressure control is being performed between points C 3  and d 3 , the control switches from third corrected pattern P 3  to fourth corrected pattern P 4  at this time. The pressure inside holding furnace  16  is thereafter controlled based on this fourth corrected pattern P 4 . Here, point C 4  of fourth corrected pattern P 4  is the time at which upper melt level detection sensor  14   a  detects that the melt surface has risen to the height of the top of cavity  14 , and the line from point C 4  to point d 4  is made by duplicating the line from point b 3  to point C 3  in third corrected pattern P 3 . As above, if points C 4 , C 3  and co are superimposed, patterns P 0 , P 3  and P 4  will map to each other. As the relationship between point C 3  and point C 4  clearly shows, when second elapsed time (from t b  to t c ) has elapsed from the first timing (b 3 ) before the melt surface reaches the top level of the cavity, the rate of pressure increase is reduced even if the melt surface has not reached the top level of the cavity. Note that in this specification, the time at which the melt surface reaches the top level of the cavity is defined as the second timing. 
     Here, said basic pattern P 0  is switched to first corrected pattern P 1  or third corrected pattern P 3  based on the program stored in control device  20  by correcting the values of the elapsed times in the control program based on the time at which the melt surface detection signal is input from lower melt surface detection sensor  14   b . In the same way, first corrected pattern P 1  is switched to second corrected pattern P 2  and third corrected pattern P 3  is switched to fourth corrected pattern P 4  based on the program stored in control device  20  by correcting the values of the elapsed times in the control program based on the time at which the melt surface detection signal is input from upper melt surface detection sensor  14   a.    
     The melt filling pressure difference control method of the casting machine relating to the present invention will now be described. 
     As shown in FIG. 1, mold  13  is fastened together and set on fixing plate  12 , whereupon control of the pressure inside holding furnace  16  is started based on basic pattern P 0  shown in FIG.  2  and FIG.  3 . As a result, the melt inside crucible  16   r  rises at high speed through stalk  15  to the height of bottom level Ka of mouth piece  14   h , and is supplied into mouth piece  14   h  relatively slowly from this bottom level Ka. Here, when the melt surface is judged to have risen to the height of bottom level Kb of the cavity by lower melt level detection sensor  14   b  while pressure control is being performed between point a 0  and point b 0  of basic pattern P 0 , the control switches from basic pattern P 0  to first corrected pattern P 1  at this time, as shown in FIG.  2 . The pressure inside holding furnace  16  then continues to be controlled from point b 1  based on this first corrected pattern P 1 , and the melt is slowly supplied into cavity  14 . Furthermore, when the melt surface is judged to have risen to the height of the top of cavity  14  by upper melt level detection sensor  14   a  while pressure control is being performed between point b 1  and point c 1  of first corrected pattern P 1 , the control switches from first corrected pattern P 1  to second corrected pattern P 2  at this time. The pressure inside holding furnace  16  then continues to be controlled from point c 2  based on this second corrected pattern P 2 , and feeder head pressure is applied to the melt filled into said cavity  14 . In this way, when the pressure control proceeds to point e 2  of second corrected pattern P 2 , exhaust valve  19   b  provided on pressurizing pipeline  19  is opened to release the pressure in holding furnace  16 , and mold  13  is opened. 
     Also, when the melt surface is judged to have risen to the height of bottom level Kb of the cavity by lower melt level detection sensor  14   b  while pressure control is being performed between point b 0  and point c 0  of basic pattern P 0 , the control switches from basic pattern P 0  to third corrected pattern P 3  at this time, as shown in FIG.  3 . The pressure inside holding furnace  16  then continues to be controlled from point b 3  based on this third corrected pattern P 3 , and the melt is slowly supplied into cavity  14 . Furthermore, when the melt surface is judged to have risen to the height of the top of cavity  14  by upper melt level detection sensor  14   a  while pressure control is being performed between point c 3  and point d 3  of third corrected pattern P 3 , the control switches from third corrected pattern P 3  to fourth corrected pattern P 4  at this time. The pressure inside holding furnace  16  then continues to be controlled from point C 4  based on this fourth corrected pattern P 4 , and feeder head pressure is applied to the melt filled into said cavity  14 . In this way, when the pressure control proceeds to point e 4  of fourth corrected pattern P 4 , exhaust valve  19   b  provided on pressurizing pipeline  19  is opened to release the pressure in holding furnace  16 , and mold  13  is opened. 
     In this way, the present embodiment is able to detect the actual melt surface at two places—at the bottom Kb of cavity  14  and at the top of cavity  14 —and corrects the elapsed times of the initially set pressure pattern based on the times at which the melt level reaches these levels. Thus, the pressure is controlled based on a suitable pressure variation pattern that is matched to the actual circumstances, so that it becomes possible not only to fill the melt into the cavity at a suitable rate, but also to achieve a satisfactory feeder head pressure after filling with melt. Thus there is no incorporation of air into the melt filled inside cavity  14 , and it also becomes unlikely that defects such as pipes will occur due to insufficient feeder head pressure. As a result, it is possible to reduce defects such as pressure leaks in pressure-resistant components. 
     Note that although the present embodiment has described a melt filling control method for a low-pressure casting machine  10 , it can—needless to say—also be applied to a low-pressure casting machine wherein the melt is filled into a mold by reducing the pressure inside the cavity. 
     The melt surface detector devices preferably used in this method are described in the following. 
     Hitherto, the method conventionally used to detect the melt surface in the cavity of a mold has involved measuring the temperature by means of a thermocouple fitted to the wall forming the cavity of the mold, and inferring the time at which the surface of the melt injected into the mold reaches the level at which this thermocouple exists from the gradient of temperature increase. 
     However, with the above-mentioned conventional method, a certain time delay arises between the melt surface reaching a certain level and the temperature of the thermocouple at that level starting to rise. Therefore, there have been problems in that it is difficult to detect the melt surface level accurately, and it is thus impossible to accurately control the filling rate of the melt based on the melt surface level. 
     Also, since a thermocouple is a temperature measuring instrument wherein two kinds of metal are joined together and which is used to measure temperature by means of the characteristics of variation of the thermoelectric power arising from the temperature of the junction, it can often become unable to make measurements due to, for example, open circuits that occur when the junction is subjected to severe thermal conditions. It is therefore absolutely essential to perform regular maintenance. 
     The melt surface detection device described in the following is able to bring a sensor directly into contact with the melt and can thus not only accurately measure the melt surface level without having to consider time delays and the like, but it is also possible to set the strength of the sensor that is brought into contact with the melt to the same level as the strength of the mold or higher, so that the durability and reliability of the sensor are improved and it requires less effort to maintain. 
     In the following, a melt surface detection device relating to a first embodiment of the present invention is described based on FIGS. 5 through 7. Here, FIG. 5 is a detailed installation diagram of detection sensor  112  of melt surface detection sensor  14   a , and FIG. 6 is a circuit diagram of melt surface detection sensor  14   a . Also, FIG. 7 is a cross-section showing the entire mold  13 . 
     Said detection sensor  112  is an upper sensor for detecting whether or not melt is filled into cavity  14 , and as shown in FIG. 5 it consists of an electrically conductive electrode  114  fabricated from Fe—Ni steel and a ceramic insulating member  116  that insulates this electrode  114  from mold  13 . 
     Said insulating member  116  is provided with flange part  116   f  formed into a cylindrical shape at a position in its center, and through-hole  116   k  along its central axis which houses said electrode  114 . Here, said insulating member  116  is a ceramic chiefly consisting of Al 2 O 3 , and is joined to said electrode  114  by silver solder after being metallized. Also, when said insulating member  116  and electrode  114  are joined together, the lower end surface of this insulating member  116  and the lower end surface of electrode  114  are positioned in the same plane. 
     Large-diameter through-hole  102   m  and small-diameter through-hole  102   s  are formed coaxially at the top of said mold  13 , and a ring-shaped step  102   d  is formed at the connecting part between through-holes  102   m  and  102   s . Next, the end part  116   a  and flange part  116   f  of detection sensor  112  are respectively housed in said small-diameter throughhole  102   s  and large diameter through-hole  102   m . Here, the length of said small-diameter through-hole  102   s  is set equal to the length of end part  116   a  of detection sensor  112 , so that the lower end surface of this detection sensor  112  is flush with wall surface  4   w  of cavity  14  when said detection sensor  112  is set in mold  13 . That is, the end surface of said detection sensor  112  constitutes a part of the wall surface  4   w  of cavity  14 , and through the use of the above materials, its strength is at least of the same level as that of mold  13 . 
     Electrode  114  of said detection sensor  112  is electrically connected to terminal T 1  of signal output unit  118 , as shown in FIG.  6 . Also, mold  13  is electrically connected both to earth and to terminal T 2  of signal output unit  118 . 
     Said signal output unit  118  is a circuit for outputting a signal that shows whether or not electrode  114  of detection sensor  112  is electrically connected to mold  13  by the melt, and consists of a constant-voltage source  118   v  and a relay  118   r . Constant-voltage source  118   v  and the coil  118   c  of said relay  118   r  are connected in series between terminal T 1  and terminal T 2 . That is, electrode  114  of detection sensor  112 , terminal T 1 , coil  118   c , constant voltage source  118   v , terminal T 2  and mold  13  are all thereby connected in series, so that a fixed current flows in said coil  118   c  when said electrode  114  and mold  13  are electrically connected by the melt. When a current flows in said coil  118   c , the contact point  118   s  of relay  118   r  is closed, and this signal is output to the control device (not illustrated) via terminals T 3  and T 4 . 
     Next, the operation of melt surface detection sensor  14   a  relating to the present embodiment will be described. 
     While the melt surface has not yet reached the position of detection sensor  112  in cavity  14 , the electrode  114  of this detection sensor  112  is insulated from mold  13  by insulating member  116 , so that no current flows through coil  118   c  of relay  118   r  shows in FIG.  6 . Therefore, the contact point  118   s  of relay  118   r  is left open. However, when the melt surface arrives at the position of detection sensor  112 , said electrode  114  and mold  13  are electrically connected by the melt, and a fixed current flows through said coil  118   c . As a result, relay  118   r  is operated and contact point  118   s  is closed, and this signal is output to said control device  20  via terminals T 3  and T 4 . 
     In this way, with a melt surface detection device  14   a  relating to the present embodiment, since detection sensor  112  constitutes a part of wall surface  4   w  of cavity  14 , the melt comes into direct contact with this detection sensor  112  and thus there are no time delays or such problems associated with the detection. Also, since electrode  114  of said detection sensor  112  is made of a material having a strength of the same or higher level than the strength of mold  13 , and since insulating member  116  is made of ceramic, it has high durability and reliability, and it requires less effort to maintain. 
     Also, in the present embodiment, detection sensor  112  is fitted at the top of mold  13  (at the uppermost part of cavity  14 ) and is used to detect whether or not cavity  14  has been filled with melt; however, it is not limited to such a use, and can—needless to say—be used by fitting it at a prescribed level in said cavity  14  and detecting whether or nor the melt surface has reached this position. 
     Next, melt surface detection device  220  relating to a second embodiment is described based on FIGS. 8 and 9. 
     Melt surface detection device  220  relating to the present embodiment constitutes an improvement on the electrical Circuit of signal detection unit  118  in melt detection device  14   a  relating to the first embodiment, and has a configuration wherein it is possible to measure the electrical resistance between mold  13  and electrode  214  of detection sensor  212 . Note that the following description is simplified by using the same numbers to signify members that are identical to those used in melt surface detection device  14   a  of the first embodiment. 
     As shown in FIG. 8, in melt surface detection device  220  relating to the present embodiment, electrode  214  of detection sensor  212  is connected to first measurement terminal T 1 , of a resistance meter  222 , while mold  13  is connected to second measurement terminal T 2  of resistance meter  222 . Also, a constant-voltage source  224  is connected to first measurement terminal T 1 , and second measurement terminal T 2  of said resistance meter  222 . With this circuit configuration, it is possible to continuously measure the electrical resistance between mold  13  and electrode  214  of said detection sensor  212 . Also, said resistance meter  222  is able to output a signal to the control device (not illustrated) when the detected value is below a previously set value (set value). 
     FIG. 9 is a graph showing the variation in electrical resistance between mold  13  and electrode  214  between the start (point S) and finish (point D 1 , D 2 ) of casting. Here, the solid line in the figure shows the variation in resistance when casting is performed with the wall surface  4   w  of cavity  14  coated with mold paint  203  that is an insulating substance, and the dotted line in the figure shows the variation in resistance when casting is performed without the wall surface  4   w  of cavity  14  being coated with mold paint  203 . As shown in FIG. 1, when mold  13  is positioned directly above holding furnace  16 , the inside of holding furnace  16  is pressurized by a compressor (not illustrated), and the melt is pushed up inside cavity  14  via stalk  15 . The present embodiment is described for the case where casting is performed after coating with mold paint  203 . 
     Point S in FIG. 9 shows the time at which pressurizing of the interior of holding furnace  16  is started. At the time the pressurizing is started, mold  13  and electrode  214  of detection sensor  212  are electrically insulated by insulating member  216 , and as shown in FIG. 8, since the lower end surface of detection sensor  212  is coated with mold paint.  203  which is an insulating substance, the electrical resistance between said electrode  214  and mold  13 —i.e., the value of the electrical resistance measured by resistance meter  222 —is at its maximum. However, as melt is supplied into cavity  14  and the melt surface rises, the electrical resistance of insulator  216 , mold paint  203  and so on gradually decreases due to the heat radiated from the melt, and as shown in FIG. 9, the resistance value of resistance meter  222  gradually decreases. Next, when the melt surface arrives near the top of cavity  14  and the melt starts to come into contact with detection sensor  212  via mold paint  203  (point A 1 ), the resistance value of resistance meter  222  begins to drop sharply. Then, when cavity  14  is filled with melt (point B 1 ), the resistance value of resistance meter  222  becomes equal to the resistance value of mold paint  203  situated between mold  13  and electrode  214  of detection sensor  212 . The resistance value attributable to the melt is extremely small. 
     Accordingly, if the resistance value (B 1 ) at point B 1  is stored beforehand as a set value, it can be determined that the melt has reached the position of detection sensor  212  at the time when the measured value of resistance meter  222  becomes equal to this resistance value (B 1 ), and it is possible to output a signal to said control device at this time. Note that point C 1  in the figure is the time at which the melt inside cavity  14  begins to solidify, and point D 1  is the time at which the resulting product is released from the mold. 
     On the other hand, when casting is performed with mold paint  203  removed from wall surface  4   w  of cavity  14  (shown by the dotted line in the figure), the measured value of resistance meter  222  is decreased by an amount corresponding to the resistance value of mold paint  203  compared with the case where it is coated with mold paint  203 . 
     Here, the method mentioned above in melt surface detection device  14   a  relating to the first embodiment is employed, whereby relay  118   r  detects the state of electrical connection between mold  13  and electrode  114  of detection sensor  112 . However, when wall surface  4   w  of cavity  14  is coated with mold paint  103 , the current flowing through coil  118   c  will be insufficient to drive relay  118   r  due to the resistance of this mold paint  103 , since mold paint  103 —which is an insulating substance—is positioned between mold  13  and electrode  114  of detection sensor  112  even when the melt surface reaches the position of detection sensor  112 . As a result, there is a limitation in that the melt surface detection device  14   a  relating to the first embodiment must be used in a state where no mold paint is applied to wall surface  4   w  of cavity  14 . 
     However, since melt surface detection device  220  relating to the second embodiment employs a scheme whereby the resistance value is measured between mold  13  and electrode  214  of detection sensor  212 , it is able to judge whether or not the melt surface has reached the position of detection sensor  212  from the variation in resistance, even when coated with mold paint  203  as mentioned above. 
     Also, even when mold  13  is used without coating it with mold paint, it is not essential to remove residual mold paint from the end surface of detection sensor  212  left over from previous usage, and there is no need to polish detection sensor  212 . Accordingly, electrode  214  of said detection sensor  212  suffers hardly any erosion, and the sensor lifetime is improved.