Abstract:
A vacuum die casting apparatus includes a casting cavity ( 10 ), which is evacuatable via a vacuum valve ( 26 ). A liquid casting material is pressable into the casting cavity by a piston ( 14 ) actuated by an actuator ( 17 ). A filling level sensor ( 20 ) detects a predetermined filling level of the casting material in the casting cavity. A control device ( 34 ) is connected to the filling level sensor for controlling the vacuum valve, and a position sensor ( 32 ) is connected to the control device ( 34 ) for detecting movement of the piston ( 14 ). The control device generates a closing signal for the vacuum valve ( 26 ) when the piston, after reaching the position (s 1 ) at which the filling level sensor ( 20 ) indicates a predetermined filling level of the casting cavity ( 10 ) with casting material has been reached, is displaced in a predetermined manner.

Description:
CROSS-REFERENCE 
   This application is the national stage filing of International patent application No. PCT/EP02/10234 filed Sep. 12, 2002, which claimed priority to German patent application No. 101 44 945.3 filed Sep. 12, 2001. 
   TECHNICAL FIELD 
   The invention concerns a method for controlling a vacuum valve of a vacuum die casting apparatus and a vacuum die casting apparatus. 
   THE RELATED ART 
   A vacuum die casting apparatus is known from DE 200 16 166 U1, in which the filling level of liquid metal injected into the casting cavity is detected by a metal contact sensor; upon coming into contact with the liquid metal, the sensor outputs a signal. A closing piston of the vacuum valve, which is driven by a linear motor, is moved in response to the signal so that the vacuum valve is entirely closed when the metal reaches the vacuum valve. In a further teaching, the known vacuum die casting apparatus utilizes a meandering vacuum and venting passage, in which a plurality of metal contact sensors are disposed and are connected to a computer; the computer controls the linear motor of the closing piston in accordance with the signals from the sensors. 
   If there is only one metal contact sensor, it is not possible to compensate for fluctuations in the speed at which the molten metal is injected during the manufacture of the castings; therefore, if the vacuum valve is not timely closed at high filling speeds, the evacuation will be prematurely ended, thereby possibly detrimentally influencing the quality of the casting. The use of a plurality of metal contact sensors is comparatively complicated and expensive. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide a method and an apparatus that can manufacture castings of improved quality. 
   According to one method of the invention, the position of a piston, which presses or injects liquid casting material into a mold cavity, is detected, and the vacuum valve is closed in dependence on the piston position; as a result, it is possible for the vacuum valve to be reliably closed just before the casting material, preferably liquid metal, reaches the vacuum valve. In that way, the casting mold or the casting cavity can be connected to a vacuum source for as long as possible; thus, polluting gases, i.e., gases originating from lubricants and separating substances and the like, are suctioned away and a casting is produced that is substantially free of pores or inclusions. 
   A vacuum die casting apparatus according to the invention includes a casting cavity that is evacuatable via a vacuum valve. A liquid casting material is pressable into the casting cavity by means of a piston actuated by an actuator. A filling level sensor detects a predetermined filling level of the casting material in the casting cavity. A position sensor detects the movement of the piston. A control device is connected to the filling level sensor and the position sensor. The control device generates a closing signal for the vacuum valve when the piston, after reaching a position at which the filling level sensor indicates a predetermined filling level of the casting cavity with casting material, is displaced in a predetermined manner. 
   In another aspect of the invention, the vacuum valve advantageously operates with a time delay that is as short as possible so that the closing signal can be generated as late as possible; as a result, the closing time is as late as possible, and is substantially independent of fluctuations that occur during the operation of the casting apparatus. 
   In accordance with another aspect of the invention, the time point at which the closing signal must be generated can be calculated from the piston speed almost in real-time. 
   Advantageous characteristic curves may be stored in the control device, and the operating condition of the die casting apparatus can be monitored. 
   Further, operating fluctuations of the casting apparatus are taken into account when generating the closing signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic view of a vacuum die casting apparatus. 
       FIGS. 2 and 3  show curves to illustrate an operational mode of the apparatus according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown in  FIG. 1 , a vacuum die casting apparatus includes at least two main bodies  2  and  4  that are movable towards and away from each other by means of a drive device (not shown) in a known manner. An associated mold portion  6  and  8  is respectively fixed to each main body. Mutually facing surfaces of the mold portions  6  and  8  are configured such that a casting cavity  10  is formed when the apparatus is closed or when the main bodies are moved towards each other. 
   A cylinder  12  opens towards the casting cavity  10  and contains a movable piston  14 . The cylinder  12  serves to introduce casting material, preferably liquid metal, into the casting cavity  10 ; the piston  14  is connected via a shaft  16  to an actuator  17  for driving the piston  14 . A filling conduit  18  passes through the wall of the cylinder  12  for introducing the liquid casting material. 
   A filling level sensor  20  is disposed adjoining the casting cavity  10 , which filling level sensor  20  generates a signal upon contacting the casting material  22  that rises in the casting cavity  10  when the casting cavity is filled with liquid casting material. A vacuum opening  24  adjoins the upper end region of the casting cavity  10  and the vacuum opening  24  is connected via a vacuum valve  26  to a vacuum pump  28 . The vacuum valve  26  is configured such that, in response to a closing signal, the valve member of the vacuum valve moves with the shortest possible delay into the valve closed position. Actuation of the valve member is effected, for example, by means of an actuator, e.g., a solenoid  30 . The actuation can also be suitably effected hydraulically or by other means. 
   The stroke position of the piston  14  or the shaft  16  is detected by a motion sensor  32 . The motion sensor  32  can be, for example, a linear sensor. In the alternative, if the linear movement of the piston is converted into a rotational movement by a suitable mechanism, the motion sensor  32  may be an incremental rotational motion sensor. 
   The motion sensor  32 , the filling level sensor  20  and the solenoid of the vacuum valve  26  are connected to a control device  34  that includes a microprocessor with associated storage devices, a display unit, for example in the form of a display, and operating elements. 
   The structure and function of the described components are known. The co-operation of the components is described in the following: 
   It will be assumed that the piston  14  has been moved towards the right in  FIG. 1  beyond the outlet port of the filling conduit  18 . At that position, liquid metal can be introduced into the evacuated casting cavity  10  via the conduit  18 . For example, the space within the cylinder  12  to the left of the piston  14  is filled to between 20% and 60% with liquid casting material, depending on the volume relationships thereof. The piston  14  is then moved towards the left beyond the outlet port of the filling conduit  18 . The vacuum valve  26  is open at this time. The vacuum pump  28  is operating so that the casting cavity  10  is subjected to a vacuum. The casting material  22  is pressed into the casting cavity  10  and eventually reaches the filling level sensor  20 . 
   As soon as the casting material  22  reaches the filling level sensor  20 , the filling level sensor  20  sends a trigger signal to the control device  34 ; at the time the trigger signal is sent, the position of the piston  14  is detected by the motion sensor  32  and the detected piston position is stored in the control device  34 . Advantageously, the actuator  17 , e.g. a hydraulic actuator, is switched to a higher speed by the trigger signal so that the casting material is injected at a higher speed and under a higher pressure into the remainder of the mold cavity. 
     FIG. 2  clarifies these relationships. The speed of the piston  14  is illustrated as being dependent upon the distance s that the piston  14  has been displaced. As can be clearly seen, the piston movement begins at a low speed until the position s 1 , at which position the casting material  22  reaches the filling level sensor  20 . The associated position s 1  is stored in the control device  34 . At the same time, the force or, if provided, the speed, with which the actuator  17  drives the piston  14 , is switched to a high value. It will be assumed that the position s 2  of the piston  14  corresponds to the position at which the casting material reaches the vacuum opening  24  of the vacuum valve  26 ; in other words, it is the piston position at which the vacuum valve  26  must be closed in order to avoid being damaged by the casting material. It will further be assumed that the piston moves by the distance Δs within the predetermined time delay period Δt that is required to close the vacuum valve  26  in response to the generation of a closing signal. Therefore, as can be readily seen from  FIG. 2 , the closing signal for the vacuum valve  26  must be generated at the time when the piston is disposed at the position s 3 . Because the position s 1  of the piston is stored in the control device  34 , the distance s 3 −s 2  or the position s 3  of the piston  14  can be accurately detected by the motion sensor  32 , and the closing signal for the vacuum valve  26  can be generated at that time. 
     FIG. 3  provides a characteristic curve indicating the change of the piston position over time.  FIG. 3  additionally shows the time delay Δt of the vacuum valve  26 . 
   It will be appreciated that the characteristic curve of  FIG. 2  can be generated by differentiation of the characteristic curve of  FIG. 3 . Depending on the particular design configuration of the motion sensor  32 , the characteristic curve of  FIG. 2  or  FIG. 3  can be recorded directly. Thus, the characteristic curve of  FIG. 3  can be generated by integration of the characteristic curve of  FIG. 2  or the characteristic curve of  FIG. 2  can be generated by differentiation of the characteristic curve of  FIG. 3 . 
   Depending upon the construction of the vacuum valve and the arrangement thereof in the casting cavity, it is possible to completely fill the casting cavity when the piston  14  reaches the position s 2  (casting material reaches the vacuum opening  24 ); or, as illustrated in the Figures, the casting cavity can be only substantially filled, so that the piston is still displaced a short additional distance. It will be appreciated that the piston can also be controlled in such a way that the piston is braked at the position s 2 . 
   It is advantageous for the characteristic curve of  FIG. 2  to be stored as a target or reference characteristic curve that corresponds to a functionally acceptable operational condition of the casting apparatus. That reference characteristic curve can then serve to determine the piston position s 3  for generating the closing signal for the vacuum valve. 
   The reference characteristic curve can be continuously displayed together with an actual characteristic curve, for example on a display screen, so that operating changes in the die casting apparatus are directly visible and any faults can be recognised at an early stage. In addition, in the series production of castings, the characteristic curve, which curve is used to ascertain the position s 3 , can be continuously updated. For example, the actual characteristic curve of an immediately preceding casting operation may be used as the characteristic curve for ascertaining s 3 , or a characteristic curve may be used that is derived from a plurality of preceding casting operations. 
   Overall, the invention provides that the casting cavity is subjected to the effect of a vacuum as long as possible during the casting operation so that high quality vacuum die castings are produced. The elimination of pores in the die castings is further improved due to the fact that the final phase of the casting procedure is performed under a high casting pressure provided by the higher piston force. Thus, if any pores are not suctioned away by the vacuum and remain in the die casting, due to the gas being contained therein, the pore volume can be reduced by the relatively high piston force utilized to force the casting material into the casting cavity during the final stage of the casting operation. 
   The apparatus according to the invention can be developed and modified in many different ways. 
   For example, a plurality of connections extending from a casting cavity  10  to vacuum valves, and a plurality of filling level sensors, can be provided in the die casting apparatus; in this case, the use of the method according to the invention assumes that no changes occur in the relationships of the degrees of filling of the individual passages, which are disposed between the filling level sensors and the vacuum valves. As is immediately apparent, visualization of the characteristic curve of  FIG. 2  is advantageous due to the good resolution thereof; however, it is also possible to display other characteristic curves. It is further possible for the closing signal not to be generated based upon the piston reaching the predetermined position (S 3 ) in  FIG. 2 , but rather by the piston moving at a predetermined speed, and the like. 
   REFERENCE NUMBER LIST 
   
       
       main body 
       main body 
       mold portion 
       mold portion 
       casting cavity 
       cylinder 
       piston 
       shaft 
       actuator 
       filling conduit 
       filling level sensor 
       casting material 
       vacuum opening 
       vacuum valve 
       vacuum pump 
       solenoid 
       motion sensor 
       control device