Abstract:
A motor-driven mold clamping device includes a toggle mechanism for advancing and retracting a movable platen toward and away from a fixed platen, and a servo motor adapted to drive the toggle mechanism via a ball screw mechanism. The toggle mechanism, the movable platen, and the fixed platen are configured such that a mold clamping force is controlled with a knicking in the toggle mechanism being in a predetermined range such that the servo motor is driven with a current which is controlled to be at or near a rated current therefore, in order to maintain the mold clamping force. A method of controlling mold clamping force includes the steps of providing the toggle mechanism, and the servo motor as discussed above, and controlling the mold clamping force by appropriately configuring the knicking in the toggle mechanism to be in a predetermined range such that the servo motor is driven with a current which is at or near a rated current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement of a motor-driven mold clamping device for a motor-driven injection molding machine as well as to an improved mold clamping method. 
     2. Description of the Related Art 
     Motor-driven injection molding machines have gradually replaced hydraulic machines, and have increasingly been used in recent years. One reason lies in their simple configuration as compared with the hydraulic injection molding machines because of the lack of need of a hydraulic pump, hydraulic tubes and valves. In addition, servo motors used for the source of power allows the easier control of the motor-driven injection molding machines. The servo motors are used in most cases for an injection device and a mold clamping device. 
     As far as mold clamping devices are concerned, it is often based on a toggle system. The toggle system uses a toggle mechanism to double the force generated by the servo motor which is then transmitted to a mold by means of a toggle link. These types of mold clamping devices are undergoing changes and refinements. An example of an improved toggle-operated mold clamping device is disclosed in Japanese Patent Publication No. 1-22135. The disclosed mold clamping device comprises a servo motor and a position detector for detecting a rotation position of the servo motor. The mold clamping device further comprises a conversion mechanism for converting the rotation movement of the servo motor into a linear movement. The conversion mechanism has a ball screw mechanism. The conversion mechanism is used for driving the toggle mechanism and the position detector detects a position of a movable mold, to carry out control operation of the mold clamping. Upon the mold clamping, the servo motor is driven with a microcurrent flowing therethrough. 
     For the toggle mechanism, the reason the microcurrent is used is to generate a sufficient clamping force with the phenomenon known as knicking reduced as much as possible. This provides a large toggle magnification factor and thus allows a smaller output of the servo motor. The mold clamping devices using the toggle mechanism of the type described advantageously require only a small electric power consumption. As will be described more in detail below, the smaller the knicking is, the shorter the distance from the dead point of the toggle mechanism. 
     However, the smaller knicking results in a larger effect of a frictional force on junctions and contacted portions of mechanical parts forming the mold clamping device. This therefore increases an operational hysteresis. Such a large operational hysteresis has an adverse effect on the accuracy of control for the mold clamping force provided by the servo motor. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to improve the accuracy of control for the mold clamping force while reducing the effect of a frictional force on junctions and contacted portions of mechanical parts forming the mold clamping device. 
     A motor-driven mold clamping device according to the present invention comprises a toggle mechanism for use in advancing and retracting a movable platen; and a servo motor adapted to drive the toggle mechanism via a ball screw mechanism. 
     According to an aspect of the present invention, the motor-driven mold clamping device controls a mold clamping force with a knicking in the toggle mechanism being in a predetermined range and the servo motor driven with a current which is not smaller than 20% of a rated current therefor. 
     A method for clamping a mold in a motor-driven manner according to the present invention clamps the mold by means of a toggle mechanism operated by a servo motor. In this method a mold clamping force is controlled with a knicking in the toggle mechanism being in a predetermined range and the servo motor driven with a current which is not smaller than 20% of a rated current therefor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference should be made to the appended drawings, wherein: 
     FIG. 1 is a view illustrating a configuration of a motor-driven mold clamping device to which the present invention is applied; 
     FIG. 2 is a block diagram illustrating a configuration of a mold clamping force feedback control system used in the present invention; 
     FIG. 3 shows a characteristic curve illustrating a hysteresis during the control for a mold clamping force; 
     FIG. 4 is a partial sectional view for describing a ball screw mechanism which is used in the motor-driven mold clamping device illustrated in FIG. 4; and 
     FIG. 5 is a characteristic for describing the relation between a theoretical toggle magnification factor and a knicking K in the toggle mechanism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a motor-driven mold clamping device to which the present invention is applied. In FIG. 1, a mold clamping device comprises a fixed platen  11  and a toggle support  12 . Four tie bars  13  (two of which are illustrated in the figure) are provided between the toggle support  12  and the fixed platen  11 . A movable platen  14  is opposed to the fixed platen  11  such that the movable platen  14  can freely be advanced and retracted along the tie bars  13 . A fixed mold (not shown) is attached to the surface of the fixed platen  11  that is opposed to the movable platen  14 . Likewise, a movable mold (not shown) is attached to the surface of the movable platen  14  that is opposed to the fixed platen  11 . 
     An ejector pin feeder  15  is provided at the rear end of the movable platen  14 . The ejector pin feeder  15  is for pushing ejector pins (not shown) in order to eject a molded product. In the ejector pin feeder  15 , a servo motor  16 , used for ejection, advances and retracts an ejector rod  17  by stroke Sa. 
     A toggle mechanism is provided between the toggle support  12  and the movable platen  14 . A servo motor  18  for mold clamping is driven to advance and retract crosshead  19  to generate a mold clamping force multiplied by a toggle magnification factor. This mold clamping force is used to advance the movable platen  14  (in the right direction in FIG. 1) to achieve the mold clamping. 
     The toggle mechanism is formed of toggle levers  20  and  21  and a toggle arm  22 . The toggle lever  20  is pivotally supported on the crosshead  19 . The toggle lever  21  is pivotally supported on the toggle support  12 . The toggle arm  22  is pivotally supported on the movable platen  14 . The toggle lever  20  is linked to the toggle lever  21 . The toggle lever  21  is linked to the toggle arm  22 . A rotary encoder  23  is provided in the servo motor  18  to detect the position of the crosshead  19  (hereinafter, referred to as a crosshead position). The rotary encoder  23  detects the crosshead position by directly detecting the rotation speed of the servo motor  18 . 
     Now, the concept of knicking will be described. Assume a line segment between the fulcrum B on the toggle lever  21  and the point of application A on the toggle arm  22  in the toggle mechanism. Assume another line segment which is parallel to the above-mentioned line segment AB and which passes the fulcrum on the toggle arm  22 , that is, the junction C between the toggle lever  21  and the toggle arm  22 . A distance K between these two line segments are referred to as the knicking or knicking distance. Therefore, the smaller the knicking K is, the shorter the distance from the dead point of the toggle mechanism. 
     The mold clamping device of the type described is disclosed in Japanese Patent Application No. 7-327017 (corresponding to Japanese Patent Laid-open No. 9-164571). The device as disclosed in this patent application is what is referred to as a “built-in driving type” and one feature thereof is that no additional drive mechanisms such as a belt is needed for the transmission of the driving force. 
     Referring to FIG. 4, the ball screw mechanism is described in brief. The servo motor  18  has a hollow output shaft  18 - 1 . A ball nut  18 - 2  is fixed to an end portion of the hollow output shaft  18 - 1 . A ball screw shaft  18 - 3  is engaged with the ball nut  18 - 2  and is inserted into the hollow portion of the hollow output shaft  18 - 1 . The crosshead  19  is attached to an end portion of the ball screw shaft  18 - 3 . Thus, the rotation motion of the hollow output shaft  18 - 1  is converted into the reciprocating motion of the crosshead  19  through the ball nut  18 - 2 . 
     Referring to FIG. 2, a mold clamping force feedback control system is described. A strain gage  30  is provided on any one of the four tie bars  13  shown in FIG.  1 . Strain gage  30  detects the mold clamping force by detecting a strain exerted on the tie bar  13  as the clamping proceeds. The detected mold clamping force is converted into a positional amount of the crosshead position in a converter  31 . The converted positional amount of the crosshead is supplied to a subtracter  32 . The subtracter  32  calculates a difference between a value of the converted positional amount of the crosshead and a crosshead position setting value supplied from a setting unit (not shown). The subtracter  32  supplies the subtraction result to a position control amplifier  33  as a difference signal. The position control amplifier  33  amplifies the received difference signal into a signal suitable for a velocity feedback system and supplies it as an amplified signal to a subtracter  34 . The subtracter  34  calculates a difference between the amplified signal and a velocity feedback signal supplied from the rotary encoder  23 . The subtracter  34  then supplies the subtraction result to a velocity amplifier  35  as a difference signal. The velocity amplifier  35  amplifies the received difference signal into a signal suitable for a current feedback system. The velocity amplifier  35  then supplies the amplified signal to a subtracter  37  via a limiter  36  which restricts the upper and lower limits of the amplified signal. The subtracter  37  calculates a difference between the signal supplied from the velocity amplifier  35  and a current feedback signal from a current detector  40  which detects an output current from a motor drive  39 . The subtracter  37  then supplies a signal indicative of the calculated difference to a current amplifier  38 . The current amplifier  38  supplies a current command value for the servo motor  18  to the motor drive  39 . 
     As described above, the mold clamping device controlled by the mold clamping force feedback control system is known to have the following problem. The ball screw in the ball screw mechanism receives no reaction force of the mold clamping force when the clamping is performed with the toggle lever  21  and the toggle arm  22  extending almost completely, that is, with the smallest possible knicking. The reaction force is taken up by toggle lever  21  and toggle arm  22 , rather than being transferred to the ball screw. Receiving no reaction force means only a small electric power is required for the servo motor  18 . However, the small knicking results in a large effect of a frictional force on the junctions and the contacted portions of the mechanical parts forming the mold clamping device, which increases the operational hysteresis of the device. 
     This large operational hysteresis increases torque through a straight line L 1  as shown in FIG. 3 because the frictional force acts as a resistance during the mold closing operation. On the other hand, the frictional force helps the torque to be reduced through a straight line L 2  during the mold opening operation. As a result, the torque has a non-linear characteristic curve, and the control performance is deteriorated. Therefore, it is not possible to dynamically control the mold clamping force when the device has a small knicking K. Again as described above, it has an adverse effect on the accuracy of control for the mold clamping force provided by the servo motor  18 . 
     A feature of the present invention lies in the timing of the mold clamping. The present invention performs the mold clamping with the toggle lever  21  and the toggle arm  22  not being extended completely, that is, with a relatively large knicking K. In this state, the servo motor  18  is required to be supplied with a higher electric current which is at or near the rated current value for the motor. Although power requirements are therefore increased, this brings some significant advantages including, but not limited to, the effect of the frictional force being reduced and that it becomes easier to control the mold clamping force with the higher accuracy. 
     Referring to FIG. 5, the description will be made with respect to the relation between a theoretical toggle magnification factor and the knicking K in the toggle mechanism. Generally, the toggle mechanism has a characteristic as shown in FIG.  5 . In FIG. 5, if the knicking K approaches the zero, the theoretical toggle magnification factor approaches the infinity. This means that, if the knicking K is a small value, it is possible to obtain a sufficient mold clamping force, even if the servo motor  18  is driven with a small current smaller than the rated current. In the embodiment, the toggle mechanism is used within a range of 20 through 80 in the theoretical toggle magnification factor. This is because the following reason. If the toggle mechanism is used with a large theoretical toggle magnification factor, the operational hysteresis becomes large and the accuracy of control for the mold clamping force is deteriorated. When the toggle mechanism is used within the range of 20 through 80, it is required that the servo motor  18  is driven with the current near to the rated current. However, it is possible to control easily the mold clamping force because the operational hysteresis becomes small. 
     The servo motor  18  may be driven with a current which is not smaller than 20% of a rated current therefor. It is preferable that the servo motor  18  is driven with the current which is at least 70% of the rated current. 
     In addition, the value of the knicking K is determined by the theoretical toggle magnification factor which is used for the toggle mechanism. If the size of the toggle lever  21  and the toggle arm  22  in the toggle mechanism is changed, the value of the knicking K is also varied. This means that the value of the knicking K is varied with the size of the toggle mechanism. For example, the value of the knicking K is determined within a predetermined range of 5 through 10 (mm). However, the present invention is no limited by the above range. A rated output of the servo motor  18  is determined by the required mold clamping force, the theoretical toggle magnification factor, a lead of the ball screw shaft  18 - 3 , and so on. 
     While the preferred embodiments of the present invention have thus been described for the case where the present invention is applied to the built-in type motor-driven mold clamping device, the present invention is also applicable to other types of the motor-driven mold clamping devices. 
     As described above, according to the present invention, it is possible to control the mold clamping force with high accuracy while reducing the adverse effect of the frictional force between the mechanical parts of the mold clamping device by means of controlling the mold clamping force with a relatively large knicking. 
     Numerous modifications may be apparent to one of skill in the art, while remaining within the spirit and scope of the invention. To determine the scope of the invention, reference should be made to the appended claims.