Patent Publication Number: US-2021170657-A1

Title: Control device and control method for injection molding machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-221328 filed on Dec. 6, 2019, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a control device and a control method for an injection molding machine. 
     Description of the Related Art 
     In the field of injection molding machines, a technique is known for preventing a molding failure in which a resin leaks from a cylinder, by reducing a resin pressure after the resin has been melted inside the cylinder. Such a technique is disclosed, for example, in Japanese Laid-Open Patent Publication No. 2008-230164. Such a molding failure in which the resin leaks from the cylinder is also referred to as drooling or leakage. 
     According to the disclosed technique, the injection molding machine performs sucking back in a sucking back step (pressure reducing step) following a metering step in which the resin is melted. Consequently, the resin pressure arrives at a set pressure (target pressure P 0 ) which is capable of preventing drooling. 
     SUMMARY OF THE INVENTION 
     Incidentally, when sucking back of the screw is performed, it is necessary for an operator to determine a suck back distance or a suck back time period in advance. However, in order to appropriately determine the suck back distance or the suck back time period, the operator is required to perform trial and error attempts while taking into consideration material properties of the resin and specifications of the injection molding machine. From the standpoint of the operator, performing such tasks has been a burden. 
     Thus, the present invention has the object of providing a control device and a control method for an injection molding machine, in which the suck back distance or the suck back time period can be appropriately and easily determined. 
     One aspect of the present invention is characterized by a control device for an injection molding machine, the injection molding machine including a cylinder into which a resin is supplied, a nozzle disposed at a distal end of the cylinder, and a screw configured to move forward and rearward and rotate inside the cylinder, the injection molding machine being configured to perform metering of the resin while the resin is being melted inside the cylinder, by causing the screw to be moved rearward to a predetermined metering position while being forwardly rotated, in a manner so as to keep a pressure of the resin at a predetermined metering pressure, the control device including a calculation unit configured to calculate, based on a target volume of the resin inside the nozzle that is drawn in from a side of the nozzle to a side of the cylinder, a suck back distance or a suck back time period that achieves drawing in of the target volume of the resin inside the nozzle to the side of the cylinder, and a suck back control unit configured to cause the screw to be sucked back on the basis of the suck back distance or the suck back time period, after the screw has reached the predetermined metering position. 
     Another aspect of the present invention is characterized by a control method for an injection molding machine, the injection molding machine including a cylinder into which a resin is supplied, a nozzle disposed at a distal end of the cylinder, and a screw configured to move forward and rearward and rotate inside the cylinder, the injection molding machine being configured to perform metering of the resin while the resin is being melted inside the cylinder, by causing the screw to be moved rearward to a predetermined metering position while being forwardly rotated, in a manner so as to keep a pressure of the resin at a predetermined metering pressure, the control method including a calculation step of calculating, based on a target volume of the resin inside the nozzle that is drawn in from a side of the nozzle to a side of the cylinder, a suck back distance or a suck back time period that achieves drawing in of the target volume of the resin inside the nozzle to the side of the cylinder, and a suck back control step of causing the screw to be sucked back on the basis of the suck back distance or the suck back time period, after the screw has reached the predetermined metering position. 
     According to the present invention, the control device and the control method for the injection molding machine are provided, in which the suck back distance or the suck back time period can be appropriately and easily determined. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings, in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an injection molding machine according to an embodiment of the present invention; 
         FIG. 2  is a schematic configuration diagram of an injection unit according to the embodiment; 
         FIG. 3  is a schematic configuration diagram of a control device according to the embodiment; 
         FIG. 4  is an example of a first table which is stored in a storage unit according to the present embodiment; 
         FIG. 5  is an example of a second table which is stored in the storage unit according to the present embodiment; 
         FIG. 6  is a flowchart showing an example of a control method for the injection molding machine according to the embodiment; 
         FIG. 7A  is a schematic cross-sectional view showing an example of a state inside a cylinder at a point in time when a metering control step is completed; 
         FIG. 7B  is a schematic cross-sectional view showing an example of a state inside the cylinder after sucking back has been executed; 
         FIG. 8  is a schematic configuration diagram of a control device according to a third modification; and 
         FIG. 9  is a schematic configuration diagram of a control device according to a fourth modification. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a control device and a control method for an injection molding machine according to the present invention will be presented and described in detail below with reference to the accompanying drawings. It should be noted that the directions discussed below conform respectively to the arrows shown in the drawings. 
     EMBODIMENTS 
       FIG. 1  is a side view of an injection molding machine  10  according to an embodiment of the present invention. 
     The injection molding machine  10  according to the present embodiment comprises a mold clamping unit  14  having a mold  12  that is capable of being opened and closed, an injection unit  16  that faces toward the mold clamping unit  14  in a front-rear direction, a machine base  18  on which such components are supported, and a control device  20  for the injection molding machine  10 . 
     Among such components, the mold clamping unit  14  and the machine base  18  can be configured based on a known technique. Accordingly, in the following discussion, descriptions of the mold clamping unit  14  and the machine base  18  will be appropriately omitted. 
     Prior to describing the control device  20  of the present embodiment, at first, a description will be given concerning the injection unit  16 , which is a control target of the control device  20 . 
     The injection unit  16  is supported by a base  22 . The base  22  is supported by a guide rail  24  installed on the machine base  18  so that the base  22  is capable of moving forward and rearward. Consequently, the injection unit  16  is capable of moving forward and rearward on the machine base  18 . Further, the injection unit  16  can move closer to and away from the mold clamping unit  14 . 
       FIG. 2  is a schematic configuration diagram of the injection unit  16 . 
     The injection unit  16  is equipped with a tubular shaped heating cylinder (cylinder)  26 , a screw  28  provided inside the cylinder  26 , a pressure sensor  30  provided on the screw  28 , and a first drive device  32  and a second drive device  34  connected to the screw  28 . According to the present embodiment, it is assumed that the cylinder  26  has a cylindrical shape. 
     The axial lines of the cylinder  26  and the screw  28  coincide with each other on an imaginary line L according to the present embodiment. Such a system may be referred to as an “in-line (in-line screw) system”. Further, the injection molding machine to which the in-line system is applied is also referred to as an “in-line type injection molding machine”. 
     Concerning advantages of an in-line type injection molding machine, several advantages are known. As examples thereof, there may be cited a point in which the structure of the injection unit  16  is simpler, and a point in which the maintainability thereof is excellent, as compared with other types of injection molding machines. In this instance, as another type of injection molding machine, for example, a preplasticating type injection molding machine is known. 
     As shown in  FIG. 2 , the cylinder  26  is equipped with a hopper  36  provided on a rearward side, a heater  38  for heating the cylinder  26 , and a nozzle  40  provided on a frontward side thereof, i.e., at a distal end of the cylinder. Among such elements, the hopper  36  is provided with a supply port for supplying a molding material resin to the cylinder  26 . Further, in the nozzle  40 , there is formed a nozzle flow path  41  that communicates with the interior of the cylinder  26 . An opening of the nozzle flow path  41  communicates with the interior and the exterior of the cylinder  26 . 
     The shape of the nozzle flow path  41  is not particularly limited, however, according to the present embodiment, the shape thereof is cylindrical. Further, the shape of the opening of the nozzle flow path  41  is circular. 
     The screw  28  includes a spiral flight part  42  provided to span across the longitudinal (front-rear) direction thereof. The flight part  42 , together with an inner wall of the cylinder  26 , constitutes a spiral flow path  44 . The spiral flow path  44  guides in a frontward direction the resin that is supplied from the hopper  36  into the cylinder  26 . 
     The screw  28  includes a screw head  46  which is on a distal end on the frontward side, a check seat  48  that is disposed at a certain distance in a rearward direction from the screw head  46 , and a check ring  50  (a ring for backflow-prevention) that is capable of moving between the screw head  46  and the check seat  48 . 
     The check ring  50  moves in the frontward direction relatively with respect to the screw  28  when the check ring receives a forward pressure from the resin located on a rearward side of the check ring  50  itself. Relative movement of the check ring  50  in the frontward direction is performed, for example, at a later-described time of metering. 
     In this case, accompanying relative movement of the check ring  50 , the flow path  44  is gradually opened. As a result, the resin can easily flow along the flow path  44  from the rearward side to the frontward side across the check seat  48 . 
     Further, upon receiving a rearward pressure from the resin on the frontward side thereof, the check ring  50  moves in a rearward direction relatively with respect to the screw  28 . Relative movement of the check ring  50  in the rearward direction is performed, for example, at a later-described time of injection. 
     In this case, the flow path  44  is gradually closed accompanying such relative movement of the check ring  50 . As a result, the flow of the resin is suppressed along the flow path  44  from the frontward side toward the rearward side across the check seat  48 . In particular, when the check ring  50  is retracted to the check seat  48 , at least the resin on the frontward side of the check ring  50  is placed in a state in which the flow of the resin to the rearward direction across the check seat  48  is maximally suppressed. 
     The pressure sensor  30 , such as a load cell or the like for sequentially detecting the pressure imposed on the resin inside the cylinder  26 , is attached to the screw  28 . Hereinafter, the phrase “the pressure applied to the resin inside the cylinder  26 ” may also be simply referred to as a “resin pressure (pressure of a resin)”. 
     The first drive device  32  serves to rotate the screw  28  inside the cylinder  26 . The first drive device  32  comprises a servomotor  52   a , a drive pulley  54   a , a driven pulley  56   a , and a belt member  58   a . The drive pulley  54   a  rotates integrally with a rotary shaft of the servomotor  52   a . The driven pulley  56   a  is disposed integrally on the screw  28 . The belt member  58   a  transmits the rotational force of the servomotor  52   a  from the drive pulley  54   a  to the driven pulley  56   a.    
     In accordance with the above-described first drive device  32 , by the rotary shaft of the servomotor  52   a  being made to rotate, the rotational force thereof is transmitted to the screw  28  via the drive pulley  54   a , the belt member  58   a , and the driven pulley  56   a . Consequently, the screw  28  can be rotated. Further, according to the above-described first drive device  32 , by changing the direction in which the rotary shaft of the screw  28  of the servomotor  52   a  is rotated, in response to the changing, the direction of rotation of the screw can be switched between forward rotation and reverse rotation. 
     A position/speed sensor  60   a  is provided on the servomotor  52   a . The position/speed sensor  60   a  detects the rotational position and the rotational speed of the rotary shaft of the servomotor  52   a . The detection result therefrom is output to the control device  20 . Consequently, the control device  20  is capable of calculating the amount of rotation (the rotation amount), the rotational acceleration, and the rotational speed of the screw  28 , based on the rotational position and the rotational speed detected by the position/speed sensor  60   a.    
     The second drive device  34  serves to move the screw  28  forward and rearward inside the cylinder  26 . In the present embodiment, unless otherwise specified, the term “forward and rearward movement of the screw  28 ” implies forward and rearward movement of the screw  28  relative to the cylinder  26  inside which the screw  28  is provided. 
     The second drive device  34  comprises a servomotor  52   b , a drive pulley  54   b , a driven pulley  56   b , a belt member  58   b , a ball screw  62 , and a nut  64 . The drive pulley  54   b  rotates integrally with a rotary shaft of the servomotor  52   b . The belt member  58   b  transmits the rotational force of the servomotor  52   b  from the drive pulley  54   b  to the driven pulley  56   b . An axial line of the ball screw  62  and an axial line of the screw  28  coincide with each other on the imaginary line L. The nut  64  is screw-engaged with the ball screw  62 . 
     In accordance with the above-described second drive device  34 , by the rotary shaft of the servomotor  52   b  being made to rotate, the rotational force thereof is transmitted to the ball screw  62  via the drive pulley  54   b , the belt member  58   b , and the driven pulley  56   b . The ball screw  62  converts the transmitted rotational force into linear motion and transmits the linear motion to the screw  28 . Consequently, the screw  28  can be moved forward and rearward. Further, according to the above-described second drive device  34 , by changing the direction in which the rotary shaft of the servomotor  52   b  is rotated, in response to the changing, the movement direction of the screw  28  can be switched between forward movement (advancing) and rearward movement (retracting). 
     A position/speed sensor  60   b  is provided on the servomotor  52   b . The position/speed sensor  60   b  detects the rotational position and the rotational speed of the rotary shaft of the servomotor  52   b , and is a similar sensor as the position/speed sensor  60   a . The detection result therefrom is output to the control device  20 . Consequently, the control device  20  is capable of calculating the forward position and the rearward position (rearward movement distance) of the screw  28  in the front-rear direction, as well as the rearward movement speed (forward and rearward movement speed) of the screw  28 , based on the rotational position and the rotational speed detected by the position/speed sensor  60   b.    
     Hereinafter, a description will be given of the plurality of steps performed in the injection molding machine  10  for obtaining a molded product. In particular, a description will be given focused on operations that can be performed by the injection unit  16 . 
     The injection unit  16  melts (plasticizes) the resin supplied to the cylinder  26  due to being heated by the heater  38  and by the rotational force of the screw  28 , while the resin is fed and compressed in the frontward direction along the flow path  44  due to forward rotation of the screw  28 . Such forward rotation of the screw  28  is started in a state in which the screw  28  has been fully advanced inside the cylinder  26  (a state in which the volume of the metering region is at a minimum). Further, the screw  28  undergoes forward rotation at a predetermined rotational speed. 
     The screw  28  is gradually moved rearward relatively with respect to the cylinder  26 , accompanying the resin being fed and compressed in the frontward direction. The rearward movement speed of the retracted screw  28  is controlled by the control device  20 , in a manner so that the resin pressure is maintained in the vicinity of a predetermined value (metering pressure) P 1 . A description will be given later concerning the configuration of the control device  20 . 
     The resin that is melted while being fed and compressed reaches a region (including the nozzle flow path  41 ) on the frontward side of the check seat  48  inside the cylinder  26 , and is accumulated inside the region. Hereinafter, the region on the frontward side of the check seat  48  inside the cylinder  26  may also be referred to as a “metering region”. 
     The forward rotation and rearward movement of the screw  28  are performed until the screw  28  reaches a predetermined position (metering position) by way of such rearward movement. More specifically, until the screw  28  arrives at the metering position, the resin inside the cylinder  26  continues to be fed and compressed toward the metering region while being melted. 
     The step of carrying out forward rotation and rearward movement until the screw  28  arrives at the metering position to thereby accumulate the molten resin in the metering region may also be referred to as a “metering step” or simply “metering”. By performing such metering, a certain predetermined amount of the resin can be accumulated in the metering region. 
     Moreover, when metering is performed, it is necessary to specify in advance a metering pressure P 1 , and a predetermined rotational speed of the screw  28  that undergoes forward rotation. The metering pressure P 1  and the predetermined rotational speed, which are specified in relation to metering, may also be referred to as “metering conditions”. 
     After the screw  28  has arrived at the metering position, a step of causing the resin pressure in the metering region to be reduced from the metering pressure P 1  to the target pressure P 0  is carried out by further causing the screw  28  to be retracted (moved rearward) from the metering position. Such a step may also be referred to as a “pressure reducing step” or simply a “reduction in pressure”. 
     Further, the operation of further moving rearward the screw  28  that has reached the metering position may also be referred to as “sucking back”. When sucking back is carried out, the volume of the metering region is enlarged corresponding to the distance over which the screw  28  is moved rearward. Consequently, an expansion in the volume of the resin in the metering region, and more specifically, a decrease in the density of the resin takes place, and as a result, the resin pressure in the metering region is reduced. 
     Sucking back is performed on the basis of a condition predetermined in relation to sucking back. Hereinafter, such a predetermined condition may also be referred to as a “suck back condition”. The suck back condition may include designation of a suck back distance L sb , or designation of a suck back time period T sb . 
     The suck back distance L sb  is a distance over which the screw  28  undergoes rearward movement relatively with respect to the cylinder  26  due to being sucked back. The suck back time period T sb  is a time period during which sucking back is continued. 
     As the target pressure P 0 , a pressure is specified which is smaller than the metering pressure P 1  (P 0 &lt;P 1 ). Although the magnitude thereof is not particularly limited, for example, the value of atmospheric pressure (zero) can be specified. 
     The resin pressure in the metering region is in the vicinity of the metering pressure P 1  immediately after the screw  28  has arrived at the metering position, i.e., immediately after metering has been carried out. By reducing the resin pressure from being in the vicinity of the metering pressure P 1  to the target pressure P 0 , it is possible to weaken the forward momentum of the resin in the metering region, which has received the pressure directed toward the frontward direction in the metering step. Consequently, flowing of the resin in the metering region in the frontward direction is suppressed, and the occurrence of drooling is prevented. 
     In addition to being sucked back, causing the pressure of the resin in the metering region to be reduced can also be achieved by causing the screw  28  to be rotated (reversely rotated) in a direction opposite to that at the time of metering. However, in the present embodiment, a description concerning such a reduction in pressure due to reverse rotation is omitted. 
     After having carried out metering and a subsequent reduction in pressure, the resin accumulated in the metering region inside the cylinder  26  is filled into a cavity of the mold  12 . Such a process is also referred to as an “injection step” or simply “injection”. 
     Injection is performed in a state in which the mold  12  of the mold clamping unit  14  and the nozzle  40  of the injection unit  16  are pressed against each other for contact with each other, so that the cavity of the mold  12  and the nozzle flow path  41  are placed in communication with each other. Pressing of the mold  12  and the nozzle  40  against each other may also be referred to as “nozzle touching”. When injection is carried out, the mold  12  is placed in a closed state, for example, by a well-known toggle mechanism provided in the mold clamping unit  14 , and a mold clamping force is applied thereto. By advancement of the screw  28 , the injection unit  16  pushes out the resin in the metering region, through the nozzle  40 , into the cavity of the mold  12  to which the mold clamping force is applied. Consequently, the cavity is filled with the resin. 
     Immediately after injection, the screw  28  is in a state of being fully advanced inside the cylinder  26 . Accordingly, after injection, the injection unit  16  can perform metering again. In this manner, the injection unit  16  is capable of efficiently and repeatedly carrying out metering, reduction in pressure, and injection in this order. 
     On the other hand, in the mold clamping unit  14 , cooling and solidification of the resin that is filled in the mold  12  by executing injection, opening of the mold  12 , and removal of the solidified resin (a molded product) are carried out. The step of cooling the resin that is filled in the mold  12  may also be referred to as a “cooling step” or simply “cooling”. Further, the step of opening the mold  12  may also be referred to as a “mold opening step” or simply “mold opening”. Further, the step of removing the molded product may also be referred to as a “removal step” or simply “removal”. 
     Between the steps of mold opening and removal, the molded product may be ejected from the mold  12  by a known ejector (ejecting pin) provided in the mold clamping unit  14 . This step may also be referred to as an “ejecting step” or simply “ejection”. By ejection of the molded product, subsequent removal of the molded product can be easily accomplished. 
     Further, by closing the mold  12  after having removed the molded product, the mold  12  can be placed in a state in which the resin can be filled therein again. Further, the step of closing the mold  12  may also be referred to as a “mold closing step” or simply “mold closing”. In the foregoing manner, the mold clamping unit  14  can repeatedly perform cooling, mold opening, ejection, removal, and mold closing in this order. 
     The plurality of steps described above can be performed routinely as a “molding cycle”. By repeatedly executing the molding cycle, the injection molding machine  10  is capable of efficiently mass producing molded products. 
     Next, a description will be given concerning matters that can be considered in order to obtain high quality molded products. In order to obtain high quality molded products, it is desirable to reduce insofar as possible the occurrence of defects during execution of the molding cycle. Defects that occur during execution of the molding cycle may also be referred to as molding defects. The aforementioned drooling is a typical example of such a molding defect. Further, mixing of air (foreign material) into the metered resin may also be cited as an example of the molding defect. 
     In order to reduce any concern over drooling, it is necessary to appropriately perform sucking back in the pressure reducing step, by appropriately specifying the suck back distance L sb  or the suck back time period T sb . For example, if the suck back distance L sb  or the suck back time period T sb  can be specified in a manner so that the resin filled inside the nozzle flow path  41  is drawn in from the nozzle  40  side to the cylinder  26  side at a certain volume amount or distance amount, any concern over drooling can be reduced. 
     However, the suck back distance L sb  or the suck back time period T sb , by which the resin inside the nozzle flow path  41  is drawn in from the side of the nozzle  40  to the side of the cylinder  26  at a fixed distance amount or a fixed volume amount, is not obvious to the operator at first glance. In addition, if the specified suck back distance L sb  or the specified suck back time period T sb  is excessive, excessive drawing in of air from the distal end of the nozzle  40  into the nozzle flow path  41  occurs when sucking back is performed. In such a case, mixing of air (foreign material) into the resin disadvantageously takes place. 
     As can be appreciated from the above, in order for the operator to appropriately specify the suck back distance L sb  or the suck back time period T sb , the operator is required to perform trial and error attempts while taking into consideration material properties of the resin and specifications of the injection molding machine  10 . From the standpoint of the operator, performing such tasks is a burden. 
     Thus, according to the present embodiment, the injection molding machine  10  causes the control device  20  to calculate an appropriate suck back distance L sb  or an appropriate suck back time period T sb  in order to achieve drawing in of the resin inside the nozzle  40  to the cylinder  26  side at a target distance amount L tar  or a target volume amount V tar . A description will be given in detail below concerning the control device  20  of the present embodiment. 
       FIG. 3  is a schematic configuration diagram of the control device  20 . 
     From among the mold clamping unit  14  and the injection unit  16  provided in the injection molding machine  10 , the control device  20  according to the present embodiment controls at least the injection unit  16 . The control device  20  is equipped with a storage unit  66 , a display unit  68 , an operation unit  70 , and a computation unit  72 . 
     Among these units, the storage unit  66  may include a volatile memory and a nonvolatile memory, neither of which are shown. The volatile memory can be configured by hardware such as a RAM (Random Access Memory) or the like. The nonvolatile memory can be configured by hardware such as a ROM (Read Only Memory), a flash memory, or the like. 
     A predetermined control program  74  for controlling the injection unit  16  is stored in advance in the storage unit  66 . Further, the storage unit  66  appropriately stores information necessary for controlling the injection unit  16 . Among such information, descriptions will be given below concerning information in the present embodiment which is deserving of particular explanation, as necessary. 
     Although not limited to this feature, the display unit  68 , for example, is a display device equipped with a liquid crystal screen. The display unit  68  appropriately displays information concerning the controls performed by the control device  20 . 
     Although not limited to this feature, the operation unit  70  comprises, for example, a keyboard, a mouse, or a touch panel that can be attached to the screen (liquid crystal screen) of the display unit  68 . The operation unit  70  can be used by the operator in order to transmit commands to the control device  20 . 
     The computation unit  72  may be configured by hardware such as, for example, a CPU (Central Processing Unit) or the like. The computation unit  72  includes a pressure acquisition unit  76 , a metering control unit  78 , a calculation unit  80 , a change-in-volume acquisition unit  82 , and a suck back control unit  84 . These respective units can be realized by the computation unit  72  executing the control program  74  in cooperation with the storage unit  66 . Hereinafter, descriptions will be given concerning each of such units. 
     The pressure acquisition unit  76  sequentially acquires the resin pressure detected by the pressure sensor  30 . Although not limited to this feature, the acquired resin pressure is stored in the storage unit  66 , for example, in the form of time series data. The data in relation to the stored resin pressure can be referred to by the metering control unit  78 . Further, the operator may be made capable of monitoring such data by displaying the data on the display unit  68 . 
     Among the controls of the injection unit  16 , the metering control unit  78  carries out a control particularly in relation to metering. More specifically, initially, in the case that the metering conditions are stored in the storage unit  66 , the metering control unit  78  acquires the metering pressure P 1  and the predetermined rotational speed by referring to the storage unit  66 . Moreover, the metering control unit  78  may acquire, as the metering pressure P 1  or the predetermined rotational speed, values that are instructed by the operator via the operation unit  70 . 
     When the metering control unit  78  acquires the metering conditions, the screw  28  is forwardly rotated at a predetermined rotational speed by supplying a drive current to the servomotor  52   a  of the first drive device  32 . Further, while referring to the resin pressure acquired by the pressure acquisition unit  76 , the metering control unit  78  adjusts the drive current supplied to the servomotor  52   b  of the second drive device  34 , thereby causing the screw  28  to be moved rearward to the metering position while maintaining the resin pressure in the vicinity of the metering pressure P 1 . 
     The calculation unit  80  calculates the suck back distance L sb  or the suck back time period T sb  in order to achieve drawing in of the resin inside the nozzle flow path  41  to the cylinder  26  side at the target distance L tar  amount or the target volume V tar  amount. The operator may select which one of the suck back distance L sb  and the suck back time period T sb  is calculated. In the present embodiment, as an example, a description will be given in which the calculation unit  80  serves to calculate the suck back distance L sb . A description will be given later in a modified example concerning a case in which the suck back time period T sb  is calculated. 
     In the calculation unit  80 , calculation of the suck back distance L sb  is carried out on the basis of the target volume V tar  of the resin inside the nozzle  40  that is drawn in from the nozzle  40  side to the cylinder  26  side. More specifically, the calculation unit  80  according to the present embodiment calculates the suck back distance L sb  based on the following Equation (1). In Equation (1), the target volume V tar  is input thereto, and the suck back distance L sb  is output therefrom. 
     In the following equation, the term dV cyl  represents the amount of change (expansion) in the volume of the resin, in the case that the pressure of the resin (metered resin) in the metering region is reduced due to being sucked back from the metering pressure P 1  to atmospheric pressure. The term D cyl  is a known numerical value, and represents the inner diameter of the cylinder  26 . The character IC represents the circumferential ratio (pi). 
     
       
         
           
             
               
                 
                   
                     L 
                     sb 
                   
                   = 
                   
                     
                       
                         V 
                         tar 
                       
                       + 
                       
                         dV 
                         cyl 
                       
                     
                     
                       
                         π 
                         4 
                       
                       · 
                       
                         D 
                         cyl 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The target volume V tar  can also be indirectly obtained based on the shape of the nozzle  40 , and more specifically, based on the shape of the nozzle flow path  41 , and the target distance L tar  over which the resin inside the nozzle  40  is drawn in from the nozzle  40  side to the cylinder  26  side. For example, in the case of the present embodiment, the shape of the nozzle flow path  41  is cylindrical. In this case, the target volume V tar , by which drawing in of the resin inside the nozzle  40  to the cylinder  26  side at the target distance L tar  amount is achieved, is obtained in accordance with the following Equation (2). In Equation (2), a function f(L tar ) is shown to which the target distance L tar  is input, and which outputs the target volume V tar . In the following equation, D noz  is a known numerical value, and represents the inner diameter of the nozzle flow path  41 . 
     
       
         
           
             
               
                 
                   
                     V 
                     tar 
                   
                   = 
                   
                     
                       f 
                        
                       
                         ( 
                         
                           L 
                           tar 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         π 
                         4 
                       
                       · 
                       
                         D 
                         noz 
                         2 
                       
                       · 
                       
                         L 
                         tar 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     As a shape of the nozzle flow path  41  other than a cylinder, for example, a tapered shape may be cited. Further, the nozzle  40 , in which the shape of the opening of the nozzle flow path  41  is not circular but elliptical, may also be provided in the cylinder  26  of the injection molding machine  10 . In the case that the present embodiment is applied to such a nozzle  40 , the function f(L tar ) corresponding to such a shape of the target nozzle  40  may be obtained geometrically. 
       FIG. 4  is an example of a first table  86  which is stored in the storage unit  66  according to the present embodiment. 
     A corresponding relationship between the shape of the nozzle  40  and the function f(L tar ) specified in accordance with the shape of the nozzle  40  can be defined in the first table  86 . The first table  86  can be stored in the storage unit  66 . As shown in  FIG. 4 , in the case that the number of types of the shape of the nozzle  40  is greater than or equal to m, the number of types of the function f(L tar ) may be greater than or equal to m (m: a natural number of 1 or greater). 
     By referring to the first table  86  and based on the shape of the nozzle  40 , the calculation unit  80  is capable of easily specifying the type of an appropriate function f(L tar ) in order to calculate the target volume V tar  from the target distance L tar . Information regarding the shape of the nozzle  40 , which serves as a key when the table is referred to, can be input by the operator, for example, via the operation unit  70 . 
     The amount of change dV cyl  included in Equation (1) is acquired by the change-in-volume acquisition unit  82 . The change-in-volume acquisition unit  82  acquires the amount of change dV cyl , for example, by way of a calculation based on the following Equation (3). In the following equation, L met  represents the length of the distance by which the screw  28  is moved rearward in the metering step. The term ρ 0  is a known numerical value, and represents the density of the resin under the target pressure P 0 . The term ρ 1  represents the density of the resin under the metering pressure P 1 . 
     
       
         
           
             
               
                 
                   
                     dV 
                     cyl 
                   
                   = 
                   
                     
                       π 
                       4 
                     
                     · 
                     
                       D 
                       cyl 
                       2 
                     
                     · 
                     
                       L 
                       met 
                     
                     · 
                     
                       ( 
                       
                         
                           
                             ρ 
                             1 
                           
                           
                             ρ 
                             0 
                           
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The change-in-volume acquisition unit  82  applies to Equation (3) the density ρ 1 , which is calculated based on the position of the screw  28 , and the pressure of the resin when the screw  28  has reached the metering position. By calculating the density ρ 1  each time that metering is performed, the change-in-volume acquisition unit  82  is capable of acquiring the amount of change dV cyl  with higher accuracy. By the change-in-volume acquisition unit  82  acquiring the amount of change dV cyl  as accurately as possible, and by assigning the target volume V tar  acquired from Equation (2) and the amount of change dV cyl  acquired from Equation (3) to Equation (1), the calculation unit  80  can calculate the suck back distance L sb  with high accuracy. 
     It should be noted that, according to the present embodiment, it is not essential that the density ρ 1  be calculated each time that metering is performed. More specifically, a value of the density ρ 1  which is obtained experimentally in advance may be applied to Equation (3).  FIG. 5  is an example of a second table  88  which is stored in the storage unit  66  according to the present embodiment. 
     In the case that the density ρ 0  and the density ρ 1  are experimentally determined in advance for each type of resin, the amount of change dV cyl  for each type of resin can be prepared in advance. In this case, as shown in  FIG. 5 , a second table  88 , in which the type of resin, and the amount of change dV cyl  corresponding to the type of resin are associated with each other, can be prepared and stored in the storage unit  66 . Consequently, by referring to the second table  88  and based on the type of resin, the change-in-volume acquisition unit  82  is capable of easily acquiring the amount of change dV cyl . For example, in the case that the type of resin is “PA (polyamide)”, the change-in-volume acquisition unit  82  can easily acquire the value of the amount of change dV cyl  (dV cyl1 ) corresponding to PA by referring to the second table  88 . Information regarding the type of resin, which serves as a key when the table is referred to, can be input by the operator, for example, via the operation unit  70 . 
     Moreover, the second table  88  may be merged with the above-described first table  86 . More specifically, in the present embodiment, a table may be created in which the shape of the nozzle  40 , the function f(L tar ) corresponding to the shape of the nozzle  40 , the type of resin, and the amount of change dV cyl  corresponding to the type of resin are associated with each other. 
     From among the controls for the injection unit  16 , in particular, the suck back control unit  84  performs a control in relation to reducing pressure due to sucking back. After the screw  28  has reached the predetermined metering position, by supplying a drive current to the servomotor  52   b , the suck back control unit  84  causes the screw  28  to be sucked back based on the suck back distance L sb  or the suck back time period T sb  calculated by the calculation unit  80 . 
     Moreover, according to the present embodiment, it is assumed that a rearward movement speed (suck back speed) U sb  of the screw  28  during sucking back is determined in advance. 
     An exemplary configuration of the control device  20  has been described above. It should be noted that the configuration of the control device  20  is not limited to the above description. For example, the control device  20  may further comprise a configuration for controlling the mold clamping unit  14 . Further, the injection molding machine  10  which is capable of being controlled by the control device  20  is not limited to being an in-line type injection molding machine. 
     Next, a description will be given below concerning a control method for the injection molding machine  10  according to the present embodiment. 
       FIG. 6  is a flowchart showing an example of the control method for the injection molding machine  10  according to the present embodiment. 
     The control method for the injection molding machine  10  according to the present embodiment (hereinafter, simply referred to as a “control method”) is executed by the above-described control device  20 . As shown in  FIG. 6 , such a control method includes at least a calculation step and a suck back control step. Hereinafter, a description will be given concerning such a control method. 
     As a premise, hereinafter, a description will be given concerning a case in which the suck back distance L sb  is calculated from among the suck back distance L sb  and the suck back time period T sb . 
     It is assumed that the control method according to the present embodiment is initiated from a metering control step (metering step). The present step is executed by the metering control unit  78  in the present embodiment. 
       FIG. 7A  is a schematic cross-sectional view showing an example of a state inside the cylinder  26  at a point in time when the metering control step is completed. 
     The metering control step is continued until the screw  28  arrives at the metering position, and more specifically, until the rearward movement distance of the screw  28  reaches the predetermined distance L met . By performing the metering control step, as shown in  FIG. 7A , the molten resin is filled in the metering region on the frontward side of the check ring  50  including the nozzle flow path  41 . 
     When the screw  28  arrives at the metering position, the change-in-volume acquisition step is initiated. The present step is executed by the change-in-volume acquisition unit  82  in the present embodiment. In the present step, initially, the density ρ 1  of the resin in the metering region under the predetermined metering pressure P 1  is calculated, based on the position (rearward movement distance) of the screw  28  and the pressure of the resin at the time of having reached the metering position. In addition, the amount of change dV cyl  is acquired based on Equation (3) which has already been described. Moreover, in the case that the second table  88  is stored in the storage unit  66  in advance, the amount of change dV cyl  may be acquired by referring to the second table  88 . 
     Subsequently, the calculation step is executed. In the present step, the suck back distance L sb  is calculated based on the target volume V tar . Such a calculation is performed based on Equation (1) which has already been described. 
     Specification of the target volume V tar  required for calculating the suck back distance L sb  is carried out by the operator via the operation unit  70 . In the following example, it is assumed that the target volume V tar  has been calculated by inputting into Equation (2) the target distance L tar  instructed by the operator. However, the present embodiment is not limited to the description given above. For example, as the target volume V tar , a default value specified by the manufacturer of the injection molding machine  10  may be automatically specified. 
       FIG. 7B  is a schematic cross-sectional view showing an example of a state inside the cylinder  26  after sucking back has been executed. 
     After the calculation step, the suck back control step is executed in which the screw  28  is sucked back based on the calculated suck back distance L sb . The present step is executed by the suck back control unit  84  in the present embodiment. The suck back control unit  84  continues sucking back the screw  28  at the predetermined suck back speed U sb  until the screw  28  is moved rearward by the suck back distance L sb . 
     According to the above-described control method, it is possible to easily calculate the appropriate suck back distance L sb , which achieves drawing in of the resin inside the nozzle  40  to the side of the cylinder  26  at the target volume V tar  (over the target distance L tar ) amount. 
     More specifically, according to the present embodiment, the control device  20  and the control method for the injection molding machine  10  are provided, in which the suck back distance L sb  can be appropriately and easily determined. The operator can easily produce high quality molded products by using the injection molding machine  10  which is equipped with the control device  20  of the present embodiment. 
     [Modifications] 
     Although an embodiment has been described above as one example of the present invention, it goes without saying that various modifications or improvements are capable of being added to the above-described embodiment. It is clear from the scope of the claims that other modes to which such modifications or improvements have been added can be included within the technical scope of the present invention. 
     (Modification 1) 
     In the present modification, as a supplement to the embodiment, an example of a case in which the suck back time period T sb  is obtained will be disclosed. 
     The suck back time period T sb  corresponding to the target distance L tar  can be obtained by the following Equation (4). In the following equation, the term U sb  is the suck back speed. Moreover, even in this case, the target volume V tar  can be indirectly calculated from the target distance L tar  based on Equation (2). 
     
       
         
           
             
               
                 
                   
                     T 
                     sb 
                   
                   = 
                   
                     
                       
                         L 
                         sb 
                       
                       
                         U 
                         sb 
                       
                     
                     = 
                     
                       
                         
                           V 
                           tar 
                         
                         + 
                         
                           dV 
                           cyl 
                         
                       
                       
                         
                           π 
                           4 
                         
                         · 
                         
                           D 
                           cyl 
                           2 
                         
                         · 
                         
                           U 
                           sb 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     By using the above Equation (4), the calculation unit  80  is capable of easily and appropriately calculating the suck back time period T sb  which achieves drawing in of the resin inside the nozzle  40  at the target volume V tar  (over the target distance L tar ) toward the cylinder  26 . 
     In the foregoing manner, according to the present modification, the control device  20  and the control method for the injection molding machine  10  are provided, in which the suck back time period T sb  can be appropriately and easily determined. 
     (Modification 2) 
     In the case that the target volume V tar  is in excess of a predetermined limit value V max , the calculation unit  80  may calculate the suck back distance L sb  or the suck back time period T sb  after having limited the target volume V tar  to a value less than or equal to the limit value V max . Such a limitation can be applied not only to the target volume V tar  instructed by the operator, but also to the target volume V tar  that is specified from the target distance L tar . 
     The limit value V max , for example, is a value specified by the manufacturer of the injection molding machine  10 . The limit value V max  may also be instructed by the operator via the operation unit  70 . 
     Consequently, in the case that the target volume V tar  is in excess of the limit value V max , any concern over an excessive suck back distance L sb  being calculated on the basis of such an excessive target volume V tar  can be reduced. Similarly, any concern over an excessive suck back time period T sb  being calculated on the basis of such an excessive target volume V tar  can be reduced. 
     (Modification 3) 
       FIG. 8  is a schematic configuration diagram of the control device  20  according to a third modification. 
     The calculation unit  80  may include a compensation unit  90  which, in the case that the calculated suck back distance L sb  is in excess of an upper limit value L. of the predetermined distance, compensates (modifies) the suck back distance L sb  to the upper limit value L max . The upper limit value L max , for example, is a value specified by the manufacturer of the injection molding machine  10 . The upper limit value L max  may also be instructed by the operator via the operation unit  70 . 
     In accordance with this feature, any concern over sucking back being performed on the basis of such an excessive suck back distance L sb  can be reduced. 
     Further, the present modification can also be applied in the case that the calculation unit  80  calculates the suck back time period T sb . More specifically, the calculation unit  80  may include the compensation unit  90  which, in the case that the calculated suck back time period T sb  is in excess of an upper limit value T max  of the predetermined time period, compensates (modifies) the suck back time period T sb  to the upper limit value T max . The upper limit value T max , for example, is a value specified by the manufacturer of the injection molding machine  10 , similarly to the case of the upper limit value L max . The upper limit value T max  may also be instructed by the operator via the operation unit  70 . 
     In accordance with this feature, any concern over sucking back being performed on the basis of such an excessive suck back time period T sb  can be reduced. 
     (Modification 4) 
       FIG. 9  is a schematic configuration diagram of the control device  20  according to a fourth modification. 
     The control device  20  may further be equipped with a notification unit  92  that issues a notification of the calculated suck back distance L sb  or the calculated suck back time period T sb . Such a notification can be performed, for example, by causing the suck back distance L sb  or the suck back time period T sb  to be displayed on the display unit  68 . 
     In accordance with this feature, the operator is capable of easily grasping the suck back distance L sb  or the suck back time period T sb  calculated by the control device  20 . 
     (Modification 5) 
     The amount of change dV cyl  need not necessarily be acquired. More specifically, in the calculation in which Equation (1) is used, the amount of change dV cyl  may always be treated as zero. Even in this case, the control device  20  can calculate a minimum required suck back distance L sb  or a minimum required suck back time period T sb  for achieving drawing in of the target volume V tar  amount (or the target distance L tar  amount) of the resin inside the nozzle  40 . 
     The present modification enables the operator to know the minimum value of the suck back distance L sb  or the suck back time period T sb  to be specified as the suck back condition. By referring to the suck back distance L sb  or the suck back time period T sb  calculated according to the present modification, the operator may newly specify the suck back distance L sb  or the suck back time period T sb . 
     According to the present modification, the burden on the operator can be significantly reduced, in that it is possible to easily grasp the minimum value of the suck back distance L sb  or the suck back time period T sb  that should be specified. Further, according to the present modification, the change-in-volume acquisition unit  82  can be omitted from the configuration of the control device  20 . Therefore, the configuration of the control device  20  can be made simpler than in the case of the embodiment. 
     (Modification 6) 
     The above-described embodiment and the modifications thereof may be appropriately combined within a range in which no technical inconsistencies occur. 
     [Inventions that can be Obtained from the Embodiment] 
     The inventions that can be grasped from the above-described embodiment and the modifications thereof will be described below. 
     &lt;First Invention&gt; 
     The control device ( 20 ) for the injection molding machine ( 10 ) is provided. The injection molding machine includes the cylinder ( 26 ) into which the resin is supplied, the nozzle ( 40 ) disposed at the distal end of the cylinder ( 26 ), and the screw ( 28 ) that moves forward and rearward and rotates inside the cylinder ( 26 ). The injection molding machine performs metering of the resin while the resin is being melted inside the cylinder ( 26 ), by causing the screw ( 28 ) to be moved rearward to the predetermined metering position while being rotated, in a manner so as to keep the resin pressure at the predetermined metering pressure (P 1 ). The control device includes the calculation unit ( 80 ) that calculates, based on the target volume (V tar ) of the resin inside the nozzle ( 40 ) that is drawn in from the side of the nozzle ( 40 ) to the side of the cylinder ( 26 ), the suck back distance (L sb ) or the suck back time period (T sb ) in order to achieve drawing in of the target volume (V tar ) of the resin inside the nozzle ( 40 ) to the side of the cylinder ( 26 ), and the suck back control unit ( 84 ) which causes the screw ( 28 ) to be sucked back on the basis of the suck back distance (L sb ) or the suck back time period (T sb ), after the screw ( 28 ) has reached the predetermined metering position. 
     In accordance with such features, the control device ( 20 ) for the injection molding machine ( 10 ) is provided, in which the suck back distance (L sb ) or the suck back time period (T sb ) can be appropriately and easily determined. 
     There may further be provided the change-in-volume acquisition unit ( 82 ) which acquires, after the screw ( 28 ) has reached the predetermined metering position, the amount of change (dV cyl ) in the volume of the resin metered inside the cylinder ( 26 ) while the pressure of the resin is reduced from the predetermined metering pressure (P 1 ) to atmospheric pressure (P 0 ), wherein the calculation unit ( 80 ) calculates the suck back distance (L sb ) or the suck back time period (T sb ) based on the amount of change (dV cyl ) and the target volume (V tar ). In accordance with such features, the calculation unit ( 80 ) can accurately calculate the suck back distance (L sb ) or the suck back time period (T sb ). 
     There may further be provided the pressure acquisition unit ( 76 ) that acquires the resin pressure, wherein the change-in-volume acquisition unit ( 82 ) acquires the amount of change (dV cyl ) based on the distance (L met ) over which the screw ( 28 ) is moved rearward during metering, and the pressure (P 1 ) of the resin when the screw ( 28 ) has reached the predetermined metering position. In accordance with such features, the change-in-volume acquisition unit ( 82 ) can acquire the amount of change (dV cyl ) with higher accuracy. 
     There may further be provided the operation unit ( 70 ) through which the operator instructs the target volume (V tar ). In accordance with this feature, the suck back distance (L sb ) or the suck back time period (T sb ), which achieves drawing in of the resin inside the nozzle ( 40 ) to the side of the cylinder ( 26 ) at the target volume (V tar ) amount instructed by the operator, can be calculated. 
     There may further be provided the storage unit ( 66 ) in which there is stored the first table ( 86 ) in which the plurality of functions are defined in association with the shape of the nozzle ( 40 ), the functions being configured to calculate the target volume (V tar ) based on the shape of the nozzle ( 40 ) and the target distance (L tar ) over which the resin inside the nozzle ( 40 ) is drawn in from the side of the nozzle ( 40 ) to the side of the cylinder ( 26 ), wherein the calculation unit ( 80 ) selects from within the first table ( 86 ) the function corresponding to the shape of the nozzle ( 40 ) provided on the cylinder ( 26 ), and calculates the suck back distance (L sb ) or the suck back time period (T sb ) based on the selected function and the target distance (L tar ). In accordance with such features, the calculation unit ( 80 ) can easily specify the appropriate function for calculating the target volume (V tar ) from the target distance (L tar ). 
     The storage unit ( 66 ) may further store therein the second table ( 88 ) in which the amount of change (dV cyl ) and the type of resin are associated with each other, wherein the change-in-volume acquisition unit ( 82 ) acquires the amount of change (dV cyl ) by referring to the second table ( 88 ), and based on the type of resin. In accordance with such features, the change-in-volume acquisition unit ( 82 ) is capable of easily acquiring the amount of change (dV cyl ). 
     In the first invention, when the change-in-volume acquisition unit ( 82 ) is provided, and in the case that the target volume (V tar ) is not calculated from the target distance (L tar ), there may further be provided the storage unit ( 66 ) in which there is stored the table ( 88 ) in which the amount of change (dV cyl ) and the type of resin are associated with each other. In accordance with such features, the change-in-volume acquisition unit ( 82 ) is capable of easily acquiring the amount of change (dV cyl ), even in the case that the target volume (V tar ) is not calculated from the target distance (L tar ). 
     There may further be provided the operation unit ( 70 ) through which the operator instructs the target distance (L tar ). In accordance with this feature, the suck back distance (L sb ) or the suck back time period (T sb ), which achieves drawing in of the resin inside the nozzle ( 40 ) to the side of the cylinder ( 26 ) over the target distance (L tar ) amount instructed by the operator, can be calculated. 
     In the case that the target volume (V tar ) is in excess of the predetermined limit value (V max ), the calculation unit ( 80 ) may calculate the suck back distance (L sb ) or the suck back time period (T sb ) after having limited the target volume (V tar ) to a value less than or equal to the limit value (V max ). In accordance with this feature, any concern over an excessive suck back distance (L sb ) being calculated on the basis of an excessive target volume (V tar ) can be reduced. 
     The calculation unit ( 80 ) may further include the compensation unit ( 90 ) configured to, in the case that the calculated suck back distance (L sb ) or the calculated suck back time period (T sb ) is in excess of the predetermined upper limit value (L max , T max ), compensate the suck back distance (L sb ) or the suck back time period (T sb ) to the upper limit value (L max , T max ). In accordance with this feature, any concern over sucking back being performed on the basis of such an excessive suck back distance (L sb ) or an excessive suck back time period (T sb ) can be reduced. 
     There may further be provided the notification unit ( 92 ) that issues a notification of the calculated suck back distance (L sb ) or the calculated suck back time period (T sb ). In accordance with this feature, the operator is capable of easily grasping the suck back distance (L sb ) or the suck back time period (T sb ) calculated by the control device ( 20 ). 
     &lt;Second Invention&gt; 
     The control method for the injection molding machine ( 10 ) is provided. The injection molding machine includes the cylinder ( 26 ) into which the resin is supplied, the nozzle ( 40 ) disposed at the distal end of the cylinder ( 26 ), and the screw ( 28 ) that moves forward and rearward and rotates inside the cylinder ( 26 ). The injection molding machine performs metering of the resin while the resin is being melted inside the cylinder ( 26 ), by causing the screw ( 28 ) to be moved rearward to the predetermined metering position while being forwardly rotated, in a manner so as to keep the resin pressure at the predetermined metering pressure (P 1 ). The control method includes the calculation step of calculating, based on the target volume (V tar ) of the resin inside the nozzle ( 40 ) that is drawn in from the side of the nozzle ( 40 ) toward the side of the cylinder ( 26 ), the suck back distance (L sb ) or the suck back time period (T sb ) that achieves drawing in of the target volume (V tar ) of the resin inside the nozzle ( 40 ) to the side of the cylinder ( 26 ), and the suck back control step of causing the screw ( 28 ) to be sucked back on the basis of the suck back distance (L sb ) or the suck back time period (T sb ), after the screw ( 28 ) has reached the predetermined metering position. 
     In accordance with such features, the control method for the injection molding machine ( 10 ) is provided, in which the suck back distance (L sb ) or the suck back time period (T sb ) can be appropriately and easily determined.