Abstract:
Method and apparatus for control of an ejector mechanism of a molding machine. A procedure is executed without operator intervention for setting limits for controlling motion of movable members of mold assemblies, the movable members linked to ejector pins communicating with the mold cavity and used to assure molded articles are dislodged from a mold element. The procedure, typically associated with a “set-up” mode of control, operates an ejector mechanism with reduced force to drive the movable members to the extremes of travel thereof, senses a “stalled” condition at each extreme, and causes measured position of each extreme to be recorded. Position values intended for use in normal (program controlled) operation of the molding machine are derived from the recorded position values.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to molding machines, more particularly, to ejector mechanisms commonly used in molding machines. The invention is directed particularly to setting travel limits for ejector mechanisms. 
     II. Description of Related Art 
     In molding machines, plastically deformable material to be molded is formed in cavities defined by mating mold sections and allowed to cure to a state wherein the material will not unacceptably deform upon removal from the mold cavity. The cured material defines molded articles that are removed from the machine upon separation of the mating mold sections. However, as it is common that articles will adhere to one of the mold sections, it is typical to provide ejector pins communicating with the mold cavity and linked to movable members in the mold assembly comprising the mating mold sections. Motion of the ejector pins is effective to dislodge molded articles from the mold section, assuring their complete removal. The movable members are typically translatable and include links to the ejector pins to move them between retracted positions whereat their free ends are flush with mold cavity surfaces and forward positions whereat the free ends protrude into the mold cavity. 
     From U.S. Pat. No. 5,639,486 it is known to provide for calibration of a control of an ejector mechanism to establish a position value corresponding to or derived from an ejector retract travel extreme. In accordance with this patent, the ejector mechanism is controlled to retract to the travel extreme where motion is mechanically restrained and record a representation of position corresponding to the travel extreme. To prevent overloading the ejector mechanism, the retraction is stopped on detection of cessation of motion by a mechanical restraint (“stopper”). The ejector may be advance away from the stopper a predetermined distance “L” to define a “calibration completion position”. 
     As mold cavity depths vary according to the articles being produced, the translation of movable members required to dislodge articles varies accordingly. Although the calibration technique known from U.S. Pat. No. 5,639,486 is suitable for establishing a coordinate value associated with a retract position, known procedures for establishing stroke length for ejector mechanisms require data entry by a user having access to information concerning a mold assembly. Consequently, errors in setting of values for control of ejectors can result, and such errors may cause malfunctions of ejector mechanisms, triggering alarms and/or damaging machine or mold elements. Consequently, there is a need for improved methods for setting ejector mechanism stroke lengths that overcome the deficiencies of known methods. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide improved methods and apparatus for setting program controlled stroke length of ejector mechanisms for molding machines. 
     It is a further object of the present invention to provide an improved method for setting stroke length of an ejector mechanism of a molding machine wherein coordinate values of advance and retract end points are determined and recorded without operator intervention. 
     Further objects and advantages of the invention shall be made apparent from the accompanying drawings and the following description thereof. 
     In accordance with the aforesaid objects the present invention provides a method for setting a program controlled stroke length of an ejector mechanism of a molding machine. The ejector mechanism imparts translation to movable members of a mold assembly, the movable members being linked to ejector pins communicating with a mold cavity defined by mating mold sections. A procedure is performed under program control to effect definition of ejector travel limit position information. To limit forces generated during execution of the limit setting procedure, the procedure causes setting of an ejector actuator control parameter to limit useful force produced by the ejector actuator. The procedure then causes the ejector mechanism to be driven to advance the movable members to the extremes of their travel range, in each direction, motion being ceased as a result of physical restraint. Travel limit position information is defined in response to detection of restraint of motion at the travel range extremes. The definition of travel limit positions for both forward and rearward travel limits establishes an ejector stroke length adapted to the peculiarities of the mold assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an injection molding machine with a power operated ejector mechanism. 
     FIG. 2 is a block diagram of a control system for the injection molding machine of FIG.  1 . 
     FIG. 3 is a flow chart of a procedure used by the control system of FIG. 1 to set ejector travel limits. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To illustrate the invention, a preferred embodiment as implemented for an injection molding machine shall be described. It is contemplated that the invention could as well be applied to other molding machines, such as, but not limited to blow molding machines. 
     Referring to FIG. 1, injection molding machine  10  includes a clamp assembly  12  and injection unit  14 . Typical of plastic injection molding machines, raw material in the form of pellets and/or powders is introduced to an extruder  16  through hopper  18 . Extruder  16  includes a barrel portion  60 , typically surrounded by external heating elements  20 , and an internal material working screw, not shown. As raw material is plasticized by a combination of heating and material working, the plasticized material advances toward the exit end of the extruder, displacing the interior screw away from clamp assembly  12 . Once a sufficient volume of material has been plasticized, the working screw is advanced within barrel portion  60  to force material through the exit end of barrel portion  60  into a cavity defined by mating mold sections  22  and  24 . Clamp assembly  12  holds mold sections  22  and  24  together during injection and thereafter until the injected material has sufficiently solidified to be removed without unacceptable deformation. Movable platen  26  is then retracted, separating mold section  22  from mold section  24  to permit release of the molded article. 
     Continuing with reference to FIG. 1, clamp assembly  12  comprises fixed platen  28 , movable platen  26 , thrust or “die height” platen  36  and a mechanism for effecting translation of movable platen  26 , such as a toggle mechanism (not shown). Forces required to overcome separation forces acting on mold sections  22  and  24  during injection are generated by the toggle mechanism in reaction with strain rod pairs  32  and  34  supported at opposite ends by fixed platen  28  and thrust platen  36 . 
     Continuing with reference to FIG. 1, movable members  42 , within mold section  22 , are connected to ejector pins  56  that communicate with the mold cavity defined by mating mold sections  22  and  24 . Movable members  42  comprise a plate as illustrated by FIG. 1, and additional couplings, guides, springs, and the like as are known to movably support the plate within the mold assembly, connect the plate with connecting rods  54  of ejector mechanism  38  and connect the plate with ejector pins  56 . The number, size(s) and placement of ejector pins  56  are chosen according to characteristics of the article(s) defined by the mold cavity. Displacement of movable members  42  away from movable platen  26  advances ejector pins  56  to cause the free ends thereof to protrude beyond the surfaces of mold section  22  intersected by their respective axes of motion, such protrusion, or like repeated protrusions, being effective to dislodge an article from mold section  22 . While shown in FIG. 1 as intersecting a vertical flat surface, the mold cavity surfaces at the points of intersection with ejector pins  56  may be curved and/or at various angles. The free ends of ejector pins  56  are made to conform to the mold cavity surface at the points of intersection therewith so that when ejector pins  56  are retracted, the free ends thereof are flush with the mold cavity surfaces. While it is known to provide mechanical linkages to effect translation of movable members  42  with separation of mold sections  22  and  24 , it is also known to provide power operated ejector mechanisms to improve the effectiveness of ejector pins  56  to dislodge articles. 
     A power operated ejector mechanism  38  is illustrated in FIG. 1 disposed between thrust platen  36  and movable platen  26 . Ejector mechanism  38  effects translation of movable members  42  in mold section  22 . Ejector mechanism  38  includes transmission  44 , motor  40 , ejector arm  50 , ejector plate  52 , and ejector connecting rods  54 . Motor  40  drives transmission  44  to effect translatory motion of ejector arm  50 . Transmission  44  is fixably supported by mounting rods or brackets  46  and  48  attached to movable platen  26 . Motor  40  is mounted to and supported by transmission  44 . Hence, transmission  44  and motor  40  move with movable platen  26 . Advantageously, ejector plate  52  may be slidably supported by support rods  46  and  48 , will move with movable platen  26 , and will move relative to movable platen  26  with translation of ejector arm  50 . Connecting rods  54  are slidably supported by movable platen  26  and connect ejector plate  52  with movable members  42 . In consequence of the connection of ejector plate  52  with movable members  42 , translation of ejector arm  50  effects translation of movable members  42  relative to movable platen  26 . While plural connecting rods  54  are illustrated in FIG. 1, it is contemplated that ejector mechanism  38  may comprise a single connecting rod coupled to an ejector actuator or ejector arm without an interposed ejector plate. 
     As shown in FIG. 1, motor  40  is a rotating machine, wherein an armature and stator are arranged for rotation of one relative to the other. As is conventional, motor  40  is preferably a servo-motor and includes or works in combination with a position measuring transducer  120  which measures relative angular position. Also, as is well known for control of servo motors, other transducers may be used with motor  40  to measure, for example, angular velocity or to detect motor element relative locations for motor current commutation. Transmission  44  converts rotation of the armature of motor  40  to translation of ejector arm  50  along its length. The motion conversion of transmission  44  and the operation of transducer  120  are such that position of ejector arm  50  within its range of translatory motion can be unambiguously determined from measurement of angular position by position transducer  120 . In the configuration illustrated in FIG. 1, position transducer  120  may be an angular position encoder. 
     It is known to use linear actuators to effect translatory motion of connecting rods  54 . Hence, ejector mechanism  38  may alternatively comprise a linear electric motor or linearly operating hydraulic actuator and suitable coupling devices to propel connecting rods  54 . Further, position transducer  120  could be a linearly operating transducer used to directly measure linear position of a translating motor armature, linear displacement of ejector plate  52 , or linear displacement of connecting rods  54 . Irrespective of the nature of transducer  120 , it is effective to measure position representative of position of movable members  42  and, hence, representative of position of ejector pins  56 . 
     The desired range of motion of movable members  42  is that motion from the point at which the free ends of ejector pins  56  are flush with surfaces of portions of the mold cavity defined by mold section  22  to a point at which the free ends of ejector pins  56  protrude sufficiently beyond such surfaces to be effective to dislodge an article from mold section  22 . As the range of motion desired for movable members  42  depends on characteristics of movable members  42 , and ejector pins  56 , it is necessary to control operation of motor  40  so as to define a stroke length of ejector mechanism  38  matched to the desired range of motion of movable members  42 . 
     A control system for the injection molding machine shown in FIG. 1 shall be described with reference to FIG.  2 . Control system  80  includes a programmed controller  82 , such as, for example, a programmable logic controller or personal computer based control system, and an operator terminal  84  including a display  100  and input devices  102  such as keys, push buttons, computer “mouse”, and the like and data reading and recording devices such as magnetic tape drives, diskette drives, and magnetic strip or stripe card reading drives. Programmed controller  82  includes operator terminal interface circuits  94 , memory  86 , one or more processors indicated by processor  88 , output interface circuits  90 , and input interface circuits  92 . Operator terminal interface  94  includes circuits for controlling display of data on operator terminal  84  and for translating between signals used by processor  88  and signals used by input devices  102 . Memory  86  may include non-volatile memory such as semiconductor read only memory (ROM), volatile memory such as semiconductor random access memory (RAM), and mass storage devices such as disk memory. Processor  88 , typically, one or more digital processors, executes programs recorded in memory to process input signals, perform logical and arithmetic functions, and produce output signals to control the operation of machine devices. Input and output interface circuits  90  and  92  may include electrical and optical devices for translating between the digital electrical signals used by processor  88  and the digital and analogue electrical signals used by machine devices. Machine control  80  produces signals for controlling the operation of motor  40 . Output signals defining, for example, position, velocity, and/or acceleration are applied to motor drive  112  to control electrical current delivered to motor  40  from a suitable power source such as a conventional three-phase alternating current source. As is conventional, motor drive  112  uses signals produced by position transducer  120  and/or other transducers in connection with the control of current delivered to motor  40 . 
     Functions performed by programmed controller  82  are controlled by operating system programs  104  which may be recorded in ROM or otherwise stored in memory  86 . Operating system programs may be entirely dedicated to particular programmed controller hardware or may comprise commercially available operating systems for personal computers such as, for example, a WINDOWS operating system available from Microsoft Corp. Operating system programs  104  typically include programmed facilities for management of hardware resources and control the execution of machine control programs  96  by processor  88 . Machine control programs  96  perform logical and arithmetic functions to monitor and control the operation of machine devices. Typically, such programs permit at least two modes of operation of the machine: (i) an automatic mode for normal production; and (ii) a set-up or manual mode, for preparing the machine and machine devices for production and for setting parameter values used by the machine control programs in production of particular articles from particular material. While the automatic mode of operation will cause motion of machine members in accordance with values established by the user during machine set-up, the set-up mode permits manually controlled motion of machine members. Hence, routines for control of machine actuators, known as axes control routines, may be used to effect controlled motion in both automatic and manual or set-up modes of operation. 
     The present invention is concerned with a particular aspect of machine set-up, that is, establishment of values of travel limits for ejector mechanism  38  to limit the range of motion of connecting rods  54  to the desired range of motion of movable members  42 , thereby establishing a program controlled stroke length for ejector pins  56 . The operator selects a set-up mode of operation via operator terminal  84 . With set-up mode selected, the operator may invoke automated ejector limit setting, causing execution of ejector limit setting programs  110  recorded in memory  86 . 
     Description of the functions of ejector limit setting programs  110  shall be made with reference to the flow chart of FIG.  3 . At step  150 , an ejector actuator control parameter is set to limit the useable force (torque) produced by the ejector actuator, in the preferred embodiment, a motor current limit value for motor  40  (ILIM(E)) is set to a low value (LO). As the automatic limit setting procedure relies on physical restraint of movable members  42  at the extremes of their travel range, setting of the motor current limit value assures that sufficient force can be generated to propel movable members  42  and ejector pins  56 , without producing excessive strain on mechanical components at the travel extremes. 
     Continuing with reference to FIG. 3, at step  152 , motor  40  is driven in the direction to translate movable members  42  away from movable platen  26  and toward the ejector forward extreme. Decision step  154  represents a program execution delay waiting for detection of a “stalled” condition of motor  40 , i.e., a condition in which further motion is prevented notwithstanding control of motor  40  to continue motion. With position controlled servo motors, a “stalled” condition is advantageously determined by the servo position error, i.e. difference between commanded and measured position, exceeding a limit value (PE(E)&gt;LIM). Alternatively, a “stalled” condition may be determined by detecting the absence of change of position indicated by transducer  120  over a predetermined interval while motor  40  is controlled to effect motion. While awaiting detection of the stalled condition, further execution of the procedure of FIG. 3 is inhibited. To insure that an indefinite delay does not occur, a timer is advantageously associated with this decision step that, on expiration of a predetermined period, will cease further execution of the procedure and cause display of a fault message at display  100  for the operator. 
     Under the circumstances established by the procedure of FIG. 3, a first “stalled” condition will occur when movable members  42  are restrained from further motion at the ejector forward extreme (most distant from movable platen  26 ) of the ejector travel range. With detection of a first “stalled” condition, motor  40  is controlled to cease motion and actual position of the ejector mechanism (POS(E)) is read from transducer  120  at step  156 . At step  158 , the value of the ejector forward extreme (HILIM) is set equal to the position read at step  156 . At step  160 , the value of the ejector forward travel limit (FWDLIM) is calculated by subtracting a forward offset value (FOFF) from the position read at step  156 . The forward travel limit value is the value that will be used to control normal operation of the ejector mechanism during program controlled operation of the injection molding machine. Consequently, the offset value is chosen to allow for variables such as dimensional changes in components of movable members  42  whether induced mechanically, as with compression of springs, or thermally, as with transfer of heat during operation, and motion overshoot that may be encountered in normal machine operation. Conversely, the ejector forward extreme (HILIM) corresponds to the forward mechanical restraint, which, in normal operation, would be reached only in the event of a fault, and is associated with presentation of an alarm message on display  100  or other programmed response to occurrences of motion faults. 
     Following step  160 , execution of ejector limit setting programs  110  continues through on-page connector  3 -A at process step  162  where motor  40  is controlled to retract movable members  42 , i.e., to propel movable members  42  toward movable platen  26  and the ejector rearward extreme. Decision step  164  represents detection of a second “stalled” condition of motor  40 , under these circumstances, corresponding to physical restraint of movable members  42  at the ejector rearward extreme of ejector travel range. Decisions step  164 , like decision step  154 , will inhibit further execution of ejector limit setting programs  110  pending occurrence of the second “stalled” condition. In a like manner, a timer is advantageously associated with decision step  164  to prevent an indefinite delay of further program execution. With detection of a second “stalled” condition, motor  40  is controlled to cease rearward motion and actual position of the ejector mechanism (POS(E)) is read from transducer  120  at step  166 . At step  168 , the value of the ejector rearward extreme (LOLIM) is set equal to the position read at step  166 . As with the ejector forward extreme, the ejector rearward extreme is associated with the rearward mechanical restraint and, in normal operation, would be reached only in the event of a fault. At step  170 , the value of the ejector rearward travel limit (RETLIM) is calculated by subtracting a rearward offset value (ROFF) from the position read at step  166 . The rearward travel limit value is the value that will be used to control normal operation of the ejector mechanism during program controlled operation of the injection molding machine. 
     At step  172  the value of motor current limit for motor  40  is set equal to a nominal value used in normal operation of ejector mechanism  38 . This completes setting of travel limits for ejector mechanism  38  and execution of the ejector limit setting procedure ends at terminal  174 . 
     During program controlled operation of injection molding machine  10 , ejector mechanism  38  is controlled by use of the forward limit (FWDLIM(E)) and rearward limit values (RETLIM(E)). These values define the stroke length effected by ejector mechanism  38 , and consequently of ejector pins  56 . By virtue of the procedure used to establish the forward and rearward limit values, the stroke length is defined without operator intervention. 
     Successful removal of articles from mold section  22  may require repeated reciprocation of movable members  42 . As is well known, the number of operations of ejector mechanism  38  may be set so that during execution of a single normal cycle of operation of injection molding machine  10 , ejector pins  56  will advance and retract repeatedly, potentially repeatedly impacting molded articles retained in mold section  22  to dislodge them therefrom. As the forward and rearward travel limits established by the ejector limit setting programs  110  limit travel of movable members to less than the extremes of travel range, the present invention is effective to reduce wear and tear on ejector components that would otherwise be produced by such repetitive operation. 
     While the invention has been described with reference to a preferred embodiment, and while the preferred embodiment has been illustrated and described with considerable detail, it is not the intention of the inventors that the invention be limited to the detail of the preferred embodiment. Rather, it is intended that the scope of the invention be defined by the appended claims and all equivalents thereto.