Abstract:
Method and apparatus for control of a toggle operated press to effectively set relative position between a fixed platen and a die height platen. The toggle crosshead is placed at a position required to achieve a desired press clamp force. The die height platen is advanced toward the fixed platen until minute motion of the toggle crosshead away from the fixed platen is detected whereat advance of the die height platen is ceased. In the relative position of the die height platen and fixed platen results in contact of the mold sections prior to the crosshead being placed at the required position, the crosshead is retracted a predetermined distance and then advanced to the required position. Desired die height setting is achieved without repeated iterations of a die height setting procedure.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to presses, more particularly, to toggle operated presses commonly used in injection molding machines. The invention is directed particularly to setting relative locations of fixed and thrust or “die height” platens in such presses. 
     II. Description of Related Art 
     In injection molding machines, material to be molded is forced into cavities defined by mating mold sections. To permit relative motion of the mold sections, typically, at least one mold section is mounted to a movable platen driven by a press mechanism. In addition to moving the press member for productive use of the machine, the press mechanism, in combination with a press structure, provides the force required to overcome the separation force produced by injection of material into the mold cavities. A commonly used press mechanism for reciprocation of the movable platen is a “toggle” mechanism, a combination of pivoting links which produces translation and substantial mechanical advantage. In addition to such mechanisms, the press mechanism typically includes devices for setting the relative locations of the press mechanism and a fixed platen, to accommodate tooling elements (mold sections and mold “bases”) having a combined thickness within a range determined by overall press size, such thickness referred to as “height” or “die height”. 
     Although it would be possible to overcome mold separation forces by consistently imposing a mechanism maximum “clamping” force, the attainment of such forces requires maximum energy consumption and increases wear of machine components. Consequently, it is preferred that the “clamping” force be matched to the expected mold separation force. Hence, it is known to provide press mechanisms which permit setting of desired clamp forces while also permitting adjustment of die height. Examples of such mechanisms are shown and described in U.S. Pat. Nos. 5,059,253 and 5,149,471. As described in these patents, desired “clamping” force is produced by elastic stretch of strain rods induced by a toggle mechanism after mating mold sections are brought into contact. Typically, force at initial contact of mold sections is controlled, by, for example, torque limit control of the toggle mechanism drive motor during press closure. Once the mold sections have made contact, greater forces are permitted for further operation of the toggle mechanism to achieve the desired clamping force. 
     As described in the referenced patents, desired clamping force is achieved by precise setting of relative position between the press mechanism and a fixed platen on which a mating mold section is mounted. It is known, for example, to use iterative procedures requiring operation of the press mechanism and press mechanism positioning devices. As devices for positioning the press mechanism typically change position at low rates, such iterative procedures may require substantial time to complete position setting. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide improved methods and apparatus for setting die height in toggle operated press mechanisms. 
     It is a further object of the present invention to provide a method for setting die height of a toggle operated press wherein the likelihood of repetition of setting steps is reduced. 
     It is a further object of the present invention to provide a method of setting die height of a toggle operated press wherein the press mechanism is operated to locate a toggle crosshead at a predetermined position required to achieve a desired clamping force. 
     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 die height of a toggle operated press. The toggle is operated to position the toggle crosshead at a position where the mold sections will make contact and from which further extension of the toggle mechanism will generate the desired clamping force, this position is referred to as “required crosshead position” or “RCP”. Thereafter, the entire press mechanism is advanced toward the fixed platen until minute motion of the toggle crosshead away from the fixed platen is detected. In the event the relative location of the press mechanism prevents initial positioning of the toggle crosshead to the RCP, the press mechanism is moved away from the fixed platen a predetermined distance and the press mechanism is again operated to position the toggle crosshead at the RCP. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an injection molding machine with a toggle operated press. 
     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 die height of the injection molding machine press. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an injection molding machine  10  includes a clamp assembly  12  and an 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 liquefied, i.e. 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 toggle mechanism  38 . Fixed platen  28  supports mold section  24  and is rigidly mounted to machine base  30 . Strain rod pairs  32  and  34  are supported at opposite ends by fixed platen  28  and thrust or die height platen  36 . Movable platen  26  is slidably supported on strain rod pairs  32  and  34  for reciprocation between “open” and “closed” positions, “closed” referring to the advanced position as shown in FIG. 1. A toggle link mechanism  38 , interposed between movable platen  26  and thrust platen  36 , is operated by a rack and pinion combination comprising a rack (not shown) and pinion (not shown) within drive case  44 . The pinion is rotated by motor  40  to translate the rack horizontally toward and away from fixed platen  28 . A rack extension (not shown) connects the rack with a toggle link crosshead  56 . The rack extension is enclosed by bellows  42  to contain lubricant dislodged from the rack externally of drive case  44  and to prevent contaminants from entering drive case  44  at the opening through which the rack extension protrudes. Toggle link crosshead  56  includes guide sleeves, such as sleeve  46 , surrounding guide rods, such as rod  58 , supported between die height platen  36  and support plates, such as support plate  62 . In response to reciprocation of the rack, toggle link mechanism  38  produces reciprocation of moveable platen  26  and provides sufficient mechanical advantage to convert torque at motor  40  to the desired clamping force. Toggle link mechanism  38  is preferably operable to a “lock-over” configuration, as shown in FIG. 1 wherein serial pivoting links between thrust platen  36  and movable platen  26  are longitudinally aligned. On opening, reciprocation of crosshead  56  pivots these links to reduce the effective length and draw movable platen  26  away from fixed platen  28 . 
     Die height setting nut pairs  48  and  50  are threadably engaged with ends of strain rod pairs  32  and  34  outboard of thrust platen  36 . Die height setting nut pairs  48  and  50  are rotated by motor  52  through a drive such as drive chain  54 . Nut pairs  48  and  50  could as well be driven by, for example, a ring gear drive, or toothed belt drive. Rotation of nut pairs  48  and  50  positions the combination of die height platen  36 , toggle link mechanism  38  and movable platen  26 , that is, the press mechanism, along strain rod pairs  32  and  34 . 
     As is conventional, motor  40  is preferably a servo-motor and includes or works in combination with a position measuring transducer  120  which produces electrical signals representing position of the motor armature. Also, as is well known for control of servo motors, other transducers may be used with motor  40  to measure, for example, armature angular velocity or to detect armature locations for motor current commutation. Further, as is conventional, motor  52  is not operated as a servo-motor, and no position transducer is fitted to motor  52  or the die height adjusting drive. In the configuration illustrated in FIG. 1, position transducer  120  may be an encoder for measuring angular position of the motor armature. 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 and position transducer  120  measures the relative angular position. Were motor  40  a linear motor, position transducer  120  could as well measure linear position of a translating motor armature. Alternatively, position transducer  120  may measure linear displacement and be mechanically coupled to crosshead  56 . 
     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 programmable control  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. Programmable control  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 motors  40  and  52 . 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. Output signals defining direction of rotation are applied to motor relay  114  to control application to motor  52  of a suitable power source, such as for example, three-phase voltage. As is conventional, motor drive  112  uses signals produced by position transducer  120  in connection with the control of current delivered to motor  40 . Conversely, motor drive  114  may include current limiting devices such as thermal overload devices or fuses to prevent excessive currents flowing through motor  52 . 
     As is conventional, functions performed by programmable control  82  are controlled by operating system programs  94  which may be recorded in ROM or otherwise stored in memory  86 . Operating system programs may be dedicated to particular programmable control hardware or may be commercially available operating systems for personal computers such as, for example, a WINDOWS operating system available from Microsoft Corp. Operating system programs  94  control the execution of machine control programs  96  by processor  88 . Machine control programs 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 setup, that is, establishment of die-height so as to achieve a desired clamp force. The operator selects a set-up mode of operation via operator terminal  84 . With set-up mode selected, the operator may invoke automated die height setting, causing execution of the die height setting programs  110  recorded in memory  86 . 
     Description of the functions of die height setting programs  110  shall be made with reference to the flow chart of FIG.  3 . At step  150 , the required clamp force value entered by the operator is read from memory  86 . At step  152  the position of toggle crosshead  56  required to produce the specified clamp force is calculated from the following relationship: 
     
       
           RCP=K   1   *CF+OFF   1   +K   2   *MPF+OFF   2   +K   3   *MH+OFF   3   Equation 1 
       
     
     Where: 
     CF=required clamp force 
     MPF=mold protect force 
     MH=actual mold height 
     RCP=required crosshead position 
     K 1 =clamp force constant 
     K 2 =mold protect constant 
     K 3 =mold height constant 
     OFF 1 =clamp force offset 
     OFF 2 =mold protect offset 
     OFF 3 =mold height offset 
     The first term of equation 1, “K 1 *CF+OFF 1 ” defines a nominal crosshead location according to the desired clamp force. The clamp force constant “K 1 ” and clamp force offset “OFF 1 ” are values determined from measurements made on machine  10  using blank mold elements of nominal thickness, referred to as “mold height”. The clamp force constant “K 1 ” and clamp force offset “OFF 1 ” are determined from measurements of cross head position to produce clamp forces equal to the maximum clamp force and at least one reduced clamp force. 
     As the clamp closes, it is desired that initial contact of the mold elements occur with reduced force. Hence, common practice is to define a mold protect force to limit further advance of the moveable platen during clamp closure. It will be appreciated that crosshead position from which desired clamp force is generated varies as a function of the mold protect force, since the mold protect force arises from contact of the mold sections as the toggle is operated toward lock-over. Hence, equation  1  includes a term to account for mold protect force, that is “K 2 *MPF+OFF 2 ”. In this term, the mold protect constant “K 2 ” and mold protect offset “OFF 2 ” are determined from measurements made on machine  10  wherein a selected clamp force is achieved with blank mold elements of nominal thickness. The mold protect constant “K 2 ” and mold protect offset “OFF 2 ” are determined from measurements of cross head position to achieve the selected clamp force for at least two values of mold protect force. 
     Equation 1 includes a mold height term “K 3 *MH+OFF 3 ” to account for actual mold element thickness which typically will differ significantly from the nominal mold thickness used to generate the constants and offsets associated with desired clamp force and mold protect force. In the mold height term, values for the mold height constant “K 3 ” and mold height offset “OFF 3 ” are determined by measurements made on machine  10  of cross head position to achieve a selected clamp force using blank mold elements equivalent to maximum and minimum mold heights. 
     At step  154 , a command is generated to drive motor  40  to move crosshead  56  to the required clamp force position. Steps  156  and  158  represent monitoring of the progress of crosshead to the required clamp force position. Position of the crosshead is conveniently measured using position transducer  120 , and arrival at the commanded position will result in generation of an “In Position” signal by the axes control routines  98  by comparison of measured position and commanded position. Occurrence of the “In Position” signal is detected at step  156 . It will be recognized by those skilled in the art that, depending on the capabilities of motor drive  112 , an “In Position” signal may be generated by motor drive  112  rather than by axes control routines  98 . In any case, the “In Position” signal represents coincidence between measured position and commanded position within an acceptable tolerance. 
     In the event the crosshead is prevented from reaching the commanded position, for example, in the event mold sections  22  and  24  come into contact before the crosshead  56  has reached an expected mold contact location, motor  40  will “stall”, that is, will cease to further advance crosshead  56 . This condition will be reflected in cessation of change of position of crosshead  56  while a position error, that is, difference between the commanded position and measured position, continues to exist. This condition may be detected within axes control routines  98  as a velocity error, that is a difference between expected velocity and actual velocity as determined from the rate of change of position. Alternatively, this condition may be detected within motor drive  112  by, for example, motor current reaching a current limit value. Step  158  represents detection of occurrence of stalled motion of crosshead  56 . 
     In the event step  158  detects that crosshead motion is stalled, commanded motion of crosshead  56  is terminated at step  160  where position command S(C) is set equal to the present crosshead position, eliminating position error. Thereafter, die height platen  36  is driven to be retracted away from fixed platen  28  a predetermined distance. As motor  52  effectively operates at constant velocity (within the tolerance of the applied power and allowing for inherent delays of acceleration and deceleration as the motor is energized and de-energized), motion through a predetermined distance can be accomplished by driving motor  52  in one direction for a predetermined period. Hence, at step  162 , a drive command is generated to retract die height platen  36  for a preset period Δt( 1 ). Step  164  detects expiration of the retract period. The die height setting procedure continues at step  154  where a position command is generated to position crosshead  56  at the required crosshead position previously calculated. It will be appreciated that steps  154 - 164  define an iterative loop to automate positioning of crosshead  56  at the required crosshead position. 
     Once crosshead  56  has been successfully positioned at the required crosshead position, die height platen  36  is driven to advance to the point of contact of mold sections  22  and  24 . At step  166 , a command is generated to advance die height platen  36  toward fixed platen  28 . On occurrence of contact of mold sections  22  and  24 , crosshead  56  will be forced away from fixed platen  28  by the forces acting on toggle mechanism  38 . Step  168  detects the occurrence of a minute change of position (ΔS(C)=MIN) of cross head  56  away from fixed platen  28  as reflected in position measured by position transducer  120 . Conveniently, the minute change of position is programmable to accommodate characteristics of the press mechanism established during commissioning of machine  10 . The minute change of position must be more than any expected fluctuation of measured position attributable to signal conversion and “holding” torque of motor  40  and must be less than would translate to an error in desired clamp force. This completes setting of die height and execution of the die height setting procedure ends at terminal  174 . 
     While the invention has been described with reference to a preferred embodiment, and while the preferred embodiment has 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.