Abstract:
A controller for a mold vacuum unit uses a programmable logic controller to provide flexible translation of pre-existing signals present on the injection-molding machine avoiding the need for modification of that machine by the addition of limit switches or the like. Simple adjustment of real time of time delays is provided by the integration of two self-contained timer units working in conjunction with the fixed programming of the programmable logic controller. Removable memories for the programmable logic controller allow the vacuum unit to be used with a variety of different injection-molding machines

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     - - -  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     - - -  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to injection molding machines and in particular to vacuum systems for such injection-molding machines.  
         [0004]     Injection-molding injects heated thermoplastic material at high pressures from an injector into a mold. The thermoplastic material cools within the mold and the mold may be opened and the part ejected.  
         [0005]     Gas trapped in the mold during the injection process may escape through the mold seams or through vents provided for that purpose. Alternatively, it is well known to apply a vacuum to the mold during the injection process to reduce gas entrapment. U.S. Pat. No. 4,573,900 entitled:  Evacuation System for Injection - molding Machines,  hereby incorporated by reference, describes a timer system controlling a pneumatic vacuum pump that may be used first, to apply a vacuum to the mold during certain periods of the injection cycle and second, to apply a positive pressure from the pneumatic line to the mold creating a “blow back” to assist in part ejection.  
         [0006]     Standard machines normally employ proprietary control systems that do not provide control signals to activate the vacuum system at the proper time and duration. Accordingly, it is known in the art to attach mechanical limit switches to the mold and its associated clamping mechanism to create the necessary signals from mechanical movement of the mold elements.  
         [0007]     This approach may not be suitable for creating more sophisticated vacuum control signals triggered by events or states not associated with a change in mold position, for example, as may occur in some two-shot molding applications. Modification of the standard injection-molding machine to attach the limit switches not envisioned by the manufacturer can be difficult and the limit switches, once in place, are subject to damage and contamination.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a mold vacuum system that employs a programmable logic controller to translate pre-existing control signals used or created by the standard injection-molding machine into the necessary vacuum control signals. In one important application, the programmable logic controller receives signals from the interface on the standard injection-molding machine used for coordination of the injection-molding machine with robotic or automated parts handling system. This interface provides some standardization and documentation.  
         [0009]     The programmable logic controller may be flexibly programmed for complex logical combinations of existing signals to eliminate the need for special limit switches. Further, replaceable memory cartridges for the programmable logic controller allow ready transfer of the mold vacuum system among machines by switching the memory as a “personality module”.  
         [0010]     In one embodiment, the programmable logic controller may work with standard electronic timers, the latter which provide a simple and familiar method of varying timing parameters in real-time without the drawbacks of connecting a programming terminal to the programmable logic controller.  
         [0011]     Specifically then, the present invention provides a vacuum system for a standard injection-molding machine having an injector unit, a mold clamp unit, and an injection-molding controller producing control signals controlling the injector unit and mold clamp unit. The vacuum system includes an interface cable connectable to the control signals of the injection-molding controller and a programmable logic controller receiving the control signals from the interface cable and executing a stored program to derive vacuum timing signals from at least some of the control signals. A vacuum pump and valve assembly receives vacuum timing signals from the programmable logic controller to provide vacuum on a vacuum line to a mold.  
         [0012]     It is thus one object of at least one embodiment of the invention to provide a control of a vacuum system without the need to install mechanical limit switches or otherwise modify the injection-molding machine. It is another object of at least one embodiment of the invention to allow arithmetic and logical combinations of standard injection-molding controller signals to provide more sophisticated vacuum control.  
         [0013]     The vacuum system may include at least one electronic timer having an integral programming panel, the electronic timer communicating with the programmable logic controller to provide a timing function for the vacuum timing signal.  
         [0014]     It is thus another object of at least one embodiment of the invention to incorporate standalone timers into the vacuum system, despite the presence of timing functions within the programmable logic controller, to provide a familiar interface to the user that allows simple, real-time modification of the operation of the vacuum system without a programming terminal.  
         [0015]     The programmable logic controller may provide the vacuum timing signal to the electronic timer where it is delayed and then passed to the vacuum pump and valve assembly.  
         [0016]     It is thus another object of at least one embodiment of the invention to provide an interface between the timers and the programmable logic controller that does not require communication of a timing value to the programmable logic controller.  
         [0017]     The timing function may be the duration of the application and vacuum to the mold.  
         [0018]     Thus it is another object to the invention to provide ready accessibility to a parameter that typically must be adjusted for a given mold while employing the programmable logic controller to handle parameters that vary only among injection-molding machines.  
         [0019]     The vacuum system may include a terminal block receiving the interface cable and providing terminals releasably holding wires communicating with the programmable logic controller and the timer.  
         [0020]     It is thus another object of at least one embodiment of the invention to allow highly flexible interconnection among the control signals, the programmable logic controller and the timers.  
         [0021]     The programmable logic controller may execute the stored program to create a control signal communicated to the injection-molding machine causing the injection-molding controller to suspend cycling of the injection-molding machine.  
         [0022]     It is thus another object of at least one embodiment of the invention to provide new functionality to a vacuum system by allowing the vacuum system to control the injection-molding system in complex vacuum applications.  
         [0023]     The vacuum system may include a pressure sensor and the programmable logic controller may receive a pressure sensor signal to create the vacuum timing signal.  
         [0024]     It is thus another object of at least one embodiment of the invention to provide sophisticated control of vacuum levels.  
         [0025]     The programmable logic controller may execute the stored program to provide pump control signals to the vacuum pump and valve assembly stopping the pump when vacuum will not be required for a predetermined period of time.  
         [0026]     It is thus another object of at least one embodiment of the invention to provide a more efficient mold vacuum system that reduces part wear and energy consumption.  
         [0027]     The control lines of the injection-molding controller may be electrical inputs or outputs providing standard signals to automatic parts handling machines.  
         [0028]     It is thus another object of at least one embodiment of the invention to make use of a standard interface signals intended for robotic or automated part handling to control the vacuum system on injection-molding machines having proprietary control systems.  
         [0029]     The programmable logic controller may include a removable memory, and the vacuum system may include multiple removable memories storing programs associated with control signals of different standard injection-molding machines.  
         [0030]     It is thus another object of at least one embodiment of the invention to allow the mold vacuum system to be quickly moved between injection-molding machines by simply swapping personality modules.  
         [0031]     The vacuum system may include an input receiving compressed air and a compressed air valve assembly communicating with the vacuum line. The programmable logic controller may further execute the stored program to create blowback timing signals from at least some of the control signals to the compressed air valve to provide compressed air to the mold.  
         [0032]     It is thus another object of at least one embodiment of the invention to provide blowback features as well as vacuum line features.  
         [0033]     These particular objects and advantages may apply to only some embodiments falling within the claims, and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  is a simplified schematic of an injection-molding machine for two shot injection showing the injector units, the mold clamp unit, the injection-molding controller, and the vacuum system of the present invention;  
         [0035]      FIG. 2  is a detailed block diagram of the vacuum system of  FIG. 1  showing the use of a terminal block to manage interconnection of a programmable logic controller having removable memory to timers, valves, regulators, vacuum pumps, and accumulators of the vacuum system;  
         [0036]      FIG. 3  is a simplified wiring diagram of a connection of the vacuum system of  FIG. 2  to control signals of the electronic injection-molding controller of  FIG. 1  showing the receipt of the signals to create various vacuum control signals and the creation of signals returning to the injection-molding controller to modify the injection cycle; and  
         [0037]      FIG. 4  is a flow chart of an example program executed by the programmable logic controller of  FIG. 2  to generate vacuum control signals for a two shot injection-molding process on the machine of  FIG. 1 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0038]     Referring now to  FIG. 1 , an injection-molding machine  10  for two shot molding includes a first injector  12  and second injector  14  for delivering two different thermoplastic materials to the mold  16 .  
         [0039]     The mold  16  includes a stationery mold portion  18  and a movable mold portion  20  that together define cavities  30  and  32 . The movable mold portion  20  may be separated along a mold separation axis  22  to pull away from stationary mold portion  18  for the ejection of parts. This motion may be accomplished by hydraulic actuators  24  providing a mold clamping and opening according to methods known in the art.  
         [0040]     For two shot molding, a hydraulic motor  26  may rotate the movable mold portion  20  about axis  22  to carry a molded part (not shown) produced in a cavity  30  after receiving material from injector  14  to a cavity  32 . At cavity  32 , the molded part may receive additional material from injector  12 .  
         [0041]     Generally, injectors  12  and  14  may communicate via runners all or partially within the stationary mold portion  18  with the injector  14  directed along the axis  22  and the injector  12  perpendicular to that axis near the part line between mold portions  18  and  20 .  
         [0042]     Control lines  36  may pass from each of the injectors  14 ,  12 , the actuators  24 , the motor  26 , and other sensors  38  (for example, a mold position sensor) to an injection-molding controller  40 . The injection-molding controller  40  may be a proprietary electrical circuit or a programmable logic controller with a proprietary program written to synchronize operation of the various components.  
         [0043]     The injection-molding controller  40  may have a terminal block  42  at which these control signals, including inputs to the controller  40  and outputs from the controller  40 , may be accessed. Generally, the control signals at terminal block  42  may include non-proprietary signals intended to assist coordinating the injection-molding machine  10  with automatic handling equipment (not shown).  
         [0044]     The injection-molding machine  10  may be provided with a vacuum system  44  typically added after purchase of the injection-molding machine  10 . The vacuum system  44  may receive electrical power  46  and a source of compressed air  48  from external sources and provides a “vacuum” line  50  that may provide either a vacuum or pressurized air therein. The vacuum line  50  communicates with the stationary mold portion  18  and channels within the mold portion  18  that may provide a path of gases out of the mold cavities  32  and  30 .  
         [0045]     The vacuum system  44  provides an interface cable  52  that connects via a releasable connector  54  with the selective control signals of the terminal block  42  that may be prewired to a separable half of the releasable connector so that the vacuum system  44  may be easily moved among injection-molding machines  10 .  
         [0046]     Referring now to  FIG. 2 , the vacuum system  44  has a housing  47  with fittings receiving the source of compressed air  48  and the interface cable  52  and providing an outlet for the vacuum line  50 . Inside the housing  47 , the line from the source of compressed air  48  may pass to a tee  60  where it branches to a first pressure regulator  62  of a type well known in the art and second to a solenoid valve  72 . The outlet of the pressure regulator  62  passes to a second tee  64 . The second tee  64  is joined with a pressure gauge  66  and a solenoid valve  68 .  
         [0047]     The outlet of the solenoid valve  72  is received by a pneumatically operated vacuum pump  74  so that when the pump  74  receives pressurized air through valve  72 , a vacuum is created and passed to accumulator  76  and then to vacuum regulator  78 . The vacuum regulator  78  provides regulated vacuum to a tee  80 , a first branch of which goes to a second tee  82  branching to a vacuum sensor  84  providing an electrical signal and a gauge  86  as will be understood in the art. The remaining branch of tee  80  provides vacuum to a third solenoid valve  90 .  
         [0048]     The solenoid valves  90  and  68  are joined through tee  92  to the vacuum line  50 .  
         [0049]     Generally, solenoid valve  72  may thus be used to electrically control turning on and off the vacuum pump  74  whereas the solenoid valves  90  and  68  may alternatively apply either a vacuum or a regulated pressure through tee  92  to the vacuum line  50 .  
         [0050]     A knob  70  of the pressure regulator  62  and a knob  88  of the regulator  78  as well as the pressure gauges  66  and  86  are accessible during use of the vacuum system  44  through apertures in a front panel of the housing  47 .  
         [0051]     Also included within the housing  47  are timers  100  and  102  such as are commercially available from Potter &amp; Brumfield under the “CNT Series” trade name. Timers of this type are freestanding electronic modules including typically an internal microprocessor for reading front panel switch inputs (for count, time value, or mode of operation). and producing a displayed output and executing contained firmware to control a the set of contacts for generating an output signal based on the switch settings and input trigger signals.  
         [0052]     Finally, the housing  47  holds a compact programmable logic controller  98 , for example, as are commercially available from Mitsubishi under the FX trade name. As will be understood to those of ordinary skill in the art, a compact programmable logic controller is an industrial computer programmable with a specialized programming language, for example, relay ladder logic language, to allowing inputs to produce outputs, both via self-contained input/output (I/O) terminals, using Boolean and arithmetic instructions and simple timer and counter functions.  
         [0053]     The programming of a compact programmable logic controller  98  normally requires connection to an external programming terminal such as a personal computer. In the preferred embodiment, the programmable logic controller  98  may include a removable memory stick  104  onto which data and/or programs developed on the external programming terminal may be retained so that reprogramming of the programmable logic controller  98  may be performed by simply changing the memory stick  104 .  
         [0054]     Referring still to  FIG. 2  inside the housing  47 , the control signals of the injection-molding machine  10  from interface cable  52  are received by a terminal block  96  of conventional design. Terminal block  96  allows these signals to be flexibly connected to I/O terminals of the programmable logic controller  98  corresponding terminals of a first timer  100  or second timer  102 , and/or for signals to be passed between the programmable logic controller  98  and the timers  100  and  102 .  
         [0055]     In normal use, control signals from the controller  40  of the injection-molding machine  10  will be provided from interface cable  52  directly to the programmable logic controller  98  and signals from the programmable logic controller  98  will be provided to one or both of the timers  100  and  102 . Generally, the signals on interface cable  52  will be signals from the controller  40  effecting a common interface with material handling equipment, however, these signals may also be obtained from other input and outputs to and from the controller  40  depending on the availability of documentation and the knowledge of the installer.  
         [0056]     Referring now to  FIGS. 2 and 3  during operation, the programmable logic controller  98  may be connected via the terminal block  96  to controller output signals  108  generated by the controller  40  and exposed at output terminals  106  of terminal block  42  of the controller  40 . These controller output signals  108 , for example, provide signals controlling the injectors and hydraulic systems of the injection-molding machine  10 . The controller output signals  108  may be combined with controller input signals  112  from input terminals  110 , for example, being from sensors on the injection-molding machine  10  and received at I/O terminals of the programmable logic controller  98  to be used as inputs thereto.  
         [0057]     The programmable logic controller  98  logically combines signals from controller output signals  108  and from controller input signals  112  according to its stored program on memory stick  104 , using user selected logic and arithmetic instructions and state dependent counter and timer functions to produce vacuum control signals  114 . The vacuum control signals  114  pass through terminal block  96  to valves  72 ,  68 ,  90  to control those and to the timers  100  and  102  to trigger the timers and to produce timer output signals that may be provided to the programmable logic controller  98  or directly to the valves  72 ,  68 ,  90  as additional vacuum control signals  114 .  
         [0058]     In addition, the programmable logic controller  98  may produce an output signal  116  forming one of the controller input signals  112  that may be received by the input terminals  110  of the terminal block  42  of controller  40  to provide control signals to the controller  40 , for example, stalling the injection cycle of the injection-molding controller  40  until the output signal  116  is released by the programmable logic controller  98 .  
         [0059]     Referring now to  FIGS. 3 and 4 , a program executed by the programmable logic controller  98  may begin, as indicated by process block  120 , by detecting a start of an injection cycle, for example, by decoding one or more signals from the controller  40  depending on the proprietary protocols of the controller  40 . The particular control signals and their combination used to produce the vacuum signals will depend on the machine and protocol.  
         [0060]     At this detection, the programmable logic controller  98  may activate valve  72  to begin charging of the accumulator  76  through operation of the vacuum pump  74 .  
         [0061]     As indicated by process block  122 , closure of the mold portions  20  and  18  may be deduced from other control signals from interface cable  52 , for example, actual sensor  38  signals, the ceasing of injector screw rotation, or the like.  
         [0062]     In one embodiment, the programmable logic controller  98  may then open valve  68  and produce a stall signal stalling the controller  40  until a sufficient vacuum level is reached within the mold  16  as indicated by process block  124 . Once that level is reached per process block  126 , the controller  40  is released and injection may begin. At this time, the programmable logic controller  98  may strobe the timer  100  to provide a timing signal (per process block  128 ) to the programmable logic controller  98  when vacuum should be released. This timing signal may be readily changed by an operator observing the injection-molding process by access to the front panel switches of the timer  100 . The timer  100  or the programmable logic controller  98  may control valve  90  to cut off the vacuum.  
         [0063]     Process blocks  130 - 134  may then repeat the processes of process blocks  122 - 128  for the second shot of plastic, but this time using a second timer  102  during process block  134  and possibly a different vacuum pressure in process block  130 , variations made possible by the internal logic of the programmable logic controller  98  which may easily deduce different shot numbers from similar mold closure signals.  
         [0064]     At process block  135 , detecting an end of the injection cycle may be performed by decoding one or more signals from the controller  40  to initiate a blow back where valve  68  is opened for a time controlled by an internal timer of the programmable logic controller  98 . The process may then be repeated.  
         [0065]     This is intended as one example of the type of control available from the present invention and to illustrate the ability to use pre-existing control signals and to provide more sophisticated coordination of the vacuum system with the injection-molding machine, and this example is not meant to be limiting.  
         [0066]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.