Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting
Reexamination Certificate
2001-09-29
2004-01-27
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
C264S040500, C425S145000
Reexamination Certificate
active
06682669
ABSTRACT:
FIELD OF INVENTION
The invention relates generally to the art of injection molding and more particularly to methods and apparatus for controlling motion and/or pressure in an injection molding machine.
BACKGROUND OF THE INVENTION
Injection and other types of molding machines are complex systems, typically operated in multiple steps or phases, in order to provide a molded part or parts in a molding cycle. Once a finished part is removed from the machine, the molding cycle is repeated to produce further parts. A typical injection molding machine operational cycle includes clamp, inject, pack and hold, recovery, and eject steps, each of which involves moving machine components and motion control thereof. The clamp phase joins the individual sides or portions of a mold together for receipt therein of plastic molding material, in the form of a melt. In the inject phase, a reciprocating screw or ram within a cylindrical barrel pushes or injects a plasticized melt through an orifice at the barrel end or nozzle, which in turn provides the melt to the interior cavity of the mold. Further material is then provided to the mold and force is maintained during a pack and hold phase, and the eject phase separates the molded part from the separated mold halves. The screw is retracted in the barrel during a recovery phase while the screw is rotated to advance new plastic material through screw flights into the barrel space forward of the screw, whereupon the cycle may be repeated.
Each of the molding machine phases may involve linear and/or rotational motion of one or more machine components, which motions are implemented using appropriate actuators in a controlled fashion. The actuators for such machine component motion can be of various forms, such as hydraulic actuators, rotary and linear electric motors, and the like. The various rotational and/or translational motions of the molding machine components are often controlled according to predetermined profiles in order to achieve the desired molded parts, while attempting to ensure complete filling of the mold, reduce cosmetic and structural problems in the molded parts, eject molded parts without damage, and to minimize cycle time to achieve acceptable machine throughput. In addition to motion control, such molding machines also include controls and actuators for controlling temperatures, pressures, and other process variables in the machine. Many contemporary injection molding machines employ a combination of open-loop command type control for non-critical motions, such as for clamping and ejection, in combination with closed-loop control for injection ram velocity, hold pressure, back pressure, and screw rotation speed control.
Controls for injection molding machines have evolved from early manual controls wherein plastic was injected into a mold when a crank wheel was turned, to programmable logic controllers operating the machine actuators in closed loop fashion using sensor inputs to implement a control law, typically proportional, integral, derivative (PID) control. Conventional injection molding machine PID controllers receive sensor inputs indicative of machine component motion (e.g., position, velocity, etc.), which are then compared with desired motion values (e.g., set point values) to derive an error value. Control outputs are derived from the error value and are provided to actuators in the machine, such as hydraulic valves, electric motors, or the like, in order to reduce the error. PID controllers provide such control output signals which are proportional to the error, the integral of the error, and/or the derivative of the error, wherein PID coefficients k
p
, k
i
, and k
d
are set to provide relative weighting for the proportional, integral, and derivative components of the control output signal. More recently, molding machine controllers have provided more advanced functionality, such as combining auto-tuned PID with a predictive open-loop term and an adaptive learned disturbance correction term, as set forth in my U.S. Pat. No. 5,997,778, the disclosure of which is hereby incorporated by reference as if fully set forth herein. The present invention provides an improvement to the conventional molding machines and controls therefor illustrated and described in U.S. Pat. No. 5,997,778.
Motion of various components in a molding machine are commonly controlled according to a user-defined profile. For instance, a user may define a desired profile for ram velocity versus position or time to be used during an injection step, sometimes referred to as a velocity profile. Alternatively or in combination, such moving components may be controlled according to a user-defined pressure profile, such as melt pressure profile defined in terms of pressure with respect to time. A machine cycle, moreover, may employ one or more such profiles for various steps therein. For example, the injection ram translation may be controlled according to a velocity profile during the injection step, after which pressure profile type control is employed during a pack and hold step. Furthermore, different profiles and/or profile types may be used to control different subsystems within a molding machine. In this regard, velocity profiling may be used to control the ram during injection, whereas pressure profiling may be used to control a clamping system operation.
Where velocity profiling is used, the user defines the desired profile, which establishes the desired velocity of one or more moving machine components with respect to position. The controller then uses the profile as a series of set point values (e.g., and/or derives further set point values therefrom) for the control law in order to implement a particular machine phase in a molding cycle. For instance, the user typically defines a desired piecewise linear translational velocity profile with respect to position, for movement of the injection ram during an injection step. Alternatively, the user may define a pressure versus time profile for the melt in the barrel during injection, which can be used to control the linear ram force during injection. Similarly, user-defined profiles may be used in controlling rotational movement of the ram, heating of the barrel, clamping of the mold halves, movement of the carriage, and/or ejecting finished parts from the separated mold.
To facilitate user entry of such desired control profiles, molding machines often include an operator station or user interface with a screen display and keyboard which sends signals to a programmable logic controller (PLC). For example, such a user interface may allow an operator to define desired ram velocities at fixed ram travel increments or zones. The operator sets the desired ram velocity at each zone, from which a series of bar graphs are assembled. The ram movement is then controlled so as to follow the bar graphs. More recently, the bar graphs have been replaced with points at each zone boundary so that the ram is not programmed to travel at constant speed within each zone. Rather, the ram is controlled according to a linear interpolation which varies from one set point at one zone boundary to another set point at an adjacent zone boundary. Thus, in defining a number of desired velocity settings at set ram travel positions which the ram is to follow, a “velocity profile” is established. The molding machine controller then attempts to cause the ram to travel at the user set speeds at the user set positions i.e., to emulate the velocity profile, by providing corresponding control output signals to one or more actuators in the machine.
In a conventional molding machine, the controller receives a velocity feedback signal from a velocity sensor on the machine and compares it with the user set velocity profile to generate an error compensated control output value by which the injection ram speed is controlled. Alternatively, a position signal is received from a ram longitudinal position sensor, and is differentiated, either in hardware or in software, in order to derive velocity information therefrom. Where the linear actuator f
Bulgrin Thomas C.
Schuplin Andrew
Fay Sharpe Fagan Minnich & McKee LLP
Heitbrink Jill L.
Van Dorn Demag Corporation
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