Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting
Reexamination Certificate
2001-09-29
2004-02-24
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
C264S040500, C264S328100, C425S145000, C425S166000
Reexamination Certificate
active
06695994
ABSTRACT:
FIELD OF INVENTION
The invention relates generally to the art of injection molding and more particularly to methods and apparatus for sensorless observation of melt pressure in an electric 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 ejected or 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, recover, 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 pressurized plastic molding material, in the form of a melt. In the inject phase, a reciprocating feed screw or ram within a cylindrical barrel pushes or injects the 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 pressure is maintained during a pack and hold step, and the eject step separates the molded part from the separated mold halves. The screw is retracted in the barrel during a recovery step 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.
In the United States, injection molding machines have traditionally been hydraulic machines. Within the last several years, there has been a general shift to electrically powered machines either in hybrid arrangements (where some machine functions are performed by electric motors while others hydraulically) or in an all-electric machine. The machine cycle is the same whether the injection molding machine is electrical or hydraulic. However, there are major differences in the hardware required to perform the sequences in the molding cycle. The hardware differences require control changes to the machine so that the user of an electric injection molding machine can perform the same types of control traditionally performed when molding with hydraulic machines.
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. 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 various improvements over the conventional molding machines and control systems therefor illustrated and described in U.S. Pat. No. 5,997,778.
Of particular importance is the motion of the in-line reciprocating screw or ram during injection and also the control of the ram during hold and pack. This is necessary to achieve acceptable molded parts, for example, by insuring complete filling of the mold, reducing cosmetic and structural problems in the molded parts, ejecting molded parts without damage and minimizing cycle time to achieve acceptable machine throughput.
Control of the ram motion during inject is typically accomplished in one of two ways. In a first method, the velocity of the ram along its travel is set at desired velocities at desired travel distances by the operator. The velocity of the ram is thus profiled and typically the control monitor will show the programmed velocity as a trace and the actual trace achieved during injection. When velocity profiling is used, the machine transitions from a motion control to a pressure control during hold. That is, the shrinkage of the melt while the plastic starts to cure is made up of additional plastic pushed into the mold as a function of the packing pressure desired to be exerted on the plastic in the mold. Still further, when velocity profiling is used, the machine control will monitor and can display the pressure on the ram, sometimes in an overlaid display, to allow the operation to better set the velocity profile. In any event, the pressure is typically monitored and values displayed when velocity profiling is used.
The second method to control the ram is by pressure and not velocity. Pressure profiling is similar to velocity profiling in that the operator sets pressure at specified ram positions to achieve desired profiling. While velocity profiling is widely used, there are applications where pressure profiling is desired. For example, gas injection mold technology may require ram pressure control to achieve a desired gas pressure in the mold.
In a conventional hydraulic machine, an inexpensive pressure sensor(s) is located in the hydraulic circuit. The hydraulic pressure is directly sensed and a good portion of the entire control system is governed by the pressure in the hydraulic circuit. Because of the mechanical arrangement used to mount the hydraulic actuator, i.e., piston, a direct correlation between the pressure used to push the ram and the pressure in the mold is obtained. Unless an intricate mold is used, the hydraulic pressure on the ram can be easily correlated to the pressure of the melt in the mold. Thus, pressure profiling is provided at no cost in a hydraulic machine. The hydraulic circuit sensor needed to operate the machine provides the sensor information for machine control.
An entirely different problem is present with an electric machine. First, the rotation of an electric motor has to be converted to linear motion, typically accomplished by a ball screw. To permit translation and rotation of the ram (screw), a tie-bar structure, not dissimilar to that used for the platens of a mold is typically employed. The ball screw pushes a plate sliding in a guided manner on the tie-bars secured to the ram (screw) to achieve translation. This mechanical arrangement does not provide the inherent pressure reading available in a hydraulic machine to sense pressure. What is done then is to provide a melt pressure sensor in the barrel or mold to sense the pressure. However, melt pressure sensors do not operate in the pristine environment of the pressure sensors in the hydraulic circuit where the sensor is exposed to clean fluid at low temperatures. Basically, the melt pressure sensor is an expensive instrument with a short finite life. Further, and somewhat surprising, the melt pressure sensor may not necessarily give a true accurate or absolute reading of the actual pressure in the mold and may not have sensitivity or response time to truly pick up variations in the melt. However, in most cases, the melt sensor will give a consistent reading. Furthermore, if a melt sensor is used on the machine and not the mold, special nozzles or nozzle adapters are frequently required to mount the pressure transducer in the machine barrel, and, in all events, the transducer needs to be calibrated upon installation and periodically thereafter. Calibration and changeout of failed barrel mounted pressure sensors are off-line procedures causing machine down-time, and are typically beyond the expertise of mold operators. This is in addition to external signal conditioning and/or amplification. Thus, several shortcomings make direct transducer measurement of melt pressure less than desirable.
The prior art has recognized this and has used a force transducer (i.e., strain gage, load cell) to measure the axial force executed on the ram (typically, vis-a-vis the mounting plate). See, for example, U.S. Pat. 4,828,473, 4,961,696 and 5,955,117.
In the prior art, for example, strain gage sensors or load cells are mounted so as to measure compression on the ram or some other co
Bulgrin Thomas C.
Uhrain, IV Michael C.
Fay Sharpe Fagan Minnich & McKee LLP
Heitbrink Jill L.
Van Dorn Demag Corporation
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