Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Positioning of a mold part to form a cavity or controlling...
Reexamination Certificate
2002-10-24
2004-11-09
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Positioning of a mold part to form a cavity or controlling...
C264S102000, C425S145000, C425S150000
Reexamination Certificate
active
06814908
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an injection molding machine for producing a molded part by injection of a plasticized material into a two part mold. More particularly, the present invention relates to an improved injection molding machine controller coupled with the hydraulic system for operating the molding machine more efficiently.
BACKGROUND OF THE INVENTION
Injection molding machines have been increasingly used in the last twenty years for molding a part by injection of a material into a two part mold. An injection molding machine converts a plastic or rubber material from one form or shape to another. Most plastic molding materials initially are in pellet form, but other materials are in strip, powder or webbed form. A molding machine conventionally includes a clamping mechanism including one or more clamping cylinders for applying a mold clamping force to the two part (or multi-part) mold and for moving the mold parts apart to open the mold, and an injection mechanism for plasticizing the material and injecting the plasticized material into the closed mold to form the molded part. The injection mechanism conventionally includes a screw member rotatable and axially moveable within a barrel having a discharge end in fluid communication with the closed mold. The material that enters the molding machine is converted to a molten state partially by heater bands applied to the injection barrel, but primarily by frictional heat generated by the screw rotation. The screw rotation thus changes the solid material into a molten and homogenous mix and transfers the molten material to the front of the injection barrel in preparation for injecting the molten material into a mold once the mold is closed and pressure has built up between the mold halves by the mold clamping mechanism. A hydraulic system is provided for powering the clamping mechanism, and typically also the injection mechanism.
During operation, the moving platen of the molding machine holding the ejection half of the mold closes to contact the injection half of the mold. At this stage, high pressure built up by the clamping mechanism presses the mold halves together. High pressure is built up and a signal is generated by a pressure sensor or limit switch to initiate the injection of the material by the screw. Air is trapped in the cavities of the closed mold that is shut at high pressure. Consequently, the injection speed must be reduced to allow the trapped air to be forced slowly about the ejector pin ports (pin clearance) or through machined vent grooves in the mold. In many applications, even slowing the injection speed will not mold a good part, since the trapped air creates burn marks due to high pressure compressing the trapped air and creating a void or short part, which may also be due to trapped air.
When the injection is completed, the screw rotation will conventionally start at full hydraulic pressure applied to the mold, with the hydraulic motor driving the screw. The screw rotation speed is conventionally controlled by a flow control valve. The screw rotation may end at a preset value established by an electrical switch or by a linear potentiometer. When the screw rotation ends, the screw decompression starts by linear movement or pull back of the screw. The stroke of the pull back may be controlled by a limit switch or a linear potentiometer. At the end of the screw pull back, the clamp may still be closed for a predetermined time to allow for the molded part to cool. When the cooling time ends, the mold opens and the part is ejected. The machine is now ready to start a new cycle.
The operation of a molding machine as described above is conventionally driven by an electric motor coupled to a hydraulic pump or pumps that pump the oil from a tank, deliver the oil to the different actuators of the molding machine, and return the oil to the tank. Most machines use AC motors with fixed volume pumps or a combination of fixed and variable pumps. Both the pumps and the motors run at a fixed speed regardless of the load requirement, which wastes energy.
U.S. Pat. No. 5,756,019 discloses an injection molding machine with a conventional screw member rotatable within a barrel, with a supplying hopper at one end and a port into the closed mold at the other end. The mold clamping force is controlled in response to molding data and position sensors for the screw member and the clamping mechanism during injection. The injection speed is controlled, so that the detected injection pressure gradually increases from the initiation of injection. This detected pressure, the position sensor data, and the molding data are input to a controller which reduces the mold clamping force to a minimum value, thereby reducing power consumption. A closed loop hydraulic system is controlled for operating the screw, and another closed loop hydraulic system is provided for the clamp. Injection molding is the most widely used process for the production of plastic parts. The molding machine ideally produces parts at the lowest possible temperatures, at the lowest applied pressures, in the shortest possible times, and with the lowest energy consumption. Present day molding machines are highly productive, but the molding operation generally faces two significant problems: (1) evacuating the air trapped in the mold cavities in which the mold material is injected, and (2) high energy consumption. Air trapped in mold slows down the injection process, taking longer for a part to fill out. Higher material temperatures and higher mold temperatures are required to keep the material flowing to fill out the part. Higher temperatures in the material and the mold require longer cycle time, thereby requiring more cooling for the mold, higher energy consumption, and longer injection time periods.
U.S. Pat. No. 5,756,019 discloses an injection molding machine with a conventional screw member rotatable within a barrel, with a supplying hopper at one end and a port into the closed mold at the other end. The mold clamping force is controlled in response to molding data and position sensors for the screw member and the clamping mechanism during injection. The injection speed is controlled, so that the detected injection pressure gradually increases from the initiation of injection. This detected pressure, the position sensor data, and the molding data are input to a controller which reduces the mold clamping force to a minimum value, thereby reducing power consumption. A closed loop hydraulic system is controlled for operating the screw, and another closed loop hydraulic system is provided for the clamp. The '019 patent detects injection pressure at a flow control valve, so that the mold clamping force is controlled, along with other parameters, by the detected injection pressure. Detected pressure may, for example, then be multiplied by a factor to determine the clamping pressure. The position sensor is used to detect the screw position, and another position sensor is used to detect the position of the movable mold. A pressure detector monitors the injection pressure to the screw, while another pressure detector monitors the mold clamp pressure. All of this information is input to a controller, and inherently the system as shown in
FIG. 5
of this patent takes a significant amount of time, e.g., in the range from 15 to 50 milliseconds, between a signal being sensed by a sensor and the time the controller operates a molding machine component in response to that sensed signal, and is subject to considerable variations from cycle to cycle. Systems which are based on multiple sensors and perform multiple functions may require a time lag of over 100 milliseconds or more between a sensed signal and a resulting action. Injection molding machines today are able to produce a part of varying size during injection cycle which may take no more than one second, the time delay between sensed signals and the resulting action according to the prior art techniques thus does not allow the injection mold operation to be performed in an efficient man
Browning & Bushman P.C.
Heitbrink Jill L.
Helmreich Loren G.
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