Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Controlling fluid pressure in direct contact with molding...
Reexamination Certificate
2003-04-29
2004-10-12
Davis, Robert (Department: 1722)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Controlling fluid pressure in direct contact with molding...
C264S564000, C264S040600, C425S141000, C425S326100
Reexamination Certificate
active
06802995
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an internal bubble cooling (IBC) air control for a plastic blown film apparatus.
When blown film is extruded, it typically is in the form of a continuous, vertically oriented tube. The tube, which is in a molten state as it exits a die, expands in diameter as it is pulled continuously upward. The diameter stabilizes to a more or less constant value when the tube cools sufficiently to solidify a short distance from the die at what is called the frost line. Air cooling systems such as external air rings and IBC systems within the tube are provided close to the exit of the die to ensure that the tube cools quickly enough to remain stable.
The tube usually passes through a bubble cage, which minimizes unwanted tube motion and also determines the final tube size if the cage is allowed to contact the tube while the tube is still molten. After solidifying, the tube passes through a flattening device, known as a collapsing frame, to convert the inflated tube into a flattened out film with no air inside. This film is pressed together by motorized nip rolls that continually draw the film upward and away from the extrusion process to form what is call “layflat.” The die and nip roll act as seals, which in steady state, form a trapped, column of air with constant volume inside the tube.
Film processors employing IBC systems realize production rate gains on the order of 20% to 50%. In known IBC systems, such as that described in U.S. Pat. No. 4,243,363, air passages are provided through the die to allow for significant air flow into and out of the tube. Air supply and exhaust systems act under the supervision of a control system in response to measured tube size. The control system adjusts the flow of air to be in balance so that a constant, desired tube size is maintained.
For IBC systems to remain stable, there cannot be a significant closed loop lag time between the time when an air flow change first occurs and when the new tube size actually gets sensed by the controller. Excessive total lag time causes a tube to oscillate in size. Typical oscillation periods induced by IBC systems are generally 4 to 6 seconds in duration. This implies that the closed loop lag time must remain less than about 1 second to 1.5 seconds or else the lag time will be greater than 90 degrees out of phase and oscillation will result. Present art IBC systems have a hardware sensor response time and actuation of corrective air flow lag time of about ½ to 1 second. Total closed loop lag time includes this hardware lag time and an additional process related sensing lag time caused by size changes taking place at a point prior to where sensing occurs.
Bubble instability prevents film processors from using IBC systems to achieve higher production rates when extruding many of the newer high performance materials. This instability is caused by a process related sensing lag time that is great enough to force the IBC system into oscillation. Sensing lag time is the time it takes for the molten region of the bubble, which reacts in size to the influence of IBC air flow changes, to move along the process until it has solidified into a final dimension that can be accurately measured at or just after the frost line. Traditionally, older resins such as Low Density Polyethylene (LDPE) react in size to air flow changes very close to or at the frost line, thus providing for minimal sensing time lag time and making it easy to control bubble stability. In the early 1980's, however, Linear Low Density Polyethylene (LLDPE) became commercially viable. LLDPE reacts just prior to reaching the frost line causing a slightly longer sensing time lag of ½ to 1 second. Processors found LLDPE more difficult to control, but by blending in small amounts of LDPE and/or by lowering the IBC size sensors to the frost line or slightly below the frost line, bubble stability could be maintained. Lowering the sensors this far, however, has a disadvantage in that the measured size is no longer accurate. This accuracy problem has been partially addressed by control systems providing for easy re-calibration of measured tube size. More recently, new materials such as metallocenes have further lowered the reaction point, making them difficult or impossible to control with IBC systems. It would be advantageous to sense directly at a reaction point that is well below the frost line without adverse effects on measured size.
An additional problem arises due to the sensitivity of sensor positioning in that the frost line does not stay in one place over time. As material and ambient conditions vary during production, the location of the frost line can change by several inches. This movement causes the processor to constantly monitor and adjust sensor positioning to track with the frost line. Presently, sensor adjustments are made manually by the operator, usually in response to tube oscillation that suddenly appears or actual tube size changes that occur due to degraded tube size calibrations. It would be advantageous to automatically reposition IBC sensors relative to the frost line to maintain sensing lag time constant, thus preventing the onset of tube oscillations. Automatic positioning would also serve to minimize the need for tube size re-calibration although bubble shape effects that can accompany changes in the location of the frost line might still warrant re-calibration, but significantly less often.
Another problem relates to a well documented characteristic that tubes naturally vary in size over short periods of time, independent of any IBC volumetrically related instability, just as processes not using IBC systems do. Experimentation has revealed that with materials in use today, tube size naturally changes in a periodic manner with a frequency of about 1 to 2 Hertz. Tube size changes by as much as ⅓ to ½ inch of layflat for processes with light or no contact with the bubble cage and from {fraction (1/10)} to ⅓ inch for processes that use the bubble cage to squeeze in just below the frost line and size the bubble. It is a disadvantage to squeeze the tube since marks and scratches routinely result from contact points with the bubble cage. Without squeezing with a cage, IBC control systems must have a total system response time (sensing lag time+sensing response time+actuator response time) of about 0.1 seconds (10 hertz) or better to control these natural fluctuations. Presently, sensing lag time, response time, and actuator response time individually are each too great to allow for control of natural tube size changes so each must be addressed. It would be beneficial if total system lag time and accuracy could be brought to a level where higher frequency natural bubble instabilities could be controlled by IBC control systems without reducing film quality due to scratches.
IBC control systems employ mechanical, optical, and acoustic sensors for monitoring tube size. Mechanical sensors cause marks on the resulting film and optical sensors tend to get dirty and unreliable in the typical blown film plant environment making them unsuitable for many applications in blown film production. Acoustic sensors are preferred because they provide non-contact sensing and are very reliable in a plant environment. Such systems, however, do have slow sampling rate and problems with sensor interference when more than one acoustic sensor is placed into service around a tube. Acoustic sensors operate by sending out a conical ultrasonic sound pulse and measuring the time it takes for the pulse to bounce off a target, such as the tube, and return back to the sensor which sent the pulse. Distance is then calculated by multiplying the time of flight by the speed of sound in the ambient air that the pulse just traveled through. Blown film bubbles tend to flutter and move around, causing the sound pulses to bounce in many different directions. If the pulse passes by a sensor other than the one that sent it, interference can and usually does occur. Additionally, an o
Addex, Inc.
Davis Robert
Del Sole Joseph S
Wilmer Cutler Pickering Hale and Dorr LLP
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