Agitating – With heating or cooling – Including temperature control
Reexamination Certificate
2002-11-12
2004-06-29
Soohoo, Tony G. (Department: 1723)
Agitating
With heating or cooling
Including temperature control
C366S078000, C366S601000
Reexamination Certificate
active
06755564
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an extruder temperature controller with a stable temperature reset and a method for controlling the temperature of a molten material in an extrusion device. Specifically, the invention relates to an extruder temperature controller and a method for controlling the temperature of molten extrudate wherein the control alarm is delayed for a predetermined time when generating a control output driver signal to the heat exchange means at or near maximum capacity of an extruder system.
2. Description of the Background Art
Extrusion devices are often used in the plastics or other industries to continuously melt, blend, form, and solidify plastics or other materials into a desired shape. Typical extrusion devices include a rotating screw housed coaxially within a heated, cylindrically-shaped barrel. The screw rotates within the barrel and drives an extrusion material such as plastic through the barrel. The extrusion material is forced through a die or aperture at the end of the barrel. A temperature drop, that occurs when the extrusion material leaves the heated barrel, allows the material to solidify in a molded shape that is determined by the profile of the die.
The temperature of the extrusion material or plastic within the extruder barrel must be controlled so as to remain as near to a desired temperature as possible. An extruder barrel can be operated to control the temperature of the extrusion material within the barrel under one or more of three conditions. An extruder barrel can (1) add heat to a material, (2) extract heat from a material, or (3) maintain the heat of a material. The third condition of maintaining a temperature of an extrusion material occurs when an extruder is operated at a speed wherein the heat from the friction of the extrusion material created, as the material is processed in the extruder barrel, is approximately equal to the heat loss from the extruder barrel. This condition of no heat gain or loss in known as an “adiabatic” condition.
Most extrusion devices have a plurality of heat exchange zones. The temperature of each heat exchange zone can be independently controlled such that one or more heat exchange zones heat the material being processed while the remaining heat exchange zones are in an adiabatic condition or are cooling the extrusion material. It is common for a heat exchange zone near the end of an extruder barrel to be used to cool an extrusion material before the material is extruded through the die. This procedure allows the extrusion material to quickly solidify upon existing the die. An extruder barrel, typically, has eight heat exchange zones, but the number of zones can vary.
An extruder device can control the temperature of its extruder barrel with heat exchange elements. The extruder barrel is surrounded by a shell containing heat exchange elements. The heat exchange elements can be (1) heaters such as resistive heaters which increase the extruder barrel temperature and (2) cooling tubes for circulating water or another coolant in order to decrease the extruder barrel temperature. Alternative heat exchange elements can be used. For example, the cooling structure can be a finned shell with a blower that circulates air past the fins.
Temperature sensors, such as thermocouples, are positioned in extruder barrels to signal the temperature at the location of the sensor. Two thermocouples per barrel zone are usually provided and are electrically isolated from one another. A first thermocouple is known as the “A” thermocouple of the pair and is placed at the inner surface of the extruder barrel. A second thermocouple is known as the “B” thermocouple of the pair and is placed in the interior of the heater/cooler shell. Each zone of the extruder is similarly provided with a pair of thermocouples, A and B, similarly placed. An air-cooled extruder system also has the B thermocouple in the interior of the shell.
An extruder temperature controller receives signals from the temperature sensors. The extruder temperature controller determines whether the temperature of a given heat exchange zone is too cool or too hot and, if necessary, signals the appropriate heat exchange elements to increase or decrease the heat in the particular zone regulated by that controller.
The extruder barrel and the heat exchange elements are heat sinks and, thereby, cause a delay between the signalling of instructions by the extruder temperature controller to increase or decrease the temperature of a zone. For example, when the extruder temperature controller instructs a heating element to cease applying heat, energy stored in the heating element continues to warm that zone of the extruder barrel. This continued warming causes the extruder barrel temperature to continue to rise in that zone. The lag between the issuance of an instruction from the extruder temperature controller and the response from the heat exchange elements causes the extruder barrel temperature to oscillate about the desired temperature.
U.S. Pat. No. 3,866,669 to Gardiner and U.S. Pat. No. 3,751,014 to Waterloo both address the problem of oscillating extruder barrel temperatures. In the systems described in Gardiner and Waterloo patents, a first temperature probe or thermocouple provides a “deep” temperature measurement representative of the temperature of the extrusion material. A second thermocouple is positioned within the shell surrounding the extruder barrel to provide a “shallow” temperature measurement representative of the temperature of the heat exchange elements. The electrical signals from the pair of thermocouples are combined to provide an average value. The extruder temperature controller monitors the average value and selectively activates the heating and cooling elements to maintain the average value at a temperature that is approximately equal to a setpoint representative of the desired temperature for the extrusion material.
The control of the heat exchange elements by an extruder temperature controller that is responsive to an average value for temperature rather than the actual temperature of the extrusion material, that is being processed, reduces temperature and/or control signal oscillations. An example of such a temperature oscillation occurs during operational conditions wherein a resistive heating element applies heat to increase the temperature of an extruder barrel. While the heating element is active, the shallow temperature measurement is higher than the deep temperature measurement. This temperature difference occurs because the shallow temperature probe is positioned in the vicinity of the activated heating element. Accordingly, the average value of the extruder temperature controller is also greater than the deep measurement or the actual temperature of the extrusion material. The average value reaches the temperature setpoint while the actual temperature of the extrusion material is still below the desired temperature. The extruder temperature controller inactivates the heating element after the average value reaches the temperature setpoint, but before the extrusion material reaches the desired temperature. The heat stored in the heating element continues to raise the temperature of the extrusion material toward the desired temperature. Such temperature oscillations can also occur during operational conditions wherein the temperature of the extrusion material is being decreased.
Inactivating the heat exchange elements before the extrusion material has reached the desired temperature prevents the temperature of the extrusion material from “overshooting” the desired temperature which can cause undesirable temperature oscillations. This advantage is achieved at the expense of a reduction in the accuracy with which the temperature of the extrusion material is controlled. More specifically, since the extruder temperature controller operates to correct the temperature only when the average temperature value deviates from the desired temperature, the extruder temperature controller may not attemp
Davis-Standard Corporation
Reitenbach Daniel
Soohoo Tony G.
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