Abstract:
An extruder includes a barrel having a die attached to one end and at least one material inlet with a feeder for providing material to the material inlet. At least one screw is rotatively mounted in the barrel, and a motor is provided for driving the screw. A plurality of sensors is mounted in the barrel for sensing passage of screw thread edges as the screw rotates. A controller for controlling operation of the extruder receives signals from the sensors and determines local torsional deformations of the screw based on the signals. The controller slows down the extruder if any one of the local torsional deformations falls outside of an optimal range.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to screw extruders and more particularly to determining torsional deformations in screw extruders.  
           [0002]    Thermoplastic resins are commonly formed using extrusion molding machines typically referred to as extruders. Both single screw and multi-screw extruders are known. For instance, a twin screw extruder includes a pair of intermeshing screws rotatively mounted within a close fitting casing or barrel. Raw material, typically in the form of powder or pellets, is fed into the interior of the barrel and is moved through the barrel by the rotating screws. The mechanical action of the screws, along with any heat that may be added, melts and mixes the raw materials. The heated and compressed material is forced out of a die at the discharge end of the barrel and assumes the desired shape.  
           [0003]    Along the length of each screw, there are many different material regimes-solids, voids and liquids of varying viscosity. Because they are not perfectly rigid bodies, the extruder screws act like long torsion springs when encountering these varying material regimes. That is, the screws will experience angular twist or torsional deformation.  
           [0004]    A major cause of customer rejection of extruded plastics is variations in viscosity of the finished product. Rejected material increases production costs and leads to dissatisfied customers. There are many factors that can effect viscosity, including the quality of the raw materials used, the amount of heated applied and the rotational speed of the screws. Monitoring the extrusion screw torque would allow better control of the process such that variations in viscosity could be reduced. However, merely measuring the torque at the drive motor will not provide optimal control of the extrusion process because of the torsional deformations along the length of the screws due to the above mentioned variations in the material regimes that the screws encounter.  
           [0005]    Accordingly, it would be desirable to be able to continuously measure rotating extruder screw torsional deformations along the length of the screw. Knowledge of the varying torsional deformations allows for better control of the extrusion process leading to reduced viscosity variations in the finished product. Reducing variations in the finished product will increase yields, thereby reducing overall production costs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The above-mentioned need is met by the present invention, which provides an extruder having a barrel including a die attached to one end and at least one material inlet with a feeder for providing material to the material inlet. At least one screw is rotatively mounted in the barrel, and a motor is provided for driving the screw. A plurality of sensors is mounted in the barrel for sensing passage of screw thread edges as the screw rotates. A controller for controlling operation of the extruder receives signals from the sensors and determines local torsional deformations of the screw based on the signals. The controller slows down the extruder if any one of the local torsional deformations falls outside of an optimal range.  
           [0007]    The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:  
         [0009]    [0009]FIG. 1 is a cross-sectional view of one embodiment of a twin screw extruder.  
         [0010]    [0010]FIG. 2 is a schematic representation of the control scheme for the extruder of FIG. 1.  
         [0011]    [0011]FIG. 3 is a side view of one of the extruder screws from the extruder of FIG. 1 rotating under no load.  
         [0012]    [0012]FIG. 4 is a side view of one of the extruder screws from the extruder of FIG. 1 rotating under a load.  
         [0013]    [0013]FIG. 5 is a cross-sectional view of an alternative embodiment of a twin screw extruder. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 shows an extrusion molding machine or extruder  10 . The extruder  10  comprises a long, substantially cylindrical barrel  12  having an internal chamber. First and second intermeshing screws  14  and  16  are rotatively mounted in the chamber of the barrel  12 . Each screw  14  and  16  is provided with one or more helical threads such that, when rotating, the screws  14  and  16  will convey raw materials through the barrel chamber. The mechanical action of the screws  14  and  16  will also heat and mix the raw materials. The pitch of the screw threads can vary along the length of the screws  14  and  16 .  
         [0015]    A motor  18  synchronously drives the screws  14  and  16  via a dual axle gearbox  20  that is mounted to a first end of the barrel  12 . As is known in the art, the gearbox  20  contains a number of gears such that the rotational speed of the screws  14  and  16  can be controlled. An inlet opening  22  is formed in the barrel  12 , typically near the first end thereof, for allowing raw materials to be extruded to be fed into the barrel chamber. Although only one such inlet opening is shown in FIG. 1, it should be noted that the barrel  12  could be provided with additional inlet openings located at various locations. A feeder  24  (shown schematically in FIG. 1) provides raw materials, typically in powder or pellet form, at a controlled feed rate to the inlet opening  22 .  
         [0016]    Heaters  26 , such as electrical resistance heaters or the like, are optionally disposed around the outer surface of the barrel  12  for providing additional heating of the raw materials in the barrel chamber. A die  28  is mounted to the second end of the barrel  12  and has an outlet  30  through which the extruded material is discharged.  
         [0017]    A plurality of sensors  32  is provided for sensing passage of screw thread edges as the screws  14  and  16  rotate such that the angular twist or torsional deformation of the screws  14  and  16  can be measured. In one embodiment, the sensors  32  are arranged in a first series mounted in the barrel  12  adjacent to the first screw  14  and a second series mounted in the barrel  12  adjacent to the second screw  16 . The sensors  32  of the first series are spaced longitudinally along the length of the barrel  12  but at the same circumferential location on the barrel  12  so as to define a line that is parallel to the rotational axis of the first screw  14 . Likewise, the sensors  32  of the second series are spaced longitudinally along the length of the barrel  12  but at the same circumferential location on the barrel  12  so as to define a line that is parallel to the rotational axis of the second screw  16 .  
         [0018]    The number and locations of the sensors  32  in each series can vary depending on a number of factors such as the length of the screws, the number of material inlets used, and the type of material being extruded among others. In one preferred embodiment, the sensors  32  are threaded into small holes formed in the barrel  12  at appropriate locations. Although other means of mounting the sensors  32  to the barrel  12  can be used, screwing the sensors  32  into the barrel wall maintains the seal-tight nature of the barrel  12 . The sensors  32  can be placed at a number of locations, but are generally located at points along the length of the barrel  12  that correspond to screw locations at which it is desired to know the torsional deformation. Some possible sensor locations include near the first and second ends of the barrel. It is also useful to place sensors  32  at locations corresponding to locations in the barrel chamber where the material viscosity is likely to change, as material viscosity is a primary factor on the torsional deformation of the screws  14 ,  16 . Accordingly, other likely sensor locations include spots immediately downstream of material inlets and heat input sources.  
         [0019]    During operation of the extruder  10 , the sensor  32  at any given sensor location would “see” the periodic passage of a screw edge, then a void, then a screw edge, then a void and so on. The frequency at which a screw edge passes the sensor  32  is determined by the screw pitch in the vicinity of the sensor  32  and the rotational speed of the screw. The sensors  32  can be any type of device capable of sensing passages of screw thread edges. This would include inductive, capacitive, eddy current, optical and sonic sensors, among others. In any case, when a screw edge passes a sensor  32 , it will generate a signal indicating that the screw edge passage has been detected. The detection signal from each sensor  32  is fed to a controller  34  that controls the operation of the motor  18 , the feeder  24  and the heaters  26 .  
         [0020]    Referring now to FIG. 2, it is seen that the controller  34  comprises a processor  36 , a timing device  38  such as a peak detector and a screw model  40 . The detection signals from the sensors  32  (shown collectively in FIG. 2) are fed to the timing device  38 , which notes the edge arrival times for each sensor  32 . The screw model  40  is a geometric (i.e., radius, length and pitch) and material model of each screw  14 ,  16 . The processor  36  receives inputs from the timing device  38  and the screw model  40  to compute local torsional deformations along the lengths of the two screws  14 ,  16 . The processor  36  also outputs control signals to the motor  18 , the feeder  24  and the heaters  26 . Thus, if the torsional deformation at any point along the length of either screw  14 ,  16  goes out of optimal range, the processor  36  can slow down the extrusion process until screw torsional deformations at all locations return to optimal ranges. This can be accomplished by slowing down the feed rate of the feeder  24 , slowing down the rotational speed of the screws  14 ,  16 , reducing the heat input from the heaters  26 , or any combination thereof.  
         [0021]    The determination of the local torsional deformations is illustrated in FIGS. 3 and 4, which compare the first screw  14  under unloaded and loaded conditions. The same discussion also applies to the second screw  16  as well. FIG. 3 shows a portion of the first screw  14  in the vicinity of one of the sensors  32  while the screw  14  is operating with no load (i.e., with the barrel  12  empty). With no load, the screw  14  will not be twisted and the screw edge passes the sensor  32  at a nominal time, as shown in FIG. 3. FIG. 4 shows the same portion of the first screw  14  operating under a load having different material regimes such that varying torsional deformations occur along the length of the screw  14 . In this instance, the load on the rotating screw  14  causes the screw  14  to be twisted in the direction opposite to the direction of rotation so that edge passage at the sensor  32  will be “late” with respect to the nominal time. In other words, the screw edge will not pass the sensor  32  at the nominal time, as shown in FIG. 4. Instead, the screw  14  must rotate a circumferential distance d before the screw edge passes the sensor  32 . The difference between the time when the screw edge passes the sensor  32  and the nominal time is referred to herein as the time delay. It is also possible for the screw  14  to be twisted in the same direction as the direction of rotation, such as when a load is suddenly released in the vicinity of the sensor  32 . In this case, the screw edge would pass the sensor  32  before the nominal time, resulting in a negative time delay.  
         [0022]    A calibration run is conducted before normal operation by running the extruder  10  empty with no load on the screws  14 ,  16  so as to establish the nominal times for each one of the sensors  32 . Then, the processor  36  is able to determine the time delays from the edge arrival times received from the timing device  38 . The local torsional deformation at each sensor location is then determined from the detected time delay, the known screw rotational speed and the appropriate screw properties from the screw model  40 .  
         [0023]    [0023]FIG. 5 shows an alternative embodiment of a twin screw extruder  110 . This extruder  110  is substantially similar to the extruder of the first embodiment in that it has first and second intermeshing screws  114  and  116  rotatively mounted in the chamber of a barrel  112  and synchronously driven by a motor  118  via a gearbox  120 . The extruder also includes an inlet opening  122 , a feeder  124 , heaters  126  and a die  128  that are the same as those described above in connection with the first embodiment. However, this extruder  110  differs from the first embodiment in that instead of having a series of sensors located adjacent to the first screw and a second series of sensors located adjacent to the second screw, it has only a single series of sensors  132  located adjacent to one of the screws. As shown in FIG. 5, the series of sensors  132  is located adjacent to the first screw  114 , although it could alternatively be adjacent to second screw  116 . The detection signal from each sensor  132  is fed to a controller  134  that controls the operation of the motor  118 , the feeder  124  and the heaters  126 . In this case, the local torsional deformations of the first screw  114  are determined in the same manner as that described above. The local torsional deformations for the second screw  116  are then estimated or assumed to be the same as the first screw local torsional deformations. Although the present invention has been described in the context of twin screw extruders, it should be appreciated that the present invention is not limited to twin screw extruders and can be implemented with other types of screw extruders including single screw extruders.  
         [0024]    The foregoing has described an extruder capable of monitoring local torsional deformations of the screws and thereby providing better control of the extrusion process. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.