Method and device for regulating pressure in a single-screw degassing extruder or in a cascade extruder

Abstract

The invention relates to a method for regulating the minimal value of the pressure in the first half of the metering zone in a direction of conveyance in a single-screw degassing extruder or in a cascade extruder during the conveying of a melt consisting of a thermoplastic. The single-screw degassing extruder or the cascade extruder each have a first screw section ( 13 ), a degassing zone ( 14 ), a metering zone ( 22 ), and a measuring control loop (M), which includes a pressure measuring sensor (P) in the first half of the metering zone, a regulator (R) and an actuator (S) for influencing the pressure in the metering zone. The pressure is measured by the pressure measuring sensor (P) and is set by the regulator (R) and the actuator (S) so that the minimal value at the location of measurement ranges from 0.1 to 10 bar.

1 . Method for controlling the minimum level of the pressure in the first half of the metering section, as seen in the transport direction, in a vented single-screw extruder or a cascade extruder during the transport of a molten thermoplastic, the vented single-screw extruder or the cascade extruder in each case having a first screw section ( 15 ), a venting zone ( 14 ), a metering section ( 22 ) and a measurement control circuit (M), which includes a pressure-measuring sensor (P) in the first half of the metering section, a controller (R) and an actuator (S) for influencing the pressure in the metering section, characterized 
 in that the pressure is measured by means of the pressure-measuring sensor (P) and is set by means of the controller (R) and the actuator (S) in such a way that the minimum level at the location of the measurement is between 0.1 and 10 bar.    
   
   
       2 . Method according to  claim 1 , characterized in that the measurement control circuit (M) includes a measurement amplifier ( 2 ) which filters out the minimum level of the pressure and provides this value for control purposes.  
   
   
       3 . Method according to  claim 1  or  2 , characterized in that the screw drive ( 9 ) is equipped with a rotation sensor, which is located on the drive itself or on a shaft which runs synchronously with the drive, the rotation signal being taken into account in the controlling of the minimum level of the pressure in the first half of the metering section ( 22 ).  
   
   
       4 . Method according to one or more of  claims 1  to  3 , characterized in that there is a second measurement control circuit (M 2 ), comprising a pressure-measuring sensor ( 19 ), which is located downstream of the screw tip of the metering section ( 22 ), a controller ( 7 ) and an actuator ( 9 ), and the pressure is measured by means of the pressure-measuring sensor ( 19 ) and is set by means of the controller ( 7 ) and the actuator ( 9 ) in such a way that the maximum level of the pressure is limited at the location of measurement.  
   
   
       5 . Method according to one or more of  claims 1  to  4 , characterized in that the extruder is a vented single-screw extruder with a melt gear pump ( 16 ), which is located downstream of the discharge zone ( 22 ), the minimum level of the pressure being controlled in the first half of the metering section ( 22 ) as a result of the rotational speed of the drive motor ( 9 ) being increased when the pressure is too low.  
   
   
       6 . Method according to one or more of  claims 1  to  4 , characterized in that the extruder is a vented single-screw extruder with an adjustable flow restrictor, which is located downstream of the metering section ( 22 ), the minimum level of the pressure being controlled in the first half of the metering section ( 22 ) as a result of the flow restrictor being closed when the pressure is too low.  
   
   
       7 . Method according to one or more of  claims 1  to  4 , characterized in that the extruder is a vented single-screw extruder with a restrictable bypass which, in the open state, returns melt from the space downstream of the metering section ( 22 ) to the first half of the metering section ( 22 ), the minimum level of the pressure being controlled in the first half of the metering section ( 22 ) as a result of the bypass being opened when the pressure is too low.  
   
   
       8 . Method according to one or more of  claims 1  to  4 , characterized in that the extruder is a vented single-screw extruder with separate control of the temperature of the barrel in the region of the metering section, the minimum level of the pressure being controlled in the first half of the metering section ( 22 ) as a result of the temperature of the barrel being increased in the region of the metering section when the pressure is too low.  
   
   
       9 . Method according to one or more of  claims 1  to  4 , characterized in that the extruder is a cascade extruder and the minimum level of the pressure is controlled in the first half of the metering section ( 22 ) as a result of the rotational speed of the drive motor for the screw part in the metering section ( 22 ) being reduced when the pressure is too low.  
   
   
       10 . Method according to one or more of  claims 1  to  9 , characterized in that the thermoplastic which is processed is a polyethylene terephthalate, a polybutylene terephthalate, a polyamide, a polystyrene, a polymethyl methacrylate, a polycarbonate, impact-modified polymethyl methacrylate or another mixture (blend) of two or more thermoplastics.  
   
   
       11 . Device which is a vented single-screw extruder or a cascade extruder, characterized in that it is equipped with a measurement control circuit (M) for carrying out the method according to one or more of  claims 1  to  10 .