Counter-rotating twin screw extruder

Abstract

A screw extruder having a body forming a chamber of two barrels housing two counter-rotating axis-parallel rotors, a supply port for the material to be mixed in the chamber at one end of the body, a discharge port for discharging the mixed material at the other end of the body, a conveying section with screws for feeding the material from the supply port downstream to a mixing section which comprises at least two mixing zones, each mixing zone having at least one forward-conveying wing and at least one backward-conveying wing downstream of the forward-conveying wing on each rotor characterized in that a throttle valve is provided in the chamber downstream of the mixing section, and downstream of the throttle valve a second conveying section with screws and a second mixing section are provided.

1 . Screw extruder having a body ( 1 ) forming a chamber ( 2 ) of two barrels ( 3 ,  4 ) housing two counter-rotating axis-parallel rotors ( 5 ,  6 ), a supply port ( 7 ) for the material to be mixed in the chamber ( 2 ) at one end of the body ( 1 ), a discharge port ( 8 ) for discharging the mixed material at the other end of the body ( 1 ), a first conveying section ( 34 ) with screws ( 12 ) for feeding the material from the supply port ( 7 ) downstream to a first mixing section ( 44 ) which comprises at least one forward conveying wing ( 14 ,  15 ;  21 ,  22 ) and at least one backward conveying wing ( 17 ,  18 ;  24 ,  25 ) downstream of the forward conveying wing ( 14 ,  15 ;  21 ,  22 ) a throttle valve ( 40 ) downstream of the mixing zone ( 44 ), and downstream of the throttle valve ( 40 ) a second conveying section ( 47 ) with screws ( 27 ) and a second mixing section ( 48 ) on each rotor ( 5 ,  6 ), characterized in that the first mixing section ( 44 ) comprises at least two mixing zones ( 45 ,  46 ), each mixing zone ( 45 ,  46 ) having at least one forward conveying wing ( 14 ,  15 ;  21 ,  22 ) and at least one backward conveying wing ( 17 ,  18 ;  24 ,  25 ) downstream of the forward conveying wing ( 14 ,  15 ;  21 ,  22 ) on each rotor ( 5 ,  6 ). 
   
   
       2 . The twin screw extruder according to  claim 1  characterized in that the downstream ends of the forward-conveying wings ( 14 ,  15 ;  21 ,  22 ) and the upstream ends of the backward-conveying wings ( 17 ,  18 ;  24 ,  15 ) of each mixing zone ( 45 ,  46 ) of the first mixing section ( 44 ) are offset to form a passage for the material to be mixed. 
   
   
       3 . The twin screw extruder according to  claim 1  characterized in that the downstream ends of the backward-conveying wings ( 17 ,  18 ) of the first mixing zone ( 45 ) on the side of the conveying section ( 34 ) and the upstream ends of the forward-conveying wings ( 21 ,  22 ) of the mixing zone ( 46 ) downstream of the first mixing zone ( 45 ) are offset to form a passage for the material to be mixed. 
   
   
       4 . The twin screw extruder according to  claim 1  characterized in that the length (L 2 ) of the backward-conveying wings ( 17 ,  18 ) in the first mixing zone ( 45 ) is shorter than the length (L 4 ) of the backward-conveying wings ( 24 ,  25 ) in the mixing zone ( 46 ) downstream of the first mixing zone ( 45 ). 
   
   
       5 . The twin screw extruder according to  claim 1  characterized in that the radial clearance (C) of the wings ( 11 ,  14 ;  17 ,  18 ) of the first mixing zone ( 45 ) is greater than the radial clearance (C) of the wings ( 21 ,  22 ;  24 ,  25 ) of the mixing zone ( 46 ) downstream of the first mixing zone ( 45 ). 
   
   
       6 . The twin screw extruder according to  claim 1  characterized in that the second mixing section ( 48 ) comprises at least one mixing zone ( 49 ) consisting of at least one forward-conveying wing ( 30 ,  31 ,  32 ) and at least one backward-conveying wing ( 34 ,  35 ,  36 ) downstream of the forward-conveying wing ( 30 ,  31 ,  32 ). 
   
   
       7 . The twin screw extruder according to  claim 1  characterized in that the forward-conveying wings ( 30 ,  31 ,  32 ) and the backward-conveying wings ( 34 ,  35 ,  36 ) of the mixing zone ( 49 ) of the second mixing section ( 48 ) are positioned to form continous flights in the shape of a “V”. 
   
   
       8 . The twin screw extruder according to  claim 1  characterized in that the number of the forward-conveying wings ( 14 ,  15 ;  21 ,  22 ;  30 ,  31 ,  32 ) and of the backward-conveying wings ( 17 ,  18 ;  24 ,  25 ;  34 ,  35 ,  36 ) of each mixing zone ( 45 ,  46 ,  49 ) corresponds to the number of the flights of the screws ( 12 ,  27 ) of the conveying sections ( 34 ,  47 ) upstream of the respective mixing zone ( 45 ,  46 ,  49 ). 
   
   
       9 . The twin screw extruder according to  claim 1  characterized in that the length (L 1 , L 3 , L 5 ) of the forward-conveying wings ( 14 ,  15 ;  21 ,  22 ;  30 ,  31 ,  32 ) of each mixing zone ( 45 ,  46 ,  49 ) is longer than the length (L 2 , L 4 , L 6 ) of the backward-conveying wings ( 17 ,  18 ;  24 ,  25 ;  34 ,  35 ,  36 ) of said mixing zone ( 45 ,  46 ,  49 ). 
   
   
       10 . The twin screw extruder according to  claim 1  characterized in that the ratio of the length (L 1  to L 6 ) of the wings ( 14 ,  15 ;  17 ,  18 ;  21 ,  22 ;  24 ,  25 ;  30 ,  31 ,  32 ;  34 ,  35 ,  36 ) to the barrel diameter (D) is 0.3 to 2.0. 
   
   
       11 . The twin screw extruder according to  claim 1  characterized in that the lead of the wings ( 14 ,  15 ;  17 ,  18 ;  21 ,  22 ;  24 ,  25 ;  30 ,  31 ,  32 ;  34 ,  35 ,  36 ) of the first mixing section ( 44 ) and/or or the second mixing section ( 48 ) is 2 to 6 times the barrel diameter (D). 
   
   
       12 . A method for compounding multimodal polymer compositions comprising the steps of contacting the multimodal polymer compositions with the twin screw extruder according to  claim 1 . 
   
   
       13 . The method according to  claim 12  characterized in that the specific energy input for compounding a multimodal olefin polymer is lower than 300 k Wh/t.