Controlling device and method for extruder

A controlling device applied to an extruder in which a roller die having upper and lower parallel rollers is arranged in front of an outlet of the extruder body rotatably having screws for extruding a material and a bank portion surrounded by the upper and lower rollers and a side guide closing both of right and left sides of the outlet is formed, comprising:

• minimum bank quantity detecting means which is provided for the side guide and detects a first bank quantity which is the minimum for a preceding material to be extruded to be succeeded by a following different material to be extruded in the bank portion;
• roller die stop control means for stopping the roller die when the minimum bank quantity detecting means detects that the bank quantity is reduced to the first bank quantity;
• said screws supplying the following different material to the bank portion;
• bank quantity detecting means for detecting a second bank quantity at which the extruder can be operated;
• and
• roller die activation control means for activating the roller die when the bank quantity detecting means detects that the bank quantity is increased to the second bank quantity.