Other Methods of Die Balancing

Balancing problems frequently occur when the extruded product has differences in wall thickness. Flowever, even without differences in wall thickness there can be distortion in the extruded product. An example is a simple rectangular or square profile the velocity distribution on a square channel is shown in Fig. 7.105. Clearly, the velocities in the corner are less than they are along the middle of the wall. As a result, there will be more drawdown at the corners than at the mid-sections of the wall. Obviously, this problem is inherent to shapes with corners, particularly corners with a small radius and corners smaller than right-angle corners. 

For this reason, the easiest shapes to extrude are circular and annular shapes. The lower velocities in the corners of a square or rectangular profile can be increased by reducing the land length in the corners. This can be done several ways as shown in Fig. 9.12. 

The examples shown in Fig. 9.12 are methods for local balancing of the land length. Another method of adjusting the shape of the die is to incorporate moveable elements in the channel such as choker bars, flex lips, and membranes. These elements are frequently used in film and sheet dies. However, they can be used in other types of dies as well. 

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