Melt flow orifice shape

As explained in Chapter 17 die configurations meet certain melt flow requirements such a drawdown ratio and draw ratio balance. Draw-down ratio (DDR) in a circular die, such as a wire die, is the ratio of the cross sectional area of the die orifice/opening to the final extruded shape. Another guide for setting uniformity and best repeatable references is the draw ratio balance (DRB) that aids in determining the minimum and maximum values that can be used for different plastics. 

Some special dies are shown in Fig. 3-20 they produce interesting flow patterns and products such as tubular to flat netting dies. For a circular output, a counter-rotating mandrel and orifice have semicircular-shaped slits through which the melt flow emerges. If one part is held stationary, then a rhomboid or elongated pattern is formed if both parts rotate, then a true rhombic mesh is formed. When the slits overlap, a crossing point is formed... 

The die has been called the brains of the operation because the product s final shape is most determined by the melt forming that occurs in the die. Many die types are available to produce various extruded products to specific geometric and performance requirements. One thing that all dies have in common is the need for properly designed internal flow geometry, including exit orifice geometry. If the die does not possess the correct flow shape, it may not be possible to produce the polymeric product to specification. 

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