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The die is placed at the discharge end of the extruder. Its function is to mold the flowing plastic into the desired shape of the extruded product. Dies can be catego- rized by the shape of the product that they produce. Annular dies are used to make tubing, pipe, and wire coating. Slit dies are used to make flat film and sheet. Circu- lar dies are used to make fiber and rod. Profile dies are used to make shapes other than annular, circular, or rectangular. Dies are also named by the product that they produce. So, we talk about tubing dies, flat film dies, blown film dies, etc.
The inlet channel of the die is usually designed to match the exit of the extruder. If the die entrance does not match the extruder exit, an adapter can be used between the extruder and the die. The three main elements of the die flow channel are the inlet channel, the manifold, and the land region, see Figure 1.16.
Figure 1.16 The three main elements of an extrusion die
The flow channel of the die should be designed such that the plastic melt achieves a uniform velocity across the die exit. The shape of the land region of the die corre- sponds to the shape of the extruded product. An example of an inline tube or pipe die is shown in Figure 1.17. The material flows into the die from the extruder; then it flows around a torpedo.
Figure 1.17 Example of inline tubing or pipe die
The torpedo is supported by spider legs that have a streamlined shape to achieve smooth flow around the support legs. From the torpedo, the plastic melt flows to the tip and die, where it is shaped into an annulus, so that a tube-shaped product emerges.
The size and shape of the land region are not exactly the same as the extruded product. There are several reasons for this: drawdown, cooling, swelling, and relax- ation. These factors are discussed in more detail in Chapter 5. Because of the sev- eral variables affecting the size and the shape of the extruded plastic, it is often difficult to predict how exactly the size and shape of the plastic changes once it leaves the die. As a result, it is also difficult to predict how the die flow channel should be shaped to achieve the desired shape of the extruded product. This is an important reason why die design is still largely based on experience rather than on engineering calculations. With the advent of improved numerical techniques and commercial die flow analysis software, this situation is improving; however, die design is still often a trial and error process.
Size and shape of the extruded product
Types of extruder drives
Another type of die used in the extrusion industry is the coextrusion die. This type of die is used to make a multi-layered product in one step. There are two main coex- trusion systems: the feed block system and the multi-manifold system. In the feed block system, the different plastic melt streams are combined in a feed block and then fed into a regular single manifold extrusion die, see Figure 1.18.
In the multi-manifold system, the different plastic melt streams enter the die sepa- rately and each material has its own manifold. The different melt streams combine close to the die exit to make the multi-layered product. Coextrusion dies are dis- cussed in more detail in Chapter 3.
Figure 1.18: Example of a feed block system