The die is placed at the discharge end of the extruder. Its function is to shape the flowing plastics into the desired shape of the extruded product. Dies can be categorized 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. Circular dies are used to make fibre and rod. Profile dies are used to make shapes other 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. The flow channel of the die should be designed such that the plastics melt achieve a uniform velocity across the die exit. Many different dies can be used.
Dies can be categorized by the shape that they produce; we have tubing and pipe dies, film and sheet dies, wire coating dies, and profile die. Profiles are extruded shapes other than circular, annular, or rectangular.
The shape of the land region of the die will correspond to the shape of the extruded product. An example of an inline tube or pipe die is shown here. The material flow into the die from the extruder, then it flows around a torpedo. The torpedo is supported by spider legs that have a streamlined shape to achieve smooth flow around the support legs. From the torpedo, the plastics melt flows to the tip and die where it is shaped into an annulus, so that a tube shaped product emerges from the die.
The size and shape of the land region are not exactly the same as the extruded product, there are several reasons for this: draw down, cooling, swelling, and relaxation. This is discussed in more detail in a later session. Because of the several variables affecting the size and the shape of the extruded plastics, it is often difficult to predict how exactly the size and shape of the plastics will change as 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 sometimes 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 involves a trial and error process.
1.10.1 Co Extrusion Dies
Another type of die that is used in the extrusion industry is the co extrusion die. This type of die is used to make a multi-layered product in one step. There are two main [list] co extrusion systems: the feed block system and the multi-manifold system. In the feed block system the different plastics melt streams are combined in a feed block and then fed into a regular single manifold extrusion die . In the multi-manifold system the different plastics melt streams enter the die separately and each material has its own manifold. The different melt streams combines close to the die exit to make the multi-layered product. Co extrusion dies will be discussed in more detail in a later session.
