Coat-hanger die

Perhaps a more common sheeting die is the so-called coat hanger die, presented in detail in Chapter 3. For a given manifold angle a we must determine the manifold radius profile, R(s), such that a uniform sheet or film is extruded through the die lips. 

Using the nomenclature presented in Fig. 6.12 and assuming a land thickness of h we can assume the land length to be described by slit flow and the manifold by the Hagen-Poiseuille flow with a variable radius as 

Here too, we can cut a small element out of the manifold area and can relate the pressure drop in the s-direction to the drop in the z-direction using 

Combining the definition of pressure gradient in the die land, eqn. (6.42), with eqn. (6.43) and using geometry we get 

A cross-head tubing die is equivalent to the coat hanger die by wrapping it around a cylinder as can be recognized in the schematic presented in Fig. 3.17. If we follow the same derivation but for a shear thinning power law melt, we get 

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To generate a uniform extrudate geometry at the die lips, the geometry of the manifold must be specified appropriately. Figure 3.15 presents the schematic of a coat-hanger die with a pressure distribution that corresponds to a die that renders a uniform extrudate. It is important to mention that the flow through the manifold and the approach zone depend on the non-Newtonian properties of the polymer extruded. Hence, a die designed for one material does not necessarily work for another. 

To overcome weld line problems, the cross-head tubing die is often used. Here, the die design is similar to that of the coat-hanger die, but wrapped around a cylinder. This die is depicted in Fig. 3.17. Since the polymer melt must flow around the mandrel, the extruded... 

Derive the die design equation (eqn. (6.50)) for a coat hanger die assuming a shear thinning power law viscosity model. 

Figure 12.29 presents the geometrical features of the coat-hanger die, on which the design equation will be developed. The manifold is a tubular, variable radius channel of curved axis /. The slit opening H is constant. The only geometric restriction is that the manifold be of a small curvature, so that the lubrication approximation can be applied in the manifold region. Also, for the same reason, dR(x)/dx [Pg.706]

Example 12.2 Coat Hanger Die Design We specify the coat hanger die manifold radius along the entire width of the die, if the manifold axis is straight and makes an angle a = 5° with the x coordinate (see Fig. 12.29). The slit opening is set at H — 0.05 cm, the half-width W = 100 cm, and the Power Law index of the polymer melt n — 0.5. 

A homopol5mier of vinylidene fluoride was ex-trudedt J using a coat hanger die with a width of 590 mm to produce a sheet at a thickness of 150 pm at a die temperature of 270°C. A chill roll at a temperature of 70°C chilled the hot film. This film was biaxially oriented by four times the original length in the extrusion (machine) direction and six times in the transverse direction (perpendicular to the extrusion direction). The film was at a temperature of 100°C while it was oriented at a stretch rate of 10 mm/min in both directions. 

The most common die configuration for production of cast film and sheet is a coat-hanger die (Fig. 7.7), named because of its resemblance to a common coat hanger. T-shaped dies are also used. The die opening is wider and thicker than the finished film. As the film is drawn down between the die and the chill roll, it contracts in width and thins, due to tension on the film being produced and polymer relaxation effects. 

Internals of coat hanger die for cast film and sheet... 

Each type of resin is melted in an individual extruder, and the melts are carefully brought together prior to or in the die, in a manner that keeps them in homogeneous layers, without mixing. The process used to combine the polymers is usually different in the cast and blown film processes. In cast processes, as illustrated in Fig. 7.12, the polymers are typically combined in an adapter, called a feed block, before they enter the coat hanger die. This permits a simpler design for the die itself. Multimanifold dies are used when plastics with widely different flow properties are to be combined, as such systems provide a shorter multilayer flow path before solidification, and thus minimize distortion of the interface. 

Fig. 3-19. Example of a coat hanger die, showing lower half and cross section.

Coat hanger die section (from Osswald and Hernandez-Ortiz, 2006). 

In flat-die extruders, slot and coat hanger die geometries (Fig. 4) predominate, whereas in circular die extruders, spiral dies (Fig. 5) predominate. A die... 

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