Coat-hanger-type die

Flat Film Extrusion In flat film extrusion, the melt is extruded through a long slot in a T or coat hanger-type die, past the die lands. In this setup, the polymer melt is forced into the slot die at its center it reaches the slot opening by way of a manifold and over the lands. The principal advantages of film casting are substantial improvements in the film s transparency, freedom from haze, improved gloss, and other optical properties. 

Fig. 2 Comparing designs of T-type die (A) [(1) constant cross-section manifold (2) constant land length] to coat hanger-type die (B) [(1) manifold cross-section decreases as distance from centerline increases (2) land length becomes shorter farther from the centerline of the die].

Figure 2.25 illustrates a T-type die and a coat-hanger-type die, which are used for both film and sheet extrusion. The die must produce a smooth and uniform laminar flow of the plastic melt which has already been mixed thoroughly in the extruder. The internal shape of the die and the smoothness of the die surface are critical to this flow transition. The deckle rods illustrated in Figure 2.25 are used by the processor to adjust the width of the extruded sheet or film. 

Fig. 1 Coat hanger-type sheet die concept (A) (1) central inlet port (2) manifold (distributes melt) (3) island (along with manifold, provides uniform pressure drop from inlet to die lip (4) die lip (die exit forms a wide slit) and schematic of sheet die (B) (1) upper die plate (2) lower die plate (3) manifold (4) island (5) choker bar (6) choker bar adjustment bolt (7) flex die lip (8) flex lip adjustment bolt (9) lower lip (10) die bolt (11) heater cartridge.

The most common extrusion die for sheet products is the coat hanger-type manifold die as shown schematically in Fig. lA and as a section view in Fig. IB. The key elements of Fig. lA are ... 

Core tube mandrel with coat hanger-type passage that splits the flow and uniformly distributes melt along the annulus between the die pin and the die land. 

FIGURE 2.25 Schematic cross-sections (a) T-type and (b) coat-hanger-type extrusion dies. 

Multilayer co-extrusion is another technique used in the preparation of starch/ synthetic sheets or films [164, 263-266], in which TPS is laminated with appropriate biodegradable polymers to improve the mechanical, water-resistance and gas-barrier properties of final products. These products have shown potential for applications such as food packaging and disposable product manufacture. Three-layer co-extrusion is most often practiced, in which a co-extrusion line consists of two single-screw extruders (one for the inner starch layer and the other for the outer polymer layers) a feedblock a coat-hanger-type sheet die and a three-roll calendering system [164]. Biodegradable polyesters such as PCL [164, 264], PLA [164, 263], and polyesteramide, PBSA and poly(hydroxybutyrate-co-valerate) [164] are often used for the outer layers. These new blends and composites are extending the utilization of starch-based materials into new value-added products. 

FIGURE 1.25 Schematic cross-sections (a) T-type and (b) coat-hanger-type extrusion dies. (After Petrothene A Processing Guide, 3rd Ed., 1965. U.S. Industrial Chemicals Co., New York)... 

Sheet Die n A heavy-walled, extremely rigid steel structure, bolted to an extruder head, whose inner passages form the molten plastic leaving an extruder screw into the shape of a flat sheet. Most modern sheet dies are of the coat hanger type with multi-zone temperature control, and contain adjustable choker bars and die lips for close control of lateral variation in the sheet thickness. See Coat Hanger Die, Choker Bar, and Flexible-Lip Die. 

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. 

Liu et al. [ 1 ] found that the flow uniformity is very sensitive to the variation of the power-law index in a linearly tapered coat-hanger die. If a die was originally designed based on n=0.5 and suddenly n was changed to 0.6, a W-type flow distribution would appear. On the other hand, if n dropped to 0.4, an M-type flow pattern would appear instead. The design equation (17) can be applied to offset the ffow uniformities caused by the variation of w. We fixed =0.5 and made proper selections of ho and h. Then n was varied to n = 0.6 and 0.4. l iy) was computed based on Eq. (17) and then a choker bar with the taper function l y) could be constructed to correct ffow nonuniformities. The effect of n on l y) is displayed in Fig. 7. For the case /I=0.6 >0.5, l y) has to decrease from the die end to the die center to eliminate the ffow nonuniformities if n=0.4 <0.5, then l iy) will increase from the die end to the die center instead. If is longer, the variation of hiy) will be smaller. 

In this chapter, we have described a novel design approach to correct flow nonuniformities caused by four types of production variations in a linearly tapered coat-hanger die. The theoretical approach is based on the onedimensional lubrication approximation and can be used to predict the taper function of an adjustable choker bar. Once a choker bar is constructed based on the prediction of the mathematical model, the flow nonuniformities can be properly eliminated by inserting this bar into the die. A choker bar can be tapered in different ways as indicated in Fig. 3. The shape displayed in Fig. 3a may be easier to machine and was selected for illustration. The four production variations we have considered include (i) enlarging the manifold, (ii) including the fluid inertial terms, (iii) varying the viscosity of the polymeric liquids, and (iv) narrowing the liquid film width to meet production requirements. All these four production variations can be properly handled, but if the fluid inertia becomes dominant, or the Reynolds number is not small, the present method may not be applicable. 

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