Die-head mandrel

Fig. 14.23 Typical blow molding die A, choke adjusting nut B, mandrel adjustment C, feed throat D, choke screw E, die head F, plastic melt G, die barrel H, heater band /, choke ring J, centering screw K, clamp ring L, die heater M, die N, mandrel. [Reprinted by permission from J. D. Frankland, A High Speed Blow Molding Process, Trans. Soc. Rheol., 19, 371 (1975).]...

FIGURE 14-9 Schematic diagram of the die head and mandrel for a single screw extruder. 

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... 

This is another ramification of incomplete response of polymers, because the experimental time is smaller than the relaxation time of the system of macromolecules. As expected, weld lines are mechanically weak and have optical properties that differ from those of the bulk, making them visible. Furthermore, they result in film or tube gauge nonuniformities, probably because of the different degree of swelling of the melt in the neighborhood of the weld line. They also induce cross-machine pressure nonuniformities. To overcome these problems, basic cross-head die designs (Fig. 12.42) have been devised in which the mandrel is mechanically attached to the die body in such a way that obstacles are not presented to the flow in the annular region. 

In the cross-head type of dies, the melt is split at the inlet to the manifold and recombines 180° from the inlet. Moreover, the flow is not axisymmetric, and fluid particles flowing around the mandrel have a longer distance to travel than those that do not. 

Forcing of Cl into the head die is a multifunctional operation leading to reduced friction of the melt against the die walls, in addition to improved orientation stretching and reinforcement of the film. The distinguishing feature of the employed extrusion head [100] (Fig. 2.24) is the presence of an annular chamber 6 in mandrel 4. The chamber communicates with Cl channel 8 and is shut off by a porous wall 5 from the side of channel 7 over which the polymer melt is fed from the extruder into the head. The outside surface (a) of the porous wall 5 and inner surface (b) of matrix 1 are made in the form of a cone whose apex faces the side opposite to the inlet to channel 7. 

The blown-film technique is widely used in the manufacture of polyethylene and other plastic films [14,15]. A typical setup is shown in Figure 2.23. In this case the molten polymer from the extruder head enters the die, where it flows round a mandrel and emerges through a ring-shaped opening in the form of a tube. The tube is expanded into a bubble of the required diameter by the pressure of internal air admitted through the center of the mandrel. The air contained in the bubble carmot escape because it is sealed by the die at one end and by the nip (or pinch) rolls at the other, so it acts like a permanent shaping mandrel once it has been injected. An even pressure of air is maintained to ensure urufoim thickness of the film bubble. 

Die spider n. In extrusion, the legs or webs supporting the die core within the head of an in-line pipe, tubing, or blown-film die. In many pipe dies, the spider legs are cored to permit application of air or water for cooling the mandrel. 

The extrusion rate is controlled by screw speed and head pressure. Since output increases with screw speed, films become thicker. Extruder size should also match the die size, as illustrated in Table 5.10. Internal air is introduced through a 6.5- to 12.5-mm-(0.25- to 0.50-in-) i diameter hole in the die mandrel. The air pressure, typically 0.7 to 34 kPa (0.1 to 5.0 Ib/in ), is used to expand the bubble, but then held constant once the bubble diameter is fixed. This ensures uniform film width, uniform film thickness, and wind-up of wrinkle-free rolls. 

When changig the mandrel, the lead slug behind the die must be removed. This is done by turning the screw-fitting h completely out of the thread and dismounting the die towards the rear. As may be seen in Fig. 42, the lead slug is milled out by a cutter-head v set on a tube w and advanced and rotated by a motor, the worm drive x and the threaded ring y. 

The use of a spider and the problems associated with weld lines can be avoided if a erosshead or an offset die is used. In these dies the molten polymer flow enters from one side and envelops the mandrel directly. One such extrusion head is illustrated in Figure 5. These types of dies are typically used in the extrusion of small diameter tubing or when an internally cooled extended mandrel is used. 

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