Crosshead die

A wire crosshead die is used to manufacture wire coatings, which is illustrated in Fig. 11.10. This specialized die turns the melt flow 90° before it leaves the die. At this turn, the wire to be coated enters the melt stream and exits the die co-axially with the polymer. This process yields a seamless polymer coating around the wire. 

Carley (C2) has used the power-function relation [Eq. (3)] in a mathematical development for slit-crosshead-die extruders. Experimental verification of what appears to be a most useful piece of work would appear to be desirable, but even without such data the qualitative trends discussed by Carley are of major significance. 

FIGURE 14-28 Schematic diagram of a crosshead die for wire coating. 

Tube extrusion is quite similar to the wire insulation process. Processing details depend on the size and type of the tube. Tube can be manufactured by both in-line and crosshead dies. FEP tubing can be produced with outside diameters as low as 1 mm to over 20 mm. This range is broken into three processing zones based on the size of the outside diameter small, medium, and large. The sizing die determines the outside diameter of the extrusion output and the line speed determines the inside diameter. Take-up speed and the die gap, and the difference between the inside diameter of the die and the outside diameter of the tip, set the wall thickness. 

The procedure for shutdown without cleanout is as follows (1) one should empty the hopper as well as possible (2) if coating, one should take the action required to remove the substrate (with wire coating, one removes the wire from the crosshead die in coating paper, film, etc., usually the extruder is on a track and can be withdrawn from the substrate etc.) (3) one reduces all heat settings to the melt heat (4) one reduces the screw speed to 2 to 5 RPM, purging the resin, if required, into a water bucket or drum prior to reducing the heat to melt heat (5) when the screw appears to be... 

Die designs depend on the product that will be formed. Typically, spiral flow and spider arm dies are used for blown film, tubing, and pipes. Crosshead dies are employed for tubing and wire coating. Wide dies with tee, coat hanger, and exponential are employed... 

Figure 9.21 Diagram of concave and convex pins in the crosshead die for maximal spreading of filaments. (From Ref. 182, with permission.)...

The difference between pipe and tubing is mainly determined by size. Small diameter products (less than 10 mm) are generally referred to as tubing, while large products are generally referred to as pipe. Annular products can be extruded on inline dies and crosshead dies. In the crosshead die the polymer melt makes a turn as It flows through the die an example of a crosshead die Fig. 9.18. 

The crosshead die is also used for wire coating. In wire coating, a conductor passes through the hollow center of the core tube and becomes coated with polymer melt close to the die exit. The conductor may be a bare conductor or it may already have been coated with one or more layers of polymer. In wire coating, one distinguishes between high-pressure extrusion and low-pressure extrusion see Fig. 9.19. 

In these dies, the center line of the die is in line with the center line of the extruder. The central torpedo is supported by a number of spider legs, usually three or more. The spider legs are relatively thin and streamlined to minimize the disruption of the velocity profile. Of course, as the polymer recombines after the spider leg, a weld line will form. Thus, the location of the spider support should be far enough from the die exit to enable the polymer to heal. The location of the die is generally adjustable relative to the pin, just as in the crosshead die. 

A simpler die is the conventional crosshead die see Fig. 9.24. This design is more susceptible to weld lines however, with the correct design good blown film can be... 

The distribution characteristics of conventional crosshead dies may not be good enough for application in blown film extrusion, where wall thicknesses are generally quite small (the typical range is 0.005 mm to 0.25 mm). Spiral mandrel dies can achieve good flow distribution and largely eliminate weld lines. As a result, spiral mandrel dies are widely used in blown film extrusion. 

A crosshead die is shown in Figure 74. The melt is split around the flow splitter or helicoid, then flows over a shoulder to the tip and the die. The tip is... 

Crosshead dies are also used for wire coating. In this case, a conductor moves through the core tube and tip, often at high speed. As the conductor exits from the tip, it is coated with molten plastic in a continuous fashion. Very high line speeds can be achieved in wire coating, as high as 3000 m/min (about 10,000 ft/min). 

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