Pipe and Tubing Dies

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 direction of the inlet flow is perpendicular to the outlet flow. The polymer melt makes a 90° turn and splits at the same time over the core tube. The polymer melt recombines below the core tube this is where a weld line will form. After the 90° turn, the polymer melt flows through the annular flow channel where it adopts more or less the shape of the final land region. 

Eximder scuew Extruder barrel Breaker plate Die adjustment bolt 

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 high-pressure extrusion. Fig. 9.19, the polymer melt meets the conductor before the die exit. This allows for good contact between the conductor and the polymer. In low-pressure extrusion. Fig. 9.19, the polymer melt meets the conductor after the die exit. The polymer is tubed down over the conductor. Low-pressure extrusion is used when good contact between the wire and the polymer is not essential, for instance, when a loose jacket needs to be extruded over a coated wire. 

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Several unique products are made by extrusion and the dies needed to make these products are classified as 1) sheet dies 2) flat-film and blown-film dies 3) pipe and tubing dies 4) profile extrusion dies and 5) co-extrusion dies. Furthermore, each product type has unique hardware downstream of the die to shape and cool the extruded melt. To aid the reader, detailed illustrations of the various die designs and the complementary downstream cooling and shaping hardware are shown. 

Pipe and Tubing. A typical die for extmding tubular products is shown in Figure 4. It is an in-line design, ie, the center of the extmded pipe is concentric with the extmder barrel. The extmdate is formed into a tube by the male and female die parts. The male die part is supported in the center by a spider mandrel. Melt flows around legs of the mandrel and meets on the downstream side. The position of the female die part can be adjusted with bolts adjustment is requited to obtain a tube with a uniform wall thickness. 

Fig. 4. Pipe or tubing die for in-line extmsion A, die body B, mandrel, pin, and male die part C, die, die bushing, and female part D, die-retaining ring E, die-retaining bolt F, die-centered bolt G, spider leg H, air hole I, seat for breaker plate J, ring for attachment to extmder and K, die land (15).

Fig. 5. Vacuum caUbrator for pipe and tubing extmsion A, molten tube from die B, tank C, hinged cover with gasket D, siting rings E, circulated and temperature controlled water E, water level G, vacuum and H, inside of pipe open to atmospheric pressure (15).

Extrusion. The main applications of this method include the production of film, sheet, pipe, and tubing. PB is usually extruded by using the same equipment (single- cu twin-screw extruders) as that used foi PP and HDPE, at melt and die temperatures of 170- 190°C. PMP is processed on extruders with a high length-to-diameter ratio at temperatures of 240-300°C. 

It is worth noting that, although in principle tube and tubular blown film dies are similar, in practice they are quite different in function, size, and complexity. Blown film dies are much longer, have a very small die lip opening, and are subject to more stringent product uniformity criteria because there is no sizing equipment downstream. Furthermore, blown film products are almost exclusively LDPE, and occasionally HDPE and PP. On the other hand, HDPE and rigid and plasticized PVC are the common polymers for pipes and tubes. 

Usually when making pipe and tubing the extrudate is inflated to the dimensions of a sizing die in such instances it must be taken that on emerging from the die the swell will be in line with the rate of shear, that the thicknesses of sections will be reduced in proportion to the amount of inflation, and with ratio of sizing die to the external diameter of the die orifice. 

Both pipe and tubing are made in dies with an annular die exit. A pipe product is defined as being greater than 1 in. in outer diameter and a tube less than 1 in. Dies for these products are made in two styles 1) in-line dies (also called spider dies) shown in Fig. 4A and... 

Figure 6.16 Typical spider-type tube die for pipe and tube extrusion.

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