Pipe or Tube Extrusion

Figure 1. Pipe or tubing extrusion die conf uration.

Soldered Joints (Fig- 10-147) These joints reqmre precise control of the diameter of the pipe or tubing and of the cup in the fitting in order to cause the solder to draw into the clearance between the cup and the tubing by capillaiy action (Fig. 10-147). Extrusion provides this diametral control, and the joints are most widely used in copper. A 50 percent lead, 50 percent tin solder is used for tempera-... 

FIGURE 2.28 An extrusion die fitted with a tapered cooling and sizing mandrel for use in producing either pipe or tubing. (After Fisher, E. G. 1971. Blow Molding of Plastics, lliffe, London.)... 

While any plastic material, irrespective of its chemical composition and character, may be made into pipe or tubing, by far the greatest amount of pipe is made from thermoplastics (TPs) that are adaptable to extrusion processes (Chapter 5). Specialty pipe is made in small amounts from TS materials such as phenolic and polyester, but very large of commercial pipe is made from polyethylene, polyvinyl chloride, acrylonitrile butadiene copolymers, and acrylonitrile butadiene styrene types of alloys. Specialty tubing in relatively small amounts is made of acrylates and acrylate copolymers, as well as other transparent materials (Chapter 3). 

Fig. 3-12a. Examples of dies with nomenclature. (1) Pipe or tubing die for in-line extrusion. (2) Pipe or tubing die for crosshead extrusion. (3) Cast film die. (4) Sheet extrusion die.

Die core n. The tapered element in an extrusion die for pipe or tubing that guides the material to the webs of the spider. Sometimes called the torpedo or spreader. 

Mandrel man-drol [prob. mod. of F man-drin (1665) n. (1) The core around which paper, fabric, or resin-impregnated fibrous glass is wound to form pipes or tubes. (2) In extrusion, an extension of the core of a pipe or tubing die, internally cooled by circulating water or other fluid, that guides and cools the internal surface of the tube as it emerges from the die proper. The mandrel is an important determiner of the final internal diameter of the tube. 

Pipe die n. An extrusion die whose lands form a circular annulus used in extrusion of plastic pipe or tubing. The outer shell of the die is usually called the die, the core is called the mandrel. Pipe dies may be side-fed or end-fed, and the mandrel may be supported by a trio of legs called a spider, or it may be supported from the rear of a side-fed die. It is easier to achieve circumferential uniformity of wall thickness with a spider die, but the splitting of the melt stream at the legs has sometimes caused weak welds because of insufficient knitting time before the pipe emerges and is chilled. 

While barrel, die, and water-bath temperatures and extrusion rate can be varied in pipe extrusion, the primary controls are the die gap, calibration rings or extended mandrel, and the take-off speed. Die gap and centering of the mandrel provide the initial wall thickness and thickness uniformity, whereas take-off speed determines the final thickness. While extended mandrels fix the inner diameter, calibration rings establish the outer diameter in vacuum and pressure cahbration systems. The other diameter varies with the take-off rate. Water-bath temperatures and temperature profiles control the cooling rate for the pipe or tubing. This alters the shrinkage, crystallinity, and retained stresses in the products. 

In the extrusion of pipe or tubing, the die bushing is generally the female part of the die and is attached to the die body by adjustable saews. Thus small changes can be made in the wall thickness if required. 

Pig. 4. Pipe or tubing die for in-line extrusion 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 extruder and K, die land (20). 

The equipment used in extrusion of various profiles is generally similar to that used for pipe or tubing. The main difference is in the die, whose shape corresponds to the contour of the particular profile. The shape of the die determines the ultimate shape of the section but the two are not the same because of the complex flow within the die. Many of the problems associated with the die design for profile can and have been solved theoretically, but a majority of dies made still rely on practical experience and trial and error to achieve the exact shape required. The most important material used for miscellaneous sections is PVC both plasticised and unplasticised. 

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