Thermoplastics Tube And Pipe Size Is Set by One of Four Techniques

Thermoplastics tube and pipe size is set by one of four techniques: the vacuum trough, the extended (internal) mandrel, the sizing sleeve, and the sizing plates (Fig. 4.47). Vacuum trough and extended mandrel are the more common techniques for holding the size of the tubes and pipes in a quench bath. These two methods are described in detail below. In the sizing sleeve method, the outside diameter of the tube is fixed as it comes in contact with a water-cooled metal sleeve (usually brass). This contact takes place by air pressure inside the tube or by drawing a vacuum through the perforated internal surface of the sleeve. The sizing-plate method predates the vacuum trough technique. In this method, the tube takes its size as it is pulled through a series of brass or stainless steel plates, similar to the way that metal wire is drawn from a metal rod. The tube is forced through the plates by a positive internal air pressure.

In the vacuum trough method illustrated in Fig. 4.47(A), the tube (or pipe) enters into one end of a closed long trough and is extruded out of the other end. The trough is filled with water, which directly contacts and surrounds the tube, providing efficient cooling. Inside the trough, vacuum is drawn over the water, reducing the pressure in the trough, thus allowing the soft tube to expand against the collars or rings at the entrance and exit. This mechanism prevents tube collapse and ensures well-rounded sizing of the outside of the tube. The pressure difference between the outside (under vacuum) and the inside of the tube that is open to the atmosphere generates the expansion force. The tube moves through fixed metal collars or rings, which give it its size. The first collar at the trough entrance is the most important one. A small stream of water is directed at the pipe just before the entrance as a lubricant. The number and positions of the rings and sleeves are usually adjustable. They become slightly smaller as the pipe passes through the trough, taking into account shrinkage that results from cooling and crystallization of the polymer[1].

Jiri George Drobny, in Handbook of Thermoplastic Elastomers (Second Edition), 2014

Figure 4.47. Methods for pipe and tube sizing: (A) Vacuum trough; (B) Extended mandrel method.