Extended mandrel

The extended mandrel technique, illustrated in Fig. 6.22b, is highly desirable for fluoropol5miers because it provides internal cooling and support to the tube. The shrinkage of the plastic as a result of cooling causes a tight contact between the tube and the metal mandrel that can extend up to 30 cm beyond the die. Longer mandrels have been difficult to use due to mechanical difficulties. Surfaces will eventually cor-... 

The most common method of fabricating tubing with outside diameters of 12-30 mm and wall thickness in the range of 0.3 to 0.8 mm is the extended mandrel technique.Aside from corrosion concerns, brass is the best choice of construction material for the mandrel and Vespel for the insulation gasket between the guide and the mandrel. The guide is heated electrically to prevent melt fracture. Mandrel temperature is controlled to allow solidification of the melt and avoid sticking. 

For monofilament samples, the apparatus shown in Figure 2 is used to load the sample in tension (11). The monofilament is wound around a split cylindrical mandrel, half attached to the fixed lintel and half attached to the loading rod. The sample is attached to the mandrel at two points and is free to move around the mandrel as the sample is extended under load. 

If the filaments are not wound onto a mandrel but simply gathered into relatively large bundles and led slowly into a heated consolidation zone, emerging from a die partly or completely cured, the primitive essentials of pultrusion are present (Fig. 2.3(b)). This process began as the thermosetting resin equivalent of extrusion, that is, it was a continuous operation for manufacturing profiles, such as rod and channels. The method has been extended to fibre reinforced thermoplastics. 

In recent years filament winding has been extended to the continuous production of pipe using a continuous steel band mandrel. In this way continuous lengths of pipe can be produced, with diameters ranging from about 0.3 to 3.5 m. 

With a spider-arm die (Fig. 5.396 ), polsuner melt is fed through the bottom of the die while air is introduced through three or more spider arms that extend from the sides of the die to support the mandrel. This creates a relatively low pressure drop, and allows the die to be used with high-viscosity materials. The weld lines are stronger than the single-weld line produced with a side-fed die, but can result in weak points in the film. Spider-arm dies are used for poly(vinyl chloride) and other high-viscosity, heat-sensitive materials. 

Figure 5.50 Calibration methods for pipe extrusion (a) extended water-cooled mandrel, (b) vacuum forming, and (c) pressure forming.

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. 

A special extrusion technique that is occasionally used is the extended mandrel. This is particularly used for thin wall tubing. In this technique the tip extends beyond the die by a considerable distance. The purpose of the extended mandrel is to obtain better shaper definition. Another use of the extended mandrel is to provide localized heating of the tip using an induction coil at the die exit. This is a variation of the G-Process discussed under special features. It is a suitable method to eliminate internal melt fracture or internal die drool. The mandrel extension should be made of a ferro-magnetic material to obtain an efficient temperature increase under the influence of the alternating magnetic field of the induction coil. 

Pipe and Tubing. A typical die for extending tnbnlar products is shown in Figure 4. It is an in-line design, ie, the center of the extmded pipe is concentric with the extruder barrel. The extrudate is formed into a tnbe 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 required to obtain a tnbe with a uniform wall thickness. 

Spider n (1) In a molding press, that part of an ejector mechanism that operates the ejector pins. (2) Within an end-fed extrusion die making a tubular section, the three or four legs extending from die to mandrel and... 

Figure 8.26 Methods of tube and pipe sizing, (a) Vacuum trough method. (6) Extended mandrel method.

Extrusion stretch blow molding is a two-stage process using two mold/mandrel sets — one for pre-blow and the other for final blow. An extruded parison is first pinched off and blown conventionally in a relatively small pre-blow mold to produce a closed-end preform. The preform is then transferred to the final blow mold where an extending stretch rod within the blowing mandrel bears on the closed preform end to stretch it axially. The stretched preform is then blown to impart circumferential stretch. Standard blow molding machines can be converted for extrasion stretch blow molding. 

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