Tube extrusion, methods

Two-stage EBM treats the parison as a true preform by separating the functions of parison preparation and BM. Parisons are produced by conventional tube extrusion methods (Chapter 5). The tube is cooled and cut into parison lengths that are stored before being reheated and BM in the normal way. 

The whole study of this research has been divided into two parts preparation of porous substrate and deposition of thin palladium membrane. This paper reveals only the first one, i.e. the fabrication of porous ceramic tubes by extrusion method. Early ceramic supports for palladium membrane were made of AI2O3 In recent work, we attempted to examine the properties of two kinds of ceramic materials, AI2O3 and YSZ (yittria stabilized zirconia). The extrusion of those ceramic materials was carried out by mixing with additives in various portions. After that, they were sintered at temperature between 1200 - 1450 °C in order to investigate the effect of sintering temperature on pore size and porosity of porous support. The mechanical strength was also inspected to clarify the most appropriate sintering temperature for each ceramics support. 

This method can be used as an alternative to melt extrusion method. The polymer powder is fed into a screw extruder, which heats up the polymer. The molten polymer from screw extruder is forced through an annular die. An inert gas is then passed through the tube, so that due to gas pressure, the tube blows into a cylindrical bubble. The bubble is then flattened by a series of rollers to form a continuous film that can be wound up by specially designed rollers. This process is shown schematically in Figure 1.35. 

Typical processing methods compression molding, electrostatic powder coating, fluidized bed coating, film/tube extrusion, flame and plasma spraying, injection molding monofilament extrusion, wire cable injection molding ... 

Combined tube and rod extrusion presses are sometimes designed so as to be suitable both for direct and inverted extrusion. In the direct or Dick extrusion method, as illustrated in Fig. 56 a, the billet lies between extrusion ram and die. Consequently, frictional resistance has to be overcome on the wall of the container which has its highest value at the beginning of the extrusion cycle and decreases in accordance with the reduction in length of the billet. When extruding by the inverted method. 

The design of the early vertical tube extrusion presses was simple and cheap. They were of the down-stroking type and built in frame construction. No special piercing equipment was provided since the billets extruded were only small ones. The mandrel w as rigidly inserted in the extrusion ram and extrusion is performed on the without shell method, when the ram has only a little clearance in the container. The heavy metal... 

Fig. 121 illustrates the tools used in various methods of tube extrusion, when - as is general practice - round billets are used. Sketch a shows the tools most frequently used when extruding tubes employing a normal mandrel, i. e. not a stepped one. 

With older rod and tube extrusion presses the container is in many cases set in the center of the counterplaten. This design has, however, proved unsuitable because of the high transmission of heat. The method presently used is a separate container-holder, either bolted to the counterplaten or used to shift the container. The container-holder is made of a steel casting and is insulated both on the inside and outside so as to prevent absorption and reflection of heat. It has furthermore proved to be very advantageous to support the container both in the horizontal and vertical plane on four ribs, as illustrated in Fig. 142, to allow for a free expansion due to heat changes. 

Tubular membranes are usually prepared by a plastic extrusion method [28]. Calcined and milled ceramic powder is mixed with several additives to make a slip with enough plasticity to easily be formed into tubes while... 

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

In vacuum and blow forming, sheets produced by extrusion are shaped by vacuum or pressure forming. Heat-softened sheet is pressed into a mould by atmospheric pressure when a vacuum is created between the mould and the sheet. Fig. 24.4(a). Plastic bottles are made by blowing instead heated tube is clamped in a split mould and expanded with compressed air to take up its shape (Fig. 24.4b). Both methods are cheap and quick, and can be fairly accurate. 

Plastic products are made by a variety of basic manufacturing processes. As an example a major method such as extrusion has subdivisions that include profile, pipe, tube, film, sheet, coating, post forming, etc. equipment In injection molding there are subdivisions such as coinjection, gas assist, foam, inmold decorating, etc. equipment. There are literally hundreds of processes used with only about the dozen, as reviewed in this chapter, that are principally used (2). 

A dilatometer is used to measure the intrusion and extrusion volumes and several methods are used to measure the change in mercury level within the dilatometer stem as intrusion and extrusion take place. One method involves the use of platinum or platinum-alloy resistance wire placed coaxially in the stem of dilatometer tube. As the mercury level decreases, the amount of resistance wire exposed increases, thereby providing a voltage which increases linearly with decreasing mercury level. Very small current is used to minimize resistive heating of the mercury. Platinum or platinum-iridium alloy may be employed as the resistance wire because they do not amalgamate with mercury. 

Heat-shrinkable sheets (thickness 0.040 to 0.120 inch, or 1 to 3 mm) and films (thickness 0.001 to 0.020 inch or 0. 025 to 0.5 mm) are fabricated from many of the same materials as shrinkable tubing.91 They are produced by extrusion as a tube, sheet or blown film. Irradiation is done by the equipment shown in Figure 8.10. Orientation (stretching) after irradiation can be done by several methods, namely by differentially heated and driven rolls (in the machine direction) or by a tenter frame (see Figure 8.11) in the transverse direction. If desired, biaxial stretching can be done. 

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. 

Mixed compounds are almost always transformed into products with required shapes and dimensions. There are several methods to accomplish this. Tubes, solid round profiles and profiles with irregular, often complex shapes, are prepared by extrusion. Sheets, slabs, and rubber-coated fabrics are made by calendering. 

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