Cross head die

CROSS BEAM, 195 CROSS HEAD DIE, 365 CROSS-POLARISATION, 101 315 CROSSLINK DENSITY, 66 168 169 262... 

This is another ramification of incomplete response of polymers, because the experimental time is smaller than the relaxation time of the system of macromolecules. As expected, weld lines are mechanically weak and have optical properties that differ from those of the bulk, making them visible. Furthermore, they result in film or tube gauge nonuniformities, probably because of the different degree of swelling of the melt in the neighborhood of the weld line. They also induce cross-machine pressure nonuniformities. To overcome these problems, basic cross-head die designs (Fig. 12.42) have been devised in which the mandrel is mechanically attached to the die body in such a way that obstacles are not presented to the flow in the annular region. 

Two types of cross-head dies are used for wire and cable coating. The first type is an annular flow or tubing die, where a thin-walled tube is extruded and the molten tube drawn onto the conductor by vacuum after it leaves the die. The vacuum is applied through the clearance between the conductor and the guide, which is usually of the order of 0.2 mm. Tubing dies are commonly used for jacketing cables or coating very thin wires with polymer melts that are very viscous. 

Extrusion of tubes, hose, and profiles is done on standard extruders for rubber. The usual temperature pattern is a gradual increase of temperature from the feed zone to the die. The die temperature is typically 100°C (212°F), and the screw temperature is approximately the same as the temperature of the feed zone [44]. Processing aids are almost always required to improve the surface appearance and to increase the extrusion rate. Extrusion represents only a small proportion (about 10%) of the total consumption of fluorocarbon elastomers [52].Cold feed extruders typically used for extrusion of rubber stocks are also suitable for the extrusion of fluorocarbon elastomers. Extrusion should be carried out at temperatures below 120°C (250°F) to avoid scorch. In most cases a breaker plate and screen pack are used to generate backpressure on the screw and to remove foreign particles from the stock. A straight head is used for the extrusion of profiles or tubing, whereas a cross-head die is used for... 

Another application of extrusion is the coating of metal wires or profiles with a plastic layer. This is done on a large scale in the manufacture of electric wires and cables. For this purpose, a cross-head die is needed (Fig. 23.8). The same principle is used for plastic profiles containing a metal core. 

With a given die cross-sectional area, there is only 1 ratio of puller speed to extruder throughput rate that produces a product with the correct cross-sectional dimensions. If the extruder throughput is increased, the puller speed must be increased proportionally to maintain the same finished product dimensions. Likewise if the throughput is decreased the puller speed must be decreased proportionally to maintain the same finished product cross-sectional area. The draw ratio and molecular orientation can only be increased or decreased by changing the die cross-sectional area relative to the puller speed assuming the final product dimensions are kept constant. This is easily done with sheet dies, cast film dies, or blown film dies that have adjustable die lips. Profile and cross head dies... 

Numerous types of dies are available depending on the extrusion process and the type of product being produced. Compounders use strand dies to make continuous strands that are chopped into pellets. Sheet and cast film producers use flat dies. A schematic of a flat film or sheet die is shown in Fig. 12.53. This produces a flat web of a specific thickness that passes through a three-roll stack or lays flat on a cast roll. Blown film producers use circular dies as shown in Fig. 12.54. Tubing and pipe cross head dies are similar to that shown in Fig. 12.55A where... 

Figure 12.55 Schematic of a cross head die for tubing or pipe extrusion and the schematic diagram of two types of wire coating cross head dies.

LCP/carbon-fibre composites have been developed and are used as secondary composites within the aerospace industry. LCP matrices offer low viscosity for the impregnation of the fibres, and excellent chemical resistance. Impregnation is achieved in a cross-head die, after which the composite panels are prepared by compression moulding the stacked prepreg sheets. 

Low-density polyethylene (LDPE, Dow 6401, 0.92 g/cc and 1 g/10 min MFI) was used in this study. The polymer was extruded using a lab-scale 19 mm, 24 1 L/D ratio extruder (Alex James Associates, Greenville, SC) equipped with a cross-head die (Wayne Machine Co., NJ) of 50 mm diameter, 0.635 mm gap and a single-lip air ring. A die temperature of 195 C and polymer mass flow rate of approximately 36 g/min was maintained throughout the experiment. The bubble was extended uniaxiallyat a take-up ratio (TUR) of 7 and a blow-up ratio (BUR) of 0.4, so that a thickness ratio of 0.4 was obtained. 

Fig. 10. Wine-coating pressure die A, die body, cross-head B, guider tip C, die D, die-retaining ring E, die-retaining bolt F, wine and G, core tube (15).

Figure 14.15 General concept of the process of forming a distributed bend using the continuous die-less forming machine in which the cross-head cylinder moves both vertically and horizontally...

To overcome weld line problems, the cross-head tubing die is often used. Here, the die design is similar to that of the coat-hanger die, but wrapped around a cylinder. This die is depicted in Fig. 3.17. Since the polymer melt must flow around the mandrel, the extruded... 

A cross-head tubing die is equivalent to the coat hanger die by wrapping it around a cylinder as can be recognized in the schematic presented in Fig. 3.17. If we follow the same derivation but for a shear thinning power law melt, we get... 

In the cross-head type of dies, the melt is split at the inlet to the manifold and recombines 180° from the inlet. Moreover, the flow is not axisymmetric, and fluid particles flowing around the mandrel have a longer distance to travel than those that do not. 

C. Gutfinger, E. Broyer, and Z. Tadmor, Analysis of a Cross Head Film Blowing Die with the Flow Analysis Network (FAN) Method, Polym. Eng. Sci., 15, 385-386 (1975). 

Generally, processing temperatures for THV are comparable to those used for most thermoplastics. In extrusion, melt temperatures at the die are in the 230°C to 250°C (446°F to 482°F) range. These relatively low processing temperatures open new options for combinations of different melts (coextrusion, cross-head extrusion. 

Another application for the cross-head-style tubing die is wire coating. The following adjustments are made to a cross-head tubing die to perform wire coating ... 

Mechanical properties (stress-strain) were measured on an Instron tester at a cross head speed of 10 mm/min. Dumbbells shaped samples were cut with a die and had an 8 mm test length and a thickness of no more than 0.5mm. Additional details may be found in the thesis work of one of the coauthors (C. W. C.) (14). 

The mechanical properties of the polylmide film were measured in tension tests. To do this polylmide films were cast and cured on glass substrates. The films were removed and tensile test specimens were stamped from the films using a die. The specimens were then pulled to fracture at a cross-head speed of 10 mm/min. The mechanical properties obtained from the load-elongation curves are given in Table I. 

Tensile tests were conducted using the ASTM D638 Type V technique at room temperature on dog bone-shaped specimens of 25 mm uge-length and 3 mm width. The specimens were 1 mm thick and were die-cut into the ASTM dog bone shape. An ATS Universal 900 tensile tester at a cross-head speed of 25 mm/min was used to test six replicates per nanocomposite type. 

The MVX has been used to make hoses with a cross-head, and sheet material with a roller die head. This method is economical when the whole of the output of the MVX can be used. If, for example, small extruder lines with an output of 50-200 kg/h are to be used, then direct compounding would be difficult to justify. But for 400-3 000 kg/h direct compound and product production has proved to be very profitable. 

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