Drawn-down processes

One of the characteristics of the extrusion process is that when the material extrudes from the die there is some swelling of the section. After the material exits the die it is usually stretched or drawn down to a size equal to or smaller than the die opening. The dimensions are reduced proportionally so that in an ideal material the drawn down section is the same as the original section but smaller proportionally in each dimension. Because of melt-elasticity effects on the material it does not draw down in a simple proportional manner so that the drawn-down process is the source of errors in the profile. These must be corrected by modifying the die and the forming jigs. 

In most spinning processes, the fiber emerging from the spinneret is drawn down to a desired dimension before complete solidification. In some production lines, a laser beam is uti1i2ed to monitor the dimensions of emerging thread any alteration is recorded and if required, the dope deHvery rate, take-up rate, and internal-injection medium deHvery rate are adjusted automaticaUy. 

Much effort has been expended to try and produce flat-top foams. In one process polyethylene sheets placed along the side-walls of the trough rise with the foam. In another technique the reactants are metered from the mixing head into a fixed trough in which partial expansion takes place. The foaming material is then drawn over a weir by a moving band of paper and then drawn down a slope so that the top surface maintains a constant level as the material expands. 

In a continuous casting machine, molten steel is poured into a reservoir at the top of the machine. It passes at a controlled rate into a water cooled mold where the outer shell of the steel becomes solidified. The steel is drawn down into a series of rolls and water sprays, which ensure that it is both rolled into shape and fully solidified at the same time. At the end of the machine, it is straightened and cut to the required length. Fully formed slabs, blooms, and billets emerge from the end of this continuous process. 

The first type to be introduced consisted of PETN or RDX in a metal sheath drawn down to a small diameter. More recently, cord made by the wet process, covered with plastic instead of metal, has been introduced in North America and is available in charges down to 0-6 g m. The methods of manufacture of these cords are believed to be similar to that of Cordeau and the wet detonating fuse process described above. 

Cables via the powder-in-tube (PIT) method The approach is especially suited to processing Bi-2223 into leads and cables for power applications. A silver or silver alloy tube, filled with the partially reacted precursor powders formulated to yield Bi-2223, is drawn down to a wire 1-2mm diameter. The wire is rolled into a tape if that is the required form, usually with a width-to-thickness ratio of approximately 10 1. The composite is then heated to 800-900 °C when the powder partially melts. The recrystallization process is controlled and the pure Bi-2223 phase develops with large grains oriented so that the Cu-O planes lie parallel to the silver surface to optimize Jc. 

The company claims easy processing results from the high compatibility of the blend components. The formulation consists of more than 10% PLA (purchased from NatureWorks LLC) plus a biodegradable co-polyester and special additives. FKuR says a special combination of compatibilisers permits coupling between the PLA and the co-polyester. The compound is homogeneous, which allows the film to be drawn down to 8 microns. Film up to 110 microns thick is 90% degraded after twelve weeks in composting conditions. 

The subtile Spirit (or Spiritus volatilis) remains in the upper part of the glass during this process, since he cannot endure the fire, and when the steam ceases, he will eventually be drawn down through a magnetic force. 

In ordinary circumstances the system concerned is not a condensed one, but there are also processes going on in solution. If, however, we cool down to — 7°, ice appears as a solid phase, and then the above reaction occurs between pure substances simply. The heat of this reaction is well known and, in addition, all the reactants have been measured to a sufficiently low temperature (that of boiling hydrogen). The heat change is first calculated at T = 234, at which temperature the mercury is solid and the heat therefore changes by 2 x 555 cals, (heat of fusion of mercury). From this point the U-curve can be drawn down to the absolute zero, and the A-curve calculated with the aid of the Heat Theorem. This calculation, for which we have to thank Pollitzer (46), gives for T = 266 an E.M.F. of 1456 volts, which is identical with the observed value. Consequently the potential, which is of course also a measure of the... 

Modem hydro- High-temperature hydrothermal vents currently active at mid-ocean ridges offer a thermal mineral- unique opportunity to study a hydrothermal mineral deposit in the process of ization at formation. The current working model assumes that cold seawater sulphate is mid-ocean ridges drawn down into sea-floor basalts, where it is heated in the vicinity of a magma chamber. Some sulphate is precipitated as anhydrite whilst the remainder is reduced to sulphide by reaction with the basalt. The fluid is vented back onto the seafloor at about 350 C laden with sulphides. On mixing with seawater these are precipitated onto the sea floor as a fine sulphide sediment whilst at the vent site itself the sulphides are built into a chimney a metre or so in height. 

Unfavorable processing conditions will induce residual stresses in the products which impair the biostability of polyurethane-based prostheses. To avoid residual stresses in polyurethane tubes, an upper limit of drawn down ratio of 1.5 1.0 are recommended for the appropriate stretching during extrusion. If water bath for tube processing is too cold, residual stresses are also induced. A recommended temperature for the water bath is between 21-27°C. 

---