Discontinuously operated melting

The discontinuously operated melting unit (see Fig. 4.13) consists of the following components melting tank (melting volumes up to 28 m ), insertion system, arch heating, electrical bottom heating, feeder, cooling system, waste gas disposal, and instrumentation and control with process computer. 

The details of the development of the EBRD process have been described by Pietsch et al.[187] There are two alternative operation modes in addition to the above continuous non-contact mode. The first one may be referred to as continuous contact atomization. In this mode, liquid metal contacts the bottom surface of the container instead of melt dripping, and then flows continuously from the center to the rim of the container. The second one may be termed discontinuous non-contact atomization. In this mode, the container is first filled by dripping melt while it is rotating at a very low speed of about 3 x 10-3 radians/s. The rotating speed of the container is then enhanced to about 14 radians/s while the metal or alloy is remelted and atomized. More than one focused electron beam may be used to provide energy for melting metal. 

Because of the low operating temperature, PTFE can be used for the cell seals which simplifies cell assembly. The high operating voltage, however, leads to problems of corrosion and the only current collector material which has been found to show any corrosion resistance is tungsten. If sulphur is not added to the melt, a nickel current collector can be used, but omission of sulphur adversely affects the operation of the positive electrode. The problems encountered with the positive electrode were so severe that the development of this cell was discontinued. 

The PGSS can be operated in continuous mode, while antisolvent techniques are discontinuous or semicontinuous. In the particular case of polymers, PGSS allow operation at significantly reduced temperature. Indeed, polymers are melted or swollen at a temperature significantly lower than their melting or glass transition point (see this volume, Part 2, Chapter 10 by Kazarian). 

Alternatively, it is possible to install fibre optic probes directly in the main stream in-line while the IR spectrophotometer remains remotely in a low vibration laboratory environment. In-line analysers, which do not remove any sample from the line, have the minimum possible lag time and do not change the sample physically or chemically from its nature in the process. Recently, bundles of 500 /xm optic fibres have been developed for the 5000-900 cm (2000-11,000 nm region), which permit transmission of IR energy over distances of several metres. Lowry et al. [76] have evaluated fibre-optic cables that might prove useful in FTIR remote sampling applications. The various optical fibres (chalcogenide, silver halide, heavy metal fluoride or sapphire) differ in their spectral window [77]. Due to the thermal stability and the spectral window, sapphire fibres are considered suitable for in-line characterisation of polymer melts in a production line (e.g. in an extruder head) as an alternative to discontinu-ously operating conventional off-line transmission IR spectroscopy of polymer films [78]. 

A soft mode is a lattice mode whose frequency becomes imaginary at some value of the density (or external pressure). See e.g. Dove, M. T. Rae, A. I. M. Structural phase transitions in malononitrile, Faraday Disc. 1980, 69, 98-106. The whole discussion, entitled Phase transitions in molecular solids , is extremely instructive reading for the theory, experiment, and simulation of second-order phase transitions, operatively defined as those transitions that occur without a major discontinuity in enthalpy and heat capacity the onset of molecular rotation is an example, as opposed to first-order transitions like polymorphic transitions or melting. 

Since a plasticiser screw normally operates discontinuously and for economy its cycle must match the time the mould takes to cool, attempts are frequently made to increase the melting efficiency of the unit by raising the temperature of the extruder cylinder. When this is the case, the temperature of the injection cylinder should be maintained below that of the plasticising cylinder so that the melt forced into the injection cylinder will not degrade during the stationary period. 

For some other applications hard TPU needs to be extruded discontinuously, e.g. for the production of cormgated tubes. The extruder is operated in a stop and go rhythm resulting in relative long residence times of the TPU melt in the extruder. It is well known that hard TPU tend to crystallize in the melt and this usually causes irregularities and surface defects in these processes. Because of their reduced crystallization tendency the use of the new E 1200 materials for these discontinuous processes is appropriate. 

---