Melt flow temperature control

Adequate PC and its associated instrumentation are essential for product quality control. The goal in some cases is precise adherence to a single control point. In other cases, maintaining the temperature within a comparatively small range is all that is necessary. For effortless controller tuning and the lowest initial cost, the processor should select the simplest controller (of temperature, time, pressure, melt-flow, rate, etc.) that will produce the desired results. 

Iodine handling appeared to require extreme care to avoid condensation and clogging. In particular, it proved difficult to consistently and routinely melt and flow the pure iodine which is fed to the Bunsen section. Special care must therefore be given to temperature control in order to make sure all points remain above iodine melting point, including during start-up and shutdown transients. 

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...

For continuous melting of domestic and technical glass, small electric furnaces have been developed a longitudinal cross section of one such type is shown in Fig. 102. At the working end, the temperature is controlled by resistance heating elements. The direction of melt flow is shown by the arrow. All-electric furnaces have to be heated up by auxiliary gas burners since the cold batch does not conduct electrically. 

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