Switchover method

However, the switchover method differs quite fundamentally. In machine-dependent switchover methods, for example depending on the screw position, it is an open control loop, which cannot react to process variations. If, for example, the viscosity of the melt changes during production, the machine always switches at the same screw position (i.e. at different process conditions). The quality of the molded parts must inevitably vary. 

The manufacturing method of making phthalic anhydride has been changing rapidly similar to the switchover in making maleic anhydride. In 1983 28% of phthalic anhydride came from naphthalene, 72% from o-... 

At the present moment, a switchover is taking place in the mode of operating the database access environment from the old C/S (client server) system to a method employing the concept of a distributed database such as the World Wide Web using the Internet. A distributed database is being constructed hat uses data and monogr hs in the... 

The proper method to control drum foamovers during a rapid switchover is to control coke drum pressure by pinching back on the outlet vapor valve to maintain the pressure in the full coke drum. While the operator at the switch valve is diverting the feed to the empty drum, he observes the full coke drum pressure. As this pressure falls during the switch, he closes off the vapor outlet valve from the full drum to hold the pressure within 5 psig of normal operating pressure. Using this technique, the switchover can be accomplished in about 20 minutes. 

Modern controis of an injection molding machine switch from a speed-controlled injection phase to a pressure-controlled holding pressure phase. For this purpose, various methods are available, which differ mainly in terms of their accuracy. For all methods, however, the goal is to switch over the target when the cavity is volu-metrically filled. If the holding pressure switchover is done too early, a momentary drop in pressure usually occurs, and the cavity is filled under holding pressure in an undefined way. If the holding pressure switchover is done too late, the melt is still compressed in the injection phase, which results in pressure peaks, overloads, or so-called residual pressure. Both phenomena usually prevent the production of an optimum molded part. 

Quite similar is the switchover to holding pressure above a pressure threshold value with using a cavity pressure sensor. Once the optimal change is determined, it cannot react to viscosity variations. Both methods have the disadvantage in that the optimal switch over point is determined with difficulty, first using of a mold-filling study, and the switch over point then changes in the course of production. 

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