Position of the melt

For the constant channel depth region in the solids section, we compute with the initial position of the melt film of 1.26 m, we can write... 

An interesting variable in all this is the position of the melting point in the sequence (here, the term melting point indicates the temperature at which the solid melts to form a disordered phase, while clearing point indicates the temperature at which the isotropic hquid forms from a mesophase), for there is no real reason for the sohd to be less stable then the mesophase(s). Thus, Fig. 31 shows three possible examples of situations that could occur. In the flrst (A), the sample melts to the mesophase (LC), which then clears to give the isotropic liquid, and on cooling, the whole things reverses. This mesophase is termed enantiotropic. In the second example (B), the sohd melts directly to the isotropic liquid and then supercools into the mesophase (LC), which then crystallizes, here, the mesophase is termed monotropic. In the... 

Fig. 5.41. A simple arrangement by which gravitational flow is avoided. The displacement of the electrolyte from the cattiode to the anode region occurs at one level. The change in position of the melt in the capillary indicates the amount of electrolyte displaced.

The above values are not sufficiently reliable for an accurate determination of the melting-point, as already explained nor do they extend over a wide enough interval of temperature. They are sufficient, however, to give a rough picture of the A- and of the U-curves, and a graphical representation will at once show that the position of the melting-point may be derived, at least approximately, from the heats of fusion and the above values of specific heat. 

This equation is used alternately with the flow equation, to update the melt temperature distribution. If the injection pressure is below the limit of the machine, the new position of the melt front is computed. The programmes output the melt front shape and melt pressure isobars at various times, hence predict whether a mould can be filled satisfactorily. If not, modifications can be made to the CAD file for the mould geometry, and the analysis repeated, before the mould cavity is machined. Figure 5.27 shows the predicted flow fronts for an instrument panel moulding. 

The position of the melting devices, from the least to the most expensive, is not the same depending on the type of alloy and the authors ... 

Figures 8-18 and 8-20 compare predicted (theoretical) and experimental data for the filling of a 180° disk cavity with edge flow. In Fig. 8-19, theoretically determined values of positions of the melt front show the same relative behavior with time as the experimental values (transducer and photographic). However, as can be seen, the theoretical values are considerably larger. Pressure changes with time and flow rate are given as functions of time in Fig. 8-20. The general theoretical prediction of pressure behavior qualitatively matches the trend of the experimental results but once more doesn t agree completely in a quantitative sense.

It can be shown that the change in striation thickness on mixing, and hence in the degree of laminar mixing, is a simple function of the total shear strain imposed on the system. However, at the end of the process the components of the mixture still exist as discrete components. The total shear strain exerted on the melt is a function of the residence time of the melt in the process. As a result of the complex velocity profile of the melt in the screw channel, the residence time of the melt in the channel varies as a function of the position of the melt in the screw channel as well as the down-channel velocity of the melt. 

A cavity temperature signal, however, is precisely measured when the melt reaches the sensor. In this way, the position of the melt is always known and can be used for control purposes as automatic switch over to holding pressure. In contrast to the cavity pressure sensor, the cavity temperature sensor can be placed where it is needed. 

Especially cavity temperature sensors are increasingly used to control the injection molding process. Here, the arrival of the melt front on the sensor is detected in real time and used for switch over operations in real time. In contrast to the cavity pressure measurement, the position of the melt is always known this way and can he optimized with the help of programmable delay times. This allows moving weld lines in a certain direction, and the meeting of the melt (e.g., in sequential molding) can he optimized [8]. 

In classical sequential molding, the opening and closing of various nozzles is usually path- or time-controlled. In this method, the position of the melt is unknown, which is why an optimization of weld lines or the melt flow is practically impossible. 

Moving cores can be selectively controlled using this technique so that they automatically open or close, depending on the position of the melt. This is for example used in automatic venting of cavities by only closing a moving core when the cavity is almost completely filled. 

An alternative approach to monitoring with cavity sensors is to use temperature sensors located in the cavity to monitor the location of the melt front relative to time. In this case, the control loop that manages the transfer point is based on position of the melt front [8]. 

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