A high frequency preheater using dielectric heating is used to raise the resin temperature. Dielectric heating is a phenomenon in which dielectric material consisting of dielectric dipoles undergo rotational motion when a high-frequency electric field is applied to this dielectric material and internal heat is generated by mutual friction.
The electrode structure of the high frequency preheater consists of parallel-plate electrodes and a roller electrodes. The roller electrodes rotate the tablets and heat them, so they are suitable for uniform preheating. Furthermore, by changing the pitch of the roller electrodes, the temperature distribution can be controlled.
In this example, evaluation of the influence of the pitch of the roller electrodes on the material (tablet) temperature distribution are shown.
Roller Electrode – Tablet Positional Relationship
The pitch of the roller electrodes is set to 90 deg and 40 deg. In addition, the distance between the roller electrodes and the tablets is fixed.
Dielectric Loss Density
Fig. 2 shows the dielectric loss density distribution and Fig. 3 shows the electric field intensity distribution.
From Fig. 2, it can be seen that when the pitch of the roller electrodes is 90 deg the loss near the circumferential edges of the tablets is high, and when the pitch is 40 deg the loss near the radial centers of the tablets is high.
From Fig. 3, it can be seen that from reducing the pitch of the roller electrodes the electric field inside the tablets concentrates toward the radial centers. Dielectric loss is affected by physical properties, frequency, and electric field. In this example, since physical properties and frequency are both factors, the differences in the electric field appears as differences in dielectric loss.
The temperature distribution during heating is shown in Fig. 4 to Fig. 6, and the temperature change between the radial centers and the circumferential edges of the tablets is shown in Fig. 7 and Fig. 8.
Due to the differences in pitch of the roller electrodes, a difference also is exhibited among the temperature distributions. It can be seen that when the pitch is 90 deg heating is more prominent towards the circumferential edges of the tablets, but when the pitch is 40 deg the temperature difference between the radial centers and near the circumferential edges of the tablets is small during heating.
In this way, it is possible to control the temperature distribution of the tablets by changing the pitch of the roller electrodes.