Induction pre-heating of tube
When manufacturing a television or computer monitor, it is essential that the cathode ray tube be correctly mounted. To achieve this, a so-called ‘shrinking frame’ is attached to the cathode tube and shrunk to fit its outer dimensions.
Once properly positioned, this frame allows the correct mounting of the tube within the monitor. Muffing material is inserted between the frame and the tube to protect the latter from cracks and other damage. The frame itself is a closed construction made from normal magnetic iron. Zinc-plated versions are also available. To obtain the required shrinking, it is first necessary to heat the frame to at least 450–500°C. The heat source for this operation has traditionally been open gas flames. However, the use of gas has major drawbacks: heat distribution is uneven, and it is not possible to dismount an already shrunk frame from the monitor. But there is a better way, one that removes the disadvantages of gas—induction heating systems. The power rating of the induction system to be used depends on the size of the frame and the monitor. But normally, a rating of 5-20 kW is needed. A low frequency system such as the EFD Induction Minac 18/25 is necessary in order to achieve uniform temperature distribution. When using induction heating for this particular application, it is crucial that the coupling distance on all four sides of the frame be identical. Following successful pre-heating, the tube is automatically immersed and the frame cooled with com-pressed air. The heating time is 5–25 seconds, depending on the size of the frame and the installed power of the induction system. Induction heating can also be used for disassembling cathode ray tubes. Moreover, disassembly with induction heating leaves the frames undamaged, meaning they can be re-used. Traditionally, the frames of faulty tubes were mechanically detached and scrapped—a costly problem that induction heating has now solved.