If the difference of moisture content between inside and outside of the blank is large after the blank is out of the mould, the magnet will foam simply. This is because the initial surface water transpiration rate of the blank is faster than the internal water dispersion rate. With the passage of time, the external water content of the blank decreases and the difference between the internal and external water content of the blank increases. If there is residual closed water in the base of the blank, it will bubbles at high temperature. If the difference of moisture content between inside and outside of the billet is gradually reduced, the foaming will also be gradually reduced.
(2) In the second fine grinding, the larger the particle size, the simpler the internal and external capillary water dispersion, and the smaller the foaming rate of the magnet.
(3) When sintering, no foaming occurs below a certain sintering temperature, which is called the critical temperature of foaming. Beyond this temperature, the foaming rate will gradually improve with the progress of sintering temperature. With the advance of pre-firing temperature, the critical temperature of foaming also improves.
(4) Another reason for bubbling is that after the formation of Fe2O3 by mixing iron due to crushing and grinding in the crushing technology, the pre-sintered material will be differentiated according to the reaction of 6Fe203 = 4Fe304 + O2 2Fe2O3 = 4FeO + O2 during sintering. When this kind of Fe2O3 is drastically reduced to FeO, the differentiated oxygen, if not discharged, will also make the billet bubbling.
The magnets of sintering furnace with good ventilation are also prone to foam.
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