埋没材の膨張におよぼす練和条件の影響 : 手練和と機械練和の相異

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It is well known that the setting expansion of the gypsum bonded investment is affected by many factors, namely it increases with smaller water-powder ratio, higher spatulation rate and longer spatulation time within certain limits. But those limits are not quite distinct, and moreover recently the mechanical mixing has widely prevailed ; thus it will be necessary for us to reconsider about the precise setting mechanism. The authors studied the effects of water-powder ratio, mixing method and their interactions on the setting expansion of the investment using hand mixing and mechanical mixing with much higher spatulation rate than that of hand spatulation. The results obtained are as follows : 1. For hand mixing for 60 sec and mechanical mixing for 15 sec and 30 sec, the setting expansion of the investment increased as water-powder ratio decreased from 0.40 to 0.32, because smaller water-powder ratios resulted in larger density of the investment. 2. For mechanical mixing for 60 sec, the setting expansion increased as water-powder retios increased, because smaller water-powder ratios resulted in very rapid set inhibiting the setting expansion. 3. For smaller water-powder ratios, larger differences in the setting expansion were found among mixing methods, and not so much as for larger water-powder ratios. 4. The percolated water was detected on the investment slurry by some difinite mixings. It may be considered that this water was imbibed into the investment at the initial set and probably reacted with non-reacted particles, causing hygroscopic expansion under a favourable condition, i.e., sufficient spatulation, slow set and abundantly percolated water. 5. For this reason, hand mixing caused higher setting expansion than mechanical mixing for water-powder ratios of 0.36 and 0.40. 6. For water-powder ratios of 0.32 and 0.36, mechanical mixing for 60 sec seemed excess, causing lower expansion than the other poor mixings.
九州歯科学会の論文
1972-09-30

埋没材の膨張におよぼす練和条件の影響 : 手練和と機械練和の相異

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