A moulding compsn. for producing casting moulds and cores consists, by wt., of (a) 10-35% soln. of hydrolysed Et orthosilicate, as binder, (b) 0.2-2% organic soln. of a polyorganosiazane, (I), as hardener bringing about the gelling of the Et orthosilicate, and (b) balance to 100%, filler. (I) has formula left brace (R2SiNH)n R'Si(NH)1.5 m right brace k (where R is Me; R' is Me or Ph; n = 1, k is 4 to 10 and m is 1 to 3). The moulds and cores can be used in precision casting. The use of (I) causes 2 consecutive reactions in moulding compsn. : (i) hydrolysis of (I) with formation of siloxane bonds, using up "ballast" water of filler, (ii) promotion of Et orthosilicate-gelling by the NH3 released. (I) is less toxic than previously used triethanolamine. The flexural strength of the hardened moulding compsn. may be 20-25 kp/cm2, i.e. 1.5-1.75 x higher than on using triethanolamine. (I) is evenly distributed and the moulding compsn. hardens evenly, without risk of cracks. (I) is introduced esp. a 20-40% soln. in an organic solvent. A pref. compsn. contains 15-25% binder soln., 0.5-1% of a 20-40% soln. of (I), balance filler. Describes the prepn. of a moulding compsn. for cores used in casting steel to form centrifugal pump wheels. The compsn. contained 30% quartz and having particle size 0.08-4.4 mm, 45% quartz dust having particle size 0.01-0.06 mm, 24.25% soln. of hydrolysed Et orthosilicate and 0.75% of a 30% soln. of (I) in toluene. After hardening 5 min., the core was heated to 900-1100 degrees C in 1-2 hrs. at 100-150 degrees C/hr and achieved flexural strength 30-80 kp/cm2.
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