| 摘要 |
Coated sand grains are obtained by heating 100 parts by weight of sand to a temperature of 90 DEG to 150 DEG C., admixing with the sand a liquid adhesive mixture comprising 1 to 5 parts by weight of a liquid urea-formaldehyde concentrate having 80 to 90% by weight of total solids and a mol. ratio of 4.0 to 6.5 mols. of formaldehyde for every mol. of urea, sufficient urea to adjust the mol. ratio to 1.5 to 2.0 mols. of formaldehyde for every mol. of urea and 1 to 5 parts by weight of an acid catalysed novolak, and rapidly cooling the mass with a blast of air until it breaks down to a free-flowing solid. An example is given. ALSO: A liquid adhesive mixture comprises (a) 1 to 5 parts by weight of a liquid urea-formaldehyde concentrate having 80 to 90% by weight total solids and a mol ratio of 4 to 6.5 mols of formaldehyde for each mol of urea; (b) sufficient urea to adjust the mol ratio to 1.5 to 2.0 mols of formaldehyde for every mol of urea; and (c) 1 to 5 parts by weight of an acid catalyzed novolak. The mixture, admixed with sufficient sand to serve as a binder may be used as a shell moulding composition, preferably in the presence of a lubricant e.g. the magnesium, calcium or zinc soaps of stearic, palmitic or oleic acids. An example is given. |
