GB789721A - Improvements relating to casting processes for metals - Google Patents

Abstract

789,721. Making moulds for casting metals. ROLLS-ROYCE, Ltd. March 24, 1955 [March 9, 1954], No. 6913/54. Class 87(2). [Also in Group XXII] An expendable pattern comprising an insert or core 11, Fig. 4 (Provisional) or refractory material and a coating 16 of wax, polystyrene or frozen mercury (which pattern is to be invested in refractory material R, Fig. 5, the wax &c. part of the pattern being then removed from R to provide a mould for casting a hollow article, e.g., a hollow turbine blade) is made by a threestage process. First, Fig. 1, the refractory core is made by casting, e.g., a gellable sillimanite composition in a two-part mould A, B, the core being strengthened by,rods 10 of silica, alumina, mullite or porcelain : the rods may be hollow. The sillimanite composition (see below) may contain alcohol, in which case the core may be removed from the mould and the alcohol burnt off before the next stage. Second, Fig. 2, the mould part A is replaced by a mould part C which leaves a space 12 into which wax &c. is injected to coat one face of the core. Third, Fig. 3, the mould B is replaced by a mould part D to leave a space 14 into which wax &c. is injected to coat the other face of the core. The resultant pattern is mounted on a base plate 17, Fig. 5, of an investment container 19 by means of a wax support 18 which forms a sprue passage for the entry of metal into the refractory mould when it is melted out, the refractory material R is cast in the container 19 to form the mould, and the wax coating 16 and wax support 17 are melted out, the refractory core 11 being left embedded in the refractory mould to form the hollow turbine blade &c. When the blade has been cast the core 11 is removed therefrom as by sand-blasting. The material R may be refractory material bound with sodium silicate or hydrolysed ethyl silicate, or concrete. Modifications and details. (1) The completed turbine blade may have not one through passage, as represented by 11, Fig. 5, but three through passages (for the flow of cooling air) ; or as shown in Fig. 7, three passages 21 may terminate short of one end of the blade, being interconnected by short passages 21A. When the passages are through passages the cores may be simple refractory rods. When the passages are as shown in Fig., 7, a trident-shaped core may be used. The cores may be cast in a mould as in Fig. 1, or the rods may be preformed and supported in a mould as in Fig. 4 (Complete), the rods being shown at 10a. (2) The mould of Figs. 2 and 3 may have three or more parts. (3) Prior to investment in the mould material R, the pattern may be coated with finely-divided refractory suspended in a liquid binder and then with dry refractory powder. (4) The gellable sillimanite composition for making the core 11 may comprise 140 gr. of sillimanite of specified grain sizes, 55 c.c. of hydrolysed ethyl silicate solution (containing alcohol and hydrochloric acid) and 2À5 c.c. of ammonia solution. According to the Provisional Specification the two lots of wax &c. which coat the refractory core are injected through passages 13A, 15A, Fig. 6, formed entirely in the mould halves C<SP>1</SP>, D<SP>1</SP> ; this Fig. 6 shows cores arranged to produce passages as in Fig. 7. Specification 789,722 is referred to.