MOLD CORE FOR INVESTMENT CASTING

Abstract: A core mold to be assembled with a shell mold for use in an investment casting process which comprises a core matrix (10) consisting of an aggregate and an inorganic binder, a binder layer (12) impregnated from the surface of the core matrix (10), a coating layer (14) formed by coating a slurry over the binder layer (12) and a paraffin wax layer (16) cover­ ing the exterior periphery of the coating layer.

Description: The present invention relates to a mold core used in an investment casting process and a process for preparing such a mold core, and further pertains to a process for preparing a mold for an investment molding process assembled with such a mold core. A ceramic mold core used or assembled within a mold for an investment casting process should have a sufficiently smooth surface, a high strength enough for withstanding the injection molding of a wax model and a sufficient strength at high temperature for retaining its integrity under high temperature environment during the sintering and/or casting steps. Prior art cores conventionally used for such purposes are molded from aggregates, such as those containing alumina, zirconium or fused silica, and then the thus molded cores are burned or sintered singly. However, such a process is low in producibility or operation efficiency, in addition to the problem that the dimensional accuracy of the finished core is inferior, particularly in preparation of a large-size core, with extreme difficulty for obtaining a large-size core of accurate dimensions as well as increase in production cost. Further disadvantages of conventional sintered core molds are that they are hardly demolished after use, and that they cannot be removed from by the application of physical vibration or impact. Thus, cumbersome and inefficient operations are required for the removal of such cores. In addition, in the production of such core molds which should be sintered for acquiring necessary strength and integrity, some of inexpensive aggregates, such as silinceous sand, cannot be used as the starting materials therefor because of the difficulty encountered in sintering them. An object of this invention is to provide a mold core having a smooth surface suited for molding a wax model and having a thermal strength enough for withstanding high temperature operation during the step of molding the wax model. According to the present invention a mold core comprising a core matrix is characterized in that it consists of an aggregate and an inorganic binder, a binder layer impregnated from the surface of the core matrix, a coating layer formed by coating a slurry over the binder layer and a paraffin wax layer covering the exterior periphery of the coating layer. Also according to the invention a process for preparing a mold core is characterized in that it comprises the steps of kneading an agregate with an inorganic binder, casting the kneaded aggregate and inorganic binder into a core molding mold to be solidified therein to produce a core matrix, dipping the solidified core matrix in a binder bath so that the core matrix is impregnated with the binder from the surface thereof, coating the core matrix impregnated with the binder with a slurry followed by drying to form a coating layer and covering the coating layer with paraffin wax. A process characterized in that the aggregate used in the kneading step is substantially composed of silica sand and the inorganic binder is substantially composed of sodium silicate. A process characterized in that the aggregate used in the kneading step contains silica sand and silica flour. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a flow diagram showing the process for preparing a mold core according to this invention; and Fig. 2(A) to 2(G) are illustrations showing the steps of preparing a mold core of this invention and the steps of investment casting process wherein the thus prepared mold core is used. An embodiment of this invention will be described with reference to Figs. 1 and 2 showing the steps of the process for preparing the mold core of this invention. At the first step, an aggregate and an inorganic binder are kneaded together. One example of the aggregate which may be used in this invention has the following composition of: Siliceous Sand 75-100 wt% Silica Flour 0-25 wt% Preferably 90 wt% of siliceous sand and 10 wt% of silica flour may be used. Preferable siliceous sand used in the composition has a particle size corresponding to No.7 grade stipulated in JIS G-5901 (1954). An example of preferable inorganic binder is JIS No.3 sodium silicate (water glass), which is added little by little to the main ingredient, i.e. the siliceous sand, in an amount of about 5 to 15 wt%, preferably about 8 to 10 wt%, based on the total weight of the aggregate, followed by kneading (Step 100). Preferably, kneading is effected at a room temperature of about 20 o C and at a relative humidity of about 55% for about 20 minutes, and immediately after the completion of kneading operation the container is sealed to prevent the kneaded mass from being hardened due to the reaction of sodium silicate with carbon dioxide in the atmosphere. The kneaded aggregate mixture is fed in a mold (not shown) for shaping a mold core so that a core matrix 10 (see Fig. 2(A)) is prepared. In this step, hot air (at about 140 o to 150 o C) is blown into the mold to facilitate solidification of the core matrix 10. Otherwise, the core matrix 10 may be solidified through the CO₂ process wherein a core matrix is molded using a wooden mold heated to 60 o to 80 o C and then carbon dioxide gas is blown through the blow holes or the slits at the splitting surfaces of the mold to solidify the core matrix contained in the wooden mold. Due to the binding force of the hardened inorganic binder, the thus prepared core matrix has a strength and integrity for retaining its shape and dimensions during the later wax model injection molding step. The next step is the step of dipping the core matrix 10 into a bath containing a binder so that the surface of the core matrix 10 is covered with the layer 12 impregnated with the binder (Step 104 in Fig. 1; Fig. 2(B)). Examples of preferable binder used in this step are ethyl silicate and colloidal silica. Such a binder impregnates from the surface of the core matrix 10 to a proper depth for increasing the strength of the core matrix at a high temperature environment. The solidified aggregate added with sodium silicate and then solidified at the preceding steps 100 and 102 has a sufficient strength at a temperature of up to about 200 o C, but the strength of the aggregate bonded by the hardened sodium silicate is abruptly lowered as the temperature is raised above 200 o C. The core matrix impregnated with the binder at the step 104 has a strength enough for retaining its integrity within the temperature range of from 200 o to 1000 o C. The core matrix impregnated with the binder is coated with a slurry (Step 106; Fig. 2(C)) which is desirously containing a binder and a filler. An example of the slurry used in this step 106 has the following composition of: Ethyl silicate (Binder) 50 wt% Zircon Flour No. 350 (Filler) 50 wt% The slurry may be coated by the dipping process wherein the core matrix 10 is dipped into a slurry container, or by the spraying method wherein the slurry is sprayed on to the surface of the core matrix, or by the electrostatic coating method wherein an electrostatic potential is applied between the core matrix 10 and a sprayer nozzle to deposite the slurry mists on to the surface of the core matrix 10. For instance, when the slurry is coated by the dipping process, the core matrix 10 is dipped in the slurry container for about 60 seconds. Prior to coating with the slurry at the step 106, the core matrix 10 impregnated with the binder to form the layer 12 may be dried. A coating layer 14 is thus formed by coating the slurry over the surface of the binder containing layer 12. The surface condition of the core matrix 10 is improved by the provision of the coating layer 14 to have a smooth surface. The mold reaction between the mold and the molten metal at the casting step is also improved by the provision of such a coating layer 14, with a further advantage that the high temperature strength of the mold core is further increased. After being coated with the slurry the mold core matrix is then dried, for example, at a temperature of 28 o C and at a relative humidity of 50% by air flowing at a rate of 1 m/sec for about 3 hours. A large size core may be additionally dried by microwave heating for about 10 minutes. The dried core matrix 10 is then coated with paraffin wax (Step 108; Fig. 2(D)). The core matrix 10 coated with the coating layer 14 is dipped in a molten paraffin wax maintained at 80 o to 90 o C for about 10 minutes to form a wax layer 16 over the surface of the coating layer 14 so that the crumbling or fall-off of the coating layer 14 is prevented. The wax layer 16 also serves to increase the strength of the core to prevent breakdown thereof during the transportation operation and to prevent the core from absorbing moisture during the storage time. The finished mold core 10A shown in Fig. 2(D) is prepared through the aforementioned steps of impregnating the core matrix 10 with the binder to form a binder containing layer 12, and then forming successively the coating layer 14 and the wax layer 16 over the exterior surface of the layer 12. The mold core 10A is fixed in position by any conventional means within a shell mold 18. A material for forming a lost model, such as a wax or foamed polystyrene, is injected into the cavity defined by the core 10A and the shell mold 18, whereby a lost model 20 is molded (Step 110; Fig. 2(E)). The lost model 20 is then removed from the shell mold 18 and a refractory material is then coated over the periphery of the lost model 20 by repeating for plural times the operation cycle each including the step of dipping the lost model in a slurry container (Step 112) and the step of applying with stucco particles (Step 114), whereby a refractory material layer 22 having a desired thickness is formed (Fig. 2(F)). After drying sufficiently the refractory material layer 22 (Step 116), the lost wax model 20 is allowed to vanish by dewaxing (Step 118), and then the refractory material layer 22 is baked (Step 120). During this dewaxing step, the wax layer 16 of the core 10A is also removed, whereupon the coating layer 14 is exposed over the surface of the core 10A. At the baking step (Step 120), the core 10A deprived of the wax layer 16 is also baked simultaneously with the baking of the refractory material layer 22 of the shell mold. As a result of the aforementioned sequential operations, a ceramic shell mold 24 containing therein the core matrix 10 having a layer 12 impregnated with the binder and being covered with the coating layer 14 is produced (see Fig. 2(F)). A molten metal is cast in the cavity of the ceramic shell mold 24, i.e. the cavity defined by the interior wall of the refractory material layer 22 of the shell mold 24 and the exterior surface of the coating layer 14 of the mold core 10A (Step 122). After cooling, the outside shell mold is removed (Step 124) and then the core matrix 10 and the coating layer 14 are removed (Step 126). The core matrix 10 and the coating layer 14 are removed by the step of removing the major portion of the core by means of physical vibration or impact, and the subsequent step of immersing the cast metal in a caustic soda solution or hot melt caustic soda to dissolve the remaining portions of the core matrix and the coating layer. A final cast product 26 is thus produced as shown in Fig. 2(G). An important advantage of the process of the invention is that the core matrix may be readily demolished to be removed easily at the step 126, since the depth of the layer 12 impregnated with the binder is spontaneously controlled to an appropriate degree so that the central portion of the core matrix 10 is not impregnated with the binder. Although the core matrix 10 is applied with the binder and the slurry by the separate steps 104 and 106, respectively for impregnating with the binder (Step 104) and for coating with the slurry (Step 106) in the aforementioned embodiment, the steps 104 and 106 may be combined to treat the core matrix 10 at a single step. This may be done by using a slurry containing the same binder as used in the step 104 and by increasing the time for dipping the core matrix in the slurry container to allow the binder to be impregnated into the core matrix to a desired depth. Although the present invention has been described by referring to an embodiment wherein the mold core prepared by the invention is combined with a ceramic shell mold, it should be apparent to those skilled in the art that the mold core of the invention may also be conveniently used in other investment casting process, such as solid mold process. The aggregate and the binder which may be used in the present invention should not be limited only to the materials specifically referred to in the aforementioned embodiment. For example, siliceous sand used as the aggregate may be replaced in part or entirely by alumina, fused silica, zircon or fused mullite. Phosphate cement may be used as the inorganic binder added to and kneaded with the aggregate. The mold core provided by the present invention has a strength for withstanding the injection molding operation for molding a wax model, and also has a sufficient strength at high temperature environments during the mold baking step and the molten metal casting step without the need of sintering the same prior to combination with the outside shell mold. Due to exclusion of the step of sintering the mold core, the total process can be simplified to improve the production efficiency and lower the cost, with an additional merit that the dimensions of the mold core may be more easily controlled. It is also possible to prepare a mold core made of materials same as those used in the outside shell mold so that the core mold has an essentially same thermal expansion coefficient as that of the shell mold to control the dimensions of the finished cast product accurately. This is particularly convenient when a large-scale cast product is produced. According to another important feature of this invention, impregnation of the binder into the core matrix is limited to an appropriate depth so that the mold core can be readily demolished or collapsed and thus easily removed after use. The coating layer serves to smooth the rough surface of the shaped core matrix and to suppress the mold reaction taking place between the molten metal and the mold core at the later casting step to prevent formation of rough surface of the cast product. The strength of the mold core at the high temperature environments during the baking step and the casting step is further increased by the provision of the coating layer, so that the yield rate of the total casting process is improved. The wax layer serves to prevent fall-off of the coating layer and to increase the strength of the mold core so that breakdown of the core during the transportation is prevented, and also serves to prevent the mold core from absorbing moisture during the storage time.

Claims: A mold core comprising a core matrix (10) characterized in that it consists of an aggregate and an inorganic binder, a binder layer (12) impregnated from the surface of the core matrix (10), a coating layer (14) formed by coating a slurry over the binder layer and a paraffin wax layer (16) covering the exterior periphery of the coating layer. A process for preparing a mold core characterized in that it comprises the steps of kneading an aggregate with an inorganic binder, casting the kneaded aggregate and inorganic binder into a core molding mold to be solidified therein to produce a core matrix, dipping the solidified core matrix in a binder bath so that the core matrix is impregnated with the binder from the surface thereof, coating the core matrix impregnated with the binder with a slurry followed by drying to form a coating layer and covering the coating layer with paraffin wax. A process according to claim 2, characterized in that the aggregate used in the kneading step is substantially composed of silica sand and the inorganic binder is substantially composed of sodium silicate. A process according to claim 2, characterized in that the aggregate used in the kneading step contains silica sand and silica flour. A process according to claim 2, characterized in that the binder used in the dipping step contains ethyl silicate or colloidal silica. A process according to claim 2, characterized in that the slurry used in the coating step comprises a binder containing ethyl silicate, zircon flour and a filler. A process according to claim 2, characterized in that the coating step of the slurry is carried electrostatically. A process according to claim 2, characterized in that the core matrix is dipped in a slurry bath so as to be coated with the slurry. A process according to claim 2, characterized in that the slurry is sprayed on to the core matrix in the coating step so that the core matrix is coated therewith. A process for preparing an investment casting mold having a mold core prepared by the process according to claim 2, characterized in that the core mold is placed in position within a shell mold and a lost model forming material is then poured into the shell mold to produce a lost model having therewithin the core mold, coating a slurry and stucco particles alternately for plural times to form a refractory layer which is then dried, allowing the lost model to vanish so as to obtain a final mold and baking the core mold and the refractory layer simultaneously.

Inventors: SASAKI, NOBUYOSHI
Applicants: SASAKI, NOBUYOSHI
Application year: 1987
Main classification: B22C9
IPC codes: B22C9, B22C1