The Ceramic Shell Process The essential key to ceramic shell lies in the interaction of two basic materials: colloidal silica, and expanded silica flour, with stucco or fused silica grains . The development of this crucial binding ingredient was first developed by the Nalco Company in the early 1930's. At this time Nalco research laboratories discovered that sodium silicate by means of ion exchange produces silica acid. This new product was utilized in the water clarification industry. Nalco was awarded a patent on this and soon found that colloidal silica was a far stronger binder than Sodium Silicate. Being a strong binder and ample refractory for metal casting, colloidal silica when added to fused silica flour and stucco began to be used in the "lost wax" casting method in the late 1960's. It is still used widely today ! Presently there are only a few companies producing this colloidal mixture of amorphous silica and water. One is Ransomand Randolph and the other is Remet. When the wax patterns are properly "gated" They are dipped into a "slurry" mixture of colloidal silica and +- 200 mesh fused silica flour. It is important to use fused silica as all the organic mater has been disapated leaving only pure silica. This slurry mix is held in suspension usually through a rotating barrel or propeller mixer. It is important that the solids of silica be continually stirred as they are heavier than the water content and will sink and separate to the bottom. Once the wax pattern has been evenly coated it is "stuccoed" with fused silica grains of various sizes depending on which dip and number of dips are needed. Once thoroughly dry the "shell" is continually re-dipped until a thickness of 1/4 inch is achieved. This process usually takes up to 8 or 9 dips depending on slurry viscosity, humidity, and slurry life. The
following is a recipe for slurry I use for Bronze and Steel Casting : Be sure to follow all MSDS sheets for proper storage, disposal, and safety hazards, that are part of this process. |