The premise of high quality shell is good wax mold quality. The standard requires that before shell manufacturing, the wax pattern quality shall be confirmed to meet the process quality requirements, and the surface of the wax pattern assembly tree shall be checked to see whether it is clean, and the wax pattern surface shall be free of wax drops and wax chips, and no seams and welds.
The quick-drying silica sol shall be prepared for the coating preparation for rapid drying. Investment casting enterprises can purchase finished quick-drying silica sol directly with refractory materials according to the ratio of powder to liquid; However, if ordinary silica sol is purchased, it is necessary to add quick-drying agent to the silica sol and make it fully wet before adding powder to prepare the coating, so that the macromolecular polarity can be fused; For some solid quick-drying agents such as polyvinyl alcohol, mixing can be achieved by alternately adding “powder-glue-powder-glue” in the glue; When the pH value is controlled, the quick-drying agent can also be dispersed with a small amount of distilled water and warm water, and then added to the silica sol by stirring.
For the configuration of quick-drying silica sol coating, in order to ensure the uniformity of the dispersion of refractory powder and related additives in the coating and improve the efficiency of the configuration process, the standard has agreed on the configuration process and method, as well as the mixing time of different layers of coating in the continuously rotating coating mixing bucket.
For ordinary silica sol, after one-time addition, slowly add quick-drying agent while rotating the mixing barrel. After all quick-drying agents are added, the mixing time shall not be less than 3 min to ensure that there is no flocculent in the glue.
The refractory powder can be added into the mixing bucket in two or three times, and the mixing time shall not be less than 5 min after each addition to ensure that there is no undispersed powder in the paint bucket; Other solid additives are mixed in the powder and added together.
When all the paint components are added into the mixing drum, start to measure the mixing time. The mixing time follows three rules, that is, the mixing time of new coating should be appropriately longer, the mixing time of surface coating should be longer, and the mixing time of powder is longer than that of major coating. The ultimate goal is to ensure that the coating is uniform, and the main parameters such as coating viscosity, coating weight and coating density meet the process requirements.
In order to ensure the stable and uniform mixing of the coating and meet the quality requirements, the coating viscosity, film thickness, coating density and other indicators should be monitored for the surface layer. Generally, the viscosity of the coating should be measured three times with a standard flow cup, with an interval of 1 min each time, and the viscosity of the coating at three points in the coating mixing barrel should be measured; If the viscosity value error is ± 2 s, it can be judged that the mixing is uniform.
The standard emphasizes the requirements for the shelf life of the coating. If the coating has not been used up for more than 7 days, the viscosity, film thickness, density and other indicators of the coating should be re-determined before re-use to determine whether they meet the process requirements. At the same time, the gelling index should also be tested. If the gelling index exceeds 20%, it should not be used again. The service cycle of quick-drying silica sol coating is shorter than that of ordinary silica sol coating, so the prepared coating should be used up as soon as possible.