According to the formation mechanism of shrinkage cavity and porosity inand based on the numerical simulation of temperature field, researchers at home and abroad put forward various criteria for predicting shrinkage cavity and porosity in sand casting. These criteria are simple and easy to combine with simulation programs, and have been widely used. The commonly used judgment methods include direct simulation method, isosolidus method, critical solid rate method, temperature gradient method and niyama method.
Niyama method is an improvement of temperature gradient method, which was proposed by Professor Yingfu shinyama of Japan. It is an important method to judge shrinkage and porosity in sand casting. Niyama method is mainly based on the ratio of temperature gradient g to the square root of cooling rate R. when the ratio is less than a certain critical value (about 0.8), shrinkage and porosity will occur in sand casting. The smaller the ratio, the more defects will appear. Where, G and R can be calculated by the following formula:
Where: G is the temperature gradient temperature; R is the cooling rate; Tupper is liquidus temperature; Tlower is the solidus temperature.
This method uses two parameters of temperature gradient g and cooling rate r in predicting defects. It not only takes into account the influence of the shape and size of sand casting, but also can accurately evaluate the flow pressure loss. It is widely used in predicting defects in the field of sand casting.
In numerical simulation research, niyama method is used to predict shrinkage and porosity, and this criterion can be set and used in ProCAST.