The volume ratio between the three phases has a great influence on the wax mold size.
The volume ratio between the three phases has a great influence on the wax mold size. The proportional relationship between the three cannot be controlled in actual production, which is also an important reason why the wax-based molding material is used to suppress the dimensional stability of the wax mold. 2. The influence of the shell material and shell manufacturing process on the dimensional stability of the casting The impact of the shell on the size of the casting is mainly due to the thermal expansion, thermal deformation (high temperature creep) of the shell during roasting and the constraints of the shell on the cooling and shrinking of the casting ( Obstruction) and other factors. (1) The thermal expansion of the shell depends mainly on the shell material. Different refractory materials have different expansion rates. Among the commonly used refractory materials, fused silica has the smallest expansion rate, followed by aluminum silicate, and silica is the largest and uneven.
According to the test, it is determined that the aluminum silicate shell is heated from room temperature to 1000 ℃, the shell can produce about 0.25% expansion, and the proportion of the overall shrinkage rate of the casting size is not large, so if you use such refractory materials, the shell Has better dimensional stability, such as fused silica will undoubtedly be better. However, if silica is used, the shell size fluctuates greatly. (2) Thermal deformation (high temperature creep) For example, the shell with water glass as the binder has a higher temperature creep degree than that of silica sol and ethyl silicate shell above 1000 ℃. Although the fused corundum itself has a high degree of refractoriness, due to the presence of impurities such as sodium oxide, the calcination temperature of the shell above 1000 ° C may worm wheel factory also cause creep, resulting in poor dimensional stability. (3) The constraint of the mold shell on the shrinkage of the casting-the yield and collapse of the mold shell This also mainly depends on the material of the mold shell. In summary, refractory material plays a major role in the influence of mold shell on the fluctuation of casting size, but the role of binder cannot be ignored. In contrast, the impact of the shell making process is small. 3. The effect of stress on dimensional stability caused by uneven casting cooling.
The cooling speed of each part of the casting (including the pouring system) is different, and thermal stress is generated to deform the casting, which affects the dimensional stability. This is often encountered in actual production. Reducing the cooling rate of castings and improving the combination of runners are effective preventive measures. 2. The key to improving accuracy-the correct value of mold shrinkage is different from the above "dimensional stability" and "dimensional accuracy", "accuracy (precision)". Dimensional stability (that is, precision) is synonymous with dimensional consistency, reflecting the degree of dimensional fluctuation or dispersion, usually measured by the standard deviation σ. The reason for the unstable size is mainly due to inadequate process control, which is a random error. Accuracy refers to the degree to which the arithmetic mean of many measured values deviates from the nominal size for a certain size on the casting, that is, the size of the average deviation. For investment casting, the reason for the poor dimensional accuracy is mainly due to the improper assignment of shrinkage during the press design, which is a systematic error and is usually adjusted by repeated repairs to the mold. T
he dimensional accuracy (precision) is a combination of the two. Therefore, to improve the dimensional accuracy of castings and solve the problem of oversize products, not only must the process be strictly controlled to reduce dimensional fluctuations, but also the shrinkage of each size of the casting must be correctly assigned when designing the molding. It is well known that the final total shrinkage rate of precision castings is a combination of wax mold, alloy shrinkage and a small amount of mold shell expansion.
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