An effective way to improve the shelling performance of silica sol precision casting shell

An effective way to improve the shelling performance of silica sol precision casting shell

The shell delamination performance of silica sol precision casting is poor, which directly affects the production efficiency and surface quality of complex structure precision castings. In the production of precision castings, some parts with complex structures, deep holes, blind holes, curved holes or narrow grooves, etc., have very difficult post-processing. Today we will talk about effective ways to improve the shelling performance of silica sol precision casting shells.

1、 Definition and measurement of shelling performance

The shelling performance of precision casting shell refers to the ability of the shell to detach from the surface of the casting after being processed by the shelling process when pouring molten metal and cooling to room temperature. At present, the most widely used, efficient, and economical first shell removal process in China is to use a "vibration shell removal machine" to remove shells, and often use auxiliary post-treatment processes such as acid immersion, shot blasting, sandblasting, and alkali explosion (boiling) to thoroughly remove residual shells and sand on the surface of castings (mainly the inner cavity). The shelling performance of castings depends not only on the shelling performance of the mold itself, but also on the complexity of the casting structure and the efficiency of the equipment in the post-processing shelling process (such as the impact energy of the shell shaking machine and the efficiency of the shot blasting machine).

Many castings with complex structures cannot completely remove residual shells and sand by simply relying on vibration to remove the shell. Therefore, when comparing the shelling performance, it is not only necessary to consider the efficiency of "shell shaking", but also to include the overall efficiency of other auxiliary shelling methods (shot blasting, sandblasting, etc.) to reasonably evaluate the shelling performance of the mold.

2、 Ways to improve the shelling performance of molded shells

1. For small castings of steel (iron, copper) (≤ 0.1kg), a coating is prepared using a silica sol with a lower SiO2 content. The surface layer coating is prepared using a silica sol with an ω (SiO2)=25%. To achieve the goal of improving shelling performance without affecting the strength of the shell. Reducing the content of silica sol SiO2 in the coating will cause a decrease in the residual σ of the mold shell, which will directly affect the strength of the mold shell at room temperature and high temperature, and will also cause casting defects such as "flying wings" or "flow lines". Its production scope is limited to small or extremely small parts, and this process has weak universality.

2. Add 5%~10% fused silica powder in the surface layer zircon powder coating, or use "mixed quartz sand" instead of zircon sand as the surface layer sanding material for small and medium-sized pieces to improve the shell shelling and permeability. This method has a certain impact on the surface quality of castings and often causes surface layer shell cracks. At present, it is only applied to some small and medium-sized parts with less complex inner cavities, and its processability and quality stability are still not as good as zircon sand. However, the risk of using it in the transition layer is much smaller.

3. The fused silica sand powder is used to replace the kaolinite sand powder in all layers of the shell, especially the transition layer and the back layer, so as to improve the shelling property, permeability and dimensional accuracy of the casting. The overall strength of the full fused silica mold shell is about 20% lower than that of the typical mold shell combination, so there are more shell damage and breakout during dewaxing, baking or pouring, and shell cracks are easy to occur due to the large difference in expansion coefficient. Although it has good shelling performance, there is a high quality risk and low cost-effectiveness, making it difficult to promote and apply.

4. Add high polymer (polyvinyl alcohol) to the surface coating or use "quick drying silica sol" to prepare the coating to improve the shelling ability of the mold. Although using "quick drying silica sol" has many advantages, its high cost is still the biggest obstacle to its widespread promotion. The correct selection of quick drying silica sol combined with refractory materials is an inevitable trend to improve the shell delamination performance in the future, and it is also the best shell combination. It has been continuously promoted and applied abroad.

The most effective method to improve the shell delamination performance of precision castings using ordinary silica sol is to use a "mixed coating" in the transition layer shell. Its biggest feature is that it will not excessively reduce the overall strength of the shell, improve the effectiveness of shell removal, increase costs less, have strong universality of castings, have little impact on the surface quality of castings, and have high cost-effectiveness.

3、 Key points of the process

The shelling performance of castings not only depends on the shelling performance of the mold itself, but also on the efficiency of casting structure, vibration, shot blasting, sandblasting and other equipment. Improve the seismic shell method to effectively transmit impact energy, and it is not advisable to use cushioning and shock-absorbing methods such as rubber pads or steel pier seats with high rigidity to place casting groups. The use of cast aluminum piers can prevent slipping without buffering or shock-absorbing, reducing vibration impact energy and production efficiency. The shell shaking machine should be equipped with a pressure regulating valve. According to the material, size, and requirements of the casting, adjust the intake pressure to prevent the casting from falling off (cast iron, high carbon steel, etc.) without shaking off the shell. Adjust the vibration frequency to be the same (near) as the natural frequency of the shell or casting, and generate "common vibration" to improve the efficiency of shell removal.

When the inner cavity of the casting is filled with no gaps by the mold shell, the vibration effect of the shell is much worse than that of the through hole inner cavity. Therefore, it is required that the operator use a special shell chiseling tool during vibration, which can accelerate the vibration of the residual shell and improve production efficiency.

Precision machining