The research team of ZHY casting decided to open thetest mold first to verify the material fluidity and the influence of gate and riser on product quality. Because the air tightness of the valve body blank can only be known after the valve body is processed and assembled with other accessories for test, the reason is that the air tightness test method needs to be consistent with the test method of the valve manufacturer. After machining, reassemble for air tightness test. The verification cycle lasts for one month, but the results are not very satisfactory. The specific performance is as follows:
a. The fluidity of the copper alloy is poor in sand casting. The fluidity can be improved by increasing the content of aluminum and iron. When the content of aluminum and iron is below 0.1%, it is not easy to form. When the content of aluminum and iron is in the range of 0.2% to 0.25%, the fluidity reaches the maximum, and the pass rate of air tightness test is the highest. When the content of the two elements is increased, the qualified rate of air tightness test does not increase.
b. The most frequent place of air tightness defect is the screw position, followed by the farthest position from the gate. In most cases, there are only small bubbles in the air tightness test.
c. The pass rate of air tightness test is 60% on average, only 30% in serious cases.
Analyze and find out the cause, and find out whether the problem point is poor liquidity. Because of the poor fluidity of the copper alloy material in the sand mold, the dense structure has not been formed at the place far away from the gate, the molten metal has cooled, and the porosity and porosity are formed at the place far away from the gate. After the thread is processed, the micro porosity will be presented, which leads to the unqualified air tightness test.
The preliminary conclusion is that the fluidity of sand casting according to the requirements of the drawing is poor, which will lead to unqualified air tightness. It is necessary to consider another casting method or add special treatment, such as infiltration, to improve the qualification rate of casting air tightness test.
The solution of low air tightness qualification rate is to replace the existing sandand look for a good fluidity casting process. It can be seen from the casting manual that the metal mold gravity casting can obtain better material fluidity than the ordinary sand casting, which can greatly improve the qualified rate of finished products, while the cost change is not great.
The research team of ZHY casting decided to open a gravity casting test mold and verify it first. The conclusion of verification is that the qualification rate of air tightness test of two batches is over 98%, which greatly improves the qualification rate of air tightness test and exceeds the requirement of 97% of the supplier’s air tightness qualification rate required by the procurement.