1. Vibration modeling
The sand used for lost foam casting of valve castings is dry sand without binder, which has good fluidity, air permeability and fire resistance. It is easy to flow into the internal cavity of the pattern, so that the residue produced by pouring can escape smoothly. At the same time, it can also resist the infiltration of liquid metal and provide support for the pattern cluster. 40 ~ 70 mesh sea sand is selected in production, and the particle shape is mainly circular. When placing the pattern cluster in the sand box, ensure that the shortest distance from the pattern cluster to the bottom and surrounding of the sand box is more than 50 mm. Sand adding molding adopts adding sand at the same time and vibrating until it is filled to the position of the gate cup, and the frequency of the vibrating motor is adjusted to 30 ~ 50 Hz
2. Negative pressure pouring
Before the valve casting is poured, the dry sand can form a certain strength by pumping negative pressure, and the vacuum degree in the sand box in production is adjusted to 0 60~ 0. 65 M Pa。 During pouring, the ladle shall be as close to the gate cup as possible, with a distance of 50 mm, and the filling shall be stable and continuous. The liquid flow cannot directly wash the root of the sprue cup, but needs to pass through the buffer dike and enter the sprue smoothly. During the pouring process of valve castings, the gate cup shall be kept full and prevent liquid disturbance. The pouring speed shall be slightly slow and kept constant to prevent secondary oxidation inclusions caused by rapid flow.
3. Pressure maintaining process
After the valve casting is poured, keep the sand box at a certain negative pressure within the specified time, that is, maintain the pressure. The pressure holding time shall consider the escape time of gas in stainless steel water under negative pressure. If the pressure is relieved prematurely and the stainless steel water is not completely solidified, the pressure will be relieved, and the residual gas in the stainless steel water will remain in the casting, so the pressure shall be relieved after the stainless steel water is solidified. In addition, the holding time mainly depends on the thickness of the valve casting. Under the condition of ensuring forming and crystallization solidification, the holding time shall be as short as possible. However, if the pressure holding time is too short, shrinkage cavities are easy to occur in the valve casting. If the time is too long, the deformation resistance will be increased and the performance of the valve casting will be affected. It is generally controlled within 5 ~ 15 min.