The phenomenon that the mold wall displaces after the molten metal is poured into the lost foam casting mold is called the mold wall movement. The molten metal in the mold always contacts the mold wall and moves with it before it solidifies into a hard shell with sufficient strength. The solidified metal will move inward due to shrinkage, while the mold wall will continue to move outward. Therefore, the movement of the mold wall will affect the dimensional tolerance and shrinkage hole of the casting The formation of shrinkage porosity has a great relationship. The reason for the movement of the mold wall is that the mold wall expands under the thermal action of the molten metal, and the sand mold is further compacted under the pressure exerted by the metal, such as the static pressure of the molten metal and the expansion force during the graphitization of the cast iron.
The surface hardness of the mold is an important factor affecting the mold wall movement. The surface hardness of lost foam casting mold was measured by sand mold hardness tester, and the surface hardness of the mold was measured by mold hardness tester.
The test method is to evenly distribute quartz sand with a thickness of more than 200mm at the bottom of the sand box during molding, and then place the EPS model of 100mmx20mmx400mm in the center of the sand box. The EPS model is covered with plastic film and vibrated and compacted with sand. Under the action of certain vacuum and negative pressure, the EPS pattern is taken out, and a cavity of 100mmx200mmx400mm is formed in the center of the sand box. The surface hardness of the sand mold at different vacuum degrees and heights is tested.
When the vacuum degree is -0.02mpa, the relationship curve between the surface hardness and the height of the casting mold is shown in Fig. 1. It can be seen from the figure that in negative pressure lost foam casting, the surface hardness of sand mold at different heights of the mold is not the same. The surface hardness of the top of lost foam casting mold is the lowest, and the surface hardness of the bottom of lost foam casting mold is the highest. There is a linear relationship between the surface hardness of lost foam casting mold and the height.
Fig. 2 shows the relationship between the surface hardness of the lost foam casting mold and the surface hardness of the top of the mold and the vacuum degree of the mold. It can be seen from the figure that the surface hardness of the lost foam casting mold gradually increases with the increase of the mold vacuum. When the mold vacuum exceeds -0.06mpa, the vacuum increases again, and the surface hardness of the lost foam casting mold changes little. The pouring test shows that the vacuum degree of lost foam casting mold is too high, which will promote the metal liquid to permeate into the lost foam casting mold, form serious penetration and sand adhesion, and affect the surface quality of the casting.