In the investment Precision casting production process, the reason for the burrs of the castings is that the casting shell has cracks during dewaxing.
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1. The shell wet strength is insufficient. Let me give an example of shell making parameters that produce burrs: 270 mesh zircon sand powder (graded powder) is used for the surface layer, the powder-to-liquid ratio is 4.5:1, and the viscosity of the No. 4 cup is 43 seconds.
Around, the temperature is 22℃, the humidity is 65%, and the drying time is 6-8 hours; the shell is made of 5 and a half layers, and the drying time of the remaining layers is also 6-8 hours. From the above shell making parameters, I feel that the powder-to-liquid ratio of the surface layer is a bit high, 4.5.
Consulted with refractory suppliers, they said that the powder-to-liquid ratio is not high because it is 270 mesh zirconium powder. However, the graded powder is used. The graded powder can be made into a high powder-to-liquid ratio.
However, a high powder-to-liquid ratio is harmful to the air permeability of the shell. According to the drying conditions of the surface layer, it seems that the problem is not very big, and the drying of the surface layer should be acceptable. However, the drying time of the sealant layer is relatively short. Generally speaking, the viscosity of the sealing layer slurry is the smallest, and it directly penetrates the first five layers to the surface layer. Theoretically, the drying time is recommended to be more than 24 hours. The relatively short point here may cause lower wet strength.
2. Design of pouring system for castings. We know that the design of the gating system also has a function of wax removal. The wax removal is smooth, and the probability of cracks in the shell will be greatly reduced. Generally, a gating system design similar to the top injection type is adopted. In fact, this type of gating system design is relatively beneficial for wax removal. If the contour of the casting is relatively large, the lower part (the top of the casting) will have fewer wax outlets, which will easily cause a certain phenomenon of poor wax discharge.
3. The pulp is often prepared first, and there is no process of waking up or the time for waking up is not enough. This is a particularly terrible thing. The prepared slurry is not uniform, the powder and silica sol cannot be fully wetted, the strength of the surface layer may be insufficient, and the slurry and sand may be selectively adsorbed (because the wettability of the slurry and the wax mold is not good, and the The wettability of sand is better than wax mold), so after sanding, some parts of the coating slurry attached to the wax mold will be "sucked" by the sand particles and separated from the surface of the wax mold, forming cavities of irregular size and shape. This may also be the reason why bumps (cucumber thorns) are formed on the surface of these products.
To sum up, the reasons that cause burrs and bump defects in castings are firstly insufficient wake-up time after slurry preparation; insufficient drying time of the shell, especially the drying time of the sealing layer, resulting in insufficient wet strength of the shell; surface layer slurry The powder-to-liquid ratio is a bit high, which may result in poor air permeability of the surface layer or even the entire shell.
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