Influence of mold filling on casting defects in lost foam casting

The pin holes, slag inclusion defects of cast aluminum parts, wrinkle of iron castings, carbon defects of steel castings, and shrinkage defects such as shrinkage and porosity are all caused by the incomplete discharge of the foam pattern under the pyrolysis of molten high temperature molten metal. The formation of these defects is affected by the flow shape and speed of molten metal in the filling process. And foam shape, negative pressure, paint performance, pouring temperature, gating system and other parameters directly affect the filling ability of molten metal.

1.The influence of bubble appearance

The type and density of foam patterns determine the amount of heat and decomposition products absorbed during the thermal decomposition of the mold, which have a great influence on the filling ability of the molten metal. The greater the pattern density, the more heat absorbed and the number of products generated during the thermal decomposition of the pattern will also increase, the lower the temperature of the liquid metal flow front and the greater the resistance of the air gap pressure to the flow front, which slows down the filling speed and filling capacity of the liquid metal. The smaller thickness of the mold, the loss of heat transfer between the metal liquid and the mold is large, which makes the temperature of the front edge of the liquid metal drop fast and the filling ability decreases. For this reason, the thin thickness pattern prevents the deformation of the mold and does not affect the filling ability, and can choose a foam with higher density.

2.Influence of vacuum degree

In lost foam casting mold filling, because the air gap pressure formed by the gas products produced by pattern thermal decomposition has resistance to the flow of liquid metal, extracting the air in the sealed sand box during pouring can speed up the mold filling capacity of liquid metal. The advantage of vacuum pumping is that it can inhibit the number of gaseous products decomposed by the pattern, accelerate the escape speed of pattern decomposition products from the coating, partially release the air gap pressure in the breath layer, and improve the filling flow speed of liquid metal. However, the vacuum degree makes the molten metal front fill preferentially along the mold wall between the pattern and the mold, and the molten metal front is concave; When the vacuum passes too large, the front of the liquid metal is concave and deformed so as to wrap the foam pattern to form a closed thin shell. The appearance of the thermal decomposition product can not escape the coating quickly, and the filling ability becomes slow.

3.Influence of pouring temperature

High or low pouring temperature will affect the type and quantity of products during pattern thermal decomposition, the way and speed of escaping from the coating, and the amount of heat lost at the front of liquid metal flow. The results show that the casting temperature of different alloys at different pouring temperatures is different from that of the sample. The pouring temperature of aluminum and magnesium alloys is between 700-800 degrees. The decomposition product of foam pattern is mainly liquid. With the increase of pouring temperature, the viscosity of liquid products decomposed by the mold will also decrease. Thus, the escape rate of liquid decomposition products from the coating increases, which reduces the obstruction of the decomposition products of the pattern to the filling ability of the metal liquid; On the other hand, the increase of pouring temperature will increase the superheat of molten metal and prolong the filling life. When the pouring temperature rises to above 800 degrees Celsius, the decomposition products of the foam pattern will further gasified. This will increase the air gap pressure and the heat absorbed by the metal in the air gap layer between the front and the metal liquid. Therefore, due to the double pressure of the pressure and the decrease of the temperature at the front of the liquid metal, the filling ability of the liquid metal will become slow. For cast steel and cast iron, when the pouring temperature is above 1500 โ„ƒ, the decomposition products of the pattern are mainly in gaseous form. The volume of decomposed gaseous products increases with the increase of pouring temperature. The speed of the coating escaping from gaseous products does not increase, but only increases the air gap pressure in the air gap layer, which hinders the filling ability of molten metal and weakens the effect of increased liquid metal superheat on the filling ability. Therefore, in the casting process of lost foam casting, the pouring temperature can not be blindly increased in order to eliminate casting defects and accelerate mold filling speed, which will only make the casting defects more serious, the grain structure of the casting coarser and the mechanical properties of the casting worse.

4.Effect of coating

The air permeability, permeability, wettability and thermal insulation of the coating are directly related to the filling ability of liquid metal. When the performance of the coating is poor, the resistance of the gaseous or liquid product decomposed by the foam pattern increases through the coating, causing air gap pressure to increase. More decomposition products are left in the liquid metal liquid, and the filling ability of the metal liquid is reduced. For castings with large pattern wall thickness, the permeability of the coating is the main influencing factor. Increasing the permeability of the coating will improve the mold filling ability of the molten metal; For castings with thin pattern wall thickness, the thermal insulation of the coating protects the heat capacity of the molten metal. Increasing the thermal insulation of the coating will improve the mold filling ability.

In addition, the mold filling ability of lost foam casting is also related to gating system, molding sand particle shape and temperature. In short, due to the interaction between metal liquid and foam pattern, the molten metal becomes more complex during mold filling. Therefore, in practical applications such as insufficient casting and cold separation, it is necessary to find out the causes based on the production results and casting filling and solidification process.