There are various types of defects in marine castings, and the reasons for their occurrence are also very complex. Casting porosity is one of the many defects, which is not only related to the casting process, but also to a series of factors such as the properties of the casting alloy, alloy melting, and the properties of the molding material. Today, we will focus on how to control the porosity defects of castings in the lost foam casting process.

1. Choose appropriate model materials

Copolymerized foam model material is used. Because the copolymer is zippered decomposition, one-time gasification degree is high, liquid phase proportion is small, small molecular gas is easy to overflow from the coating, white mold density is as small as possible under the premise of ensuring strength and smoothness, so as to reduce the total amount of gas generation. The pouring system uses hollow sprues and adds filters to mechanically block slag and absorb comb flow of the metal liquid, so as to make the filling more stable.

2. The use of pouring system and insulation riser

The opening of the internal runner should be conducive to the smooth, stable, and rapid filling of the alloy, without forming blind spots. An atmospheric pressure insulation riser should be installed at the top of the casting to collect slag and supplement it. The relevant parts should be equipped with shape separated sand cold iron to achieve the sequential solidification of the casting. The gas, slag, and gasification residue in the metal liquid should be quickly floated to the riser, reducing the probability of porosity, slag holes, and shrinkage. 3. Improve the breathability of coatings

The particle size of refractory aggregate should be appropriate, using composite binder suspension agent. The prepared coating has good high and low temperature strength, wear resistance, easy to apply, no cracking, strong exhaust ability, good breathability, uniform sintering, easy to peel off when opened, and does not react with metal wetting agents and chemical reactions. Generally, castings are coated twice with a coating thickness of about 2mm. The quality management of lost foam coatings is crucial. During mass production, regularly check the breathability of coatings and adjust the particle size of aggregates in a timely manner. Due to the presence of organic substances in the binder and suspension of coatings, special attention should be paid to the fermentation enzyme changes of coatings in summer and autumn seasons. Qualified castings cannot do without better coatings, and the quality of coatings cannot be controlled lightly.

4. The pouring temperature should be appropriate

Due to the exothermic vaporization of foam during the filling process of EPC, the pouring temperature of steel is 30-50 ℃ higher than that of sand casting. The thermal interaction between the molten metal and foam is restricted by the hot pouring temperature. The pouring temperature is suitable for full pyrolysis. The pyrolysis products of the pattern are mainly small molecular gas, which is easy to be discharged from the mold cavity under the effect of negative pressure field. When the pouring temperature is low, the pattern is not fully pyrolyzed, and liquid residues will block the coating layer. The discharge of pyrolysis gas is blocked, and the back pressure is formed in the mold cavity, which reduces the filling fluidity. In addition, the solidification speed is fast, The gas slag near the liquid cannot float and collect into the riser in time, increasing the probability of forming pores. Of course, the pouring temperature should match the smelting material to avoid other casting defects caused by high temperature or bottom.

5. Reasonable pouring position

The better pouring position for castings is to obtain good gasification and exhaust conditions during mold filling, which is convenient for gasification and slag removal. Vertical or oblique pouring should be chosen, which follows the principle of larger side surface area. The pouring system should have slag blocking, gas collection, and shrinkage filling functions.