Mold collapse defects, also known as casting collapse or casting disintegration, have been extensively studied with the growing maturity of lost foam casting technology. Research has shown that the causes of mold collapse defects are multifaceted rather than resulting from a single factor. The main causes can be summarized as follows:

a. During the pouring process, excessive and rapid gas generation from the decomposition of the lost foam pattern, combined with insufficient mold exhaust and inadequate vacuum suction capacity, can easily result in mold collapse and disintegration.

b. Molten metal flashover is another major cause of mold collapse defects. This occurs when molten metal that has already filled part of the lost foam cavity is diverted to other areas under external influence, leaving the original area empty or insufficiently filled. Such problems are common in top pouring systems, castings with large flat surfaces, and molds containing multiple patterns.

c. Excessive buoyancy from the molten metal may cause deformation of the molding sand at the top of the mold, leading to localized collapse. In general, insufficient sand thickness on the top surface and inadequate negative pressure can result in poor casting quality or incomplete formation.

d. Inadequate refractoriness and poor high-temperature strength of the coating can also lead to mold collapse defects in lost foam castings. During pouring, the lost foam pattern helps buffer the filling and cooling process while reducing erosion of the mold by molten metal. After the molten metal replaces the pattern and fills the cavity, the dry sand mainly depends on the coating for support. If the coating lacks sufficient strength or refractoriness, localized collapse may occur, particularly above the ingate area of large castings.

Box collapse defects in lost foam castings generally occur during the pouring or solidification stages. These defects are mainly characterized by localized mold collapse, resulting in incomplete casting formation or excess material in certain areas. To prevent box collapse defects, the following measures can be adopted:

a. When the buoyancy generated by molten metal is excessively high, the molding sand at the top of the mold can deform and cause localized collapse. Increasing the amount of sand on the top surface or adding pressure irons to the mold can effectively reduce this risk.

b. If the decomposition of the lost foam pattern generates gas too rapidly during pouring and the mold cannot vent efficiently, mold collapse is likely to occur. Therefore, low-density foam materials should be selected to minimize gas generation.

c. After the molten metal replaces the foam pattern and fills the cavity, the dry sand relies primarily on the coating for structural support. If the coating has insufficient strength or refractoriness, localized collapse may occur. Therefore, coatings with high strength, excellent refractoriness, and good permeability should be used whenever possible.

d. The gating system should be designed properly, with suitable sprue and ingate dimensions. The pouring process should also be optimized by reducing pouring temperature where possible, controlling pouring speed, and avoiding interrupted flow during casting.

e. To prevent mold collapse caused by molten metal flashover, areas subjected to severe molten metal erosion can be reinforced using ceramic inserts or self-hardening sodium silicate sand.

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