Six Common Causes of Defects in Wet Casting (Part 1)

Sep. 23, 2020

The small gray cast iron scraps in the wet small workshop are mainly caused by defects such as blisters, sand sticking, and pores. Among them, the most common are casting defects such as porosity, iron bean porosity, shrinkage porosity, and porosity shrinkage. In addition, sand sticking, interlayer, and poor surface quality are also important factors affecting product quality. Analyze the causes and preventive measures of waste products. The main points are as follows. OEM Metal Parts manufacturer will introduce you.

1. Sand

The casting defects caused by the thermal expansion of the sand surface are the most typical in the form of sand inclusions. Under the production conditions of wet casting, sand inclusions are very common, but dry castings rarely produce such defects. It is common on the upper surface of castings, and the larger the upper surface, the easier it is to produce sand.

(1) Reason for generation

1) The thermal conductivity of the molding sand is poor, or even the sand crystal form changes to increase the expansion rate, which causes a large temperature gradient between the layers of the sand mold wall, which causes the uneven expansion of each layer and produces compressive stress.

2) To improve the resistance of the high-humidity and weak abrasive belt on the back of the dry sand layer, the hot-wet tensile strength of the high-humidity and weak abrasive belt can be increased to make the dry sand layer on the surface of the mold wall difficult to bulge and warp.

(2) Countermeasures

1) Reduce the macroscopic expansion of the sand mold (core);

2) Increase the thermal and wet tensile strength of the moisture condensation zone between the dry sand layer and the sand body;

3) Increase the thermal deformation of the dry sand layer so that the thermal deformation exceeds its thermal expansion value.

Green Sand Casting

Green Sand Casting

2. Trachoma

(1) Reason for generation

Mainly from molding materials, molding sand and core sand have high ash content, low strength, or excessive moisture and gas-producing substances. The molding sand will fall due to insufficient strength of the sand mold or core or even boiling of the molten metal during pouring.

(2) Preventive measures

Uneven compaction during modeling, improper repair, and accidental collisions during core setting and box closing will cause sand particles to fall and cause blisters. To prevent sand holes, besides paying attention to the operation of modeling and closing the box and minimizing the excessive erosion of the sand mold by the molten metal, the main reason is to control the moisture of the sand reasonably to improve the toughness and strength of the sand.

3. Stoma

During the solidification process of the molten metal, the pores formed by the bubbles trapped in the metal are called pores. The pores belong to the hole-type casting defects with smooth pore walls.

(1) Reason for generation

It is different from shrinkage porosity, shrinkage cavity, blisters, and slag inclusion, and the reason for its formation is also very complicated. Or because the moisture content in the sand mold is too high, or the exhaust of the cavity is not good, the ventilation between the sand cores is not smooth, or the charge is not dried, etc., it is easy to cause pore defects. According to the mechanism of pore formation, it can be divided into intrusion pores, entrapment pores, precipitation pores, endogenous reaction pores, exogenous reaction pores. , Sand core, etc.) The gas generated invades the molten metal in the cavity, forming bubbles and pores.

(2) Countermeasures

1) Reducing the gas pressure in the capillary between the sand grains is mainly to increase the aeration capacity of the sand mold wall or sand core, reduce and control the actual gas output of the gas source such as the sand mold or sand core to reduce the gas pressure in the capillary between the sand grains, and avoid external gas intrusion In molten metal, we must first control the moisture permeability of the molding sand and the surface hardness of the wet mold, and artificially enhance the aeration capacity of the sand mold wall and core-sand mold exhaust system. In addition to relying on the permeability of the molding sand and core sand to ensure the aeration capacity, In production, measures to enhance the ventilation capacity are mainly adopted to prevent and eliminate the intrusion of stomata. Reduce and control the actual gas generation volume during pouring of sand molds or sand cores. Prevent local concentration of gas-producing substances in sand molds or sand cores. In addition, the height from the top surface of the upper box of the wet gray cast iron to the gate, that is, the effective height of the sprue should not be less than 65mm.

2) Make bubbles formed by the gas invaded into the molten metal in the cavity, and then discharged from the molten metal.

3) The entrapment of air holes is due to the fact that the metal stream in the pouring system entrains bubbles during pouring, and the bubble stream enters the cavity, or the stream impacts the metal surface in the cavity, bringing the bubbles into the molten metal. Can not be excluded from the cavity metal liquid, it will cause the casting to produce porosity. The formation of pores can be eliminated by controlling the gas content of the molten metal, degassing treatment, using metal molds, and solidifying the molten metal after pouring under pressure.

Because it is green sand casting, neither the mold nor the core is dried, so it is easy to form pore defects, which is the biggest disadvantage of green sand casting. Therefore, the moisture content of the molding sand should not be too high, the amount of gas-producing substances should not be too high, the hardness of the sand mold should not be too high, the compactness of the upper mold can be slightly lower than that of the lower mold, and the air permeability of the molding sand should be appropriately improved. A small amount of molding sand can reduce the metal Protective agent for liquid oxidation.