Анализ десяти пунктов усадки и дефектов пористости в отливок в литейных цехах

Causes and preventive measures of shrinkage and porosity in castings.

1. Casting and pattern design

(1) The cross-sectional dimensions of the casting have changed significantly. If the cross-sectional size of the casting changes too much in the design, the cooling rate of the thin section is much faster than the condensation rate of the adjacent thick section, making it difficult to achieve sequential solidification of the casting and also difficult to perform shrinkage repair. During design, this situation should be avoided as much as possible, otherwise cold iron should be used to achieve sequential solidification of castings and facilitate shrinkage.

(2) If the cross-section of the casting is too thick and no corresponding measures are taken to compensate for it, shrinkage holes may form due to poor compensation.

(3) The rounded corner is too small. The concave corner radius of the casting is too small, which can lead to a decrease in the heat transfer ability of the molding sand and a decrease in the solidification speed. At the same time, due to the strong heating effect of the molding sand in this area, the gas pressure is high, and the released gas can infiltrate into the uncured metal liquid, causing gas shrinkage in the casting.

(4) The rounded corners are too large. If the fillet is too large, the fillet will become a thick section. If the adjacent sections are thinner, it will be difficult to achieve effective shrinkage, resulting in poor shrinkage

2. Appearance

(1) The wear of the appearance or core box leads to a thinning of the casting section, resulting in a reduction in the thickness of the casting section and hindering shrinkage repair.

(2) Improper size or structure of the pattern can result in excessive thickness or thinness of the casting section. When designing, attention should be paid to controlling the thickness of the pattern, and efforts should be made to maintain the maximum thickness of thin sections adjacent to thicker sections.

3. Sandbox

(1) The upper box is too shallow. The common cause of shrinkage defects is the use of upper boxes with insufficient height in production to save sand usage or reduce sand box and molding costs. If the upper box is too shallow, it will reduce the static pressure of the metal liquid, making it difficult to perform shrinkage repair. Insufficient shrinkage repair pressure can lead to shrinkage or porosity, or both. If it is necessary to use a shallower upper box, a riser sleeve or sprue cup should be used to compensate for the problem caused by the shallower upper box. Alternatively, a heating riser or continuous addition of metal liquid to the riser can be used to improve the shrinkage efficiency.

(2) The sand box is too small. In order to save sand and reduce costs, sometimes undersized sand boxes are used for molding. If the size of the sand box is too small, it will affect the reasonable arrangement of the pouring system or cause the riser to be too close to the sand box wall, and the metal liquid in the riser will cool and solidify prematurely before completing the shrinkage. You can try using heating risers and rain pouring gates to solve these problems.

(3) The box cover is not suitable. Due to the mismatch in the size of the casing, it cannot support the sand mold well, resulting in displacement or deformation of the mold wall, forming a thicker casting section, and the originally designed riser is insufficient to make up for it.

4. Riser system

When shrinkage defects occur, the problem should be identified from the perspective of the pouring system, as shrinkage defects are closely related to shrinkage repair.

(1) The design of the sprue failed to promote sequential solidification. The position and size of the sprue and riser must be designed according to the different conditions of the casting section. It is necessary to consider which sections solidify first and which sections solidify later without sprue and riser. The final solidified section must have a still liquid sprue or riser for shrinkage.

(2) Insufficient number of sprues or improper sprue design. Insufficient number of sprues is a common cause of shrinkage defects, and the shrinkage distance and range of each sprue are limited. The size of the shrinkage range depends on the thickness of the cross-section, the temperature of the molten metal, and the composition of the metal. Thin sections condense quickly and cut off the contraction channel. Improper design of the sprue, such as a small cross-section of the sprue, can lead to premature condensation, and even if there is a lot of molten metal in the sprue, it cannot be replenished into the casting.

(3) The riser is too small. Too small a riser is also a common mistake. In general, the cross-sectional size of the riser should be larger than the cross-sectional size of the casting that needs to be shrunk. Another mistake is to increase the height of the small diameter riser to improve the shrinkage effect, which is often unsatisfactory because the riser diameter is small and the metal liquid in the riser is prone to condensation. Even increasing the height of the riser is of no use. The ideal shape of the riser should be spherical because it has the smallest heat dissipation area. In some cases, chilling thick sections is an effective and feasible method.

(4) Improper ratio of riser neck and casting size. The size of the riser neck is very important for the feeding efficiency. If the cross-sectional size of the riser neck is too small, it will affect the feeding effect. It is necessary to ensure that the metal liquid flows smoothly in the riser neck.

(5) The sprue is too large. Shrinkage at the entrance or on the sprue is usually caused by the sprue being too large. Because the entrance of the sprue is surrounded by hot sand, if its size is too large, the molten metal at the entrance will remain until it solidifies. At this time, there is no other metal solution to supplement it, so shrinkage cavities will occur at the entrance of the sprue. The correct design of sprue and riser should first solidify the casting, then solidify the sprue and riser neck, and finally solidify the riser. If this solidification sequence is disrupted, shrinkage defects will occur.

5. Molding sand

The displacement of sand mold walls, especially irregular displacement, can affect the shrinkage effect and cause shrinkage defects. The greater the displacement of the wall, the greater the possibility of shrinkage. The low strength of the molding sand and the softness of the pounding sand are the main reasons for the displacement of the mold wall. It is necessary to improve the strength of the sand mold and the compactness of the pounding sand to reduce the creep or displacement of the mold wall.

6. Core making

The displacement of the sand core will cause thickening of the casting section, resulting in uneven wall thickness of the casting section. The displacement of sand cores is caused by factors such as too few core supports, soft core heads or insufficient support surfaces, and low strength of sand cores leading to warping. Changes in the cross-sectional area of castings caused by changes in the position of sand cores during production may result in shrinkage defects.

7. Metal composition

The influence of metal composition on shrinkage defects has two aspects. One is the direct influence of alloy composition on shrinkage defects themselves. For example, the total shrinkage of low-carbon cast iron is greater than that of high carbon cast iron; By changing the metal composition in non-ferrous metal casting to broaden the solidification range and promote sequential solidification, shrinkage defects can be reduced; Adding grain refining agents such as titanium and boron to aluminum alloys can reduce shrinkage defects. The influence of alloy composition on shrinkage defects is mostly direct, and these factors should usually be taken into account when designing sprue and riser systems. The second reason is that the pressure of the metal solution causes the mold wall to retreat, resulting in mold wall displacement. The pressure of the metal solution is related to the metal composition, and the pressure of gray cast iron on the mold wall is smaller than that of alloy cast iron and ductile iron. To some extent, the higher the carbon equivalent, the greater the pressure of the metal solution on the mold wall.

8. Melting

Hasty loading or material handling can cause deviations in metal composition. When dealing with shrinkage defects, melting factors should be considered, as dirt, rapid solidification, and easily oxidizable metals all have an impact on the early shrinkage of metals. If the temperature of the molten metal is too high, it will heat the sand mold to the point where it slides, thereby exacerbating the displacement of the mold wall. The more gas there is in the molten metal, the greater the pressure released during the solidification process on the mold wall, and the more likely it is to cause displacement.

9. Pouring

(1) Low pouring temperature can reduce the shrinkage efficiency of the riser system and cause shrinkage defects.

(2) If the pouring temperature is too high, it will increase the heat transferred to the molding sand, causing high-temperature deformation of the molding sand and exacerbating the displacement of the mold wall.

(3) When producing large castings, the absence of hot metal liquid to supplement the sprue can also result in shrinkage defects. Heating risers or enlarged risers can be used as needed to replace supplementary pouring.

10. Other

The fracture of sand core or mold wall can cause burrs or mold leakage, resulting in the loss of molten metal. At this point, the demand for the amount of molten metal in the mold cavity has changed. If there is no excess molten metal available for replenishment, or if the replenished molten metal is insufficient, shrinkage defects may also occur.