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China street lighting cast part supplier combined with many years of production practice experience, talk about the causes and preventive measures of shrinkage and shrinkage of castings.
The displacement of the sand mold wall, especially the irregular displacement, will affect the shrinkage effect and cause shrinkage defects. The larger the displacement of the mold wall, the greater the possibility of shrinkage. The low strength of the molding sand and the softness of the sand are the main reasons for the displacement of the molding wall. The strength of the sand molding and the compactness of the sand should be increased to reduce the creep or displacement of the molding wall.
6. Core making
The displacement of the sand core will thicken the cross section of the casting and cause uneven wall thickness. Too few core braces, soft core heads or insufficient support surfaces, and low core strength caused by core warpage, etc., are all causes of core displacement. Changes in the cross-section of the casting due to changes in the position of the sand core during production may cause shrinkage defects.
7. Metal composition
The effect of metal composition on shrinkage defects is twofold. The first is the direct effect of alloy composition on the shrinkage defect itself. For example, the total shrinkage of low-carbon cast iron is greater than that of high-carbon cast iron; in non-ferrous metal casting, by changing the metal composition to broaden the solidification range, promote sequential solidification, and reduce shrinkage defects; add grain refiners to aluminum alloys , Such as titanium and boron, can reduce shrinkage defects. The effect of alloy composition on shrinkage defects is mostly direct. When designing the riser system, these factors should usually be taken into account. The second is that the pressure of the metal solution causes concession to the molding wall, causing displacement of the molding wall. The pressure of the metal solution is related to the metal composition. The pressure of the gray cast iron on the profile wall is smaller than that of alloy cast iron and nodular cast iron. To some extent, the higher the carbon equivalent, the greater the pressure of the metal solution on the mold wall.
Sloppy loading or weighing can cause deviations in metal composition. For shrinkage defects, consideration should be given to melting factors. Fouling and solidification of metals that are fast and easy to oxidize have an impact on the early shrinkage of metals. If the temperature of the molten metal is too high, the sand mold will be heated to the extent that it will cause slippage, which will aggravate the displacement of the mold wall. The more gas in the metal liquid, the greater the pressure of the gas released by the solidification process on the mold wall, and the easier it is to cause displacement.
(1) Too low pouring temperature will reduce the shrinkage efficiency of the feeder system and cause shrinkage defects.
(2) If the pouring temperature is too high, it will increase the heat transferred to the molding sand, cause the molding sand to deform at high temperature, and exacerbate the displacement of the molding wall.
(3) In the production of large-scale castings, shrinkage defects will also occur if the riser is not supplemented with hot metal liquid. You can use a heating riser or increase the riser as needed instead of supplementary pouring.
Rupture of sand core or mould wall can cause burrs or mould leakage, resulting in fluid loss. At this time, the cavity's demand for the amount of metal liquid has changed. If there is no excess metal liquid to be replenished, or the metal liquid for replenishment is insufficient, shrinkage defects will also occur.
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