Shrinkage and shrinkage are common castings casting defects. Shrinkage is the casting in the condensation process of contraction, not the addition of metal solution generated by the hole, irregular shape, hole wall rough, generally located in the hot section of the casting. Shrinkage and stomata are often very similar in appearance, often confusing. In general, the inner wall of the stomata is smooth, while the inner wall of the shrinkage hole is dendrite-like crystal. Shrinkage is the final solidification of the casting area is not obtained by filling the metal solution to form scattered and small holes, often appear in the casting of the thick section and the thickness of the intersection or hot junction. The distribution area of shrinkage is much larger than the shrinkage, often hidden in the casting of the internal, sometimes naked eye imperceptible. Shrinkage and shrinkage in castings waste in a large proportion, must be given enough attention to improve the casting yield.
I combine years of practical experience in production, to talk about cast shrinkage and shrinkage of the causes and prevention measures.

1. Casting and pattern design
(1) casting cross-sectional size change is too large. If the cross-sectional dimension of the casting is too large in the design, the cooling rate of the thin section is much quicker than that of the adjacent thick section, so it is difficult to solidify the casting and make it difficult to fill. Design to try to avoid this situation, or should be used cold iron, in order to achieve the order of casting solidification and conducive to feeding.
(2) casting cross-section is too thick, if not take appropriate measures to its filling, will form a shrinkage due to poor filling.
(3) fillet is too small. Casting concave corner fillet radius is too small, will lead to reduced heat transfer capacity of sand, solidification rate decreased, and because of the role of sand heated, the gas pressure, the precipitation of gas can be poured into the non-solidified liquid metal, casting Air hole.
(4) The fillet is too large. Fillet is too large, then the fillet becomes thick section, if the adjacent section is thin, it is difficult to get effective filling, resulting in poor filling

Look like
(1) pattern or core box wear caused by casting thin section, resulting in reduced casting thickness of the cross-sectional hinder feeding.
(2) the appearance of improper size or appearance of improper structure, resulting in casting cross-section is too thick or too thin. Design should pay attention to control the thickness of the appearance, as far as possible to make the thin section near the thicker section to maintain the maximum thickness.

3. Sand box
(1) the box is too shallow. In order to save the amount of sand in production or in order to reduce the cost of molding and sand box to use the height of the box is not enough, which is caused by shrinkage defects common cause. The upper case is too shallow to reduce the static pressure of the metal liquid, so that it is difficult to make up the filling, the filling pressure is not enough, will lead to shrinkage or shrinkage, or both. If you have to use a shallow box on the situation, you should adopt the riser or gate cup to make up for the box is too shallow to bring the problem, you can also use the riser riser or continue to add metal To improve the filling efficiency.
(2) the flask is too small. In order to save sand, reduce costs, sometimes using less than the size of the flask shape. The size of the flask is too small to affect the proper placement of the casting system or to cause the riser to be too close to the wall of the flask. The liquid metal in the riser cools and solidifies prematurely before completion of the filling. Attempts can be made to solve these problems with hot riser and rain gates.
(3) Case is not suitable. Due to the size of the box does not match, can not support the sand, resulting in displacement or deformation of wall to form a thick casting cross-section, so that the original design of the riser is not enough to fill it.

4. Riser system
When a shrinkage defect occurs, the problem should be sought from the casting system, since the shrinkage defect is strongly related to the filling.
(1) pouring riser design failed to promote the order of solidification. The position and size of the riser must be designed according to the different conditions of the casting cross-section. It is necessary to consider which cross-sections are solidified and which cross-section is solidified without the riser, and finally the solidified section must have a liquid Mouth or riser to fill.
(2) the number of pouring risers or gate design is not appropriate. The number of pouring riser is a common cause of shrinkage defects. The filling distance and range of each gate are limited. The size of the filling range depends on the thickness of the section, the temperature of the liquid metal and the metal composition. Thin section of the condensate quickly, will cut off the filling channel. Inadequate gating design, such as the gate section is too small will lead to premature condensation, this time despite the riser in a lot of liquid metal can not fill into the casting to go.
(3) riser is too small. Riser too small is a common mistake. Under normal circumstances, the cross-sectional size of riser should be greater than the need to fill the casting section size. Another mistake is to increase the small diameter of the riser height to improve the filling effect, this approach is often unsatisfactory, because the riser diameter is small, the liquid metal in the riser is easy to condense, even increase the height of the riser Also to no avail. Ideally, the shape of the riser should be spherical because the area of heat dissipation is minimal. In some cases, chilling of the thick section is an effective and practical approach.
(4) riser neck and casting size ratio improper. Riser neck size is very important to the efficiency of feeding, feeder neck cross-sectional size is too small will affect the filling effect, to ensure that the liquid metal in the riser neck flow.
(5) the gate is too large. In the gate or the gate at the entrance to produce shrinkage is usually caused by too much gates. Because the inlet gate is surrounded by hot sand, if the size is too large, the entrance of the liquid metal will remain until the last solidification, because at this time no other metal solution to its filling, so The gate entrance will produce shrinkage. The correct riser design should be solidified castings, followed by solidification gate and riser neck, and finally solidification riser. If this solidification sequence is destroyed, shrinkage defects occur.

5. Molding sand
Sand-type wall displacement, especially the irregular displacement will affect the filling effect, resulting in shrinkage defects. The greater the displacement of the wall, the greater the possibility of shrinkage. The strength of the sand and the compaction degree of the sand should be increased to reduce the peristalsis or displacement of the wall, which is the main reason of the displacement of the wall.

6. Core preparation
Sand core displacement, will make the casting cross-section thickening, resulting in uneven wall thickness of castings. The number of core struts is too small, the core is soft or the supporting surface is not big enough, and the core strength is low and the sand core is warped, which is the reason of the sand core displacement. Production due to changes in the position of the core caused by changes in casting cross-section, are likely to cause shrinkage defects.

7. Metal composition
Metal components on the impact of shrinkage defects in two ways. First, the direct impact of the alloy composition on the shrinkage defect itself. For example, the total shrinkage of low-carbon cast iron is larger than that of high-carbon cast iron; in the non-ferrous metal casting, it is possible to reduce shrinkage defects by widening the solidification range by changing the metal composition and promoting the solidification of the sequence; in the aluminum alloy, , Such as titanium and boron, can reduce shrinkage defects. The effect of alloy composition on the shrinkage defects is mostly direct, and these factors should be taken into account when designing the riser system. Second, the pressure of the metal solution to make the wall yield, resulting in wall displacement. Metal solution pressure and the size of the metal composition, gray cast iron wall pressure than the alloy cast iron and ductile iron to be small. To some extent, the higher the carbon equivalent, the greater the pressure of the metal solution on the walls.

8. Melting
Filling or material loading can cause deviations in the metal composition. Shrinkage defects should be considered for melting factors, dirt and rapid solidification, easy to oxidize the metal have an impact on the early contraction of the metal. The temperature of the molten metal is too high, the sand will be heated to produce the degree of slip, thereby increasing the wall displacement. More liquid metal in the gas, solidification process to release the gas pressure on the wall the greater the more easily cause displacement.

9. Pouring
(1) pouring temperature is too low will reduce the feeding system feeding efficiency, resulting in shrinkage defects.
(2) pouring temperature is too high, will increase the heat transfer to the molding sand, so that high-temperature deformation of sand, type wall displacement.
(3) the production of large castings, there is no hot metal liquid on the riser to make pouring will produce shrinkage defects. Can be used as required, the use of heating riser or riser instead of pouring.

10. Others
Sand core or wall fracture can cause burr or type leakage, resulting in the loss of molten metal. At this point the number of metal cavity on the demand for liquid metal has changed, if there is no excess liquid metal can be filling, or filling the lack of molten metal, will produce shrinkage defects.