The process by which metal goes from a liquid to a solid causes it to undergo a process known as solidification shrinkage. Once it reaches room temperature, metal also experiences additional thermal contraction. Cast parts are consequently designed with allowances for shrinkage so that the finished parts will have the desired dimensions. Cast steel, for example, will shrink by about a quarter of an inch per foot and produce castings that have a rough appearance. When designing a mold, an experienced mold designer will take into account the metal's known shrinkage allowances and will do so because shrinkage allowances are known for a variety of metals. Please read our article on the various processes used in casting if you would like more information on casting in general. In addition, shrinkage can cause defects in cast products, which can ultimately result in failure, leakage, and other undesirable outcomes. Sometimes these flaws will manifest themselves on the surface of the casting, and they can be discovered visually, through the use of dye penetrant, or through other non-destructive procedures. Sometimes the flaws are located within the casting itself, and in order to discover them, an X-ray inspection or destructive testing is necessary. The term "open and closed shrinkage defects" refers to both of these types of flaws collectively.
Pipes may form on the surface of the casting and extend into the body of the casting if the metal cools and contracts while there is insufficient liquid available to fill any voids caused by the contraction
Cave defects are another name for defects that begin on the surface and spread across the face
In a similar vein, sinks are another name for cave defects
In both cases, the defects are exposed to the surrounding air, and the molten metal has been replaced by the surrounding air
Cracks and hot tears usually form in the final stages of solidification, and they can be localized around abrupt changes in areas where there is a concentration of stress, such as a thin web connecting two heavy sections. Cracks and hot tears can also form during the final stages of solidification. In areas of the part where there is insufficient draft, as well as in heavy sections where heat pools, they are also able to occur. Defects Caused by Closed Shrinkage Porosity is one of the most common flaws in castings, and it can be caused in two different ways: first, by gases that become trapped in the molten metal, and second, by the casting itself shrinking as it cools. It is possible to identify shrinkage porosity on the surface of a cast part by looking for what looks like small holes or cracks. This type of porosity is by far the most common type. Although they may have the appearance of being round, these holes are actually angular in shape and have a propensity to form branching internal fractures. This type of shrinkage, which takes place when the metal cools and solidifies in a non-uniform pattern, is most likely to affect thick parts with multiple angles. It's possible for there to be porosity within a casting even if it doesn't manifest itself on the exterior, but that's not always the case. This happens when liquid metal is surrounded by solid metal and molten metal cannot fill in behind the liquid as it cools and shrinks. This prevents molten metal from filling in behind the liquid. The casting sprue is the channel through which liquid metal is poured into a mold. Because of this, shrinkage is most frequently caused by factors that are associated with the casting sprue. It takes the metal longer to contract and solidify in some areas, such as the heavy sections of the mold. This decreases the amount of feed material that is available and increases the likelihood that there will be shrinkage, particularly if the sprue is too small for the amount of flow that is being directed through it. The shrinkage cavity can be filled by a properly sized sprue that is attached directly to the heavy section. This will provide the feed material that is necessary to counteract shrinkage as the cooling process takes place. In addition, reducing the risk of forming defects on the sprue by using a rounded gate rather than a flat or square one can be done by rounding the corners of the gate. When filling the cavity with molten metal, using a sprue that is too narrow or too tapered can cause the metal to spray into the cavity rather than pour. When this occurs, particular regions of the workpiece begin to solidify before the mold is completely filled with material. It is important that the flow of molten material into the cavity be as even as is humanly possible. A larger central sprue or an arrangement with multiple sprues can help achieve this even supply of material. The use of risers ensures that a sufficient amount of molten material is available to fill in the spaces left behind by the part as it solidifies and contracts. The risers in a structure ought to have dimensions that make them the very last components to freeze. Insulation is sometimes added to ensure that this is the case. The use of local heat dissipation techniques, such as chills (metal that is inserted in the mold and melts during the pour), can help to reduce shrinkage defects in places where heat has a tendency to pool, such as in thick and heavy sections of the casting. The filling of cavities can be improved with the help of simulation software, which also has the ability to predict the occurrence of shrinkage porosity and maximize the efficiency with which cavities are filled. It is possible to exert control over the movement of material as it passes through the mold by employing methods such as directional solidification and having a well-designed mold.