Metal shrinks as it transforms from liquid to solid (known as solidification shrinkage) and undergoes additional thermal contraction as it cools to room temperature. Cast parts are therefore designed with shrinkage allowances to result in parts of the desired dimensions. Cast steel, for example, will shrink about ? in. per foot and produce rough-looking castings. Shrinkage allowances are known for various metals and an experienced mold designer will take the shrinkage into account when designing a mold.
Shrinkage can also cause defects in cast products which can lead to failure, leakage, and so on. Sometimes these defects appear on the surface of the casting and can be detected visually or through dye penetrant or similar non-destructive procedures. Sometimes the defects are internal to the casting and require X-ray inspection or destructive tests to uncover. These defects are collectively known as open- and closed-shrinkage defects.
Open Shrinkage Defects
If the metal cools and shrinks with insufficient liquid available to fill any voids, pipes may form in the surface and extend into the body of the casting. Similarly, defects that form on the surface and spread across the face are sometimes called cave defects, or sinks. In both instances, the defects are open to atmosphere and air takes the place of molten metal.
Cracks and hot tears usually form in the final stages of solidification and can be localized around abrupt changes where stress concentration exists such as a thin web connecting two heavy sections. They can also occur where insufficient draft in the part exists and in heavy sections where heat pools.
Closed Shrinkage Defects
Porosity is one of the main defects in castings and is caused by both trapped gases in the molten metal and as the casting shrinks during cooling. Shrinkage porosity is by far the most common type and can usually be detected on the surface of a cast part by what appear to be small holes or cracks. These holes may seem round but are actually angular in shape and tend to form branching internal fractures. Thick multi-angled parts are most susceptible to such shrinkage, which occurs as the metal cools and solidifies in a non-uniform pattern. Porosity can exist in the interior of a casting as well, without necessarily showing on the surface of a part. This occurs when liquid metal is surrounded by solidified metal and molten metal is unable to fill in behind the liquid as it cools and shrinks.
The most common causes of shrinkage are related to the casting sprue, which is the passage through which molten metal is poured into a mold. In some areas, such as the heavy sections of the mold, the metal takes longer to contract and solidify, which reduces feed material availability and increases the likelihood of shrinkage, especially if the sprue is too small for the volume of flow. A properly sized sprue attached directly to the heavy section can fill the shrinkage cavity and provide the feed material necessary to counteract shrinkage as cooling occurs. In addition, using a rounded, rather than a flat or square, gate on the sprue can further reduce the risk of forming defects.
Using a narrow or tapered sprue can result in the molten metal being sprayed rather than poured into the cavity. When this happens, certain sections of the workpiece begin to solidify before the entire mold is filled. Molten flow into the cavity should be as uniform as possible, and a larger central sprue or a multiple-sprue arrangement can help achieve the even supply of material.
Risers are used to ensure that sufficient molten material is available to fill in where the part is solidifying and shrinking. Risers should be sized such that they are the last parts to freeze. Sometimes insulation is added to ensure this.
Shrinkage defects can be reduced by employing local heat dissipation, such as chills (metal inserted in the mold that melts during the pour), in areas where heat tends to pool--in thick, heavy sections, for example.
Simulation software can be used to optimize the filling of cavities through improved runner and gate designs and can predict the occurrence of shrinkage porosity. The flow of material through the mold can be controlled by good mold design and employment of techniques such as directional solidification.
Other shrinkage defects include:
? Sponge shrinkage usually arises in the thicker mid-section of the casting product and causes a thin lattice texture similar to filament or dendrites to develop.
? Filamentary shrinkage results in a network of continuous cracks of various dimensions and densities, usually under a thick section of the material. It can be difficult to detect, and the fracture lines tend to be interconnected.
? Dendritic shrinkage fractures are narrow, randomly distributed lines or cavities that are often unconnected. They are typically thinner and less dense than filamentary cracks.
How Temperature Affects Casting Shrinkage
To reduce the potential for metal casting shrinkage, it is helpful to work within a delineated temperature range. Metal should be heated to achieve appropriate molten characteristics, generally to 100°F above its flow point. Any overheating should be avoided.
Maintaining mold temperatures of 800-1000°F below the melting point of the metal is a generally accepted rule. Another useful rule to know is the casting cooling rate, which can be around 100°F per minute after pouring is complete.
Attention paid to the fluidity of the molten pour can also influence the manner in which shrinkage is addressed.
Post cast repairs
A process called vacuum impregnation is sometimes used to fix porosity problems in castings. Here sealants are introduced into the casting by pressure or vacuum to eliminate any leaks due to porosity that runs through a casting. It is used where leaks would otherwise cause a casting to be rejected, in pump and compressor housings, for instance.
ABOUT KEVA
Keva Casting is a professional casting company that focuses on Construction, Machinery, Automobile, Hardware, Pipeline and many other industries.
We provide top quality products with the best service based on our complete production chain including our own mold design, advanced production techniques, and full management of the supply chain. Keva Casting has two factories located in Zhucheng, China's Shandong province.
We use "shell molding", which is a new technique that has been developed in recent years.This new technique has simplified production procedures and reduced air pollution compared with traditional sand casting and investment casting. Shell making and core making are faster to ensure production in enormous quantities. The dimensions and surface finishing are satisfactory for all kinds of industrial requirements. Besides supplying our own formwork & scaffolding systems, our casting products also supply OEM services in various industries.