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Porosity refers to a hole or void in an otherwise solid cast metal part. The size of pores can range from microscopic (micropores) to larger voids, measured in cubic millimetres or more. A porous is not always circular in cross-section, but can also occur in the form of irregular linear cracks.
There are two major causes of porosity: solidification shrinkage and gas porosity. In the following sections, we will explore the root causes of these two types of violence and suggest strategies to fight them.
Mold cavities solidify when the molten metal comes into contact with relatively cooler walls. As the metal solidifies, it shrinks; however, the rate of shrinkage will vary depending on the geometry of the mold tool and the type of alloy.
In this semi-liquid metal, pore formation is more likely to occur further away from the tool wall, which is called slush. The solidified metal might also block the flow of liquids to other areas in the mold once it becomes solidified. It is possible to create pores when preventing full liquefaction of all design features.
A gas pocket can form in a few different ways. With aluminum, hydrogen may fall out of suspension and fill voids with hydrogen gas.
It is possible that the mold tool may have been partially or completely vacated or vented as the cavity filled. Air that is trapped in this manner is called entrained air.
In addition to the liquids mixed with the molten metal, other liquids may be injected along with it. Among these might be mold release agents, hydraulic fluid leaks or even humidity. It is possible to vaporize and create gas bubbles from any of these substances. A liquid or oil that does not evaporate can become a contaminant which can lead to particle inclusions in the final product.
• Pinholes
There are very small holes in the casting about 2 mm in size that appear on the surface. In molten metal, hydrogen gases dissolve, resulting in this defect. During the solidification process, as the solubility of hydrogen gas decreases in the molten metal, it escapes through tiny holes called pinholes. As the molten metal solidifies, the hydrogen gas can no longer dissolve as easily and escape.
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• Blowholes
Blowhole is formed when gases are trapped on the surface of casting because of solidifying metal. The defect always occurs in the cope part of the mold.
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• Shrinkage Cavity
During volumetric contraction, a hollow spot forms in a cast. This is referred to as a shrinkage cavity.
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• Swell
The molten metal pressure results in an enlargement of the mold cavity, which results in either localised or overall enlargement of the casting.
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• Drop
Drop defects occur when sand pieces fall into molten metal due to cracks on the upper surface.
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• Sand Holes
This refers to the holes made on the casting's exterior or internally. It occurs when loose sand is washed into the cavity of a mold and sets up or when molten metal is quickly poured into a mold cavity.
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• Dirt
A dirt defect is caused when dust and sand embed themselves in the casting surfaces.
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• Honeycombing / Sponginess
It is an external defect characterized by many small cavities closely adjacent to one another present in metal casting.
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• Hot Tears / Hot Cracks
Castings fail when molten metal cools because of residual stress (tensile) left in the metal when it's hot. In this case, casting failure appears as cracks or hot tears.
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• Hot Spot / Hard Spot
The hot spot defect occurs when an area on the casting cools faster than the surrounding materials. The hot spots on a casting are areas which are harder than the nearby areas. Alternatively, it is referred to as a hard spot.
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• Metal Penetration
A rough and uneven surface of the casting is indicative of these casting defects. If the size of the sand grains is large, the molten metal will fuse into the sand and solidify, causing a metal penetration defect.
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• Cold Shut / lap
The defect appears as a line on the surface. A cold shut occurs when two streams of molten metal meet at the junction of the mold with low temperatures and do not fuse together (appear as lines on the casting). This appears to be a crack with rounded edges.
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• Misrun
This occurs when the molten metal solidifies before fully filling a mold cavity and leaves a space. when the molten metal solidifies before completely filling the mold cavity and leaves a space in the mold called misrun.
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• Shift or Malmatch
A defect as a result of the upper and lower parts of the casting being misaligned and the core being misplaced at the parting line.
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• Slag Inclusion (scab)
The defect occurs when slag particles and molten metal are poured into the cavity of the mold and then solidified.his defect is caused when the molten metal containing slag particles is poured into the mold cavity and solidifies.
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• Warpage
During or after solidification, it causes an uncontrolled deformation in the casting. Consequently, the product dimension changes as a result of this defect.
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It can be prevented by producing large areas with wavy, corrugated construction, or add sufficient rib-like shape, to provide equal cooling rates in all areas.
Proper casting designs can reduce these defects more efficiently.
• Fins
Fins or fins are thin projections of metal that are not considered as part of the casting process. This usually occurs when the mold or core section is being parted.
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ProCAST is an outstanding finite element solution for simulation of metal casting processes. Designing and developing processes with this tool will help improve yields and casting quality.
With comprehensive finite element solvers, accurate and efficient output, ProCAST meets the manufacturing industry’s needs. By reducing manufacturing costs, improving mold development time and improving casting quality, the software is the ideal alternative to traditional trial-and-error methods.
In addition to predicting residual stress, ProCAST is an expert at predicting deformations and it can also be used for more specific processes, such as sand casting, investment casting, die casting, lost foam casting, etc.
Simulations help foundries analyse how metals expand and contract in the process of casting, so they can predict where shrinkage porosity will be located. There are several possible defects that can occur during a casting process, causing a final product to be imperfect. Therefore, an effective gating and feeding system is essential for reducing or eliminating these defects.
By seeing how castings fill and solidify within a mold, manufacturers can validate and improve the locations and sizes of the down sprue, runners, gates, vents, chills, and feeders. ProCAST is used as a tool to simulate casting processes and ensure that molds are made correctly the first time.
With Omnidex ProCAST Software, our customers will not recevie any scrap cast parts in the future. ProCAST has served as their go-to tool to predict porosity, filling, and solidification of castings for decades. It can also predict complex issues like deformations and residual stresses, and it is capable of dealing with most castable alloys, most casting processes such as sand castings, die castings, investment castings, and the variants that go with each process.
There are no 100% perfect casting processes. Porosity in metals must be considered despite excellent properties.
It is almost always sand casting that is the most favorite and popular foundry process. The ability of sand casting to casting large volumes, high-quality and low-cost make it an exceptional machining process. Yet, examination of sand casting defects is vital to determining the customer’s tolerance as well as expectations.
We at Omnidex are transparent about casting defects that can occur during the process with our customers. We always strive to minimize the number and severity of casting defects so that we can supply the best quality products.
Furthermore, we monitor the gates and pouring process continuously in order to ensure that they are working properly in accordance with each casting requirement. In this way, we can ensure our customers receive the best quality of metal casting services and products.
The Omnidex Casting team is committed to providing quality machining and manufacturing services with no compromise. If you are interested in our metal casting services, feel free to contact us for a quick quote.
Contact our Customer Service and let us know how we can help to catapult your next project to success.
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