Different types of casting defects are unwanted occurrences on a casting during the casting process. It may lead to bad shape, weak casting, and poor functionality. Casting defects are broken down into five main categories: gas porosity, shrinkage, mold material, pouring metal, and metallurgical Casting defects. Some casting defects can be repaired while some are tolerated.
To avoid any of these types of casting defects, one needs to have a clear understanding of its types and their remedies. These defects are less when the technical level of each operation is perfectly done. That could be achieved by skills of the operator, management quality, and good and maintained equipment.
- 1 Types of Casting Defects
- 1.1 Gas porosity casting defects
- 1.2 Shrinkage casting defects
- 1.3 Mold material casting defects
- 1.4 Join our Newsletter
- 1.5 Metallurgical defects
- 1.6 Pouring defects
- 1.7 Casting shape defects
- 1.8 Kindly Share!
Types of Casting Defects
The following are types of casting defects and their remedies:
Gas porosity casting defects
Gas porosity takes place on casted metal when it traps gasses like nitrogen, oxygen, or hydrogen during casting. The casting defect appears on metals as small holes, rounded, or circular cavities. The holes are formed when the casting cools and solidifies because the solid form of metal cannot store a lot of gas as of the liquid form.
Gas porosity casting defects are caused by an escape route on mold and cores at which gases can pass and when mold and cores do not allow dry before use. Gas porosity casting defects are classified into three:
pinholes are very tiny holes found in the cope outer part of the mold, in a poorly vented pocket. These casting defects are sometimes referred to as porosities. Pinholes usually appear in large numbers at the surface of the casting.
blowholes are larger cavities located inside the casted metal. These types of gas porosity casting defects are difficult to locate before machining and it requires harmonic, ultrasonic, magnetic or x-ray analysis.
Read more: How sand casting is done
These blowholes appear on the surface of the casted metal which makes it easier to detect than the subsurface blowholes.
Shrinkage casting defects
These types of casting defects occur during the solidification of the casting. Its porosity appears with angular edges when compared with round surfaces of gas porosity. Shrinkage casting defects happen because metals are less dense as a liquid than a solid. these branches of casting defect might also be paired with cracks. Shrinkage with large cavities can cause the casting to eventually break under stress.
Read more: 4 Different Types of Metal Casting Process
These defects should always be expected during solidification. So, one should factor shrinkage allowance into the pattern design during casting. Specifically, alloys, shrink when changing from molten to solid because their density in a molten state is lower than that of a solid state. One can prevent shrinkage casting defects by improving the overall casting structure which includes, designing a running (gate) system with risers to enables a continuous flow of molten metal, reduce casting temperature to reduce the total volume deficit, and by increasing local heat dissipation by inserting cooling ribs or coils, and internal chills. Shrinkage types of casting defects are classified into two:
Open Shrinkage defects:
These casting defects are located at the surface as pipes and burrow into the casting. It is open to the atmosphere and allows air to enter as the shrinkage cavity is formed.
Closed shrinkage defects:
these casting defects are also known as shrinkage porosity. It usually appears at the top of the hot spots or isolated pools of the hot liquid. Closed shrinkage defects are formed within the casting. Some can be seen with the bare eye while but some cannot.
Mold material casting defects
Mold material casting defects occurs on sands and can always be prevented by modifying the mold. These casting defects can affect the cast in several ways:
Cuts and washes:
these casting defects occur when a molten metal erodes the molding sand. It is shown on the casting as areas of excess metal. Cuts and washes can be caused by a molten metal flowing at an extremely high velocity, allowing the flow of too much metal through the gate. Cuts and washes are prevented when the gating system is properly designed, improving the mold and core strength and adding more binders to the facing and core sand.
Read more: Different Types of molding sand
fusion is a thin crust with a brittle, glassy appearance firmly joined to the casting. This is caused when sand grains are fused with molten metal. It takes place when molten metal with extreme hot velocity is poured and low refractoriness of clay or sand. Silica sand is mostly used during casting due to its high refractoriness. Remember, refractoriness is the ability of molding material to resist the temperature of liquid metal to avoid fusion. To prevent fusion casting defects is by reducing the temperature of the pouring metal and improving the refractoriness of molding materials.
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these casting defects occur when the molten metal escaped through the mold, which could lead to incomplete or missing casting. Run-out casting defects are caused by the mold or flask and they can be prevented by designing the casting mold with precision. One should also inspect and repair any defective molds before casting. Reducing the temperature of molten metal can stop the wear and tear of the mold. Using quality raw materials for mold work will help in resisting high temperatures.
these casting defects occur on the vertical face of the castings caused by low strength mold and improper or bad ramming of the mold. To prevent swell casting defects, mold should be built to withstand the pressure of liquid metal. Otherwise, the mold shape changes by moving a bit back and causes swells. These casting defects can also be prevented when the mold is properly rammed.
these casting defects occur when the casting is still liquid. It is caused when pieces of metal drop into the liquid metal. It appears as an irregular-shaped projection on the surface of the casting. Using sand with lower strength, soft ramming, insufficient fluxing of molten metal, and absence of reinforcement of sand projection in the cope can lead to drops casting defects.
Rattail, Veins, and Buckles:
these casting defects occur in the casting as irregular lines or cracks. The rat tail appears on casting when the hotness of the metal causes the sand to expand which may be caused by poor expansion properties of the sand, the hot pouring temperature of the metal, and poor design.
These casting defects occur when molten metal penetrate gaps in the molding sand. These types of casting defects appear as the rough and uneven surface finish of the casting which is visible to the naked eye. Metal penetration is caused by various issues which include: using sand with low strength and high permeability, using large or coarse sand grain, soft ramming of sand, and lack of mold wash. All these can be prevented when high strength, small grain size, low permeability, and hard ramming of sand. Metal penetration can be removed from the surface of the casting by grinding down the affected parts.
Metallurgical casting defects come in two types, they are, Hot tears and hand spots. These types of casting defects may fall in any of the two.
these casting defects take place in the branching pattern as narrow cracks. These cracks are obvious while some are required. These cracks are seen after the casting cools when it’s about to solidify. The hot tear occurs if the casted metal does not have sufficient strength to resist tensile forces during solidification and it is mostly caused by poor mold design.
Hot/ hard spots:
these casting defects are spots that are harder than the surrounding area. This spot cools quickly than the surrounding material. Hard spots casting defects are results of improper cooling practice and can be avoided by correcting cooling practice and considering changing the metal’s chemical composition. The casting defects can interfere during machining and increase tool wear.
Read more: Types of foundry patterns (casting patterns)
Pouring casting defects occurs during the pouring process. These defects are categories into four:
Cold shut/ lap:
these casting defects appear on the surface of the casting. It occurs as a line or cracks with a round edge on the casting surface. The defect leads to a full rejection of casting because it creates a weak spot. It is caused when molten metal enters the mold from two gates, the steams will meet at a junction. Low temperatures can prevent fusion at the junction, causing the steams to solidify before fusion, creating a cold shut. These types of casting defects are usually caused as a result of lacking fluidity of the molten metal, or a poor design of the gating system, which is prevented by increasing fluidity of the molten metal.
these casting defects are closely related to cold shut defects. It occurs when the liquid metal is too cold to flow through the mold cavity before freezing and solidifying. The liquid metal does not fill the mold cavity. The misrun casting defect is an unfilled portion of the mold. To prevent misrun casting defects, the molding design, gating system design, and molten metal fluidity have to be check.
these casting defects are solid globules of the surface of the casting. It is caused by the splattering during the pouring of the liquid metal. These cold shuts are typically ball, drop, or pearl-shaped. To prevent splattering and cold shots, the pouring procedures should be modified, and adjusting the gating system designs by reducing the gate speed.
Slag inclusion (scab):
these casting defects are irregular metallic crusts located on the casting surface. They are typically thick but can be tough and seen with the naked eye. Scabs are usually sharp edges, irregular shapes and are firmly bonded to the casting. Removing scabs from the cast will expose a rat tail underneath. These types of casting defects are caused when molten metal is containing slag particles is poured into the mold cavities and solidifies together. Slag can be removed in several ways which include: melting the metal with flux in a vacuum, inert atmosphere, or adding a ceramic filter into the gating system.
Also check: Different types of Foundry tools and equipment
Casting shape defects
These types of casting defects occur in the overall shape of the casting. These include:
these casting defects occur when the mold shift due to misalignment of the cope (upper) and drag (lower) parts of the mold. Mold shift is usually reflected as a horizontal displacement. These casting defects occur when the box pins loose, inaccurate pattern dowel pins, or carelessness in placing the cope on the drag, leading to misalignment. To avoid misalignment, these three causes must be examined.
Flash, Fin, and Burrs:
these casting defects are one of the most occurring casting defects. This defect is also called fin and burrs which are unwanted or excess materials attached to the cast. Flash is a waste material that turns into dross after being melted. Flashes are caused due to crack and gap on the core surface which is caused due to insufficient weight on the mold or improper clamping of the flask can lead to the gap. To avoid flash, there should be enough weight on the top part of the mold so the two parts fit together tightly.
these casting defects are unwanted casting deformity that occurs over time that may lead to change in dimensions of the product. These types of defects take place during solidification. They are caused by the different rates of solidification of different sections, which causes stress in the adjoining walls. Heat treatment can help in removing the residual stress in iron casting.
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