- 1 Types of Casting Defects
- 1.1 Gas porosity casting defects
- 1.2 Shrinkage casting defects
- 1.3 Mold material casting defects
- 1.4 Metallurgical defects
- 1.5 Casting shape defects
Types of Casting Defects
Different types of casting defects are unwanted occurrence 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.
Avoiding any of this types of casting defects, one needs to have clear understanding on its types and their remedies. This defects are less when technical level of each operation is perfectly done. That could be achieved by skills of the operator, management quality, and good and maintained equipment.
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 cool 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 are not allow dry before used. 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.
These blowholes appear on the surface of the casted metal which makes it easier to detect than of the subsurface blowholes.
Shrinkage casting defects
This types of casting defects occurs during the solidification of the casting. It porosity appears with angular edges when compared with round surfaces of gas porosity. Shrinkage casting defects happens because metals are less dense as a liquid than a solid. these branch of casting defect might also be paired with cracks. Shrinkage with large cavities can cause the casting to eventually break under stress.
This defects should always be expected during solidification. So, one should factor shrinkage allowance into the pattern design during casting. Specifically alloys, they shrink when changing from molten to solid because it density in a molten state is lower than that of solid state. One can prevent shrinkage casting defects by improving the overall casting structure which includes, designing a running (gate) system with a 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 hit liquid. Closed shrinkage defects is formed within the casting. Some can be seen with bear 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.
fusion is a thin crust with a brittle, glassy appearance firmly joined to the casting. This is caused when sand grains 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 use during casting due to it 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 temperature of the pouring metal and improving the refractoriness of molding materials.
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 it can be prevented by designing the casting mold with precision. One should also inspect and repair any defective molds before casting. Reducing temperature of molten metal can stop wears and tears of the mold. Using quality raw materials for mold work will help in resisting high temperature.
these casting occurs 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 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 metals drop into the liquid metal. It’s 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.
Rat tail, Veins and Buckles:
these casting defects occur in the casting as an irregular line or cracks. The rat tail appear on casting when hotness of the metal causes the sand to expand which may be caused by poor expansion properties of the sand, hot pouring temperature of metal, and poor design.
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these casting defects occur when molten metal penetrate gaps in the molding sand. These defects appear as 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 comes in two types, they are, Hot tear and hand spots. This casting defects may fall in any of the two.
these casting defects take place in the branches pattern as narrow cracks. These cracks are obvious while some required. These cracks are seen after the casting cools, when it’s about to solidify. 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. These spot cools quickly than the surrounding material. Hard spots casting defects are results of improper cooling practice and can he avoided by correcting cooling practice and considering changing the metal’s chemical composition. The casting defects can interfere during machining and increases tool wear.
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 crack with a round edge on the casting surface. The defect leads to 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 casting defects is 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 is the 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 pouring of the liquid metal. These cold shut are typically ball, drop or pearl shaped. Preventing 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 naked eye. Scabs are usually sharp edges, irregular shapes and are firmly bonded to the casting. Removing scabs from the cast will exposed a rat tail underneath. These casting detects is caused when molten metal are containing slag particles is poured into the mold cavities and solidifies together. Slag can be removed in several ways which includes: melting the metal with a 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
This types of casting defects occur on the overall shape of the casting. These include:
these casting defects occurs when the mold shift due to misalignment of cope (upper) and drag (lower) part of the mold. Mold shift is usually reflected as a horizontal displacement. These casting defects occurs when the box pins looses, 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 cause due to insufficient weight on the mold or improper clamping of the flask can lead to 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. This types of defects take place during solidification. They are caused by different rate of solidification of different sections, which causes stress in the adjoining walls. Heat treatment can help in removing the residual stress in iron casting.
That is it for this article, types casting defects and their remedies. You can ask question or add to the knowledge via our comment box, I think that will be helpful to others. Or you can easily answer these any these questions using the comment box:
- Gas porosity casting defects are caused by what?
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