Most Encountered Metal Casting Defects And Solutions

Metal casting processes generally are not easy processes. It might sound easy because the general principle of metal casting processes depends on, melting metal then pouring it into molds. If you visit our articles about expandable-mold metal casting processes and permanent mold metal casting processes, you will understand that there are a lot of kinds of differences between different casting processes. And the process parameters are also very different and tough for each casting process type.

Because of this difference in both parameters and processes themselves, defects can occur in different ways. Preventing casting defects is one of the toughest topics of casting processes. Here, we explain these general defects that are occurring in general metal casting processes below.

Fundamentals of Modern Manufacturing: Materials, Processes and Systems, Seventh Edition

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What Are The Metal Casting Defects?

You can find information about 14 casting defects that are encountered mostly in metal casting processes generally.

Shrinkage Cavity Problem

Shrinkage is a phenomenon that occurs in metal casting processes because of the difference in volume between the liquid state and the solid-state of metals. In the liquid state, the volume of metal will be higher than in the solid-state. When metal transforms into the solid phase in solidification in casting, internal voids can occur because of this volume reduction.

Shrinkage problem illustration.

Solution Of Shrinkage:  A proper riser design for the casting process will generally solve this problem.

Misruns In Casting

Misruns are general problems in casting processes that are generally occurring because of the unsuitable gating and mold design, insufficient fluidity of molten metal, very thin sections in mold, and low pouring temperature of molten metal which is being poured.

Illustration of misrun.

Solution Of Misruns: You might understand the solution actually above information about misrun reasons. Proper design of mold sections, good calculation of required pouring temperature of molten metal, and suitable design of gatings will solve the problem of misruns occurring in metal casting processes.

Microporosity Problem

Microporosity is caused by local shrinkage which leads to contraction of unsolidified sections near the solidified sections dendritically. Because of these localized contractions, little porosities inside the volume may occur.

Microporosity on cast metals.

Solution Of Microporosity: In casting methods, you need to design the solidification manner of metals also. You need to use chillers for critical sections inside the mold cavity.

Cold Shots

This is a casting phenomenon that generally occurs. Splattering of metal during the pouring of metal created metal globules that will be entrapped inside the mold.

Illustration of cold shots(Image Source: Tfgusa).

Solution Of Cold Shots Problem: A proper gating system design must be accomplished to prevent the formation of metal globules via splattering.

Cold Shuts

In metal alloys, generally, two types of metals are used. When there is insufficient mixing of liquid metals, fluidity of metal can prevent the fluxing of other metal that has low fluidity.

Cold shut illustration(Image Source: Mechanical Booster).

Solution Of Cold Shuts: Actually, you can find the answer from the information about cold shuts above. You need to mix portions of different metals at sufficient pouring temperature.

Hot Tearing

The hot tearing problem takes place in specific shapes of molds. When metal is poured and solidified inside the mold, shrinkage of metal inside mold can cause some crackings because of the tensile stresses that occur on part because of the mold shape. Hot tearing is also called ‘hat cracking’.

Various versions of hot tear problems.

Solution Of Hot Tearing Problem: In expandable-mold casting processes, mold is collapses after the full solidification is completed. In permanent-mold casting processes, part is removed immediately after solidification. Because hot tearing takes place in the early stages of cooling after solidification.

Sand Wash Problem

Sand wash problem is generally encountered in sand casting operations. The form of sand mold surfaces can be eroded because of the hot fluxing metal. And this erosion can be seen on the surface of metal after solidification.

Solution Of Sand Wash Problem: Obtaining required mold strength and providing non-turbulence flux of molten metal inside the mold is important for preventing sand wash problems.

Mold Crack

This is also another problem that occurs because of the insufficient strength of mold. Generally, molten metal infiltrates into crackings of mold.

Mold crack problem(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution Of Mold Crack Problem: The solution is straightforward. Sufficient strength must be established for mold structure.

Pinholes

This problem is also related to sand casting operations. Because of the entrapped gases inside molten metal and mold cavity which is filled with molten metal, pores can be seen beneath the surface of metal parts.

Solution Of Pinholes: The permeability of mold sand must be sufficient for the evacuation of gases. Also, small holes for venting inside the mold cavity can be drilled on sand molds.

Place of pinholes in mold(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217)

Penetration Problem In-Mold

When the fluidity of molten metal is high, it can penetrate the inside surface of the sand mold. The defect of penetration can be seen at the surface of the metal part as a mixture of sand and cast metal.

Penetration of molten metal inside sand(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217)

Solution Of Penetration Problem In-Mold: Strick packing of sand mold is important to alleviate the penetration problem. This will increase the strength of sand mold.

Sand Blow

This is the most critical sand casting defect. In sand blow defects, there can be big voids just beneath the surface of solidified metal parts. This is generally caused by mold gasses entrapped inside the mold.

Sand blow illustration(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution Of Sand Blow Problem: The moisture content of mold sand must be optimized correctly. Low permeability is also the reason.

Core Shift

Because of the buoyancy of core material inside high-density molten metal, the core can be displaced vertically inside the sand mold.

Lateral view of core shift(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution Of Core Shift Problem: Required consolidation of core placement regions must be done to prevent core shift problems.

Scabs

At the inner surfaces of mold, mold and can take a shape of crust which can penetrate inside the molten metal.

Scab(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution Of Scabs: More rigid design of sand mold could prevent this problem.

Mold Shift

Mold shifting is an external problem caused by the displacement or misplacement of cope or drag according to each other on parting line.

Cope and drag shift illustration(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution Of Mold Shift Problem: Placement of cope and drag must be double-checked before the pouring of molten metal.

Conclusion

These are the problems and defects that are generally encountered in sand casting and other metal casting methods. You can compensate for all these problems with the application of the proper solution to them. You need to assess the solution according to your system and consider your parameters.

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