Metal casting processes generally are not easy processes. I might sound easy because the general principle of metal casting processes depend on, melting metal then pouring it into molds. If you visited our articles about expandable-mold metal casting processes and permanent mold metal casting processes, you will understand that there are lot of kinds of differences between different casting processes. And the process parameters are also very different and tough for each casting process types.
Because of this difference in both parameters and processes itself, defects can occur in different ways. Preventing casting defects is one of the toughest topic of casting processes. In here, we explain these general defects that are occuring in general metal casting processes below.
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 occur in metal casting processes because of the difference in volume between the liquid state and solid state of metals. In liquid state, the volume of metal will be higher that solid state. When metal transforms into solid phase in solidification in casting, internal voids can occur because of this volume reduction.
Solution Of Shrinkage: A proper riser design for casting process will generally solve this problem.
Misruns In Casting
Misruns are general problems in casting processes that are generally occuring because of the unsuitable gating and mold design, insufficient fludity of molten metal, very thin sections in mold and low pouring temperature of molten metal which is being poured.
Solution Of Misruns: You might undertstand 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 occur in metal casting processes.
Microporosity is an actually caused by local shrinkage which leads to contraction of unsolified sections near the solidified sections dendritically. Because of this localized contractions, little porosities inside the volume may occur.
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 mold cavity.
This is a casting phenomenon generally occurs. Splattering of metal during the pouring of metal created metal globules which will be entrapped inside mold.
Solution Of Cold Shots Problem: A proper gating system design must be accomplished to prevent the formation of metal globules via splattering.
In metal alloys, generally two types of metals are used. When there is insufficient mixing of liquid metals, fludity of a metal can prevent the fluxing of other metal that has lowe fludity.
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 in sufficient pouring temperature.
Hot tearing problem takes place in specific shapes of molds. When metal poured and solified inside mold, shrinkage of metal inside mold can cause some crackings because of the tensile stresses occur on part because of the mold shape. Hot tearing is also called as ‘hat cracking’.
Solution Of Hot Tearing Problem: In expandable-mold casting processes, mold is collapsed 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-turbulance flux of molten metal inside mold is important for preventing sand wash problem.
This is an also another problem occurs becuase of the insufficient strength of mold. Generally, molten metal infiltrates into crackings of mold.
Solution Of Mold Crack Problem: Solution is actually straightforward. Sufficient strength must be established for mold structure.
This problem is also related to the sand casting operations. Because of the entrapped gases inside molten metal and mold cavity which is filled with molten metal, pores can be see beneath the surface of metal parts.
Solution Of Pinholes: The permability of mold sand must be sufficient for evacuation of gases. Also small holes for venting inside the mold cavity can be drilled on sand molds.
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 metal part as mixture of sand and cast metal.
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.
This is the most crtitical sand casting defect. In sand blow defect, there can be big voids just beneath the surface of solidified metal parts. This is generally caused by mold gasses entrapped inside the mold.
Solution Of Sand Blow Problem: The moisture content of mold sand must be optimized correctly. Low permeability is an also reason.
Becuase of the buoyancy of core material inside high density molten metal, core can be displaced vertically inside sand mold.
Solution Of Core Shift Problem: Required consolidation of core placement regions must be done to prevent core shift problem.
At the inner surfaces of mold, mold sand can take a shape of crust which can penetrate inside molten metal.
Solution Of Scabs: More rigid design of sand mold could prevent this problem.
Mold shifting is an external problem caused by the displacement or misplacement of cope or drag according to each other on parting line.
Solution Of Mold Shift Problem: Placement of cope and drag must be double checked before the pouring of molten metal.
These are the promlems and defects that generally encountered in sand casting and other metal casting methods. You can compensate all these problems with the application of proper solution to them. You need to assess the solution according to your system and considering your own parameters.
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