lecture 4 mold defects design considerations

Lecture 4Foundry Processes

Successful students will be able to know:

• Know the types of defects common in sand mold.

• the different types of inspection methods for foundry processes.

• common metals used for casting.• guidelines and considerations in

designing casting parts.

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Foundry Processes

Sand Mold DefectsSome defects are related to the use of sand molds and therefore they occur only in sand castings.

1. Sand Blow• This defect consists of a balloon-

Shaped gas cavity caused by release of mold gases during pouring.

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Foundry Processes

Sand Mold Defects1. Sand Blow• It occurs at or below the casting

surface near the top of the casting. • Low permeability, poor venting, and

high moisture content of the sand mold are the usual causes.

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Foundry Processes

Sand Mold Defects2. Pinholes • A defect similar to a sand blow involves

the formation of many small gas cavities at or slightly below the surface of the casting.

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Foundry Processes

Sand Mold Defects3. Sand Wash• A wash is an irregularity in the surface

of the casting that results from erosion of the sand mold during pouring.

• The contour of the erosion is imprinted into the surface of the final cast part.

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Foundry Processes

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Foundry Processes

4. Scabs• This is a rough area on the surface of

the casting due to encrustations of sand and metal.

• It is caused by portions of the mold surface flaking off during solidification and becoming embedded in the casting surface.

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Foundry Processes

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Foundry Processes5. Penetration• When the fluidity of the liquid metal is

high, it may penetrate into the sand mold or sand core.

• After freezing, the surface of the casting consists of a mixture of sand grains and metal.

• Harder packing of the sand mold helps to alleviate this condition.

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Foundry Processes

6. Mold Shift• This is manifested as a step in the cast

product at the parting line caused by sidewise displacement of the cope with respect to the drag.

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Foundry Processes

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Foundry Processes

7. Core Shift• A similar movement can happen with

the core, but the displacement is usually vertical.

• Core shift and mold shift are caused by buoyancy of the molten metal.

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Foundry Processes

8. Mold Crack• If mold strength is insufficient, a crack

may develop into which liquid metal can seap to form a fin on the final casting.

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Foundry Processes

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Foundry ProcessesInspection Methods

1. Visual inspection2. Dimensional measurements3. Metallurgical, chemical, physical and

other testsa) Pressure testb) Radiographic methods, magnetic

particle tests, use of fluorescent penetrants, and supersonic testing to detect either surface or internal defects in the casting.

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Foundry Processes

Inspection Methods

4. Mechanical testing to determine properties such as tensile strength and hardness. If defects are not serious, use welding, grinding, or other savage methods to save the casting.

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Foundry Processes

Metals for Casting1. Ferrousa) Gray cast ironb) Nodular ironc) White cast irond) Malleable iron

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Foundry Processes2. Nonferrous Casting

Alloysa) Aluminum alloysb) Magnesium alloysc) Copper alloysd) Tin-based alloyse) Zinc alloysf) Nickel alloysg) Titanium alloys

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Foundry ProcessesDesign Considerations in Casting Parts

1. Risers• The riser must not solidify before the

casting; • The riser volume must be large enough to

provide sufficient liquid metal to compensate for shrinkage in the casting;

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Foundry ProcessesDesign Considerations in Casting Parts

• Junctions between the casting and feeder should not create a hot spot where shrinkage porosity can occur;

• Risers must be placed so that liquid metal can be delivered to locations where it is most needed;

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Foundry Processes

Design Considerations in Casting Parts

• There must be sufficient pressure to drive liquid metal into locations in the mold where it is needed;

• The pressure head from the riser should suppress cavity formation and encourage complete filling of cavities.

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Foundry Processes

2.Corners, angles, and section thickness. • Sharp corners, angles, and fillets should be

avoided, because they may cause cracking and tearing during solidification of the metal.

• Fillet radii should be selected to reduce stress concentrations and to ensure proper liquid-metal flow during the pouring process.

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Foundry Processes

• If the fillet radii are too large the volume of the material in those regions will also be large, and, consequently, the rate of cooling will be lower.

• Section changes in castings should smoothly blend into each other.

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Foundry Processes3. Flat Areas• Large flat areas (plain surfaces) should

be avoided, as they may warp, because of temperature gradients during cooling, or develop poor surface finish, because of uneven flow of metal during pouring.

• Flat surfaces can be broken up with ribs and serrations.

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Foundry Processes4.Parting Lines • The parting line should be along a flat plane,

rather than a contoured plane. Whenever possible, the parting line should be at the corners or edges of castings, rather than on flat surfaces in the middle of the casting.

• In this way, the flash at the parting line (i.e., the material running out between the two halves of the mold) will not be as visible.

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Foundry Processes4. Parting Lines• The location of the parting line is

important, because it influences mold design, ease of molding, number and shape of cores, method of support, and the gating systerm.

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Foundry Processes

4. Parting Lines• Preparation of dry-sand cores requires

additional time and cost, so these cores should be avoided or minimized; this consideration can usually be made by reviewing and simplifying the design of castings.

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Foundry Processes

5. Draft • A small draft (taper) is provided in sand-

mold patterns to enable removal of the pattern without damaging the mold.

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Foundry Processes

5. Draft• Depending on the quality of the pat-

tern, draft angles usually range from 0.5° to 2°. The angles on the inside surfaces of molds are typically twice this range; they have to be higher than those for outer surfaces, because the casting shrinks inward toward the core.

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Foundry Processes

6. Dimensional tolerances. • Dimensional tolerances depend on the

particular casting process, size of the casting, and type of pattern used.

• Tolerances are smallest within one part of the mold and, because they are cumulative, increase between different parts of the mold.

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6. Dimensional tolerances. • Tolerances should be as wide as

possible, within the limits of good part performance; otherwise, the cost of the casting increases.