Manufacturing Chapter 11.5-11.7

Defects possible in all casting processes

1. Misruns 2. Cold shuts 3. Cold shots 4. Shrinkage cavity 5. Microporosity 6. Hot tearing

Inspection tests

1. Pressure testing to locate leaks in a casting 2. Radiographic methods, magnetic particle tests, use of fluorescent penetrants, and supersonic testing 3. Mechanical tests on tensile strength and hardness

Defects possible only in sand casting

1. Sand blow 2. Pinholes 3. Sand wash 4. Scabs 5. Penetration 6. Mold shift 7. Core shift 8. Mold crack

Causes of misruns

1. The fluidity of the molten metal is insufficient 2. Pouring temperature is too low 3. Pouring is done too slowly 4. Cross-section of the mold cavity is too thin

Foundry inspection methods

1. Visual inspection of defects 2. Dimensional measurements to ensure tolerances have been met 3. Metallurgical, chemical, and physical tests to test quality 4.

Geometric simplicity

Although casting is a process that can be used to produce complex part geometries, simplifying the part design will improve its castability. Avoiding unnecessary complexities simplifies mold making, reduces the need for cores, and improves the strength of the casting.

Pinholes

Caused by the release of gas during pouring, consist of many small gas cavities formed at or slightly below the surface of the casting

When splattering occurs during pouring, solid globules of metal can become entrapped in the casting; this defect is called which one of the following?

Cold shot

Mold shift

Defect caused by a sidewise displacement of the mold cope relative to the drag

Core shift

Defect caused by a vertical displacement of the core relative to the drag

Sand blow

Defect consisting of balloon-shaped gas cavity caused by a release of mold gases during pouring. It occurs at or below the casting surface near the top of the casting. Low permeability, poor venting, and high moisture content are the usual cases

Mold crack

Defect that occurs when old strength is insufficient, and a crack develops, into which liquid metal can seep to form a "fin" on the final casting

Hot tearing

Defect that occurs when the casting is restrained from contraction by an unyielding mold during the final stages of solidification or early stages of cooling after solidification

Cold shots

Defects that occur when two portions of metal flow together but there is a lack of fusion between them from premature freezing

Shrinkage cavity

Depression in the surface or an internal void in the casting, caused by solidification shrinkage that restricts the amount of molten metal available in the last region to freeze

Sand wash

Irregularity on the surface of the casting that results from erosion of the sand mold during pouring, and the contour of the erosion s formed on the surface of the final cast part

Machining allowances

Machining allowances. Tolerances achievable in some casting processes are insufficient to meet functional needs in many applications. Sand casting is the most prominent example of this deficiency. In these cases, portions of the casting must be machined to the required dimensions. Almost all sand castings must be machined to some extent in order for the part to be made functional. Therefore, additional material, called the machining allowance, is left on the casting for machining those surfaces where necessary. Typical machining allowances for sand castings range between 1.5 mm and 3 mm (1/16 in and 1/4 in)

A misrun is which one of the following defects in casting?

Metal that solidifies before filling the cavity.

Microporosity

Network of small voids distributed throughout the casting caused by localized solidification shrinkage of the final molten metal in the dendritic strucutre

Cold shuts

Occur when two portions of the metal flow together but there is a lack of fusion between them from premature freezing. Causes are similar to a misruns

Scabs

Rough areas on the surface of the casting due to encrustations of sand and metal. They are caused by portions of the mold surface flaking off during solidification and becoming imbedded in the casting surface

An irregularity in the surface of the sand casting caused by erosion of the sand during pouring is called which one of the following?

Sand wash

Section thickness

Section thicknesses should be uniform in order to avoid shrinkage cavities. Thicker sections create hot spots in the casting, because greater volume requires more time for solidification and cooling. These are likely locations of shrinkage cavities

Corners

Sharp corners and angles should be avoided, because they are sources of stress concentrations and may cause hot tearing and cracks in the casting. Generous fillets should be designed on inside corners, and sharp edges should be blended

Penetration

Surface defect that occurs when the fluidity of the liquid metal is high, and it penetrates into the sand mold or sand core

Castabiltiy

The ease with which a cast part can be produced. Depends on: 1. Part design 2. Metal used for the casting 3. Proper selection of casting process to economically satisfy production requirement

Dimensional tolerances

There are significant differences in the dimensional accuracies that can be achieved in castings, depending on which process is used

Surface finish

Typical surface roughness achieved in sand casting is around 6 μm (250 μ-in). Similarly poor finishes are obtained in shell molding, while plaster-mold and investment casting produce much better roughness values: 0.75 μm (30 μ-in). Among the permanent-mold processes, die casting is noted for good surface finishes at around 1 μm (40 μ-in).

Misruns

castings that solidify before completely filling the mold cavity