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The Soundness of Precision Castings

Soundness of metal components refers to the level of freedom from impurities and discontinuities such as sand inclusions, slag inclusions, macro porosity, and shrinkage.

Steel castings begin to solidify at the mold wall, forming a continuously thickening envelope as heat is dissipated through the mold-metal interface. In general, the volumetric contraction that occurs within a solidifying cast member must be compensated by liquid feed metal from an adjoining heavier section, or from a riser which serves as a feed metal reservoir and which is placed adjacent to, or on top of, the heavier section. The lack of sufficient feed metal to compensate for volumetric contraction at the time of solidification is the cause of shrinkage cavities. They are found in sections which, owing to design, must be fed through thinner sections. The thinner sections solidify too quickly to permit liquid feed metal to pass from the riser to the thicker sections.

Testing ensures that the material meets the requirements of the specification, consequently, testing may be mandatory. More frequent testing or other tests may be imposed, by use of supplementary requirements of material specifications or general requirement specifications. In addition to specifying test methods, acceptance criteria must be agreed upon between the purchaser and the foundry. The more testing and tighter the acceptance criteria the more expensive the product will be - without necessarily increasing quality or serviceability. Hence, the extent of testing and acceptance criteria should be based on the design and service requirements.

As a matter of fact, it is impossible to produce a defect free casting, only castings with defects of varying degrees of acceptability. The acceptance or rejection of such castings can only be determined by examination and analysis of parts based on customers' formal engineering requirements and in accordance with standards. A defect in one application may not be a defect in another application and it is impossible to make a casting without some kind of flaw.

The size of flaw(s) can vary significantly, and what is acceptable and what is defined as a rejectable defect depends on agreement between the supplier foundry and the client prior to production. Large cavities often exist in thick-section castings and can be perfectly acceptable depending on the application and the location within the Precision Casting . On the other hand, some applications are very critical and tiny flaws may be considered as defects that may be detrimental to the intended use of the product.