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More tips on casting

By Udo O.J. Huff, M.ED.

October 2011 - The previous article in this series, in September 2012 FFJournal,  focused on the process of forming metal parts by casting. In it we offered some explanation about an old technique used in the metal trades to master complex components for single or mass production purposes.

Casting deals with the chemical components of the ore, the alloy, the heat source, the impurities and the cast itself.

FFJ-1012-training-figureNevertheless, the skills of the professional shop floor personnel and the foundry equipment also figure significantly in casting quality. The desired result is a smooth-surface casting that is well contoured and not very porous.

As mentioned in the previous article, shrinkage of the casting must be anticipated and accounted for in constructing the mold, and that depends on the material being cast. During the casting process, the ingate must remain full or other slag particles of air from eddy formation will be carried into the mold. The casting will be complete when the riser is full. After sufficient cooling, the mold is broken and the casting taken out and cleaned.

Process of solidification

In the process of cooling, the casting material goes through three distinguishable stages: first turning viscous, then plastic and finally becoming solid. Shrinkage occurs in degrees throughout the cooling process.

1. Liquid shrinkage: This is the state of the poured material, which manifests itself in the level of the liquid sinking in the feeder.

2. Solidification shrinkage: This takes place during solidification and leads to “funnel” formation. The funnels formed should be in the feeder and not in the casting, so that the casting will be free of shrinkage cavities.

3. Linear shrinkage: This is the last stage of shrinkage as the material cools to room temperature. This shrinkage is taken into consideration in the construction of the pattern itself by using a contraction rule. The extent of linear shrinkage for cast steel is 2 percent, for gray cast iron 1 percent, for cast aluminum 1.25 percent and for casting copper-zinc alloys about 1.5 percent.

Possible fault types

During the casting process there are other casting faults to consider. For example, there may be slag inclusions, which occur when slag and metal oxides are not held back during casting because the ingate has not been kept full during casting.

Piping in a cast structure develops when the cast material does not solidify uniformly. The solidification takes place from the surface inward. A cavity that forms in the thick parts of the casting as a result of non-uniform cooling is known as piping. In order to avoid piping, the casting should be of uniform wall thickness, which is a basic construction principle for casting design.

Further faults include bubbles in the cast structure. That can happen when the wet molding sand releases hydrogen and oxygen at casting temperature and the gases penetrate the material of the casting.

The casting also may crack, due to abrupt changes in various sections, causing the solidification to take place too slowly at the points of maximum thickness. For that reason, iron chills are embedded in the mold at points of maximum cross-section to control the cooling process and avoid cracks and piping.

A fault can happen if the core has shifted during the casting process, resulting in unequal wall thickness of the poured casting at two opposite points.

Last but not least, someone responsible for the casting needs to check for thickened points and rounded bulges. This type of fault occurs if the mold has not been compacted sufficiently, allowing it to give under the pressure of the casting material.

Most of the casting faults described here can be avoided if the technical personnel involved in the casting are properly trained.

It is essential that a well-developed curriculum and syllabus are included in the training package. Hands-on training should include stepwise progression from an introduction, to a beginner level, then through intermediate and advanced performance.

The hands-on-training content offers good learning implementation by doing it on the shop floor. The academic related instructions should include science of materials, including in-depth instructions of the iron-carbon diagram; technical mathematics; and design engineering applications.

The student should receive good guidance through either a complete in-house training or a program linked with a technical college or, if necessary, with online training courses. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.

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