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Defect Detective: Common Green Sand Flaws


Green sand quality control, including good critical visual inspection, helps avoid scrapped castings. AFS INSTITUTE (SCHAUMBURG, ILLINOIS)


Q


uality control begins with prevention. Met- alcasters can control


five areas of green sand mold mak- ing in order to prevent mold defects: • The selection of type of sand and clay and the amount of moisture needed to make the mold.


• Consistent mulling, using the right sequence of raw materials, and selecting the right tempera- ture of the sand.


• The cleanliness and condition of horizontal and vertical patterns, as well as core placement.


• Selection of scrap metal and determining what can be reused and what must be discarded.


• Testing for quality ensures the part meets the requirement of the buyer. While castings can be tested many ways, two one of the major casting quality tests used after the casting is made is visual inspection. Nine types of defects can be visu-


ally identified on a casting: cracks, holes, rough surfaces, tear-up/drop, friable edges, excess parting spray, missing components, loose sand


and sand contamination (Fig. 1). Usually, the causes of defects include shrinkage, gas porosity, sand expan- sion, sand adherence, and weak sand strengths.


Shrinkage Shrinkage is a depression in the


cured metal due to uneven solidifica- tion. A shrinkage cavity is a void left in cast metals as a result of solidifica- tion shrinkage and the progressive freezing of metal toward the center. Excessive moisture (high com- pactability) in green sand molds can result in mold wall movement. Tis can cause dimensional variation and an increased chance of shrink defects. Elevated gas content in the metal can magnify the size of the shrink.


Gas Porosity Te two types of gas porosity


defects are entrapped and soluble. Entrapped Gas When molten metal is poured into the mold, it heats up the green sand. If the sand is too moist, it creates steam, causing air to become entrapped in the mold. Metal cannot displace air in the mold. Air can also be a by-product of some sand addi- tives when they are heated. Without venting this entrapped air,


blowholes can be produced. Keeping the sprue full while pouring helps to prevent air from being trapped in the metal. Soluble Gas Gas-related defects can also be


Fig. 1. Shown are nine common types of green sand defects. 24 | MODERN CASTING July 2018


caused by soluble gasses in metals. • Aluminum alloys will dissolve hydrogen.


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