This page contains a Flash digital edition of a book.
REMEDIES FOR SAND EXPANSION DEFECTS S


• Make an addition of cellulose or cereal to the sand to provide a place for expansion to occur.


• Lower the moisture content of the molding sand, which increases the overall mold strength.


• Lower the pouring temperature of the metal, which reduces the amount of sand expansion.


• Lower the temperature of the molding sand from the return sand system to increase the strength properties of the sand.


• Increase the sand’s clay content of the sand for better hot strength properties.


• Decrease the amount of fines in the sand. Fines tend to Rattails are surface defects. Tey’re


an irregular line on the casting result- ing from thermal deformation of a sand mold during filling with molten metal. Rattails are common on flat surfaces, and look like a rat walked across the surface dragging its tail. Buckles are an indentation on a cast-


ing, resulting from expansion of the sand. A scab is a rough thin layer of metal


partially separated from the body of the casting by a thin layer of sand.


soak up water, increasing the overall mold moisture.


• Avoid over-ramming. This pushes moisture closer to the mold surface, increasing the probability for defects.


• Improve the sand mulling practice to create a more homogenous sand mixture with a better developed bond.


• Fill the mold faster by increasing the flow rate of the gating system to leave less time for the heat to act on the sand without pressure from the metal.


• Improve the sand distribution to stagger the expansion and create a linear expansion curve.


REMEDIES FOR LOW HOT STRENGTH


• Increase sodium bentonite levels in the mold to provide better hot strength properties.


• Introduce better mulling practices to allow for better and more consistent distribution of clays, sand, and other additives throughout the mold.


• Reduce the combustibles in the sand mix because they increase the mold moisture content and gas-producing ability, but usually do not strengthen the mold.


26 | MODERN CASTING July 2018


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68