SAND FACTORS IN HIGH PRESSURE MOLDING

19710408
Journal:
Author: D.A. Taylor

Two important aspects of high pressure molding are considered: (a) Molding materials suitable for high pressure applications. (b) The dimensional accuracy of castings produced in molds compacted by high pressure squeezing. It is possible to summarize briefly the factors necessary for high pressure molding and the advantages that can be gained: 1. Sands of high clay content are necessary to obtain high strength molds, of high mold hardness and density. For desirable molding properties and also economy there are practical limits to this clay content and a maximum of 18 percent appears to be satisfactory. 2. A close control of water content is necessary if consistent bulk densities and hardness are to be obtained. Water contents should be lower relatively than those used for conventional molding. 3. It is necessary for some organic additions such as dextrine or starch to be made to the sand if its "brittleness" is not to be increased at high pressures. 4. Squeeze pressures in the range 200 - 400 p.s.i. are necessary to develop the full potential of the molding sand. Pressures above this range appear to offer little additional advantage. 5. Sharp definition is obtained on a high pressure mold and thus any imperfections in pattern equipment are reproduced. Good pattern equipment is absolutely essential. 6. By high pressure molding, molds of uniform hardness and density are obtained. This is difficult with other methods of molding. 7. Close dimensional accuracy can be maintained for a particular molding sand composition than is usually observed with normal molding machines. This accuracy increases as squeeze pressure is raised.