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Unformatted text preview: Single patterns: simplest and inexpensive to make; duplicate of the part to be cast with shrinkage allowances; used when the shape to be made is relatively simple and the number of duplicate castings is rather small Split patterns: used when moderate quantities of a casting are desired; pattern divided into two sections along a single parting line, which corresponds to the parting plane of the mold; bottom is produced then the top and then pins hold the two pieces together Match plate patterns: simplify the process; can be coupled with modern molding machines to produce large quantities of duplicate castings; holes that align with pins on the flask, the match plate is positioned between the upper and lower segments of the flask; mold material packed on both sides of match plate then removed and mold put together Loose piece patterns: used when the product has a geometry that a one piece for split pattern could not be removed from the molding sand; separate pieces are held to a primary pattern segment by beveled grooves or pins; after molding the primary segment of the pattern is withdrawn; expensive; careful maintenance, slow molding process, increase molding costs Green sand ingredients: mostly silica; glass and beach sand; clay, water, wheat (cereals) iron oxide Permeability: the ability to permit gases to escape through sand Compactability: determined by sifting loose sand into a steel cylinder, leveling off the column, striking it three times with a standard weight, and then measuring the final height; low compactability is usually associated with too little moisture Green compressive strength: strength to maintain the integrity of the mold cavity as the mold is handled between molding and pouring: must withstand the erosion of the liquid metal as it flows into the mold and the pressures induced by a column of molten metal Moisture content: usually determined by washing the clay from a 50-g sample of molding sand using water that contains sufficient sodium hydroxide to make it alkaline; remaining sand is dried and weighed to determine the amount of clay removed from the original sample Modern Methods of Green Sand Compaction Jolt: a flask is placed over the patter, filled with sand, and the pattern, flask, and sand are then lifter and dropped several times Squeeze: use an air operated squeeze head, a flexible diaphragm, or small individually activated squeeze heads to compact the sand Air blow: Flaskless molding: vertical; cope and drag patterns are incorporated into opposing sides of a vertical mold; sand deposited between the patterns and squeezed with a horizontal motion; get an assembled mold Chemically Bonded Sands- Sand with a binder that acts like a glue; the binder is activates by catalyst (liquid or gas) and temperature makes it harder Shell Process- Gives good surface finish; a heated pattern is placed over a dump box containing sand and resin; the box is inverted and the shell partially cures around the pattern; the box is righted, the top is removed, and placed in an oven to further cure the shell; the shell is...
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- Spring '07
- Metalworking, molten metal, Powder Metallurgy