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Metal Casting Technologies : March 2009
Foundry practice Equipment - for starters, the most suitable and practical furnace to use is a crucible gas-fired furnace. The choice may be between a 70kg capacity (No. 45) crucible and a 150kg capacity (No. 100) crucible, depending on the sizes of the castings that will be produced. Pouring ladles, likewise, would depend on the casting sizes, with ladles either used by two men working together as a team or by one man pouring alone. Or a bull ladle carried by an overhead crane may be used. Everything depends upon the size of the castings to be poured. Every foundry, however, should have all these equipment available to give it much flexibility. Melting – raw materials would be: (1) composition ingots or scrap, or virgin copper metal – like copper wire or magnet wire, (2) tin ingot, (3) lead ingot, and (4) zinc ingots. Note that if scrap is used, the resulting casting composition would most probably be unknown and casting rejects would be quite high due to gases; while the use of composition ingots or virgin metal assures the casting composition and casting rejects would be much lower. If virgin copper and ingots are used as charge materials, the crucible shall be filled in this sequence: copper first, tin second, lead third, and zinc last and stir well after the zinc is added. Efficient stirring is essential, as lead will segregate rapidly when molten metal containing it is left in a quiescent state. In the case where composition ingots or scrap are used, which already contain the major elements in their composition, the given charging sequence is not needed. Deoxidize the virgin copper well with “deoxidizing tubes” or “phosphor copper” before tin is added. Where pressure tightness is not a consideration, or maximum density not a requisite, these alloys may be melted without flux in an oxidizing atmosphere, taking the usual precautions during melting. However, for use as bearings, porosity is beneficial to the castings as it helps retain an oil film on its machined surface. To have an oxidizing atmosphere, adjust the oxygen valve in the gas-burner so that there is a yellowish flame indicating a slightly oxidizing atmosphere in the furnace (a greenish flame indicates a reducing atmosphere). The purpose of this is to prevent hydrogen from entering the melt as the excess oxygen will react with hydrogen, turning the combination into water vapor. Hydrogen in the metal causes gas holes which are very difficult to remove. The best approach is to prevent it from entering the melt by introducing oxygen through the flame. Then there would be an of excess oxygen which gets into the melt and this, in turn, would cause the gas holes; but then oxygen is very easy to remove by using deoxidizers thus leaving a gas-free melt. In cases where pressure tightness and maximum density are desired, the following procedure should be followed, based on the so-called Walpole’s flux method. 1. Prepare the flux by mixing equal parts by weight of dry copper mill scale (this is the cheapest form of CuO), dry some clean silica sand, and powdered fused borax. Keep in a suitable container to avoid absorbing moisture as this mixture is hygroscopic. 2. Use enough flux to maintain a cover of about 6 mm. (¼-inch) thick over the molten metal under the conditions of melting. Generally, flux of about 3% of the weight of the metal to be treated is sufficient. Where the phosphorus content of the charge exceeds 0.05%, a larger amount of flux may be required. 3. Put the flux in with the metal as it is charged into the melting unit. 4. Melt as rapidly as possible under an oxidizing atmosphere. 5. When the metal has reached the desired temperature, stir the flux well into it, to further the oxidizing process. 6. Transfer to pouring container and throw dry silica sand onto the metal to thicken the flux and facilitate its removal by skimming. 7. Skim and add 115 grams (4 oz.) of 15% phosphor copper for every 45 kgs. (100 lbs.) of metal for the purpose of deoxidizing the melt. 8. Stir well, exposing as little of the metal to the atmosphere or air as is practicable, and wait for at least 2 minutes before pouring. Note that a residual phosphorus content of 0.05- 0.03% is desirable. Molds - tin bronzes may be cast in either green or dry sand molds. Unless the casting is unusually large or complicated in design, green sand containing not more than 6% moisture is all that is needed. Use a fine natural sand (with about 15% clay substance) of AFS grain fineness number of about 200 for small work and equal parts of this sand and natural sand (with about 12% clay substance) of AFS grain fineness number 130 for casting up to 200 kgs. (400 lbs); and for castings that are larger than this, use the natural sand of AFS grain fineness number 130 alone. White silica sand may also be used, but then a binder has to be added to supplant the clay substance inherent in the natural sand. Pouring – the usual precautions should be taken – namely, pour as close as possible to the sprue head or pouring cup, keeping the sprue head full all the time, pour as slowly as will prevent “cold shuts,” regulate the feed of the metal by some suitable construction between the sprue, riser and gate to the casting (that is, placing a “choke” in the gating system). Pouring temperature as close to 1035° C (1900° F) as possible should be the general rule, but this is a function of the casting size and the alloy that is being poured. Experience will help determine, through previous trial and error, the optimum pouring temperature for what or which casting is indicated. METAL Casting Technologies March 2009 55