All castings, including manhole covers, are made in large factories called foundries. Scrap steel comes into the foundry, is melted and alloyed, and leaves as iron casting. Cast iron is everywhere. A typical home in the United States contains around 2,000 lb (900 kg) of iron castings, mostly as pipe and pipe fittings, but also in furnaces and air conditioners. The casting process consists of five steps, pattern making, mold preparation, melting/pouring, and cooling and finishing.
1 Manhole patterns are either carved out of wood or machined out of aluminum. Aluminum models are used for large production runs because of their greater durability. Patterns are designed to be slightly larger than the finished manhole cover to allow for shrinkage as the castings cool. Two patterns, one for the top half of the cover and the other for the lower half, are required for each manhole. The top half of the pattern is usually provided with a decorative design, though the design is usually limited to a basic waffle, basket weave, or concentric circle pattern in modern times. Prior to 1950, the patterns could have been anything from shooting stars to city skylines. The bottom half of the mold may simply be flat, or may be designed in a three-dimensional spider web pattern to provide much greater strength without increasing the cover’s weight to a degree that would make moving the cover impractical.
2 The sand molds are created by placing the two halves of the manhole model into boxes called flasks so that the models form the base of the box. The upper flask is known as a cope while the bottom flask is known as a drag. Green sand is tightly packed into the flasks to create the two molds. The upper mold contains holes (known as risers or sprues), into which the molten iron will be poured, and vents that allow gases to escape from the mold. For a manhole cover, these risers and vents can be created by simply placing a piece of wood vertically into the flask and removing it once the sand has been packed into the flask. The riser does not usually lead directly into the mold. The riser connects to runners, horizontal channels at the “parting line” (the plane where the two halves of the mold are joined). Using runners allows the molten metal to be fed into the mold at more than one location which helps prevent voids from forming in the final casting.
3 Once the patterns are removed, the bottoms of the flasks are then a hollow image of the upper and lower halves of the manhole cover. The bottom and top halves of the mold are then assembled in a “drag flask,” a large metal frame.
4 Some castings are made with sand bound together with a chemical resin that is thermoset, which means it must be heated to become fixed. This process has some advantages in that the molds can be constructed very quickly and require less labor. These types of molds are ideal for automation when large numbers of casting are to be made. However, manhole covers are not usually produced in the quantities that would justify automation.
5 Cupola, electric arc, reverberatory, induction, and crucible furnaces are commonly used to melt the scrap steel that most foundries use to produce cast iron. The The manufacturing of a manhole cover using a sand mold.
The manufacturing of a manhole cover using a sand mold.
scrap steel is placed into the furnace and melted at about 2,700°F (1,500°C).
6 Any required alloying metals and flux are then added to the molten iron. The purpose of the flux is to bind with any impurities creating a waste product called “slag.” Because the slag is lighter than iron, it floats to the top of the molten iron and can be removed.
7 The molten iron is collected into a large metal ladle. Working from a distance to avoid being splashed with molten iron, foundry workers tip the ladle so that the iron pours into the sand mold through the riser (or sprue). The riser is designed to hold extra molten iron. As the casting cools and shrinks, the excess metal fills in the mold. Because the temperature of the molten iron is much higher than the autoignition temperature of the organic materials in the green sand, the organic materials burn and use up all the oxygen present in the mold. This prevents oxidation of the manhole cover. Foundry workers watch for the exhaust products jetting from the mold to make sure the gases are not trapped in the mold where they might cause bubbles in the casting.
8 It takes about an hour and a half for the metal to cool sufficiently so that it can be removed from the mold. Complete cooling takes about a day.
9 In large foundries, the cooled casting and mold are placed on a vibratory grate and shaken until all of the sand has been shaken off. In a small foundry, the same process might be accomplished by a worker with a wire brush.
10 Handling the used sand from the w molds can be a major headache for foundry personnel as enormous quantities of it can be generated during the casting process. After each use, the sand is sorted in a cyclone to remove any that is too fine to be reused and to sift out all of the metal slag that might be present.
11 While finishing can be a large part of the casting process for intricate castings, manhole covers do not require a lot of finishing. For the most part, all that is required is to remove the runners, gates, and risers (the channels into which the molten iron was poured become little stalagmites on the finished manhole covers), shotblast the surface, and then machine the bearing surfaces to assure that the cover will lie flat in its frame.
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