The proper disposal of human waste has been solved by the sewage systems built across the various parts of our nation. And what keeps the sewage system separate from the actual city are the manhole covers. The history of the manhole covers is rather varied – what started as stone manholes covers are now made of cast iron.
How are Traditional Manhole Covers manufactured?
Most top manhole covers manufacturing is carried out cast iron foundries as the modern-day manholes are made out of cast iron. The process is fairly simple theoretically – the cast iron is melted and then poured, or cast, into a mold. In many cases, ductile cast iron is also preferred because of its greater strength and is generally found in and around airplane terminals. What adds more strength and durability to the manhole covers made of gray cast iron is the alloying element(s) – carbon and silicon. And in the case of ductile cast iron manhole covers, manganese is added to the molten iron that provides greater strength and malleability.
What are the Raw materials used besides Cast Iron?
Apart from iron, the other raw materials required in manhole cover manufacturing is green sand -which is sand bound together with clay. It is mainly used to produce the molds which are further used to store the molten iron. The sand mixture is about 90% silica sand, 4-10% clay, 2-10% organics and 2-5% water.
Design & shapes
All manhole covers are first structured in aluminum or wood as a model. This model serves as the key design and is used as a reference to make the mold and then the molten cast iron is poured. The intricate designs on the surface are often created by skilled artisans who personalize them. Coming to the shape of it, all manhole covers are round since it cannot go beyond its assigned diameter. Round manhole covers are convenient to move around as they can be rolled.
Assessment of the Quality
The key to controlling the quality of manhole covers is assessing the proper percentages of iron, carbon, and alloying metals. Once the casting is complete, the strength and ductility of the cast irons are tested to assure reliability. And lastly, the strength and ductility are determined by casting bar test specimens from the same metal used to cast the manhole covers.