Every manhole cover, from the simplest to the most ornate, is first modeled in wood or aluminum. The model is used to make the mold into which the molten cast iron will be poured. The designs that have been created for the surface of the manholes are as varied as the skilled artisans who created them. All manhole covers are round because a round object cannot be dropped into a round hole of the same diameter. This is vital since the weight of the manhole could easily kill a worker standing underneath it. Round manhole covers are also easier to move around on the surface as they can be rolled. There are rectangular utility box covers, but they are not installed over manholes.
Drawing Designers use CADto make various drawings required for pattern making, machining, finishing and inspection.
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..
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.
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, 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
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. Some castings are made with sand bound together with a chemical resin that is thermo set, which means it must be heated to become fixed. This process has some advantages in that the molds can be constructed very quickly and requires 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.
Manhole covers are made out of cast iron. Cast iron means that the iron is melted and then poured, or cast, into a mold. Typical manhole covers are cast using gray cast iron. Ductile cast iron, because of its greater strength, is used for special manhole covers, like those that would be found near airplane terminals. Gray cast iron consists of the element iron and the alloying elements carbon and silicon. The alloying agents, chiefly carbon, give cast iron its strength and durability. Ductile cast iron is produced by adding Mg to the molten iron. The Mg causes the carbon in the iron to form nodules instead of flakes, giving ductile cast iron its greater strength and malleability.Manhole covers are made out of cast iron. Cast iron means that the iron is melted and then poured, or cast, into a mold. Typical manhole covers are cast using gray cast iron. Ductile cast iron, because of its greater strength, is used for special manhole covers, like those that would be found near airplane terminals. Gray cast iron consists of the element iron and the alloying elements carbon and silicon. The alloying agents, chiefly carbon, give cast iron its strength and durability. Ductile cast iron is produced by adding Mg to the molten iron. The Mg causes the carbon in the iron to form nodules instead of flakes, giving ductile cast iron its greater strength and malleability.
Besides iron, the other raw material required to make manhole covers is green sand, which is sand bound together with clay. The green sand is used to produce the molds into which the molten iron is poured. The sand mixture consists of about 90% silica sand, 4-10% clay, 2-10% organics (e.g., coal), and 2-5% water. The sand is not colored green. Green refers to the fact that it is allowed to remain wet during the casting process.
Melting / Pouring
Induction, and crucible furnaces used to melt the scrap cast iron that use to produce cast iron.
The Scrap cast and iron are placed into the furnace and melted at about 2,700°F (l,SOO°C). 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.
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. 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 auto ignition 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.
While finishing can be a large part of the casting process for intricate castings, 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), By grinding machine, we are bears surfaces to assure that the cover will lie flat in its frame. After the above procedure the last and finishing of our manhole is painting which we paint the manhole cover.
Ensuring our customers' needs and expectations are met
From receipt of enquiry, through presentation of a fully detailed technical and commercial offer, production planning/liaison and manufacturing processes we aim to provide the highest and most professional service to our clients.
To perfect the method of manufacture, the cast ability and integrity, the components are tested using computer models to simulate the solidification pattern.
The prediction of any problem areas allows the gating and feeding system to be optimized and any preferred shape changes to be confirmed prior to first casting production.
Castings are produced to meet highly demanding operational requirements. In addition to chemical, mechanical testing and close process controls, the quality of the iron casting is proven by Non Destructive Testing (NDT),machining or sectioning.
The surface quality of the casting can be confirmed by either Magnetic Particle or Dye Penetrant testing. Internal soundness can be checked by radiographic (X-ray) or Ultrasonic testing. All NDTis carried out by fully qualified personnel working to national, international or customer's specific standards.
We have in-house Load Testing facility and we can test the product with load rating from 1.5 tons to 45 Tons.
It is unlikely that the production process for manhole covers will change much in the future. Nor is it likely that alternative materials will be used to produce manhole covers as cast iron is extremely economical. The exciting prospect for manhole covers involves computer-aided design and computer-aided manufacturing (CAD-CAM). With CAD-CAM, manhole cover designers can produce intricate patterns that can be cut out of plastic molds by automatic machinery. It will not be necessary for a highly-paid artisan to spend days or weeks creating particularly intricate models for special manhole covers. Once the design is ready, the model can be cut in just a few minutes.We are trying the fallow process for better manufacturing:
- Modification of chemistry to improve properties.
- Improved target control of chemistry by modification of melting practice
- Develop methods to manufacturing low cost alloys
- Develop melting, hot-working, cold-working, and heat-treating processes