Manhole cover and examination of its manufacturing methods – from traditional casting to modern technologies

You may have come across circular metal or concrete protrusions many times while crossing the street or sidewalk. These covers, which are called manhole cover in engineering literature, play a vital role in the safety of citizens and access to vital underground arteries. Manhole cover are actually the entrance and exit doors for technicians to underground water and sewage, telecommunications, gas and electricity channels, which, in addition to preventing people and vehicles from falling into the well, are also responsible for protecting the facilities from environmental factors. In this article, we will look behind the scenes of the production of these products and the engineering methods for their construction.

Variety of materials in the construction of manhole cover

The first step in understanding the construction method is to identify the raw materials that are selected based on the needs of the project. The production process is completely dependent on the type of cover, and each material has a unique production line.

In traditional methods, concrete manhole cover are produced using reinforced concrete molding. In the structure of this manhole, rebar or polymer fibers are used to increase tensile strength to make brittle concrete resistant to impacts. Cast iron valves, which have long been popular, are also produced by melting a cast iron alloy (usually ductile or gray iron) and casting it in sand molds. Although cast iron has excellent resistance, its high price and risk of theft have led engineers to think of new methods.

The new generation of manhole valves includes composite and polyethylene valves. Composite models are made from a combination of polymer resins and reinforcing fibers (such as fiberglass or silica), which, in addition to being lightweight, have very high chemical resistance. Polyethylene valves, which are mostly used for household purposes and meters (non-traffic), are made by injecting polymer materials into a mold and are not a good option for high-traffic roads.

Production Line Stages and Standards

Regardless of the type of material, there is a general algorithm in the engineering of manufacturing these parts. First, the engineering team determines the raw material analysis based on market needs and load-bearing standards. After accurate dimensional calculations, the critical molding stage begins. The molds, which are usually made of aluminum, wood or steel, must be designed with micron precision so that the valve and the frame are completely locked together and do not, so to speak, loosen.

In the main stage, or shaping, the raw material (molten metal or composite paste) is injected or poured into the mold. After the mold is filled, it is time for curing and cooling operations. At this stage, the part must be placed under controlled temperature conditions to stabilize its molecular structure. In some methods, to increase density and strength, the part is pressed under very high pressure or placed in compacted sand beds to eliminate its porosity. Finally, all valves must pass the quality control unit and pass pressure tests.

Modern Methods of Manufacturing Composite Manhole Cover

Today, composite manhole cover have a large market share due to their anti-theft and high durability. These covers are mainly produced by three specialized methods:

１) BMC method

In the Bulk Molding Compound method, the raw materials are prepared in the form of a thick paste. In this paste, short reinforcing fibers (between 10 and 15 mm) are randomly distributed. This paste mixture is placed in hot press molds. The covers produced by this method are usually suitable for sidewalk applications and green spaces (semi-traffic).

２) SMC method

The Sheet Molding Compound method is the most advanced production method. In this method, the raw materials are prepared in the form of sheets containing long fibers (30 to 60 mm). These sheets are placed in large steel molds and baked under very high pressure and heat. The presence of long fibers and hydraulic press pressure make SMC valves have excellent mechanical resistance and are considered the best option for main streets and heavy traffic.

3) FRP method

In the Fiber Reinforced Polymer method, which is sometimes done manually or semi-automatically, unsaturated polyester resins along with fibers and fillers such as silica are applied layer by layer in the mold. This method has high flexibility in design and is used for special orders.

The importance of the EN 124 standard in the manufacturing process

One of the most important points that buyers should know and should also be considered in the manufacturing process is the product’s compliance with the international EN 124 standard. Many people think that all valves are the same, but the manufacturing method must guarantee the weight class of the valve. In this standard, hatches are divided into the following categories, each of which has a different production process:

• Class A15: 1.5 ton load capacity (for pedestrians and cyclists)
• Class B125: 12.5 ton load capacity (for light vehicle parks and parking lots)
• Class C250: 25 ton load capacity (for roadsides)
• Class D400: 40 ton load capacity (for main streets and truck traffic areas)

When manufacturing a D400 hatch (heavy traffic), the manufacturer cannot use simple methods or short fibers and must use heavy pressing methods such as SMC or ductile iron. Therefore, when purchasing, do not just pay attention to the material of the hatch, but also check the weight class engraved on the hatch, which is a direct result of its manufacturing method, to prevent sudden breakage and accidents.

Conclusion

The manhole hatch manufacturing industry is a combination of metallurgy and polymer chemistry. From sand castings for cast iron to heat presses for advanced SMC composites, the ultimate goal is to produce a product that ensures traffic safety in the city. Understanding these production methods will help you make smart choices tailored to your project needs.