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Manhole covers, those iron things that we all walk over each day of our lives. Have you ever stopped to think of what mysteries lie behind them? For example, in Finland they are mostly round, why is that? It is said that the reason for manhole covers being round is that the cover could not fall through the frame into the well. With square covers you would have this theoretical risk on the diagonal. Despite this urban legend, there are covers in all shapes and forms around the world. Round, square, rectangular, triangular.
The products are, despite a unified European standard EN124, quite local. And often each market is quite emotionally attached to their traditional product types and choices. After all, we have all grown accustomed to seeing the covers during our daily walks. But have you ever thought about the surprisingly vibrant world behind them?
So why do we actually have manhole covers?
The main purpose of the manhole covers is to act as an access hole to the underlying rain or wastewater systems. The infrastructure will remain under ground for tens of years. And in order for maintenance to be possible, we need access holes to the systems. Since the systems are transporting a large amount of water and any clogs or disruptions would have quickly impact our daily lives. Our company’s mission is therefore “To modernize and digitalize infrastructure, and to provide products that support communities and protect the planet”.
In another blog, we will write about the history of manhole covers, but suffice it to say that it also is quite intriguing.
So where do they come from and how do they appear on our streets?
There are foundries all around the world manufacturing manhole covers.
Many of the largest specialized manufacturers are today in India and China. But there are also large foundries around Europe and USA not to mention other parts of the world.
Many European countries have a foundry manufacturing manhole covers to the local taste, but they cannot produce enough to cover all the market needs. And foundry business is often not very appealing to younger people in developed countries which leads to labor shortages.
A reason for Asia’s strong foundries is of course labor and production costs, but there are also other drivers and the manufacturers have invested over time in automated and specialized production lines. What used to be a rather heavy and dirty type of manual labor has seen more automation in the past years and many of the rather large foundries are family-owned businesses. The local legislation has also improved in the recent years and brought environmental improvements. There is however still more to be done on this front and we are in a good position to support in this.
Before the products are sold to municipalities and construction companies, they are either stocked in large warehouses like the ones we in Combi Works have. In these plants we also have our assembly lines for making our telescoped manhole covers. We install pipes onto the covers that allow for installation into the wells and chamber systems underneath.
How are manhole covers then manufactured?
There are a number of materials and methods, but the overwhelming majority of manhole covers are made by casting and in iron. The primary production method is sand casting. This means that a special type of resin sand is packed into a steel box. The box has a bottom and a top part. Then what is called a pattern, a copy in wood or aluminium of the product to be made, is placed onto the sand in one of the boxes. The pattern’s other side or half is placed into the second box.
They are then pressed to compact the sand. The pattern is then removed from the box and a void or cavity is left. This is called the mold, a negative impression of the part to be made. Liquid iron is then poured into the mold through a gate or channel, into the cavity that is left after the pattern was removed.
Historically, casting as a manufacturing method was quite hard and manual. The manufacturing method itself has been used for thousands of years. Nowadays the foundries have varied levels of automation. The mold making can be automated just as the metal pouring and the following fettling (cleaning) process and so on. The process depends alot on the manufacturing batches. Products with low volumes are made manually with floor molding. The more popular products are made in automatic lines.
After cleaning, the seating surface where the cover is lying on the frame would mostly be machined for better fitting. And after this, the covers will be painted by a bituminous paint, packaged and shipped into the centralized warehouses, like Combi Works’.
This is the path of the manhole cover.