The new casting technology was piloted at Oshaug Metall in Molde [Credit: Arnt J�rgen S�nslien]

Manufacturing greener ship propellers

A foundry in Norway has successfully trialled a new method of manufacturing gigantic ship propeller blades that cuts the energy required by a third.

Oshaug Metall in Molde has collaborated with research institute SINTEF to develop the energy-saving casting technology that reduces the amount of metal that needs to be melted for each blade as well as increasing production capacity.

The technology, devised by SINTEF researcher Arne Nordmark, has now successfully completed pilot trials at the foundry's manufacturing centre, which creates propellers for vessels ranging from naval vessels to cruise ships and where the smelting furnaces consume as much electricity as 100 average Norwegian households – 1.5 million kilowatt hours a year.

"The results of the project so far indicate that we can reduce electricity consumption by half a million kilowatt hours a year, and in this way reduce both costs and negative impacts on the environment," said Stein Berg Oshaug, managing director at Oshaug.

The key to the energy saving has been an alteration to the design of feeders – molten metal reservoirs attached to the mould that compensate for the fact that the casting shrinks in volume as it solidifies by supplying additional molten metal.

To avoid the metal in the feeder solidifying before the casting is complete, foundries have traditionally made the feeders very large due to the principle that large volumes solidify more slowly than small ones. As a result, a blade weighing 1,000kg may require a feeder weighing as much as 800kg.

But as shrinkage during solidification constitutes only about 10 per cent of the weight and volume of the propeller blade, as much as 700kg of the molten metal in the feeder will not be used in the final casting.

The new invention increases the efficiency of the process by surrounding the feeder with an induction coil that supplies heat to keep the metal from solidifying, meaning the feeders only have to be large enough to contain the small amount of molten metal needed to compensate for the shrinkage in the casting.

This drastically reduces the amount of metal that needs to be melted for each casting, saving energy but also allowing the foundry to cast more propeller blades in a given time, or cast bigger blades, than its current production capacity permits.

"The pilots reveal that the method can save as much as 35 per cent of the electricity currently used to melt the metal we need to cast a propeller," said SINTEF researcher, and creator of the invention, Nordmark.

On top of this there is an extra benefit. Because the invention comes close to halving the amount of metal that has to be melted, Oshaug will now be able to cast more propeller blades in a given time (or indeed bigger blades) than its current capacity permits.

"The new technology enables us to increase production capacity by as much as 50 per cent, and in cash terms this is even more than we save in electricity," said Oshaug.

The new process also means less post-casting work. After the casting process is complete and the metal has solidified the feeder metal has to be cut from the casting for re-melting. This cutting process is very time-consuming for feeders of large diameter, but during the pilot trials of the new technology in Molde the diameter of the feeder was reduced by 60 per cent, thus reducing cutting work.

"Less post-casting work means lower costs and, moreover, the environment in the foundry is improved when cutting is reduced," added Oshaug.

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