Carbon fibre is noted for its combination of strength and light weight, however, its low bulk density and fibrous particle shape can make it resistant to flow. Careful handling is therefore required to ensure a reliable feed during production. In 2019 electrical power and advanced materials company, Mersen, approached solids handing equipment specialist, Ajax Equipment, to enhance carbon fibre handling from a milling process at its Eurocentral, UK facility near Glasgow.
Even where the general properties of a material are widely known it is valuable to determine the exact behavioural characteristics of the material to be handled. “Milled carbon fibre is quite low bulk density, which is often a concern for flowability, but it is the other bulk properties like shear strength and wall friction against mild steel which need to be measured so that the correct design of storage and feed control can be established,” commented Eddie McGee, managing director, Ajax Equipment.
“With the knowledge obtained from material testing at Ajax, our engineers designed an integrated carbon fibre storage and feed system, formed of a hopper and screw feeder, that would provide a dependable onward supply of material from the line’s rotary valve and vacuum loader above.”
The new hopper’s steep walled vee shape encourages the flow of carbon fibre powder towards its elongated outlet and the screw feeder below. Once out of the hopper, the screw’s progressive geometry, including variable pitch and stepped shaft, ensures a reliable and controlled flow of reworked fibre powder. In addition, the feeder’s cover is hinged to allow Mersen’s manufacturing team to easily retrieve a sample of carbon fibre powder and inspect the machine internally.
Commenting on the line upgrade, Scott Keil, manufacturing manager at Mersen, said “As one of several partners in this project, Ajax were a delight to deal with. They tested our unusual material in their lab to determine its flow characteristics, before designing and supplying the specified equipment within tight deadlines. They also coordinated and provided technical support to our other partners. In short, we are very pleased with Ajax’s work.”
Sextuple Screw Feeder for New Line
Following the completion of the rework line upgrade, Ajax was asked to design and supply a carbon fibre handling system for a new line as part of Mersen’s ongoing projects to enhance and expand its production capacity at the same Eurocentral, UK facility. “After the success of working with Ajax to upgrade the existing line we were happy to work with them again on this new line,” said Scott Keil.
Eddie McGee again, “Working on a new line provides the opportunity to design each component of the system together, ensuring efficiency of process and plant configuration. The carbon fibre product behaves very differently to the milled version, it is extremely low density and very fibrous with long thin particle shapes. Flow property tests were performed and informed the design of the storage and transfer system needs for carbon fibre on Mersen’s additional line. Ajax designed a system comprising silo, six screwed feeder, collecting screw conveyor with declumping features and an inclined screw.”
The silo needed vertical walls and was fitted with a six screwed feeder which actively extracts poor flow carbon fibre from the large outlet of the silo above. The integrated design of the feeder and silo means that any arching and hold up of fibre is completely avoided and a positive, regulated feed is consistently achieved. The output is fed into a collecting screw which transfers the product towards the outlet. To ensure the most even of feeds to the onward process the outlet section incorporates blades to comb the fibre through a grille, breaking up any tendency for the fibre to clump and hold together.
The conveyor has outlets at each end so that the existing and a future, planned mill can be served. The conveyor has the declumping feature at each outlet, with supply controlled by which direction the collecting screw is run in. When constructing reversible discharge screws, it is essential to ensure the flights are completely uniform otherwise the screw may compact material when running in one of the directions. The elevator features a long single span screw to avoid any potential blockages, effectively configured for pushing the poor flow fibre up the incline and minimise service needs.
“Commissioning with the new multi-screw system has gone exceptionally well with production totally satisfied that the Ajax feed of product to our mill is consistent and indeed superior to our original Silo set up,” comments, Scott Keil.