University of Nottingham commissions Slack & Parr precision metering technology as part of short fibre compression moulding solution for composite parts

The short fibre compression moulding approach has been developed by a team from the EPSRC Future Composites Manufacturing Research Hub at The University of Nottingham, led by Professor Nick Warrior and Dr Anthony Evans.

Their objective has been to develop an out of autoclave (OOA) manufacturing technology that accommodates lower cost, short fibre formats as a way of making composites more viable and cost-efficient and supporting the transport industry’s ambitions to become more fuel-efficient and sustainable. The solution is being used to manufacture components including complex-shaped under bonnet and suspension parts for automotive.

The university’s technology uses a liquid resin spray (LRS) system to apply the resin matrix to short form (25mm) carbon fibres activated by an air stream. In this way, the resin and the fibres make complete and uniform contact with each other without the need for mechanical application.

Professor Nick Warrior explains how Slack & Parr’s precision external gear pumps are key to the resin metering and spraying stages of the process:

“Slack & Parr’s metering pumps allow us to control precise amounts of the resin’s constituent parts to achieve a matrix with the exact characteristics we need.

“Being able to accurately meter each ingredient means we can mix the resin matrix to a viscosity that can be sprayed reliably and effectively without having to rely on the overuse of solvents. Varying the percentages even slightly at this stage can result in a resin that may not spray or cure correctly, so accuracy is crucial here.

“We can also adjust the metering pump to create new resin systems with different characteristics, for example faster curing and better mould release to help us achieve our target 30-second cure cycle.”

Slack & Parr’s metering pumps are built around hardened steel involute gears which are machined to precise tolerances of one or two microns. This results in extremely small, controlled clearances – measuring significantlyless than a human hair – between the gear and mating components, ensuring almost no internal slip even at high pressures. This level of precision means the pump is able to control or meter the flow with extreme accuracy and repeatability.

In the university’s system, each part of the resin matrix is held separately to prevent curing and mixed only at the point of application. The metered matrix is then sprayed directly from the resin mixing tank onto the short form carbon fibre filaments which are simultaneously propelled into a controlled air stream via a robot head. The resin-coated carbon fibres are subsequently moulded to form a composite component.

The University of Nottingham’s application requires three different fluids with different viscosities to be brought together at the same rate and in the right ratios to achieve the desired resin matrix and to ensure consistency and repeatability of the process. Using Slack & Parr’s gear pump technology, the university team was able to achieve a consistent flow throughout the process despite varying pressures linked to the different viscosities of the fluids.

Neil Anderton, Hydraulics & Industrial Director at Slack & Parr, said: “We are delighted to be working with the University of Nottingham on this exciting development. Our gear pumps are recognised as the most accurate in the world. Their ability to accurately meter again and again with various resins and in varying conditions is absolutely vital to the project’s success and commercial viability.

“Our pump solutions are relevant to every area of the composites industry where the need to precisely determine the fibre-to-resin ratio is key to the manufacture of consistent composite materials and parts. Importantly, the technology can also be extended out to other polymer resins and plastics to offer the same level of performance.”