Viscous resins … it’s flow time

Louise Smyth

When it comes to pumping and metering, viscous resins pose many challenges for the aerospace, automotive and renewable energy industries. It’s a tricky proposition to get absolutely right

For years, pumps and flow meters have been used as important components in the painting and coating of products. However, with breakthroughs in technology, more advanced pumps and flow meters are now being used to dispense more complicated fluids, such as special resins, carbon fibres and plastics which help manufacturers meet the demand for faster, lighter, stronger, quieter and more fuel-efficient products.

But viscous resins can pose a number of challenges for engineers. First, their thick and sticky consistency means they lose pressure when pumped through a pipe. Any bend in the pipe will take away even more pressure, so it’s essential that the pump is capable of generating enough pressure to allow the viscous resin to flow freely at the desired flow rate.

Secondly, while in a pipe, viscous resin has a tendency to harden, especially if it contains materials such as aluminium, talc or fibre. Finally, viscous resin is sensitive to temperature. If it gets too hot, it becomes runnier and less thick; too cold and it can set.

Given these challenges, what type of pump and flow meter works best for viscous resin? “In our experience, it is best to avoid standard or traditional gear pumps and flow meters as they tend to be large, expensive and unreliable, due to their very low output frequencies which can make them difficult to control,” states Andrew Biggs, from UK Flowtechnik. “Instead, our pump of choice is the German-made Beinlich, which has a range of positive displacement pumps specifically designed to deal with both high and low viscosity, thereby making the pumping of thick and sticky resins much easier.

So what makes the Beinlich so superior, in his view? “To start with, Beinlich’s pumps operate at a constant flow and pressure at a given viscosity, so they are able to be extremely accurate, and are ideal for dispensing and dosing, as the flow rate will remain proportional to the speed. The pump’s innovative design can also cope with a wide range of viscosities, but is ideally suited to thicker fluids, as the volumetric efficiency actually increases improving its accuracy.

“And finally, as is generally well known, a bi-product of pumping thicker fluids is the possibility of high system pressures within pipework and fittings, but again the Beinlich pump is more than apt to cope with pressures up to 200 bar,” adds James Lees

Spot-on ratio

Typically viscous resins range from one to one million centipoise (cPs). So distilled water measures one centipoise, whereas more viscous liquids, like motor oil, generally measure between 1,000 to 2,000 cPs, and an extremely thick and sticky resin, like window putty, measures around 1,000,000 cPs. But whatever the viscosity, when pumping and metering viscous resin, it’s essential the materials that make up the resin are mixed to the correct ratio and their combined flow rate must be spot on.

And that’s where the Beinlich pump comes in, says James Lees. “Starting at very low flow rates of two millilitres per minute and rising up to 3,000 litres per minute, the Beinlich works well when used as part of on a closed-loop-control system, allowing engineers to check the output via the speed of the flow meter against the pump, so they can adjust the flow of the resin and hardener until the right ratio is achieved. Indeed, it’s crucial that the ratio of resin to hardener is accurately controlled, otherwise the strength of the composite assembly is reduced or, in the worst case, the resin does not harden, leading to components being scrapped.”

So how does viscous resin behave when it comes to Resin Transfer Moulding (RTM), a manufacturing process typically used to make from very small components to large carbon fibre parts such as wing panels, car doors & components and wind turbine blades? “As you’d expect, pumps and flow meters play a key role,” adds James Lees “Any pump used in RTM must have the ability to be fine-tuned, so it can fill moulds over a range of speeds. This is critical as moulds need to be filled quickly enough, so that the resin is not cured before it goes into the mould – but not too quickly. Otherwise it will leave voids in the components which then have to be discarded.

“And when it comes to bonding parts, it’s imperative that the right amount of resin is in the right place, so a high accuracy flow is essential. After all, no car manufacturer worth their salt would produce a car that started to fall apart, due to inadequate bonding!

“This is done by fitting the (also German-made) VSE flow meter with innovative interpolation electronics, which produces extra signals and pluses, so the engineer can control the variables in a more responsive way and alter the amount of resin that is pumped out, thereby ensuring every finished product is of the same high quality, no matter what the hiccups on the production line.”

And if all this wasn’t challenging enough, varying the temperature of a viscous resin also changes its dynamics. Even turning the heat up or down by 10-20 degrees can have a big impact on viscosity making the resin more or less viscous. According to James Lees, the Beinlich pump comes into its own here as well, as its closed loop control sensor is specifically designed to monitor and adjust the temperature as required.

“Yes, viscous resins are a tricky customer at the best of time,” Andrew Biggs .“But our experience over 25 years has taught us that that, when it comes to pumping and metering such resins – be it in the aerospace, automotive or renewable energy industries – a specifically designed Beinlich pump and the VSE flowmeter, with its interpolation electronics, can help to significantly reduce the challenges they pose.”

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