Challenges of pumping and metering viscous fluids
When it comes to pumping and metering, viscous resins pose a number of significant challenges for the aerospace, automotive and energy industries. Here, Andrew Biggs and James Lees of UK Flowtechnik explain why it’s such a tricky customer.
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 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. Firstly, its thick and sticky consistency means it loses 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, whilst in a pipe, viscous resin has a tendency to harden, especially if it contains materials such as aluminium, talc or fibre. And thirdly, 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 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. 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? 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 we all know, 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.
Typically viscous resins range from one to one million centipoise (cP). Distilled water measures one centipoise, whereas more viscous liquids, like motor oil, generally measure between 1000 to 2000 centipoise and an extremely thick and sticky resin like window putty, measures around 1,000,000 centipoise. 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. Starting at very low flow rates of two millilitres per minute and rising up to 3,000 litres per minute, the Beinlich works on a closed-loop-control, allowing engineers to check 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 everything from very small components to large carbon fibre parts such as wing panels, car doors and components, and wind turbine blades.
As you’d expect, pumps and flow meters play a key role. 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 a fine art 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 its salt would produce a car that started to fall apart due to inadequate bonding.
This is done by fitting the 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 ten to twenty degrees can have a big impact on viscosity making the resin more or less viscous. But again, the Beinlich pump comes into its own here as its closed loop control sensor is specifically designed to monitor and adjust the temperature accordingly.
So, as we’ve shown, viscous resin can be a challenge. It's thick and sticky, prone to hardening, sensitive to temperature and loses pressure. But our 25 years of experience have taught us that when it comes to pumping and metering viscous resin – be it in the aerospace, automotive or energy industries – a specifically designed pump such as the Beinlich with its interpolation electronics can help to significantly reduce these challenges. Maybe viscous resin doesn’t have to be such a tricky customer after all.
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