Small scale fluid control in high precision production processes

Controlling and dispensing precise volumes of fluid, down to the microlitre and nanolitre range, is a common challenge in production processes. The experts at the Lee Company discuss design considerations to control the key variables in two applications.

Whether you’re designing a high throughput production line for lateral flow assays, consumable manufacturing, drug discovery or inkjet printing, the success of the system is often based on control of three main variables: pressure, nozzle or orifice precision, and valve speed.

Within inkjet printing, the quality of the print output hinges on the precision with which ink droplets are formed and land on the substrate. Controlling the geometry and stability of the meniscus, that is, the surface curve of the ink droplet when in contact with the nozzle, is most critical for proper droplet formation and jetting. Ensuring a stable meniscus requires precise control over the negative fluid pressure in the ink reservoir.

Too high a vacuum can lead to air ingestion and starvation, while too low a vacuum can result in nozzle plate flooding or ‘wetting-out’. Pulsatility in the ink reservoir can lead to periodic changes in print output. These effects lead to irregular jetting and compromised print quality.

Many current systems use a motor-driven diaphragm pump to provide the negative pressure in the ink reservoir. Such pumps are pulsatile and offer limited controllability, meaning that additional components such as pulsation-dampers and proportional valves are required to achieve acceptable printhead performance.

The pulsation-free output of the Lee Company’s piezoelectric disc pumps and their infinite turn down ratio ensure there is no pressure variation in the printhead, eliminating the need for those additional components. At just 29 mm in diameter and 5 grams, the disc pump is small enough to be directly integrated with the printhead. This enables a modular approach, with the ability to tailor vacuum levels for each print head, ultimately reducing system complexity and size while increasing design freedom.

To provide a more plug-and-play solution, the Lee Company also offers precision orifices, bleed valves and solenoid valves which can be integrated with a disc pump into a fully tested and assembled manifold solution.

Precision reagent dispensing

The consistent control of dispensed fluid volume, or droplet size, is a critical factor when dispensing reagents in the production of lateral flow assays or rapid diagnostic tests. For example, reagent volumes dispensed on porous media or into consumables directly relate back to the sensitivity and specificity of those tests, hence the emphasis on precision dispensing during production.

For these types of applications, time-metered dosing systems offer simplicity while providing accurate liquid dispensing across a wide range of dispense volumes – from nanolitres to millilitres. In these systems, a constant pressure is generated and a dispense valve, also known as a microvalve, opens and closes to control the volume being dispensed through the nozzle or orifice.

The precision of the nozzle or orifice has a significant impact on the precision of dispensed volumes. Manufacturing tolerances of the orifice can lead to differences between various dispensing channels. Burrs can lead to splitting streams or unwanted satellite droplets.

The speed, or response time, of a dispense valve controls how quickly fluid is ejected from a nozzle, and how little fluid can be dispensed at a time. A faster actuation speed creates more predictable dispenses because flow starts and stops very quickly. These faster, consistent solenoid valves can also eject droplets, breaking the surface tension of the liquid.

Pressure can be generated by several sources but most common is the ‘air over liquid’ system, which uses air pressure supplied by a compressor and regulated to a dispense pressure. New pumping technologies, such as the Lee Company’s piezoelectric disc pumps, generate pressure and use a closed loop feedback system to act as an all-in-one pump and regulator solution.

This eliminates the need for separate compressors and regulators, thus reducing system costs and size. Their pulsation-free output and infinite turn down ratio enable maximum accuracy when holding a target pressure above the fluids, resulting in a fine-tuned, highly repeatable system that redefines what it means to generate and hold a constant pressure.