What is the function of a piston when fluid dispensing?

Syringe barrel pistons perform four key functions in pneumatic fluid dispensing systems:

• Preventing air pulses from tunneling a hole through thick pastes
• Eliminating suck back of thin fluids
• Ensuring consistent and well-controlled deposits of fluid
• Minimizing the amount of fluid remaining in the syringe reservoir after the piston has bottomed out

What is a syringe barrel piston?

In a pneumatic fluid dispensing system, a dispenser pulses air pressure on a fluid column in a filled syringe to precisely dispense the contents.

A syringe barrel piston is a polyethylene component that rests on top the fluid column inside the syringe (see Figure 1). Although pneumatic dispensing systems can be operated without these small components, syringe barrel pistons can significantly improve performance, reduce costs, and extend equipment lifetime.

Figure 1: A syringe barrel piston (white) is a plastic component that rests on the top of the fluid column inside the syringe barrel to assist with the fluid dispensing process.

A syringe barrel piston consists of a cone topped by a sidewall that interfaces with the inner diameter (ID) of the syringe barrel (see Figure 1).

The cone is precisely shaped to match the curvature of the bottom of the syringe barrel. The sidewalls above it can take a variety of forms, from straight and smooth to slightly concave.

Figure 2: Each of the syringe barrel pistons in the Nordson EFD Optimum® product line has specialty features that address specific dispensing challenges.

Now that we know what syringe barrel pistons are, let’s review their functions.

Syringe barrel pistons prevent tunneling

To dispense high viscosity fluids, the dispenser sends a series of air pulses into the syringe barrel to force the thick paste out through the dispenser tip.

In the absence of a piston, the air pulses are confined to a very small area in the center of the syringe barrel. In a short time, the pressure can tunnel through the paste in the center while the rest of the paste remains in the syringe reservoir.

After tunneling, air pulses cause the release of a mixture of air and paste, leading to inconsistent deposits and increased scrap or rework.

A syringe barrel piston converts localized air pulses into a uniform force across the full surface area of the fluid column, preventing tunneling and creating consistent, high-quality deposits.

Syringe barrel pistons eliminate suck back

When dispensing low-viscosity fluids, the system often needs vacuum after a deposit to prevent dripping and oozing. If the vacuum is too strong, the fluid can be sucked back into the air tubing, or worst-case scenario, into the dispenser itself.

When this happens, the dispenser can be permanently damaged, which leads to downtime while waiting for a replacement.

A syringe barrel piston can prevent “suck back” into the dispense controller.

Syringe barrel pistons ensure consistent and well-controlled fluid deposits

We’ve already talked about how the prevention of tunneling leads to more accurate and controlled application of high viscosity fluids. In thin fluids, a piston can make vacuum adjustment less sensitive as the volume changes in the syringe.

Syringe barrel pistons minimize residue

Wipers on the piston contact the internal diameter of the syringe barrel, removing residue as the fluid is dispensed. The cone of the piston is shaped to conform to the contours of the end of the barrel (see Figure 3). The combination of the two does a very effective job of dispensing nearly all of the material.

Figure 3: The Nordson EFD Optimum® White SmoothFlow™ piston features wipers to minimize residue left on the sidewalls of the syringe barrel after dispensing.