Why am I getting fluid around the piston in my syringe barrel?

If fluid is getting past the piston, the reason could be:

• Incorrect piston type for the application
• Improperly chosen process parameters
• Wrong dispensing tip for the application

Additionally, fluid residue on the barrel walls is a necessary trade-off of some processes and for some very loose-fitting pistons.

Incorrect piston type for the application

A piston consists of a cone topped by a sidewall. Sidewalls are typically designed with wipers that provide a complimentary fit to the inner diameter (ID) of the syringe barrel (see Figure 1). As the piston is pushed down the syringe barrel by air pulses from the dispenser, these wipers block the movement of fluid upward. (Note: there are exceptions to this rule – we will discuss one below.)

Figure 1: A syringe barrel piston consists of a cone that matches the shape of the syringe barrel, topped by a sidewall. Sidewalls typically feature wipers designed to fit the inner diameter of the syringe barrel, preventing the passage of fluid.

If fluid is bypassing your piston, you may not be using the appropriate piston for the fluid type and process parameters. For information on selecting the right piston for your dispensing applications, see What are the differences between syringe barrel pistons? Keep in mind that certain pistons are designed to allow fluid to build up around the piston due to the nature of the fluid being dispensed.

Improperly chosen process parameters

Not all dispensing process parameters can be applied successfully. There has to be a balance between pressure, tip diameter, and cycle time. Using very high pressure to force a thick fluid through a very small tip at high speeds, for example, can be problematic. In many cases, the fluid is just too thick to be quickly compressed and forced out of a narrow tip. This set up may bury the piston in the fluid, leading to inconsistent deposits.

Carefully consider each element in the system for each process and be prepared to make trade-offs in pressure and cycle time. You may also need to choose a different dispensing tip. This brings us to our next point.

Wrong dispensing tip for the application

Choosing a dispense tip for an application is equal parts art and science. Many dispense tips feature a metal cannula bonded into a plastic hub. The fluid undergoes compression when passing from the syringe barrel into the much smaller hub, and then again when passing from hub to cannula. This compression is a particular problem for thick pastes and particle-filled materials.

For thick pastes and room temperature vulcanizing (RTV) materials, see “What is the best dispensing tip for a thick paste?”

Figure 2: Nordson EFD’s SmoothFlow tapered tips are designed to deliver higher flow rates even with highly viscous materials to prevent fluids from bypassing the piston. They can be made with UV-blocking additives, which prevents premature curing of UV-sensitive materials.

Fluid residue on the barrel walls is a necessary trade-off of some dispensing processes

Thick, stringy fluids such as RTVs can be challenging to dispense cleanly. The best choice of piston for these fluids is one with a flat sidewall that leaves a narrow gap between the piston and the ID of the syringe barrel (see Figure 3). The gap makes it possible to dispense these materials smoothly.

Figure 3: For dispensing thick, stringy fluids like RTVs, use a piston with a flat sidewall that leaves a narrow gap between piston and the syringe barrel inner diameter.

The drawback is that use of this piston type will leave some residue on the syringe barrel wall, but that’s an expected result. It’s part of the trade-off for successfully applying these materials.

Don’t hesitate to reach out to an EFD application specialist at [email protected] if you have any questions.