What is the correct way to insert a piston into a syringe barrel or cartridge?

Piston insertion techniques vary depending upon whether the material being dispensed is a thick paste, a room temperature vulcanizing fluid (RTV), a pourable fluid, a thin fluid, or a cyanoacrylate. Depending on whether the vessel being filled is a syringe barrel or a cartridge there may be additional variations. Let’s take a closer look.

How do I properly install a piston for thick pastes, RTVs, and pourable fluids?

For thick pastes, RTVs, and pourable fluids, the piston should be placed so that the cone is completely submerged in the fluid up to the lower wiper (see Figure 1). This positioning enables the piston to apply consistent force across the full surface of the fluid column, accurately depositing the desired amount of fluid with each pulse from the dispenser. If air gets trapped between the piston and the fluid column, it will lead to piston bounce and inconsistent deposits (see How can I prevent piston bounce?)

Figure 1: For thick pastes, RTVs, and pourable fluids, the cone of the piston should be submerged in the fluid up to the lower wiper.

Avoiding piston bounce starts with proper insertion of the piston. Follow these steps to install pistons for thick pastes, RTV’s, and pourable fluids. Note: this is only if you are filling the syringe from the top opening, not from the bottom. For more information on how to fill syringe barrels see Can I manually fill an EFD syringe barrel?

Step #1:
Insert the piston into the top of the barrel, pushing it down with the eraser tip of a pencil or some other sort of long, thin implement. Be careful to avoid damaging the inside of the syringe barrel, as that can compromise the seal between the piston and the barrel, allowing air to bypass the piston during dispensing.

Step #2:
Squeeze the sides of the syringe barrel to create a small amount of space between the piston wipers and the sidewalls. Use your implement to push the piston down until it contacts the fluid column.

Tip:
If squeezing does not release the air, try a different technique. Use a small, flexible shim like a piece of fishing line to open up a small amount of space between piston and sidewall. Dangle the fishing line in the syringe barrel before inserting the piston. Use the implement as in step one to push the piston down until it contacts the fluid. The shim should allow air to escape throughout this process; then just remove the fishing line. Note: be sure to use a soft shim and not something rigid like a metal wire that could damage the wipers on the piston and cause air blow by during dispensing (another way to cause piston bounce).

Installing a piston into a cartridge (Figure 2) uses basically the same technique. The plastic of the cartridge barrel is thicker than for a syringe barrel, however, which means it takes more force to deform enough to let air escape. Consider using two people for the procedure – one to squeeze the barrel and the other to push the piston down with the implement.

Figure 2: Nordson EFD’s Optimum Optimum Cartridges are available in clear, UV-blocking amber, and light-blocking black. Capacities include 2.5 oz, 6 oz, 12 oz, 20 oz, and 32 oz.

How do I properly install a piston for thin fluids and cyanoacrylates?

Thin fluids and cyanoacrylates require an entirely different approach. Thin fluids are able to flow under force of gravity, so they can be fed using very slight variations in air pressure, from pressure to vacuum. Thin fluids require the use of barrier pistons with air space intentionally left between the piston and the fluid column (see Figure 3). The dispenser pulses apply alternating pressure and vacuum to the piston. The piston applies pressure to the fluid column via air pressure rather than through direct contact.

Figure 3: Unlike conventional pistons, which maintain contact with the fluid column (left), barrier pistons are designed to be positioned above the fluid (right).

Although this technique can work using a solid piston, a better option is to use a piston with a small perforation, like the Nordson EFD blue LV barrier pistons (see Figure 4). The perforation allows direct airflow from the dispenser to the fluid column.

Figure 4: Nordson EFD blue LV Barrier pistons feature a small hole (circle) allows air to pass through for very precise pressure control.

Follow these steps to install pistons for thin fluids and cyanoacrylates:

Step #1:
Use a barrier piston with a hole in it to allow airflow.

Step #2:
Insert the piston into the top of the barrel, pushing it down with the eraser tip of a pencil or some other sort of long, thin implement. Be careful to avoid tilting or shaking the syringe barrel to prevent the liquid from splashing up onto the piston. This is particularly important in the case of cyanoacrylates, as their quick curing properties could cause the piston to permanently adhere to the side of the syringe barrel.

Step #3:
Leave at about an inch of space between the piston and the fluid column.

Step #4:
Store the syringe upright.

Have a question? Please reach out to an application specialist at [email protected] today.