How do you purge air from a syringe barrel?

Air in the syringe barrel reservoir of a pneumatic precision dispensing system can compromise dispensing performance, causing oozing, drooling, or incomplete deposition. Here, we will tell you how to purge air from a filled syringe barrel.

How to purge air from syringe barrels containing low- to medium-viscosity fluids (pourable fluids)

As with many dispensing issues, the solution varies depending on fluid viscosity. Here are the steps for purging air from syringe barrels holding pourable fluids:

1. Invert the syringe barrel so that the threaded tip that connects to the dispense tip (the luer) is facing upward (see Figure 1).
2. Tap the side of the syringe barrel to release air bubbles so that they can rise toward the tip.
3. Remove the tip cap and push the piston upward slowly to purge the air bubbles.

Figure 1: To remove air bubbles from a syringe of pourable fluid, turn the syringe barrel upside down (left). Tap the side of the syringe barrel to encourage bubbles to rise to the top. Remove the tip cap and push piston upward gently to force air out (right).

How to purge air from syringe barrels containing high-viscosity fluids

For thick pastes and particle-filled materials, air bubbles can be trapped in the fluid during syringe filling. These bubbles will compress during the dispense cycle, then expand after the cycle terminates. This can cause oozing, which can lead to inconsistent deposits.

Because of the nature of these high-viscosity fluids, simply tapping won’t remove the air bubbles. Here are the steps for removing air from a syringe barrel of high-viscosity fluid:

1. Place the syringe barrel reservoir in the centrifuge according to the instructions of the centrifuge.
2. Run the centrifuge per the instructions. This should collect any air bubbles at the luer end of the syringe barrel. The exact duration and RPM setting varies depending on the fluid properties and syringe dimensions.
3. Remove syringe barrel from the centrifuge, holding it with luer end up.
4. If a piston is installed, gently squeeze the sides of the syringe barrel where the piston sits. Using the eraser tip of a pencil or a similar implement, gently press the piston upward until it contacts the fluid column. Continue applying pressure until all air has been expelled at the luer end of the barrel.
5. Replace the tip cap at the luer end. Replace the syringe barrel cap at the other end of the barrel.
6. Store syringes vertically with the luer end facing down.

How to store filled syringes for easy removal of air bubbles

Always store syringes vertically, with the luer end facing down (see Figure 2). This allows air pockets to migrate upward. Next, remove accumulated air using the following steps:

1. Store filled syringe barrels vertically, right side up.
2. Before use, squeeze the sides of the barrel and push the piston down to purge the air.

Figure 2: Store syringe barrels filled with high-viscosity fluids and pastes vertically so that air migrates upward when ready to use, pinch the barrel to create a gap between piston and sidewall and press the piston down to release air (right).

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