Why do I keep getting air bubbles in my syringe?

Air bubbles in a pneumatically controlled syringe dispensing system can cause oozing and drooling, resulting in inconsistent deposits, reduced throughput, and increased scrap or rework. There are three main causes of air bubbles:

• Bubbles trapped in the fluids
• Improperly thawed frozen fluids
• Air bypassing piston wipers during high-speed dispensing of thick pastes

Bubbles trapped in the fluids

Air bubbles can be introduced in fluids of all viscosities. Air bubbles can form during filling of the syringes. In the case of low viscosity fluids, holding the syringe at an angle during filling can help minimize introduction of bubbles. In high viscosity fluids, using a piston with precision channels can help guide any bubbles up and around the piston to be purged around the side walls (see Figure 1).

Figure 1: Nordson EFD white SmoothFlow™ pistons feature precision molded channels designed to guide air bubbles upward during syringe filling.

Prefilled syringes from manufacturers are typically degassed to remove bubbles. Still, pressure changes and vibration during shipping can lead to the formation of new ones. For low-viscosity fluids, inverting the syringe with the threaded end (luer) facing up and tapping the barrel causes air bubbles to accumulate beneath the luer cap, where the excess air can be released (see Figure 2).

Figure 2: To remove air bubbles from a syringe filled with low- to medium-viscosity fluid, invert the barrel and tap gently. The bubbles will accumulate at the tip, where they can be released by removing the tip cap.

Alternatively, simply store the filled syringe barrel reservoirs in a vertical orientation with the tip down and let them settle for about 24 hours (see Figure 3). Air will accumulate at the piston; gently squeezing the barrel at the piston will create a gap to release the excess air.

Figure 3: Storing filled syringes with the tip down for 24 hours (left) allows air to migrate up to the piston. Squeezing the barrel just below the piston creates a gap to release the excess air (right).

High-viscosity fluids can be degassed with a few minutes in a centrifuge to collect air bubbles at the luer end for release.

Improperly thawed frozen fluid

Another source of air bubbles is not following proper procedure in the thawing of frozen fluids. Frozen fluid thaws at a different rate than frozen plastic. Trying to accelerate defrosting using elevated temperatures can lead to the formation of free thaw voids (FTVs) – air bubbles between the fluid and the inside wall of the syringe barrel. To avoid this issue, always follow manufacturer’s instructions for thawing.

High-speed micro dispensing of thick pastes

High pressure micro dispensing of thick pastes can lead to air bubble formation, particularly when performed at high cycle rates. Air bypasses the piston wipers to accumulate beneath the piston. This buildup of air can lead to piston bounce, which impacts dispensing performance. To avoid this issue, choose a piston with a tight, yet flexible fit (see Figure 4)

Figure 4: With a tight yet flexible fit, EFD’s Clear Flex pistons prevent air from bypassing the wipers and accumulating between the piston and the fluid column.

The orange flatwall piston and beige SmoothFlow piston also have features that help customers prevent air bubbles from forming under the piston. See What are the differences between syringe barrel pistons? for more details.