How can I prevent piston bounce?

Piston bounce occurs when air accumulates between the piston and the fluid column (see Figure 1). Because air is compressible, pressure applied to the piston no longer correlates directly with the amount of fluid dispensed. When pressure is removed, the piston bounces up, giving the phenomenon its name.

Figure 1: Piston bounce occurs when air accumulates beneath the piston. The air compresses when pressure is applied by the dispenser. When that pressure is removed, the air expands, causing the piston to rebound (bounce).

There are four causes of piston bounce:

• Incorrect choice of piston
• Air not removed during piston insertion
• Air bubbles in the fluid prior to dispensing
• Excessively high dispensing pressures or dispensing speeds

Let’s review how to prevent or fix each of these issues.

Incorrect choice of piston

One of the most common causes of piston bounce is air accumulating between the piston and the fluid column as a result of bypassing piston sidewalls. This can usually be prevented by proper choice of piston. In most cases, the general-purpose piston will provide the right fit to the inner diameter of the syringe barrel (see Figure 2) to prevent piston bounce.

Figure 2: Nordson EFD’s white SmoothFlow™ pistons feature a dual wiper sidewall design for a complimentary fit. If air does accumulate under the piston, precision-cut channels guide it upward to escape.

For processes that require extremely high pressures for cycle rates, look for pistons that are specially designed to operate effectively with these parameters (see Figure 3).

Figure 3: Ultra flexible Clear Flex pistons deliver a positive interference fit, making them ideal for extremely thick materials dispensed at high pressures and/or cycle rates.

Sometimes, the issue is not viscosity but another material property. Room temperature vulcanizing fluids (RTVs) can be very difficult to dispense because their quick curing properties make them stringy and clumpy, as well as thick. Rather than using a piston with a tight interference fit, the solution is to use a piston with flat sidewalls and a narrow gap between sidewall and syringe barrel (see Figure 4). This gap prevents air from accumulating during dispensing. The design leaves more residue on the walls of the syringe barrel, but this is a necessary trade-off for well-controlled deposits.

Figure 4: EFD orange Flatwall pistons are designed to leave a slight gap between the sidewall and the inner diameter of the syringe barrel. The gap let’s in the air below the piston escape to prevent piston bounce

For more information about the different types of pistons and how to select the right one for your application, see What are the differences between syringe barrel pistons?

Air not removed during piston insertion

Air can be trapped beneath the piston when it’s inserted into the syringe barrel for high viscosity fluid. Gently squeeze the barrel where the piston meets the barrel and use an object to press it down until the piston is in contact with the fluid column. Squeezing the barrel provides enough space for air to bypass the piston and escape.

Air bubbles in the fluid prior to dispensing

Although prefilled syringes are treated to remove air bubbles, new air pockets can form during shipping. Air bubbles can also be introduced during on-site filling when fill rates are too high. To prevent the introduction of air bubbles during filling, follow best practices (see Can I manually fill an EFD syringe barrel?) If bubbles exist, there are easy ways to remove them (see Why do I keep getting air bubbles in my syringe?)

Excessively high dispensing pressures or dispensing speeds

If the dispensing pressure is too high, air will bypass even the tightest fitting piston. This can occur in cases of trying to dispense very thick pastes or particle-filled fluids through an overly narrow cannula. It can be difficult or impossible to compress the fluid enough to pass through the cannula. Maxing out pressure won’t necessarily create the desired deposits, but it may very well force air past the piston to cause piston bounce.

A better approach is to use a tapered plastic tip (see Figure 5). The tapered shape and wider diameter lead to more accurate deposits with much lower dispensing pressures. For more tips on better dispensing of thick pastes, see What is the best dispensing tip for a thick paste?

Figure 5: Nordson EFD’s SmoothFlow tapered tips are optimized for dispensing thick pastes and particle-filled materials at much lower pressures.

For more information, or if you have any questions, don’t hesitate to contact us at [email protected].