What are the differences between syringe barrel pistons?

In a pneumatic fluid dispensing system, a dispenser is used to apply pressure or vacuum to a column of fluid in the syringe barrel. Polyethylene pistons inserted in the syringe barrel on top of the fluid help manage the effect of air pulses on the fluid column. The result is accurate fluid dispensing.

A syringe barrel piston consists of a cone contoured to match the shape of the dispensing end of the syringe, topped by an annular sidewall (see Figure 1).

There are six different types of syringe barrel pistons for different fluid properties and dispensing challenges. They are differentiated by the shape of the sidewall, the fit to the syringe barrel, and physical attributes like channels and holes. Some pistons are designed for low-viscosity fluids. Some are designed for thick, particle-filled fluids. The most common challenge these different pistons are designed to control is piston bounce.

Figure 1: Nordson EFD’s white SmoothFlow™ pistons are general-purpose pistons designed to work well with a wide range of fluids.

Pistons can be separated into six types

• General dispensing pistons
• Pistons for high viscosity fluids
• Pistons for thick, stringy fluids
• Pistons for expensive high viscosity fluids
• Pistons for extremely thick materials dispensed at high pressures and cycle rates
• Pistons for mechanical dispensing

Read on to learn how to use them to get the best results from your dispensing process.

General dispensing pistons

A general-purpose dispensing piston features a sidewall with a concave contour that creates an upper and lower wiper (see Figure 2). The outer diameters of the wipers are sized to form an effective interference fit with the inner diameter of the syringe barrel. As the piston moves down the barrel during the dispensing process, the wipers push the fluid along with it, minimizing residue and fluid waste.

Channels cut into the cone help guide air that may be underneath the piston so that it can contact the fluid column effectively, minimizing air bubbles that impact fluid deposit consistency.

Figure 2: Wipers created by the convex contour of the white SmoothFlow™ piston’s sidewall both block air from moving into the region below the piston and also prevent the buildup of fluid residue, minimizing waste.

Pistons for high-viscosity fluids

One of the most common issues in the dispensing of thick pastes or particle-containing materials is piston bounce.

What is piston bounce?

During the dispensing of high-viscosity fluids, air sometimes gets trapped below the piston, separating it from the fluid (see Figure 3). Because air is compressible, motion of the piston does not necessarily equate to motion of the fluid column.

In between air pulses, the piston “bounces” back upward, leading to the origin of the term. From the standpoint of dispensing, piston bounce can lead to oozing and drooling after the termination of the dispense cycle.

Figure 3: Piston bounce occurs when air bypasses the sidewalls of the piston to create space between piston and fluid column. Because this air is compressible, moving the piston no longer dispenses a predictable amount of fluid.

Pistons for thick, stringy fluids

Thick, stringy fluids like RTVs can lead to piston bounce. To avoid this, use a straight-walled piston with a negative interference fit.

A negative interference fit means that the sidewall has no wipers and is not in contact with the inner diameter of the syringe barrel, so that it doesn’t “interfere” with piston motion (see Figure 4). Making room for air to flow helps prevent air from accumulating underneath the piston, which is the primary cause of piston bounce.

The trade-off is that some fluid residue remains on the sidewalls of the syringe barrel, leading to waste and higher costs.

Figure 4: To prevent piston bounce with thick, stringy fluids, use EFD orange Flatwall pistons. These pistons have negative interference fits – a slight gap between the sidewall and the inner diameter of the syringe barrel. The gap allows air to pass as the piston applies pressure to the fluid column, helping to prevent air buildup.

Pistons for expensive high-viscosity fluids

Leaving residue on the sidewalls of the syringe may be acceptable for low-cost materials like silicone caulk, for example, but some assembly fluids can be extremely expensive.

In these cases, it’s better to use a piston with a dual-wiper sidewall and a tighter interference fit than the orange piston above (see Figure 5). The design still addresses piston bounce but leaves less residue, reducing waste and costs.

Figure 5: To prevent piston bounce while minimizing residue for expensive materials, use EFD beige SmoothFlow pistons. They feature a tighter interference fit than our orange pistons while incorporating the same dual-wiper system as our white SmoothFlow pistons.

Pistons for extremely thick materials dispensed at high pressures and cycle rates

Dispensing thick paste or particle-filled materials at extremely high pressures and/or high cycle rates requires many air pulses for each deposit. This process can cause air to accumulate under the piston, leading to piston bounce.

For these kinds of materials, look for high-flexibility pistons engineered for a tighter contact with the syringe barrel inner diameter (see Figure 6). The sidewalls should be designed with dual wipers to block the passage of air.

Figure 6: Clear Flex pistons are extremely flexible and provide a positive interference fit, making them effective for extremely thick materials dispensed at high pressures and/or cycle rates.

Barrier pistons for thin fluids, including thin cyanoacrylates

Watery fluids and low-viscosity cyanoacrylates dispense under air pressure, and so do not need syringe barrel pistons to equalize pressure across the fluid column.

Syringe barrel pistons do play a key role in successful dispensing, however. Dispensing low-viscosity fluids involves alternating applications of positive pressure to dispense the fluid and vacuum to prevent dripping. This is where a barrier piston can help.

Barrier pistons are designed to stay in place in the syringe barrel, well out of contact with the fluid being dispensed. The sidewall of the piston has the tightest interference fit with the ID of the barrel, so that the barrel piston does not move as the fluid level changes.

The key to successful dispensing is a tiny hole in the piston cone that allows just enough air to pass through to permit very precise pressure control (see Figure 7).

Figure 7: Nordson EFD blue LV Barrier pistons feature sidewalls with dual wipers and a high positive interference fit to maintain the position of the piston even during pressure changes. The small hole allows air to pass through for very precise pressure control.

Pistons for mechanical dispensing

Manual or mechanical dispensers involve very high forces. The syringe barrel pistons used with them need to have a very tight fit to stand up to these mechanical forces. The sidewalls of the pistons should have dual wipers to reduce fluid waste (see Figure 8).

Figure 8: Red SmoothFlow pistons feature sidewalls with dual wipers and a very tight interference fit that can stand up to the extreme forces involved in manual or mechanical dispensing.