Syringe barrel pistons perform four key functions in pneumatic fluid dispensing systems:

• Preventing air pulses from tunneling a hole through thick pastes
• Eliminating suck back of thin fluids
• Ensuring consistent and well-controlled deposits of fluid
• Minimizing the amount of fluid remaining in the syringe reservoir after the piston has bottomed out

What is a syringe barrel piston?

In a pneumatic fluid dispensing system, a dispenser pulses air pressure on a fluid column in a filled syringe to precisely dispense the contents.

A syringe barrel piston is a polyethylene component that rests on top the fluid column inside the syringe (see Figure 1). Although pneumatic dispensing systems can be operated without these small components, syringe barrel pistons can significantly improve performance, reduce costs, and extend equipment lifetime.

A syringe barrel piston consists of a cone topped by a sidewall that interfaces with the inner diameter (ID) of the syringe barrel (see Figure 1).

The cone is precisely shaped to match the curvature of the bottom of the syringe barrel. The sidewalls above it can take a variety of forms, from straight and smooth to slightly concave.

Now that we know what syringe barrel pistons are, let’s review their functions.

Syringe barrel pistons prevent tunneling

To dispense high viscosity fluids, the dispenser sends a series of air pulses into the syringe barrel to force the thick paste out through the dispenser tip.

In the absence of a piston, the air pulses are confined to a very small area in the center of the syringe barrel. In a short time, the pressure can tunnel through the paste in the center while the rest of the paste remains in the syringe reservoir.

After tunneling, air pulses cause the release of a mixture of air and paste, leading to inconsistent deposits and increased scrap or rework.

A syringe barrel piston converts localized air pulses into a uniform force across the full surface area of the fluid column, preventing tunneling and creating consistent, high-quality deposits.

Syringe barrel pistons eliminate suck back

When dispensing low-viscosity fluids, the system often needs vacuum after a deposit to prevent dripping and oozing. If the vacuum is too strong, the fluid can be sucked back into the air tubing, or worst-case scenario, into the dispenser itself.

When this happens, the dispenser can be permanently damaged, which leads to downtime while waiting for a replacement.

A syringe barrel piston can prevent “suck back” into the dispense controller.

Syringe barrel pistons ensure consistent and well-controlled fluid deposits

We’ve already talked about how the prevention of tunneling leads to more accurate and controlled application of high viscosity fluids. In thin fluids, a piston can make vacuum adjustment less sensitive as the volume changes in the syringe.

Syringe barrel pistons minimize residue

Wipers on the piston contact the internal diameter of the syringe barrel, removing residue as the fluid is dispensed. The cone of the piston is shaped to conform to the contours of the end of the barrel (see Figure 3). The combination of the two does a very effective job of dispensing nearly all of the material.

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.

Pistons can be separated into six types

1. General dispensing pistons
2. Pistons for high viscosity fluids
3. Pistons for thick, stringy fluids
4. Pistons for expensive high viscosity fluids
5. Pistons for extremely thick materials dispensed at high pressures and cycle rates
6. 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.

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.

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.

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.

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.

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).

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).

Syringe-based pneumatic dispensing systems use syringe barrel pistons to manage pressure applied to the fluid. In the case of high-viscosity fluids, pistons directly contact the material to distribute pressure evenly across the fluid column. This prevents tunneling by the air pulses of the dispenser, ensuring consistent deposits and presenting waste.

Although general-purpose pistons with an effective fit to the syringe barrel inner diameter work well for many fluids, they are not appropriate for thick, stringy fluids like RTVs.

Dispensing systems for thick, stringy fluids can suffer from a phenomenon known as piston bounce, which can significantly impact dispensing performance. The best piston to prevent piston bounce during dispensing of thick, stringy fluids like RTV’s is a loose-fitting piston with flat sidewalls.

What is piston bounce and why is it a problem?

In pneumatic dispensing, extensive testing takes place prior to production to relate the amount of pressure applied to the piston – and the fluid column – to the amount of fluid dispensed.

In theory, once this relationship has been established, dispensing should be easy. Unfortunately, the fluid in prefilled syringes often contains air bubbles.

During the dispensing process, this can cause air to build up between the RTV fluid column and the piston (see Figure 1). Because air is compressible, pressure on the piston no longer applies an equivalent amount of pressure to the fluid column.

This leads to discrepancies in the amount of fluid dispensed. When pressure is removed, the piston rebounds – bounces – giving the phenomenon its name.

To prevent the accumulation of air, it’s important to use the correct piston. We no longer want to have an effective fit between the piston sidewalls and ID of the syringe, nor do we want to have wipers.

The piston should have straight side walls and a negative interference fit – a small amount of space between the sidewall and the internal diameter (ID) of the syringe barrel (see Figure 2). This narrow gap enables air to bypass the piston, preventing piston bounce.

The trade-off for this benefit is that a certain amount of fluid residue will remain on the sidewalls of the syringe barrel, but this is a small price to pay for accurate dispensing in critical industrial applications.

Thin fluids and cyanoacrylates present an entirely different set of material characteristics and challenges than higher viscosity fluids.

Low-viscosity fluids require precise pressure control for accurate dispensing. Because they are thin enough to feed under gravity, thin fluids respond to the slightest pressure variation, and typically require application of suction, or vacuum, immediately after the deposit to prevent dripping.

To support this type of pressure control, low-viscosity fluids and cyanoacrylates require a special type of piston known as a barrier piston.

One of the key features of the barrier piston is a small hole that permits bidirectional airflow (see Figure 1). This enables the dispenser to apply a precise amount of pressure to make a deposit.

Barrier pistons differ from other piston types in another important way. Rather than being put into contact with the fluid column, like pistons for high-viscosity fluids, barrier pistons are positioned about an inch or less above the fluid (see Figure 2).

This position is particularly important for cyanoacrylates, which are quick-curing adhesives. If the piston comes into contact with the adhesive, it will be instantly bonded to the sides of the syringe barrel reservoir.

Barrier pistons are designed with very tight interference fits to maintain their position in the barrel, despite being cycled repeatedly between positive pressure and suction.

Another important consideration in the dispensing of cyanoacrylates is that moisture can cause the adhesive to cure inside the barrel reservoir. To prevent this, consider installing a filter regulator with a coalescing filter (see Figure 3).

Finally, the hole in the barrier piston means that not only air but also fluid can pass through if the applied suction is too high. In such cases, the liquid could move into the air lines, or even into the dispensing controller, potentially causing catastrophic damage.

To protect the controller, use a syringe adapter with a filter trap in the line (see Figure 4).