What kind of pistons should I use for thin fluids and cyanoacrylates?

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.

Figure 1: Nordson EFD blue LV Barrier pistons feature a small hole that allows air to pass through for very precise pressure control.

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).

Figure 2: Unlike conventional pistons, which maintain contact with the fluid column (left), barrier pistons are designed to be positioned above the fluid (right).

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).

Figure 3: To prevent cyanoacrylates from curing within the barrel reservoir, use a filter regulator with a coalescing filter.

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).

Figure 4: During the vacuum phase of the dispense cycle, thin fluids can be drawn back through the tubing and into the dispense controller, a phenomenon known as suck back. Using a filter trap (white cylinder) can prevent this issue and protect the equipment.