What kind of pistons should I use for thick, stringy fluids like RTVs?

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.

Figure 1: In piston bounce, air accumulates between the fluid column and the piston, reducing the accuracy of dispensing.

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.

Figure 2: EFD Orange Flatwall pistons have negative interference fits – space between the syringe barrel and the piston sidewall – that prevents air from accumulating underneath the piston and causing 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.