This question is asked often, and my answer is based on comparing an air-powered syringe system to a positive-displacement syringe system. I’ll answer your question in the order in which you listed the criteria.

Repeatability

Air-powered dispensing is based on a time-pressure system. The operator steps on a foot pedal or actuates a finger switch, thereby sending a pulse of air to the syringe reservoir for a preset period of time, usually measured in milliseconds. The air pushes the fluid through the dispensing tip, a precise amount of which is applied to your part.

In a positive-displacement syringe system, the operator steps on a foot pedal that actuates a motor, usually a stepper motor. The motor is attached to a rod that pushes down on the fluid in the syringe reservoir, usually moving a predetermined distance or motor “steps.”  Once the rod has moved its preset distance, the stepper motor reverses direction, retracting the rod backwards to relieve pressure against the fluid and prevent dripping or excess flow.

Here’s the thing: both systems have their place. If you are working with a two-part epoxy with a limited working time-usually 30 minutes or longer-and you don’t want to make constant adjustments over the working life of the epoxy, then the positive-displacement system is probably the best way to go…for now.  New technology in air-powered dispensing is coming that will allow for automatic changes for fluids that change viscosity. Until then, you can continue to use the positive-displacement syringe system for your two-part epoxy. For all other fluids, I think air-powered dispensers are the way to go. They are easier to use and to set up than positive-displacement syringe systems.  More importantly, both systems are comparable in repeatability. The theory of how positive-displacement syringe systems work is sound; however, there are some flaws that might not make it as accurate as most people would think.

Speed of Operation

Hands down, air-powered dispensers are faster and can provide higher cycle rates than a positive-displacement syringe system…full stop!

Ease-of-Use

This is always a subjective topic. What might be difficult for one person is easy for someone else. Based on feedback from hundreds of companies that have tried both technologies, air-powered dispensers always win the “ease-of-use” debate. Again, this doesn’t mean that positive-displacement dispensers don’t have their place; it just means that a majority of people think that air-powered dispensers are easier to use, easier to set up, and easier to train their operators on.