There could be several causes, depending on what type of fluid you are dispensing. If you are dispensing a particle-filled material, you could potentially get some partial clogging in the dispensing tip, causing the variations. One of the most common factors to look for is your plant air supply. If you have fluctuations in the air line going into the dispenser, you will most certainly get variations in your deposit size. Make sure you have an in-line filter regulator between your plant air and the dispenser. If you do have plant air fluctuations, you should set the filter regulator approximately 5–10 psi lower than your lowest plant air fluctuation point.

When a fluid drips out of the dispensing tip at the end of the dispense cycle, it usually means one of three things: air in your fluid, the vacuum feature isn’t set properly, or a faulty solenoid valve. Generally speaking, as long as you’re using dry, filtered air to supply the dispenser, the solenoid valve should work fine. In fact, there are very few instances of faulty solenoid valves.

That leaves us with air in the fluid and the vacuum feature. When dispensing a watery fluid, the vacuum feature prevents dripping between dispense cycles. When setting the vacuum feature for a particular fluid, you want to increase the vacuum slowly until the fluid completely stops dripping from the dispensing tip. If you notice that bubbles are being sucked back into the syringe reservoir, you have too much vacuum. In that case, simply back off the vacuum a bit and you should be fine.

We typically find that the main cause of dripping is from air in the material or from an air bubble trapped in the hub of the dispensing tip. If you are dispensing a watery fluid and using a small-gauge dispensing tip, sometimes it is difficult for an air bubble to purge itself out of the dispensing tip. In this case, we recommend filling the hub of the tip with your fluid first, then attaching it to the bottom of the syringe reservoir. For thicker pastes that have air pockets, the best way to eliminate dripping is by centrifuging the material inside the syringe to remove most of the air pockets. This procedure will ensure trouble-free, drip-free dispensing.

PTFE-coated tips are primarily used for fluids that have a higher surface tension and tend to wick up the outer diameter of the cannula of a standard general-purpose tip. When trying to make a small deposit, if part of that deposit wicks up the outer diameter of the cannula, then the result will be an inconsistent deposit. After a few dispenses, the build-up of the fluid that has collected on the outer diameter of the cannula will fall off, creating a larger deposit on the part. The PTFE coating lowers the surface tension of the cannula, so that the fluid being dispensed will not wick up the tip, resulting in very consistent dispenses.

This question is asked often…and my answer would be based on comparing an air-powered syringe system to a positive displacement syringe system. I’ll answer the question in the order of criteria that you listed.

1. Repeatability – Air-powered dispensing is based on a time-pressure system. The operator steps on a foot pedal or actuates a finger switch and a pulse of air is sent to the syringe reservoir for a pre-set time, usually in milliseconds. The air pushes the fluid through the dispensing tip and a precise amount of is applied to your part.

   In a positive displacement syringe system, the operator steps on a foot pedal which 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 a predetermined distance or “steps” of the motor. Once the rod has moved it’s pre-set distance, the stepper motor reverses direction which retracts the rod backwards to relieve the pressure against the fluid to 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 be making constant adjustments over the working life of the epoxy, then the positive displacement system would probably be the best way to go…for now. New technology in air-powered dispensing is coming to 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 the air-powered dispensers are the way to go. First of all, they are easier to use and easier to set-up than a positive displacement syringe system. More importantly, both systems are comparable in repeatability. The theory of how positive displacement syringe systems work is good, however, there are some flaws that might not make it as accurate as most people would think.

2. Speed of Operation– Hands-down, air powered dispensers are faster and can provide higher cycle rates than a positive displacement syringe system…period!
3. Ease-of-Use – This is always a subjective topic…what might be difficult for one person is easy for another person. Based on feedback from hundreds of companies that have tried both, the air-powered dispensers always win the “ease-of-use” debate. Again, this doesn’t mean that the 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.

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