What you describe is a common issue. Placing a viscous material into a syringe after you’ve vacuum degassed it generally can create air pockets. Centrifiging the material once you’ve placed it into the syringe should take care of this problem. Here is a link to the Nordson EFD centrifuge. It might take up to three minutes of centrifuging to remove the air pockets; however, the centrifuge has worked extremely well in the past for this situation.

Regarding your question about using positive displacement to minimise the air bubbles on your dispense bead: If you see air pockets in the dispense bead, that means air is trapped in the material. If that’s the case, a positive displacement dispenser will not be a benefit to you. Once you eliminate all of the air in the fluid, the air powered dispenser will work fine—the piston inside  the syringe will act as a barrier to prevent air from penetrating into your fluid.

That depends on the type of fluid you are dispensing. If you are dispensing a particle-filled material, partial clogging in the dispensing tip could cause the variations.

One of the most common factors to consider is your plant air supply. If you have fluctuations in the air-line going into the dispenser, you will almost certainly get variations in your deposit size. Make sure you have a in-line filter regulator between your plant air supply and the dispenser. If you do have plant air fluctuations, you should set the filter regulator approximately 5 to 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: there is air in your fluid, the vacuum feature isn’t set properly or you have 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 a faulty solenoid valve causing this. That leaves us with the vacuum feature and air in the fluid.

When a watery fluid is being dispensed, the vacuum feature prevents dripping in-between dispense cycles. When setting the vacuum feature for a particular fluid, slowly increase the vacuum until the fluid completely stops dripping from the dispensing tip. If you notice that bubbles are being sucked back into the syringe reservoir, then you have too much vacuum; simply back off the vacuum a bit and you should be fine.

We typically find, however, that dripping is caused from air in the material or by 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, it can be 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 with air pockets, the best way to eliminate dripping is by centrifuging the material inside the syringe to remove most of the air pockets.

The simple answer is “it depends.” If you are working with a two-part epoxy that has a working life of approximately 30 minutes or more, and you want to perform an initial setup of the dispenser and not make any changes over the working life of the epoxy, then a positive displacement syringe system would be more accurate than an air-powered dispenser. However, if you are using a standard fluid that does not change viscosity over time, then an air-powered dispensing system and a positive displacement syringe system are comparable in accuracy.

Some people have a difficult time believing this, so let’s take a closer look at how a positive displacement syringe system works. In either an air-powered system or a mechanical positive displacement system, the syringe side is identical: a fluid-filled syringe, a plastic piston inside the syringe that rests on top of the fluid and a dispensing tip attached to the bottom of the syringe. During the operation of a positive displacement system, a rod pushes against the piston inside the syringe to force out the fluid from the dispensing tip. The conventional thinking is that the rod moves down the syringe at a given distance, displacing a specific volume of fluid . . . and that is partly true. The rod does travel a certain distance, but when it gets to the preset distance, it retracts, relieving pressure on the fluid to alleviate any dripping or excess flow from the dispensing tip. To be a “true” positive displacement system, once the rod gets to its preset distance, it would remain there until all of the fluid displaced by that movement has flowed from the dispensing tip. This is virtually impossible because the fluid would flow out of the dispensing tip at an extremely slow rate, which would be impractical in real-world dispensing applications.

A second issue is the compressibility of fluid.  Many arguments have been made regarding what constitutes a compressible fluid, but the reality is that most fluids used in dispensing systems are compressible. Since the dispensing tip provides a restriction at the end of the syringe, the compressibility of the fluid is going to come into play with regards to the accuracy of the system.

The third issue is the retraction of the rod. Again, the rod retracts so that you get the achieved shot size in a reasonable period of time while eliminating excess flow or dripping from the dispensing tip, but the rod retraction is actually “cheating.” As I mentioned earlier, a true positive displacement system would advance the rod a certain distance and remain at that position until all of the fluid has been displaced.

To summarise, although each system uses different means of force (air pressure versus mechanical pressure) to push out fluid from a syringe, the concept for each is the same and their accuracy is comparable. There really is no significant difference between the two systems.