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.

Yes, we have both an aseptic dispense valve and spray valve that can be autoclaved, cleaned-in-place or steamed-in-place.  The aseptic dispense valve is used for making dots, stripes, or small volume fills of fluids. The aseptic spray valve is used for spray coating small areas with inner diameters 1/8 in. and larger. Both valves are primarily used on automated machines.

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.

PTFE-coated tips are primarily used for fluids with a higher surface tension that tend to wick up the outer diameter of the cannula of a standard general-purpose tip. When you’re trying to make a small deposit, and part of it wicks up the cannula’s OD, the result will be an inconsistent deposit. After a few dispense cycles, the fluid buildup that has collected on the cannula’s OD will fall off, creating a larger deposit on your part. The PTFE coating lowers the surface tension of the cannula and prevents the fluid from wicking up the tip, resulting in very consistent dispenses.

First of all, if by “manual process” you mean a squeeze bottle or squeeze tube, you can forget about getting any type of process control. The control of the deposit is predicated on the “squeeze” of the operator. All operators will squeeze differently and even the same operator will squeeze differently as the day progresses. This type of method might be acceptable for a noncritical application, but medical device companies need process control, repeatability and predictability, none of which you will get by using a squeeze tube or squeeze bottle.

Using a time-pressure dispensing system is a great way to take control of a watery cyanoacrylate application. The most common dispensing system for a watery cyanoacrylate is a time-pressure system using a syringe as the fluid reservoir. To achieve the best process control, you want to choose a dispensing system that has a digital readout of all the important parameters: pressure, time and vacuum control. These digital settings can be documented and added to the work instructions for the specific project that you are working on.

The next choice will be the size of the reservoir you want to use for the application. A good rule of thumb is the smaller the deposit size that you are making, the smaller the reservoir you should use. Typically, a 3-cc reservoir can dispense hundreds to thousands of small dots of cyanoacrylate, depending on the dot size. If you are making larger dots or beads of cyanoacrylate, you will want to use a larger reservoir. The recommended fill level for watery fluids in the reservoir is two-thirds full.

Once the reservoir is filled, a piston should be inserted directly above the fluid, leaving approximately a 1/8-in. gap between the bottom of the piston and the fluid. Special pistons with a small hole are best for watery cyanoacrylates. In these cases, the piston serves two primary purposes: the hole allows air to flow through, pushing the cyanoacrylate out of the reservoir, and the piston acts as a barrier to prevent the cyanoacrylate from getting drawn back into the dispensing system.

The inner diameter (ID) of the dispensing tip should be based on the dot size that you are trying to make. Again, a good rule of thumb is to choose an ID that is approximately one-half the dot size. For watery cyanoacrylates, Teflon-lined or polypropylene tips are the most chemically compatible and will resist clogging.

Finally, the general parameters for dispensing a watery cyanoacrylate are low pressure (usually less than 3 to 5 psi), a pulse time that achieves the appropriate dot size (this is usually a trial-and-error process; it will probably end up at approximately 100 milliseconds), and an appropriate vacuum setting that will prevent the cyanoacrylate from dripping out of the dispensing tip.

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.

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.

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