Fluid Dispensing Q&A
It is possible to use a hydraulic ram with an air cylinder to generate the force needed to displace fluid. While this type of system can generate a lot of force to push any type of fluid, it is limited in speed and accuracy. Depending on the size of the system, it will take time to charge and discharge the air cylinder. The closer to the end of the stroke you are, the longer it will take. Also, there is still energy in the system when the pressure is removed from the air cylinder. This results in fluid drooling from your output port.
I would recommend using some sort of valve on the output port to control the flow of the fluid. The type and size of valve is determined by the type of fluid and flow requirements that you have. This will increase the accuracy of the amount of fluid you are dispensing by shutting off the output and reducing drooling. Using a valve also increases speed by allowing you to keep the air cylinder continuously energised and using the valve to meter the dispensed fluid.
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.
So many companies asked for help with this problem that we developed the HP High-Pressure Dispensing Tools, which make it easy to apply RTV silicones, epoxies, medical-grade adhesives and other thick fluids through dispense tips as small as 0.004 in. diam.
These tools are designed to work with EFD precision fluid dispensers, and can multiply the output of a standard 100-psi dispenser up to 7X to produce a maximum dispensing pressure of 700 psi. This provides fast, fatigue-free application of thick assembly fluids, minimises the risk of repetitive stress injuries and increases yields by enabling workers to easily apply the correct amount of material in much less time.
I assume there is some sort of plunger or piston that needs to move freely inside the cylinder, and would suggest either a precision spray valve or a radial spinner valve.
The spray valve uses a low volume low pressure (LVLP) design to apply an even coating of material, while the radial spinner dispenses fluid onto a rapidly spinning disk to produce a uniform band. I have seen both produce excellent results -it just depends on whether you want to apply lubricant over a larger area or put it in a specific location.
Depending on your production volume and budget, the valve could be stand-mounted and controlled by the operator or incorporated into an automated dispensing station.
As the volume of fluid in the syringe decreases, the volume of air increases, so that it takes longer to build up the pressure needed to push the desired amount of fluid through the dispensing needle. Shot size can be easily maintained by:
- Increasing either the air pressure or the length of time air pressure is applied to the fluid. Most companies will choose to adjust air pressure and keep process time the same.
- Using a smaller syringe (5 or 3 cc) and filling it only halfway to minimise the change in air volume.
A third option is the recently introduced Optimeter. Used in place of the standard adapter that links the syringe to the dispenser, the Optimeter is a mechanical device that automatically adjusts air flow as the volume of fluid in the syringe decreases, so that shot size stays the same.
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.