Dispensing Drops: A Look at the Factors
Regardless of the material or the application, achieving a consistent flow of drops from a dispensing system can be much more challenging than designers may realize. There are a number of factors to consider as well as changing variables that can occur during the process. This article will offer considerations for engineers in need of a dispensing solution.
Dispensing a droplet of liquid is a simple task. Conversely, repeatedly dispensing a consistent drop of liquid is not an easy task. Droplet formation is dependent upon a number of factors, including fluid properties, system design, and system dynamics. System designers must pay close attention to these factors to maximize the consistency and repeatability of the droplet.
Dispensed volume = ƒ(pressure, system restriction, fluid properties, time).
Fluid properties, such as viscosity and surface tension, are usually dictated by the application. The designer must consider how the viscosity will change over the operating temperature range of the fluid. As the temperature increases and the fluid thins out, the droplet volume will increase unless something else in the system compensates for this change in viscosity.
The surface tension of the fluid will affect drop quality. If this is not considered carefully, drops may “wick” up onto the sides of the nozzle. Some of these drops may not break off, and some may have double volumes. This can result in a dispense stream that “drools” instead of drops.
A basic dispensing system consists of a fluid reservoir, a pressure source, a means to control the pressure (valve), and an outlet restriction (nozzle). The pressure source may be gravity, a pump, or a pressurized vessel. The key here is to keep the pressure constant. Any change in pressure will result in a change in the drop volume. The designer also needs to consider elevation changes with gravity systems, and recovery times on pressurized vessels. The accuracy of the pressure source or regulator must also be considered.
The outlet restriction, or nozzle, should act as the governing restriction in the system. It needs to be more restrictive than the rest of the system to “filter” out dynamic changes upstream. A common problem is designing a system where the restriction from the tubing is a significantly high percentage of the overall restriction. This will not be an issue on a single unit but if multiple systems are made, the tubing tolerance on the ID may affect the dispense volume.
The final consideration is the control valve of the system. This is the dynamic portion of the dispensing system. The open time, or on-time, of the valve is the final determining factor of dispensed volume. The longer the valve stays open, the larger the volume dispensed. To decrease the fluid volume, the valve should be opened for a shorter period of time.
When a valve is actuated, there is a delay between the electrical signal and the actuation of the valve. This is referred to as response time. When the valve is opened for very short periods of time, the response time becomes a significant portion of the overall on-time.
This leads to inaccurate dispenses with drops that do not have very repeatable volumes. When the valve is opening, it is operating on a very steep portion of the curve. Very small changes in on-time can have a significant effect on the plunger position. An on-time that is too short may actually prevent the valve from fully opening. This is a very unstable situation. A well-designed system will have a minimum operating on-time that is long enough to ensure the valve has fully opened, thus allowing a repeatable volume for that given time period.
Controlling Drop Volume
There is a triad of factors that can be used to control the drop volume. Each has its benefits and limitations.
As mentioned previously, if the on-time is too short, the valve may not fully open, which can result in erratic drop volumes. Some valves are also limited by a maximum on-time. The designer must determine if the valve is rated for continuous duty or has a limited allowable on-time. Exceeding this limitation may overheat the valve, causing permanent damage. Running the valve for extended on-times may also result in unwanted heating of the fluid. This could damage the fluid or change the viscosity, again altering the dispense volume.
Pressure is the motivating force on the fluid. Increases in pressure will increase the drop volume while decreases in pressure will reduce the volume. If the pressure is too high, it can result in high exit velocities at the nozzle. This can lead to splattering or splashing when the droplet makes impact with the surface. This can also cause problems with the stability of the droplet.
A minimum drop velocity is needed to ensure that the drop properly “jets” from the nozzle. Reducing the pressure too much leads to drops that do not cleanly break off from the nozzle. These drops will continue to build up, and eventually gravity will force the drops to dispense from the nozzle. This phenomenon is often referred to as “drooling.” This scenario causes missed dispenses and then a sudden “large drop,” which is actually several drops all at once.
The final factor is the diameter of the nozzle, or discharge orifice. Typically, this diameter is determined during the system design phase and is not easily adjusted during dispensing. The nozzle must not only be sized to handle the range of required dispenses but it must also allow for the pressure and valve on-time to remain in their stable ranges. There are also physical limitations that need to be considered.
Nozzles that are too large in diameter will not be restrictive enough to compensate for variations in the upstream components. Properly designed nozzles will have a high enough restriction (typically 10X more than the upstream system), which will make small system variations invisible to the overall dispense volume.
Nozzles sized too small will be very susceptible to clogging. Even systems that are very clean may have random wear particles that can clog a very small diameter nozzle. Exit velocities may also be a problem if the nozzle diameter is too small.
A robust dispense system can be achieved if steps are taken to ensure consistent, repeatable dispensing of droplets. Designers must consider the properties of the fluid being dispensed, as well as the dynamics and design of the overall system, to ensure consistent performance and reliability.
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