Positive Displacement Piston Pump
What is a positive displacement?
Positive displacement can mean a few things: there is a physical property – the correct meaning, then there is a bunch of marketing hype designed to convence you to part with your money. First the physical property:
Positive displacement is actually a concept you know very well. It refers to a solid replacing a liquid. For example, lets assume you fill a bath tube up to the very top – there is no room for anything else. Then step in the bath. What happens? Water flows over the sides onto the floor – right? That is positive displacement! The amount of water that comes out is exactly the volume of your foot and leg.
Practical use of Positive displacement
Lets switch our example around and look at it in a more industrial way. It you have a cylinder full of liquid, with an opening at the top and a piston coming from the bottom, and the piston moved into the cylinder – the amount of volume the cylinder displaces is the exactly amount that volume. From this, we can design a dispensing valve (Actually several have been designed)
What is a positive displacement piston pump ?
This pump is the first and in many ways an extremely clever example. It is from Creative Automation.
It uses an air soleniodand compressed air (about 100 psi) to move up and down. There is a shaft though the center. On the top of the shaft is threaded and a knurled stop can be lowed to stop the shaft at any point. Below the solenoid is a piston that goes through an alignment bushing and into a fluid chamber, then into a hub with a cylinder slightly bigger than the diameter of the piston.
How it works.
How do piston pumps work ?
A constant air pressure of 15 to 20 PSI is applied to a syringe of material forcing the material (Glue) down a material feed tube. The feed tube is mounted to the piston chamber. The chamber is filled when the piston is in it’s up stroke.
When the piston is driven down into the hub of the needle the material is forced out the needle. The timing of the piston’s up stroke is programmed into the software controlling the equipment and allows just enough time to let the chamber fill.
When the piston is in the down stroke the material feed path is closed off so that no additional material can get out the needle. Because the material is forced out the needle mechanically the deposition time is extremely quick and is dependent on the piston stroke cycle time. That makes it a “positive displacement pump”
The length of the piston’s stroke and the diameter of the piston are what determine the volume of the material displaced. It is displaced out the needle or dispensed. Because the volume of the material is determined by two easily controlled parameters (diameter and stroke) the viscosity of the material plays a minimal role in the volume of material dispensed. The diagrams below show the sequence of steps involved in piston pump dispensing.
How it works up close
Step 1 - The piston is in the up stroke position and material is allowed to fill the piston chamber. As the piston moves upward it will create a vacuum which helps suck the material down the material feed tube and into the piston chamber.
Step 2 - The material has filled the piston chamber and the piston is ready to start the down stroke.
Step 3 - The piston begins to seat into the needle hub. The actual volume of material that will be dispensed is determined by the piston displacement at this stage..
Step 4 - The piston has completely seated inside the needle hub and material has been dispensed onto the PCB.
And that is the concept
If the piston is not seated properly inside the needle hub at the end of it’s down stroke, then low viscosity materials can ooze past the piston resulting in unwanted material dripping or drooling out of the needle. Drooling can occur if the piston is not seated far enough down into the needle hub or if the fit of the piston in the needle hub is not tight enough. Drooling will also occur if the air pressure supply is turned up to high on the syringe.
Why is constant air pressure important for the syringe feed on piston pumps ?
Constant air pressure maintains a constant force on the syringe plunger so that the piston’s up stroke timing will not have to compensate for the time it would otherwise take to compress the air volume between the air inlet and the plunger. There are other benefits to maintaining constant air pressure besides maintaining a ready supply of material. It also reduces the number of parts needed to supply and regulate the air pressure to the syringe. A constant air pressure of 15 to 20 PSI is recommended. Excessive air pressure or insufficient air pressure will cause adverse effects such as drooling or cavitation.
Insufficient Air Pressure - If the air pressure on the syringe is too low there will not be enough force on the plunger to move the wanted volume of material out the needle. If the piston fires before the material has filled the chamber then the piston will displace an inconsistent volume of material resulting in cavitation, insufficient or missing dots
Excessive Air Pressure - If the air pressure on the syringe is too high it will force unwanted material between the piston and the needle hub causing material to drool from the needle. Excessive air pressure can also cause the chamber to fill too quickly . The piston up stroke length would then need adjustment to prevent material from flowing out the needle prior to seating of the piston.
How does material viscosity effect dispensing consistency ?
Material viscosity has little effect on dispensing consistency of positive displacement piston pumps. The only effect material viscosity has in piston pumps is on the length of time needed for the piston up stroke. This is because lower viscosity materials will tend to flow quicker than that of higher viscosity materials allowing the piston chamber to fill quicker. The graph below shows how resilient the piston positive displacement pump is to viscosity changes. It shows volumes of material dispensed over a wide range of temperatures or viscosities since viscosity is directly proportional to temperature.
How does syringe level effect piston pumps ?
Changes in levels of material within the syringe effect the rate of material flow into the piston chamber. The syringe level effect on material flow rate is caused by the changing surface friction area of material wetting the syringe sidewall. This change in surface friction directly effects the amount of pressure required to move a consistent volume of material out of the syringe. As the syringe empties it requires less pressure to obtain the same flow rate as when the syringe was full and therefore the piston chamber fills quicker. Syringe level differences in piston pump dispensing systems ultimately have very little effect on dispensing consistency since the air pressure is only used to fill the piston chamber and not to force material out of the needle. The actual change in deposition volumes is less than one percent over the entire syringe. This is because the piston is only in the up stroke position long enough for the material to fill the piston chamber and the volume dispensed is metered mechanically through piston stroke length piston diameter.
How does the piston stroke and diameter effect volume ?
As the piston enters the needle hub the material inside the needle hub is displaced. Therefore the distance the piston travels inside the needle hub along with the it’s size or diameter determines the actual volume of material that is dispensed. The piston stroke can be adjusted by moving the collar on the piston cylinder. The collar limits the down stroke of the piston into the needle hub and determines the distance the piston travels inside the needle hub. Improper adjustment of the piston stroke can cause drooling or cavitation. It is important to use the correct piston diameter and stroke so that the optimal volume of material is dispensed with each piston stroke. The piston stroke length can be set up using feeler gauges or by experimentation. Adjusting the piston collar up or down will directly change the volume of material dispensed. The selection of piston diameters is limited because the needle and the piston must be matched together and the number of resulting combinations would be difficult to control. It is common to control the specific volume needed by utilizing one of the standard piston diameters and setting the piston stroke accordingly. Specific material volume needs can designed into the piston and needle combinations and be specially designed, ordered and manufactured.
How often do piston pumps need to be cleaned ?
Materials left in the piston pump will tend to dry and cure eventually blocking the material flow path. This dry materials can flake off and become lodged in the needle hub blocking flow into the needle. Regular maintenance is needed to ensure proper performance and best results. Manufacturers of positive piston displacement pumps recommend cleaning the needle daily and cleaning the pump weekly.
The actual volume dispensed can be calculated mathamatically using piston diameter and piston stroke length and can also be experimentally validated. There are some tolerance losses, but there is good correlation between the actual volume displaced and the mathematical model results.
Advantages : There are some very distinct advantages in using mechanical actuation to meter and maintain the volume of material dispensed. Mechanical actuation dispensing is very consistent and is insensitive to changes in material viscosities and syringe level.
Disadvantages The major disadvantage of the piston positive displacement pump is that the volumes are determined by the piston stroke which is manually set by adjusting the collar. Manual adjustments are generally less accurate and less easily repeatable.
Some materials (like silicones) compress. A positive displacement pump counts on the density of the material relating directly to the volume. Compressable materials allow this relationship to change – not good.
Filled materials (like solder paste) have particles. These can get trapped in the pinch between the piston and the cylinder edge or wall, causing problems. Solder pastes “coin” or smash solder balls together and cold weld a plug in these pumps.