Auger (or Screw) Valve

What is an Auger pump?
An Auger pump utilizes a screw (an auger) to turn fluid down a cylinder. The rotation of the screw creates a shearing force on the fluid which forces the fluid down the threads of the screw. This technology has been around since the Egyptian times where the technology was used to move water from the Nile River. An example of an Auger pump is shown here. I think of this like a drill. Drills cut into solids (like wood) and moves the saw dust out the groves. An auger valve is fixed and moves the fluid through the threads.

 

How do Auger pumps work?
Air pressure on a syringe forces fluids down a tube and into the valve. The fluid hits the (auger) screw. The screw is turning and drives the fluid down along the threads. This turning, or rotation, is driven from the motor above. The motor is controlled in a couple of ways:  It can be turned on for a set amount of time or a set rotation (like 2 turns or 3 turns). These two types are explained in a little more depth below.

The rotation of the screw creates a shear force on the fluid which forces the fluid to move down the screw. The speed and how much rotation control the amount of fluid going through.
As the fluid reaches the bottom of the screw, the flow dynamics change. It takes some force to get the fluid to flow through the needle and the “hub” right before the needle starts. This is a resistance to flow. The screw will continue to turn creating a needle pressure which is high enough to force fluid through the needle. Narrow, long  needles have a lot of resistance and short, big needles have much less. This restriction creates back flow. The screw is pushing fluid into the hub and needle, faster than the fluid can move through the needle, so it starts to flow back up the sides toward the syringe. This back flow is proportional to the pressure drop from the needle hub to the syringe. The flow out the needle is equal to the pump shear flow minus the back flow. If the needle requires a higher pressure to flow than the screw can deliver then back pressure is created. The back pressure is the pressure necessary to create a flow of fluid between the screw and the screw housing. The back pressure created by the turning of fluid by the screw is equal to the pressure relieved through the needle minus the pressure of the fluid created by the screw.  It other words there are a couple of huge differences between positive displacement pumps and auger valves: One: The needle configuration – that is how big the opening is and how long it is – dramatically affect how much fluid comes out. And Two: if the pressure is too much, the fluid churns in the valve and gets beaten up – this can cause real problems depending on the fluid – like curing glue, or coining and cold welding solder balls, and wearing out the screw and “sleeve. -the cylindrical hole around the screw. Cavitation can also happen if the motor over powers the fluid and creates a small void

What is an encoder and why use one?
An encoder is found on better auger valves. One is show here.  As you can see it is a disc mounted on the motor with a bunch of slits in it.  There is a sensor that sees these slits go by. The little PC board counts the slits.  These slits are “encoder counts”.  There are 3 ways to drive a motor.

1. The motor can be turned on and off for a time – say 5 seconds. This works well enough, but there is a resistance on the screw from the fluid.  Thick fluids make it hard for the motor to turn quickly, they need more “motor on” time to turn the screw and dispense the fluid.  Thin fluids don’t resist as much and they need little “motor on” time to dispense fluids. Unfortunately most fluids can be shear thinned so they are thin sometimes and thick sometimes. An auger with a motor “time on, time off” system is not exact enough to get consistent dispensing all the time. To get better consistency an encoder helps
2. There are a couple of ways to use an encoder. One way is to measure the speed or the acceleration of the encoder (and because it is a attached to the screw) the speed or the acceleration of the screw.  This lets a computer program know – This fluid is really resisting so the speed is low and the acceleration is low too. The computer can then increase the current to the motor and power through. It is like a car going up hill or down hill. When one drives up hill you give it the gas, and when you go down hill – let your foot off the gas and let it cruise on down.
3. The other way to use an encoder is to count the encoder counts.  If one turn is 360 counts (1 degree per count) and we need 3 turns – give it  1080 clicks – that is 360 X 3.

Encoder control gives the valve a tremendous ability to control the dispense. It is required for exacting applications

Rotation Speeds- The screw speeds must be matched with the fluid supply pressure to ensure that the fluid is not cavitating or by passing the screw. Motor speeds that are too fast (above  800 RPM) require higher air pressure applied to the fluid supply to ensure that the screw threads are completely filled while turning. To get the resolution needed for smaller depositions such as 0402 type components the motor speed should be reduced to around 200 RPM or less.

What is Drool?
Auger valves are not positive displacement valves and there is an open path from syringe to needle tip. When the screw is not moving and the air pressure to the syringe is turned off, low viscosity fluids can drip from the needle. Gravity pushes the fluid to the lowest level – generally on the machine, or the piece (board) the valve is parked over. This phenomenon is like water in a straw. If the straw is not restricted or blocked then the water will empty out of the straw. The same thing is happening, the fluid flows through the syringe, down the auger and out the needle. High viscosity fluids take some force to start moving, and resist this problem, but low viscosity fluids will flow out of the needle.  Drooling happens fast if the syringe pressure is on. This is common between dispensing cycles.

Why is constant air pressure important for Auger pumps ?
Constant air pressure ensures even flow of fluid to the screw. The air pressure pushes fluid into the screw so if the amount changes, the volume of fluid coming out the needle changes too. If air pressure varies – the consistency caries with it.  

Air pressure – normal air pressure is between 10 and 20 PSI to the syringe. Small syringes (like 10cc syringes) need less pressure than bigger (like 30cc) syringes. High viscosity materials need more pressure to force them through the syringe and into the screw. The actual amount of air – the pressure – varies, but using the minium required to get a good flow is the key. This depends on several settings and properties: fluid viscosity, screw rotation speed, screw diameter, needle and hub sizes and screw cut depth.

Air Pressure too low -  If air pressure is set to low the screw will cavitate causing inconsistent fluid deposition and missed dots. Inconsistent depositions due to cavitation are difficult to track due to the difficulty of observing the defect.  Defective depositions due to low air pressure frequently look good at first but deteriorate over time. These problems are often incorrectly blamed on air bubbles entrapped within the fluid or inconsistant board heights.

Air Pressure to high - If the air pressure on the syringe is set to high it will force fluid down the screw when the screw is not turning. High air pressure causes fluid to drool from the needle. Inconsistent dispensing will occur due to the fluid creeping up around the needle and bridging the gap between the mechanical standoff.  High air pressure causes fluid to flow through the pump down the auger and eventually the  fluid will flow out the needle even though the auger is stopped.

Higher air pressures are difficult to control for screw driven systems due to the tendency for the higher feed pressures to overdrive the surface tensions or the retention ability of the screw thus the fluid is forced down the screw and out the needle causing excessive drool.

What is viscosity and how does it effect dispensing consistency?
Viscosity is a measure of a materials resistance to flow. Viscosity is a dynamic rheological property of a material which can be measured in different ways giving different results.  Viscosity is commonly measured using a Brookfield viscometer which correlates shear stress to shear rate.  Viscosity measurements are not absolute since the value obtained will vary with temperature and shear rate.  In general, as the shear rate increases the viscosity of the material decreases.

Archemedes pumps are reliant upon material flow characteristics such as viscosity and shear behavior to force material down the screw and out the needle. Material viscosity directly effects the volume of material dispensed and is directly effected by the dispensing temperature and the rate of shear due to the rotation of the auger screw.  Any changes in the material flow characteristics will directly effect the consistency of the volume of material dispensed.  Therefore, it is important to maintain a stable dispencing cabinet and material temperature in order to maintain consistent material viscosity.

How viscosity and Rheology affect dispensing consistency?
Augers valves depend on the character of the fluid to get a consistent dispense.
Viscosity and Rheology are two big words that basically describe a characteristic of a fluid. Thin fluids dispense faster, thick fluids require more time or force to direct the fluid down the screw.

Viscosity is defined as a fluid’s resistance to flow when sheared.  An auger valve has a screw that shears the fluid. So the measurement of a viscosity directly relates back to how it will dispense.

Viscosity measures a fluid’s resistance under one set of conditions, Rheology measure the same resistance over a range.  The types of things (in an auger valve) you control that affect a fluid’s resistance (or viscosity) include:

1. Heating the fluid – A needle heater or (more rare) a syringe heater will normally drop the resistance to flow and dispense more.
2. Increasing the speed of the screw – this increases the shear force and normally increases the amount dispensed

Syringe level’s affect Auger valves?
Yes, as a syringe empties the amount of fluid coming out of the syringe drops and changes the amount of fluid dispensed. This characteristic is much more noticeable when using low constant air pressures. This trend is completely opposite of Air Over systems because they are pulsing the air instead of keeping the air pressure constant. With constant pressure as ring can dispense correctly, and then drift out of the desired spec – the dots shrink, as the level goes down. The auger valve levels this out compared to air over dispensing, the affect is the same. This can account for 10 percent change in the volume of the depositions from the beginning to the end of the syringe.

Screw designs – What can change?
When one looks at the screw and the cylinder it rides in there are some parameters that can be specified.  These are:

1. Clearances - The clearance between the OD of the screw and the ID of the screw housing is one of the factors in determining maximum flow rate of material. With a tight clearance of less than 1 mil the screw is able to build a higher pressure in the needle hub (the area between the end of screw and needle shaft). Screw systems that have larger clearances are not able to build up as much pressure in the needle hub due to the tendency of the material to back flow up the sidewall of the screw.  Tight clearances and tolerances however are not always good, for instance, screw systems that have tight tolerances have problems with malleable materials such as solder paste. Small particles get lodged between the screw and the screw housing causing the balls to get flattened out. Eventually these metal particles will work there way to the needle shaft and clog the needle.

2. Pitch - The pitch of the threads on the Auger screw will have a direct effect of the screws ability to both create the necessary needle pressure and retention of material when not turning. Finer pitch screws will have more material retention and be able to create higher needle pressures than that of screws with courser pitch. Thread angle, like pitch can give better accuracy or more speed (trading off these two attributes)

3. Depth of cut - Deeper cut screws will help in moving more material faster  however; they also reduces the surface tension of the material within the screw which makes it easier to drool.  Deeper cut screws have larger cross sectional areas which makes it easer pressure flow.  Less ability to create higher pressure at needle hub.  Smaller gauge needles will have higer material flow rates when used with a shallower depth cut screw.
   
   
4. Contour  - Rounded screw cuts will give more contact surface area allowing more material to be delivered per rotation compared to a screw with sharp cuts and all other parameters the same.
   
   
5. Surface Tension - The “no slip “ condition of fluid dynamics.  Material travels down the auger screw with the greatest acceloration in the center of the auger. Material along the screw sides moves the slowest.  Relate fluid flow to build up of colestoral in blood or in water pipes.

   The surface tension properties of the screw and material is what allows the material to be moved down the screw while moving and retain the material when not in motion. These properties can be changed by several different parameters. Some of these properties are listed below.

6. Surface Finish - Rough surfaces will tend to move more material than that of a highly polished surface.

7. Screw material - metal and plastic materials all have different porosity and other characteristics that will affect the way epoxies are attracted to them. These characteristics will produce different flow rates from the needles.

There are PhDs dreaming about this stuff. The actual way it works is through experimentation -  trying different screw designs until the desired result is found

How often must the Archemedes pumps be cleaned ?
Materials left in Archemedes pumps will tend to dry onto the screw causing layers of material that will flake off and clog the needle. Regular maintenance is needed to ensure best results most manufactures will recommend cleaning at least once a week

Summary
After Air Over systems, Auger valves are on most of the dispensers used for solar and electronic applications. They are the first choice for silver epoxies, and solder paste. They are also a good general solution for dispensing.

Advantages
The auger valve offers a lot better consistency and control than the air over systems. It can be adjusted quickly and dispense a range of dots and lines.

Disadvantages
Cleaning is the biggest, cost and wear are also issues.

 

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