The Holy Grail: Achieving true volumetric dispensing

Peter Swanson, managing director at adhesives specialist Intertronics, discusses how manufacturers can achieve true volumetric dispensing.

When dispensing materials like adhesives, coatings and lubricants, manufacturers aim for a process that is both repeatable and accurate to within the specified tolerances. Choosing the right dispensing system is key to achieving this, especially when the requirements are more exacting.

Technology manufacturing businesses are demanding ultimate precision in their processes in order to meet the highest quality expectations. But all engineers dislike variability in their production, and that includes the application of materials.

In the early 1970’s, fluid dispensers based on a time/pressure principle started taking over from techniques like brushing, dabbing or hand syringes. They delivered quality, efficiency and health & safety benefits, and remain ubiquitous in factories around the world to this day. Since then, advances in dispensing technology have been progressing to deliver a true volumetric process; the delivery of exactly the same specified volume of material every time. This is The Holy Grail.

Time/pressure fluid dispensers

These popular machines work by applying a pulse of air to a syringe barrel, which forces liquid out through a needle or nozzle. Control of the process relies on the applied pressure, the time of the air pulse and the diameter of the needle. These work reasonably reliably but without the accuracy that more sophisticated assembly demands. Variations in output come from changes in material viscosity — caused by changes in ambient temperature or the curing occurring with mixed two-part materials, for example. In addition, as the syringe barrel empties and the material is replaced by air, each air pulse to the syringe delivers progressively less material; air is compressible and liquid is not, and an increasing volume of air needs to be compressed by each pulse before pushing out the liquid.

These effects which limit accuracy can be somewhat mitigated by the use of a dispensing valve. For industrial applications like adhesive dispensing, these valves are usually pneumatically controlled. The advantage is being able to turn the material on and off like a tap, with the inherent improved control. There are many types of dispensing valve on the market, including needle valves, spool valves, and diaphragm valves. Valves which produce a spray pattern instead of dots or beads are also available. Valve choice is based on material chemistry and cure mechanism, and size and geometry of output. Pneumatic dispensing valves are popular, robust, and provide increased dispensing control, often at modest cost. But they are still based on time/pressure principles, and the variable results based on factors like viscosity change may not be acceptable for more critical applications.

The precise control of a pressurised flow of material will always be susceptible to changes in the material rheology. In consequence, industrial dispensing technologies have been developed based around a positive displacement concept. An amount of material is physically established in the pump or valve, which is then ejected or displaced — generating a reproducible volume. Volumetric dispensing is achieved in a number of ways.

Positive displacement

Dispensing valves are available designed around an auger screw in a metal tube. Material is fed under pressure into the tube and is propelled by the motor-driven screw. These are more effective for higher viscosity materials like filled epoxies; low viscosity materials can just fall through the valve, and can be affected by the supply pressure, with the resultant loss of control.

In contrast, low viscosity materials may be dispensed using peristaltic pumps. The fluid is contained in a flexible tube fitted inside a circular pump casing. Rollers or wipers are fitted to a rotor and they process along the inside circumference of the casing as the rotor turns, pinching the tubing and pushing the material along. The tubing is subject to wear and needs regular replacement to avoid failure and the output from a peristaltic pump is pulsed, especially at low speeds and volumes. In both scenarios, increasing the motor speed will increase the flow of material. They can both be seen as positive displacement methodologies but are limited by viscosity and deposit size.

Jetting valves

In this positive displacement technology, a small cavity in the valve body is filled with material, which is then ejected from the valve by a tappet. The tappet is driven pneumatically or by piezo, and can be activated at very high rates — so whilst the amounts dispensed are small, and can be in the nanolitre range, the dispensed shots can occur at hundreds of times per second, building up dots or beads. Jetting valves like the Vermes MDS 1560 find uses in very small deposit applications, high speed manufacture, or where complex part geometry benefits from its non-contact nature — for example, the application of an annular ring of lubricant inside a cylinder.

Progressive cavity pump

Manufacturers can now achieve true positive displacement dispensing and dosing by opting for technology based on the progressive cavity pump principle. Typically, a progressive cavity pump consists of a single‐helix metal rotor and a double‐helix hole in an elastomeric stator. The rotor seals against the stator, forming a series of spaces or pockets, which translate along as the rotor rotates, keeping their form and volume. The pumped material is moved inside the pockets. The pockets are shaped such that they taper and overlap; the output is continuous, even and non‐pulsing.

The flow rate is directly proportional to the rate of rotation (which can be reversed), and the volumetric output of the pump is directly proportional to the number of rotations. Due to the rotor/stator seal, input pressure has no effect on the pump. It is able to pump at very low rates, and low levels of shear are applied to the pumped fluid. Vitally, the output is viscosity independent — the material rheology changes caused by ambient temperature variations, for example, have no effect.

One example of this technology is the preeflow eco-PEN, which uses a progressive cavity pump principle to dispense a wide range of material viscosities with absolute control, achieving a volumetric dispensing process with an accuracy of ±1%, >99% of the time. These pumps are capable of precise, stable doses as small as 1 µl and applications include electronics, medical devices, laboratories and more. The preeflow eco-PEN can be integrated into a robotic system for an automated dispensing process.

The delivery of exactly the same specified volume of material every time, requires a volumetric approach. Progressive cavity pumps, such as the preeflow eco-PEN, give manufacturers more control over their dispensing process than ever before.