IDS Recommends Non-Contact Dispensing Methods

Non-contact dispensing methods allow for more efficient production processes, according to Integrated Dispensing Systems (IDS). With the ever-increasing miniaturisation of industrial processes, manufacturers are continually asking for higher demands in production-process accuracy. In the field of precision micro-dispensing, the demands are getting tougher as dosed quantities of oils, greases, adhesives and soldering fluxes must be dispensed with pinpoint accuracy and within a very short period of time - typically less than 2ms per shot or dose.

IDS, representing Liquidyn, claims to have a solution for these demands. The current known and widespread method of dispensing is by the traditional contact dispensing method, where the material is volumetrically dispensed by varying time and pressure. However, this method is prone to inaccuracies and potential mess, according to IDS. Liquidyn dispensing valves utilise a non-contact dispensing technique that avoids any contact between the dispenser and the workpiece. This ensures that the dispended media is 'shot' over a distance of several millimetres onto the workpiece target area, thereby keeping the dispense nozzle clean and ensuring a clean cut-off of material.

There are various benefits of using non-contact dispensing: accurate and repeatable dosed volumes; very fast dispensing; a constant nozzle and workpiece distance; no damage to the workpiece as the nozzle is kept clear at all times; no damage or bending of the precision dispense nozzle; the dispensed volume is unaffected by the distance between the nozzle and the workpiece; and closed-loop process control is simple as flying drops can be detected and counted.

There are disadvantages to using traditional contact dispensing techniques: the high-precision movement of the nozzle is required in the Z direction to prevent damage to the workpiece; in most cases, a distance control system is necessary (additional investment costs); the Z-directional movement of the nozzle slows down production; potential damage to the workpiece is possible owing to variations in workpiece tolerances; in almost all cases, a delicate needle-type nozzle is required; the damage and bending of the needle-type nozzle is possible; the varying distances of the nozzle to the workpiece as a result of tolerances can affect the accuracy of the dosing volume; media builds up on the nozzle causing inaccuracies in the dispensed volume; and process control in most cases is only via a visual monitoring system.

Non-contact dispensing is the high-velocity ejection of fluid-like or paste-like media at the exit of a nozzle, where the distance between the nozzle and the workpiece is great enough that the ejected material can, with the help of natural cohesive forces, form a perfect droplet on separation from the nozzle before impacting the target workpiece. Non-contact dispensing falls into two categories: jet-forming (jetting) and dynamic drop-forming (dot) methods. The drop, formed at the exit of a nozzle, is deposited directly onto the workpiece, without drop separation from the nozzle.

Dynamic drop dispensing is characterised by the separation of a drop of material from the nozzle exit as a result of a dynamic process, as the static pressure of the liquid medium is insufficient to form a solid fluid jet. The best known example for non-contact drop dispensing is found in inkjet printers. In this application, the volume of a small dispensing chamber with an adjoining nozzle becomes reduced through a short impulse, whereby the ink is ejected through the nozzle at high speed. The nozzle chamber, the nozzle and the ink reservoir are interconnected without the use of any valves.

During the dispensing process, some of the medium is also flowing in the reverse direction, back into the reservoir. In between dispensing, the surface tension of the fluid at the nozzle exit prevents air from being sucked in, as well as preventing fluid from exiting the nozzle. The principle of this process is only useful for low-viscosity fluids; therefore, this principle is not applicable with higher fluid pressures.

Inkjet systems have the following inherent properties: very small single-drop volumes are achievable (circa eight pico-litres); high dispensing frequencies are possible (low kilohertz range); low costs for mass production; only certain low viscous media are dispensable (no volatile media); and they are not principally not leak proof. For industrial production, the dispensing volumes and the viscosity range of inkjet systems are too small for most applications. In these areas of production processes, specially designed valves with high-speed tappet drives are used instead.

These micro-dispensing systems are characterised by the following properties: single-drop volumes from 10 to 200 nano-litres; dispensing frequencies up to 100Hz; a dispensing accuracy of less than one per cent; and media viscosities up to 200,000mPas (thixotropic). For non-contact dispensing in an automated industrial production processes, Liquidyn has developed a range of electro-pneumatic actuated micro-dispensing valves, which has undergone rigorous testing in many demanding applications. These valves are equipped with a high-speed actuator system, which enables rapid switching times of less than 1ms.

During the designing of these valves, much attention was given to standardising connection interfaces. The electro-pneumatic switching valve operates on an industry-standard 24VDC power supply - a voltage that is available in most industrial SPS process controls and is utilised to control the actuator system. The micro-dispensing system has been designed for the simple operation, cleaning and maintenance of the fluidic components, so no specialist knowledge or tools are required.

By sacrificing a piezo-electrical actuator system in favour of a more robust electro-pneumatic drive system, the following benefits are realised: in its inactive state, it is normally closed, which is safer; high safety thanks to low voltage in the operating area; simple maintenance by the end user; simple nozzle change (within seconds); no readjustment after cleaning/nozzle change; a reliable low-failure rate; insensitive to environmental conditions (shock and moisture); high life expectancy (one billion cycles are achievable): low-cost control systems; and low-cost maintenance.

Examples of applications for Liquidyn micro-dispensing valves include: saving on an additional cleaning process after soldering, through accurate application solder flux; the accurate and selective bonding of electronic components; the application of instant glue (super glue) in a 0.4mm-wide groove; the application of electrically conductive silver-filled adhesive; the joining of precision-machined components with anaerobic adhesive; and the precision deposition of silicone oil on medical components.


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