Jetting Adhesives: Full Steam Ahead in Microdispensing Technology
In the course of ever progressing miniaturization, increasingly small and sensitive components need to be joined. Increasing functionality comes along with increasing material mix and higher quality requirements – this development makes it necessary to provide for joining methods that ensure reliable fixing of miniature components. In many cases, bonding is the ideal solution to meet the requirements relating to material diversity and narrow installation space. The bonding of small components, in turn, requires accurate application of tiny amounts of adhesive.
Furthermore, companies often seek to reduce costs, mainly by speeding up their production processes. The challenges to be faced by microdispensing technology are thus clearly defined: ever smaller amounts of adhesive need to be dispensed at ever increasing precision and speed.
The printing industry has already shown how this can be achieved: dot matrix printers transferring the ink to the paper by impact were extremely slow and created broad lines. The development of inkjet systems placing tiny ink droplets without contact on the paper has significantly increased both speed and resolution.
Jetting Beats Needle Application
These advantages can also be used for the dispensing of adhesives, when “jetting” free-flying adhesive droplets from a certain distance. The time-consuming placing of a dispensing needle onto the substrate for application of the medium, as is done with traditional dispensing, is no longer necessary, resulting in a time advantage that directly reflects in higher cycle times.
Position tolerances of components in z-direction are irrelevant when jetting, since the free-flying adhesive droplet will reach the substrate surface anyway due to the functional principle of the method. Consequently, the risk of collision between needle and component posed during needle dispensing does not exist in jetting mode, so there is no need to perform camera measurements before and after the application process. The closing mechanism prevents unwanted dripping as it happens with needle valves, thus avoiding dragging of the adhesive.
To put it simpler, companies will opt for needle dispensing when tiny quantities of just a few milligrams need to be dispensed with maximum positional precision. They will prefer jetting when the focus is on speeding up their production.
Pneumatic or piezo actuator
In the latter case, you can choose between two driving systems for your dispensing unit. The requirements for the actuator generating the pulse pressure for ejecting medium- to high-viscous adhesives are much higher compared to applications with low-viscous ink. During the closing process, for example, the valve piston moves with high momentum towards the outlet. The dynamic pressure at the piston head allows overcoming the surface energy and the high cohesive forces within the liquid, causing a free-flying droplet to detach from the valve opening. High actuator forces and speeds are decisive factors in ensuring that this process works. However, in order to make full use of the speed advantages provided by this method of contact-free dispensing, the valve must be operational most of the time. And that is exactly where common systems often fail.
They use either pneumatic actuators with sliding sealing elements that involve friction, or piezoelectric ceramic actuators based on a lever mechanism and thus also subject to friction. Friction combined with high speeds and fragile ceramics combined with high driving forces lead to severe wear and reduce the service life. This partly negates the speed advantage of the jetting method and causes substantial costs.
Pneumatic: The New Microdispensing Valve from DELO
DELO is the only manufacturer of adhesives that develops its own jet valves. For its new DELO-DOT PN3 microdispensing valve, the company relies on a pneumatic actuator system. In contrast to conventional pneumatic actuators, the new system does not use any sliding or frictional elements. It is extremely robust and has an exceptionally long service life of up to one billion dispensing cycles.
Compared to its predecessor, DELO-DOT PN2, the new valve features increased jet performance. It allows, for example, many adhesives to be dispensed in significantly smaller quantities. On an average, the minimum dispensing volume is reduced by the factor three. Certain products can even be jetted permanently in quantities of 2 nl. In contrast, the maximum dispensing volume per drop is almost 5000 nl, making the valve extremely flexible in the production process. This is made possible thanks to an optimized fluid system geometry and two different piston types, one for very small dispensing amounts and one for small to medium drop sizes.
Thanks to the modular design, the actuator is completely separated from the resistant fluid system which, in turn, can be dismantled into its individual parts. This allows complete and easy cleaning of all components that come in contact with adhesives and the relevant areas are easily accessible without any special tool. To keep system downtimes to a minimum, for example when changing the adhesive, it is possible to replace the fluid system as a whole. Single components such as the chemical-resistant fluid system body made of stainless steel, nozzles of different diameters and geometries and the nozzle heater are available separately and can quickly be exchanged by the user on site without any special effort. This is rarely the case with comparable devices and maximizes the availability of the new jet valve.
The redesigned nozzle heater guarantees permanently stable temperature and viscosity conditions for the dispensing medium. The built-in thermal insulation of the heater which has no open electric contacts and is screwed onto the valve ensures that only the medium-transporting part at the valve outlet is heated, thus reducing the time during which sensitive dispensing media are exposed to a defined thermal load.
Plug & Play Installation
Especially in high-throughput systems, components need to be designed as compact as possible. The compact outer dimensions of the jetter (min. 20.5 mm × 153 mm × 62 mm) in combination with a media supply that can be positioned at a 90-degree angle enable flexible adaptation to small installation spaces and easy integration into production systems. Mounted on a movable axis, its low weight of less than 350 g is ideal for high travel speeds and ensures, in practice, an operating frequency of up to 330 drops per second.
When installed, in particular, the microdispensing valve is much easier to handle than its predecessor. Many parameters can be read from a PCL, which is not the case with a mechanical system. A rotary wheel is used to set the stroke of the piston by selecting one of five different locking positions without any additional tool. The respective position is indicated by an LED, thus making it easier to check the stroke setting, in particular after integration into a production system.
The new DELO-DOT PN3 jet valve features a highly resistant fluid system, separated from a pneumatic actuator. This combination results in an extremely robust and reliable system guaranteeing a long service life and minimized operating costs. At the same time, it allows meeting the requirements in microdispensing technology, where tiny quantities need to be dispensed with maximum precision and speed.