Automating the application of adhesives is not as straightforward as it might appear at first glance. Like many of life’s big decisions, these choices should be well-considered so that the required investment will accomplish the original goal.
“There are many aspects to consider when considering moving from a manual setup to a semi-automatic or automatic setup. Some people may assume it is only down to the production volumes and the requirements of the business; how many products in a minute or hour. But it’s not as simple as that,” states Ahmed Khan, Nordson EFD global product line manager for dispensing solutions.
First, the decision isn’t only about productivity, he points out. “I have seen more and more companies, especially with market segments such as electronics, media and life sciences and aerospace, which are depending on the application, consider semi- and automated processing from the start. Return on investment is one reason, but product quality also comes into play. If there is a human element involved in putting a line of adhesive down, that depends on the linear speed of their arm; if they go fast, they put down less, if they go slow, they put more down. A tabletop XYZ robot takes that element away.”
He continues: “We have applications in high-end blue-chip customers where these factors are not critical, where they just need to dispense a blob of grease, and that’s it. Even though the product may be complex and intricate, since the role of the fluid where it is being put in is not going to define the overall performance of the assembly, they stick with manual [application]. Whereas on the other hand, even start-ups with very tight budgets go towards a semi- automatic set-up, if the fluid they want to be dispensed is in a critical position, both functionally and from the point of view of aesthetics, if adhesive plays a significant role in performance in the end assembly.”
Another reason why specification is not always simple is that customers’ views are likely to be neither objective nor fixed; as they gain experience, their own goalposts move. “Automation is usually a learning experience for the customer, prompted by a large order for what was a low-volume product. They can’t physically get enough parts made manually, so they need some way to do it quicker. They get a robot and see how easy it is to use, and then they have different process applications,” says Sam White, Ellsworth Adhesives dispensing equipment and automation specialist.
That insight also means that a process that works in the lab under R&D conditions might not be at all appropriate in high-volume production. In other words, an efficient automated system must have defined parameters. Khan states: “Production, especially in automatic or semi-automatic lines, is very process-specific. If you are not at that stage, manual is the way to go. If you are developing something with an unknown fluid, and you don’t know the geometry of the component for dispensing, do not invest at that stage in an automatic setup; it will be too high a risk.”
And, finally, Khan warns, automation is not by any means always the solution. He points to medical/life science applications where the sheer variety of stents and the physical flimsiness of medical assemblies means that the task of automating gluing processes will be very complex and require a huge amount of investment. He adds: “There is a big concentration of these manufacturers in Ireland, and if you go to a factory there you see hundreds of people working with tabletop dispensers.”
Such set-ups are partially automated in fact, in that they have replaced finger or thumb pressure on the back of a syringe with air pressure, for example, to force the adhesive out of the nozzle. Time/ pressure dispense controllers (such as pictured below left) modulate the flow of air from a shop supply line to dispense, when actuated, a continuous feed or a pre-set single shot.
Peter Swanson, managing that there are four factorsinfluencing their performance: the viscosity or rheology of the product, the size of the needle orifice, pressure and time. “So if you’re applying the same parameters to water or toothpaste, you’ll get lots more water out than toothpaste,” he quips.
Workers are still responsible for positioning of the needle on the part in space, but these feed systems will control the amount of adhesive applied. But as that takes a little time to react, such dispensers have a certain tolerance, points out White at Ellsworth (which Swanson estimates to be an error rate of 1-10%, provided the adhesive is thermally stable).
A small error may be entirely appropriate for the application, White states. “Generally speaking, we try to keep it simple. If the customer has a loose tolerance, we don’t need to jump into complex valve systems. If it doesn’t need to be super-fast or super-accurate, we need to understand that. We don’t want to price ourselves out of projects, or over-deliver.”
Unlike the absence of pressure, which is the de-facto stopping mechanism on such controllers, valves provide more accuracy because they actually stop flow completely.
Adds Swanson: “There are different types of valves: spool valves, cartridge valves, diaphragm valves, all of which involve material flow going on and off by some mechanism. It gives more control.”
Systems using valves can be more flexible in terms of how the adhesive is stored, and can supply from a syringe, cartridge, pressure pump or extrusion pump. Whatever the reservoir, feed systems rely on the way the adhesive is stored, Swanson observes: “You need to think about what the material you need to dispense is delivered in. If it’s in 54-gallon drums, you need to think of a way of getting it out of there first beforethe next stage. That’s why materials suppliers who are sympathetic with customers supply materials in dispensing- friendly containers. If you have to decant before you dispense, that adds another process.”
As a loose approximation, users can calculate the flow rate of material that time/ pressure dispensers dispense by combining the system pressure and the viscosity of the adhesive. But a long feed hose might reduce the flow rate, since pressure drops in long lengths, points out White.
While thick adhesives can be extracted with hydraulic extrusion pumps (limited at the upper end by the burst pressure of the container), thin adhesives behave completely differently. In some cases they can be dispensed by gravity.
Thin or thick, they are affected by what White calls ‘full to empty syndrome’, in which flow rates tend to be higher when the vessel is full than when it is nearly empty. What he’s referring to is a fundamental limitation of such dispensers: they cannot dispense a constant volume of product every time, in any conditions. Swanson explains that the phenomenon is due to the fact that, while air is compressible, liquid is not. That means that as the syringe must supply a greater quantity of air to dispense the same amount of product. As a result, dispensing of nearly empty syringes takes more time, and the dot size dispensed reduces in size. Another drawback is that the system won’t prevent an adhesive that softens in warmer temperatures from dispensing at a faster rate.
If those issues matter, users can turn to volumetric dispensers, which deposit an exact volume every time. Normally, progressive-cavity pumps are used for this purpose, according to Swanson. They incorporate a globular-shaped rotor and an elastomeric stator, and work like an Archimedes screw. When the two parts are put together, they form a sequence of cavities, and as the rotor turns, its shape pulls material through those cavities one by one until it falls out the end. Dispensed volume does not depend on temperature, speed or input pressure, but only on the number of turns. They accommodate both one- and two-part adhesives.
ROBOTS
Larger-capacity dispensing systems require a stable platform to apply the adhesive. As Khan quips, you can’t hold a jet valve in your hand. Semi-automated systems involve loading parts on a tray by hand underneath a robot. For example, an entry-level 4200 robot offered by Ellsworth offers a positioning accuracy of +/-20mm.
Robots offer better dynamic ability too. “Humans can’t draw a bead around a corner easily. They can dispense in a line, but it’s almost impossible to change speed, but the robot compensates automatically,” Swanson says.
Users can program robot motions from 3D CAD files, or by driving the robot through space using a teaching pendant control.
And robots can be set to adapt automatically to conditions. For example, the F4000 Advance incorporates a vision system that can detect, and compensate for, production deviations from part to part (another vision system is pictured above). Nordson EFD and Intertronics also offer such technology.
Swanson says: “The real source of inaccuracy is not normally the dispenser or the robot; it’s the tooling. Parts are not completely repeatable; plastics might have a bigger tolerance than that. So a vision system looks for fiducials to make sure the system is aligned; if not, it will alter the programme for dispensing.”
On the other hand, integrating an adhesives cell into fully-automated production (pictured, left), which might involve conveyors, often requires specialist integrators.
BOX: Connected dispensing control: UltimusPlus-NX
Two years ago, Nordson EFD launched a dispensing robot with a USB slot to enable operators to quickly load adhesive application programme updates to many units arrayed on a shopfloor, the UltimusPlus. Its latest machine comes with a connectivity module to allow set-up to be done remotely. That does away with the need to physically visit each station. Two versions are available: one for thin and one for thick adhesives, both of which rely on a remote PLC for programming.
