Cyanoacrylate adhesives, also known as CAs or cyanos, are highly effective at bonding many types of materials together in assembly processes in a variety of industries, including automotive, electronics, life sciences, defense, and consumer goods. Often referred to as super glues, they exhibit high bond strength and fast cure times. Though highly beneficial, these moisture-cure adhesives can be a challenge, especially when your assembly process requires precise, repeatable dispensing.
by Claude Bergeron
Unlike other types of adhesives that require heat, light, or catalysts to initiate curing, humidity in the air alone is sufficient to cure cyanoacrylates. For this reason, CAs are often called rapid-cure, quick- bonding, or instant adhesives. When a CA is exposed to moisture, the molecules in the adhesive and the molecules in the surface to which it’s applied start a change reaction. This reaction creates a bond or adhesion, which can often occur in less than a minute and reach full strength within a few hours. Adding heat or light speeds the curing process even more.
Light-cure instant adhesives combine the advantages of UV technology with the rapid-curing features of cyanoacrylates. Exposure to UV or visible light often provides a tack-free surface in less than five seconds. CAs provide several manufacturing advantages, including strong bonding to many non-porous surfaces, such as plastics, metals, ceramics, and glass; no special mixing or controlled-storage requirements other than keeping air out. Only small amounts are needed for a strong bond. Furthermore, rapid cure time initiated simply by exposure to ambient air: the lower the viscosity, the faster the cure time. Though the advantages of using CAs far outnumber the disadvantages, it’s important to understand the dispensing challenges given this material’s superfast curing time and reactive qualities.
Cyanoacrilates dispensing challenges
The main challenge when dispensing cyanoacrylates is keeping unwanted moisture out. Any moisture that contacts the CA will start the bonding reaction prematurely. This can cause costly rework and rejects, in addition to slowing production. Unfortunately, some of the advantages of CAs contribute to its dispensing challenges, including:
Correct placement. Due to its fast-setting nature, a CA must be applied accurately the first time, since even small adjustments after curing are generally not possible.
Precise deposits. The strongest bond is created with a minimal amount of cyanoacrylate. This presents a challenge for operators because it’s not easy to apply thin deposits consistently. Since most cyanos are low viscosity, this can be particularly challenging because thin fluids are often prone to dripping.
Migration. This occurs when CAs seep beyond the desired deposit location, creating chalky, white filaments that can impair part quality. Preventing migration is a major dispensing challenge and is especially tricky with low-viscosity cyanos. Safety. Since CAs can adhere to skin, skin contact should be avoided. Dispensing systems must safely contain the fluid and reduce the amount of handling.
Cost. Because many CAs are expensive, inconsistent dispensing can lead to costly material waste and part rejection.
Manual hand-lever valves as well as benchtop systems
Nordson EFD recommends dispensing systems for cyanoacrylates including hand-lever valves, benchtop dispensers, semi-automated dispense valves and automated dispensing robots. These systems provide controlled, repeatable dispensing results. Some reduce material use by as much as 60% and virtually eliminate rejects.
When using a manual hand-lever valve (Figure 1), the operator determines the amount of cyanoacrylate needed by looking at the deposit size as it is dispensed. Material is fed to the valve directly from a 1-l tank containing a 1-pound bottle of CA. The amount of adhesive dispensed is not metered by the system. The material is pumped with consistent pressure to deliver uniform deposits. Handheld valves are best for low-volume production and applications that require larger deposits or stripes.
With a benchtop system with an air-powered fluid dispenser (Figure 2), an operator holds a syringe barrel and guides the dispense tip to the correct location, then presses the foot pedal or finger switch to release the fluid. This is a much more controlled dispensing method than squeeze bottles or hand-lever valves. The system is best for low- to medium-volume production and applications that require smaller, more precise deposits. Using a benchtop system with a dispense valve, valve stand and valve controller, an operator places the part or workpiece under the valve, which is fixed to a valve stand. The operator actuates the dispense valve, which is connected to the valve controller. This option is ideal for medium-volume production processes.
Semi-automated systems and fully automated systems on a production line
With a semi-automated system with a tabletop dispensing robot, dispense valve, and valve controller (Figure 3), the operator places a batch of parts on a fixture plate and presses the run button. The dispense valve is mounted to the dispensing robot. The valve controller is the interface between the robot actuation signal and the valve. With this system, the valve dispenses a controlled amount of CA in a pre-programmed pattern onto the workpiece. A dispensing robot can dispense CAs in complex patterns. This is the best option for higher volume production or applications that require extremely precise, repeatable deposit placement. Another option is the semi-automated system with a tabletop dispensing robot, jet valve, and valve controller (Figure 4). As noted above, the operator places parts on a fixture plate and presses the run button. The valve is mounted to the robot and the controller actuates the valve. With jet valves there is no contact between the dispense nozzle and the workpiece. This allows for faster deposit placement on tough-to-reach or uneven surfaces and reduces the risk of part damage and contamination. Jet valves also dispense very small amounts at fast speeds with optimal repeatability.
Finaly, in a fully automated system with parts that advance on a production line (Figure 5), parts placed on a conveyor or rotary table are detected by sensors and automatically receive a deposit of CA from the dispensing valve. The valve may be mounted on the line and controlled by a valve controller or PLC. It may also be mounted on an automated dispensing robot, which is programmed and controlled via a PC.
(Claude Bergeron is Product Line Manager Nordson EFD)