Even the best of operators can't control exactly how much assembly fluid they put on components with squeeze bottles, hand syringes, and swabs during medical-device production. Pneumatic fluid dispensers eliminate this problem by using a precisely timed air pulse to govern fluid amounts. Workers can focus on applying fluid in the right place instead of trying to match a sample deposit or worrying about how much fluid to use.

These compact, benchtop systems handle UV-cure adhesive, cyanoacrylate, solvent, silicone, and many other assembly fluids. They improve process control by making it possible to document settings, cut rework from too much or too little adhesive, and reduce operator injuries due to repetitive hand motion.

From watery solvent to thick silicone

Pneumatic fluid dispensers are easy to use. Operators connect a disposable syringe barrel that contains assembly fluid (loaded by the user or prefilled by a supplier) to the dispenser by a flexible air line. They also fit the barrel with a precision dispense tip. (Syringes and tips come in a wide variety of sizes and styles for almost any application.) Producing uniform fluid deposits as small as 0.004-in. in diameter is then merely a matter of holding the syringe barrel like a pen, placing the tip in position, and tapping an electric foot pedal.

The amount of material the system deposits comes from a combination of time, air pressure, and tip size. Adjusting one or more of these parameters lets users apply fluids from watery solvents (for bonding tubes) to thick silicones (for sealing pacemaker electrodes) with exceptional accuracy and repeatability.

System set-up

System set-up is simple but follow these tips for best results:

Fill syringe barrels no more than 11/42 to 21/43 full for maximum dispensing accuracy. Ask your supplier about filling barrels to these levels when using prepackaged fluids.

Use the smallest barrel practical for small deposits. Most barrel styles come in 3, 5, 10, 30, and 55-cc capacities.

Keep the dispense tip at a 45° angle and lift it straight up after making a deposit to get a clean cutoff. A common mistake is to lift the tip while still dispensing, which can create a “tailing“ or oozing effect.

Match the syringe piston to the material for maximum fluid control and accurate, repeatable deposits. General purpose, double-wiper pistons are best for most fluids. Their close-tolerance fit prevents dripping between shots and minimizes fluid waste by wiping the syringe walls clean as the material dispenses.

However, the snug fit of the double wipers might make them bounce as air pressure is applied and released when dispensing thick materials such as silicone and grease. The bouncing can let material ooze from the tip between shots. To counter the oozing, it's helpful to switch to a looser-fitting, flat-walled piston.

Lastly, insert a barrier piston near the top of the syringe to prevent fluid from being accidentally drawn back into the dispenser when applying solvents and other watery fluids that typically require vacuum to keep them from dripping.

Fixture the syringe barrel for a convenient “third hand,“ which gives operators greater flexibility. Operators with both hands free can more easily position small parts and bring them to the dispense tip.

Ensure component quality and cleanliness because dust and debris inside syringe barrels can migrate to the fluid and get in the product. Also, residual molding flash and metal burrs inside dispense tips restrict fluid flow and cause unpredictable variations in fluid-deposit size. Both conditions increase rework and reject rates, so using magnification and other methods to thoroughly inspect components is time well spent.

Use a benchtop centrifuge to purge syringes of bubbles and air pockets that might compromise the accuracy of fluid deposits. Trapped air is a common problem when dispensing medium to high-viscosity fluids such as UV-cure adhesives, silicones and grease.

Install an external air regulator to monitor the plant air going into the dispenser when fluctuations in the plant air-supply result in variations in the fluid-deposit size. Air regulators ensure steady air pressure and remove residual moisture from the air supply before it enters the dispenser. This is especially important when using cyanoacrylates and other reactive fluids.

Consider switching to a dispenser with an internal pressure regulator, which is used to control shot size, that operates at lower psi when using thin fluids for most assembly work. Standard 0-100 psi air regulators can handle everything from solvents to silicones, but a 0-5 or 0-15 psi regulator allows finer adjustment for small deposits of thin materials.

Upgrade to a dispenser with all-digital display for a higher degree of process control. This lets users accurately adjust time, pressure, and vacuum settings in increments as small as 0.0001 second and more easily document these parameters. Some equipment manufacturers give a trade-in allowance to offset upgrade costs.

Purchase a benchtop, 3-axis dispensing system when assembling complex part geometries or transitioning from prototyping to production. There are always slight operator variations in placing fluid with pneumatic dispensers even though the units standardize shot size. Three-axis systems, also called dispensing robots and XYZ systems, eliminate this variation in critical or high-volume applications.

The systems consist of a dispenser integrated with an electronically controlled positioning platform. After a user programs the device with a PDA or a teaching pendant with deposit size, placement, and configuration (dot, line, circle, arc, or fill), the system deposits the correct amount in the correct location every time, without operator intervention. This lets operators, for example, perform additional value-added operations while the rack of parts is processed.

Use a system that that displays in multiple languages and measurement standards (for example, psi and bar) to reduce the risk of translation error and simplify operator training when manufacturing devices in several countries.