Robotic dispensing devices offer assemblers many capabilities that increase productivity, enhance quality and prevent rejects. But these capabilities come at a cost that many assembly applications don't warrant. For low-volume production, and applications that don't have critical dispensing specifications, handheld, powered dispensers provide the answer to many assemblers' needs.
Handheld, powered dispensers are not only much easier to use than hypodermic syringes, squeeze bottles, oilers, brushes or sticks, they also yield superior results. But these types of dispensers are not suitable for every assembly application. This review of fundamental characteristics can help you determine whether these handheld dispensers are right for your application.
Types of Handheld Dispensers
Handheld dispensers range from basic, manual units that rely totally on the user to control dispensing, to more sophisticated units that allow the user to program dispensing time and pressure settings. However, the basic methodology of dispensing fluids from a handheld unit is the same as dispensing fluids from an automatic dispenser attached to a robot. "The major difference between the two is that people's hands shake, so if you need to place the amount of material, which could be minute, to a precise location, you may miss it. Whereas a robot will not," says Vladimir Siroky, president of I & J Fisnar, (Fairlawn, NJ).
At the basic end of the spectrum are totally manual dispensers, such as dispensing pens. These pinch-tube pens are suitable for manually dispensing continuous beads or applying dots of low-viscosity fluids, such as solvents and cyanoacrylate adhesives, in noncritical applications. The disposable pinch-tube is the only part of the pen that is in contact with the material. It is discarded when contaminated. The pinch-tube pen can be connected via a wide range of interchangeable fittings to gravity-fed or pressurized reservoirs.
"If you are talking about making a line, the effectiveness of this type of dispenser depends on the user's steadiness in order to maintain a straight line. Usually, you cannot do a straight line. It is either wavy or has some bumps in it. Or if you change the speed that you are moving, you will change the bead size. As your hand moves faster, the constant flow rate will translate into thinner beads," says Siroky.
Up the handheld dispensing ladder are air-powered and positive-displacement dispensers.
Handheld air-powered dispensers use precisely timed air pressure pulses to control the amount of fluid to be dispensed. This enables the user to make a series of identical dots, apply neat beads or fill cavities with uniform amounts of material.
To begin dispensing, fluid is loaded into a syringe reservoir that is fitted with a special piston and a dispense tip. A flexible adapter connects the reservoir to the dispenser. Time and pressure settings are adjusted to produce the desired deposit size.
The operator typically presses an electric foot pedal to initiate a dispensing cycle, leaving both hands free to manage parts. When making dots or filling cavities, the goal is to duplicate the deposit repeatedly. Pressing the foot pedal starts the timer, dispensing material for the set time and producing consistent deposits. The timer can also be turned off, allowing the user to control the length of the dispensing cycle directly with the foot pedal. Many of these dispensers also offer teach functions and multiple memory cells to store dispensing parameters for specific jobs.
Air-powered dispensers use air pressures up to 100 psi to apply various types of materials, including thick pastes and nonleveling materials. Many also incorporate a suck-back feature to prevent dripping. Dot size is determined by the tip diameter, output air pressure setting and pulse time.
Dispensers using positive-displacement technology have a mechanical piston that pushes the fluid out. Typical positive-displacement dispensers use a stepper motor to drive the piston at a precise rate and force. "So what we can do with the positive-displacement dispenser is actually set the amount of fluid that we want to dispense. So if I want to dispense 1 cc, I can set the control for a cc, and it will dispense that amount precisely," says Tom Muccino, global business development manager at EFD Inc. (East Providence, RI).
Choosing a Dispenser
The type of material to be dispensed is the first point to consider in selecting a handheld dispenser. The positive-displacement dispenser is geared toward two-part epoxy materials that require mixing. The viscosity of these materials increases with time, and this characteristic makes them unsuitable for air-powered dispensers. If a user attempts to dispense a two-part fluid with an air-pressure dispenser, and if no changes are made to the dispenser, the amount being dispensed will get smaller and smaller due to the fluid becoming increasingly thicker. "The positive-displacement dispenser overcomes that change in viscosity, and it will give you the same amount every time," says Muccino. "Virtually, all other fluids are ideal for air-powered dispensers."
Air-powered units are most suitable for dispensing dots as small as a microdot. Ideal applications for the positive-displacement dispenser are those that require dispensing amounts larger than a dot.
Know the Assembly Application
The number of products to be assembled is another very important criterion to consider in selecting a handheld dispenser, according to Nick DiDonato, North American sales manager at Liquid Control Corp. (North Canton, OH). The design of the part and how the material should be applied to the part is also important.
When choosing a dispenser, the user should also consider the features that the dispenser comes equipped with. The dispenser should allow the user to fine-tune the dot size for better process control. The air-management system should hold the air pressure accurately. If you have fluctuations in the air supply or fluctuations in the air in the dispenser, the shot size is not going to be consistent. Therefore, the precision of the regulator plays a very important role in dispensing consistency.
Dispensers that incorporate a teach function allow the user to put the dispenser in a teach mode. By using the foot pedal, the user can teach the dispenser the shot size. Once the dispenser is taught the shot size, it will repeat that same deposit size.
However, "If you are looking at course dispensing, where it doesn't matter where you are dispensing the material, then handheld dispensing is fine," says Siroky. "But when you have a certain pitch that you have to maintain, then you need some mechanization to provide repeatable dispensing."
Handheld Dispensing Advice
Maintaining process control during the dispensing cycle is not magic. Following a few simple rules will ensure that dispensing is not the weak link in the assembly process. Users should ensure that they have a good air supply. The air supply must be free of moisture or oil. A filter regulator can be used to ensure that the air is free of contaminants before the air gets to the dispenser.
• Using quality dispensing components is the key to ensuring that the dispenser will provide consistent results repeatedly. High-quality tips, syringe reservoirs and pistons that go inside the reservoir should be used. The piston wipes the contents of the syringe clean. Once the piston has come to the bottom of the syringe, if it has done its job, it should have dispensed 97 percent of the contents of that syringe.
• Users should also use a syringe that isn't tapered. Most syringes are slightly larger at the top and taper down at the bottom. However, when these types of syringes are used, the piston has a tendency to get caught in the tapered area. This can seriously affect dispensing quality. Users should look for a syringe with zero draft-with no taper to it at all.
• Most of the dispenser components are molded. And most molded products have a mold release agent that is used when molding plastic parts. These mold release agents can actually be a contaminant to the fluid that is being dispensed. Users should be sure that there is no mold release agent on the plastic parts.
• Expendable components should also be replaced on a daily basis. Daily replacement can help to avoid contamination of the dispensing fluid.