Dispensing Thick Materials
Precise valves and powerful pumps are essential to properly dispense one- and two-component high-viscosity materials.
Comparative claims can be positive or negative, subjective or objective. But, in every instance, their main purpose is illustrative. A common example is when someone claims that a person or process is “as slow as molasses” (which, by the way, has a viscosity of only 5,000 to 10,000 centipoise [cps]).
In the world of assembly, however, it would not be surprising to hear assemblers and other plant workers say that someone or something is as slow as peanut butter (150,000 to 250,000 cps) or lard (1 million to 2 million cps). The reason is because many of the materials they need to dispense are this thick.
Grease is one such material. Until recently, an automotive parts manufacturer struggled daily when trying to dispense a 0.6-millimeter-wide band of very thick grease (2 million cps) onto an O-ring that is 0.5 inch in diameter.
The company tried a Nordson EFD contact dispense valve with a small gauge tip, but the amount to be dispensed was too little and the grease was too dry to transfer to the part. Then the company tested Nordson EFD’s noncontact PICO Pµlse jet valve system. Cary Long, product application specialist at Nordson EFD, says the system proved successful because of the force at which the jet valve pushes the grease out of the nozzle and onto the O-ring.
Insulated glass (IG) manufacturer Viracon also dispenses thick materials. To build large IG units (165 inches tall by 60 inches wide) for the Freedom Tower in New York City, Viracon workers had to use two-part structural glazing sealant with a viscosity of 250,000 to 300,000 cps.
The design specs required the material to be accurately mixed at an exact ratio and then dispensed. Unfortunately, Viracon’s older proportioning and dispensing system produced material that could be out of ratio, and the system could not alert operators of this problem. To resolve the issue, the company switched to the ExactaBlend AGP (advanced glazing proportioner) from Graco Inc. The new system shuts down if the material is mixed off-ratio. More importantly, it increased production by 20 percent.
Thick material—be it grease, sealant, solder paste, filled epoxy, thermally conductive gels and paste, or form- or cast-in-place gaskets—poses special challenges for dispensing equipment. Knowing this, manufacturers rely on precise valves and powerful pumps and delivery systems to provide accurate, repeatable and drip-free deposits in every application.
A Slow Start
The history of dispensing thick materials goes back many decades. Graco founder Russell Gray developed a grease gun powered by air pressure in 1926 when the hand-operated guns at the gas station where he worked proved unreliable in Minnesota’s cold temperatures. Within 25 years, Russell and his brother Leil had formed the Gray Co. Inc. and developed lubrication pumps for the automobile industry and military, as well as a direct-from-drum pump for heavy-duty industrial fluids.
Since then, several other suppliers have developed valves, pumps, delivery systems and related components specifically to handle material with a viscosity of 100,000 to more than 5 million cps. Techcon Systems, for example, started as an RTV silicone-sealant packaging company for the aerospace industry in California in the early 1960s, but within a few years developed equipment to dispense this thick material.
“If the material isn’t pourable, it must be pressurized with air to be dispensed,” explains Pete Linder, North American sales manager for advanced fluid dispensing at Graco. “Some materials less than 100,000 cps can barely be poured. But, any material north of 750,000 cps is considered thick.”
Long points out that flowable materials tend to self-level, and most materials over 100,000 cps do not have this characteristic. Instead, they cavitate (form voids throughout) and require a mechanical force to self level. Without the proper force, cavitating thick materials can cause a great deal of noise in the dispensing system, lessen its efficiency and quickly damage components.
Thick material is dispensed from a syringe, cartridge, 5-gallon pail or a 55-gallon drum. The syringe has a nozzle or needle at the end and contains 3 to 30 cc of material. The standard syringe can dispense materials up to 150,000 cps with 100 psi air pressure. It may be handheld for manual dispensing (time-pressure) or used as a reservoir for a valve mounted on a small X-Y-Z Cartesian robot for automated dispensing.
The Fisnar F4000N.1 series of three-axis benchtop robots can be used as part of a dispensing system that accurately dispenses thick sealants. Made by Fluid Research Corp. (an Ellsworth Adhesives company), four models are available (F4200N.1, F4300N.1, F4400N.1, F4500N.1), with work areas of 200, 300, 400 and 500 square millimeters, respectively. The robots offer 0.02-millimeter repeatability, 0.001-millimeter resolution and a USB port that allows easy data transfer to a PC. They store up to 100 dispensing programs (50,000 points per program).
Graco’s UniXact machine features a multifunctional platform that enables precise high-volume dispensing and real-time process monitoring. Its motion control software is easy to program (at the machine or offline) for precise valve movement along the X-Y-Z axes. The machine handles various one- and two-component materials, including those that are highly viscous and abrasive. Applications range from dispensing thermal interface materials (with a consistency of grease to heavy paste), to bonding, potting and gasketing parts used in automotive electronics, medical devices and other products.
Cartridges contain 2.5 to 32 ounces of material, which is dispensed manually by squeezing a trigger (similar to a caulk gun) or automatically via a robot. Like syringes, cartridges are rated to only withstand about 100 psi of air pressure. Unlike syringes, they often may require the addition of a valve.
Brad Smith, dispensing equipment and automation specialist with the ESR group at Ellsworth, says the higher a material’s viscosity rating, the more likely it will need a shut-off valve and a controller to actuate the valve.
Single-component materials in pails and drums (such as RTV silicone) are pumped to a valve with an attached nozzle or needle. For two-component materials, Smith says a meter, mix and dispense system is required to properly proportion the materials to the dispense valve. He also notes that using a needle with a too-small orifice in a pump-based system can cause excessive back pressure, resulting in drooling material.
Valves, Pumps, Challenges
Manufacturers can use spool, auger, needle or spray valves to dispense thick materials. Spool valves are robust and provide suck-back that prevents material waste due to drooling or stringing. This valve features a cylindrical stopper (at the end of a spool) that is sealed with a gland or O-ring and actuated by a pneumatic cylinder. The pressurized material closes the valve at the end of each dispense cycle.
A positive displacement technology, an auger valve ensures shot size accuracy and repeatability, making it a favorite of electronics manufacturers to dispense precise amounts of solder paste. Manufacturers in other industries use the valve to dispense epoxy and other types of filled material.
With the auger valve, material enters through one port under pressure and is then forced out through the dispensing orifice by a motor-driven Archimedes screw (auger). How much material gets dispensed is determined by the screw, not the pressure level.
“Positive displacement valves are helpful when the viscosity of a material changes due to environmental conditions,” claims Linder. “For example, the centipoise of a material can be higher in the morning when the plant is cold, but lower in the afternoon when the plant warms up.”
Nordson EFD’s xQR41 MicroDot needle valve dispenses material dots as small as 100 to 150 microns wide at a rate of up to 400 per minute, or 80 per second with an optional actuator. It features a 60 percent smaller form factor than standard valves, an exchangeable modular design for greater customization and process control, and a quick-release clasp that allows easy removal of the fluid body to replace wetted parts in seconds. The valve’s small profile (2.6 inches long by 0.93 inch wide) enables multiple valves to be mounted close together for greater output per batch. Weighing only 5 ounces, the valve reduces the tooling payload for Cartesian robots, enabling for faster arm movement and improved positioning stability.
Thick materials are nearly always dispensed in a dot or a bead. Rarely are they sprayed. However, nonstick material (such as grease) that’s between 75,000 and 100,000 cps may be sprayed with a heavy-duty valve. Its tip size should be four times greater than that of the grease’s filler particles to prevent blockages and protect the valve’s seals. Equally important, highly pressurized air must be added to the material to cause some atomization. Otherwise, the material will probably splatter onto a part rather than provide a nice coating.
Positive-displacement pumps are the most effective at moving thick materials. Commonly used types include the progressive cavity, piston, pail and ratio. A progressive cavity pump transfers material through a sequence of small, fixed-shape, discrete cavities as its rotor is turned.
Techcon’s TS8100 positive displacement pump dispenses materials ranging from low-viscosity coatings to high-viscosity greases, paste and flux. The pump creates a volumetric flow that does not change the shape or size of the material, and offers repeatability of ±1 percent. Easy to clean, the pump comes with syringe and mounting brackets, a Luer lock fitting and a dispensing tip selection pack.
In a piston pump, the piston forces material from the inlet side to the outlet side of the pump. As the piston moves upwards, it reduces the pressure in the pump body, causing the suction valve to open and permit material to flow into the pump.
Pail pumps feature a piston that pressurizes and draws the material up to the discharge nozzle. These pumps were originally designed for manually pumping material out of 5-gallon pails, but today include automated models for pails, as well as large models for one or more 55-gallon drums. The pumps handle material with a viscosity up to 1 million cps.
Ratio pumps are also for pails and drums. Long says this type of pump increases thick-material flowability by multiplying the air pressure level placed upon the material by a specific ratio. For example, if the pump ratio is 2-to-1, and 50 psi of air pressure enters the air motor, material will leave the pump at 100 psi.
Nordson EFD’s Rhino ratio pump comes in 5-gallon and 55-gallon models, both of which are available with a ratio of 48- or 65-to-1. Volumetric displacement of the lower ratio is 8 cubic inches per stroke, with a maximum output of 4.2 liters per minute. For the higher ratio, it’s 5.8 cubic inches per stroke, and an output of 2.8 liters per minute.
All models work with the company’s 736HPA high-pressure valve (up to 2,500 psi). They are modular in design and allow quick changeover of the air motor for uniform material output. Options include automatic depressurization, an air regulator and casters.
Pumping thick materials presents two major challenges for manufacturers. One is preventing air entrapment in hoses during priming. Linder says this can be achieved by using a level sensor that indicates when the pail or drum is nearly empty, and replacing it at that point. Another way is for the operator to carefully bleed ports as the new pail or bucket is being installed. Should bubbles form in thick material, it will likely splatter upon leaving the nozzle, resulting in an unusable part and wasted material.
The other challenge is thoroughly mixing two-component materials where one is highly viscous and the other is less viscous. Long recommends manufacturers use a fixed-ratio system that properly delivers both materials to the metering device. The system ratio can be set from 1-to-1 to 10-to-1.
To prevent problems, it’s often better to dispense several narrow beads of thick material rather than one wide bead. Smith recalls one customer that needed to cover a 1-inch-wide area on a part with thick sealant. Instead of using a large nozzle to deposit it all at once, the manufacturer learned it was better to place three 0.25-inch beads of material and then flatten them together to fill the 1-inch-wide area.