Static Mixers and Meter-Mix Equipment

A static mixer is a tube containing a series of fixed elements that split, rotate and recombine two fluid streams as they flow through, mixing them without moving parts. Meter-mix equipment, often called meter-mix-dispense (MMD) equipment, uses pumps to deliver two components at a fixed ratio into a static mixer, then to a spray gun, dispensing valve or nozzle. Static mixing is used for two-component epoxies, slower polyurethanes, sealants, adhesives, grouts and joint fillers whose reaction is slow enough to remain liquid in the mixer and hose for a short period.

How static mixing works

Each mixing element divides the flow and turns it, typically by 90 or 180 degrees, before passing it to the next element. With each element, the number of layers grows, so a short mixer with many elements produces thorough mixing. Element designs include:

• Helical elements. Alternating left and right twists. Common in disposable cartridges and dispensing.
• Square or lattice elements. Shorter mixers with high mixing efficiency per length.
• Bayonet and specialty designs for high-viscosity or filled materials.

Mixers come in disposable plastic versions, used for cartridges and short runs, and reusable metal versions for spray systems that are flushed with solvent.

Comparison with impingement mixing

Fast-set polyurea coatings need impingement mix spray guns because they would cure inside a static mixer.

Meter-mix system components

• Metering pumps. Positive-displacement pumps, often piston or gear, linked to maintain ratio. Fixed-ratio pumps are mechanically linked; variable-ratio systems use separate drives and electronic control.
• Material heaters to reduce viscosity where needed.
• Mix manifold that brings the components together, with check valves to prevent crossover.
• Static mixer.
• Integrated or remote solvent flush to clean mixed material from the mixer, hose and gun.
• Spray gun or dispense valve.

Pot life considerations

Once mixed, the material begins to react. The time available to move it through the mixer, hose and gun is limited by its pot life. Important points:

• Mixed-material volume. The volume from the mixer to the gun tip should be small enough to be sprayed or flushed well within pot life.
• Stoppages. If spraying stops for longer than the safe interval, the system must be flushed.
• Temperature. Heated material reacts faster, shortening usable time.

Many systems include alarms or timers that remind the operator to flush.

Ratio control

Off-ratio material can remain soft, sticky or brittle. To check ratio:

• Take a ratio check sample by dispensing each component separately into containers and weighing or measuring volumes.
• Use ratio monitoring on electronic systems.
• Watch for color changes if the components are tinted differently.

See mixing and induction time for related concepts.

Typical applications

• Epoxy floor and lining systems applied by spray or dispensing.
• Joint fillers and sealants in concrete floors. For example, semi-rigid polyurea joint fillers are commonly dispensed through cartridge static mixers.
• Structural adhesives in manufacturing.
• Grouting and anchoring compounds.
• Potting and encapsulation of electronics.
• Slower aromatic and aliphatic polyurethanes in shop finishing.

Cartridge dispensing

For small jobs, two-component materials are supplied in side-by-side or coaxial cartridges with disposable static mixers. Manual, pneumatic or battery tools push both components through the mixer at a fixed ratio. Before use:

1. Balance the cartridge by dispensing a small amount before attaching the mixer.
2. Attach the mixer and discard the first portion until color and consistency are uniform.
3. Replace the mixer after pauses longer than the material’s open time.

Maintenance

• Flush mixers, hoses and guns at the end of work or before pot life expires.
• Inspect check valves and seals for wear.
• Keep isocyanate components protected from moisture if used.
• Verify pump ratio and calibration regularly.

See spray equipment maintenance.

Safety

• Mixed epoxies and polyurethanes can be skin sensitizers; wear gloves and eye protection (see epoxy skin sensitization).
• Flushing solvents may be flammable; ground equipment and ventilate.
• Large masses of mixed material can heat up rapidly; do not leave them in containers.

Key terms

• Static mixer. A disposable tube with fixed internal elements that blends two components as they flow through.
• Mixing elements. The helical or square baffles that split and recombine the stream.
• Meter-mix machine. Equipment that measures components at a set ratio before sending them to a mixer.
• Cartridge dispensing. Side-by-side cartridges pushed by a hand or pneumatic gun through a static mixer.
• Ratio check. Dispensing each side separately into containers to confirm the volume ratio.

Frequently asked questions

Can I spray polyurea through a static mixer?

Not fast-set polyurea. Some slower polyurea or polyaspartic products are designed for static mixing; check the data sheet.

How many mixing elements are needed?

It depends on viscosity, ratio and material. The material manufacturer usually specifies the mixer type and element count.

Related entries

• Plural-component spray
• Mixing and induction time
• Epoxy coatings
• Concrete surface preparation
• Pot life and gel time (new)
• Impingement mix spray guns (new)