Impingement Mix Spray Guns
An impingement mix spray gun combines two reactive components by colliding high-pressure streams in a small mixing chamber at the gun. It is the standard gun type for fast-set polyurea and spray foam.
An impingement mix spray gun is a plural-component spray gun that mixes the A side (isocyanate) and B side (resin) by forcing them through small opposed ports into a mixing chamber at high pressure. The streams collide, or impinge, and the turbulence mixes them thoroughly in a fraction of a second. The mixed material leaves the gun immediately through the tip. Because mixing happens at the last moment, impingement guns can handle very fast-reacting materials such as polyurea coatings and spray polyurethane foam that would cure inside a hose or static mixer.
Why impingement mixing is needed
Pure polyurea can gel in seconds. Mixing it before the gun, as with static mixers, would block the hose. Impingement mixing keeps the two components separate until they reach the mixing chamber, which is cleaned after each trigger release. Heated material from the plural-component spray proportioner and heated hoses arrives at a controlled temperature and pressure, so viscosities are low enough for good mixing.
Main components
- Side blocks or side seals. Admit the A and B components into the mixing chamber.
- Check valves and screens. Prevent one component from flowing back into the other side, which would cause a crossover and cured material in the gun.
- Mixing chamber. A precisely sized bore with small impingement ports. Chamber size sets output and influences pattern.
- Spray tip or flat tip. Shapes the pattern, often round for foam and fan for coatings.
- Purge system. Clears the mixing chamber when the trigger is released.
- Manifold. Connects the hoses and often includes valves to isolate each side.
Purge types
Air purge
When the trigger is released, the mixing chamber is closed to the components and a blast of compressed air clears the remaining mixed material. Air-purge guns are lighter and popular in spray foam and many polyurea applications. Clean, dry air is essential (see compressed air cleanliness).
Mechanical purge
A close-fitting valving rod moves through the mixing chamber when the trigger is released, physically pushing out mixed material. Mechanical purge guns tolerate very fast systems and reduce air consumption. They need careful maintenance of the rod and chamber.
Solvent flush
Some guns or systems include solvent flushing for slower materials. This is less common for fast-set polyurea.
Mix chamber and tip selection
Chamber and tip sizes are matched to:
- Output rate of the proportioner.
- Material viscosity and reactivity.
- Desired pattern width and film build per pass.
- Spray pressure at the gun.
Too large a chamber for the available output gives poor mixing and a weak pattern. Too small a chamber raises pressure and can limit production. Equipment and material manufacturers publish recommended combinations.
Pressure and temperature
Impingement mixing depends on energy. Many pure polyureas require high pressure and elevated temperature at the gun to mix properly. ArmorThane’s data, for example, states that its ArmorShield-class polyureas must be sprayed at no less than 2,800 psi and above 66 °C, which rules out low-pressure equipment for them. Each material’s data sheet gives its own requirements; see also low-pressure vs high-pressure polyurea equipment.
Spray technique
- Hold the gun at the distance recommended for the tip, usually perpendicular to the surface.
- Overlap passes by about half the pattern width.
- Build thickness in multiple passes rather than one heavy pass.
- Use cross-hatch passes on large areas for uniform thickness.
- Watch the pattern for “fingers,” tails or color streaks, which indicate pressure imbalance or a partly blocked port.
See spray gun technique for general guidance.
Signs of trouble
| Symptom | Possible cause |
|---|---|
| Streaky color, soft or tacky spots | Off-ratio, pressure imbalance, blocked port |
| Pattern narrows or tails | Low pressure, low temperature, worn tip |
| Material spits at trigger release | Purge problem |
| Cured material in the gun | Crossover, check valve failure |
| Excessive overspray | Tip too small, pressure too high, distance too great |
Many of these trace back to the proportioner rather than the gun; see proportioner troubleshooting.
Maintenance
- Grease the gun and mixing chamber at the end of every day as the manufacturer instructs. Grease prevents moisture from reaching isocyanate residues.
- Clean impingement ports with the correct drill bits and tools; never enlarge them.
- Inspect and replace side seals, O-rings and check valves on a schedule.
- Store the gun with the manifold valves closed and capped.
- Keep a spare gun or rebuild kit on site.
See spray equipment maintenance.
Safety
- Isocyanate aerosols are generated during spraying. Supplied-air respiratory protection is commonly required (see isocyanate safety).
- High-pressure fluid can penetrate skin. Never point the gun at anyone, and release pressure before servicing.
- Engage the trigger safety lock when not spraying.
Key terms
- Mixing chamber. The small chamber where A and B streams meet at high velocity.
- Mechanical purge. A rod that pushes through the chamber when the trigger releases, clearing mixed material.
- Air purge. A gun design that clears the chamber with a blast of air.
- Side seals. Seals between the side blocks and mixing chamber that keep the two components apart.
- Check valves. Valves that prevent material from one side backing into the other.
Frequently asked questions
Can an impingement gun spray epoxy?
Not typically. Epoxies react slowly and are usually mixed with static mixers or batch mixed. Impingement guns are designed for fast-reacting systems.
Why does the gun need grease every day?
Grease seals the mix chamber and ports from air and moisture, which would otherwise react with isocyanate residues and lock the gun.
Related entries
- Plural-component spray
- Proportioner troubleshooting
- Spray equipment maintenance
- Polyurea coatings
- Heated hoses for plural-component spray (new)
- Low-pressure vs high-pressure polyurea equipment (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
