Plural-Component Proportioner Troubleshooting
A systematic way to diagnose pressure imbalance, off-ratio material, heat problems and gun faults on plural-component proportioners before they ruin a job.
Key takeaways
- The A-side and B-side pressure gauges are the most useful diagnostic tool: watch which side moves and in which direction.
- As a rule of thumb, a side reading high points to a restriction downstream on that side; a side reading low points to a supply problem or leak on that side.
- Off-ratio material shows up as soft, tacky, brittle or discolored film. Stop spraying and diagnose — do not keep spraying through it.
- Moisture contamination of the isocyanate side causes many recurring problems, from plugged filters to failed seals.
Plural-component proportioners meter, heat and pressurize two reactive components — typically an isocyanate (A side) and a resin or curative (B side) — and deliver them to a gun where they mix. Because the chemistry depends on the correct ratio, temperature and mixing energy, small mechanical problems quickly become coating failures. Polyurea, spray polyurethane foam and fast-set epoxies are especially unforgiving because they cure in seconds to minutes; there is no chance to correct a bad mix after it hits the surface.
This article assumes familiarity with how these machines work; for the fundamentals see plural-component spray equipment. Specific procedures, set points and error codes vary by manufacturer — always use your unit’s manual.
A systematic diagnostic approach
- Stop spraying on the substrate. Move to a test panel or cardboard so you do not apply more bad material.
- Read the gauges. Note A and B pressures both static (gun closed) and dynamic (spraying). Note any error codes on the controller.
- Check temperatures. Compare actual primary heater and hose temperatures to set points and to the material data sheet.
- Check supply. Confirm drum levels, transfer pump operation, inlet pressures and inlet screens.
- Isolate the gun. Inspect impingement ports, screens and mix chamber; see whether the problem follows the gun or stays with the machine.
- Verify ratio. If doubt remains, perform a ratio check (below) before returning to production.
Reading pressure imbalance
When both sides are healthy, A and B dynamic pressures stay close to each other, typically within a range the manufacturer specifies. A widening gap is the classic early warning.
| Symptom | Likely causes | Checks and fixes |
|---|---|---|
| One side pressure high while spraying | Plugged gun screen, partly blocked impingement port, restricted hose or fluid filter on that side | Clean gun screens and ports; inspect manifold filter; check for cold, thickened material |
| One side pressure low while spraying | Material starvation, cavitating pump, failed transfer pump, worn check valve or leak on that side | Check drum level and transfer pump; clean inlet screen; inspect check valves and seals |
| Pressure drops at each pump changeover | Worn inlet or outlet check valves, worn piston seals | Rebuild the affected pump |
| Static pressure creeps down with gun closed | Internal leak past seals or valves; external leak at fittings | Inspect for external leaks; test and rebuild pump |
| Both pressures low; motor working hard | Insufficient power or air supply, undersized generator, wrong set point | Verify supply voltage or air; review pressure settings |
Viscosity drives pressure. If one component is colder than intended, it thickens, its side pressure rises and the ratio can drift. Many “pressure” problems are actually heat problems.
Recognizing off-ratio material
For many aromatic and aliphatic polyureas and polyurethanes, the general pattern is:
- Resin-rich (B-rich) material tends to be softer, tacky or “cheesy,” with poor tensile strength.
- Isocyanate-rich (A-rich) material tends to be harder and more brittle, may cure with a glassy surface and can show color change or poor elongation.
- Poor mixing — as distinct from bad ratio — appears as streaks, color variation, soft spots or lumps.
These are tendencies, not certainties; formulations differ. When in doubt, sample the cured film and compare it with known-good material, and consult the manufacturer.
Performing a ratio check
Most proportioners allow the A and B streams to be dispensed separately into cups, through a ratio-check valve or with the gun’s mix module removed. Weigh each sample and convert to volume using the specific gravities on the data sheets. The result should match the specified volume ratio (often 1:1 for polyurea) within the manufacturer’s tolerance.
Dispensing isocyanate into open containers creates exposure risk. Wear the respiratory protection and PPE your program requires, and handle samples and waste according to the SDS. See isocyanate safety.
Heater and hose heat problems
Heat lowers viscosity for good atomization and mixing. Symptoms of inadequate heat include coarse pattern, poor mixing, rising pressures and rough, orange-peel texture. Common causes:
- Spraying faster than the primary heaters can deliver at set point
- Failed heater element, thermocouple or RTD, or a tripped over-temperature switch
- Damaged hose heat wire or loose connector, giving cold hose sections
- Cold drums: material entering the machine far below the recommended inlet temperature
Keep drums at the temperature the manufacturer recommends with drum heaters or a heated storage area, especially in cold weather.
Gun, supply and contamination problems
Gun faults
Impingement guns mix by colliding two high-pressure streams. Partly plugged ports, worn side seals or a damaged mix chamber cause lead/lag (one component arriving first), pattern deflection and crossover — material from one side migrating into the other. Cleaning ports with the correct drill bits, replacing side seals and keeping the gun greased at shutdown prevent most problems.
Cavitation and air
If the feed pump cannot supply the proportioner fast enough, the proportioning pump cavitates, producing a low or fluctuating pressure on that side and spitting at the gun. Check transfer pump air pressure, inlet hoses and screens, and agitate filled B-side materials as the data sheet directs so pigments do not settle.
Moisture and crystallization
Isocyanate reacts with moisture to form solid crystals. Symptoms include plugged A-side filters and screens, scored pump rods and seals, and recurring leaks. Prevention includes desiccant dryers on drum vents, nitrogen or dry-air blankets, sealed containers and keeping the A-side pump lubricant clean. See spray equipment maintenance for routine care.
Frequently asked questions
Why is my polyurea tacky after spraying?
Common causes are a resin-rich (off-ratio) mix, poor mixing from low heat or pressure, or contaminated components. Stop, check gauges and temperatures, and perform a ratio check.
Is a small pressure difference between A and B a problem?
Some difference is normal because the components have different viscosities. Watch for a change from the normal pattern or a gap larger than the manufacturer’s guidance.
How do I tell a gun problem from a machine problem?
Swap in a known-good gun or mix module. If the symptoms go away, the gun is at fault; if not, investigate the proportioner, heaters, hoses and supply.
Should I keep spraying if the pressure imbalance alarm trips?
No. The alarm exists because off-ratio material is likely. Resolve the cause, verify the ratio and spray a test panel before returning to the job.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
