On a plural component rig, the gun wears first and by a long way. Mix chambers, side seals, O-rings and check valves sit where the two chemicals meet and begin reacting, so they are consumed far faster than anything in the proportioner. After that, the A side and B side fail from completely different causes: iso crystallizes when it meets moisture, resin attacks elastomers and settles out. The most expensive failures on these machines are not wear at all. They are moisture reaching the iso side and off ratio material caused by hose or gun problems.
Plural component equipment punishes the assumptions that work on single component airless. On a 395, a worn part means a worse finish and you keep going. On a Reactor, a worn mix chamber or a cold hose zone means material that never reaches its designed properties, and foam that does not cure correctly has to come off the building.
This page covers what genuinely wears on spray foam and polyurea rigs, why the two sides of the machine fail differently, and where the expensive mistakes actually happen.
1. Why Plural Component Is a Different Machine
A single component sprayer moves one material from a bucket to a wall. A plural component rig keeps two reactive chemicals completely separate, heats both to temperature, delivers both at a controlled ratio and pressure through a heated hose, and combines them only at the last possible moment inside the gun.
Everything difficult about maintaining these machines follows from that. There are two of nearly everything, they carry chemically different materials, and the moment they meet, a reaction starts that will happily continue inside your equipment if anything is wrong.
| System | Job | What failure looks like |
|---|---|---|
| Supply and transfer | Move material from drums to the proportioner | Cavitation, starved pumps, off ratio |
| Proportioner pumps | Meter both sides at the correct ratio and pressure | Pressure imbalance between sides |
| Heaters | Bring both chemicals to spray temperature | Poor mixing, wrong cure, bad yield |
| Heated hose | Hold temperature all the way to the gun | Cold zones, off ratio at the gun |
| Gun and mix chamber | Combine the chemicals correctly at the last moment | Crossover, clogging, poor pattern |
| Iso moisture protection | Keep humidity away from the A side | Crystallization, scored seals, clogs |
Isocyanates are respiratory sensitizers. Once someone is sensitized, further exposure can trigger severe reactions at levels that previously caused no symptoms, and the sensitization does not reverse. Spray polyurethane foam application requires appropriate respiratory protection, skin and eye protection, and controlled ventilation and re-entry times. Follow your material supplier's safety data sheets and applicable regulations rather than shop habit. No maintenance advice on this page substitutes for that.
2. The A Side and B Side Wear Differently
This is the single most useful thing to understand about maintaining these machines, and it is the reason a service schedule that treats both sides identically wastes parts on one side and misses failures on the other.
A Side
Isocyanate · ISOThe reactive half. Its defining problem is that it reacts with atmospheric moisture, forming hard crystalline solids inside your equipment.
- Crystals score pump surfaces and seals
- Crystals clog filters, screens and small passages
- Damage is cumulative and not reversible by cleaning
- Requires a functioning desiccant dryer on the drum vent
- Throat seal liquid must be maintained, not topped up occasionally
- Drums left open even briefly cause real damage
B Side
Resin · POLYOLThe formulated half. Its problems come from what is blended into it rather than from the air around it.
- Amine catalysts attack elastomer seals over time
- Fillers and flame retardants can be abrasive
- Blowing agents can come out of solution if left undisturbed
- Requires agitation so components do not settle out
- Settled resin sprays off ratio even at correct pressure
- Seal material compatibility matters more than on the A side
Service the A side against moisture exposure, and the B side against chemical attack and settling. If you are replacing A side seals unusually often, the answer is almost never better seals. It is a moisture control problem upstream: a spent desiccant, a leaking drum seal, or humid air reaching the iso.
3. What Actually Wears, in Order
| Order | Component | Why it goes | Consequence of ignoring |
|---|---|---|---|
| 1st | Mix chamber | Reaction happens here; abrasion and buildup | Poor pattern, clogging, off ratio |
| 2nd | Gun O-rings and side seals | Chemical attack plus pressure cycling | Crossover between sides, gun seizure |
| 3rd | Gun check valves | Constant cycling, exposure to both chemicals | Crossover, contaminated supply lines |
| 4th | Air cap or purge components | Buildup and abrasion | Incomplete purge, chamber clogs |
| 5th | Y-strainers and inlet screens | Catch crystals and settled solids | Starved pump, pressure imbalance |
| 6th | Heated hose RTD sensors and zones | Flexing, connector fatigue, heat cycling | Cold spots, off ratio material |
| 7th | Proportioner pump seals | Normal wear plus side-specific attack | Pressure imbalance, off ratio |
| 8th | Transfer pump seals | Continuous duty from drum to machine | Cavitation, starved proportioner |
| 9th | Heater elements and thermocouples | Thermal cycling over years | Wrong temperature, poor yield |
| Rare | Drive, motor, control board | Long life under normal conditions | Machine down entirely |
The pattern is worth noticing. The top four are all in the gun. On most rigs, gun consumables account for the large majority of parts spend and nearly all unplanned stoppages.
4. Gun Components: Where the Money Goes
Graco's plural component guns split by how they keep the mix chamber clear, and that choice drives your consumable pattern more than anything else.
- Air purge guns blow the chamber clear with compressed air after each trigger pull. Simple and widely used. Air cap and chamber consumables dominate the parts list.
- Mechanical purge guns drive a rod through the chamber to clear it physically. Fewer air related issues, more mechanical wear parts.
- Cartridge based guns replace the entire fluid section, including side seals and O-rings, as one unit. Higher unit cost, dramatically faster rebuild, less scope for reassembly error.
On a crew where a gun rebuild happens in the field, the cartridge approach often wins on total cost even though the part costs more, because the labour and the risk of a misassembled seal both drop. If you rebuild guns on a bench with a trained tech, individual seals are usually cheaper. Which is better depends on how your crew actually works, not on which part number costs less.
Mix chamber sizing matters as much as condition
Mix chambers are sized by orifice, and the correct size depends on your target output and material. Running a chamber that is too small for your output raises pressure and accelerates wear. Too large and you lose mixing energy, which shows as poor cure even when everything else is correct. If output has changed since the rig was set up, chamber sizing should be revisited rather than assumed.
5. Heated Hose and the Off Ratio Problem
Heated hose is not a delivery component with a heater attached. It is part of the proportioning system, because material temperature affects viscosity, and viscosity affects how the two sides behave at the gun.
A cold zone partway down a hose run changes the viscosity of that side only. Both pumps are still delivering correctly, the gauges look right, and the material arriving at the mix chamber is off ratio anyway. This failure is difficult to diagnose precisely because nothing on the proportioner looks wrong.
- RTD sensors and their connectors take mechanical abuse and are a common failure point.
- Zone to zone connections on long runs are where problems usually appear first.
- Hose whip sections near the gun flex constantly and fatigue faster than main sections.
- Insulation damage lets a section run cold even with the heater working correctly.
Foam that sprays off ratio does not cure to its designed density, adhesion or thermal properties. It generally has to be removed and reapplied, at your cost, including the labour of removing it. That is why hose temperature monitoring and gun condition matter more on this equipment than output figures do, and why a $200 sensor failure can become a five figure remediation.
6. Proportioner Wear: Reactor and E-Flo
Graco's proportioner range spans electric, air driven and hydraulic machines, in foam and coatings configurations, from small touch-up units up to high output production rigs. E-Flo covers the electric circulation pump side of the equation on larger installations.
Whatever the drive type, the wear logic stays consistent.
- Pump seals and packings wear per side and per the chemistry described in section 2. Expect different intervals on A and B, and track them separately.
- Check valves in both fluid sections determine whether pressure holds. Imbalance between the two sides is usually a valve before it is a pump.
- Filters and strainers protect everything downstream. On the iso side they also tell you something: recurring clogs are a moisture symptom, not a filter problem.
- Heater elements and temperature sensors fail slowly enough that people adapt to a machine that no longer holds setpoint. Verify against the display rather than trusting it.
- Pressure transducers drive the control system. A drifting transducer produces an imbalance that is not actually there, or hides one that is.
A pressure difference between the A and B sides tells you something is wrong, not what. The honest order is: check filters and strainers, then check valves, then pump seals, then transducer calibration. People replace pump seals first because that feels like the real repair. It is usually the fourth most likely cause.
7. Symptom Diagnosis
Pressure imbalance between A and B
One side reads noticeably higher or lower than the other and will not settle.
Check in order: Strainer or filter on the low side → supply level and transfer pump on that side → check valves → pump seals → transducer calibration.
Repeated A side clogging or seal failure
Iso side filters clogging often, or A side seals failing far sooner than B side.
Cause: Moisture reaching the iso, almost without exception. Check: desiccant dryer condition, drum seals, how long drums sit open, ambient humidity in the rig. Replacing seals without fixing this repeats the failure.
Foam cures soft, brittle or with poor adhesion
Material sprays acceptably but does not develop correct properties.
Check in order: Hose temperature along the full run → material temperature at the gun → ratio accuracy → mix chamber condition and sizing → B side agitation and settling. Stop spraying until resolved; remediation costs far more than downtime.
Gun clogging shortly after each stop
The chamber blocks up during pauses rather than during continuous spraying.
Cause: Purge not clearing the chamber fully. Check: air supply pressure and moisture on air purge guns, purge rod condition on mechanical guns, chamber wear, and whether the chamber is correctly sized for current output.
Crossover between sides
One chemical found in the other side's line or supply. Hard reacted material in a fluid section.
Cause: Failed gun check valve or failed side seal. This is serious. Stop, isolate, and clear it properly. Cured material inside a fluid section is a far larger job than the valve that caused it.
8. A Realistic Service Schedule
| Interval | Task |
|---|---|
| Every job, before starting | Hose condition and temperature across all zones; gun chamber and air cap condition; throat seal liquid levels; desiccant dryer colour; A and B pressures at idle |
| Every job, at shutdown | Purge and clean the gun properly; grease or seal the gun per its type; seal drums; never leave iso exposed |
| Weekly in production | Strainers and inlet screens on both sides; transfer pump operation; B side agitation; verify heater setpoints against actual temperature |
| Monthly | Full gun rebuild or cartridge change; hose RTD and connector inspection; check valve inspection both sides |
| By hours, tracked | Proportioner pump seals, logged separately for A and B; heater element condition; transducer verification |
| Always on the shelf | Spare mix chambers, a full gun seal or cartridge set, strainers, spare desiccant |
One service log with a single set of intervals will always be wrong for one of the two sides. Two columns, two histories. It takes almost no extra effort and it is the difference between predicting a failure and reacting to one.
Sourcing Plural Component Parts
Reactor, E-Flo and Fusion parts are not listed in our online catalog, but we are an authorized Graco distributor and can quote and source them directly. Send us your machine model, serial number and what you need.
Request a Quote Call +1 713-931-41029. Frequently Asked Questions
What wears out first on a Graco Reactor?
The spray gun wears first by a wide margin. Mix chambers, side seals, O-rings and check valves in the gun are consumed far faster than anything in the proportioner, because they sit where the two chemicals meet and begin reacting. Most plural component downtime is gun related rather than machine related.
Why do the A side and B side of a proportioner wear differently?
They carry chemically different materials. The A side carries isocyanate, which reacts with atmospheric moisture and forms crystalline solids that score seals and pump surfaces. The B side carries resin, which often contains fillers, blowing agents and amine catalysts that attack elastomers differently and can settle out if not agitated. The result is that the two sides fail from different causes on different schedules.
What is the most common cause of Reactor downtime?
Moisture reaching the isocyanate side. Iso reacts with humidity in the air to form crystals that damage pump seals, clog filters and score surfaces. A neglected desiccant dryer on the iso supply, or a drum left open, will cause more expensive failures than normal running wear ever does.
How often should heated hose be inspected on a spray foam rig?
Inspect before every job. Heated hose carries both chemicals at pressure and temperature, and its RTD temperature sensors and heat zones are common failure points. Uneven hose temperature causes off ratio material at the gun, which is a product quality failure rather than just a maintenance issue.
Can off ratio spray foam be fixed after it is applied?
No. Off ratio foam does not cure to its designed properties and generally has to be removed and reapplied, which makes it far more expensive than the maintenance that would have prevented it. This is why proportioning accuracy, hose temperature and gun condition matter more on plural component equipment than raw output does.
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