Ratio drift during low-pressure PU machine start-up is usually a warm-up, recirculation, or inlet-pressure problem, not an immediate formulation problem. When the machine has been idle, component temperatures, pump clearances, dissolved air, and line pressures are all different from steady production. Starting to meter before those conditions settle transfers the transient condition into the first shots. The practical fix is a start-up sequence that treats temperature and pressure as ratio checks, not utility checks.
Why the First Shots Drift More Than the Rest
A low-pressure metering machine depends on stable viscosity, full pump chambers, and balanced back pressure. At start-up, those conditions are not yet present. Cold polyol and isocyanate move differently through gear or piston clearances. Air still trapped in the recirculation loop compresses and delays component delivery. A partially closed valve or loaded filter changes suction pressure. The result looks like ratio drift even when the recipe and pump settings have not changed.
Use the symptom pattern to separate a true calibration fault from a start-up condition.
| Start-up symptom | Likely cause | Confirm before changing ratio |
|---|---|---|
| First shot is component-rich despite a correct recipe | One material is not at target temperature | Compare tank and line temperature with the recipe set point |
| Ratio drifts for the first three to five shots, then stabilizes | Air or unmixed material still leaving the system | Purge into waste and repeat shot-weight verification |
| Component pressure is low at start-up | Suction restriction or partially open valve | Check filter, suction valve, drum or IBC condition |
| Drift appears only at small shot size | Pump running below its stable flow range | Confirm the minimum continuous output for the pump |
| Ratio stays correct after recalibration, then drifts again | Loose coupling, worn seal, or air leak | Recheck calibration under load and inspect seals and check valves |
| Both component flows move, but the ratio shifts | Unequal back pressure between A and B lines | Compare head or line back pressure at the same flow |
Stabilize Temperature and Recirculation Before You Meter
The first decision rule is to start the thermal and flow loops before the metering loop. The material in a day tank may be at set point while the line, filter housing, and metering head are still cold. Heat exchangers and jackets only help if material is actually moving through the circuit. Open the recirculation valves, start agitation where it is specified for filled material, and run both components back to their tanks or through the recirculation path.
For filled polyol systems, agitation must begin before material is drawn into the metering pump. If filler has settled overnight, the first shots may contain a non-representative ratio of filler to liquid. Recirculation should continue long enough for both return-line temperatures and pressures to stabilize, not just until the HMI displays the target tank temperature.
Remove Air, Check Inlet Pressure, and Prime the Pump
A metering pump can only deliver what reaches its inlet. When the inlet pressure is insufficient, the pump chamber does not fill completely on each stroke or revolution. Dissolved air expands under low pressure, adding another variable. The result is a flow that is present on the HMI but missing at the mixing head.
Check the suction filter and any partially closed inlet valves before adjusting ratio settings. A vacuum or low inlet-pressure alarm should be treated as a stop condition for ratio verification. If the machine has a solvent or purge circuit, confirm that the check valves seat properly before material is introduced. A small internal leak between the A and B circuits can produce a large ratio error at low flow.
Inlet pressure and pump clearance problems show up during start-up because the hydraulic balance has not settled. <PU Casting Machine Mechanics: Mastering Metering Pumps & Hydraulic Pressure [Part 4: Mechanics & Fluid Dynamics]> covers how pump clearance, suction conditions, and discharge pressure interact.
Verify Ratio at the Mixing Head, Not Just on the HMI
The HMI displays commanded flow, calculated flow, or a corrected signal depending on the control architecture. It does not automatically prove what the head delivered. Start-up verification should measure each component separately, either by catching timed samples before the mixer or by weighing the A and B outputs from a calibration port.
For a target ratio such as 100 parts A to 55 parts B, calculate the required component weight from the actual shot weight. If the measured A or B weight differs from the target proportion by more than the acceptance limit, do not adjust the recipe first. Check temperature, suction pressure, and back pressure. Only recalibrate after those inputs are stable. A calibration performed with cold material or trapped air becomes a new error source.
The same start-up procedure still needs a machine configured for the correct shot size and flow range. <Low Pressure PU Foam Injection Machine for Custom Molding: Process & Parameters> covers the process parameters that affect low-pressure custom molding.
Start-Up Sequence That Keeps Ratio Stable
A repeatable sequence matters more than a single perfect start. The following order prevents the most common transient ratio errors.
- Start tank agitation and bring both materials into the specified temperature band.
- Open recirculation and run both components until return temperature and pressure stabilize.
- Confirm inlet filters and suction valves are clear and fully open.
- Vent or purge the mixing head and static mixer until material appears without visible air.
- Set A and B back pressures to their normal operating values before calibration.
- Run a wasteful shot or timed component sample and weigh both components.
- Compare the measured ratio with the target before allowing material into production.
- Record the first production shot weight and ratio as the start-up reference.
Step 7 is the release gate. If the measured ratio is outside tolerance, return to the temperature or inlet-pressure check. Do not offset the recipe to force a cold machine into tolerance.
HMI Alarms and Trending That Catch Early Drift
Start-up drift should be visible before it becomes a production defect. A useful HMI setup includes a ratio deviation alarm, A and B pressure low alarms, temperature band alarms, and a pump speed-versus-flow mismatch limit. Trend data from the first five shots is more useful than a single pass or fail reading because it shows whether the system is converging or still hunting.
Closed-loop servo metering can reduce drift by comparing commanded and actual flow and correcting the pump speed in real time. Haifeng low-pressure machines use this direct-drive compensation approach to keep component ratio stable during start-up and production. Even with closed-loop control, the first-shot check remains necessary because the control loop needs a stable hydraulic starting point.
If ratio drift remains after the start-up sequence is corrected, the larger question is whether the machine type matches the production task. <High Pressure vs. Low Pressure Polyurethane Foam Machine: How to Choose the Right One for Your Production> covers the pressure and output conditions that make each machine class the better fit.
Send Us the Right Start-Up Data
If the ratio cannot be held stable across the first five shots, send the shot weights, material temperatures, inlet pressures, back pressures, and the current ratio target. That data separates a control fault from a hydraulic or thermal start-up condition. Contact Haifeng Polyurethane Machinery at info@haifeng-automation.cn or WhatsApp: 86 13566296633.
FAQ
Should I wait until the day tank reaches set point before starting the pump?
Yes. Tank temperature alone is not enough. Start recirculation, wait for return-line temperature and pressure to stabilize, and only then begin metering or calibration. Trapped cold material in the lines can still shift the first shots.
Can I trust the HMI ratio reading immediately after start-up?
No. The HMI reading may show commanded or corrected flow, but it does not prove what the mixing head delivered. Verify with a timed sample or shot-weight check before releasing the line to production.
How long should warm-up take?
There is no single safe number. It depends on line volume, material viscosity, heat exchanger capacity, and whether the material is filled. Use stable return temperature and stable pump pressures as the release condition, not a fixed timer.
What should I check first if the ratio drifts only on Monday mornings?
Start with air ingress, settled filler, and cold lines. Check suction filters and inlet valves, then run the recirculation and venting sequence before recalibrating. The pattern of drift disappearing after the first few shots usually points to a start-up condition rather than a pump fault.
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