Most ratio problems on a polyurethane line are discovered after the damage is already measurable: foam density drifts a few kilograms per cubic meter, parts come out soft or brittle at the edges, and scrap climbs a percentage point at a time. The mixing head catches the blame. It is rarely the mixing head.
What usually happened is simpler and cheaper to fix: the machine runs a ratio that was copied from an old recipe, scaled from a brochure, or set once and never re-verified against the raw materials now sitting in the day tanks. This article explains how to determine the A/B mixing ratio from the chemistry of the materials you actually run — not from assumptions — and how to confirm on the floor that the number the machine is delivering is still the number you intended.
What the A/B Mixing Ratio Actually Means
In Haifeng PU foaming documentation, A is the formulated polyol blend and B is the isocyanate. The A blend can contain polyols, approved chain extenders, catalysts, water or other blowing agents, surfactants and fillers. Other suppliers may use different letters. For CPU prepolymer/curative systems, state the actual materials and follow the configured tank labels.
The ratio itself is a statement of parts by weight per 100 parts, most often expressed as grams of A per 100 grams of B, or the reverse. Weight is the formulation basis. The machine, however, meters mostly volumetrically — gear pumps or piston metering delivering a flow — and only delivers the correct weight ratio when both sides are at the correct density and temperature. That distinction is where a large share of floor-level ratio errors begin.
Because the A side is a formulated blend rather than a single stream, knowing what each component contributes matters before any mixing math. What Is Spray Polyurethane Foam Composition And Application covers the composition of the two-component system and how each stream behaves in equipment.
The Chemistry That Sets the Ratio
Determine the target ratio from all reactive ingredients: hydroxyl and other active-hydrogen demand in the polyol formulation, water consumption and available NCO in the isocyanate. Hydroxyl number and NCO mass fraction are relevant TDS or batch-test inputs. The supplier-approved index sets the intended stoichiometric offset.
For polyols, hydroxyl number is determined per ASTM D4274 [1] or ISO 14900 [2]. For the isocyanate side, NCO content is measured by titration per ASTM D2572 [3] or ISO 14896 [4]. From those values, two standard identities convert the data sheet numbers into equivalent weights:
- Equivalent weight of a polyol = 56,100 ÷ hydroxyl value (mg KOH/g)
- Equivalent weight of an isocyanate = 42.02 × 100 ÷ NCO content (%)
Equivalent weight is the number of grams that carries one reactive equivalent. Once both sides are in those units, the ratio falls out of a simple mass balance.
The isocyanate index is the supplied NCO equivalents divided by the theoretical NCO demand, multiplied by 100. Include the active-hydrogen ingredients and any other NCO demand specified by the approved formulation. An index of 100 is stoichiometric on that stated basis. Material suppliers define the usable recipe window; index alone does not determine a finished property.
How to Calculate the Target Ratio From the TDS
Include every reactive ingredient. For water-blown systems, water creates an additional isocyanate demand: one mole of water ultimately consumes two NCO groups. Its effective demand equivalent is approximately 9 g/eq. This is a stoichiometric accounting value, rather than a measured polyol hydroxyl number.
Take a simple water-blown flexible system at index 105: 100 parts of a polyol with hydroxyl value 56, plus 2.5 parts of water, reacted with pure 2,4-TDI. The TDI equivalent weight is about 87 (NCO content near 48.3%).
- Polyol OH equivalents: 100 ÷ (56,100 ÷ 56) = 0.0998
- Water OH equivalents: 2.5 ÷ 9 = 0.2778
- Total OH equivalents: 0.3776
- Grams of TDI required: 0.3776 × 87 × 1.05 = 34.5
For this simplified example, the A blend is 100 g polyol + 2.5 g water = 102.5 g. With approximately 34.5 g TDI on the B side, the mass ratio is A:B = 102.5:34.5, or approximately 100:33.66 after normalisation. These rounded values exclude other additives; include the complete production blend before setting the machine.
For a prepolymer/curative system, use the measured NCO content and the supplier-approved curative equivalent and stoichiometry. BDO and DEG are diols; MOCA is an amine curative and should not be treated as a polyol in an OH-number calculation. Record the actual material names rather than transferring foaming-system letters blindly.
The lesson is consistent: the ratio changes whenever a raw material is substituted, even at the same phr loading, because the equivalent weight changes. A different polyol with the same nominal hydroxyl number ±5 mg KOH/g is already a measurable ratio shift.
Once the target ratio is settled, converting it into a clean shot weight for each mold is the next floor-level check. How To Calculate Shot Weight In Polyurethane covers the per-mold math and how shot weight is used for line-side verification.
Verifying the Ratio on the Floor
Three checks catch most ratio problems before they become a full day of scrap.
First, use the machine manufacturer’s approved component-output verification procedure and designated test mode. Collect and weigh unmixed streams with the approved fixtures; do not disconnect pressurised components. Define temperature, flow and dispensing time, then compare the measured masses with the target ratio under the selected configuration’s agreed accuracy and repeatability criteria.
Second, run the formulation’s approved cup or process test with accurately weighed components, defined sample size and controlled temperature. Compare cream, rise, gel and tack-free endpoints only where they apply to that material. A changed profile can reflect temperature, ratio, moisture, catalysts or material condition; it is not a diagnosis from timing alone.
Third, for foaming systems, measure free-rise density under the agreed sampling and conditioning procedure. Compare it with the formulation reference. Density alone cannot determine whether the ratio should be increased or decreased.
If repeated verification shows the target ratio is right but the machine cannot hold it, that is a metering problem, not a formula problem. Haifeng’s metering engineers can review your pump configuration, temperature control, and calibration method against the viscosity and output your formulation demands. Contact Info@chinahaifeng.com or WhatsApp 86 13566296633 with your current ratio, material temperatures, and output rate.
Where the Ratio Is Lost: Metering, Temperature, and Viscosity
The ratio is lost most often during the transition from a good static calculation to a moving process.
Actual gear-pump output depends on pressure differential, operating viscosity, speed, clearances and inlet conditions. Higher viscosity can reduce internal leakage but increase suction and line losses. Compare both streams under their actual conditions and verify component masses; neither equal pump speed nor one viscosity value establishes the mass ratio.
Temperature is the second lever. A material temperature difference of even 5°C between the two tanks changes density and viscosity differently on each side, which shifts a volumetric metering system’s effective weight ratio even when pump strokes stay identical. An MDI-grade isocyanate cooled from processing temperature toward room temperature can roughly double in viscosity, so the same pump setting no longer means the same mass. Both tanks must be held at the formulation’s specified processing temperatures, not merely “warm.”
For high-pressure impingement mixing, configure injection pressures, nozzle geometry and stream flows together to obtain the required mixing conditions. Equal pressure by itself does not establish correct mixing or local ratio. Inspect injection and start/stop behaviour if component-output tests pass but the part shows streaks or uneven properties.
When the process moves beyond simple metering into high-pressure impingement or reinforced systems, mixing quality and ratio stability come from head design as much as pump accuracy. RRIM Vs RIM Which Process Suits Your Structural Part covers how high-pressure mixing preserves the ratio during the injection window and where reinforced materials change the demands on equipment.
Ratio-Related Defects and How to Trace Them
The following symptoms are prompts for investigation. They do not identify a unique ratio direction without measured component outputs and a material-system check.
| Symptom | Likely ratio cause | First check |
|---|---|---|
| Dense, overweight parts | Shot mass, mold fill, formulation or sampling basis | Weigh the shot; confirm A/B outputs and the density method |
| Soft, tacky surface | Ratio deviation, mixing, temperature, moisture or incomplete cure | Verify component masses, material condition and cure schedule |
| Shrinkage, cracking, brittle feel | Formulation, fill, cure or thermal-history factors | Confirm ratio and index; investigate the material and process before adjusting |
| Density swings hour to hour | Temperature drift on one tank | Log both tank temperatures for a full shift |
| Ratio good at startup, drifts during run | Pump wear, seal leak, or line restriction | Repeat ratio check mid-run; inspect pumps |
Compare trends with measured component outputs, temperatures and material tests. A constant or drifting defect does not, by itself, distinguish formulation, calibration and mechanical causes.
A Ratio-Determination Checklist
- Confirm the B-side identity and A-side identity from the machine manual, not from habit.
- Pull the current TDS for every A-side component: polyol, chain extender, water, catalyst, surfactant.
- Convert hydroxyl values to equivalent weights; include water as an OH component.
- Pull the NCO content or prepolymer NCO from the B-side TDS.
- Set the index from the formulation. Do not back-calculate it from a previous machine setting.
- Calculate the target weight ratio and record it alongside the total shot weight.
- Run a machine ratio check. Record the measured A and B masses and the deviation.
- Run a cup test at the target ratio and compare reactivity against the reference profile.
- Check free-rise density at controlled temperature.
- Re-verify after any raw material lot change, tank temperature change, or pump service.
Get the Ratio Right Before It Costs Another Batch
A ratio that is calculated, verified, and held within the machine’s metering window protects density, hardness, and surface quality at once. If you are commissioning a new line or chasing a ratio that drifts despite correct settings, send Haifeng the details and we will review it against the equipment rather than the brochure.
- Your formulation: target ratio, index, and shot weight
- Your B-side and A-side viscosities and processing temperatures
- Your current output rate and equipment type
- The defect you are chasing, with photos or density data
Contact Info@chinahaifeng.com or WhatsApp 86 13566296633.
Frequently Asked Questions
What is the A/B ratio in polyurethane?
For Haifeng foaming documentation, A:B is the mass proportion of polyol blend to isocyanate. State the direction, complete formulation and chosen index. CPU systems should additionally identify the prepolymer and curative by name.
How do I calculate the mixing ratio from a TDS?
Use the OH values and other reactive equivalents of the complete polyol blend, include water and other active-hydrogen demand, then apply the isocyanate equivalent weight and supplier-approved index. Divide by the total blend mass when normalising to 100 parts; do not normalise only to its base polyol.
Does the ratio change when I switch from MOCA to BDO?
Yes. MOCA is an amine and BDO is a diol. A compatible prepolymer, curative equivalent and approved recipe are required; the same mass ratio should not be copied between them.
Why does my ratio drift during long production runs?
The usual causes are pump wear or seal leakage, temperature drift on one tank, or a developing line restriction. A mid-run ratio check and B tank temperature log separate mechanical drift from thermal drift.
What happens if the index is above or below 100?
In the stated isocyanate-index convention, above 100 supplies more NCO than the theoretical demand and below 100 supplies less. The useful range and effects on properties depend on the material system. Do not infer hardness, density or a defect from index alone.
References
[1] ASTM D4274-21, Standard Test Methods for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols, ASTM International.
[2] ISO 14900:2017, Plastics — Polyols for use in the production of polyurethane — Determination of hydroxyl number, International Organization for Standardization.
[3] ASTM D2572-19, Standard Test Method for Isocyanate Groups in Urethane Materials or Prepolymers, ASTM International.
[4] ISO 14896:2009, Plastics — Polyurethane raw materials — Determination of isocyanate content, International Organization for Standardization.
If you’re interested, check out these related articles:
Causes Of Shrinkage In Polyurethane Foam How To Control It
SRIM Explained Structural Reaction Injection Molding Basics
The Role Of Rise Time In Polyurethane Foaming
Measuring Polyurethane Foam Resilience A Practical Test Guide
