Walk into almost any machine-room discussion about a new low-pressure polyurethane foaming machine and the same question comes up early: “What ratio range does it cover?” The brochure answer is usually a spread — 1:1 to 5:1, 100:5 to 100:100, or wider — and the wider number looks like insurance. In practice, a machine that advertises the widest ratio window can be exactly the machine that performs worst on your formula. The question is not how wide the range goes; it is how tightly the machine holds your ratio inside the range you actually run. At the end of this article is a checklist you can use to pressure-test a supplier’s ratio spec before it reaches the acceptance stage.
What “Mixing Ratio” Means in a Low-Pressure PU System
In a low-pressure polyurethane system, the mixing ratio is simply the delivery ratio between the polyol side (component A, often carrying catalysts, water or blowing agents) and the isocyanate side (component B). It is usually stated by weight, which is why a machine with volume-based pumps has to be calibrated against component density before anything else. The ratio fixes the NCO index — the relationship between available isocyanate groups and the hydroxyl and water groups they react with — and that index is what determines foam density, hardness, cell structure and the amount of unreacted material left in the part.
A low-pressure machine sets this ratio by running two or more metering pumps at commanded speeds. The quoted “ratio range” is the adjustment span — for example, the ability to move the B side from 30 parts to 70 parts against a fixed 100 parts of A. It is a configuration envelope, not a single running point. What the specification rarely says is whether the published accuracy applies at every point inside that envelope, or only near the middle.
Ratio range and metering logic change across machine architectures. <Polyurethane Foam Machine: Types, Working Principle and Applications> covers how low-pressure equipment differs from high-pressure and dispensing variants.
Why the Widest Advertised Ratio Is Not the Spec to Chase
A ratio window that reaches from 100:5 to 100:100 sounds flexible, but it has a hidden cost: pump sizing. To deliver 100 parts of isocyanate, the B-side pump and its drive must be sized for the upper end of that span. If your formula actually runs at 100:8, the same pump spends most of its life near the bottom of its speed range. Gear-pump slip and piston-pump leakage do not scale linearly down to a crawl; at low speed the metered volume per stroke or per revolution becomes more sensitive to back-pressure, temperature and clearance wear. The result is that the widest-range machine can be the least stable machine at your specific ratio.
There is a second effect. Ratio range and turndown range are linked. A pump specified for a 20:1 flow span cannot also hold tight accuracy across the bottom 10 percent of that span without careful calibration, because internal leakage becomes a larger fraction of the displaced volume. This is why minimum-output problems — small flow being harder than high flow — show up most often in machines that were bought with extra ratio headroom “just in case.”
The Real Mixing-Ratio Range Worth Specifying
The useful starting point is not a machine’s catalogue span but the spread of your own formulations. Collect the A:B ratios for every product group you plan to run, then add a working margin for formula revisions. A flexible-foam plant that runs 100:35 to 100:55 today may only need a machine adjustable from 100:30 to 100:65. An elastomer caster running a TDI prepolymer with MOCA around 100:11 and a BDO system around 100:6 has a different problem: the ratios are small, so a one-percentage-point error is a larger relative swing on the curative side.
Elastomer systems use a separate curative metering lane, and the practical ratio window narrows considerably. <The Ultimate Guide to Low-Pressure Foaming Machines for PU Elastomers> covers how those machines are configured for small curative ratios.
Typical starting points for low-pressure equipment selection:
| Application family | Typical formulator A:B by weight | Practical machine window to request |
|---|---|---|
| Flexible molded foam | 100:35–100:60 | 100:30–100:70 |
| Rigid foam / pour-in-place | 100:100–100:160 | 100:80–100:180 |
| Cast elastomer (MOCA/BDO) | 100:5–100:12 | 100:4–100:15 |
| Microcellular / integral skin | 100:40–100:75 | 100:35–100:85 |
The table is a selection guide, not a universal rule; your material supplier’s technical data sheet is the controlling document. What matters is that you specify a window centered on your actual chemistry — plus about 20 percent headroom — not a fantasy span.
Ratio Width vs. Ratio Accuracy: The Number That Matters More
Once the ratio window is set, the more important number is how well the machine holds the target inside that window. On the LJJ-series low-pressure machines we build, metering accuracy is specified at ±0.5% per component — and the working value that matters in production is shot-to-shot repeatability, not a one-time calibration result. A machine that advertises a huge ratio range but drifts half a percentage point through a shift will produce harder and softer parts, density variation across a day, and scrap that looks random rather than consistent.
The causal chain is direct. Because a polyurethane part’s final density is measured by the apparent-density method in ISO 845 [1], any deviation in the A:B delivery becomes a measurable change in mass per unit volume. In many flexible and microcellular systems, a ratio deviation of one percentage point can move final foam density by more than 3 kg/m³, which is enough to push a part outside a customer tolerance. ASTM D3574 [2] gives the property test methods most downstream buyers use to catch that shift. Ratio width does not protect you from this; ratio stability does.
Some machines use closed-loop servo speed compensation to reduce ratio drift as viscosity or temperature changes during a run; others rely on open-loop pump curves. Ask which one you are buying, and ask for the repeatability test data — not just the maximum ratio figure.
Worth checking before you commit: If you are comparing two suppliers’ ratio specs, send the relevant material TDS pages and your current shot sizes to info@haifeng-automation.cn or WhatsApp +86 13566296633. A quick review of pump sizing against your smallest shot usually shows whether a wide quoted ratio range will hold up at the low end.
Specifying and Verifying the Ratio Window in FAT
Do not accept a ratio range as a certificate. Turn it into a test. In the factory acceptance test, ask for gravimetric verification of the ratio at three points: your minimum requested setting, your production target and your maximum requested setting. For each point, weigh the A and B outputs over a realistic shot time, then repeat enough shots to see scatter and drift. ISO 8655-1 [3] separates labeled volumetric accuracy from the repeatability achieved in practice; for the smallest shots, repeatability is the value that controls. ISO 4185 describes the weighing method used for liquid flow measurement, and the same principle transfers directly to weighing A/B shots on a calibrated balance [4].
A practical FAT block covers:
- Ratio setpoint vs. measured average ratio at min/target/max.
- Shot-to-shot coefficient of variation over 10–20 consecutive shots.
- Drift over a continuous 4–8 hour run at the production target ratio.
- Performance at your real material viscosities and temperatures, not water.
- Recheck after a solvent flush cycle.
Most ratio problems surface later as intermittent density variation, surface defects or dimensional instability — often written off as a “bad batch” of raw material. The fix is usually the same: verify the metering lane, not just the mixer.
Chronic ratio drift eventually becomes density variation and surface defects. <Continuous PU Foam Production Line Maintenance: How to Prevent Costly Downtime and Defects> covers how those problems are caught in continuous production.
Closing note. If you are still holding two ratio specs side by side, reduce them both to the same test: what is the measured ratio error at your formula, your smallest shot and your real material viscosity — not at the brochure’s widest setting.
To have a Haifeng applications engineer check your ratio window against your material TDS and shot list, contact:
- Email: info@haifeng-automation.cn
- WhatsApp: +86 13566296633
- Include: target A:B ratio, component viscosities, smallest and largest shot weight, and the number of product grades you plan to run.
Frequently Asked Questions
What is the typical mixing ratio for polyurethane foam?
The formulator’s ratio, not the machine’s maximum, is the number to anchor on. Many flexible foam systems land between 100:35 and 100:60 by weight; rigid pour-in-place systems may run 100:100 and higher; cast elastomers often sit at 100:5 to 100:12 because the curative is added in a much smaller proportion. Your material supplier’s technical data sheet is the controlling value.
Do I need a low-pressure PU machine that covers 1:1 to 10:1?
You need that wide a range only if you genuinely run it across product grades. Buying a very wide range usually means oversized pumps and worse low-end accuracy at the ratio you actually use. A window of about 20 percent around your real formulations is usually the more sensible specification.
Is the mixing ratio adjustable after installation?
Yes, within the machine’s designed envelope. The more useful question is whether the adjustment stays accurate. After changing ratio, re-verify gravimetrically rather than trusting the HMI setpoint only. Some machines offer recipe-linked ratio changes that also compensate for pump and viscosity changes.
What is more important: mixing-ratio range or metering accuracy?
Metering accuracy and repeatability. A wide range is a configuration capability; accuracy is what holds your product inside its density and hardness tolerance. Specify both, but weight the acceptance test toward measured ratio error at your actual operating points.
How is the mixing ratio verified during machine acceptance?
By weighing the A and B outputs over timed shots and comparing the measured ratio with the setpoint, repeated at minimum, target and maximum requested settings. The same test is then repeated after a run to expose drift. A calibrated balance is the core tool; the principle is the same one described in ISO 4185 for the weighing method [4].
References
[1] ISO 845:2006, Cellular plastics and rubbers — Determination of apparent density.
[2] ASTM D3574-17, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams.
[3] ISO 8655-1:2022, Piston-operated volumetric apparatus — Part 1: Terminology, general requirements and user recommendations.
[4] ISO 4185:1980, Measurement of liquid flow in closed conduits — Weighing method.
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