A ratio error of one percentage point may look small on the control panel, but it can shift final part density enough to change compression, rebound, and demold behavior. In polyurethane processing, the metering pump is where that error is either controlled or locked into every shot. Choosing a metering pump begins with a process question, not a catalogue question: what flow range, viscosity, pressure, and repeatability does the line actually need?
How a Metering Pump Affects Ratio Before the Mixing Head
On a two-component machine, isocyanate and polyol are kept separate until the mixing head. A metering pump does not simply move material; it establishes the A:B ratio that becomes part density, hardness, cell structure, and cured physical properties. If one pump drifts while the other holds steady, the formulation shifts. A deviation of one percentage point in the component ratio can shift final foam density by more than 3 kg/m³ [1].
That is why pump selection should not start from “kg per hour” capacity alone. It should start from the smallest regular shot, the largest regular shot, the pour time available, and the ratio tolerance on the finished part. A pump that performs well at full flow may still lose metering stability on a small molded part if it is asked to run far below its rated capacity.
Step 1: Calculate Flow Rate from Shot Weight and Turndown
The first selection number is not annual output. It is the required mass flow at the moment of pouring.
Required mass flow = shot mass ÷ usable pour time.
For continuous panel or slabstock lines, convert that to kg/min and add the recirculation flow that keeps material temperature and filler suspension stable. The pump must cover both pour flow and bypass flow without dropping out of its reliable control window.
If the pump is sized from a rough “kg per hour” estimate instead of from shot weight plus pour time, the result is usually a machine that overshoots on small parts and starves on large ones. How To Calculate Shot Weight In Polyurethane covers the shot-weight side of that calculation.
Two practical rules follow. First, the required turn-down ratio should be based on the smallest and largest shots on the same line, not on the pump’s maximum range. Second, the pump should stay inside its stable operating band for at least 80% of routine shots. Operation at the extreme bottom of a wide flow range is where small suction variations and valve timing errors become visible on the part.
Step 2: Match the Pump to Viscosity, Temperature, and Pressure
Polyurethane components do not arrive at the pump as one convenient viscosity. Formulated polyols, prepolymers, chain extenders, color pastes, and additives can range from low-viscosity liquids to materials that require heated lines and inlet pressure. What matters for pump selection is viscosity at the pump inlet, not the value printed on the raw-material data sheet under room temperature.
For moderate-viscosity components, externally mounted gear pumps with variable-frequency drive are a common low-pressure metering option. For very high viscosity, abrasive fillers, or high discharge pressure, piston or plunger metering designs become more appropriate. Inlet temperature control, day-tank conditioning, and short suction lines are often more important than the pump model itself.
Spray systems add long heated hose and higher discharge pressure, so the same pump curve reads differently in a spray rig than in a bench casting unit.
Spray systems add long heated hose and higher discharge pressure, so the same pump curve reads differently in a spray rig than in a bench casting unit. What Is Spray Polyurethane Foam Composition And Application covers the formulation and process conditions that affect that calculation.
Low-pressure and high-pressure machine architectures also place different demands on the metering loop.
Low-pressure and high-pressure machine architectures put different loads on the metering loop. RRIM Vs RIM Which Process Suits Your Structural Part covers the process-selection side before you lock in a pump type.
A straightforward pressure comparison looks like this:
| Parameter | Low-pressure mechanical mixing | High-pressure impingement mixing |
|---|---|---|
| Typical metering pressure | 0.5–3 MPa | 10–20 MPa |
| Common pump approach | External gear pump or servo-driven gear pump | High-pressure axial piston or plunger metering |
| Best fit | Elastomers, small shots, filled systems, wide viscosity | High-output foam, RIM, spray, structural parts |
| Main risk | Flow instability at low turn-down | Heat and wear if not matched to inlet pressure |
For a given chemistry, select the pump after the mixing head architecture is fixed. The reverse approach often forces a compromise: a pump that cannot hold pressure at the mixing head, or a high-pressure pump that is unnecessarily expensive and heat-generating on a low-pressure line.
Specifying a metering pump for a new or upgraded line? Send your component data — pump type, viscosity at process temperature, shot weight, flow range, mixing head pressure, and current ratio deviation — to Info@chinahaifeng.com or WhatsApp: 86 13566296633. A short operating-data list gets a more useful answer than a model-number request.
Step 3: Specify Accuracy as Repeatability, Not Just Rated Flow
Metering accuracy is not one number. It separates into steady-state accuracy, linearity, repeatability, and turn-down behavior [3]. A catalogue figure quoted as “±0.3% full range” still needs a follow-up question: repeatable under what inlet pressure, temperature, and minimum flow condition?
For two-component polyurethane, the more useful specification is ratio deviation during production. A gear pump driven by an open-loop variable-frequency drive can meter well when inlet conditions are stable, but may drift as viscosity changes with temperature or batch. A servo-driven pump with encoder feedback maintains tighter flow control across a wider turn-down. On critical lines, a Coriolis mass flow meter closes the loop by measuring mass directly rather than calculated volume.
In one polyurethane sole line upgrade, the turning point was not replacing the mixing head. It was replacing fixed-ratio metering with servo-driven closed-loop control. Density variation became a maintenance question instead of a daily quality problem.
Step 4: Check Material Compatibility, Recirculation, and Cleaning
Wetted parts matter because isocyanates are moisture-sensitive and many polyols are hygroscopic. Seals, pump clearances, and reservoirs must be compatible with both components, including any flame retardant, filler, or pigment additive. A pump that meters well in a clear demo can fail quickly on filled systems if clearances are too tight or if the pump body cannot tolerate abrasive solids.
Material recirculation supports temperature control and filler suspension, but it also consumes pump capacity. The recirculation loop should bring components back to the day tank without creating dead zones where material can gel or settle. When color or formulation changes are frequent, cleaning and purge access should be evaluated before purchase. Pump units also need to meet the applicable safety requirements for liquid pumping installations [4].
Selection Checklist for Polyurethane Metering Pumps
Before approving a metering pump, verify these parameters:
| Checklist item | Required input |
|---|---|
| Minimum and maximum shot mass | From molded part weight, not from barrel output |
| Usable pour time | From cycle time and mold movement |
| Component viscosity at process temperature | Measured at the pump inlet, not room temperature |
| Required discharge pressure | Matched to mixing head type |
| Ratio tolerance and density tolerance | Convert to acceptable pump flow error |
| Recirculation flow | Added to pour flow for total capacity |
| Turndown window | Confirmed against smallest and largest production shots |
| Control strategy | Open-loop VFD, servo, or mass-flow feedback |
| Wetted material compatibility | Seals, clearances, and pump body vs. all additives |
| Cleaning and changeover access | Purge ports, drain points, seal replacement |
Need a second set of eyes on the pump spec? Email your target shot weight, line speed, viscosity, and current ratio deviation to Info@chinahaifeng.com or WhatsApp: 86 13566296633. We will check whether the quoted flow window, turndown, and control loop hold up for your component ratio — no catalogue-only estimate.
Frequently Asked Questions
What is the difference between a metering pump and a transfer pump?
A transfer pump moves material from a drum or bulk tank into the day tank or recirculation loop. A metering pump controls the fixed ratio and shot volume during the actual pour. Transfer duty is about supply pressure; metering duty is about repeatable flow.
Can a gear pump meter high-viscosity polyol?
It depends on inlet pressure, heating, and the pump’s internal clearances. An external gear pump can meter moderate-viscosity polyol well when temperature is controlled and suction conditions are stable. For very high viscosity or shear-sensitive formulations, a piston or plunger metering design is usually more suitable.
What metering accuracy is realistic for two-component polyurethane?
Full-range dynamic metering accuracy around ±0.3% is common on production machines, but the practical number to validate is repeatability at the smallest regular shot. Ask whether the quoted accuracy holds across the full turn-down window, not only at rated flow.
Is a Coriolis mass flow meter necessary?
Not always. For stable formulations, fixed mold sizes, and moderate turn-down, a well-tuned servo-driven pump may be sufficient. For wide flow windows, filled systems, or parts where density variation is costly, mass-flow feedback reduces the effect of temperature and viscosity drift.
Does pump selection change between spray foam and casting elastomers?
Yes. Spray applications add higher discharge pressure, longer heated hose, and quick on/off flow dynamics. Elastomer casting typically involves lower pressure, smaller shots, and tighter shot-weight control. The pump and control strategy should follow the process architecture, not the other way around.
References
[1] G. Oertel, Polyurethane Handbook, 2nd ed. Munich, Germany: Hanser Publishers, 1994.
[2] M. Szycher, Szycher’s Handbook of Polyurethanes, 2nd ed. Boca Raton, FL, USA: CRC Press, 2012.
[3] API Standard 675, Positive Displacement Pumps—Controlled Volume for Petroleum, Chemical, and Gas Industry Services, 3rd ed. Washington, DC, USA: American Petroleum Institute, 2012.
[4] EN 809:1998+A1:2009, Pumps and pump units for liquids — Common safety requirements.
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