Challenges of Pumping High-Viscosity Adhesives
Industrial adhesives are not simple liquids. They are engineered formulations with extreme viscosity, temperature sensitivity, and chemical reactivity. Selecting the wrong pump technology leads to inconsistent flow, product degradation, line blockages, and production delays.
Extreme Viscosity
Industrial adhesives range from several thousand centipoise (cSt) to beyond 100,000 cSt. Standard centrifugal pumps cannot generate sufficient pressure to move such viscous fluids. Progressive cavity pumps may work but with efficiency losses and heat generation. Gear pumps are designed to handle any viscosity within their pressure rating.
Temperature Sensitivity
Adhesive viscosity changes dramatically with temperature. Hot melt adhesives are liquid at 180–200°F but become plastic at room temperature. A pump system must either heat the adhesive to maintain pumpability or be capable of handling viscosity swings without losing output consistency.
Tendency to Set Up
If a pump stops moving adhesive, the fluid cools and begins to set or cure. This can plug the pump, nozzles, and feed lines within minutes. A pump capable of self-priming and rapid restart is essential to prevent production interruptions.
Types of Adhesives and Their Pumping Challenges
Hot Melt Adhesives (Thermoplastic)
Hot melt adhesives are solid at room temperature but melt at 180–210°F to become pumpable. Upon cooling, they solidify to form bonds. The adhesive is extremely temperature-dependent: a 20°F change can double or halve viscosity.
Pumping challenge: The system must maintain heater temperatures within a narrow band and handle rapid viscosity changes. If the adhesive cools below the melting point, it solidifies and blocks lines.
Solution: NAPCO gear pumps paired with heated suction lines and supply tanks provide consistent delivery. The pump's positive displacement ensures reliable output even as viscosity fluctuates.
Reactive Adhesives (Epoxy, Polyurethane)
Reactive adhesives cure through chemical reaction (two-part epoxy) or exposure to moisture (polyurethane). Once mixed or exposed to air, they begin to harden. The "pot life" or working window may be only minutes.
Pumping challenge: The pump must move the adhesive quickly from storage to application without delay. Any interruption shortens pot life and risks curing in lines.
Solution: Gear pumps are fast and self-priming, enabling rapid transfer. Some reactive systems use metering pumps to dose exact ratios of two-part components, and NAPCO gear pumps support this with consistent volumetric output.
Water-Based Adhesives
Water-based adhesives have lower viscosity than hot melt but are more corrosive due to aqueous content. They are used in wood products, cardboard, and textile applications. Viscosity is generally 5,000–50,000 cSt.
Pumping challenge: The moderate corrosion risk requires attention to material compatibility. Ductile iron is typically acceptable; stainless is used where longevity is critical.
Solution: NAPCO ductile iron pumps (PA300C, PA200C) are standard for water-based adhesive transfer in the wood products industry.
Epoxy Systems (Two-Part Metering)
Structural epoxy adhesives require precise ratio mixing of resin and hardener. Too much or too little hardener changes cure time and strength. Industrial metering systems use separate pumps for each component.
Pumping challenge: Each pump must deliver consistent, repeatable flow to maintain proper ratio over extended operation.
Solution: Gear pumps are ideal for two-part metering because their volumetric output is highly consistent. Equal displacement at equal RPM ensures correct ratios throughout the day.
Why Gear Pumps Excel for Adhesive Applications
Positive Displacement Handles Any Viscosity
Centrifugal pumps work by accelerating fluid; their capacity drops drastically as viscosity increases. Gear pumps use mechanical displacement—each revolution of the gears displaces a fixed volume regardless of viscosity. This means a gear pump delivers usable flow with 100,000 cSt adhesive just as reliably as with 10,000 cSt.
Consistent Volume per Revolution for Accurate Batching
Because displacement is fixed, NAPCO gear pumps are used in metering systems where accurate volume output is critical. Many adhesive dispensing systems use flow meters and pump speed controllers to dose exact amounts. The gear pump's consistency makes this possible.
Self-Priming from Drums and Totes
NAPCO gear pumps are self-priming and can draw directly from 55-gallon drums or totes without external assist. This is essential for adhesive operations where rapid changeover between products is common.
Simple Design Resists Plugging
The simple internal design of a gear pump—two gears, two shafts, a housing—means minimal tight clearances where adhesive could accumulate and harden. If an adhesive does begin to set up, backpressure in the pump rises, alerting the operator to flushing needs. Progressive cavity pumps and centrifugal pumps have more complex internal passages where adhesive can lodge and plug the pump.
Housing Material: Ductile Iron vs. Stainless Steel
Ductile Iron: The Standard for Most Adhesives
Most industrial adhesives are non-corrosive to iron. Hot melt, reactive epoxy, and water-based adhesives all have excellent compatibility with ductile iron. Because adhesives are not corrosive, ductile iron is the cost-effective choice and standard in the industry.
The PA300C ductile iron pump is the most popular NAPCO pump for adhesive applications. It delivers 158 GPM at 600 RPM and is used by major wood products manufacturers (like Boise Cascade) for continuous-duty resin and adhesive transfer. The PA300C has become the industry standard because it reliably handles the viscosity and volume of adhesive operations.
When to Use Stainless Steel
Stainless steel pumps are recommended for specialty adhesives that contain corrosive additives: solvent-based adhesives (containing thinners or solvents), UV-cure adhesives with chemical modifiers, or adhesives with aggressive pH (very acidic or basic). In these cases, the investment in stainless steel extends service life and avoids corrosion surprises.
For typical hot melt and water-based applications, ductile iron is more economical and equally reliable.
Gear Material Selection for Adhesive Operations
Nitrile (NBR): Standard for Room Temperature and Moderate Heat
Nitrile elastomer gears are the standard offering for NAPCO pumps in adhesive applications. Nitrile has excellent chemical compatibility with mineral oil hot melts and water-based adhesives. The material remains elastic to 240°F, making it suitable for most adhesive operations.
Nitrile gears are cost-effective and provide reliable service life in standard applications.
Viton (FKM): High-Temperature Hot Melt Applications
When hot melt adhesives operate above 200°F sustained, or when the pump is positioned near heaters or heated supply tanks, Viton elastomer gears are recommended. Viton maintains elasticity to 400°F and provides superior thermal stability compared to nitrile.
Viton gears are commonly specified for high-temperature dispense systems and applications where the pump operates in hot environments.
System Design Considerations for Adhesive Transfer
Pipe Diameter and Line Sizing
Adhesives require larger suction and discharge line diameters than water or oil to minimize pressure drop. A suction line sized for typical industrial fluids may create excessive backpressure on a gear pump when moving high-viscosity adhesive, reducing output. NAPCO can provide sizing guidance based on your flow rate and viscosity.
Suction Lift and Tank Placement
NAPCO gear pumps can draw from sources up to 8 feet above the pump centerline (in ideal conditions with proper line sizing). For adhesives, suction lines should be short and direct. Long suction lines or elbows create friction losses that reduce priming ability. Positioning the adhesive drum on a raised platform directly above the pump suction minimizes cavitation risk.
Heating and Cooling Jackets
Hot melt applications often use heated supply tanks and heated discharge lines to maintain pumpability and delivery temperature. The pump itself can be fitted with a thermal jacket to maintain adhesive temperature while in the pump. For cooler environments, an insulated suction line prevents excessive heating loss.
Strainers and Filtration
Contaminated adhesive (dust, hardened particles from previous batches) can damage the pump. Suction-side strainers (typically 100–150 micron) remove debris before it enters the pump. Discharge strainers are less common because backpressure must remain within pump limits.
Performance Advantage: Higher Viscosity Improves Efficiency
A unique characteristic of gear pumps: volumetric efficiency improves as viscosity increases. In lower-viscosity fluids, internal "slip" (leakage between the gears and housing) reduces output slightly. In higher-viscosity adhesives, the viscous seal between gears and housing is tighter, and slip is minimal. This means a gear pump operating at 100,000 cSt adhesive may deliver 98% of theoretical displacement, while at 10,000 cSt it delivers 95%. The consistency of output makes gear pumps ideal for metering and batching.
This is why NAPCO gear pumps are standard in adhesive metering systems where accurate, repeatable volume output is essential to production quality.
NAPCO Pumps for Adhesive Applications
PA300C (Ductile Iron)
158 GPM, 350 PSI. The industry standard for high-volume adhesive transfer. Used by major wood products manufacturers.
View Specifications →PA200C (Ductile Iron)
69 GPM, 350 PSI. Compact option for smaller operations or mobile equipment.
View Specifications →Related Technical Resources
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