Rheology describes how an adhesive flows under force.
Designing for Throughput: How Adhesive Rheology Impacts Line Speed in Converting
Estimated reading time: 7 minutes
Your web can only move as fast as the adhesive will let it. Too thin and you’ll mist, bleed, and clean constantly; too thick and you’ll starve coat weight, chatter dies, or stall the nip. Throughput is constrained—and unlocked—by adhesive rheology tuned to your unit operations and held steady on the floor.
Why Rheology Sets the Ceiling for Speed
Rheology describes how an adhesive flows under force. In slot-die, gravure, curtain, bead, and lamination nips, the fluid experiences shear rates from tens to thousands of s⁻¹. Viscosity must be low enough to pump and meter yet high enough to resist sling-off and hold edges after laydown. Yield stress and thixotropy govern start/stop behavior: you want a fluid that “holds” at rest, thins under shear, and recovers quickly as the web exits the nip.
Substrate stack-ups complicate things. Paper, film, and foil present different porosity and surface energies, so a single “cup number” doesn’t predict line behavior. Design a curve—viscosity vs. shear rate plus recovery after shear—so you can raise speed without trading quality for scrap.
Pick an Adhesive Profile that Matches your Equipment
Metered coaters (slot-die, gravure) reward predictable viscosity in the shear band your hardware actually sees. Strongly shear-thinning products can meter beautifully at speed but drip at low speed or startup. Hence, moderately shear-thinning profiles are easier to control across ramp rates. Laminations benefit from a hint of elasticity to preserve strand integrity through the nip; too much elasticity causes ribbing and edge “smiles.”
When your product mix changes—foil lidding today, film/film snacks tomorrow—select application-specific adhesives for each substrate/coater/coat-weight combination. So, your process window scales as speed climbs rather than shrinking as you accelerate.
Translate rheology into speed: viscosity, yield stress, thixotropy
Treat viscosity as a function, not a single spec. Two products labeled “2,000 cP at 25 °C” can behave very differently when one is nearly Newtonian and the other is strongly shear-thinning. Shear-thinning reduces apparent viscosity at higher shear. Further, enabling lower die pressure and cleaner lips; too much thinning and edges can feather after the nip. Yield stress minimizes sag at rest but increases startup pressure and pump load. Thixotropy—time-dependent recovery—controls leveling during accelerations and decelerations.
These behaviors are temperature-sensitive. A 5–8 °C shift over a long shift can flip a line from “no misting” to “hourly cleanup.” Many of the easiest speed gains come from thermal control—jacketed day tanks, heat-traced hoses, and die-outlet temperature monitoring. Rather than from bigger motors or wider nips.
For consistency and communication across shifts, ground your measurements in a standard and then extend them with production-rate testing. ASTM D1084 defines viscosity test methods for free-flowing adhesives so teams can compare lots and correlate to rheometer sweeps. For a plain-language refresher that helps operators connect “pump is cavitating” or “nip is foaming” to the shape of the flow curve, point them to NIST’s overview of rheology.
Coating Modes: How the Same Formula Behaves Differently
Slot-die coating: Shear is high and uniform. Target a moderately shear-thinning profile to reduce die pressure without sacrificing edge stability. Guard against elastic effects; too much elasticity creates die-lip chatter or stringing at high drawdowns. A small, tightly controlled die-temperature increase can drop viscosity just enough to add 10–20% line speed while holding coat weight.
Gravure/flexo: Doctor-blade and cell geometry impose cyclic shear. Lower viscosity improves cell emptying at speed but risks misting and foaming. Add a touch of yield stress (via a compatible modifier) to suppress fly-off while keeping release clean. When jumping from 200 to 300 m/min, maintain the same cup time via temperature or solvent adjustments. Then confirm under a strobe that no mist halo appears downstream.
Bead-on-laminate/curtain: Continuity matters most. Elasticity and surface tension decide whether your bead necks or forms waves at a speed. Slightly higher viscosity and lower temperature stabilize the bead; too high and you’ll starve the bondline. If you run frequent stops, select faster thixotropic recovery so restarts don’t leave rib marks.
For background reading on the engineering mindset behind manufacturing process choices (and how thermal/flow control interact well beyond packaging), this ENTECH Online primer on LED production methods offers accessible parallels to heat management, adhesive selection, and clean process windows in high-volume lines.
Line-Speed Levers you can Pull Without Reformulating
Staging and delivery: Move totes into a temperature-controlled area 12–24 hours before use. Heat-trace the last 10–15 m of hose to prevent cold plugs that spike the die pressure. Watch pump inlet pressure; sudden drops signal air entrainment that masquerades as “thin” behavior at the die.
At the coater: Track three signatures as you raise speed: die pressure (should rise predictably), coat-weight variation (stay within ±3–5% of spec), and defect counts (misting particles, ribbing frequency, edge weeps). If pressure climbs faster than speed, you’re off the curve—reduce viscosity 5–10% via temperature. And observe whether defects stabilize without increasing mist.
At the laminator: Measure bond build at fixed times (10 min, 1 hr, 24 hr) as you raise speed. If early bond falls while ultimate bond holds, rheology is fine. But transfer is slipping—add nip load or tune web tension before touching the formula.
Operator training: A short rheology module tied to illustrations from NIST’s resources helps techs interpret shear-rate sweeps. Also, recovery curves rather than chasing “cup numbers” alone. Reinforce with photos of misting halos, rib spacing, and edge weeps at known settings so teams can diagnose in seconds.
For a general science refresher that many new hires recognize from school (and that supports conversations about heat, energy, and viscosity control), ENTECH’s introduction to thermodynamics is a practical, evergreen reference to share during onboarding.
Troubleshooting Speed-Related Defects
And the adhesive rheology tweak that fixes them
Misting at higher web speeds: Raise viscosity slightly (drop die temperature 2–3 °C), add a small yield-stress bump with a compatible modifier. And verify exhaust so atomized droplets don’t recycle to the web. If misting appears only at accelerations, increase the thixotropic recovery rate.
Edge weeping/feathering: Increase viscosity 5–10% or reduce shear-thinning so the post-nip fluid holds the edge. Narrow die lips or lower die-to-web gap to limit lateral spread. If it worsens over long runs, log bulk temperature drift and solvent balance.
Ribbing or chatter at high drawdown: Reduce elasticity (tune polymer MW distribution or modifier dose), slightly increase die temperature to flatten the viscosity curve. And tighten web-tension uniformity to avoid resonant coupling with the coating bead.
Starved bonds at the laminator: If nip pressure at speed squeezes too much out, increase bond line via micro-spacer content or reduce nip load slightly. Confirm thixotropy allows rapid structure rebuild after shear, so the adhesive doesn’t continue to flow out of the interface.
Put Structure Around Measurement and Control
Codify the method: which ASTM method you use for lot checks, cup type and time, rheometer program (steady-shear sweep 1–5,000 s⁻¹; 60-s rest; 3-point recovery), and temperature setpoints from tote to die. Train operators to log die pressure, coat weight, and defect counts at each speed step. Pair these with simple “go/no-go” visuals (strobe mist check, raking-light edge inspection) on every speed trial. Keep adhesive lots tied to rheology curves and production outcomes; when speed hits a wall, you’ll see whether the curve drifted or the mechanics moved.
Don’t skip the data handshake between the lab and the line
Validate the lab’s viscosity points against production. The same product that reads “on spec” in a cup can diverge on the die if its curved shape has changed. A brief rotational rheometer sweep at the temperatures and shear rates your line actually uses pays back fast. Especially when debottlenecking from 180 to 240 m/min.
Example: Turning 180 m/min into 240 m/min—No Reformulation Needed
A film/foil snack web ran 28 gsm via slot die at 180 m/min, limited by intermittent ribbing and faint mist beyond 190 m/min. Instead of rewriting the formula, the team mapped viscosity from 1–3,000 s⁻¹ at 22, 24, and 26 °C. The curve shifted ~1.5 cP/°C at 1,000 s⁻¹—enough to explain rib onset as the die warmed. They installed a die-outlet temperature sensor, tightened control to ±1 °C, and bumped the die to 25 °C while increasing web tension 5%. Result: Smooth edges, no ribbing, stable coat weight, and certified 240 m/min with the same cure and bond build as before.
Bottom line
Design speed around the adhesive’s flow curve. Choose and tune an adhesive rheology profile for your coater and substrates. Further, measure it at production shear and temperature, and control it tightly on the floor. Do that, and throughput rises without sacrificing edges, coat weight, or bond quality.
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