Hot Melt Adhesive Nozzle Stringing: Causes and Corrections

Updated November 2026

Hot melt adhesive nozzle stringing occurs when glue extends into a thread rather than separating cleanly at cutoff. Take note of when the thread forms, verify the approved temperature and nozzle setup, then evaluate the controlled change by its effect on stringing and finished-bond performance.

A messy tail from the rear of a carton in motion calls for a different first check from glue that remains flowing after the dispense command terminates. The pair can coexist. Recording that timing helps maintenance decide whether to adjust the process, repair the module or test an adhesive.

Hot melt adhesive nozzle stringing: start with the symptom

Begin hot melt troubleshooting by recording when and where the defect appears and whether it involves a stretched filament, a dripping tip, a damaged bead, or a bond that opens later. Note if only one applicator or the entire production line changed, because that fact points us toward the right comparison to identify the root cause.

A supplier troubleshooting guide separates stringing from hot melt flowing out when the applicator is idle. Apply that clue as a real guideline the next time you troubleshoot the causes of stringing.

Sources: supplier troubleshooting guidance and the bead-application study below.

Imagine a line with the left applicator suddenly leaving long tails while the right one remains clean. The operator has a spare nozzle on hand, but maintenance first compares the last few records of the setup. Did they move the left station when they changed cartons?

Returning the station to the recorded location gives a clear comparison of the problem. If adhesive still drips while that station is idle, then the module needs attention. A replacement tip could otherwise hide the symptom briefly while the closure problem persists.

The Cutoff Evidence Ladder

1. Observe the event. Record the thread, bead end and dispense timing from outside the guards.
2. Check the approved setup. Compare actual readings and installed parts with the existing recipe.
3. Test one change. Keep the other relevant conditions matched.
4. Choose the intervention. Use the result to support parts service or a grade trial.

This unwanted-tail sequence concerns intermittent bead dispensing; the process aims are different for intentional spray fiberization and printer extrusion. Hot adhesive can burn, and equipment can retain pressure: trained maintenance must isolate, depressurize and service it according to the equipment manufacturer’s instructions.

Check temperature and airflow before changing adhesive

Compare the temperature of the hot melt with the application window of the selected grade, then observe tank, hose and gun readings after the system stabilizes. Direct airflow and the time adhesive is exposed to heat also affect application. A setpoint adjustment is only advantageous if the bead and bond both improve while the operating conditions are unchanged.

Low temperature can raise viscosity and make separation harder. Yet “increase the temperature” is an incomplete solution. In a 2022 bead-application study of eight adhesives, the researchers varied temperature, nozzle configuration and the gap to the substrate. Jowat supplied the materials and funded the work; the results concern the tested formulations and apparatus.

“No clear correlations were found for the influence of temperature and viscosity.”

Todorovic and colleagues, Chemie Ingenieur Technik, 2022

One supplier says its low-temperature grades typically apply at about 250°F (about 121°C), not 350°F (about 177°C). Another supplier describes low-temperature hot melt adhesives designed for application between 250°F and 325°F (about 121–163°C). The second example comes from 2023 guidance; both illustrate why a temperature setting suitable for the previous glue may be incorrect for its replacement.

Record the grade’s temperature range beside the actual readings. Check drafts where the adhesive leaves the tip; cold air can change the material before it reaches the substrate. The tank temperature checks should also include the hose and applicator, rather than treating the tank display as the temperature everywhere.

Heat history gives the comparison another dimension. ISO 10363’s thermal-stability method repeats viscosity and softening-point measurements at intervals of 4–6 hours while observing changes such as skin formation and gelation. Those are laboratory intervals, not a permitted tank holding time. On the line, record idle periods and replenishment as well as temperature, so a fresh fill is not compared casually with adhesive held hot through a long stop.

Check nozzle distance, geometry and module cutoff

Nozzle distance, internal geometry and module shutoff impact various elements of the adhesive delivery. Compare the installed assembly and gap to the validated setup, and then look at the closure system when flow proceeds past cutoff. Swapping the visible tip won’t resolve every cause inside the applicator.

Increasing the distance leaves more space for a filament to stretch. The bead study largely showed lengthening strings with wider gaps. Its comparison of 0.31 mm and 0.41 mm orifices, however, showed no systematic diameter effect within the tested nozzle families.

Varying orifice size without checking the bead delivered can therefore miss the cause and alter bond formation.

A nozzle distributor lists engagement lengths of 1.3 mm, 3.8 mm and 7.6 mm for compatible styles. Engagement is an internal geometry dimension, separate from nozzle-to-substrate clearance. Its guide associates longer engagement with lower flow and often cleaner cutoff. Match the replacement to the assembly and its required output, not just the thread connection.

Keep compressed air pressure and adhesive fluid pressure separate in the record. One supplier guide calls for incoming solenoid air greater than 50 psi. The 2019 guide refers to actuation air, not a direction to set every pump to 50 psi. Use the installed module’s requirements and approved limits for either circuit.

How does a hot melt adhesive machine solve the problem of adhesive stringing?

A hot melt adhesive machine can reduce stringing by holding a suitable adhesive temperature, delivering the required bead and closing the applicator at the intended time. The proper correction relies on the lost function. Stable heat won’t offset a faulty shutoff; likewise, a clean nozzle isn’t able to correct a mismatch with the material.

In a needle and seat arrangement, dirty contact surfaces can interfere with closure. Char or even other debris can also clog the flow route. The module supplier’s explanation can point to viscosity, closing force, internal seals and the added tip.

Test those parts using the service procedure before declaring the adhesive unsuitable.

For example, a maintenance team fits a smaller nozzle to reduce adhesive consumption. The bead looks neater, but intermittent gaps appear at the existing line speed. Raising delivery pressure might restore the deposited amount, yet it changes the comparison again. A more useful trial records the original and replacement geometry, measures the bead output and checks closure separately. If the new part can’t maintain a uniform bead in the approved operating window, its attractive cutoff photo isn’t enough to keep it. The melter selection for pellets also needs to suit the grade’s processing characteristics.

Run a controlled line trial and check the bond

A controlled glue trial evaluates both stringing and bond test results. The substrate, line speed and bead requirement are kept consistent. Record results during stable running and restarts separately.

Approve the candidate only when the cleaner application still meets the agreed bond test requirements; the reduction of visible stringing alone isn’t enough for approval.

The bead study considered 105 processing scenarios with 15 measurements of string length in each scenario. At a conveyor speed of 60 m/min, it adjusted delivery pressure to maintain a target of 1.5 g/m on 75 µm PET film; module control air was 4.5 bar.

The important lesson here’s matched output and multiple observations, not transplanting lab settings into production.

A packaging practitioner in an Institute of Packaging Professionals discussion recommends starting an adhesive-reduction change on one batch or one line and testing before converting the rest. Too little deposited material can compromise the bond, even when the nozzle appears clean. Adhesive that starts to solidify before compression creates another bonding problem.

Record open time and set time with the compression sequence.

The worksheet and cost calculation below are filled out with assumptions to show how the method works. These aren’t Teng Yang test results or a sampling standard. In this example, 10,000 pieces are inspected in every run; visible stringing is recorded separately from the final reject decision.

Calculate stringing share as affected pieces divided by inspected pieces: 200 ÷ 10,000 = 2.0%, while 40 ÷ 10,000 = 0.4%. A piece with a visible thread isn’t automatically a rejected piece; apply the product’s actual acceptance criteria. Keep all the photos and bond-test result with the record of each run.

Is stringing causing line stoppages?

Yes, adhesive stringing may cause stops when accumulated glue interferes with equipment or products, but the stop log must indicate which stops were caused by stringing on your line. Log the reason and duration of each downtime event. This distinguishes time spent clearing strings from time spent on other jams, replenishment, or scheduled maintenance.

A published packaging-line example shows that 40 micro-stops of three minutes each consume 120 minutes, equivalent to 25 percent of an eight-hour available shift.

Check service heat as well as the fresh bond. One adhesive supplier’s EVA/PO troubleshooting discussion flags softening above 80°C as a bond-weakening risk for traditional thermoplastic adhesives. Include the intended transport and service conditions in the grade-specific bond test, rather than treating a clean room-temperature sample as the whole qualification.

Decide whether to replace the nozzle or change the adhesive

Repair or replace a nozzle when damage, blockage or an incompatible part is diagnosed; test another adhesive when the approved equipment set-up works but the current grade is still underperforming. Only compare costs after both options meet the same bond requirements, using accepted output rather than the price of purchased adhesive alone.

For procurement, a cheap spare is attractive only if it solves the identified fault. For production, a more expensive adhesive may be worthwhile if it reduces avoidable stops and rejects. Quality owns the bond acceptance criteria. A vendor-authored trade article on hot-melt systems highlights the distinction between cost per pound and the quantity consumed per unit; the worksheet below carries that distinction through to accepted output.

They’re broken down as follows: Calculation = allocated run cost ÷ accepted pieces × 1,000. Baseline: ($80 + $90 + $20) ÷ 9,900 × 1,000 = $19.19. Candidate: ($90 + $30 + $4) ÷ 9,980 × 1,000 = $12.42.

In this case the total higher adhesive bill is compensated for by lower allocated losses.

Keep the cost buckets separate: adhesive already counted in consumption must not appear again in reject loss, and the stop allowance must not duplicate another charge. List individual replacement parts, qualification, changeover, and unusable old stock separately in the comparison when deciding whether to continue.

What to send a hot melt pellet supplier

Send the pellet supplier a brief record of the adhesive grade, substrate, applicator setup and defect timing, together with the required bond performance for your specific application. Send them actual thermal readings and a trial worksheet. That package allows the supplier to recommend a particular grade and describe the suggested application window rather than choose based on polymer family alone.

• Material: current grade, lot, storage condition and recent substitutions.
• Application: substrate and coating, bead requirement, line speed and compression sequence.
• Equipment: melter, hose, gun/module identification, nozzle geometry and installation position.
• Conditions: tank/hose/gun readings, hold periods, replenishment and restart observations.
• Evidence: a safely recorded event video, stringing counts, bond-test result and intended service conditions.

Teng Yang provides hot melt adhesive pellets in the families TPU, EVA, PA, PES and PO. Use the pellet grade selection guide to frame the material request, then obtain the proposed grade’s datasheet and a sample for the matched trial.

Ask for the test temperature and method behind the viscosity figure. ASTM D3236 describes apparent viscosity and emphasizes equivalent shear conditions for comparison. The manufacturer’s value becomes more useful when the spindle and speed conditions are recorded, especially when two datasheets appear to quote the same viscosity. For recurring application issues, keep both the trial result and the original setup record so the next comparison starts from the same conditions.

Frequently asked questions