How to Run a Hot Tack Test Method Per ASTM F1921 (With Application-Specific Settings)

Hot tack strength is the resistance of a heat seal to peeling forces before the seal cools. Cold seal strength -- measured under ASTM F88 -- tells you how strong a bond becomes after it has fully solidified. These are different properties that answer different questions. For a detailed breakdown, see the differences between hot tack and heat seal strength.

A hot tack test produces three measurable outputs that belong on every material spec sheet:

So if you already know your seal strength numbers, why run a separate hot tack test? Because seal strength after cooling tells you nothing about what happens at line speed when a product drops into a still-hot seal. That drop happens in milliseconds, and the seal must hold before it solidifies.

Request all three outputs -- threshold temperature, peak force, and temperature range -- in every test report. A single peak-force number without the temperature window is incomplete data.

ASTM F1921 defines two distinct test protocols. Both measure hot tack force, but they control the delay between seal formation and peel initiation differently.

If you are qualifying a new sealant layer against two alternatives, Method A gives you the cleaner comparison -- every specimen sees the same delay, so force differences reflect material differences alone. If you are validating that your current film survives a high-speed FFS cycle, Method B is the only protocol that matters, because it lets you dial the delay down to match your actual line timing.

Before requesting a test from an external lab, specify Method A or Method B in your purchase order. Most labs default to Method B when the PO is silent.

Follow these steps at the bench. Each instruction is written for an operator running ASTM F1921 on a hot tack tester with standard jaw geometry.

Specimen Preparation (Steps 1-3)

1. Cut specimens to width. Cut film strips to the width specified in your test plan (typically 25.4 mm). Use a precision cutter or template to keep width consistent across all specimens. Inconsistent width is the most common source of force scatter.

2. Condition the specimens. Store cut strips at 23 +/- 2 degrees C and 50 +/- 5% relative humidity for a minimum of 24 hours before testing. Skipping conditioning introduces moisture and temperature variables that contaminate your data.

3. Inspect and prepare seal bars. Verify that the seal bar surfaces are clean, flat, and free of scoring. Worn bars create uneven pressure across the seal width, which distorts peak force readings.

Running the Test (Steps 4-7)

1. Mount the specimen. Load one strip into the upper and lower clamps so that the sealant surfaces face each other. Align the strip so the seal area is centered between the jaws.

2. Enter test parameters. Set the sealing temperature, dwell time, sealing pressure, and delay time according to your test plan. Set the sealing pressure per your material specification and the ranges in ASTM F1921. Typical equipment ranges allow temperatures up to 250 degrees C, dwell times from 0 to 99 seconds, and pressures from 0.1 to 0.8 MPa.

3. Run the seal-and-peel cycle. The instrument seals the specimen at the set temperature and pressure for the set dwell time, then separates the jaws after the programmed delay and measures the peel force.

4. Record force data. The tester outputs a force-displacement curve. Record the peak force (N) for each specimen. Instruments with Level 0.5 measurement accuracy provide the resolution needed for meaningful material comparisons.

Building the Hot Tack Curve (Steps 8-9)

1. Repeat across the temperature sweep. Run replicates (typically 3 to 5) at each temperature point. Step the temperature in increments of 5 to 10 degrees C across your target range.

2. Plot the hot tack curve. Plot peak force (Y-axis) against sealing temperature (X-axis). Identify the threshold temperature, the peak force, and the temperature range where force exceeds your minimum acceptance criterion.

Before running your temperature sweep, run three replicate seals at your target production temperature first. This baseline anchors the curve and confirms your setup is producing consistent data.

The ASTM F1921 procedure stays the same. What changes across applications is which parameters you prioritize, which method you select, and how you interpret the results.

Form-Fill-Seal Lines: Where Delay Time Decides Everything

On an FFS line, hot tack performance at short delay times is the variable that predicts seal failures. Lines running above 80 cycles per minute push product weight into the seal before the sealant layer solidifies. Method B with delay times under 0.5 seconds is the protocol that replicates this condition.

The hot tack temperature range at short delay is the number that determines how much temperature drift your sealing station can tolerate before seals start failing on the line.

Medical Device Packaging: Reproducibility Over Peak Force

Sterile barrier systems shift the priority from speed to documentation. Medical device packaging engineers should use Method A with fixed parameters traceable to the validated sealing process. Regulatory submissions under ISO 11607 require demonstrated process control -- not just a peak force number, but evidence that the sealing process produces consistent results across replicates.

Hot tack data generated this way supports IQ/OQ/PQ protocols for sealer validation. The fixed-delay approach ensures that any variation in the data reflects material or process variation, not test method variation.

Film R&D Labs: Building the Full Hot Tack Curve

Film development labs screening sealant candidates need the complete hot tack curve, not just a single peak force number. Run a full temperature sweep -- typically 80 to 200 degrees C in 5 to 10 degree increments -- using Method A. Equipment capable of reaching 250 degrees C covers virtually all thermoplastic sealant chemistries.

Overlay the curves of candidate materials on a single chart. Compare threshold temperature, peak force, and working range side by side. The hot tack curve becomes a material fingerprint -- two resins with identical peak forces can have completely different temperature ranges, and that difference drives the production decision.

These four errors show up repeatedly in failure investigations. Check each one before your next test run.

Print this checklist and post it next to your tester. A two-minute pre-run check eliminates the errors that waste the most film and time.

• A hot tack test method measures seal strength while the sealant is still molten -- a different property from cold seal strength measured under ASTM F88.
• ASTM F1921 offers two protocols: Method A (fixed delay) for material ranking and validation, Method B (variable delay) for production-speed simulation.
• Match your protocol to your application: FFS lines need Method B at short delays, medical packaging needs Method A with statistical replication, and R&D labs need full temperature sweeps for material fingerprinting.
• Every test report should include three outputs: threshold sealing temperature, maximum peak force, and hot tack temperature range.
• Specimen preparation discipline -- consistent width, proper conditioning, clean seal bars -- determines whether your data is trustworthy or noise.