ASTM D1921
Hot Tack Testing

In modern packaging, especially for high-speed form-fill-seal (FFS) applications, ensuring the hot tack strength of thermoplastic films is vital to prevent seal failures during handling. The ASTM F1921 test method provides a standardized procedure for measuring the strength of heat seals before they have cooled. This guide explores how to perform the hot tack test according to ASTM F1921 and how instruments like the Cell Instruments hot tack tester support consistent, reliable testing.

Understanding ASTM F1921: Purpose and Importance

ASTM F1921 is a widely accepted standard developed by ASTM International to determine the hot tack strength of thermoplastic films used in flexible packaging. In form-fill-seal processes, seals are often stressed while still warm. A weak hot seal can rupture, resulting in product loss, packaging failure, or contamination.

This test is essential for packaging engineers, film manufacturers, and quality control teams to:

Evaluate and compare film performance
Optimize sealing conditions
Ensure process consistency and product integrity

Hot Tack Test and Hot Tack Strength Explained

The hot tack test measures the maximum force required to peel a heat seal apart while it is still hot. This property, known as hot tack strength, reflects the material’s ability to hold together immediately after sealing—before full crystallization or cooling.

There are two test approaches under ASTM F1921:

Method A (Fixed Delay): Uses a manufacturer-defined delay time before the grips separate the sealed sample.
Method B (Variable Delay): Uses user-defined delays, offering more flexibility for different materials.

Key Testing Procedure Based on ASTM F1921

The testing process consists of the following steps:

Specimen Preparation
- Width: 15 mm, 25 mm, or 1 inch
- Length: 25–35 cm
- Direction: Machine or transverse direction (must be documented)
- At least 3–5 specimens per condition
Sealing Process
- Heated jaws apply pressure to two overlapping film strips
- Typical dwell times:
- ≤25 μm films: 0.5 s
- 25–64 μm films: 1.0 s
- Pressure: 15–30 N/cm²
- Temperature: Starts from seal initiation temperature and increases by 5–10 °C per test
Separation & Measurement
- After sealing, the jaws open and the grips automatically pull the seal apart at a constant speed
- The machine records the force required to separate the seal (hot tack strength)
Data Collection
- At each temperature, a hot tack curve is generated
- Failure modes (peel, delamination, stretch, tear) are visually inspected and documented

Understanding Hot Tack Curves

A hot tack curve is a graphical representation of seal strength across a temperature range. These curves help identify:

Optimal sealing temperatures
Performance trends
Material limitations at high or low temperatures

Proper analysis of the curve can guide packaging engineers in choosing suitable materials and adjusting machine settings to reduce failure rates in production.

Common Interferences and Seal Failures

Interferences such as delamination, incomplete fusion, or coating transfer can distort results. ASTM F1921 emphasizes documenting the failure mode of every specimen to ensure accurate interpretation of hot tack performance.

Recommended Equipment: Cell Instruments Hot Tack Tester

To perform precise and repeatable ASTM F1921 testing, Cell Instruments offers a fully automated hot tack tester designed to meet and exceed standard specifications. Key features include:

Dual heated jaws with fine control of temperature, dwell time, and pressure
Automated seal and pull cycle with real-time force measurement
User-friendly interface with programmable delay modes for both Method A and B
Generation of hot tack curves and statistical data for quality assurance

The instrument’s high accuracy and reproducibility make it ideal for film manufacturers, packaging R&D, and quality control labs.

Best Practices for Accurate Hot Tack Testing

Always inspect films for defects like gels or nicks—discard flawed samples
Maintain constant sealing conditions across replicates
Clearly label seal direction and specimen dimensions in reports
Use at least three replicates per temperature point for averaging
Avoid passive errors—manually inspect failure modes in each sample

Conclusion

ASTM F1921 provides a comprehensive and repeatable methodology for assessing hot tack strength in thermoplastic packaging films. By adhering to this standard, quality control professionals can ensure packaging durability during high-speed processing and handling. Using precision equipment like Cell Instruments’ hot tack tester further enhances test consistency, making it easier to develop reliable, high-performance flexible packaging solutions.

ASTM D1921 Hot Tack Testing