Hot Tack vs. Seal Strength: How to Build a Complete Flexible Packaging Seal Profile

Why One Seal Number Is Not Enough

In flexible packaging, the heat seal is the last barrier between the product and the outside world. Many plants still rely on a single “seal strength” value for quality control, but this approach overlooks how seals behave immediately after sealing and after cooling. When lines run at high speeds, packages may be stressed before seals fully crystallize or develop their final strength.

To prevent open packages, leaks, and product returns, engineers increasingly use both hot tack and cooled seal strength measurements. Together, these tests create a more complete seal profile that reflects real production conditions.

Hot Tack: Seal Performance Before Full Cooling

Hot tack testing evaluates the strength of a heat seal while the materials are still hot, typically seconds after the sealing jaws open. This is crucial for high‑speed lines, where filled pouches may be dropped, squeezed, or transported while seals are still developing.

Key concepts in hot tack testing include:

• Seal jaw temperature: Often lower than for maximum cooled seal strength to avoid sticking.

• Dwell time and pressure: Carefully controlled to simulate line conditions.

• Delay before testing: A short, defined time between sealing and testing to capture the “hot state.”

Hot tack results help determine whether current sealing conditions can withstand the mechanical handling that occurs immediately after the seal is formed.

Cooled Seal Strength: Long‑Term Integrity Under ASTM F88

Cooled seal strength tests, such as those based on ASTM F88, measure the force required to peel a fully cooled and conditioned seal. These tests characterize long‑term package integrity in distribution, storage, and end‑use.

A typical cooled seal test includes:

• Preparing sealed strips with known width and seal geometry.

• Allowing seals to cool and condition at controlled temperature and humidity.

• Peeling seals at a defined speed and angle, recording average force and failure mode.

Cooled seal strength reflects how well sealing parameters, film structure, and sealant formulations support the intended shelf life and handling conditions.

Building the “Seal Window”: Temperature, Time, and Pressure

To create a practical seal profile, engineers often map seal performance across a range of sealing temperatures, dwell times, and pressures. For each combination, both hot tack and cooled seal strength are measured.

Plotting these values reveals:

• Minimum temperature for acceptable hot tack.

• Temperature range where cooled seal strength reaches a plateau.

• Conditions where seals become too strong and cause film or laminate break instead of cohesive seal failure.

This “seal window” guides both process development and ongoing production settings, helping to avoid weak seals at low temperatures and brittle or over‑sealed areas at high temperatures.

Failure Modes: Reading What the Seal Is Telling You

In both hot tack and cooled seal tests, failure mode is as important as numeric strength. Engineers should categorize:

• Cohesive seal failure: Sealant splits within itself, often considered ideal for robust seals.

• Adhesive seal failure: Separation at the interface between layers, indicating issues with temperature, dwell, pressure, or contamination.

• Material break: Film tears outside the seal region, which may signal over‑sealing or inappropriate film selection.

By linking failure modes to sealing parameters and line events, plants can pinpoint the root causes of seal‑related defects.

Implementing Hot Tack and Seal Strength Testing in the Lab

A well‑equipped lab typically uses:

• A temperature‑controlled heat sealer or form/fill/seal simulator.

• A universal tester that can perform peel tests at defined speeds and angles.

• Methods that define sample width, distance from seal to grips, and conditioning times.

For each product, the lab establishes a standard set of seal conditions and acceptance criteria. Over time, trending hot tack and cooled seal strength data by lot, supplier, and line helps maintain process stability and supports rapid troubleshooting when complaints occur.

Using Seal Profiles for Process Optimization and Risk Reduction

Once a robust seal profile is in place, plants can:

• Adjust sealing parameters when materials change or new suppliers are introduced.

• Identify early drift in heat sealer performance before it causes large‑scale defects.

• Provide objective evidence to brand owners and regulators that seal performance has been characterized and is under control.

By combining hot tack and cooled seal strength testing, packaging teams move from single‑point checks to a more powerful, process‑oriented view of seal quality.