Setting Seal Strength Specifications: Lessons from ASTM F88 and Process Validation

Why Seal Strength Limits Are Often Mis‑Specified

Many seal strength specifications are inherited from historical documents or copied from supplier datasheets without a clear link to product risk or process capability. Some limits are too strict, causing unnecessary rework and scrap, while others are too loose and fail to prevent field complaints.

ASTM F88 provides a standardized way to measure seal strength, but it does not dictate numerical limits. Setting meaningful specifications requires combining test data with knowledge of the packaging system, filling process, distribution environment, and clinical or consumer use.

Understanding What ASTM F88 Actually Provides

ASTM F88 describes how to measure the force required to separate seals in flexible barrier materials under controlled conditions. The method defines:

• Sample geometry, including strip width and seal configuration.

• Test speeds and angles.

• Data reporting conventions, such as average force per unit width and failure mode.

What it does not do is specify a minimum acceptable force value. That responsibility lies with the manufacturer or brand owner, based on risk analysis and intended use.

Linking Seal Strength to Package Function and Risk

To define robust seal strength limits, teams should first ask:

• What is the primary function of the seal (sterility, leakage prevention, tamper evidence, or simple closure)?

• What are the realistic forces the package will experience during filling, handling, shipping, and opening?

• What are the consequences if a fraction of seals are weaker than intended?

For sterile medical packaging, a seal breach can compromise patient safety, so limits are usually higher and validated more rigorously. For consumer snack packs, the focus may be on preventing leaks while still allowing easy opening without tools.

Using Process Capability and Variability Data

Real‑world sealing processes show variability in seal strength due to changes in film, coating, temperature, pressure, dwell time, and machine condition. Instead of setting arbitrary limits, manufacturers can:

• Collect a statistically meaningful data set of ASTM F88 results at nominal sealing conditions.

• Calculate the mean and standard deviation of seal strength.

• Evaluate process capability (for example Cp, Cpk) against candidate upper and lower limits.

This analysis helps determine whether proposed specifications are realistically achievable without excessive scrap or rework. It also identifies whether process improvements are needed before tightening limits.

Defining Lower and Upper Limits: Avoiding Over‑Sealing

Seal specifications should usually include both a lower and an upper practical limit:

• Lower limit: Ensures that seals withstand expected mechanical stresses and protect the product.

• Upper limit: Prevents seals from becoming so strong that opening becomes difficult or that the film breaks instead of the seal.

Field feedback often reveals issues with over‑sealed packages, such as difficulty opening sterile pouches in operating rooms or tearing of film pouches that spill contents. These observations should be fed back into specification setting.

Incorporating Failure Modes into Specifications

Numeric force limits alone do not capture the full picture. Specifying acceptable failure modes is equally important. For example:

• “Seal must fail cohesively within the sealant layer” may be appropriate for certain designs.

• “No adhesive failure at the interface” could be required to avoid abrupt seal pop‑opens.

• “No film or laminate break outside the seal” may be critical for patient safety and usability.

By combining force ranges with failure mode criteria, specifications better reflect real‑world performance expectations.

Validation, Re‑Qualification, and Continuous Improvement

Seal strength specifications should not be static. As materials, suppliers, and equipment evolve, periodic re‑qualification is needed. A typical approach includes:

• Revisiting ASTM F88 data during major changes or every few years.

• Reconfirming that process capability remains acceptable at current limits.

• Updating risk assessments to account for new distribution patterns or regulatory requirements.

Treating seal strength limits as part of a living validation file, rather than a one‑time exercise, helps maintain alignment between lab testing and field performance over the life of the product.