Your seal just passed a strength test. The numbers looked good. Three weeks later, a customer calls — the package leaked in transit. What happened?
Seal strength testing told you how hard it is to pull the seal apart. It didn’t tell you whether the package would hold up under real-world pressure, temperature changes, or distribution stress. That’s a different test entirely.
Seal strength testing and seal integrity testing are both important parts of a complete QA program. But they measure different things, and confusing them — or relying on one when you need both — can leave real gaps in your quality control. This guide breaks down exactly what each test does, where each one falls short on its own, and how to build a program that actually protects your product.
What Does Seal Strength Testing Actually Measure?
Seal strength testing measures the mechanical force required to separate two sealed package components — nothing more, nothing less. You’re pulling the seal apart and recording how much force it takes. The result tells you whether your sealing process is producing seals that meet a defined strength specification.
The primary standard for seal strength testing is ASTM F88, which covers peel and tensile testing of flexible barrier materials. Samples are cut from a sealed package, placed in a tensile testing machine, and pulled apart at a controlled rate. The force required to separate the seal is measured in newtons or pounds per inch of width.
Burst testing is another common form of seal strength evaluation. Under ASTM F1140 and ASTM F2054, a package is inflated with air or water until it fails — either at the seal or through the substrate. The result is a burst pressure, typically measured in kilopascals or pounds per square inch. Burst testing evaluates the package as a whole unit under internal pressure rather than testing a sample coupon.
Both tests are effective for monitoring daily process consistency — confirming that your sealing equipment is calibrated correctly and that seal quality isn’t drifting between shifts or across material lots. In tightly regulated industries, hitting defined seal strength specifications is often a compliance requirement.
What seal strength testing doesn’t do: it doesn’t test the entire sealed package, and it doesn’t confirm that the package is actually leak-free.
What Is Seal Integrity Testing?
Seal integrity testing evaluates whether a complete, finished package will remain sealed under real-world conditions — distribution stress, temperature and pressure changes, handling, and the full product lifespan.
Where seal strength testing samples a small section of a seal, integrity testing evaluates the entire package. A package can have strong seals and still fail integrity testing if there’s a channel defect, a pinhole, or a compromised area that wasn’t captured in the strength sample. The two tests are measuring different things.
Seal integrity is particularly critical in industries where a breach doesn’t just mean a leaky bag. In pharmaceutical and medical device packaging, a failed seal can compromise sterility and render a product unsafe. The package must maintain a microbial barrier from the point of manufacture through the end of its shelf life — not just survive a pull test on the production line.
For food packaging, integrity failures accelerate spoilage, shorten shelf life, and create the conditions for recalls. A package that holds together mechanically may still allow oxygen ingress through a microscopic channel defect — enough to affect product quality long before it’s visible.
Seal integrity testing evaluates the package’s performance as a whole, under the kinds of stresses it will actually face.
Seal Strength vs. Seal Integrity: Side-by-Side
The distinction matters in practice. Here’s how the two approaches compare:
| Seal Strength Testing | Seal Integrity Testing | |
|---|---|---|
| What it measures | Force required to separate a seal | Whether a complete package is leak-free |
| Test scope | Sample coupon or section of seal | Full, finished package |
| Key ASTM standards | F88 (peel/tensile), F1140/F2054 (burst) | D3078, F2096, D5094, D6653 |
| Destructive? | Yes — sample is destroyed | Varies by method (D3078: No; F2096: Yes) |
| What it confirms | Sealing process is consistent and meets spec | Package will hold up in distribution and use |
| What it misses | Channel defects, pinholes, whole-package failures | Seal strength specification compliance |
| Best for | Daily process monitoring, specification compliance | Distribution validation, sterility assurance, QA sign-off |
Neither test makes the other redundant. They answer different questions, and a complete QA program uses both.
Why Seal Strength Testing Alone Isn’t Enough
Seal strength testing is a process control tool. It tells you that your sealing equipment is running consistently and that your seals are meeting a defined mechanical specification. That’s genuinely useful — and in many regulated industries, it’s required.
But strength testing has a structural limitation: it only evaluates a small section of the seal. A typical F88 test sample is a 25mm-wide strip cut from the seal area. If a channel defect, pinhole, or uneven seal bar is producing compromised seals elsewhere on the package, that section may not end up in the test coupon. The strength test passes. The package leaks.
Here’s where it gets important: distribution stress amplifies small defects. A package that holds together under normal handling may fail when subjected to the pressure changes of air freight, the vibration of ground transport, or extended cold-chain conditions. Seal integrity testing is designed to catch these failures before the product ships — not after it reaches your customer.
The practical framework is straightforward. Use seal strength testing (ASTM F88, F1140) to confirm your process is in control and your seals are meeting specification. Use seal integrity testing (D3078, F2096) to confirm that finished packages are actually leak-free. Seal integrity depends less on package type and more on matching your test conditions to real shipping and storage environments.
In pharmaceutical and medical device manufacturing, this pairing is commonly expected under quality frameworks like ISO 11607. In food manufacturing, running both gives you the documentation and confidence to defend your QA program through audits and customer requirements. Neither test replaces the other — they each catch what the other misses.
How to Test Seal Integrity: FlexPak Methods
FlexPak package leak detectors support four ASTM-compliant seal integrity testing methods, covering flexible and rigid packaging across food, pharmaceutical, and medical device applications.
Bubble Emission Testing — ASTM D3078
The sealed package is submerged in water inside a transparent vacuum chamber. As vacuum is drawn, headspace gas inside the package expands and escapes through any leak points — producing a visible, steady stream of bubbles. This is a no-damage test for packages with headspace gas, and it identifies the exact location of any failure.
ASTM D3078 detects gross leaks and visible channel defects under defined vacuum and hold-time conditions. Sensitivity depends on vacuum level, headspace volume, product type, and packaging material. For vacuum-sealed packages with little or no headspace, the VAC Attachment introduces a small amount of air to make the package testable.
Learn how to perform a bubble leak test →
Internal Pressurization — ASTM F2096
The package is punctured, inflated with air underwater to a predetermined pressure, and observed for a steady stream of bubbles at failure points. This is a destructive test — it requires puncturing the package — and is typically used for tray and pouch packages in pharmaceutical and medical device applications.
Based on published round-robin data, ASTM F2096 detects defects down to approximately 250 µm with around 81% probability. Test pressure must be established per package type using a control sample with a known defect. The FPIPA attachment enables F2096 testing on FlexPak units.
Altitude Simulation — ASTM D6653
ASTM D6653 is a conditioning method, not a detection method. It simulates the pressure differentials packages experience during air shipment or high-altitude ground transport — stressing the package under controlled conditions to reveal seals that were already near their failure limit.
D6653 must be paired with a detection method afterward. The standard workflow is D6653 conditioning followed by ASTM D3078 bubble emission testing to identify any resulting leaks. Specimens are typically conditioned to around 42°F before testing to reflect cold-chain shipping environments.
ASTM D6653 altitude simulation guide →
Dry Chamber Testing — ASTM D5094
Dry chamber testing is designed for rigid and semi-rigid containers with threaded or lug-style closures — bottles, jars, condiment sachets, and similar formats. The container is placed in the chamber on an absorbent material without liquid in the tank. When put under vacuum, any leaks will result in the presence of liquid within the dry chamber. Detection is visual.
ASTM D5094 uses vibration combined with storage (Method A) or vacuum exposure (Method B). It is not a vacuum decay method — detection is visual, not pressure-based.
Explore FlexPak testing equipment →
Frequently Asked Questions
What is the difference between seal strength and seal integrity testing?
Seal strength testing measures the mechanical force required to separate a sealed package component — typically a coupon sample tested under ASTM F88. Seal integrity testing evaluates whether a complete, finished package is actually leak-free under real-world conditions. Strength testing confirms process consistency; integrity testing confirms the finished package will hold up.
What ASTM standard covers seal strength testing?
The primary standard for seal strength testing is ASTM F88, which covers peel and tensile testing of flexible barrier materials. ASTM F1140 and ASTM F2054 cover burst testing — a related strength evaluation where the package is inflated until it fails. These standards are separate from the integrity testing standards (D3078, F2096, D5094) that evaluate a complete, sealed package.
Can seal strength testing replace seal integrity testing?
No. Seal strength testing evaluates a sample coupon — a small section of the seal. It can confirm your sealing process is consistent and meeting specification, but it won’t detect channel defects, pinholes, or compromised areas elsewhere on the package. Seal integrity testing evaluates the whole finished package and is the only way to confirm it will remain sealed through distribution, storage, and use.
What seal integrity testing methods does FlexPak support?
FlexPak equipment supports four ASTM-compliant methods: bubble emission testing (ASTM D3078) for flexible packages with headspace; internal pressurization (ASTM F2096) for trays and pouches via the FPIPA attachment; altitude simulation conditioning (ASTM D6653) paired with a detection method; and dry chamber testing (ASTM D5094) for rigid containers with threaded or lug closures.
A seal strength test tells you your process is working. A seal integrity test tells you your package is ready to ship. Both questions matter — and the answers aren’t always the same.
If you’re building out a QA testing program or want to confirm which integrity testing method fits your packaging format, FlexPak can help you find the right solution.