Last updated: March 2026
A seal can be strong enough to survive a peel test and still be leaking. Those are not contradictory statements. Strength and integrity measure different things, and a QA program that only tests one of them has a real blind spot.
Seal strength testing is one of the most common quality checks in flexible packaging. It tells you how much force a seal can withstand before it separates. What it does not tell you is whether that seal is currently allowing gas or liquid to escape. For some products and some programs, strength testing is exactly what you need. For others, it’s only half the answer.
This guide explains what seal strength testing actually measures, how it relates to seal integrity testing, and how to build a QA program that covers both.
What Is Seal Strength Testing?
Seal strength testing measures the force required to pull a package seal apart. A sample is cut from the sealed area of the package, clamped into a tensile testing machine, and pulled at a controlled rate until the seal separates. The result is a force measurement, typically in Newtons per 15mm or pounds-force per inch, which tells you how mechanically strong that seal bond is.
It is a destructive test. The sample is consumed in the process. And it is a direct, reliable measure of one specific thing: the mechanical strength of the seal at the point it was tested.
What it does not measure is whether the seal is continuous, complete, or free of channels and pinholes. A seal can register a strong peel force on average and still contain a localized weak point or micro-channel that allows contamination or gas loss. Strength testing won’t find that. Leak testing will.
Seal Strength vs. Seal Integrity: Two Different Questions
These two terms get used interchangeably in QA conversations. They shouldn’t be.
Seal strength answers: how hard is it to pull this seal apart? It reflects the quality of the bond itself – the temperature, dwell time, and pressure your sealing equipment applied at that moment. A high seal strength means the bond is mechanically robust. It is a useful indicator of sealing machine performance and process consistency.
Seal integrity answers: is this package actually sealed? It tells you whether gas or liquid can escape through the seal area, including through pinholes, channels, or localized failures that average strength measurements won’t catch. A package can have acceptable average seal strength and still fail an integrity test.
The practical implication: if your product is sensitive to oxygen ingress, moisture loss, or contamination, seal strength data alone does not give you confidence that the package is protecting what’s inside. You need integrity testing to close that gap.
Here’s a useful way to think about it. Seal strength testing is process monitoring – it tells you how your sealing equipment is performing. Seal integrity testing is product verification – it tells you whether the individual package in front of you is actually sealed.
| Seal Strength Testing | Seal Integrity Testing | |
| What it measures | Force required to separate the seal | Whether the package is leaking |
| Standard | ASTM F88 | ASTM D3078, ASTM F2096 |
| Destructive? | Yes | D3078: No. F2096: Yes |
| Detects pinholes/channels? | No | Yes |
| Monitors sealing equipment? | Yes | Indirectly |
| Confirms package is sealed? | No | Yes |
| Best used for | Process control, equipment validation | Product release, QA verification |
ASTM F88: The Standard Behind Seal Strength Testing
ASTM F88 is the primary standard for measuring the peel strength of flexible barrier materials. It covers the test procedure, specimen preparation, and reporting requirements for flexible packaging seal strength evaluation.
The standard defines three test techniques based on how the unsealed portion of the specimen is clamped and supported during the test. Technique A is unsupported, Technique B uses a supported substrate, and Technique C uses a restrained substrate. The choice of technique affects the measured result, which is why consistent technique selection matters when comparing data across time or across facilities.
A few things worth knowing about ASTM F88. First, the standard measures force to separate, not force to leak – those are different failure modes, and F88 is specifically designed for the former. Second, F88 results are sensitive to specimen width, grip separation, and test speed, so meaningful comparisons require consistent test setup. Third, F88 is frequently used alongside – not instead of – leak detection methods in complete QA programs, particularly for medical device packaging where both seal strength and sterile barrier integrity must be demonstrated.
When Seal Strength Testing Is Enough — and When It’s Not
Seal strength testing is well-suited to process control. If your goal is to monitor sealing machine performance, catch drift in seal bar temperature or dwell time, and verify that your sealing process is operating within spec, peel testing is the right tool. It’s fast, quantitative, and directly reflects what your equipment is doing.
It’s also appropriate for development and validation work: confirming that a new packaging material seals adequately, establishing minimum acceptable seal strength values, or comparing materials against each other.
Where it falls short is product-level verification. Testing a sample from a production run tells you that the seal at that specific location, on that specific specimen, had a certain strength. It does not tell you whether other packages on that run have pinholes, whether a seal bar is creating intermittent channel defects, or whether the package in question will maintain its integrity through distribution.
For products where the sealed environment matters – food with a modified atmosphere, pharmaceutical products requiring sterile barrier integrity, or anything sensitive to oxygen or moisture – seal strength data needs to be paired with leak detection. The two methods answer different questions, and both questions matter.
A practical combined program for most food and CPG applications looks like this: use seal strength testing at regular intervals to monitor sealing machine performance and catch process drift early, and use bubble emission leak testing to verify that packages leaving the line are actually sealed. Brands like Nestlé, Hershey’s, and Smucker’s use exactly this combination – seal strength to monitor the machine, bubble emission to verify the product. Neither method alone gives you the full picture. Together, they cover process control and product verification.
Want to know which leak detection method pairs best with your seal strength program? Get a quote in 24 hours – we’ll map out the right combination for your line.
How to Build a Testing Program That Covers Both
The goal is not to test everything twice. It’s to make sure your program answers both questions – is the process running well, and is the product sealed – without creating redundant work.
Start of shift and end of shift are the two most important checkpoints. Run seal strength tests before production begins to confirm the sealing machine is set correctly and catch any overnight drift before it affects a full run. At end of shift, pull one final sample for leak testing to confirm quality held through the run. Reviewing seal strength data from the shift alongside end-of-run leak test results gives you a complete picture of how the line performed.
During production, run bubble emission leak tests at regular intervals – at minimum, one sample per hour. This is your product-level verification step. If something goes wrong with a seal bar or film roll, these tests catch it in real time rather than at end of shift.
After any equipment adjustment or material change, run both. A change to seal bar temperature, dwell time, pressure, or film specification is a process change – confirm the new settings produce both adequate strength and intact seals before resuming normal production.
For manufacturers following ISO 2859-1 or customer-specified AQL sampling plans, seal integrity testing is typically the relevant method for critical defects. A breached seal is a critical defect. Seal strength testing may supplement this but does not substitute for it.
For guidance on building a complete testing cadence, FlexPak’s package testing standard guide walks through the decision framework in detail.
Setting Up Seal Strength Testing with FlexPak
Seal strength testing per ASTM F88 requires a dedicated tensile testing machine – that’s separate from leak detection equipment, and it’s not something FlexPak manufactures. If you’re adding peel testing to your program, you’ll need a tensile tester alongside your leak detector.
What FlexPak provides is the other half of this program: the product verification side. The FlexPak FPFAT runs ASTM D3078 bubble emission testing – package submerged in water, vacuum drawn, escaping gas produces a visible bubble stream at the exact leak location – with results in around 30 seconds. It stores up to 24 repeatable test recipes, so operators run a validated protocol every time without manual setup. For tray and pouch applications requiring ASTM F2096 internal pressurization testing, the FPIPA attachment handles that as well.
Most customers pair a tensile tester for seal strength monitoring with a FlexPak unit for leak detection. Together, they cover both questions your QA program needs to answer.
If you’re evaluating how seal strength testing fits into your program alongside bubble emission or internal pressurization testing, FlexPak’s team can help map out the right combination of methods for your product type, package format, and compliance requirements.
Get a quote in 24 hours and we’ll help you build the right testing setup for your packaging line.
Frequently Asked Questions
What is the difference between seal strength and seal integrity?
Seal strength measures the force required to pull a seal apart, reflecting the mechanical quality of the bond. Seal integrity measures whether the package is actually leaking – including through pinholes or channel defects that strength testing cannot detect. A package can pass a seal strength test and still fail a seal integrity test.
What does ASTM F88 measure?
ASTM F88 measures the peel strength of flexible barrier material seals – specifically, the force required to separate the seal under controlled test conditions. It does not measure leak resistance or confirm that a package is free of defects. It is a process monitoring tool, not a leak detection method.
Does seal strength testing detect leaks?
No. Seal strength testing is a destructive peel test that measures bond force. Detecting gas or liquid escaping through a seal requires a separate method such as ASTM D3078 bubble emission testing or ASTM F2096 internal pressurization testing.
When should I use seal strength testing vs. leak testing?
Use seal strength testing to monitor sealing machine performance and process consistency. Use leak testing to verify that individual packages are actually sealed and free of defects. For most food and pharmaceutical packaging applications, a complete QA program uses both: seal strength for process control, leak testing for product verification.
What equipment do I need for seal strength testing?
Seal strength testing per ASTM F88 requires a tensile testing machine capable of applying a controlled separation rate and measuring peel force – this is separate from leak detection equipment. FlexPak specializes in bubble emission and leak detection equipment; for seal strength testing you’ll typically need a dedicated tensile tester alongside a FlexPak leak detector.
Seal strength testing is a valuable tool. It tells you a lot about your sealing process and whether your equipment is performing consistently. What it does not tell you is whether the package in front of you is sealed.
That’s not a criticism of the method. It’s just the honest answer to what the test was designed to do. A complete QA program uses seal strength testing and leak testing together: one to monitor the process, one to verify the product. When both are in place, you’re not guessing about either question.
Ready to add leak detection to your seal strength program, or build a combined testing setup from scratch? Get a quote in 24 hours and we’ll help you figure out the right configuration for your line.
About the Author: Gordon Bruce, Sales and Application Expert, FlexPak Inc. Gordon has spent years helping food, pharmaceutical, and industrial manufacturers build leak detection and seal integrity programs that work on the production floor. He works directly with QA teams to match testing methods and equipment to real compliance and operational requirements.