There is no single seal-strength value that works for every flexible packaging application. The required seal strength depends on the product, package design, film structure, filling process, distribution conditions, and whether the package needs to be easy to open. The goal is to establish a seal that is strong enough to maintain package integrity but appropriate for the way the finished package will be handled and opened.
What Does Seal Strength Mean?
Seal strength is a measurement of how much force is required to separate a sealed portion of flexible packaging.
It is commonly expressed as force per unit width, such as N/15 mm or lb/in, depending on the testing method and specification. ASTM F88/F88M measures the force required to separate a test strip containing a seal and also identifies the mode of failure.
Seal strength is useful for more than determining whether a package is “strong enough.” It can also be used for process validation, quality control, and monitoring consistency between production runs.
Does Stronger Always Mean Better?
No. A stronger seal is not automatically a better seal.
Some packages need a high-strength seal because they will experience significant stress during filling, transportation, handling, or storage. Other packages are intentionally designed with a peelable seal so the consumer can open them without excessive force.
The desired result is therefore not maximum seal strength. It is an appropriate and consistent seal strength for the application.
For a consumer package, for example, making the seal substantially stronger than necessary could make opening difficult. For a package containing a heavy or demanding product, insufficient seal strength could increase the risk of package failure.
What Determines the Required Seal Strength?
Several factors should be considered when establishing a seal-strength requirement.
Product weight and characteristics
The product inside the package affects the stress placed on the seals.
A heavier product can place greater loads on seals, particularly during filling and handling. Products with sharp edges, irregular shapes, or other characteristics that can stress the package may require additional consideration.
Liquids and products that generate internal pressure can create different sealing requirements than dry products.
Package size and design
The size and geometry of the package also matter.
A larger package can place different stresses on its seals than a small package. Features such as gussets, corners, zippers, spouts, and other package components can also influence how forces are transferred through the package.
The seal should therefore be evaluated as part of the complete package rather than as an isolated strip of film.
Distribution and handling
A package that only needs to survive gentle handling may have different requirements from one that will experience vibration, compression, drops, temperature changes, or long-distance transportation.
The expected distribution environment should be part of the seal-strength specification.
What Is Hot-Tack Strength?
Hot tack is the ability of a seal to resist separation while the sealant is still hot and has not fully developed its final strength.
This can be particularly important on form-fill-seal equipment. For example, on a VFFS machine, a freshly made bottom seal can be exposed to the weight of the product while the package is still being formed. If the seal does not have sufficient hot-tack strength, it can be stressed before it has fully cooled.
Hot-tack performance is therefore related to seal performance but is not the same thing as final seal strength.
🔧 Note from the Engineer: When evaluating a flexible packaging film, don’t look only at the final seal-strength number. A film can produce a strong finished seal but still create problems during production if the seal does not develop strength quickly enough. Temperature, dwell time, pressure, film structure, contamination in the seal area, and machine speed can all affect the sealing process.
How Is Seal Strength Tested?
A common approach is to make representative seals using the actual film, sealing equipment, and process conditions. Samples are then tested to determine the force required to separate the seal.
ASTM F88/F88M provides a standardized method for measuring seal strength in flexible barrier materials. The method also recognizes that the way a specimen is held during testing can affect the result, which is why consistent testing procedures are important when comparing results.
Testing should ideally represent the actual production process. A laboratory seal made under conditions that are substantially different from the packaging machine may not accurately represent production performance.
How Do I Set a Seal-Strength Requirement?
Start by identifying what the package needs to survive and how it needs to open.
Consider:
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Product weight and characteristics
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Package size and construction
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Film and sealant structure
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Filling and sealing conditions
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Machine speed
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Expected transportation and handling
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Storage conditions
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Potential seal contamination
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Required consumer opening force
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Whether the package needs to be peelable or tamper-resistant
The resulting specification should include an acceptable range rather than simply a maximum target. Production testing should then confirm that the packaging process can consistently achieve that range.
The right seal strength for flexible packaging is ultimately the strength required to protect the product and maintain package integrity under actual conditions, while still meeting any opening or usability requirements. Rather than choosing a universal number, establish the requirement from the product, package, film, sealing process, and distribution environment, then verify it through controlled testing.
Information provided by Adeneli Packaging
Eugene, Oregon | adenelipackaging.com
