What Is Seal Strength & Why Does It Matter?

Seal strength is a critical performance metric in packaging that determines how well a package remains closed and protected throughout its lifecycle. From food and pharmaceuticals to medical devices and industrial goods, seal strength directly impacts product safety, shelf life, regulatory compliance, and brand reputation.

In this comprehensive guide, we’ll explore what seal strength is, how it’s measured, the factors that influence it, relevant testing standards, and why it matters across industries.

What Is Seal Strength?

Seal strength refers to the force required to separate two sealed materials that have been joined through heat sealing, pressure sealing, or adhesive bonding. In flexible packaging, it measures how strongly two layers of film are bonded together.

Seal strength is typically measured in:

• Newtons (N)
• Pounds-force (lbf)
• Newtons per 15 mm width
• Pounds per inch (lb/in)

A properly sealed package must be strong enough to:

• Prevent leakage
• Resist contamination
• Withstand shipping and handling
• Maintain sterility (if required)
• Preserve freshness

But it must also allow easy opening when intended, without excessive tearing or product damage.

Why Seal Strength Matters

Seal strength is not just a technical metric — it directly affects product integrity, safety, and customer experience.

1. Product Protection

Weak seals can lead to:

• Oxygen infiltration
• Moisture ingress
• Bacterial contamination
• Product spoilage
• Leakage during transport

For industries like pharmaceuticals and medical packaging, seal failure can compromise sterility and patient safety.

2. Regulatory Compliance

Organizations such as:

• ASTM International
• International Organization for Standardization
• U.S. Food and Drug Administration

set standards that require validated seal strength testing, especially in medical and food packaging applications.

Failure to meet these standards can result in recalls, legal issues, and loss of certification.

3. Shelf Life & Barrier Integrity

A compromised seal reduces the barrier performance of packaging films. Even if the material itself is high-quality, a weak seal can:

• Shorten shelf life
• Increase oxidation
• Allow aroma loss
• Cause freezer burn

Seal integrity is just as important as material selection.

4. Transportation & Distribution Durability

Packages undergo:

• Compression
• Vibration
• Temperature fluctuations
• Drop impacts

Strong seals ensure packages survive the supply chain without rupture.

5. Consumer Experience

Overly strong seals can frustrate consumers, while weak seals can leak or burst. The goal is optimized seal strength — secure but accessible.

How Seal Strength Is Measured

Seal strength testing is performed using tensile testing equipment that pulls the sealed materials apart under controlled conditions.

The most widely recognized standard for seal strength testing is:

• ASTM International Method F88 – Standard Test Method for Seal Strength of Flexible Barrier Materials

Common Testing Methods

1. ASTM F88 (Peel Test)

Measures the force required to peel apart sealed flexible materials at a constant speed.

Key outputs:

• Average seal strength
• Maximum force
• Failure mode

2. Burst Testing

Evaluates how much internal pressure a sealed package can withstand before bursting.

3. Creep Testing

Measures seal durability under constant load over time.

4. Dye Penetration Testing

Identifies channel leaks or incomplete seals.

Types of Seal Failures

Understanding failure modes helps optimize seal performance.

1. Adhesive Failure

The seal separates cleanly at the interface.

2. Cohesive Failure

Material tears within one of the substrates.

3. Delamination

Layers of laminated film separate.

4. Channel Leaks

Small pathways allow air or liquid penetration.

Each failure type reveals different issues in sealing parameters or material compatibility.

Factors That Affect Seal Strength

Seal strength depends on multiple variables:

1. Sealing Temperature

Too low:

• Incomplete bonding

Too high:

• Material degradation
• Burn-through

2. Dwell Time

The time heat and pressure are applied. Insufficient dwell time reduces bonding quality.

3. Sealing Pressure

Adequate pressure ensures intimate contact between layers.

4. Material Compatibility

Not all polymers seal well together. Common sealing materials include:

• Polyethylene (PE)
• Polypropylene (PP)
• PET laminates
• Foil structures

5. Surface Cleanliness

Contamination like powder, oil, or product residue can weaken seals.

6. Environmental Conditions

Humidity and temperature during sealing can affect bond quality.

Industries Where Seal Strength Is Critical

Food Packaging

• Modified atmosphere packaging (MAP)
• Vacuum-sealed products
• Frozen foods
• Snack packaging

Seal strength ensures freshness and prevents spoilage.

Medical & Pharmaceutical Packaging

Medical device packaging must maintain sterility. Seal strength validation is mandatory under regulatory bodies like the U.S. Food and Drug Administration and ISO standards set by the International Organization for Standardization.

Industrial & Chemical Packaging

Prevents hazardous leaks and ensures safe transport.

E-commerce & Consumer Goods

Ensures tamper evidence and durability during shipping.

Optimal Seal Strength: Finding the Balance

Seal strength must be:

• Strong enough to prevent failure
• Weak enough for easy opening (if consumer-facing)

This balance is often achieved through:

• Peelable seal layers
• Controlled heat-sealing parameters
• Engineered film structures

For medical packaging, peelable seals are common to ensure aseptic opening.

Seal Strength vs. Seal Integrity

Although related, they are not identical:

• Seal Strength = Force required to separate seal
• Seal Integrity = Absence of leaks or channels

A seal can be strong but still contain micro-leaks. That’s why multiple tests are often used.

Best Practices for Improving Seal Strength

1. Validate sealing parameters (temperature, pressure, dwell time)
2. Use compatible material structures
3. Implement regular ASTM F88 testing
4. Conduct failure analysis
5. Maintain equipment calibration
6. Monitor environmental conditions
7. Train operators thoroughly

Routine quality control reduces recalls and waste.

The Cost of Poor Seal Strength

Weak seals can lead to:

• Product recalls
• Food contamination
• Loss of sterility
• Damaged brand reputation
• Increased returns
• Legal liability

In highly regulated industries, the consequences can be severe.

Future Trends in Seal Strength Testing

The packaging industry continues to evolve with:

• Smart testing automation
• Real-time seal monitoring
• Advanced multilayer barrier films
• Sustainable materials requiring optimized sealing profiles

As eco-friendly films grow in popularity, validating seal strength becomes even more critical due to differences in material behaviour.

What Is Seal Strength & Why Does It Matter?

Seal strength is one of the most important yet overlooked aspects of packaging performance. It ensures product protection, regulatory compliance, customer satisfaction, and supply chain durability.

Whether you operate in food packaging, pharmaceuticals, medical devices, or industrial goods, understanding and optimizing seal strength is essential for delivering safe, reliable products.

By implementing proper testing methods, adhering to standards from organizations like ASTM International and International Organization for Standardization, and continuously monitoring production processes, businesses can safeguard product quality and brand integrity.