How to Evaluate Side-Seal Strength in PE Bags

Side-seal failure can expose or release the packed product during filling, handling, storage or transport. However, a burst side seal does not identify a single cause. A useful investigation considers the complete bag specification, sealing process, packed load and distribution conditions.

Buyers should define how the finished bag will be used and agree on measurable acceptance criteria before production. Visual appearance alone cannot establish seal strength or transport suitability.

What affects PE bag side-seal performance?

Film formulation and structure

LDPE, LLDPE, HDPE and other PE formulations can produce different stiffness, elongation, clarity, sealing behaviour and puncture performance. These properties depend on the selected grades, blend, additives, film structure and processing conditions.

Virgin or recycled-content labels do not independently predict seal performance. A recycled-content construction may require different process settings or acceptance criteria, while a virgin-resin construction can still fail if its specification or production process is unsuitable.

Thickness and thickness distribution

Nominal thickness is only one input. The specification should define whether thickness refers to one film layer or the combined thickness of two layers, the permitted tolerance, measurement locations and sampling procedure.

Local thickness variation near the seal can affect performance, but a single micrometer reading cannot characterize an entire bag or production lot.

Sealing conditions

Seal formation can be influenced by temperature, contact time, pressure, cooling, line speed, sealing-surface condition and film tension. Appropriate settings depend on the equipment and the specific film construction.

Both insufficient and excessive heat input can produce an unacceptable seal. Machine display values should therefore be treated as process settings, not as proof of finished-bag performance.

Seal design and contamination

Seal width, alignment, folds, wrinkles, gussets, zipper placement and contamination in the sealing area may affect load distribution. Oil, powder, fibres or packed-product residue can also interfere with sealing when a bag is closed after filling.

Packed load and distribution conditions

A seal that is adequate for an empty-bag pull check may not be adequate for the packed application. Product weight, sharp edges, headspace, compression, drop height, vibration, temperature, handling method and carton configuration should be considered.

How to investigate a side-seal failure

1. Preserve representative samples. Retain failed bags, unused bags from the same lot, cartons and relevant labels.
2. Record the failure location. Distinguish a seal opening from film tearing adjacent to the seal, puncture damage or closure failure.
3. Confirm the approved specification. Review dimensions, material structure, nominal thickness, tolerance, seal design and intended load.
4. Identify the production lot. Link samples to the applicable production, inspection and packing records where available.
5. Review use conditions. Document the packed product, fill weight, closing method, carton configuration, handling and point at which failure was observed.
6. Repeat an agreed test. Compare failed, retained and reference samples using the same conditioning, equipment and procedure.
7. Document containment and corrective action. Separate affected stock and record the confirmed cause, correction and verification evidence.

Choose an appropriate verification method

The test method should match the intended decision. Depending on the bag and application, evaluation may include:

• visual inspection for wrinkles, incomplete seals, contamination or misalignment;
• seal-strength testing under an agreed method and conditioning procedure;
• leak, burst or internal-pressure testing where relevant to the construction;
• tensile, tear or puncture testing of the film;
• a loaded-bag lift, drop, compression or handling trial;
• a packed-product distribution trial for higher-risk applications.

A hand-pull check can identify an obvious difference, but it is not a controlled method for determining resin composition, quantifying seal strength or approving an entire lot.

Define acceptance criteria before production

There is no universal rule that every seal must be a fixed multiple of the bag’s rated load. The required performance depends on the product, bag construction and use conditions.

An order-specific quality agreement may define:

• the applicable test method and equipment;
• sample conditioning;
• sampling locations and sample quantity;
• minimum, average or other acceptance limits;
• the permitted failure mode;
• inspection frequency and lot-release records;
• requirements for packed-product validation.

Results apply to the tested sample, construction and conditions. They should not be generalized automatically to different dimensions, gauges, formulations, loads or distribution environments.

Discuss a side-seal packaging specification

Flexon Pack can review custom PE bag dimensions, material options, thickness, seal design, packed load and proposed inspection criteria. Final specifications, test methods and acceptance limits should be confirmed for the individual order.

Contact Flexon Pack to discuss a side-seal application or request production-representative samples for evaluation.