Тест прочности шва гибких упаковочных пакетов для пищевой упаковки
Важность тестирования прочности шва: качество термосваривания, герметичность, разрыв, стабильность при транспортировке, совместимость с машиной.
Introduction
In flexible packaging manufacturing, the heat seal is the most critical functional area of a pouch or roll film package. If the seal is too weak, the bag may leak, burst during transport, fail on the filling line or allow moisture and air to enter — all of which can lead to product loss, customer complaints and brand damage.
Seal strength testing is the process of evaluating how well the sealed area of a flexible packaging bag resists pulling force, internal pressure and handling stress. It is one of the most frequently performed quality checks in food packaging production, alongside visual inspection and barrier testing.
What Is a Seal Strength Test?
A seal strength test measures the force required to separate a heat-sealed joint under controlled conditions. The most commonly referenced standard is ASTM F88 — Seal Strength of Flexible Barrier Materials, which describes a method where a test strip cut from the sealed area is pulled apart at a constant speed and the peak force is recorded.
The result is typically expressed in N/15mm (newtons per 15 millimeters of seal width). For different product types and packaging formats, acceptable seal strength ranges vary. A lightweight dry snack pouch may need lower seal strength than a heavy liquid-filled spout pouch. The supplier should be able to share typical reference ranges based on the material structure, bag type and product application.
Seal strength can be checked during production quality control, and third-party laboratory testing can be arranged when buyers or target markets require formal test reports.
Why Seal Strength Matters for Food Packaging
Seal strength directly affects multiple aspects of packaging performance. Weak seals increase leakage risk, especially for liquid, sauce or powder products. Air ingress through weak seals accelerates oxidation in coffee, nuts and oily snacks, reducing shelf life. Moisture absorption through seal gaps causes clumping in dry powders and texture changes in crispy snacks.
Pouch bursting can occur when internal pressure from filling, stacking or transport exceeds the seal strength. For products shipped over long distances, seals must withstand vibration, temperature changes and stacking pressure. On automatic packing lines, inconsistent seal strength can cause machine stoppages, product waste and reduced production efficiency.
Common Causes of Weak Seals
Multiple factors can contribute to insufficient seal strength. Incorrect sealing temperature — too low and the seal layer does not fully activate; too high and the film may burn or distort. Insufficient sealing pressure prevents proper bonding between the seal layers. Short dwell time does not allow enough heat transfer for a complete seal. Using the wrong sealing layer material for the product or machine can lead to poor compatibility.
Contamination on the sealing area — from product residue such as oil, powder or liquid — physically blocks the seal layers from bonding. Inconsistent film thickness across the web can create weak spots. Poor lamination stability between layers may cause delamination near the seal area. Machine setting mismatch between the film specification and the actual packing machine parameters is another common cause of seal problems on production lines.
Seal Strength for Different Packaging Types
Different pouch types and packaging formats have different sealing requirements. Understanding how sealing performance varies by format helps buyers ask the right questions during supplier evaluation and pre-production sampling.
Three Side Seal Pouch
Three side seal pouches have seals on three edges, making seal strength and consistency especially important. Common issues include edge seal weakness, tear notch area integrity, and powder or seasoning contamination in the seal zone. For sachet, sample pack and small portion packaging, even a small seal defect can affect the entire batch.
Buyers of three side seal pouches should confirm seal width, seal strength reference range and whether the material is compatible with powder or granular filling, which can leave residue in the seal area.
Stand Up Zipper Pouches
Stand up zipper pouches have multiple seal areas — bottom gusset seal, side seals and the zipper attachment area. The bottom gusset must support the full weight of the product when the pouch stands on the shelf. Zipper seals need to withstand repeated opening and closing without delamination.
For products such as nuts, candy and dry snacks, the combination of seal integrity and zipper function determines the consumer experience. Testing should cover both the pouch body seals and the zipper seal performance over multiple open-close cycles.
Spout Pouches
Spout pouches present unique sealing challenges because the rigid spout fitment must bond to the flexible pouch film. The spout welding area is often the most critical seal point — if the spout-to-film bond is weak, liquid can leak during filling, transport or consumer use. Liquid pressure, cap tightening torque and drop impact all stress the spout seal area.
For juice, sauce, puree, beverage and refill packaging, buyers should confirm spout position, cap type, spout diameter and whether the supplier performs spout weld integrity checks as part of production quality control.
Retort Pouches
Retort pouches must maintain seal integrity under high-temperature sterilization conditions — typically 121C to 135C under pressure. The sealing layer must withstand not only the sterilization temperature but also the internal pressure generated during the retort process.
After retort processing, seals can weaken if the material structure or sealing parameters were not designed for retort conditions. Common post-retort seal issues include reduced seal strength, seal edge waviness and delamination near the seal zone. Buyers of retort pouches should confirm that the material structure and sealing specification are suitable for the target sterilization temperature and process.
Food Packaging Roll Film and Machine Compatibility
For roll film used on automatic VFFS and HFFS packing machines, sealing performance depends on more than material structure. The heat seal window — the temperature range within which the film seals reliably — must match the machine sealing jaw temperature, speed and pressure.
Not every film runs well on every packing machine. Film thickness, sealing layer type, coefficient of friction (COF), film tension and eye mark registration accuracy can all affect sealing consistency at production speed. Before bulk production, buyers using automatic packing machines should share their machine model, sealing temperature range, film width and machine speed so the film specification can be reviewed for compatibility.
How Buyers Can Reduce Seal Failure Risk
Buyers can take practical steps to reduce seal-related quality risks: confirm product type and filling weight before material selection; choose a material structure suitable for the product and packing method; test sealing performance on pre-production samples before committing to bulk production; provide machine model and specifications if using automatic roll film; confirm storage and transport conditions so the material can be specified accordingly; and check samples before mass production to verify sealing quality under real filling conditions.
When changing material structure, bag size or packing machine parameters between orders, re-checking seal performance helps avoid unexpected problems on the production line.
What to Prepare Before Quotation
To help the supplier recommend a suitable material structure and sealing specification, buyers should prepare: product type, bag type or roll film type, bag size, filling weight, material structure preference if known, thickness if known, estimated order quantity, printing colors and artwork file, packing machine type and model if applicable, storage conditions, and destination country or region.
If you are not sure about the material structure or sealing layer, send us your product details and packing machine information, and our team can help review suitable options.
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Часто Задаваемые Вопросы
What is a seal strength test for flexible packaging bags?
A seal strength test checks how well the sealed area of a pouch or film resists pulling, pressure and handling stress. It helps buyers understand whether the package can remain stable during filling, storage and transportation. ASTM F88 is a commonly referenced method for this type of testing.
Why is heat seal strength important for food packaging?
Heat seal strength affects leakage risk, moisture protection, air ingress and transport stability. A weak seal can cause product loss, shelf life problems or customer complaints. For liquid, powder and long-shelf-life products, consistent seal strength is particularly important.
What causes weak seals in flexible packaging?
Common causes include unsuitable sealing temperature, insufficient pressure, short dwell time, wrong sealing layer, contamination on the sealing area, inconsistent film thickness, poor lamination stability and mismatch with the packing machine parameters.
Is seal strength important for automatic roll film packaging?
Yes. Roll film must match the sealing temperature, machine speed, sealing jaw design, film tension and cutting accuracy of the automatic packing machine. The heat seal window of the film should be compatible with the machine specifications for reliable performance at production speed.
Do spout pouches need special sealing checks?
Yes. Spout pouches need attention to both the pouch edge seals and the spout welding area. The spout-to-film bond is often the most critical seal point, especially for liquid, sauce or beverage products where leakage can occur under pressure or during transport handling.
What information should buyers provide before quotation?
Buyers should provide bag type, size, filling weight, material requirement, quantity, printing colors, artwork file, packing machine details if roll film is required, storage conditions and destination country. This helps the supplier recommend a suitable material structure and sealing specification.