Analysis of Causes for Soft Packaging Bag Damage

How to avoid damages to the finished products under sachet packing machine

Soft packaging refers to multi-layer composite films made from materials such as polyester, aluminum foil, and polyolefin, bonded together with adhesives through heat-sealing and cut into flexible containers of specific sizes. These are widely used for packaging food, pharmaceuticals, daily chemicals, and agricultural products.

Common Scenarios of Soft Packaging Bag Damage

Damage to soft packaging bags is one of the primary issues affecting packaging quality. The Production process of sachet packaging machine includes the following steps: custom packaging film → bag formation from rolled film → filling → vacuum sealing → heat-sealing → inspection → intermediate packaging → boxing.

Soft packaging bags are most prone to damage in the following scenarios:

First, during the filling process, the contents can exert a strong impact on the bottom of the bag. If the bottom cannot withstand this force, it may crack, compromising both production cleanliness and filling efficiency. Second, during stacking or transportation, if the bag cannot endure the increased internal pressure caused by stacking or other factors, it may rupture. Issues such as inadequate pressure resistance, high-temperature tolerance (for retort bags), flexibility (for vacuum-sealed bags), freeze resistance (for frozen packaging), or puncture resistance can also lead to bag breakage.

Analysis of Causes for Soft Packaging Bag Damage

1. Damage During the Filling Process

In modern sachet packing production lines, soft packaging bags are formed and filled almost simultaneously. As a result, the heat-sealed bottom of the bag may not have fully cooled when the contents are filled, significantly reducing its ability to withstand impact.

The term "heat-seal strength" typically refers to the bonding strength of two films after heat-sealing and complete cooling. However, on production lines, materials often lack sufficient cooling time. Therefore, heat-seal strength is not suitable for evaluating material performance in this context. Instead, hot tack strength—the peel force of the heat-sealed area before cooling—should be used for material selection.

Testing and Application of Hot Tack Strength:
The relationship between hot tack strength and heat-sealing temperature/time follows a general pattern. There is an optimal temperature point for achieving the best hot tack strength; exceeding this temperature reduces the strength. Additionally, at the same temperature, extending the heat-sealing time increases hot tack strength. For example, at a given temperature, a sealing time of 0.3 seconds yields significantly lower hot tack strength compared to 0.5 seconds.

In practice, the impact force exerted on the bag bottom during filling should be calculated. A hot tack tester can then be used to adjust heat-sealing temperature, pressure, and time to plot a hot tack curve. Based on this data and production line conditions, the optimal heat-sealing parameters can be selected.

2. Bag Damage During Storage

Except for vacuum-sealed bags, soft packaging bags are prone to rupture during storage or transportation due to increased internal gas pressure caused by stacking. Since ruptures often occur at heat-sealed edges, the issue can be mitigated by testing the heat-seal strength of these edges and adjusting materials or heat-sealing parameters.

Heat-seal strength can be categorized into tensile heat-seal strength and burst heat-seal strength. Tensile heat-seal strength, measured with a tensile tester, evaluates the resistance of the heat-sealed area to separation under uniform force. While useful for assessing bag opening performance, it is less applicable for addressing rupture issues, as internal pressure direction and magnitude are unpredictable. Burst heat-seal strength, tested with a leak and seal strength tester, is more suitable. This method pressurizes the bag internally to identify weak points (not limited to heat-sealed areas).

Testing and Application of Burst Heat-Seal Strength:
The tester measures the maximum rupture force and can simulate rupture times under specific pressures. Operators can use this data to optimize stacking designs, adjust heat-sealing parameters, or analyze structural weaknesses in the packaging. For highly flexible materials (e.g., those stretchable to several times their original length), a constrained plate test device may be needed to limit expansion during testing.

3. Retort Bag Damage Issues

For cooked food packaging, Raoult’s Law states that the vapor pressure of a liquid in a confined space depends on the amount of the substance. For example, 1 mole of liquid vaporized in 22.4 L of space exerts 1 atm of pressure. Standards like GB/T 10004-1998 (Retort-Resistant Composite Films and Bags) require filling bags with solutions (e.g., 4% acetic acid, 1% sodium sulfide, 5% sodium chloride) or refined vegetable oil, followed by degassing, sealing, and retort testing. Despite precise temperature control and counterpressure during cooling, bursting still occurs, indicating higher internal than external pressure.

Key Observations:
(1) For water-filled bags: If air is not fully removed before sealing, heating from room temperature to 98°C causes water expansion, air expansion, and steam formation, increasing internal pressure. This pressure, combined with polymer creep at heat-sealed edges, leads to bursting.
(2) For retort bags: Similar expansion occurs below 100°C. Above 100°C, internal pressure always exceeds external pressure in retort chambers.

Preventive Measures for Retort Bag Damage:

① Material Selection:

• Ensure sufficient thickness; thin materials compromise strength.

• Limit recycled material content, as it weakens the composite film.

• Blend LLDPE or mPE into PE films to enhance strength/flexibility.

• For frozen environments, use low-temperature-resistant materials (e.g., BOPA).

② Bag-Making Process:

• Avoid excessive heat-sealing temperatures, which degrade material performance.

• Excessive pressure during sealing squeezes out sealing material, weakening edges.

• Ensure heat-sealing bars are smooth; sharp edges can damage bags.

③ Composite Film Peel Strength:

• Low peel strength prevents even force distribution across layers, causing rupture during drops.

• For retort bags, use heat-resistant adhesives to prevent delamination (e.g., in BOPA15/AL7/CPP70 structures).

④ Other Considerations:

• For heavy products, evacuate air before sealing to reduce wall stress.

• Verify puncture resistance, pressure tolerance, drop resistance, temperature stability, and food safety compliance.

  
  

  
  

  
  

  
  

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Key Notes

Adjusting heat-sealing parameters based on material testing can reduce damage during filling, storage, or transportation. However, note the following:

(1) Contamination Risk: Ensure filling materials do not contaminate seals, as pollutants drastically reduce hot tack or heat-seal strength.

(2) Safety Margin: Design parameters should exceed theoretical requirements to account for material/production inconsistencies.

(3) Parameter Optimization: Combine hot tack and burst heat-seal test data to select the best heat-sealing settings for specific products and lines.