In the food packaging industry, the integrity of packaging bags is the * line of defense to ensure product safety, extend shelf life, and win consumer trust. However, in the production and circulation process, small leaks caused by fluctuations in heat sealing technology, material stress, or external damage are not uncommon. Although these microscopic defects are difficult to distinguish with the naked eye, they can become channels for microorganisms and oxygen invasion, directly leading to the deterioration of the contents. Therefore, establishing a scientific and rigorous sealing testing scheme has become a core link in packaging quality control in the fields of food, medicine, and daily chemical products.

The root cause of sealing failure and the national standard testing framework

The sealing failure of packaging bags usually originates from two aspects: one isThe barrier performance of the material itself is insufficientThis needs to be addressed through material selection and structural combination during the packaging design phase; SecondThe overall sealing integrity of the packaging has been compromisedThere is a physical leakage channel. The latter is the focus of attention in the production and quality inspection process, and its leakage points mainly include: those caused by improper temperature, pressure, or time in the heat sealing areaLeakage and weak sealingSharp contents or transportation wear and tearPress through, pinholeAnd the inherent existence of composite materials themselvesMicrocracks or pores.

To scientifically evaluate the integrity of seals, China has developed and improved a corresponding national standard system. among which,GB/T 15171 "Test Method for Sealing Performance of Flexible Packaging Components"It is a widely adopted core standard. This standard specifies the basic requirements and testing methods for testing equipment, providing a legal basis for objectively and uniformly evaluating the sealing performance of packaging.

Core detection methods: vacuum attenuation method and water bubble method

According to the GB/T 15171 standard, the mainstream sealing testing method is based on the principle of "internal and external pressure difference", and is judged by monitoring the leakage performance of the packaging under a specific pressure difference. Here are two commonly used methods:

1. Vacuum attenuation method
This is a non-destructive, quantitative and precise testing method, especially suitable for detecting extremely small leaks.

• Test PrinciplePlace the tested packaging in a sealed chamber of a specialized sealing tester. The instrument evacuates the chamber to a pressure lower than the normal pressure (or set pressure) inside the packaging, thereby forming a stable pressure difference inside and outside the packaging.

• Leakage determinationHigh precision pressure sensors continuously monitor pressure changes within the testing chamber. If there is a leakage in the packaging, external air will enter the cavity through the leakage point, causing the pressure inside the cavity to rise at a rate exceeding the predetermined threshold. By analyzing the slope of the pressure time curve, the instrument can accurately determine whether there is a leak and estimate the size of the leak aperture based on the model.

• Method advantages：Objective, quantitative, no need for immersion in waterIt can be digitally recorded and judged, with high repeatability, suitable for various forms of rigid, semi-rigid, and flexible packaging (such as bottles, bags, boxes).

2. Bubble method in water (negative pressure method)
This is an intuitive, economical, and widely used qualitative testing method, which is one of the important methods specified in the standard.

• Test PrincipleImmerse the packaged sample in the transparent observation liquid of the sealed tester water tank. Vacuumize the enclosed space above the water tank to reduce the external pressure of the packaging submerged in water. If there is a leak in the packaging, the internal gas will escape through the leak point, generating a continuous stream of bubbles in the water.

• Leakage determinationDetermine the presence or absence through visual observation or auxiliary camera recordingContinuous and stable bubble flowGenerated. A single isolated bubble is usually considered as gas adhering to the surface of the sample and is not considered a leak.

• key equipmentThe testing instrument that meets the standards should include a transparent vacuum chamber, a sample fixture, a vacuum system, a pressure regulating and measuring device (with a vacuum gauge accuracy of not less than 1.5 levels), and a control system.

• Method advantages：Intuitively display the location of the leakage pointThe equipment cost is relatively low, the operation is simple, and it is suitable for rapid screening of flexible packaging bags.

Guidelines for Standardized Testing Process Operations

To ensure the reliability and reproducibility of the test results, standardized operating procedures should be strictly followed. Taking the routine testing based on GB/T 15171 as an example:

Step *: Sample Preparation and Device Inspection
Confirm that the packaging sample to be tested is intact and undamaged. Inject an appropriate amount of clean water or specialized observation liquid into the transparent vacuum chamber of the sealed tester, with a liquid level high enough to submerge the sample.

Step 2: Parameter Setting and Sample Placement
Set appropriate values on the instrument based on the strength of packaging materials, product standards, or customer requirementsTest vacuum degreeandholding timeImmerse the sample in water and cover it with a sealed lid. For lightweight packaging bags, it is necessary to use sample fixtures to fix them and prevent them from floating up and affecting observation.

Step 3: Conduct testing and observation
Start the vacuum program. The pressure in the vacuum chamber will reach and stabilize at the preset value. During this period and the subsequent pressure holding phase, it is necessary toCarefully observe the packaging surface, especially the heat sealed edge areaSearch for a continuous source of bubbles.

Step 4: Result determination and recording
If no continuous bubbles are observed during the pressure holding period, the sample is judged to be "well sealed". On the contrary, record it as a "leak" and label the approximate location of the leak. After the test is completed, slowly release the vacuum and remove the sample. A sufficient number of samples should be taken from each batch for testing to ensure statistical validity.

Common technical questions and answers for sealing testing

1. Question: How to set a reasonable testing vacuum degree based on product characteristics?
Answer: Testing vacuum degree requires a balance between "effectively detecting leaks" and "avoiding abnormal packaging rupture". For inflatable packaging of puffed foods, potato chips, etc., a lower initial vacuum degree (such as -20kPa to -30kPa) should be used based on their inflation pressure. For ordinary vacuum packaging or solid food packaging, the range of -30kPa to -90kPa is commonly used. A reliable method is based on the specific industry standards of the product or determined through consultation with the customer.

2. Question: Is it considered qualified if a single bubble or intermittent bubble appears during testing?
Answer: According to the spirit of GB/T 15171 standard and industry practice,Single or intermittent, discontinuous bubbles are usually not considered as packaging leaksThis may be due to the release of gas adsorbed on the surface of the material. The key indicator for determining leakage is the presence ofFixed position, continuously generated bubble flowIf it cannot be determined, it can be retested at the same location or retested using a higher precision vacuum attenuation method.

3. Question: In addition to sealing, what other tests should a complete food packaging quality control system include?
Answer: Sealing is only one aspect of packaging safety. A complete evaluation system should be multidimensional:

• Barrier performance testThe use of oxygen and moisture permeameters to measure the material's ability to block oxygen and water vapor directly affects food oxidation and moisture.

• Physical and mechanical strength testingUsing an intelligent electronic tensile testing machine to test materialsTensile strength, peel strength, heat sealing strengthEnsure that the packaging is not easily damaged during transportation and stacking.

• Other specialized testsSuch as headspace gas analysis inside the packaging (monitoring oxygen/carbon dioxide content), friction coefficient testing (evaluating packaging machine film performance), impact resistance testing, etc.

4. Question: How to conduct sealing testing for irregular packaging or large packaging that cannot be immersed in water?
Answer: For non-standard or large packaging, the following alternative solutions can be adopted:

• Local testing methodCut the key sealing parts of the packaging (such as bottle mouth and easily bendable heat sealed edges) into small samples for testing.

• Vacuum chamber customizationCollaborate with instrument suppliers to customize large or irregular vacuum chambers with matching dimensions.

• High sensitivity pressure attenuation methodThis is an alternative solution as it does not require immersion in water and can adapt to weak leak detection in large volume packaging by improving sensor accuracy.

DisclaimersThe purpose of this article is to introduce the general technical principles and standard methods for testing the sealing of food packaging bags, for learning and reference purposes only. Please refer to the official version of the national standard (GB/T) cited in the text. The specific product testing methods, qualification criteria, and instrument operation should strictly follow the relevant product standards, regulatory requirements, and equipment manufacturer's operating procedures. If you have any questions or find any areas in the article that need to be discussed, please feel free to communicate with us.