1. Definition of sealing strength
Sealing strength, also commonly known as heat sealing strength or sealing strength in the field of packaging engineering, is a core indicator to measure the quality of the sealing part of flexible packaging materials. It specifically refers to the maximum tensile force that can be withstood by the sealing interface per unit width, usually in Newtons per 15 millimeters (N/15mm) or Newtons per inch (N/in). This parameter intuitively reflects the ability of the packaging seal to resist external force opening, and is a key guarantee to ensure that the contents are not leaked or contaminated during storage, transportation and sales. Sealing strength is not an isolated physical constant, but a comprehensive embodiment of material properties, heat sealing process conditions and test methods.
2. Test principle of sealing strength
The principle of testing seal strength is based on tensile testing in material mechanics. The core idea is to simulate various tensile stresses on packaging seals in actual use, such as the impact of the contents on the seal during handling or the tearing force when the consumer opens the package. During the test, the two ends of the prepared specimen with a standard width (usually 15.0 mm or 25.4 mm) and a sealing area are clamped on the upper and lower fixtures of the electronic tensile testing machine. The fixture moves in the opposite direction at a constant speed (typically 300 mm/min ± 30 mm/min), applying a gradually increasing tensile load to the specimen sealing site. The instrument records the change of force value throughout the stretching process in real time through a high-precision force value sensor. When the tensile force acts on the sealing interface, the stress is concentrated in the sealing area. The test curve usually goes through a phase where the force value rises rapidly until the seal begins to break, at which point the maximum force value recorded is the sealing strength of the specimen.
3. Measurement method of sealing strength
The measurement of seal strength follows strict standardized procedures to ensure comparability and reproducibility of results. The measurement process mainly consists of the following stages:Sample preparationIt is the first link, which needs to be sealed face to face with two layers of film of the same material at the set temperature, pressure and time on a standardized heat sealing tester, and then cut into standard specimens with a width of 15 mm using a special sampling knife. The specimen sealing area should be in the middle, and the edges should be flat without gaps.State adjustmentIt is critical that the prepared specimen is placed under standard atmospheric conditions (temperature 23°C ± 2°C, relative humidity 50% ± 5%) for at least four hours to eliminate the stress and humidity effects introduced by the sample preparation process.Tested on the machineThe two ends of the specimen are clamped on the fixture of the tensile testing machine to ensure that the longitudinal axis of the specimen coincides with the direction of the force, and the sealing area is located in the middle of the two fixtures. Start the testing machine and record the maximum force that the specimen can withstand at the sealing site during the stretching process. For each sample, it is usually necessary to test at least five valid specimens and calculate their arithmetic average as the final result.
4. Calculation formula of sealing strength
Based on the data directly collected by the test, the calculation of seal strength is a relatively straightforward process. The basic calculation formula is as follows:
S = Fmax / W
Among them:
S — sealing strength in Newtons per unit width (e.g., N/15mm or N/25.4mm);
Fmax — The maximum force value recorded during the stretching of the specimen seal in Newtons (N);
W — The initial width of the specimen in millimeters (mm).
In the actual test report, it is usually directly expressed in the form of "XX N/15mm". It should be noted that this formula calculates the tensile destructive force of the sealing interface perpendicular to the sealing direction. In some studies and more in-depth analyses, the pattern of seal failure is also focused, such as whether it is the separation of the sealing interface, tearing of the material body (breaking after stretching and thinning of the heat sealing layer), or the heat-affected zone that occurs at the edge of the heat seal. The observation of the failure mode and the value of sealing strength together constitute a comprehensive evaluation of sealing quality.
5. The main factors affecting the measurement results of sealing strength
The interaction of sealing strength can be summarized into three categories: material characteristics, heat sealing process parameters and test conditions.Material propertiesIt is the foundation, including the resin grade, thickness of the film, the content of additives (such as slip agents, anti-adhesion agents), and the peel strength between the layers of the composite film, all of which directly affect the formation and strength of the seal.Heat sealing process parametersIt is a key variable: too high temperature may cause the film to melt excessively or even burn, forming a false seal or causing the seal to become brittle; If the temperature is too low, the interface cannot be fully fused. The pressure is too small to make the material fit tightly; Excessive pressure may extrude the melt and reduce the sealing strength. The heat sealing time determines the degree of heat transfer and molecular chain diffusion. These three parameters are interrelated and together determine the final quality of the closure.Test conditionsEqually important: the gripping force and parallelism of the fixture, the tensile speed, the ambient temperature and humidity, and the way the specimen is held all have a systematic impact on the measurement results.
6. Application of sealing strength in the industrial field
As a core indicator for evaluating packaging sealing reliability, sealing strength has a wide range of key applications in multiple industries. inFood & Pharmaceutical PackagingIt is a key barrier parameter to ensure product shelf life and prevent microbial intrusion, whether it is retort pouches, stand-up pouches or blister packaging, it must have sufficient and stable sealing strength. inMedical device packagingSeal strength is not only about the integrity of the sterile barrier system, but also about the ability of the end user (e.g., operating room nurses) to open smoothly without particulate contamination during aseptic operations. inMaterial R&D and quality controlsealing strength testing is used to optimize the formulation of heat sealing resins, evaluate the performance consistency of different batches of laminated films, and set the appropriate heat sealing process window for packaging production lines. In addition, inDaily chemical product packagingIn the field, such as shampoo bags, laundry detergent stand-up bags, etc., the sealing strength is directly related to whether the product will leak from the seal during extrusion and use.
7. Summary
In summary, seal strength is much more than a simple test data, it is both the guardian of packaging integrity and the bridge between material science, processing technology, and end-user experience. From definitions and principles to rigorous measurement methods to meticulous consideration of many influencing factors, every aspect reflects a deep understanding of packaging functionality. For packaging engineers, quality controllers, and material developers, a deep understanding of sealing strength means being able to more scientifically predict the performance of packaging in circulation, more accurately diagnose quality problems such as leakage, and optimize production processes more reliably. It is through continuous research and strict control of the basic and critical indicator of seal strength that we can ensure that every well-packaged product, from everyday food to high-value pharmaceuticals, reaches consumers safely and reliably.