Sealing temperature is the most critical yet often overlooked parameter in the process of producing flexible packaging bags. Incorrect setting of sealing temperature can result in weak seals causing leakage or fluid loss, or even burn through the film leading to complete batch rejection. Different materials require significantly different sealing temperatures, with PE, PP, CPP, metallized film, and aluminum composite film each having their own unique characteristics. This article systematically explains the principles of sealing temperature, the typical sealing parameter ranges for common materials, and key quality control points during production, helping buyers and process engineers avoid common pitfalls.
Heat sealing principle: three elements – temperature, pressure, and time
Heat sealing involves using heat to melt the heat-seal layers of two film layers, allowing the molecular chains to interpenetrate under pressure and fuse together upon cooling. Heat sealing temperature, pressure, and time are interdependent: higher temperatures require less pressure and time; however, if the temperature exceeds the material’s tolerance limit, the film will shrink or burn through, thereby reducing the seal strength. Therefore, higher heat sealing temperature is not always better. Instead, the optimal balance point should be found within the material’s melting window.
The material of the heat-seal layer determines the lower limit of the heat-sealing temperature. Among common heat-seal layer materials, LDPE has a melting point of 105 to 115°C, with heat-sealing temperature generally ranging from 120 to 150°C; CPP has a melting point of 140 to 160°C, with heat-sealing temperature typically between 150 and 180°C; whereas materials like PET and PA cannot be heat-sealed themselves and must be combined with a heat-seal layer to form bags. This is why the sealing process for steam-sterilized bags is more precise than for ordinary bags—it must withstand high-temperature sterilization while ensuring sufficient sealing strength.
Reference Thermal Sealing Temperature Parameters for Common Materials
The following are commonly used reference temperature ranges for heat sealing in flexible packaging. In actual production, the temperature should be adjusted according to film thickness, machine speed, and sealing method: PE film heat sealing temperature is 120 to 150°C, suitable for three-side sealing and back-sealed bags; CPP film heat sealing temperature is 150 to 180°C, suitable for high-temperature and steam sterilization applications; metallized film composite bags require heat sealing temperature of 130 to 160°C, with attention to avoid damage to the metallized layer at the sealing area; aluminum foil composite film requires heat sealing temperature of 140 to 170°C, offering high sealing strength but prone to aluminum foil breakage, requiring pressure control; paper-plastic composite bags require heat sealing temperature of 120 to 150°C, with excessive temperature causing yellowing of the paper.
It should be noted that the above parameters are just a starting point. In actual production, the heat sealing temperature must be adjusted according to the machine speed: the faster the machine speed, the shorter the time the film passes through the heat sealing blade, so the temperature should be increased accordingly; conversely, if the machine speed is slower, the temperature should be reduced. The sealing method also affects the parameters. Three-side sealing, four-side sealing, and eight-side sealing have different requirements for the contact time and pressure distribution of the heat sealing blade.
High or low heat sealing temperature can lead to adverse effects.
When the heat sealing temperature is too low, the heat-sealing layer is not fully melted, resulting in insufficient seal strength. The package can be easily torn apart with a light pull, and it is prone to leakage after inflation or vacuum packaging. This is one of the most common customer complaints in packaging printing and bag-making processes. When the heat sealing temperature is too high, the film becomes excessively melted and shrunk, causing wrinkles and scorching on the sealed area. In severe cases, a hardened and brittle crack zone may form at the edge of the seal, leading to fragments from the seal falling into the food when consumers open the package.
To determine if the heat sealing temperature is appropriate, factories usually conduct two tests: the first is the seal strength test, where a tensile tester measures the peel strength of the seal, and it is consideredQualified only if it meets the material standards; the second is the appearance inspection of the seal, observing whether the sealed area is flat and checking for bubbles or scorch marks. Before mass production, a sample is first tested to confirm the parameters, and then the machine is used for mass production, which is the most effective way to control the quality of heat sealing.
Heat Sealing Quality Control in Production and Common Misconceptions
The core of heat sealing quality control lies in temperature uniformity and stability. The temperature difference between the ends and the middle of the heat sealing blade should be controlled within ±5°C; otherwise, the sealing strength on both sides of the same bag will be inconsistent. During production, the surface temperature of the heat sealing blade should be measured regularly with a thermometer, rather than relying solely on the displayed value from the equipment. Additionally, the film absorbs moisture during storage, which reduces its heat sealing performance. It is recommended to store the film in a constant temperature and humidity environment for more than 24 hours before production.
For bag types and processes, you may also refer to our previously published content.Three-Side Sealing Bag ExplanationandHigh Temperature Steam Sterilization Bag Material Combination SolutionsUnderstand the specific requirements of different bag types for heat sealing. If your product has special requirements for the seal strength, it is recommended to communicate directly with the factory to verify the heat sealing parameter validation plan.
Need custom packaging bags?Consult Huashu Packaging NowOur process team will recommend suitable heat-sealing materials and parameters based on your product characteristics.
For more information on flexible packaging solutions, please visitOm Packaging Chinese Website。