How to Optimize Vacuum Packaging by Adjusting Heat Sealing Temperature and Heat Sealing Time
In vacuum packaging production, seal leakage is one of the most common and frustrating quality issues. Even when the vacuum level is adequate and the seal appears smooth and intact, packages may begin to leak slowly—or even split open—after transportation, stacking, or storage.
In most cases, the root cause is not the packaging film itself but the improper combination of heat sealing temperature and heat sealing time. Heat sealing temperature determines whether the sealant layer can melt sufficiently, while heat sealing time determines whether the polymer chains have enough time to diffuse across the interface and form molecular entanglements. These two parameters are closely interdependent. Only by finding the optimal balance can manufacturers achieve strong, reliable, and leak-proof seals.
1. Heat Sealing Temperature and Heat Sealing Time: The Golden Pair for Seal Quality
The essence of heat sealing is the interdiffusion and entanglement of polymer chains between two film layers under heat and pressure. After cooling, these molecular networks solidify to create a durable seal.
During the sealing process, low-molecular-weight polymer chains melt and diffuse first, acting as "pioneers" that initiate interfacial bonding. Longer molecular chains then follow, creating a stronger entangled network. Heat sealing temperature determines whether the sealant layer reaches a sufficiently molten state, while heat sealing time determines how long the molecular chains have to complete this process.
When the heat sealing temperature is too low, the sealant layer does not melt completely. Polymer chains remain insufficiently mobile, allowing only a small number of short chains to diffuse across the interface before cooling occurs. Although the seal may appear bonded on the surface, its peel strength is extremely low and it can be opened easily by hand. This condition is commonly known as a false seal (cold seal). Such seals are highly susceptible to cracking during transportation or pallet stacking, ultimately leading to package leakage.
When the heat sealing temperature is too high, the sealant layer becomes excessively molten. The film in the sealing area thins as molten material is squeezed out of the seal zone, resulting in a phenomenon commonly referred to as heat cutting. In mild cases, the seal edges become brittle and impact resistance decreases. In severe cases, the film may burn through completely. Excessive temperature may also cause the outer film layer to shrink and deform, creating wrinkles that negatively affect both package appearance and sealing performance. Therefore, the heat sealing temperature should remain below the film's heat shrink initiation temperature to prevent deformation.
Heat sealing time also requires precise control. If the sealing time is too short, polymer chains cannot fully diffuse and entangle, even if the temperature is adequate. If the sealing time is too long, the resin at the seal may oxidize and degrade, reducing seal strength while increasing the risk of wrinkles and deformation.
The key principle is simple: use the shortest possible heat sealing time while maintaining the required seal strength. This not only improves production efficiency but also minimizes thermal shrinkage and keeps seals flat and visually appealing.
2. The Relationship Between Heat Sealing Temperature and Heat Sealing Time
In actual production, heat sealing temperature, heat sealing time, and sealing pressure are three interdependent parameters. Proper adjustment of one parameter can often compensate for the limitations of another.
When the heat sealing temperature cannot be increased because of the limited heat resistance of the film's outer layer, extending the heat sealing time allows more heat to penetrate the film and reach the sealant layer.
Conversely, when high-speed packaging lines require shorter sealing times—for example, reducing the dwell time from 1.0 second to 0.5 second—the heat sealing temperature must be increased to ensure that the sealant layer melts sufficiently within the shorter cycle.
According to industry technical data, for every additional 25 μm of film thickness, the required heat sealing time should increase by approximately 50%. Thicker films require more time for heat to transfer from the sealing bar to the inner sealing interface. If production speed is fixed and sealing time cannot be extended, increasing the heat sealing temperature becomes necessary to compensate for slower heat transfer.
Sealing pressure, the third critical parameter in heat sealing, should not be overlooked. Insufficient pressure prevents complete contact between the film layers, leaving air gaps that inhibit polymer chain interaction. Excessive pressure, however, squeezes molten resin out of the sealing area, thinning the sealant layer and producing brittle seals that crack more easily. The ideal sealing pressure ensures full interfacial contact without excessive material displacement.
3. Typical Problems Caused by Heat Sealing Temperatures That Are Too Low or Too High
In production, incorrect heat sealing temperature generally results in one of two opposite failure modes.
When the sealing temperature is insufficient, the sealant layer does not melt completely, preventing adequate molecular entanglement. Although the seal appears bonded, its strength is extremely low. The seal can be torn apart easily by hand, leaving a smooth fracture surface without any signs of film stretching.
The risk becomes even greater when packaging products containing powders, liquids, or oils, where small particles can contaminate the sealing area. Case analysis has shown that one packaged chicken feet product exhibited a seal strength of only 13 N/15 mm at leaking locations, compared with 55 N/15 mm in properly sealed areas, resulting in liquid leakage during transportation.
Excessive heat sealing temperature causes the sealant layer to over-melt, allowing molten material to be squeezed out of the seal area. The resulting seal becomes thinner and more brittle.
Typical symptoms include whitening, hardening, or wrinkling along the seal edges. Molten material may also adhere to the sealing bar, causing "stringing" that disrupts continuous production.
In one inspection case involving packaged black sesame paste, vacuum testing revealed leakage at only −18 kPa. Further examination showed significant seal shrinkage and deformation, confirming a typical case of excessive heat sealing.
4. Frequently Asked Questions
A simple on-site hand-tear test can provide a preliminary assessment.
If the heat sealing temperature is too low, the seal separates easily by hand, leaving a smooth fracture surface without stretching marks. The separation occurs along the original interface between the two film layers, indicating a typical false seal.
If the temperature is too high, the seal edges become white and brittle, or the sealed area becomes noticeably thinner. When torn apart, the seal fractures rather than stretching, and the break often occurs near the edge of the sealing area.
When the heat sealing temperature is correctly adjusted, significantly greater force is required to tear the seal. Visible film stretching occurs before failure, and the film itself generally tears instead of separating along the original seal interface.
Although useful for rapid evaluation, the hand-tear test provides only qualitative results. Regular laboratory testing of heat seal strength is recommended for reliable quality control.
According to industry technical references, every 25 μm increase in film thickness requires approximately a 50% increase in heat sealing time.
For example, if a 50 μm film requires a sealing time of 1.0 second, a 75 μm film may require approximately 1.5 seconds.
If production speed is fixed and sealing time cannot be extended, increasing the heat sealing temperature by approximately 10–20°C can compensate for the slower heat transfer.
Thicker films also require higher sealing pressure to ensure complete interfacial contact. However, excessive pressure can squeeze molten resin from the seal area, making the seal thinner and more brittle.
For best results, manufacturers should determine the optimal combination of temperature, time, and pressure through small-scale production trials.
Different packaging machines vary in sealing bar heat transfer efficiency, pressure distribution, heating methods, and cooling system design.
For example, impulse heat sealers and constant-temperature heat sealers transfer different amounts of thermal energy to the film even when set to the same temperature.
The machine's thermal accumulation state also affects actual sealing performance. U.S. Patent US 12,252,288 specifically addresses this issue by proposing a method that dynamically adjusts heating time based on the interval between consecutive sealing cycles.
During continuous high-speed operation, residual heat accumulates in the sealing bar, causing the actual sealing temperature to exceed the set value. Conversely, a cold machine requires a longer heating period before reaching stable operating conditions.
Therefore, whenever a packaging machine is replaced, manufacturers should perform new heat sealing curve tests and establish machine-specific sealing parameters rather than reusing settings from previous equipment.
Conclusion
More than 90% of vacuum packaging seal leakage problems result from an improper match between heat sealing temperature and heat sealing time.
Temperatures that are too low cause false seals, while excessive temperatures lead to heat cutting and seal deformation. Insufficient sealing time prevents adequate molecular entanglement, whereas excessive sealing time may cause resin oxidation and degradation.
Only by performing heat sealing curve testing and optimizing the combination of temperature, time, and pressure for specific packaging films, film thicknesses, and packaging equipment can manufacturers consistently achieve strong, stable, and leak-proof seals.
