If you're not careful, plastic packaging bags can easily have sealing problems.

1. Heat Sealing Temperature

The selection of the heat sealing temperature for composite films is closely related to the properties and thickness of the composite substrate, the model and speed of the bag-making machine, and the hot air pressure, directly affecting the heat sealing strength.

The initial sealing temperature of the composite film is determined by the viscous flow temperature or melting temperature of the heat-sealing material. The high temperature of heat sealing cannot exceed the decomposition temperature of the heat-sealing material.

The temperature range between the viscous flow temperature and the decomposition temperature is the heat sealing temperature range of the heat sealing material, which is a key factor affecting and controlling the heat sealing quality.

The wider the heat sealing temperature range, the better the heat sealing performance, and the easier and more stable the quality control.

At the same time, the heat sealing temperature of the composite film cannot exceed the heat setting temperature of the printing substrate. Otherwise, it will cause shrinkage and wrinkles at the heat sealing part, reducing the heat sealing strength and the impact resistance of the bag.

The printing substrate has good temperature resistance, such as BOPET, BOPA etc., increasing the heat sealing temperature can increase the production rate; the printing substrate has poor temperature resistance, such as BOPP It is best to use a lower heat sealing temperature, and ensure the heat sealing strength by increasing the pressure, reducing the production speed, or selecting a low-temperature heat-sealing material.

2. Heat Sealing Pressure

The hot air pressure is provided by the pressure spring on the bag-making machine.

The magnitude of the heat sealing pressure is related to the performance, thickness, and heat sealing width of the composite film. Polar heat sealing materials have higher activation performance, and the increase in temperature has a greater impact on the decrease in their viscosity, so the required heat sealing pressure is smaller, preventing the molten material at the heat sealing part from being squeezed out, affecting the heat sealing effect.

And PE, PP are non-polar materials with extremely low activation performance, requiring higher pressure, which is beneficial to heat sealing strength and interfacial sealing.

The heat sealing pressure should increase with the increase in the thickness of the composite film.

If the heat sealing pressure is insufficient, the two layers of film are difficult to heat seal, and it is difficult to exhaust the air bubbles trapped in the middle of the weld; if the heat sealing pressure is too high, it will squeeze away the molten material, damage the weld, and cause breakage. When calculating the heat sealing pressure, consider the required width and actual surface area of the heat sealing bar.

The wider the heat sealing bar, the greater the pressure required. If the heat sealing bar is too wide, it is easy to cause air bubbles in the heat sealing area, making it difficult to seal firmly. Generally, a hollow heat sealing bar can be used to strengthen the sealing strength in the last seal.

For heat sealing bars of the same width, if the surface is engraved, the actual contact area is greatly reduced, and the unit area pressure increases accordingly, which is beneficial for packaging bags with a larger heat sealing width.

3. Heat Sealing Speed

The heat sealing speed reflects the production efficiency of the bag-making machine and is also an important factor affecting the heat sealing strength and appearance.

The faster the heat sealing speed, the higher the heat sealing temperature should be to ensure that the heat sealing strength and heat sealing state reach the optimal value; under the same heat sealing temperature and pressure, the slower the heat sealing speed, the more fully and firmly the heat sealing material will be fused, but it cannot cause breakage.

For domestically produced bag-making machines, the length of the heat sealing time is mainly determined by the speed of the bag-making machine. To increase the heat sealing time, the bag-making speed must be reduced, reducing production efficiency.

If an independent variable frequency motor is used to control the lifting and feeding of the heat sealing bar and independently adjust the heat sealing time without changing the bag-making speed, it will greatly facilitate the operation and quality control of the bag-making machine.

4. Cooling Condition

The cooling process is to shape the weld that has just been melted and sealed under a certain pressure, eliminate stress concentration, reduce weld shrinkage, improve the flatness of the bag appearance, and improve the heat sealing strength.

The cooling water of the bag-making machine is generally tap water or circulating water at about 20℃. Too high water temperature, insufficient cooling bar pressure, poor cooling water circulation, insufficient circulation, etc., will all lead to poor cooling and reduced heat sealing strength.

5. Number of Heat Sealings

Most bag-making machines use hot plate welding for longitudinal and transverse heat sealing. The number of longitudinal heat sealings depends on the ratio of the effective length of the heat sealing bar to the bag length, and the number of transverse heat sealings is determined by the number of sets of the machine's heat sealing device.

Good heat sealing generally requires more than 2 heat sealings. Most transverse heat sealing devices are 3 sets. In order to meet the heat sealing requirements of wide edges, the transverse heat sealing device is often increased, and the number of heat sealings is increased to reduce the heat sealing temperature and reduce the necking phenomenon.

For packaging bags of longer specifications, multiple feeding technology can be used to reduce the feeding length each time to one-half or one-third of the bag length, thereby increasing the number of heat sealings and improving the heat sealing effect, but it will reduce production efficiency, so some bag-making machines have increased the length of the longitudinal heat sealing bar to increase the number of heat sealings and ensure heat sealing quality.

6. Heat Sealing Bar Gap

The heat sealing bar gap refers to the pressing distance of the predetermined hot air pressure transmitted to the heat sealing surface when the upper heat sealing bar contacts the bottom plate.

Under the same film thickness and heat sealing speed, the smaller the heat sealing bar gap, the smaller the relative heat sealing time, and the lower the heat sealing strength of the product.

Generally, the heat sealing bar gap is set at 1.0~1.5mm, which is related to the film thickness, transmission performance, and bag-making speed.

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