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What are the key points of heat sealing of composite film?
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时间:2021-01-08 14:35:13

1. Heat sealing temperature

The selection of the heat sealing temperature of the composite film is closely related to the performance and thickness of the composite substrate, the model, speed, and hot air pressure of the bag making machine, which directly affects the strength of the heat sealing.

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

The temperature between the viscosity flow temperature and the decomposition temperature is the heat sealing temperature range of the heat sealing material, which is a key factor that affects and controls the quality of heat sealing.

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

At the same time, the heat sealing temperature of the composite film cannot be higher than the heat setting temperature of the printing substrate. Otherwise, it will cause shrinkage and wrinkling of the heat-sealed part, reducing the heat-sealing strength and the impact resistance of the bag.

The temperature resistance of the printing substrate is good, such as BOPET, BOPA, etc., increasing the heat sealing temperature can increase the production rate; the temperature resistance of the printing substrate is poor, such as BOPP, try to use a lower heat sealing temperature, but by increasing the pressure, reducing Production speed or choose low-temperature heat-sealable materials to ensure heat-seal strength.

2. Heat sealing pressure

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

The size of the heat sealing pressure is related to the performance, thickness, and heat sealing width of the composite film. Polar heat-sealable materials have higher activation performance, and the increase in temperature has a greater impact on the decrease in viscosity. Therefore, the required heat-seal pressure is relatively small to prevent the molten material at the heat-sealed part from being extruded and affect the heat-sealing effect.

However, PE and PP are non-polar materials with extremely low activation performance and high pressure required, which is beneficial to heat sealing strength and interface sealing.

The heat sealing pressure should increase as the thickness of the composite film increases.

If the heat sealing pressure is insufficient, the two films will be difficult to heat together, and it will be difficult to exhaust the bubbles sandwiched between the welds; if the heat sealing pressure is too high, the molten material will be squeezed away, the weld edge will be damaged, and the root will be broken. When calculating the heat sealing pressure, the width and actual surface area of the required heat sealing rod should be considered.

The wider the width of the heat sealing rod, the greater the pressure required. If the width of the heat-sealing rod is too wide, it is easy to entrain bubbles in the heat-sealed part, and it is difficult to heat-seal firmly. Generally, hollow heat-sealing rods can be used to strengthen the heat-sealing fastness in the last seal.

For heat-sealing rods of the same width, if the surface is engraved, the actual contact area is greatly reduced, and the pressure per unit area increases accordingly. This is beneficial for packaging bags with larger heat-sealing widths.

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 strength and appearance of the heat sealing.

The faster the heat-sealing speed, the higher the heat-sealing temperature to ensure that the heat-sealing strength and heat-sealing state reach the best value; under the same heat-sealing temperature and pressure, the slower the heat-sealing speed, the better the fusion of the heat-sealing material Full and firmer, but can not cause root breaking.

For domestically produced bag making machines, the length of 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 and the production efficiency must be reduced.

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

4. Cooling

The cooling process is a process of shaping the weld just melted and sealed with a lower temperature under a certain pressure, eliminating stress concentration, reducing the shrinkage of the weld, improving the appearance of the bag, and improving the strength of the heat seal.

The cooling water of the bag making machine is generally tap water or circulating water at about 20°C. Too high water temperature, insufficient cooling rod pressure, poor cooling water circulation, insufficient circulation, etc. will lead to poor cooling and reduced heat seal strength.

5. Number of heat sealing

The longitudinal and transverse heat sealing of most bag making machines use hot plate welding. The number of longitudinal heat sealing depends on the ratio of the effective length of the heat sealing rod to the bag length. The number of transverse heat sealing is determined by the number of heat sealing devices on the machine. .

Good heat sealing generally requires more than 2 heat sealing times. Most transverse heat sealing devices consist of 3 groups. In order to meet the heat sealing requirements of the wide side, transverse heat sealing devices are often added, and the number of heat sealing is increased to reduce the heat sealing temperature and reduce the shrinkage phenomenon.

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

6. Heat sealing rod gap

The gap of the heat-sealing rod refers to the distance that the predetermined hot air pressure is transferred to the heat-sealing surface when the upper heat-sealing rod touches the bottom plate.

At the same film thickness and the same heat sealing speed, the heat sealing rod gap is small, the heat sealing time is relatively short, and the heat sealing strength of the product will be reduced.

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