Traditionally, we call heat sealing temperature, heat sealing time, and heat sealing pressure as the three elements of heat sealing bag making.
1. Heat sealing temperature
The function of the heat sealing temperature is to heat the adhesive film layer to a more ideal state of viscous flow.
Since the polymer has no definite melting point, it is a melting temperature range, that is, there is a temperature range between the solid phase and the liquid phase. When heated to this temperature range, the film enters the molten state. The viscosity flow temperature and decomposition temperature of the polymer are the lower limit and upper limit of the heat seal, respectively. The difference between the viscosity flow temperature and the decomposition temperature is an important factor to measure the difficulty of the heat seal.
The heat sealing temperature is set according to the characteristics of the heat sealing material, the thickness of the film, the number of times of heat sealing and pressing, and the size of the heat sealing area. When the number of pressings on the same part increases, the temperature can be reduced appropriately. When the heat sealing area is large, the temperature can be slightly higher.
In the process of heat-sealing compound bag making, the heat-sealing temperature has the most direct influence on the heat-sealing strength. The melting temperature of various materials directly determines the minimum heat-sealing temperature of the compound bag. In the actual production process, the heat sealing temperature is affected by factors such as the heat sealing pressure, the speed of the bag making machine, and the thickness of the composite substrate. Therefore, the actual heat sealing temperature is often higher than the melting temperature of the heat sealing material. If the heat sealing temperature is lower than the softening point of the heat sealing material, no matter how to increase the pressure or extend the heat sealing time, the heat sealing layer cannot be truly sealed. Generally speaking, as the heat sealing temperature increases, the heat sealing strength will also increase, but after a certain temperature, the strength will not increase. If the heat sealing temperature is too high, it is easy to damage the heat sealing material at the welding place, and the "extrusion" phenomenon occurs during melt extrusion, which greatly reduces the heat sealing strength of the seal and the impact resistance of the composite bag.
2. Heat sealing time
Heat sealing time refers to the time that the film stays under the heat knife, and it is also a key factor affecting the strength and appearance of the heat seal. With the same heat sealing temperature and pressure, the heat sealing time is longer, the heat sealing layer is more fused and the bonding is stronger. However, if the heat sealing time is too long, it is easy to cause wrinkling and deformation of the heat seal weld, which affects the flatness and appearance. At the same time, too long heat sealing time will also cause the decomposition of macromolecules and deteriorate the sealing performance of the sealing interface.
3. Heat sealing pressure
The function of the heat sealing pressure is to make the polymer resin film in the viscous flow state effectively penetrate and diffuse between the heat sealing interfaces, so as to achieve a certain heat sealing effect.
In order to achieve the ideal heat seal strength, it is necessary to apply appropriate pressure. For general thin and light medical packaging bags, the heat sealing pressure must reach at least 20N / cm2, and as the total thickness of the composite film increases or the width of the heat seal increases, the required pressure should also increase accordingly. If the heat sealing pressure is insufficient, it is difficult to achieve true bonding and mutual fusion between the two layers of plastic film heat sealing materials, resulting in partial heat sealing, or it is difficult to eliminate the air in the heat sealing layer, resulting in virtual sealing or unevenness.
However, when the heat sealing pressure is too high, the molten material will squeeze out, and part of the heat sealing material will be squeezed away, causing the weld edge to form a semi-cut state, and the weld is brittle, affecting the heat sealing effect and reducing the heat seal. strength. Generally, after heat sealing, the strength loss of the sealing part shall not be greater than 10% to 15%.
Changes in pressure can change the heat sealing characteristics. Obviously, the greater the pressure, the required heat sealing time or heat sealing temperature can be reduced, but at the same time the heat sealing range will become narrower. In actual operation, the pressure can be adjusted, and a higher operating temperature is adopted.
Increase the output by shortening the heat sealing time, but the operation control is more difficult, and special care must be taken to avoid negative effects.
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