At present, heat sealing is a common and practical way of making bags in composite packaging materials. It uses various external conditions (such as electric heating, high-frequency voltage and ultrasonic waves, etc.) to make the plastic film sealing part heated to a viscous state, and with a certain pressure, the two films are fused into one, and the strength is maintained after cooling. There are many types of heat sealing, such as fuse sealing, ultrasonic sealing, high frequency sealing, hot air sealing, rod heat sealing, etc. According to the characteristics and requirements of medical packaging, most of medical flexible packaging bags use stick heat sealing, which is also a common heat sealing method in the entire packaging bag industry. This heat sealing method is mainly to coordinate the temperature of the heating rod, the heat sealing time, and the pressure between the heating rod and the silicone plate to achieve a satisfactory sealing effect. Generally speaking, a good sealing effect depends on whether it has a good heat seal strength and an intact appearance.
Three elements of heat sealing bag making
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 region between the solid phase and the liquid phase. When heated to this temperature region, 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 a great 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 temperature of the heat seal increases, the strength of the heat seal will also increase, but after a certain temperature, the strength will not increase (see the relationship between the heat seal strength and the temperature in Figure 1). 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. The same heat sealing temperature and pressure, and long heat sealing time, the heat sealing layer is more fully 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.
Generally speaking, the heat sealing time is mainly determined by the speed of the bag making machine. In the old-style bag-making machine, the heat-sealing time can only be adjusted by changing the speed of the bag-making machine. To extend the heat-sealing time, the production efficiency must be sacrificed. In recent years, domestic and foreign bag making machine manufacturers have used independent frequency conversion motor technology to control the heat sealing knife drop and feeding, so that the bag making machine can independently adjust the heat sealing time or control the heat sealing time without changing the bag making speed. The bag-making speed can be adjusted independently under the same conditions, which greatly facilitates the operation and quality control of the bag-making machine and improves the production efficiency of the bag-making machine.
3. Heat sealing pressure
The function of the heat sealing pressure is to make the polymer resin film in the state of viscous flow 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, appropriate pressure must be applied. For general thin and light medical packaging bags, the heat sealing pressure must be at least 20 N / 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 is 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. A higher operating temperature is used to increase the output by shortening the heat sealing time. However, it is difficult to control the operation and special care must be taken to avoid negative effects.
Factors of the composite membrane material itself
In addition to the process parameters of the three elements of heat-sealing and bag-making, which have a great influence on the bag-making, the performance of the composite material is also one of the important factors that directly affect the performance indexes of the heat-sealing and bag-making and the equipment operating parameters during the bag-making process.