Plastic flexible packaging packaging composite film, bag, in the production process is usually sealed by heat sealing to seal a product into a closed environment. Heat sealing is to use various external conditions (such as electric heating, high-frequency voltage and ultrasonic waves) to change the sealing part of the plastic film into a viscous flow state, and to fuse the two layers of film into one by a certain pressure, and keep it constant after cooling. strength.
In order to ensure that the goods can withstand certain external forces during packaging, transportation, storage and consumption, to ensure that the goods do not crack, leak, and achieve the purpose of protecting the goods, the sealing parts are required to have sufficient strength and sealing performance. The heat seal strength reflects various comprehensive physical and mechanical properties of composite packaging materials and is one of the most important indicators of packaging materials.
The heat sealing performance of various materials is very different. The heat sealing performance is actually the melting temperature reaction of the material. The polymer with different molecular weight and structure, the remote structure and the short-range structure have great reference significance for the heat sealing performance of the material. ,
The heat sealing of plastic is the result of the combination of heat and force. On the one hand, the surface of the plastic sealing in the molten state, the polymer segments are mutually diffused, infiltrated and intertwined to achieve sealing and sealing; on the other hand, the molten viscous state is high. The polymer molecules deform under pressure and are close to each other, and sufficient van der Waals force is generated between the molecules to seal the heat seal surfaces.
1, heat sealing temperature
The function of the heat sealing temperature is to heat the adhesive film layer to a relatively ideal viscous flow state. Since the polymer has no defined melting point, it is only a melting temperature range, that is, a temperature region between the solid phase and the liquid phase, and when heated to the temperature region, the film enters a molten state. The viscous flow temperature and decomposition temperature of the polymer are the lower and upper limits of the heat seal, and the difference between the two temperatures is an important factor for measuring the difficulty of heat sealing of the material. Different materials have different melting temperatures. According to the segmentation mechanism of the polymer, generally, the molecular chain flexibility is better, the energy barrier in the chain is low, the flow unit segment is short, the melting temperature is low, and the heat sealing temperature is also It is low.
2, heat sealing pressure
The function of the heat sealing pressure is to make the film which has been in the viscous flow state to effectively penetrate and diffuse the polymer segments between the sealing interfaces, and also to make the distance between the polymers close to each other to produce an intermolecular force. If the heat sealing pressure is too low, the heat seal may not be tight; if the pressure is too high, some effective segments of the viscous flow state may be squeezed out, resulting in a half-cut state of the heat-sealed portion, the strength is weakened, and the high pressure limit should be the strength after sealing. Not less than 15% before sealing. Heat sealing pressure as the external force of molecular motion, its increase is essentially to offset the thermal motion of the molecular segment in the opposite direction of the external force, increase the probability of the chain transition along the external force direction, and effectively shift the center of gravity of the molecular chain.
3, heat sealing time
The heat sealing time refers to the time when the film stays under the sealing knife. The heat sealing time determines the production efficiency of the heat sealing device. In recent years, a controllable multiple heat sealing device has appeared, which can increase the heat without reducing the production efficiency of the device. Seal time. On the one hand, the extension of the heat sealing time can increase the heat transfer, so that the heat seal layer reaches the set temperature of the heat seal knife as soon as possible, and on the other hand, the increase of the external force action time also contributes to the viscous flow of the polymer segment. The purpose of the seal.
Because the above factors act on the outer surface, different thicknesses and different composite structures have a great influence on the effect. The thicker the film, the longer it takes for the heat seal to reach the relevant heat seal temperature, and it can be adjusted by increasing the temperature and pressure. The heat sealing conditions of materials of different structures are also different. If the composite part has aluminum foil material, because the metal has good thermal conductivity, the heat sealing time can be shortened appropriately; if the layer contains ink with temperature difference, adhesive, etc., it is necessary to lower the temperature, increase the heat sealing pressure, and increase Heat seal time to solve.
To determine the heat sealing factor, it is necessary to use the heat sealing test machine and the electronic tension machine to explore and detect, determine the upper and lower limits of temperature, pressure and time, and then adjust the relevant process to the corresponding heat sealing equipment.