The drying process of heat shrink film adhesive refers to the process of evaporating the solvent or moisture in the adhesive layer coated on the substrate to cure the adhesive. Due to the time required for solvents or water to evaporate, drying is a relatively slow step in the production process of protective films. The drying speed not only determines the production speed but also greatly affects the quality of the product.
In actual production, manufacturers consider heat sealing strength as an important quality indicator for both bag making products and roll products used in automatic packaging production lines. Based on practical production experience, a brief analysis will be conducted on the factors that affect the heat sealing strength of heat shrink films
1、 Heat sealing time: The heat sealing time of automatic packaging machines or old-fashioned bag making machines is determined by the machine speed. Under constant heat sealing temperature and pressure, the longer the heat sealing time, the stronger the bond between the heat sealing layers. But if the heat sealing time is too long, wrinkles and unevenness will appear in the heat sealing area of the composite film, seriously affecting the appearance quality of the product.
2、 Heat sealing pressure: In order to ensure the heat sealing strength of plastic composite flexible packaging, a certain heat sealing pressure is essential. Therefore, moderate and uniform heat sealing pressure is required during the heat sealing process. If the heat sealing pressure is insufficient or uneven, it will cause bubbles and virtual welding at the heat sealing area, leading to poor heat sealing.
3、 Heat sealing temperature: The heat sealing temperature directly affects the heat sealing strength of plastic composite packaging products.
Once the adhesive of the heat shrink film is applied, the solvent will quickly evaporate before entering the drying oven. However, due to the heat absorption caused by evaporation, the surface temperature of the adhesive layer decreases, resulting in a decrease in evaporation rate. When the protective film enters the oven, the heat absorbed by solvent evaporation is supplemented by the heat conducted from the outside. When the heat reaches equilibrium, the surface temperature of the adhesive layer and the evaporation rate of the solvent can also remain constant.
The viscosity and other rheological properties of heat shrink film adhesives are also factors that affect the coating process of protective films. The greater the deviation of the adhesive's rheological properties from Newtonian fluids, the more likely various quality problems are to occur during the coating operation, especially when the adhesive exhibits the properties of an expanding body. Due to the rapid increase in viscosity with the increase of coating speed during the coating process, a large mechanical force must be used to unfold the adhesive. Therefore, the faster the coating speed, the less likely it is to obtain a uniform coating.