Flatness is one of the indicators to measure the appearance quality of food packaging bags.In addition to material factors, the flatness of food packaging bags is also related to factors such as heat sealing temperature, heat sealing pressure, heat sealing time and cooling effect.The heat sealing temperature is too high, the heat sealing pressure is too high, and the heat sealing time is too long, which will cause the composite film to shrink and deform.Insufficient cooling will cause insufficient shaping after heat sealing, which cannot eliminate the internal stress and cause the bag to fold. Therefore, the process parameters should be adjusted and the cooling water circulation system should be ensured to work normally.