These are produced using flexographic printing presses to print plastic films. Flexographic printing presses are often integrated with dry laminating machines to form production lines. Flexible packaging materials generally do not use a single material but are primarily composite films. The main lamination methods are as follows:
1. Wet lamination method for square-bottom valve bags: A water-soluble adhesive is coated onto the substrate surface, then laminated with other materials (paper, cellophane) using pressure rollers, and finally dried in a hot drying tunnel to form a composite film. Square-bottom valve bags produced using this method are suitable for packaging dry foods.
2. Dry lamination method: A solvent-based adhesive is first evenly coated onto the substrate of the square-bottom valve bag, then fed into a hot drying tunnel to allow the solvent to fully evaporate before immediately laminating with another layer of film.
3. Extrusion lamination method for square-bottom valve bags: Molten polyethylene extruded from a T-die slit is pressed by clamping rollers and poured onto paper or film for polyethylene coating, or other films are supplied from the second paper feed section, using polyethylene as an adhesive layer for bonding.
