Mastering the Printing Characteristics of Food Packaging Materials to Create Exquisite Packaging Products

Printability of Common Food Flexible Packaging Materials

PE film has poor ink adhesion and requires corona treatment to improve it. Wetting tension is commonly used as an indicator to judge the quality of corona treatment. Generally, LDPE and LLDPE films are optimal at 38~420mN/m, and HDPE film at 40~44mN/m; polyamide inks are commonly used for printing. LDPE film has low tensile strength and high tensile elongation, making color registration adjustment difficult. Therefore, during printing, tension control should be low, and tension at all points should be constant. For multi-color overprinting, to ensure registration accuracy, it is best to use a large-cylinder-type printing press. LDPE film softens with a small amount of heat. It is extremely easy to stretch and deform; therefore, during printing and drying, the film surface temperature should not exceed 55°C. To prevent ink blocking, the film temperature should be cooled to below 35°C during winding.

Like PE, PP film has poor ink adhesion and also requires corona treatment before printing; its wetting tension is optimal at 10~14mN/m. Polyamide inks and chlorinated polypropylene inks (for lamination) are commonly used for printing. When polyamide inks are used to print PP film, ink adhesion is relatively low, and good adhesion is only exhibited after about one day of aging; if polyurethane adhesive is used at the same time for film lamination, a failure of low peel strength will occur.

PET film is not easily stretched, and the tension control requirement is lower than that for PE film. Reverse printing is commonly used, and it is laminated with CPP to make retort pouches. PET film has good insulation, so static electricity is easily generated during printing and is not easily dissipated. In severe cases, the film may stick to the roller and printing cannot proceed. Therefore, during high-speed printing, a static eliminator is required. The drying temperature for PET film can be controlled relatively high, but its film surface temperature should not exceed 85°C. During winding, the film should be cooled to room temperature or slightly above room temperature.

Printing Methods for Food Packaging Materials

Food flexible packaging printing is mainly gravure and flexographic printing, but compared with general gravure and flexographic printing used in publishing and commercial printing, there are many differences. For example: flexible packaging printing is carried out on the surface of a roll-shaped substrate; if it is a transparent film, the pattern can be seen from the back, and sometimes a layer of white coating needs to be added, or reverse printing is used.

Reverse printing refers to a special printing method that uses a printing plate with a reverse image to transfer ink to the inner side of a transparent substrate, thereby presenting a positive image on the front of the printed matter. Reverse-printed prints are brighter and more beautiful than surface-printed ones, with vivid colors, no fading, and are moisture-proof and abrasion-resistant; moreover, since the ink layer is sandwiched between two films after lamination, it will not contaminate the packaged items.

The printing color sequence of the reverse printing process is opposite to that of ordinary surface printing. For example, ordinary printing prints the base color first, while reverse printing prints the base color last. Reverse printing is a process unique to plastic composite packaging printing, and printing equipment has tended toward diversification, multi-functionality, and serialization. For example, various types of plastic packaging gravure printing production lines that have been introduced have linked multiple processes such as blow molding, printing, lamination, slitting, and bag making.

Next is the use of flexographic printing presses to print plastic films; flexographic printing presses mostly form production lines with dry laminators.

Lamination Methods for Food Packaging Materials

Flexible packaging materials generally do not use a single material but are mainly composite films. The main lamination methods are as follows:

① Wet lamination. A layer of water-soluble adhesive is coated on the surface of the substrate (plastic film, aluminum foil), laminated with other materials (paper, cellophane) through a pressure roller, and then dried through a hot air tunnel to become a composite film. This method is suitable for packaging dry foods.

② Dry lamination. First, a solvent-based adhesive is evenly coated on the substrate, then sent into a hot air tunnel to fully evaporate the solvent, and immediately laminated with another layer of film. For example, stretched polypropylene film (OPP), after reverse printing, is generally laminated with other materials using the dry lamination process; a typical structure is: biaxially oriented polypropylene film (BOPP, 12μm), aluminum foil (AIU, 9μm) and uniaxially stretched polypropylene film (CPP, 70μm). The process is to use a roller coating device to evenly coat the solvent-based "dry adhesive powder" on the base material, then send it into a hot air tunnel to fully evaporate the solvent, and then laminate it with another layer of film using a lamination roller.

③ Extrusion lamination. The curtain-like molten polyethylene extruded from the T-die slit is pressed by nip rollers and cast onto paper or film for polyethylene coating, or other film is supplied from the second feed section, with polyethylene used as the bonding layer for adhesion.

④ Hot-melt lamination. Polyethylene-acrylate copolymer, ethylene-acrylic acid copolymer, and paraffin wax are heated and melted together, then coated on the substrate, immediately laminated with other composite materials, and cooled to produce the product.

⑤ Multi-layer extrusion lamination. Multiple plastic resins with different properties are extruded into a die through multiple extruders and composited into a film. This process uses no adhesive or organic solvent between layers; the film has no odor and no harmful solvent penetration, making it suitable for food packaging with a longer shelf life. For example, the general structure LLDPE/PP/LLDPE has good transparency and a thickness of generally 50~60μm; if a longer shelf life is required, a five-layer or more high-barrier co-extruded film is needed, with the middle layer being high-barrier materials PA, PET, and EVOH.