The core material structure and characteristics of in-mold label technology
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In-mold labels differ from other types of labels. The core of the in-mold label is not the printing, but the material itself. In-mold label material has the use characteristics in addition to the printing suitability of the printing machine. Therefore, the in-mold label has many differences in the formulation and structure of the material from the common label material. The following is a brief introduction to the types, structural features, manufacturing methods, and development trends of in-mold label materials.
Material classification
Label materials are basically divided into two categories according to physical properties, namely paper and film.
Paper materials may deform due to water or moisture, causing the ink to fall off and the strength to be lowered, which affects the use effect, and the label must be removed when the bottle (barrel) is recycled, which brings inconvenience to the packaging and reuse. Therefore, in developed countries, paper-based in-mold labels are gradually being replaced by film materials. In China, the in-mold label has just started, and by using foreign experience, plastic film materials are used. Due to the different raw materials and processing methods of the in-mold label material, the properties of the obtained materials are also different. Figure 1 compares the surface smoothness of the three materials.
Material structure
Regardless of the type of film material, although the manufacturing process is different, the basic structure of the material is the same, that is, composed of a printed layer, an intermediate layer and a binder layer, as shown in FIG. The printing layer receives the ink to form the graphic; the intermediate layer plays a supporting role, so that the material has sufficient strength, does not deform under the action of printing tension and high temperature, ensures accurate overprinting, and can have good transparency or opacity as needed; adhesive The layer is the key to ensuring the use of the label. This layer can be melted under high temperature, so that the label is integrated with the wall of the package (barrel, bottle).
In order to make the printed layer have good ink repellency, the surface coating or corona method is often used to improve the surface adsorption performance; and the surface is required to be flat, wrinkle-free and bubble-free.
The adhesive layer has two structures, one is a flat structure, suitable for small labels, as shown in Figure 3, it is directly in contact with the blank when in use, and naturally vents; the other is a textured structure, suitable for large labels, and the gas passes through the net when in use. The exhaust passage on the groove can effectively prevent the generation of bubbles. In addition, the adhesive layer also has antistatic and sliding properties. Antistatic properties prevent double sheets in printing; sliding properties ensure smooth label separation and smooth printing and finishing.
In-mold label material manufacturing process
The in-mold label material is produced in three parts, coextrusion - biaxial stretching (uniaxial stretching) - post treatment.
(1) Coextrusion
The resin particles of different compositions or formulas are respectively placed in different extruders, and enter the same extrusion port under the action of high temperature and pressure to extrude a composite film material composed of resins of different compositions.
(2) Biaxial stretching
The coextruded material is stretched in the axial direction and the circumferential direction by the action of the mechanical device to cause plastic deformation of the crystal lattice. The two-dimensionally stretched material has improved strength and hardness, which is beneficial for processing and application. Generally, BOPP uses a biaxial stretching process.
(3) Unidirectional stretching
After the uniaxial tensile deformation of the film, the strength and hardness are improved, and the transverse direction is elastically deformable, which is more suitable for an irregular bottle body. Generally, the PE+PT material adopts a uniaxial stretching process.
(4) Post processing
Post-treatment refers to corona or coating treatment of the surface of the material, as well as web rolling of the surface of the adhesive layer. Corona or surface coating can improve the printability of the material, and the web rolling can cause the surface of the adhesive layer to be textured and easy to vent. After the processed materials are rewinded, the large rolls are further divided into small rolls or single sheets, which can be sent to the customer as finished products.
Development trend of in-mold label materials
In recent years, the global in-mold label market has developed rapidly, with an annual growth rate of about 8% to 10%. In-mold label materials are a new type of printing material that is developed to meet both printing requirements and plastic bottle blowing machines. Feature requirements. At present, on the basis of ensuring the use effect of the in-mold label and having certain physical and chemical properties, the label material presents the following development trends:
(1) The thickness of the material is developed to a thin shape, which can reduce the cost and facilitate recycling;
(2) The back of the material is not coated with adhesive, which can eliminate the texture and improve the printing quality;
(3) Fully transparent materials will be widely used;
(4) Diversified materials, easy to recycle and reuse.