In-mold labels are different from other types of labels. Its core lies not in printing, but in the material itself. In addition to satisfying the printing suitability of the printing machine, the in-mold label materials must also have use characteristics. Therefore, in-mold labels have many differences from ordinary label materials in terms of material formulation and structure. The following is a brief introduction to the types, structural characteristics, manufacturing methods and development trends of in-mold label materials.
Material classification
Label materials are basically divided into two categories according to their physical properties, namely paper and film.
Paper materials will be deformed when they are exposed to water or moisture, which will cause the ink to fall off and reduce its strength, which will affect its use. In addition, the labels must be removed when recycling bottles (barrels), which will cause inconvenience to the reuse of packaging. Therefore, in developed countries, paper in-mold labels are gradually being replaced by film materials. In China, in-mold labels are just getting started. By drawing on foreign experience, plastic film materials are used. Due to the different raw materials and processing methods of in-mold label materials, the characteristics of the resulting materials are also different. Figure 1 is a comparison of the surface smoothness of the three materials.
Material structure
Regardless of the type of film material, although their manufacturing processes are different, the basic structure of the material is the same, that is, it consists of a printed layer, an intermediate layer, and an adhesive layer, as shown in Figure 2. The printing layer accepts ink to form graphics; the middle 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 ensure the use effect of the label. This layer can be melted under the action of high temperature, so that the label is integrated with the wall of the packaging (barrel, bottle).
In order to make the printed layer have good ink-injecting properties, the method of coating surface coating or corona is often used to improve its surface adsorption performance; and the surface is required to be smooth, without wrinkles and no bubbles.
The adhesive layer has two structures, one is a flat structure, suitable for small labels, see Figure 3, direct contact with the blank when using, natural exhaust; the other is a mesh structure, suitable for large labels, see Figure 4, use When the gas is exhausted through the exhaust channel on the mesh, it can effectively avoid the generation of bubbles. In addition, the adhesive layer also has antistatic and sliding properties. The antistatic property can prevent double sheets from printing; the sliding property can ensure the smooth separation of the label and ensure the smooth progress of printing and post-press processing.
Manufacturing process of in-mold label material
The manufacturing process of the in-mold label material includes three parts, co-extrusion-biaxial stretching (unidirectional stretching)-post-processing, as shown in Figure 5.
(1) Co-extrusion Put resin particles of different compositions or formulations into different extruders respectively, enter the same extrusion port under the action of high temperature and pressure, and extrude a composite film material composed of resins of different compositions.
(2) The biaxially stretched co-extruded material is stretched in the axial and circumferential directions under the action of a mechanical device, causing its lattice to plastically deform. The biaxially stretched material has improved strength and hardness, which is beneficial to processing and application. Generally, BOPP uses a bidirectional stretching process.
(3) The uniaxially stretched film is improved in strength and hardness after unidirectional stretching and deformation, and can be elastically deformed in the transverse direction. It is more suitable for irregular bottles. Generally, PE + PT materials use unidirectional stretching process.
(4) Post-processing Post-processing refers to corona or coating treatment on the surface of the material, and textured rolling on the surface of the adhesive layer. Corona or surface coating can improve the printing suitability of the material, and the texture rolling will make the surface of the adhesive layer produce texture, which is convenient for exhaust. After the processed materials are rewound, the large roll is divided into small rolls or single sheets, and can be sent to customers as finished products.
The 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%. The in-mold label material is a new type of printing material that is developing. Feature requirements. At present, on the basis of ensuring the use effect of in-mold labels and possessing certain physical and chemical properties, label materials have shown the following development trends:
(1) The thickness of the material has been developed to be thin, which can reduce costs and facilitate recycling;
(2) No adhesive is applied on the back of the material, which can eliminate the texture and improve the printing quality;
(3) Fully transparent materials will be widely used;
(4) The variety of materials is diversified to facilitate recycling and reuse.
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