The materials used for in-mold labels are divided into paper and film. The advantage of the former is that there is no static interference, which reduces the loss of labels when blowing bottles. However, the paper is easily deformed when exposed to water or moisture, and the surface of ordinary synthetic paper is low in smoothness, and its strength and elastic properties make it unsuitable for high-end color printing.
At present, the film is generally selected as the material of choice for in-mold labels at home and abroad. Film materials have good stretch properties, especially materials such as Primax and FasClear that can stretch and shrink. In practical applications, the material can be deformed as the shape of the bottle body changes, and the surface of the bottle body will not wrinkle the label. In addition, the ink absorption of the film material is also much lower than that of paper, so the labels printed with the film material are bright in color, strong in three-dimensional sense, and color effect. Moreover, the film material and the plastic bottle body are homogenous materials and have reusability.
The material structure of the in-mold label consists of three parts, namely: the printing surface, the middle layer and the adhesive layer. The main function of the printing surface is to carry ink and form corresponding color graphics. The printed surface materials are generally divided into three types: PE, BOPP and PE + PP. Since most plastic containers currently use PE materials, the use of PE materials for in-mold labels is more conducive to recycling.
The main function of the middle layer is to support the printing surface, giving the material sufficient stiffness, so that it still maintains its original shape under the high temperature and high pressure of the printing machine, ensuring the accuracy of overprinting. The role of the adhesive layer is to melt at high temperature, so that the label material and the plastic container are firmly adhered together. The adhesive layer has two surface structures: flat non-textured structure and reticulated structure. The former is in direct contact with the blank and is suitable for small-area labels, while the latter has a textured structure imprinted on the surface to avoid the production of air bubbles. The adhesive layer has good sliding properties to ensure the smooth separation of the label.
Printing technology of in-mold label
In-mold labels can be completed by web printing or sheet printing.
Roll paper printing processing can be divided into two types: on-line, one-time processing and multiple-time processing. The former is suitable for printing graphics and texts that require bronzing, and there is a round pressing bronzing and die cutting device on the printing machine. The latter applies to the situation where there is only a separate hot stamping machine and die cutting machine. The biggest advantage of simple paper printing processing is that it is suitable for mass production, with fast production speed and high efficiency.
Sheet-fed printing is usually screen printing and offset printing. Compared with the former, this kind of processing is suitable for small batch production. The plate-making cost is low, the processing is more flexible, and the printing effect is good. The field of silk printing and the level of offset printing are better than those of web printing.
In-mold label materials are also suitable for a variety of printing processes, including: lithographic offset printing, flexographic printing, gravure printing, and letterpress printing. Offset and flexographic printing are currently the most common.
Offset printing process
The advantage of the offset printing process is that it is superior to other printing methods in the printing quality of small dots, and it has high flexibility for the low production cost of the short version business. However, the color saturation of offset lines and the ground is poor, and the color difference is difficult to control. In addition, offset printing cannot achieve inline printing like flexo printing. Offline printing, cold stamping, and glazing must be performed after printing. Pay attention to the amount of powder sprayed when using offset printing, and the accumulation of printed products should not be too high, and the ink should be fully dried. Due to the poor hygroscopicity of the substrate, the amount of Fountain solution should be strictly controlled.
Flexo process
The advantage of the flexo printing process is that it can realize inline printing once forming or a combination of multiple processes. Through the CDI platemaking technology of direct platemaking, a higher color saturation can be obtained, and the printing quality of the dot has also been greatly improved. However, its ink adhesion is small and its gradual change effect is poor, which is suitable for medium and large-scale production.
Gravure process
The advantage of the gravure printing process is that the field printing effect and color saturation are better, the printing color transition is natural, the color difference is easy to control, and it is suitable for long batch processing of large quantities. But its duty is long and the cost is high. On the gravure printing machine, the paper path of the in-mold label material is longer than other printing methods, so the chance of static electricity is greater, which should be paid attention to in the actual production process.
Embossing process
The embossing process is more convenient in controlling the color difference. UV ink printing is used to make the printing colors more colorful, dry faster, and the cost of plate making is lower. However, it is inferior to other methods in printing small dots, and the printing plate has low printing durability. Generally suitable for medium and large-scale production. [next]
Post-print processing of in-mold labels
After the in-mold label is printed, the corresponding post-press processing is also required according to customer needs, which mainly includes three parts: glazing, bronzing and die cutting.
Glazing can greatly improve the gloss and surface decoration effect of the in-mold label, and at the same time, it can also improve the surface wear resistance and high temperature resistance of the in-mold label. Therefore, when labeling, if you use a vacuum to absorb the in-mold label, you should add antistatic agent and adjust the smoothness of the varnish: if you use the electrostatic adsorption of the in-mold label, it is not suitable to add antistatic agent to the varnish; if the mold After printing, the inner label still needs to be gilded, then you should choose a gilt UV varnish: if the varnish is added with additives, it must be tested before glazing to avoid scrapping the entire batch of products.
Bronzing
Bronzing is divided into cold blanching and hot blanching. Two different bronzing processes can be used on the in-mold label material: first bronzing and then oil or first oil and then bronzing. Among them, cold bronzing is mainly used for flexographic printing, and good visual effects can be obtained through the cold bronzing process, but the process requirements are higher. In the actual production process, the method of cold bronzing before printing is often used. In this way, the thickness of the ink can be used to effectively cover the edge burrs caused by the cold bronzing. For hot stamping of in-mold labels, there are many influencing factors, including: anodized aluminum, hot stamping temperature, hot stamping pressure and hot stamping speed. When hot stamping, you should choose a hot foil with a melting point lower than the melting point of the in-mold label material. At the same time, you must strictly control the stamping time and stamping pressure, which can effectively avoid the uneven deformation of the in-mold label material . Pay attention to the following points when performing hot stamping. First, adjust the size and speed of the stamping plate according to the material characteristics of the in-mold label and the suitability of the anodized aluminum stamping, and then determine the stamping temperature and stamping pressure. In the actual mediation process, the bronzing pressure should be adjusted first, and then the bronzing temperature should be adjusted, so as to obtain a good bronzing effect.
Die cutting
The die-cutting method of in-mold labels is cut-off die-cutting, which is also the last process of post-printing processing of in-mold labels. Static electricity has a great influence on the die-cutting effect. Excessive static electricity will directly lead to uneven stacking of labels, which will cause problems such as inaccurate labeling position and double sheets. Therefore, the static electricity removal is very important in the die cutting process. The main measures to eliminate the static electricity of the in-mold labels are: try not to use a corona processor when printing; add antistatic agents to the varnish and large solid ink; install a static eliminator The humidity in the workshop is controlled at about 60%.
Die-cutting of in-mold labels is generally divided into web die-cutting and sheet-fed die-cutting. The choice of die-cutting method depends on the manufacturer's die-cutting equipment and the type of in-mold label material. In these two die cutting methods, web die cutting is divided into two types of line die cutting (Line Diecut) and offline die cutting (Offline Diecut), can also be divided into flat according to the different die cutting equipment Two die cutting methods: flattening and rounding.
Among them, the flat die cutting method is an offline die cutting method, which has the characteristics of high die cutting accuracy. This die cutting method has two paper feeding modes, one is intermittent paper feeding, the die cutting device only moves up and down; the other is continuous paper feeding, the die cutting device not only moves up and down, but also swings back and forth. After die-cutting by these two working methods, the labels will be transferred to the stacking station through the conveyor belt. The circular die cutting method has two die cutting methods: in-line die cutting and offline die cutting. At present, circular die cutting is considered to be the most perfect die cutting method, and its die cutting accuracy is very high, but there is a problem of higher cost of making die cutting rollers. The round die cutting method is suitable for large-scale in-mold label processing. It is used in flexographic printing line processing and production, with high efficiency.
For the single-sheet flat-bed die-cutting method, it is currently mainly applied to the single-sheet die-cut label material. It is an off-line die cutting method, and the processing accuracy is also average. In the actual production process, the paper is transported through the automatic paper feeder, and the finished products are stacked on the station by the relevant machinery.
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