Talking about the printing process of the label inside the film
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Material selection for in-mold labels
The materials used in in-mold labels are divided into two categories: paper and film. The advantage of the former is that there is no static interference, which reduces the loss of the label when blowing. However, the paper is easily deformed after being exposed to water or moisture, and the surface of the ordinary synthetic paper is low in smoothness, and its strength and elastic properties make it unsuitable for high-grade color printing.
At present, thin films are generally used as the material of choice for in-mold labels at home and abroad. The film material has good stretchability, especially for stretchable shrinkage materials such as Primax and FasClear. In practical applications, the material can be deformed as the shape of the bottle changes, and the label wrinkles are not present on the surface of the bottle. In addition, the ink absorption of the film material is also much lower than that of the paper. Therefore, the label printed with the film material has bright color, strong stereoscopic effect and color effect. Moreover, the film material and the plastic bottle body are homogenous materials with recyclability.
The material structure of the in-mold label consists of three parts: a printing surface, an intermediate layer, and an adhesive layer. The primary role of the printed surface is to carry the ink and form a corresponding color graphic. Printing 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 in in-mold labels is more advantageous for recycling.
The main function of the middle layer is to support the printing surface, giving the material sufficient stiffness to maintain the original shape under the high temperature and high pressure of the printing machine, and to ensure the overprinting precision. The role of the adhesive layer is to melt at high temperatures, allowing the label material to adhere firmly to the plastic container. The adhesive layer has two surface structures: a planar non-mesh structure and a textured structure. The former is directly in contact with the blank and is suitable for small-area labels, while the latter embosses the textured structure on the surface to avoid the production of bubbles. The adhesive layer has good slidability to ensure a smooth separation of the label.
In-mold label printing technology
In-mold labels can be processed by web printing or sheetfed printing.
Roll paper printing can be divided into online, one-time processing and multiple processing. The former is suitable for printing graphics and needs to be bronzing, and the printing machine has round pressing and bronzing and die cutting device. The latter applies to the case of only a single hot stamping machine and die cutting machine. The biggest advantage of roll paper printing processing is that it is suitable for mass production, with high production speed and high efficiency.
Sheetfed printing is usually silk screen and flat offset printing. In contrast to the former, this type of processing is suitable for small batch production. The plate making cost is low, the processing is more flexible, the printing effect is good, and the silk screen printing and offset printing layers are better than the roll printing effect.
In-mold label materials are also suitable for a variety of printing processes, including: lithographic offset, flexographic, gravure and letterpress printing, currently in offset printing and flexo printing.
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 has high flexibility in the production cost of the short-run business. However, the color saturation of the offset line and the solid is poor, and the chromatic aberration is difficult to control. Moreover, the offset printing cannot be performed by the inline printing process like the flexo printing, and the process of offline die cutting, cold ironing, glazing, etc. must be performed after printing. When using offset printing, pay attention to the amount of powder sprayed, and the printed matter should not be too high, and the ink should be sufficiently dried. Due to the poor hygroscopicity of the substrate, the amount of dampening solution should be strictly controlled.
Flexographic process
The advantage of the flexo process is that it can be used for one-line printing or a combination of processes. High color saturation can be achieved by direct plate-making CDI plate-making technology, and the printing quality of the dots has also been greatly improved. However, its ink adhesion is small and the gradation effect is poor, which is suitable for medium and large batch production.
Gravure process
The advantages of the gravure printing process are that the solid printing effect and color saturation are good, the printing color transition is natural, the color difference control is convenient, and it is suitable for large-scale long-length processing. However, it has a long duty time and a high cost. On the gravure press, the in-mold label material has a longer paper path than other printing methods, so the chance of static electricity is greater, and attention should be paid in the actual production process.
Embossing process
The embossing process is convenient in color difference control, and the application of UV ink printing makes the printing color more vivid, dry and fast, and the plate making cost is low. However, its printing on small dots is worse than other methods, and the printing plate has low printing durability. Generally suitable for medium and large batch production. Post-press processing of in-mold labels
After the in-mold label is printed, the corresponding post-press processing is also carried out according to the customer's requirements, mainly including glazing, bronzing and die-cutting.
Glazing can greatly improve the gloss and surface decoration of the in-mold label, and at the same time improve the surface wear resistance and high temperature resistance of the in-mold label. Therefore, when using the vacuum adsorption in-mold label when labeling, the antistatic agent should be added appropriately and the smoothness of the varnish should be adjusted: if the method of electrostatically adsorbing the in-mold label is used, the varnish should not be added with an antistatic agent; The inner label should be bronched after printing. Then, you should use a bronzing UV varnish: If the glazing oil is added with additives, it must be tested before glazing to avoid scrapping the entire batch.
Hot stamping
Hot stamping is divided into cold stamping and hot stamping. In the in-mold label material, two different bronzing processes can be used, such as first pressing the gold and then passing the oil or bronching the oil first. Among them, cold bronzing is mainly used for flexographic printing, and a good visual effect can be obtained by the cold bronzing process, but the process requirements are high. In the actual production process, the method of first performing cold stamping and then printing is used, which can effectively cover the edge burrs caused by cold stamping by using the thickness of the ink. For the hot stamping of in-mold labels, there are many factors affecting them, including: anodized aluminum, bronzing temperature, bronzing pressure and bronzing speed. In the hot stamping, the hot foil with the melting point of the hot melt lower than the melting point of the hot-melt of the in-mold label material should be used. At the same time, the hot stamping time and the hot stamping pressure should be strictly controlled, 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 of the bronzing plate and the bronzing speed according to the material characteristics of the in-mold label and the bronzing suitability of the anodized aluminum, and then determine the bronzing temperature and bronzing pressure. In the actual mediation process, you must first mediate the bronzing pressure, and then mediate the bronzing temperature, in order to get a better bronzing effect.
Die cutting
The die-cutting method of the in-mold label is the cutting die-cutting, which is also the last process of the in-mold label post-press processing. Static electricity has a great influence on the die-cutting effect. Excessive static electricity will directly lead to irregular stacking of labels, which may cause problems such as inaccurate labeling position and double sheeting. Therefore, electrostatic removal is very important in the die-cutting process. The main measures to eliminate static electricity in the mold are: do not use corona processor when printing; add antistatic agent to varnish and large solid ink; add static eliminator; The working workshop humidity is controlled at around 60%.
Die-cutting of in-mold labels is generally divided into two types: die-cutting and single-sheet die-cutting. The die-cutting method is selected according to the manufacturer's die-cutting equipment and the type of in-mold label material. In the two die-cutting methods, the die-cutting of the web is divided into two types: die cutting (Line Diecut) and offline die cutting (Offline Diecut), and can also be divided into flat according to different die-cutting equipment. Two die-cutting methods: flattening and round pressing.
Among them, the flat die cutting method is an off-line die cutting method, which has the characteristics of high die cutting precision. This die-cutting method has two paper feeding modes, one is intermittent paper feeding, the die cutting device is only moving up and down; the other is continuous paper feeding, and the die cutting device not only moves up and down, but also swings back and forth. After these two working modes are die-cut, the labels are transferred to the stacking station via the conveyor. The circular die cutting method has two die cutting methods: wire die cutting and offline die cutting. At present, the circular die cutting is considered to be the most perfect die cutting method, and the die cutting precision is very high, but there is a problem that the cost of making the die cutting roller is high. The circular die-cutting method is suitable for high-volume in-mold label processing, and is used in the production of flexographic printing and is highly efficient.
For sheet-fed flat die-cutting methods, it is currently mainly suitable for sheet-fed die-cut label materials. It is an off-line die-cutting method with a good machining accuracy. In the actual production process, the paper is fed through an automatic paper feeder, and the finished product is stacked on the workstation by the relevant machinery.