Film and its quality control measures

The concept and significance of a film

Film coating, that is, film, is to apply plastic film adhesive, and paper prints after heating, pressure, make it glue together to form a paper and plastic products a processing technology. The laminated film prints have a thin and transparent plastic film on the surface, and the surface is smoother and brighter. This improves the glossiness and fastness of the printed materials. The graphic and text colors are more vivid and full of three-dimensional, and they are also waterproof. Antifouling, wear-resisting, folding-resistance, chemical resistance, etc.

Second, film classification introduced

According to the process of coating is divided into three types: dry, wet and oily. According to the different raw materials and equipment used, it can be divided into coated film and pre-coated film process.

1. coating film

That is, the coating film is coated with a binder on the film first and then hot pressed, but the production process is prone to fire.

2. Pre-coated film

This process does not require adhesive heating and drying system for film coating equipment. It greatly simplifies the film coating process and is easy to operate. It can be used with the start, the production flexibility, no solvent smell, no environmental pollution, more importantly, it can be completely To avoid problems such as air bubbles and delamination, the transparency of its finished products is extremely high, and it has broad application prospects and promotion value.

The precoat structure consists of a substrate and a subbing layer. The substrates are usually PET and BOPP films. Considering the material cost and processing technology, most of the precoated substrates use BOPP film with a thickness of 12-20 μm. Thickness is 5-15 microns. According to the different processing equipment and process conditions, choose different thickness of the glue layer, glue layer is divided into hot melt adhesive and organic polymer low temperature resin. The difference between the two lies in that the hot melt adhesive is made by blending a main adhesive resin, a tackifier and a regulator, and the organic polymer resin is a single polymer low temperature copolymer.

Due to the limitations of technology, production equipment and raw materials, pre-coated films (mostly hot-melt adhesives) produced by domestic equipment still have quality defects. Since the production process is based on a process similar to the coating film, the colloid is dissolved with an organic solvent, and the roller is coated on the substrate film with a concave anilox roller. Therefore, there is a problem that the solvent volatilization is insufficient, and an odor is generated during the operation. After the film is coated, the solvent evaporates and the surface is likely to foam. At the same time, since the hot melt adhesive is blended by several kinds of polymer materials, during the use process, poor temperature control will cause degradation and cross-linking of the high-molecular polymer in the hot melt adhesive, and make the surface of the coating film poor. Some precoat films collide with the outer surface of the substrate, resulting in poor winding. Since the adhesive layer is thin and the surface is not activated, defects such as insufficient adhesion to the printed matter are likely to occur. Since the hot melt adhesive is made of a mixture of several materials, the transparency after coating is significantly lower than that of a low temperature pure resin type.

3. Waterborne film

It is the method of processing the surface of the printed matter to improve the surface glossiness and strength of the printed matter and enhance the aesthetic effect. To measure the quality of the coating film, it is also necessary to examine its brightness, strength, adhesion and so on. Waterborne wet coating is favored by customers because of its high strength, easy recycling, and no pollution.

4. Curved surface film

It is a new transfer technology that can deal with complex shaped parts and is suitable for different materials. It also increases the added value of products greatly because of vivid patterns and colors. This technology is a special chemical treatment of the film, after printing a certain color pattern, flat on the surface of the water, the use of water pressure, the pattern will be written on the surface of the product, and the cover film is automatically dissolved, cleaned and bake After drying, a layer of transparent protective coating is applied. At this time, the product has presented a completely different visual effect.

Film coating process can be divided into cold (cold pressure) film, heat (heat live) film and liquid film according to the cold and heat, pressure conditions and materials. The hot film technology refers to the heat activation of the pre-coated adhesive on the film, which bonds the film and the printed product together; and the cold film presses the adhesive directly to bind the film and the printed product. Together. Both of these methods can produce special films such as UV protection film, waterproof or anti-friction film. Recently, there has also emerged a new technology called "liquid coating". The transparent liquid is brushed on the surface of the product directly with a brush, spray gun, roller or special laminator. Although the technical operation is simple and the price is low, it is still immature.

Third, the film material selection

Commonly used plastic films are: polyvinyl chloride (PVC), polypropylene (BOPP) and polyester (PET) films. Among them, BOPP film (15-20pm) is flexible, non-toxic, high transparency, and low price, and is an ideal composite material in the film coating process.

According to the properties of the film itself and the purpose of use, the thickness of the film is preferably between 0.01-0.02 mm, and the surface tension of the treated surface should be 4 Pa ​​in order to have better wettability and adhesion performance. The corona treatment surface should be uniform and the higher the transparency, the better, so as to ensure the best clarity of the printed products. The transmittance of PET is generally 88-90%, and the transmittance of other types of films is usually between 92-93%. The film needs to have good lightfastness, it is not easy to discolor under long-term irradiation of light, and the geometric dimensions must be stable.

Since the film should be in contact with solvents, adhesives, inks, etc., it must have a certain chemical stability. The appearance of the film surface should be smooth, free from unevenness and wrinkles, and the film should be free from bubbles, shrinkage holes, pinholes, and pits. Due to the limited capacity of the laminating machine, it is also required that the rewinder be neat and the two sides should be loosely and tightly aligned to ensure even coating. Of course, the cost is low.

Fourth, composite strength

Refers to the adhesive fastness between the plastic film and the paper (board) print in the film product, and its size depends on the adhesive force between the plastic film, the paper (board) print and the adhesive (or hot melt adhesive). When laminating, the adhesive force is mainly derived from the mechanical bonding force and physicochemical bonding force between the plastic film, the paper (plate) and the adhesive (or hot melt adhesive).

The key to film fastness lies in the affinity of the glue with the print (ink and substrate). As a film coating industry, printing inks are not optional, that is, only the film that is adapted to the ink market is the only choice, so as a pre-coating film, only the performance of all inks can be considered as high quality. The composite strength of the film products is affected not only by the process parameters in the film production process, such as the temperature, pressure, speed, coating conditions of the adhesive, film tension, and environmental factors of the film coating process. At the same time, it is also affected by the printing process, such as the surface state of the printed product after printing, such as the thickness of the ink layer, the area and distribution of the graphics, the ink characteristics, the use of diluting agents, dusting, and the state of ink drying.

Fifth, the factors affecting the quality of the film

1. The status of the print ink

Refers to paper properties, ink properties, ink thickness, graphic area and print graphic density. When the printed ink layer is thick and the graphic area is large, many paper fiber capillary pores are closed, which impedes the penetration and diffusion of the adhesive, making it difficult for the printed matter and the plastic film to be bonded, and problems such as delamination and blistering tend to occur.

2. Environmental humidity

Between the adhesive and the plastic film and print, the moisture content changes with the relative humidity of the air. Humidity-sensitive prints can cause internal stress due to dimensional changes. For example, the longitudinal stretch rate of printed materials is 0.5%, and the longitudinal thermal shrinkage of BOPP film is 4%. If the water absorption capacity of the printed material is too large and the film stretches, the internal stress caused by heat shrinkage of the film will cause the film products to curl, wrinkle, and bond. Not strong. In addition, in a relatively high humidity environment, the equilibrium moisture content of the printed matter will also change, and a large amount of water absorbed from the air medium will be released from the surface during the hot-pressing compounding process, stagnating at the adhesive interface, and locally forming non-adhesion. phenomenon. Moreover, changes in the equilibrium moisture content of the printed matter (absorbing moisture from the air medium or releasing moisture into the air medium) mostly occur at the edge of the printed matter, making it to form a flounced or tight edge, and it is not easy to form a good film with the film in the hot press composite. Bonding and creases cause production to fail.

3. Temperature control

Heating can soften the film and allow the paper to adhere to the plastic film. However, the temperature is too high, the film is easily broken, and the quality of the product is seriously affected. Therefore, measures should be taken to cool the fan and close the heating wire.

Sixth, the plastic film green development trend

The coated paper becomes a kind of white pollution because it cannot be recycled, and toxic substances such as toluene and thinner etc. evaporate during the film coating process, and harm people's health. In advocating "protecting the environment and protecting resources" today, plastic film coating technology has been eliminated in Europe and the United States and other countries. European and American countries have even refused to import plastic packaging films. Although the water-based plastic coating process and the hot-melt pre-coating dry plastic composite process are currently available, the former still fails to solve the problem that non-degradable and non-recoverable waste of the film-covered printed matter. The latter requires a large amount of heat in the production process and it is abandoned. Materials need to be separated from paper and plastic in water at 60°C, and the cost is high.

With the improvement of environmental awareness, new scientific and technological achievements will eliminate old products, and make the coating process undergo qualitative changes. “Non-toxic, harmless, and green environmental protection” will naturally become the inevitable development trend of the coating process. In addition, the glazing process is also optimistic about it. Glossing is the use of painting, spraying or printing methods to add a layer of colorless and transparent coating on the surface of the printed matter, that is, coating oil, after calendering, drying treatment, increase the gloss of the surface of the printing product, but also improve the stiffness, while After the glazing, the printed waste can be recycled or self-decomposed and does not pollute the environment. Therefore, glazing is a processing technology with great development potential, and it may gradually occupy the market of plastic film coating technology. Although the dilution machines used for oily and alcoholic glazing on the market are toluene and alcohol respectively, which are also harmful to the human body and unfavorable to the environment, waterborne glazing has gradually become the mainstream of glazing processes. The water-based UV glazing process that has been developed so far is an ideal environment-friendly glazing process and is also the development direction of glazing processes in the future.

Reprinted from: Ke Yin Network