With the continuous development of China's packaging industry, a lot of low-end packaging is moving towards middle and high-grade. Many cloth products are changed to woven packaging, such as: seed packaging, rice packaging, fertilizer packaging, etc. The surface of the package printed with woven cloth is now mostly printed with laminating film, which makes the image of the entire package more dazzling and improves the grade. At the same time, the packaging factory is also difficult to eliminate due to many failures in the coating film, so that the product quality does not meet the requirements. The following is the analysis and elimination of common faults in the printing process and coating process.
1. Printing process:
1. Steel rolls and plate rolls made by the gravure plate-making factory of the gravure plastic printing machine are printed (back-printed) by the printing machine into coils, and then coated with a laminating woven cloth.
2. Printing inks can be divided into two types: one is special composite ink and then coated after printing; the other is printed on the surface, such as: surface printing pearlescent film is coated after printing, so that surface printing ink is easy There is poor firmness, some colors are easy to fade and change, and it is not easy for multiple inks to overlap. After printing the finished product in the printing, then process the aluminum plating, for example: use the surface printing ink transparent series to print the aluminum plating to get a bright and colorful pattern, and then coat the film.
3. Different types of inks are used, and different diluents are used. The thinner for composite ink is benzene and fat; the thinner for surface printing ink is benzene and alcohol. The two diluents cannot be mixed.
2. Coating process:
The coating machine should mix and melt the coating materials and evenly plasticize. The temperature of the coating machine is generally around 220 ℃. Too high a temperature will affect various physical properties. Too low a temperature will also affect the peel strength, so it should be set according to the equipment used by the user. Set the optimal temperature and then apply it on the woven cloth substrate through the double die, and then quickly cool it through the cooling steel roller, so that it can form a composite film on the surface of the woven cloth to achieve the desired effect.
Coating material selection: one is high-density polyethylene (coating grade), one is low-density polyethylene (coating grade) and a special PP coating material is used together, at present, the type of coating material used is Ethylene Seven A and Ethylene Seven B are more extensive. It is best to control the coating material to about 15 grams per square meter. The ratio of polyethylene and PP special materials is generally 4: 6, the increase of PP materials will make the coating film more brittle and hard.
3. Common faults and troubleshooting
A. No peel strength after coating
After casting the finished product, the product has no strength at all, and the delamination is separated.
Causes and troubleshooting methods:
1. Check whether the layered film corona treatment meets the requirements and determine whether the ink reaches the peel strength. Generally speaking, printing inks use composite inks, but some composite inks cannot achieve peel strength. At present, many ink manufacturers specialize in printing BOPP coated woven cloth special ink to solve this problem, which will increase the peel strength firmly.
2. The delamination is separated from the coating material and the woven cloth. The corona treatment of the woven cloth and the selection of the coating material should be checked to find out the cause and eliminate it in time.
B. Poor peel strength
After coating the film, the peel strength did not meet the requirements, and the peel strength was uneven.
Causes and troubleshooting methods:
1. The compound pressure is small and the coating material melts and the woven cloth substrate is not tightly bonded, which reduces the peel strength. It is necessary to increase the pressure of the pressure roller, so that the gap between the coating material and the woven cloth is tight and tight, and the peel strength is improved.
2. Check whether the coating of the coating material is too thin or uneven to reduce the peel strength; the thickness of the coating layer should be increased.
3. The extrusion compound temperature is slightly lower, the resin plasticization is not good, the molten resin flowing out from the die can not be well compounded with the woven substrate, which will cause the peel strength to decrease.
4. Unqualified coating materials or unreasonable ratio will also cause the peel strength to decrease. The temperature of each part of the extruder should be adjusted to make the resin plasticize well and increase the peel strength.
C. Uneven coating layer after coating
After coating, the coating layer is thick in some places, and the coating layer is thin in some places, which makes the packaging uneven and the peel strength is uneven.
Causes and troubleshooting
1. Improper section temperature control of the extruder, uneven temperature of the die head causes uneven coating layer thickness. The segmented temperature of the extruder should be adjusted in time, and the temperature of the die head should ensure that the temperature of each part is uniform and constant, so that the coating material is evenly coated.
2. The gap between the die openings is not uniform or there are some foreign materials attached to the die openings, so that the amount of extrusion coating in this part is reduced compared with other parts, and the coating layer is thin. The gap of the die mouth should be adjusted in time to eliminate the local attachments of the die mouth and keep clean, so that the coating amount is consistent with other parts.
3. The unevenness of the composite pressure roller or the surface of the rubber pressure roller is aging, the hardness is inconsistent, the surface is uneven, there are pits, scratches, etc., resulting in the inability to maintain a constant value, so that the coating material cannot be evenly coated. To adjust the compound pressure consistent. If it is determined that the rubber roller does not work, it should be replaced in time to ensure uniform hardness of the rubber roller surface.
D. Wrinkle phenomenon occurs when coating the film
Causes and troubleshooting methods:
1. When the position of the substrate is deviated, it will cause wrinkles during the coating and compounding, and with the accumulation of the declination, the degree of wrinkles will relatively increase. It is necessary to adjust the position of each substrate on the line, not to deflect, to avoid wrinkling.
2. After the printing substrate is seriously damp, it will also cause wrinkling and poor peel strength. The printed substrate should be properly stored to prevent moisture.
3. Improper tension setting and control make the tension of each part uncoordinated and mismatched, causing wrinkling. The tension setting and control of each part should be consistent and synchronized. Adapt and match unwinding and rewinding.
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