Analysis of Milk Powder Packaging Bag Printing Process (3)

Dry compound process control

1. Adhesive coating amount

In dry composite processing, the amount of adhesive applied to a certain extent determines the performance of the composite packaging product. The amount of coating is insufficient and the appearance of the coating is liable to occur. Insufficient adhesion. Tunneling, peeling, various resistances (resistance to cooking, resistance to content, etc.) are low. Leakage, broken bags and other issues. Especially when there are micropores on the surface of the ink, the adhesive easily permeates into the ink, causing various problems. Pay special attention to multi-color printing and field printing.

If the amount of coating is too large, adhesive overflows easily, and the appearance of the composite is poor (stretching, CO2 generation and bubble generation). Tunneling effect. Increased residual solvent. Curing speed and other issues. The determination of the optimum coating amount of the adhesive should fully consider the structure of the film, the properties of the ink, and the post processing conditions. End use and other factors.

In the compounding process, the depth of the cell depth of the texture coating roller. The number of screen lines and the shape of the cells have been determined so that the amount of coating depends on the solids content and viscosity of the binder formulation. The solid content of the binder is basically in direct proportion to the coating amount. When the solid content is high, the coating amount is correspondingly large. For the same binder, when the solid content is increased, the viscosity of the binder is increased, thereby increasing the transfer rate of the binder. When the viscosity is increased to a certain extent, the transfer rate reaches the maximum. As viscosity increases, the transfer rate decreases. Therefore, each coating roller corresponds to a maximum coating viscosity.
When the depth of the cell depth and the solid content of the binder are determined, the coating amount of the anilox roller is also affected by the pressure and hardness of the rubber roller and the flatness. If the rubber roller used is soft and the pressure acting on the roller is large, the amount of coating is small, and vice versa. This is because the rubber roller squeezes into the depth of the anilox roller net under pressure and displaces a part of the adhesive solution, and the coating amount of the adhesive on the film is accordingly reduced. Therefore, the hardness of the rubber roller should be controlled around the HS85. It should also be noted that the hardness and pressure on both sides of the coating press roll should be uniform, otherwise it will cause problems such as uneven coating.

2. Adhesive coating control

In the compound type processing, the uniformity of the coating is very important, and it has a great influence on the peeling strength, transparency and heat seal strength of the film. Many appearance defects, such as air bubbles and white spots, are often caused by uneven coating. In addition to the effects of pressure and hardness uniformity of the coating roller, the coating effect of the adhesive is also affected by the following factors.

(1) Coating performance of adhesive
The coating performance of the vacuum aluminum plating adhesive is superior to that of the general-purpose adhesive. The leveling properties of adhesives produced by different manufacturers vary greatly. In order to ensure excellent coating conditions, they should be confirmed through experiments prior to formal use, so that the solid content of the adhesive can be adapted to the anilox roller. It should be pointed out here that when the binder is used for a long time or when the temperature is too high, some of the particles will react and form particles before being coated. In this case, the adhesive should be replaced.
(2) Squeegee flatness and mobility
Squeegee flatness is good, then the coating amount is uniform. The squeegee's degree of fusibility depends on how the squeegee is mounted. If a blade or lining has foreign material on the turret, it will also affect the squeegee's flatness.
The movement of the scraper left and right plays an important role in reducing the scraping line, increasing the scraper utilization, and reducing the wear of the anilox roller. The net wall wears less, the net hole is relatively deep, and more liquid is contained, which is favorable for achieving the required coating amount.

(3) Smooth Roller
The smoothing roller can further evenly bond the adhesive to eliminate network traces. When using a smooth roller, one should pay attention to the mutual adaptability of ink and adhesive, do not damage the printing ink layer; Second, pay attention to the speed of the smooth roller and the rotation speed of the coating roller, should use a smooth roller after reaching a certain line speed .

(4) Dilution solvent (ethyl acetate)
Ethyl acetate is a common solvent for diluting the binder. The content of water or alcohol in ethyl acetate has a great influence on the coating state of the binder. The volatilization rate of water is much lower than the volatilization rate of ethyl acetate. If the water content exceeds the standard, the volatilization rate of the solvent will be seriously reduced, and the spreading speed and hardening speed of the binder will be reduced. In addition, the curing agent in the adhesive and water or alcohol can quickly form CO2, thereby limiting the infiltration of the adhesive, resulting in bubbles, so the amount of water and alcohol in the ethyl acetate solvent should be strictly controlled.

(5) When the surface tension of the base film film is too low and the difference in surface tension with the adhesive is too small, the adhesive may not be sufficiently dispersed and wet on its surface. The difference between the surface tension of the film and the surface tension of the adhesive should be greater than 1.2 × 10-2 N/m. Otherwise, poor wetting may occur. Coating hollow and other issues.

(6) Composite Roller System
The temperature and pressure of the compound press roll system can further activate the adhesive to fully spread the adhesive and eliminate the fine defects of the adhesive.
(7) Workshop environment
1 humidity. Humidity greater than 70% will significantly affect the coating quality. In the high temperature and high humidity season, if the workshop temperature is high, the evaporation of ethyl acetate will cause the temperature of the coating system to decrease, and the moisture will condense on the roller and the anilox roller to make the adhesion. The phenomenon of whitening of the agent not only destroys the coating effect of the adhesive, but also seriously affects the performance of the adhesive.
2 dust. The dust in the air of the workshop will adhere to the adhesive film layer, destroying the uniform and continuity of the adhesive film layer, thereby affecting the appearance; the air supply from the drying box can also cause the above result if it contains dust.

(8) Cleaning of the system
1 Clean the blade. The scraper should be cleaned after each coating, otherwise the residual dry adhesive on the scraper will affect the coating effect and form scratches.
2 Clean the anilox roller. The cleaning of the anilox roller is very important, and the clogging of the web site will affect the coating effect of the adhesive and the uniform continuity of the adhesive film layer. Therefore, in addition to daily daily cleaning, it is necessary to insist on regular use of a dedicated cleaning agent for regular cleaning. At the same time, when changing the adhesive type, it must be carefully cleaned to prevent the adhesive interaction and affect the adhesive performance and coating effect.
3 Cleaning of other parts The guide roller, compound rubber roller, compound roller and other parts of the 1000-type compound machine should also be wiped regularly to avoid adverse effects on the adhesive film layer.