Summary of gravure printing quality issues for packaging bags, with solutions

I. Misregistration

In multicolor printing, each color does not completely overlap, there is a certain deviation.

1. Cause Analysis

Inaccurate setting of the machine's register system.

The printing substrate has blistering or curled edges.

Inaccurate processing of the printing circumference, and non-standard design of the register marks.

2. Solutions

Reset the machine parameters accurately, and align the photoelectric eye with the register marks.

Replace with qualified and defect-free printing substrates.

The diameter difference of the printing plate should gradually increase from the first color to the last color, because the substrate will undergo slight stretching during processing due to the influence of the material roll, tension, and drying temperature. Therefore, the diameter difference of the printing plate must be considered during plate making. In addition, the register marks used for registration must be very standard and standardized for better registration.

II. Scratches

Scratches refer to the phenomenon where the ink in the non-image areas of the plate is not completely wiped clean and transferred to the substrate, resulting in line-shaped dirt where ink is not allowed.

1. Cause Analysis

There are notches on the ink doctor blade.

Impurities in the ink are brought to the ink doctor blade during operation, forming scratches.

Poor surface treatment of the printing plate.

2. Solutions

Re-use 500-grit water sandpaper to polish the blade until the notch is completely eliminated, or replace with a new blade.

There are two reasons for the existence of impurities.

The ink itself contains undissolved solid substances, forming impurities.

When polishing the printing plate, using 800-grit water sandpaper, the sand grains on the sandpaper fall into the ink, forming impurities. Therefore, it is recommended that ink should not be poured into the ink trough before polishing the printing plate. After polishing, pour the ink in. Before the ink is used on the machine, filter it with a 120-mesh nylon net to remove impurities, which can effectively reduce the occurrence of scratches.

When the plate maker performs chrome plating and polishing on the printing plate, if the surface smoothness does not meet the requirements, it will create resistance during the inking process, resulting in scratches and scumming. Therefore, before the printing plate leaves the factory, it must be carefully polished to achieve the smoothness that meets the customer's production requirements.

III. Scumming

Uniformly printing a dull color on the entire non-image area of the substrate is called scumming.

1. Cause Analysis

Poor ink printability.

Too little pressure on the ink doctor blade.

The surface treatment of the printing plate does not meet the requirements.

2. Solutions

Use high-quality ink, although it is expensive, it has high printability and will effectively reduce the defect rate.

Adjust the air pressure of the air doctor blade to remove scum. For example, if the original blade pressure is set to 0.1~0.15MPa, it can be adjusted to 0.2MPa, and then observe the printing situation and make fine adjustments.

The surface treatment method of the printing plate is the same as the method for solving scratches.

IV. Color Difference

Color difference refers to the phenomenon where the color of the printed matter is inconsistent with or differs from the customer-approved sample, color draft, or sample.

1. Cause Analysis

Incorrect angle of the doctor blade.

Incorrect ink hue.

The printing plate is not cleaned or wiped clean.

2. Solutions

Adjust the angle of the doctor blade. If the color is too dark, increase the angle of the doctor blade; if the color is too light, decrease the angle of the doctor blade, and push the doctor blade holder forward to increase the inking amount and improve the saturation of the color.

Replace the ink with the same color or re-mix the ink color. If the ink color is too dark, you can add some thinner to reduce the color density to achieve color consistency.

Ink clogs in the printing plate cells and dries and hardens, affecting ink transfer. For slight clogging, use powder to clean the clogged ink in the cells; for severe clogging, use plate washing solution to clean and use a copper brush dipped in solvent to clean.

V. Solvent Residue Exceeds Standard

1. Cause Analysis

Inferior ink with impurities.

Inferior solvent with insufficient purity.

Improper printing machine temperature setting, resulting in false drying.

Insufficient drying capacity of the printing machine, incomplete solvent volatilization.

2. Solutions

The binder used in inferior ink is of poor quality and cannot effectively release the solvent. Instead, it causes solvents such as toluene to be adsorbed in the ink. Coupled with insufficient drying temperature, it is difficult to volatilize completely. Therefore, it is recommended to use high-purity, low-viscosity, and high-flowability high-quality ink.

Conduct gas chromatography testing on the purchased solvent to check whether its purity is qualified. The purity standards for commonly used chemical solvents are as follows: Toluene ≥99.5%; Xylene ≥95%; Isopropyl alcohol ≥99.7%; Ethyl acetate ≥99.5%; Methyl isobutyl ketone ≥99.5%. Solvent residue standard: Toluene ≤3㎎/㎡; Total solvent ≤100㎎/㎡.

The drying temperature of the printing machine is set, generally from high to low for surface printing products, with smaller areas set at 30~50℃, and larger areas set at 40~45℃. Surface printing products generally have large-area printing in the first few units. For reverse printing products, it is generally from low to high, with the lowest point generally set at 45~55℃. For large-area printing patterns, it can be set at 60~65℃. The temperature is set according to the printing order and area size, with large areas at high temperatures and small areas at low temperatures. This ensures that the solvent is fully utilized, reducing odor and preventing exceeding solvent residue standards.

VI. Printing Defects

Printing defects refer to incomplete printing patterns or text caused by uneven printing rollers and printing roller pressure.

1. Cause Analysis

凹孔 on the surface of the printing roller can easily lead to printing defects.

Insufficient printing roller pressure can easily lead to printing defects.

2. Solutions

Polish the surface of the roller with 凹孔; in severe cases, consider re-plating.

Increase the printing pressure of the roller to improve the transfer rate of the intaglio ink.

VII. Adhesion

Adhesion refers to the ink on the printed surface sticking to another contact surface (usually the back of the film) or ink adhering to another contact surface after the printing is wound up.

1. Cause Analysis

Insufficient drying capacity of the equipment and excessive tension during winding can easily cause adhesion.

Hot and humid weather, and excessive moisture in the ink and solvent can easily cause adhesion.

2. Solutions

Improve the drying conditions of the ink and adjust the winding tension during winding, ensuring that the film roll does not show any bursting phenomenon as a minimum requirement.

In hot seasons, the solvent of the ink should be stored in a sealed container after use to prevent the ink from absorbing water and affecting the printing quality. If the ink solvent absorbs water, the printing speed and drying temperature can be increased to prevent ink adhesion.

VIII. Plate Clogging

Plate clogging can cause blurred printing patterns and text, changes in printing color, and in severe cases, it may even prevent further printing.

1. Cause Analysis

Inappropriate mixing ratio of the mixed solvent.

The temperature in the workshop is too high.

The air intake of the equipment is too large, blowing the printing plate dry and affecting the ink transfer.

2. Solutions

Readjust the solvent ratio and add slow-drying solvent.

Lower the workshop temperature.

Control the air intake to reduce plate drying.

IX. Burrs

Burrs mainly appear at the edges of text and patterns. Dry air friction with the substrate film generates static electricity, repelling the ink transferred to the substrate, causing ink spatter and burrs.

1. Cause Analysis

The working environment is too dry.

No anti-static agent is added to the ink.

The printing machine is not grounded or equipped with an anti-static brush.

2. Solutions

Spray water around the printing machine to increase humidity to over 30%.

Use high-quality ink or add anti-static agent to the ink.

Grounding the printing machine and installing an anti-static brush can effectively reduce burrs.

X. Poor Adhesion

When printing on polyolefin films, the ink film falls off or partially falls off after drying, when rubbed, or when peeled off with adhesive tape.

1. Cause Analysis

Incorrect ink used for printing film.

The surface tension of the polyolefin does not meet the requirements.

The ink film is not completely dry or insufficiently heated.

2. Solutions

When using ink, adhere to the principle of using dedicated ink and not mixing different types.

Prepare dyne water and check the surface treatment of the film.

The printing object, especially materials with high hygroscopicity, should not be stored in a humid environment; when printing, the thinner should be adjusted according to the temperature and humidity.