What can I do if the white and black inks on the packaging bag are not opaque enough?

Many packaging bags require a white ink base during printing, but the white ink is not opaque enough. In addition, some require full-coverage printing with black ink, which also lacks sufficient opacity. To address these two issues, Shunxingyuan Packaging, a professional flexible packaging manufacturer, believes that solving such problems requires starting with the main factors affecting ink opacity, such as ink color concentration, ink layer thickness, and pigment dispersion, and then conducting corresponding experimental analysis.

1. Increase Ink Color Concentration

Increasing ink color concentration is a major method to increase ink opacity. During printing, a specially formulated titanium white ink with a color concentration of 55% was used (ordinary white ink has a color concentration of 25%, and titanium white pigment is currently the whitest and best-covering white pigment). Although its opacity is better than ordinary white ink after printing, it still does not meet the requirements. Due to its high color concentration, less solvent, and fast drying, it is easy to cause plate clogging, poor leveling after ink printing, and powdering. Therefore, other methods should be considered.

2. Ensure the Ink Layer Thickness Meets the Standard

We know that the gravure ink layer thickness can reach 8-15μm, but if the gravure white ink layer is only 3-4μm and does not meet the standard, problems will occur. This can be investigated from the following aspects.

1. Plate Roller Production

The engraving depth of the plate roller from the supplier has reached the maximum (60μm), but due to technical limitations, the engraved cells are conical. Conical cells are prone to plate clogging, resulting in poor ink transfer. Therefore, although the ink storage capacity is increased, the ink transfer capacity is not increased. At present, for better ink transfer, the cells should be trapezoidal or honeycomb-shaped. These two shapes have a large ink storage capacity and are easy to transfer ink. In addition, the smoothness of the cell edges and the thickness of the chrome plating also affect the transfer of ink, so the method of printing twice can be considered to solve the problems caused by the plate roller.

2. The Hardness of the Impression Roller is Too High

The printing material is relatively soft, resulting in too little ink transfer from the cells. A softer impression roller should be used.

3. The Printing Pressure is Too Low

This results in low squeezing pressure on the ink in the cells and poor transfer. The printing pressure should be increased.

4. The Film Has Been Stored for Too Long After Surface Treatment

This results in poor ink wetting and adhesion. Therefore, printing should be done promptly after surface treatment of the printing material.

5. The Pigment Concentration of the Ink is Too High

This results in too fast drying speed, affecting inking. The concentration and viscosity of the pigment should be adjusted appropriately.

6. Excessive Static Electricity in the Film During Printing

Consider eliminating static electricity and then using an electrostatic inking device to improve the ink transfer rate.

7. Consider the Performance of the Printing Machine

After printing with the above measures, the ink layer thickness has been significantly increased, but the opacity of the white ink still does not meet the expected requirements. Therefore, the problem of gravure ink itself needs to be considered again.

3. Control the Pigment Dispersion

The opacity of the ink essentially depends on the ratio of the refractive index of the pigment to the refractive index of the binder. When this ratio is 1, the pigment is transparent; if the ratio is greater than 1, the pigment is opaque, i.e., it has opacity. The refractive index of the pigment is affected by the pigment dispersion and the difference in refractive index between the pigment and the binder. The higher the dispersion of the pigment, the smaller the above difference, and the better the transparency of the pigment.

The pigment used in the experimental white ink is titanium dioxide. Titanium dioxide is a white crystalline powder, the most opaque of all white pigments, with a refractive index of 1.84-2.55. It has high dispersion, good light resistance, heat resistance, and alkali resistance.

Other printing methods use ordinary pigments, and a thinner ink layer and lower color concentration can achieve the required opacity, but gravure printing cannot. The particle size of gravure ink pigments should be less than 5 μm, and gravure inks are thinner than inks used in other printing methods, so they have high dispersion and high transparency (i.e., low opacity). If the titanium dioxide particles used are fine and have high dispersion, the opacity will be weak. If the desired opacity cannot be achieved by increasing the particle size, other printing methods must be used.

If ink made with titanium dioxide with a slightly larger particle size is used for printing, the opacity of the white ink will increase. This method is more time-saving and labor-saving than increasing the ink color concentration and ink layer thickness, and it can also reduce production costs.