Using Postpress to Control Print Color and Gloss (II)

The commonly used polymeric film BOPP, which has no polar groups in its molecule, is a typical non-polar polymer material. After biaxial stretching, the crystalline molecules are thermodynamically stable, but the surface energy of the film is usually very low. The critical surface tension of polypropylene (BOPP) film is 3.4 x 10-2 N/m. When the ink is transferred from the printing plate to the film, the ink in the viscoelastic fluid state drives off the existing air between the film surface and the printing plate, but the high surface energy ink cannot be applied to the surface of the low surface energy film. wet. This adhesion without a base causes the ink in the viscoelastic fluid state to be covered between the upper surface air state and the suspended solid film of the lower back. The molecular density of the viscoelastic fluid state liquid ink is far greater than the molecular density of the gas phase, so the molecules on the upper and lower surfaces of the ink are more attracted by the viscoelasticity liquid ink molecules than the gravitational force of the gas or solid phase molecules. A considerable amount and point to the inner side of the ink print, forming a kind of pulling force that the material wants to agglomerate. When the pressure of the printing plate is eliminated, the ink layer on the surface of the film cannot be wetted, and the in-place auxiliary layer on the surface of the film will have a stress that will automatically shrink to make the surface a spherical tendency, and the transferred ink layer will break. , Pad printing failed.

At the same time, since the plastic film stretches, slits, and rolls up, it is rubbed between the rollers of various equipments, or between the layers when the film is rewound, and the film itself has a 2.0-2.6×. 106/cm high-volume electric rent, the resulting static charge can not be accumulated in time to discharge emissions to the earth. The presence of electrostatic charge not only makes the film use electricity when it is used, but also easily absorbs dust, moisture, oil, and other substances in the air medium, and it also hinders the wetting of the surface of the plastic film by inks, adhesives, and the like.

The color of the print is represented by the specific relationship between ink layer density and reflectivity. The reflectance is equal to the ratio of the reflected light to the incident light; the density is equal to the product of the ratio of log10 to 1 and the reflectance. When the density of the ink layer reaches a certain limit within a certain limit, the color starts to saturate, and the density cannot rise again. With standard density, the maximum contrast can be achieved, the color of the prints is close to the original, the level is clear, the bright area reflects a light level, and the dark tone area can reproduce the texture of the subtle traces. The entire print shines. Otherwise, the ink guide is too thick, the color is darker, and the layers are dull; the ink layer is too thin, the color is shallow, and the layers are gray and dull.

The gloss of prints comes from the drying of the conjunctiva after drying. It can reflect the amount of light in one direction when exposed to light. This is to ensure that the ink layer is properly dried. In order to control the optimal density of the ink color to achieve the best color performance, there should be a certain ink layer thickness (such as when printing on coated paper, the best ink layer thickness is: black ink is 1.8-2.0μm, cyan ink For 1.45-1.7 μm, the magenta ink is 1.25-1.5 μm and the yellow ink is 0.9-1.05 μm).

In the printing press high-speed work efficiency and the paper high-speed collection stacking production conditions, the ink's dry conjunctiva is undoubtedly not have the effective air contact time. The reflection density of the ink will decrease with the slow drying of the ink (the binder is gradually absorbed by the paper fiber or paint layer, and the oxidized conjunctiva is behind), which is often referred to as the "dry denudation phenomenon." Taking black ink as an example, when the coated paper was printed, the reflection density in the field was 1.9. The reflection density measured after 3 days was only 1.55, and the decay rate was 18.42%. When the offset paper was printed, the reflection density on the ground was 1.35, which was measured after 3 days. The resulting reflection density was 1.16 and the decay rate was 14.07%. To ensure that the ink layer is dried in a timely manner without crystallization, it is necessary to rely on the surface purification technology to ensure the color, gloss, and subsequent reprocessing of the printed material.

The color of the print is represented by the specific relationship between ink layer density and reflectivity. The reflectance is equal to the ratio of the reflected light to the incident light; the density is equal to the product of the ratio of log10 to 1 and the reflectance. When the density of the ink layer reaches a certain limit within a certain limit, the color starts to saturate, and the density cannot rise again. With standard density, the maximum contrast can be achieved, the color of the prints is close to the original, the level is clear, the bright area reflects a light level, and the dark tone area can reproduce the texture of the subtle traces. The entire print shines. Otherwise, the ink guide is too thick, the color is darker, and the layers are dull; the ink layer is too thin, the color is shallow, and the layers are gray and dull.

The gloss of prints comes from the drying of the conjunctiva after drying. It can reflect the amount of light in one direction when exposed to light. This is to ensure that the ink layer is properly dried. In order to control the optimal density of the ink color to achieve the best color performance, there should be a certain ink layer thickness (such as when printing on coated paper, the best ink layer thickness is: black ink is 1.8-2.0μm, cyan ink For 1.45-1.7 μm, the magenta ink is 1.25-1.5 μm and the yellow ink is 0.9-1.05 μm).

In the printing press high-speed work efficiency and the paper high-speed collection stacking production conditions, the ink's dry conjunctiva is undoubtedly not have the effective air contact time. The reflection density of the ink will decrease with the slow drying of the ink (the binder is gradually absorbed by the paper fiber or paint layer, and the oxidized conjunctiva is behind), which is often referred to as the "dry denudation phenomenon." Taking black ink as an example, when the coated paper was printed, the reflection density in the field was 1.9. The reflection density measured after 3 days was only 1.55, and the decay rate was 18.42%. When the offset paper was printed, the reflection density on the ground was 1.35, which was measured after 3 days. The resulting reflection density was 1.16 and the decay rate was 14.07%. To ensure that the ink layer is dried in a timely manner without crystallization, it is necessary to rely on the surface purification technology to ensure the color, gloss, and subsequent reprocessing of the printed material.

In the printing process, the sticking of the spray-resistant powder particles on the surface of the printing layer not only affects the color and gloss of the printed product, but also increases the troubles of the post-print coating processing and coating treatment. Commonly used inks have different fixing times and drying times due to different manufacturers.

For dusting, the method used in the past was often wiped off manually. Even the use of a feather duster or a soft cloth can cause a certain hardness of the paint particles to cause scratches on the surface of the ink blot. And due to the incomplete overlap of the contact surface, and the angle of starting and ending the departure angle, the force of the wiping process cannot be even and the dust cannot be completely removed.

In order to increase the brightness of the ink, dry vegetable oil such as linseed oil is often used as a binder. When these substances are in contact with air, the unsaturated fatty acids react with the oxygen in the air, causing the original linear structure of the viscoelastic fluid state liquid ink to polymerize into a network structure, gradually thickening and eventually becoming a solid.

Although this oxidized conjunctiva process has a required time standard, it does not have actual conditions of full contact with the air, and can only be completed slowly in the latter part of the printing process, failing to meet the glossiness that the production quality standards should have. If too much desiccant is added to the ink, the ink's hue properties will be affected and the resulting gloss will not be satisfactory. Therefore, the color under high-speed printing conditions ensures the ink thickness-reflectance-dryness-gloss performance. It is always in a state where it is necessary to follow laws and regulations, and it is difficult to achieve a satisfactory solution. The various efforts that have been made for this purpose have always yielded some kind of benefits, but they still have considerable deficiencies.

Source: HC Network Printing Industry Channel