Effect of Postpress on Color and Luster of Prints

The color and gloss of prints are reflected by the surface of the printing medium, the thickness of the ink layer, and the degree of ink drying. When its amount has been timed, through innovative surface purification technology, it can improve the effect of prints and enhance the color and luster of prints. The printing surface is the general name of the printing substrate and the surface of the ink layer, and is also the material basis for the color and luster of the printed matter. The printing substrate is mainly composed of paper and polymeric film.

a. Paper The smoothness and gloss of the paper are mainly determined by the papermaking raw materials and the papermaking process. When the surface smoothness and gloss of the paper are high, no pressure is required to carry out embossing, and the contact area between the printing plate and the blanket can be maximized, and the transfer of the dots can be completed more uniformly and completely. Clear, rich levels, soft tone, full color, bright gratifying.

In order to improve the smoothness and gloss of the paper, paper mills apply water-based paint to the surface of the base paper and heat and dry it. There are two methods for heating and drying water-based coatings: hot-air drying and drying on the cylinder surface. At the same time, since the recessed portion of the paper surface needs the ink to reach the surface of the paper before it is dried, the thickness of the ink layer is actually reduced, the density of the ink layer is changed, and the thickness of the ink layer is different from that of the original layer. In this state, the absorption of the binder in the ink by the paper fiber is also caused, and the uneven surface enlarges the surface area, and the ink is reflected from the surface of the pigment particles and is suspended after the binder is absorbed by the pores of the paper fiber. In the meantime, the poor refractive index of the pigment causes the ink transparency to decline. When performing color printing, if the transparency of the bottom layer ink is poor, it will affect the light transmission on the paper surface and reflect the gloss of the paper surface; when the transparency of the top layer ink is poor, it will affect the light through the upper layer ink and penetrate into the bottom layer ink. Can not be fully and selectively absorbed by the lower layer of ink, the performance of the color should not be formed after the overprint.

A large number of high-speed printing presses at home and abroad, in order to prevent the paper from entering a new process when the ink blots are not dry, cause the backside of the printed sheets to be adhered, resulting in the destruction of the surface of the imprinted imprints, and the occurrence of defects on the back of the printed sheets and sticking faults in the printing. During the paper collection process, dusting devices are installed. Air dusting devices are usually used (in addition to electrostatic powder spraying devices and liquid powder spraying devices, because of the high cost of the device or only for special blocks, which are currently used less frequently), powdery particulate matter is sprayed with compressed air. On the printing surface, the powder particles are adhered to the imprinted ink layer with a strong ink under the strong force of compressed air to form a new surface of the granular particle structure on the surface of the imprint.

If a white (Y) ink is printed on white paper, dust is applied to prevent sticking. Another layer of magenta (M) ink was printed, and the standard inter-color red (Y+M=R) was originally obtained after two equal amounts of ink. Due to the influence of dusting particles, the color of the entire print is caused. Permanent magenta light that blocks opaque mineral particles and can only reflect the color of the upper layer of ink. After the cyan particles that meet the surface of the underlying ink, it penetrates through the upper layer of ink. , thereby affecting the color of the next color sequence superimposition and dot overlap. Similarly, when a color is printed on a white paper, that is, after the powder is sprayed, and another color is printed, the transmittance of the other color is changed due to the influence of the color and transparency of the dusting particles themselves. The change of reflectivity of the printed dots is inconsistent with the appearance of color. In addition, the barrier properties of the dust particles will change the surface energy of the print, which will further affect other processing after printing.

b. Polymeric films Polymeric thin products such as PE, BOPP, PET, etc., are new-type printing substrates that have been rapidly applied in the past 20 years. For example, the biaxially-stretched polypropylene BOPP film causes the microcrystallizing effect of the crystals in the stretching direction (stretching of the plastic melt during the recrystallization process) to cause plastic sliding of the crystals in the film. Arrangement, surface structure change, turbidity from non-oriented (NO) of 0.078-0. 117% mm, reduced to biaxially oriented (BO) of 0.039-0.078% mm, so that increased refractive index, with high transparency (≥ 95 %) and high gloss (≥ 85%) surface gloss.

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, and the critical surface tension of the polypropylene (BOPP) film is 3.4×10 −2 N/m. Since the plastic film stretches, slits, and winds, it is rubbed between the rollers of various equipments, or between the layers when the film is rewinded. The film itself has a 2.0-2.6×10 6 / Cm high-volume electric rent, the resulting static charge can not be accumulated in the earth to discharge emissions. 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. When the density of the ink layer reaches a certain limit, the color begins to saturate, and the density can no longer rise. To control the standard density, the maximum contrast is achieved, the color of the print is close to the original, the level is clear, and the bright area reflects a bright level. Really reproduce the texture of 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 colors are 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. Due to the incomplete overlap of the contact surfaces, and the start and end of departure angles, the force of the wiping process cannot be even and 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. Oxidation and conjunctiva process, although the required time standards, but there is no actual contact with the air of the actual conditions, can only be completed slowly in the late printing, and can not reach the production quality standards should have gloss. 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.

Source: China Design Printing Network