Postpress and print color and gloss

"Chinese character information processing and printing revolution" was rated as China's major engineering and technological achievements in the 20th century, marking that China has successfully completed the second printing technology revolution from lead line printing, lead printing to photocopying and offset printing. The large-scale technological transformation covering the entire printing industry and related industries was nearly a millennium away from the first printing revolution in our country, which resembles the printing of engravings and the invention of movable type printing. “In the past thousands of years, some traditional industries have disappeared under the impact of new technologies, and the printing industry has always been able to quickly integrate with the new technologies of various dynasties and gain new opportunities for development from the use of new technologies” ( Excerpted from the Director of the Press and Publication Administration, Mr. Shi Zongyuan, at the 7th World Printing Conference on the theme of "The Challenge of the 21st Century Knowledge Economy on the Printing Industry".

1, surface purification and print color, gloss

The surface purification process is to generate strong ionization and excitation on the printed matter by the electrostatic electric field, which can automatically absorb dust, fluff, accelerate the oxidation of the ink, and impact the surface of the crystallized conjunctiva.

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.

1 printing surface and color, gloss

The printing surface is the general name for the surface of the printing carrier and the ink layer, and is also the material basis for the color and gloss performance of the printed matter. In today's rapid development of modern printing, printing carriers are mainly paper and polymeric film two categories.

a, paper

The smoothness and gloss of 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. In general, the heating and drying methods for water-based paints include hot-air drying and dry-cast drying. Coating papers have great differences in the mechanisms and processing methods used to remove water from applied coatings. The hot air surface is volatilized and dried. In the drying stage, hot air is used to force drying from the surface. Most of the water is evaporated on the coated surface. The molecular structure of the coating is arranged in an ordered manner along the evaporation direction toward the surface under the effect of heat, and the backside of the cast coating is infiltrated and dried. The drying stage is when the coating is in a wet state, it is pressed with a roller and pressed against a mirror hot cylinder for drying. The moisture of the coating is transmitted through the pores of the fiber. The molecular structure of the coating under the thermal action also follows the evaporation direction, but it is back-facing. Ordered. Regardless of the molecular structure that is arranged on the surface or on the back, there are always some more active molecules that are more active under heating conditions, and there is considerable energy to break away from the attraction of other molecules to the direction of the original row or in the opposite direction. Disorderly movement. When these coating molecules free from intermolecular gravitational motion reach the working surface of the applicator roll or cast coating cylinder, the enormous pressure generated by the application roller or the working interface of the cast coating cylinder forces them to continue to move and escape the coating in the coating. Stay on the surface. This kind of static on the surface of the coating is exactly the same as that of the application roller or the cast coating cylinder, although it is chrome-plated and throws off the river, but there are still fine lines and uneven positions of rough marks, which results in the absence of molecules when the hot-press coating layer is dried. As a result of the sequence movement, the surface of the paper should have a slight dent in the smooth surface, or increase some of the powder peaks, and thus appear different rough. When the light propagates to the surface of the paper, which is not so conspicuously noticeable, the uneven surface makes the reflection angle change, and the visual signal from the paper surface is weakened, ie, the glossiness is not strong.

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. This actual working condition will also cause the paper fiber to absorb the binder in the ink, and the uneven surface will increase the surface area and increase the surface area, so that the ink will reflect from the surface of the pigment particles and the binder material will be greatly contained in the pores of the paper fiber. After absorption, the refractive index of the pigment suspended between them is poor, resulting in a decline in the transparency of the ink. 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 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 avoid the paper entering a new process when the ink blots are not dry, can easily cause the back side of the printed sheets to be adhered, resulting in the destruction of the surface of the imprinted imprints, and the occurrence of defects in the backing and sticking of the backside of the printed sheets. In the printing press delivery process, a powder spraying device is provided. Commonly used air dusting devices (other electrostatic powder spraying devices and liquid powder spraying devices, because the device cost is higher or only for special blocks, currently used less), is the powdered particulate matter with the compressed air Spray on the printing surface, so that these powder particles adhere to the imprinted layer of viscous ink on the surface of the surface of the imprint.

Based on the theory of particle-size Jones-Higgins,

LaJones and CCHiggins use the density difference between adjacencies (density

The value of differences represents the granularity, and the difference in density between adjacencies is obtained according to the formula of SΔD=IDa−IDa1. The application of the theory to prints is consistent with the grainy reflection of continuous tone photographs made from silver particles, which creates a coarser, lighter reflection of the light's surface by the particulate new surface of the print. The pulverized particles remaining on the surface of the print not only affect the gloss of the printed product, but also become an isolated layer of granular structure. The penetration of light due to the presence of the coated particles causes obstacles. If we print a layer of yellow (Y) ink on white paper, we spray it for stickiness. On this basis, a layer of magenta (M) ink was printed, and the standard inter-color red (Y+M=R) was originally obtained after the same amount of ink was used twice. Due to the influence of dusting particles, the color of the entire print is caused, and at the same time, the ideal red light appears. The resident permanent opaque mineral particles block only the magenta light reflecting the color of the upper layer ink, and the cyan color just touching the surface of the underlying ink. After the particles, it penetrates the ink on the upper layer, which is reflected by three primary colors, such as white light and the like, thus affecting the adhesion of the next color sequence superimposition and the color of the dot overlap. Similarly, print on the white paper firstly. After the color is sprayed and the other color is printed, the printed dots are inconsistently colored due to the influence of the color and transparency of the dusting particles, which causes the change in transmission density of the other color to change the transmittance and reflectance. Phenomenon, so there are various forms of color change. In addition, the barrier properties of dust particles make the surface energy of the imprint change, which will further affect other processing after printing.

b. Polymer film

Polymeric thin products such as PE, BOPP, PET and other high-molecular materials are new-type printing carriers 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. The critical surface tension of polypropylene (BOPP) film is 3.

4×10-2N/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.

2 imprinting status and color, gloss

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.

3 purification treatment and color, gloss

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 if it is used

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