Effect of Raw Material Performance on Print Quality Control

The selection of raw materials in the printing process will have both artistic effects and technical performance on the printed image, and it will also affect the stability of the printed image quality. In addition, the performance of raw materials can also affect production efficiency. Raw materials that do not meet quality requirements can reduce productivity and waste operator extra energy.

The most important raw materials in the printing process are paper or other printing materials and inks. Of course, less important materials such as adhesives, stamping foils, varnish, and binding materials are also included.

The purpose of using other materials in the printing process is to form an auxiliary material for the final image on the substrate, such as a developer solution, a proofing material, a PS plate and other materials used as an image carrier, and a material used in a printer to assist in forming an image. The components in the offset fountain solution; there are also some consumable materials, such as tapes, polyester bases, lubricants, and some developer-fixing drugs; these can be classified as consumables and are not discussed here.

Many properties of raw materials can be quantified by appropriate tests. However, these tests sometimes require sophisticated equipment and skilled operators. The testing of raw materials takes a lot of time to test and perform data analysis and interpretation of the results. In addition, the results of the tests do not necessarily serve as a predictor of the quality of printed products. Therefore, how to selectively detect raw materials becomes an important issue. The key issues for the management personnel of printing companies must grasp the quality control of raw materials are: what important printing performance should be monitored for raw materials; what monitoring items should be selected as a means of predicting the printing performance of raw materials; how much should be invested in testing in order to achieve the maximum benefit of printing enterprises? .

The following sections discuss the printing properties of raw materials, the detection of raw materials, and some related inspection indicators, method principles, and other aspects.

(A) The aesthetic and functional requirements of the quality of printed products

The design of modern packaging and decorating products has become more and more upscale, and more emphasis has been placed on the aesthetic requirements of products. To this end, many high-grade materials and new printing processes have been developed that meet the aesthetic requirements. The choice of substrate and ink before printing determines the aesthetic effect of the quality of the printed product. The choice of materials includes: the artistic and technical aspects of raw materials. The design of print product designers is always subject to certain conditions. Unlike newspapers and magazines, raw materials have been identified, and the choice of materials for packaging and printing not only needs to consider aesthetic requirements, but also its structural and physical properties. When the choice of materials is not limited, only the artistry of the product is taken into consideration without considering technical issues. The artistic choice of the product is not a matter for the printer to consider, but the discussion here will involve technical factors that produce visual effects and serve as artistic components. From the point of view of the raw materials themselves, the technical aspects of aesthetics can be divided into three parts: apparent, chemical and physical.

1. Appearance factors and their quality requirements

(1) Color: The comprehensive choice of ink and substrate should make the color representation of the critical area of ​​the print as large as possible (especially saturation and brightness). In production practice, it is sometimes necessary to choose between standard four-color inks. The color of the printing material should be neutral, ie white. However, in reality, the human eye can conform to a slightly color-shifted printing material (such as a newspaper) and use it as neutral white. The human eye then uses this "white" as a reference to perceive other colors.

(2) Gloss: For the glossiness of paper and ink in image reproduction, it is required to be as high as possible in order to maximize the color saturation and brightness of the print. For writing supplies, paper and ink have lower gloss requirements. The drawings and suede photo manuscripts have no definite requirements for the gloss of the paper base and the ink.

(3) Lightness: The lightness of printed materials should be as high as possible in order to obtain maximum contrast. The so-called lightness refers to the total reflectance for all visible spectra, and the darkness of black ink should be as low as possible. We generally use the optical density to describe the "lightness" of the ink. The higher the density, the lower the lightness.

(4) Opaqueness: The opacity of the printing material is the ability to prevent the image on the other side of the printed sheet from being reflected. It should be as high as possible, especially for publications such as books and magazines.

2, chemical properties and quality requirements

(1) Lightfastness: The ink and substrate should have the ability to resist discoloration and discoloration. This is important for outdoor advertising posters, but not for magazines, newspapers, and product catalogs.

(2) Chemical resistance: The ink and substrate should have resistance to acids, alcohols, and alkalis. This is important for labels, package printing, and less important for other prints.

3, physical properties and quality requirements

(1) Strength: The tensile strength, tear resistance, crack resistance, and folding resistance of the substrate should be as high as possible. In production practice, some of the above-mentioned properties are very important, such as the crack resistance of corrugated cardboard boxes, the crack resistance of children's books or banknotes, and the folding resistance of maps. The stiffness of the cardboard is also important for the carton. It must have a sufficiently high stiffness but it cannot be so high that it is difficult to bend.

(2) Smoothness: For most text and image prints, printing materials are required to be as smooth as possible. For embossing and gravure printing, the smoothness is particularly high. For lithographic offset printing and filter printing, high quality prints do not require much smoothness. Sometimes textured paper or uncoated paper is intentionally used to obtain the surface effect or texture of the original.

(3) Texture: The texture of the printing materials should be as uniform as possible to facilitate the printing of uniform images.

(4) Quantitative and Thickness: Generally, as the ration of paper increases, the translucency of the paper also increases, and users generally do not want to increase the weight of a printed matter such as a magazine to be mailed.

(5) Abrasion resistance: The wear resistance of the ink should be taken into account when designing the printed product. This indicator should be as high as possible. Especially for those who are in the process of transportation, the printing products tend to produce friction, such as cartons or product catalogs.

(B) Process requirements for image quality

For raw materials, the process factors that affect the quality of the prints are those that affect the image of the substrate. They include dot size, uniformity of coverage, blots, and smudges. From the three aspects of appearance, chemistry, and texture, the above factors are analyzed.

1. Appearance factors and their quality requirements

(1) Viscosity of ink: Multi-color offset printing adopts wet and wet printing, so the ink viscosity affects the overprint of the ink film. In terms of process requirements, the ink prior to printing the color sequence should have a higher viscosity than the subsequent ink. Otherwise, the reverse pulling phenomenon will occur.

(2) Viscosity: For typographic and embossing processes, low-viscosity inks are generally not as clear as high-viscosity imprints. For gravure and flexo printing, too low viscosity of the ink will reduce the density of the print.

(3) Particle fineness or dispersibility: Large aggregates or other particles of the pigment in the ink may cause unevenness in the printed image or additional plate wear, which may lead to image destruction.

(4) Pigment concentration: When the same ink film thickness is formed, the ink density of the low pigment concentration is much lower than the ink density of the high pigment concentration, and the thickness of the transfer ink film must be increased. However, low pigment inks can form higher gloss than high pigment inks.

(5) Emulsification: In the lithographic offset process, the degree of emulsification of the ink affects the dot gain, overprint, and drying time and other print quality. The volume percentage of the water-containing emulsion after embossing ink must not exceed 50%. Otherwise, it is difficult to achieve the balance of ink and wash.

(6) Absorption: Paper with high absorbency will have problems such as transmission, low gloss, and powdering. Conversely, low-absorption papers may have problems such as long drying times, back-scratching, or scabbing.

In addition, the sensitivity properties of film and plate emulsions affect the resolution of the printing process.

2, chemical properties and quality requirements

(1) pH (pH): The pH of the paper should be 7.0, which is neutral, otherwise it will affect the drying time and make the back sticky.

(2) Temperature and Relative Humidity: The relative humidity (RH) and temperature of the paper should be consistent with the temperature and humidity of the workshop. The temperature and humidity differences between the workshop and the paper can cause problems such as overprinting and wrinkling.

(3) Drying time: The performance of the ink must be taken into account when it is dry. It must not be dried on the printing roller, but must be quickly fixed on the paper.

3, physical properties and quality requirements

(1) Dust off and hair loss: Flocking can lead to defects in the traces or graphic parts of the non-graphic part. Sheets must be free from falling powder on paper.

(2) Paper Strength: Paper with low surface strength When high-viscosity inks are used, the paper surface will be napped and even cracked, which requires the surface strength of the paper to be high.

(3) Length of ink: If the ink is too short, a pile of paper will be produced. If the ink is too long, ink mist will splash and the image quality will be affected.

(III) Stable and consistent quality

The key to evaluating raw materials is to consider how much the raw material properties can change within the limits of the print quality. It is also necessary to consider that this variation range should not affect the printing speed or cause other problems in the production process.

The properties of the raw materials described above require further testing. If one can specify a specific request value and be able to determine the control range of the data, then the ink or paper properties can be measured and controlled. However, it is difficult to determine the control range.

The final test method for the raw materials in the printing process is used under actual printing production conditions. However, this method is only suitable for large-scale paper mills or ink factories, and most print shops cannot afford such tests. Most laboratory tests more or less lack the ability to predict the actual printing production conditions.

Source: Ink Information