I. Introduction
The difference between bar code printing and general graphic printing is that the printing must meet the requirements of the bar code national standards on optical properties and dimensional accuracy so that bar code symbols can be correctly identified.
Barcode printing is generally divided into on-site printing and off-site printing. Off-site printing refers to pre-printing bar code labels for later use. This kind of printing is suitable for the printing of bar code marks (such as serial serial numbers, etc.) that need to be used in large quantities, with stable code structure, identical marks, or regular marks. On-site printing refers to the need to generate the required bar code identification from a dedicated device at the location where bar code identification is required. On-site printing is suitable for bar codes with a small number of prints, many types of labels, or emergency use, and on-site printing is also used in store codes. On-site printing can be accomplished using a general graphic printer or a special bar code printer. This article describes several off-site printing principles and their impact on readability.
Second, the printing process
1. Printing principle
(1) Offset: Offset is based on the principle of repulsion of oil and water. The graphic part and the blank part of the printing plate are almost on the same plane. During printing, the printing plate surface is first immersed in water and wetted, the ink is applied to the printing plate, and the graphic part is inked, and then transferred onto the surface of the printing material via a blanket. The centre of the line of printed matter is darker in ink, and its edges are slightly subdued.
(2) Gravure: The printing image part is a recessed ink hole. When printing, the entire printing plate layout is first covered with ink, and the blank part of the ink is scraped with a squeegee, and the ink left in the depressions of the printing plate is transferred to the surface of the printing object by the printing pressure. The size, depth, and distribution of the ink holes affect the thickness of the ink layer on the printed product. When the number of the screen lines is low, zigzag lines are easily generated.
(3) Flexo printing: Flexo printing is a kind of embossing, and its graphic part is higher than the blank part. During printing, the ink roller transfers the ink to the protruding portion of the printing plate, and under the effect of the printing pressure, the ink on the printing plate is transferred to the printing material to complete the printing. Due to the flexographic plate and platemaking conditions, especially the rubber plate, its hardness is low and it is easily deformed. For bar codes that require high precision, this can only satisfy the printing of larger magnification factors.
2. printing material
(1) Print carrier
Materials commonly used for packaging, such as paper, cardboard, plastic film, aluminum foil, tinplate, and corrugated cardboard, can be used as barcode printing carriers. Since bar code prints require high optical properties and dimensional accuracy, production should be strictly controlled.
First of all, attention should be paid to the reflection, transparency and translucency of the material. White ink is used for the transparent plastic and dark aluminum and tinplate surfaces to increase the reading rate.
Secondly, from the viewpoint of maintaining the dimensional accuracy of ingots, materials that are affected by temperature and humidity and tension, small in size, stable in color, good in color fastness, smoothness, and good finish should be selected. For example, when the carrier is paper, a coated paper, an offset paper, or a white plate paper may be selected. In plastics, plastic films with high tensile strength and small deformation can be used. Surface treatment is generally required to ensure the fastness of bar code symbols. Corrugated cardboard for large packaging, often used after the printing paste or flexographic printing, in order to ensure its reading rate.
(2) Printing ink
Printing should choose the ink that matches the carrier, paying particular attention to the uniformity and diffusivity of the ink. 1 Poor ink uniformity, will cause uneven ink, lack of ink, line edge blur, etc.; there will be defects or stains between the symbol strips and space; 2 When you must use transparent materials, you can use two kinds of completely different light absorption properties The inks are printed overlapping to meet the PCS values ​​required for reading.
3. Printing process control method
In the bar code printing process, if the bar code symbol original film is made according to its bar width nominal size (design size), and then it is used for plate printing, there will be a phenomenon of printing increments, that is, the printed bar is always wider than the one produced in the original plate. Strips on film. Table 1 is the printing increment measured on Zhongshan Zhongxing Hung Hing Offset Printing Co., Ltd. on April 12th, 2003 for different printing methods on the same substrate.
The reasons for the above results are: the quality of the printing plate, the printing process, the properties of the printing materials, and the viscosity of the ink. Therefore, before the printing, the printability test of the bar code needs to be performed, and the printing increment and the amount of change thereof are evaluated. The (a) of the bar code printing suitability test is to find out: (1) the average G of printing increments; (2) the allowable printing error V. If the width of the bar on the original bar code symbol is B and the printed width is W, then: W=B+G+V.
The printing average increment G can be corrected at the time of plate making by reducing the width of all the bars on the bar code original plate symmetrically from both the left and right sides.
Barcode printability tests should be conducted using a special "printing accuracy scale". The rules of design and production are very precise, there are positive and negative two forms. The ruler consists of 11 levels of printing accuracy marks. Each level consists of two sets of parallel lines and are oriented perpendicular to each other so that the printing accuracy can be evaluated both in the printing direction and in the vertical direction. The width between adjacent two parallel lines in the printing accuracy mark is uniformly reduced from the A level to the K level, and the value is determined based on the test.
During the test, the ruler can be co-processed with the actual print and printed using the same printing method and printing conditions. (Note: The printing accuracy rule is made specially, must use according to the given size, can't change its enlargement factor arbitrarily) ,And observe with the magnifying glass more than 15 times, in order to confirm that those lines in the printing printing precision rule have begun to contact The range of grades. When 50% or more of the parts along the length of the line are in contact, it is assumed that the two lines have come into contact. For example, when the AD-level lines are very clear, the GK-level lines are completely filled with ink, and the EF-level lines are just beginning to touch, the printing accuracy range can be determined as EF-level.
If the print quality of the line in the printing direction of the ruled printed sample is better than the quality of the line in the vertical direction, it can be determined that the direction is the best printing direction, and the accuracy of the bar code also depends on this direction. Based on the bar code printing suitability test, according to the determined range of printing accuracy, the magnification factor and strip width reduction to be selected are determined from Table 2-3 or 2-4 at the time of plate making.
4. Quality inspection of product barcode prints
The quality inspection items for product barcode print include: appearance, consistency of bar code symbol, strip reflectivity and print contrast (PCS value), strip dimension deviation, blank area size, strip height size, inspection code, magnification factor, printing thickness , printing position and so on.
Among them, the optical characteristics and dimensional accuracy of the stripe are the key to correctly identify the barcode symbol. Therefore, the strip reflectivity and the PCS value and the strip dimension deviation are the most important items for the quality inspection of barcode printing products.
The inspection of product barcode printed products shall be conducted in accordance with the method specified in GB/T 14258, and the inspection results shall meet the requirements of GB 12904 spearhead GB/T 14257.
Measurement method: Take five positions in the direction parallel to the strip to measure.
Judgment of results: The maximum and minimum values ​​of the difference between the measured value and the nominal value of each empty dimension at the same position are taken as the dimensional deviation of the corresponding position and compared with the national standard to determine its conformity.
Third, the conclusion
The importance of the printing process for bar code quality. (1) The printing plate has low resistance to printing, resulting in lack of strokes when printing, so that the barcode scanner cannot scan the complete bar code symbol, resulting in a decrease in the reading rate. (2) The poor printability of printed materials and inks will also lead to a decrease in the reading rate. (3) Different printing methods When printing on the same kind of substrate, the reading rate is also quite different.