Plastic bottles made of polypropylene material through injection molding are basically outer packagings of light industrial products, such as cosmetic bottles, detergent bottles, drinking water bottles, and oil and fat bottles. The weather resistance, chemical resistance and mechanical strength of these bottles are very good, and they are commonly used in the packaging industry. Among them, the smaller bottles are made into cylindrical, conical and elliptical shapes. The bottles of these curved bodies are generally printed with a domestic semi-automatic curved surface printing machine, and the color of overprinting is no more than 3 to 5 colors.
The monochromatic printing of the curved surface is supported by four rollers to support the printing material, and the screen plate moves to rotate the printing plate to complete the printing. However, the plastic bottle trapping printing cannot use this method. It is based on moving and static fixtures to clamp the substrate. Through the process of meshing the rack and gear, it drives the screen to move and the substrate rotates to complete the printing. The following outlines the process points in the overprint process.
First, all kinds of screen version need rules plate
The so-called rules plate contains two aspects: the same number of blank mesh screens with uniform tension are all coincident with the same position of the printing table; the positioning cross lines on several negatives are overlapped on the same position on the printing table. on.
1. Positioning of the screen: The positioning of the screen is relatively simple, and the positioning is generally performed using a square ruler or a three-point positioning block. The positioning block needs to have a certain thickness, can be made of 3~5 mm plexiglass, and is fixed on the printing platen with double-sided adhesive tape. If the frame does not have a 90° angle, the longest side of the frame must be placed with the positioning block or square Cushioning to ensure the flatness of printed graphics.
2. Positioning of the film: There are many methods for overlapping the film and the following two methods can be used. (1) Replacement method: When the first screen printing is finished, do not remove the negative film from the printing platen, place the second negative film on the first negative film, and make the positioning crosshairs overlap. One side of the transparent tape is fixed, and then the first negative is peeled off from the printing table. The replacement method is also called the left and right side shortage positioning method, that is, the same side of the bottom two negatives, the size should be changed. As shown in Figure 2, the size of the first film (A film) is slightly smaller (5~10 mm) than the size of the second film (B film), so that the second screen plate can be cured when the second plate is printed. A negative film can be smoothly removed from the platen. A number of negatives are positioned in the same way. (2) Center line extension method: When using the computer to separate the originals, each color separation sheet does not use the cross-line overlapping positioning method, but adopts the center line positioning method, that is, the center position of several sheets of the color registration film 4 positioning short lines, short length 5~10 mm, width 0.3~0.4 mm. When making a plate, lay the screen with the printing surface facing up and place it on the desktop. Draw the center line on the frame. Then place the film on the screen and use a ruler to connect the four short lines in the vertical and horizontal directions. Dislocation, correction is completed, with a transparent tape to fix the film on the screen, and then turn over the screen and the printing plate three-positioning block or positioning angle bracket can be aspiration of the print version.
Since the squeegee blade and ink-recovering blade of the curved surface printer occupy a large area of ​​the screen (about 200 mm), the screen is generally rectangular. In the printing process, only four points in the longitudinal direction are often connected accurately. , and the four points in the horizontal direction, there are 3 points or 2 points can be connected together (the frame squareness deviation). In this way, it can be ensured that the printed picture will not be tilted.
In addition, the rules and regulations also include two factors that cannot be taken lightly, that is, the correct choice of the number of screen meshes and the thickness of the photosensitive film. Mesh screens generally use 300-500 mesh yellow polyester mesh, solvent-based inks use 300 mesh screens, and light-cured inks should select 355 mesh screens. If there are long lines and bar code lines in the printed graphics, 22.5~30° diagonal stretch nets are also required. The smooth substrate film can be thinner, and the matte and grain-surface substrate film should be slightly thicker to increase the ink coverage.
Second, the gear and positioning pin sets of reasonable preparation
The surface color printing is a process in which the gear and the rack are engaged and the substrate rotates to complete the printing task. The rack is an inherent part of the press, and the gear is a component that changes at any time depending on the size of the substrate. The following principles should be followed in the selection of gears: The diameter of the substrate = the diameter of the gear, that is, the rotation perimeter of the gear should be consistent with the rotation perimeter of the substrate, and the tooth profile of the gear should also be perfectly matched with the rack. Screen printing plants generally do not have the ability to make gears themselves. The solution is to remove the rack and pinion holder from the press and send it to the mold factory or gear factory to ask them to process a gear of the right size. Because the ellipse is a part of a perfect circle, when equipped with a gear, be sure to find out the center and radius according to the actual object and then customize the gear. There are two points to note when making positioning fixtures: (1) The position of the positioning pin must match with the plastic positioning groove; (2) The fixture is generally made into a round shape. There is a bushing at the center of the disc. The bushing is tightened with the screw and the gear seat. The stainless steel band around the disc must also be welded into a retaining wall to prevent the plastic bottle from being pulled out of the chuck. Retaining wall height of 5 ~ 10 mm, oval plastic bottles should also be welded inside the retaining wall, the inner wall of the arc and the plastic bottle consistent.
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