Technical and Precautions for Pad Printing Originals and Films

Photographic originals are opaque figure parts with black solid blocks in a white, clean background. The resolution of the text output from the laser printer must be at least 600 dpi to be a photographic original. Duplicated photos or graphic documents output from a fax machine cannot be used as photographic originals because the density of these two types of images is not large enough to produce positive pictures.

High-quality manuscripts and high-quality positive films created from this manuscript determine the quality of the typographic plates because the originals are poor. There is no way to compensate for the printing plate.

The size of the original is preferably twice the size of the printed product. Because if the size of the original is large, a lot of bleed can be reduced in the reduction of the photograph, so that the resulting positive picture image is clear and the edge resolution is good.

Most pad printing manufacturers have the ability to modify the draft, or use computer scans to process unmanifested manuscripts to produce an ideal photographic film.

Imaging system

There are many methods for making positive films, such as dry film, direct film, photographic film, and laser output. If the use of graphics software, such as CorelDRAW, Adobe, Illustrator and Freehand make 1:1 originals, you can use a laser printer or imagesetter output film.

Dry film output system uses thermal energy to copy images on paper or film, the same principle as the fax machine imaging on fax paper. One drawback of the dry film output system is that the imaged paper cannot be kept opaque black and it fades to brown. If it is used only once, brown is not a problem, but if you want to save the paper for later use, you should use it instead. Film, or saved as an electronic file.

The direct film output system uses a computer to directly output photographic film without a photographic process. When the image is imaged on a film, the carrier removes the film from the image-running device and places it in an automatic developing device. Because the film is never exposed to the air and is not contaminated by airborne dust, this film-out system ensures the cleanliness of positive images. This film can be stored for a period of time without fading problems.

Laser printers with a resolution of 600 dpi or more can make photographic originals. The higher the resolution, the better the quality of the output film. Coated paper works best. Inkjet printers are not suitable for outputting photographic originals because the density of the output documents is not sufficient.

There are many kinds of special paper or transparent materials on the market for laser printer output. The images output from these special papers are of good quality, high density, and good coverage. It is best not to use a transparent acetate base for laser output because the output image density is not enough and the image hiding is not good. It is also possible to stack two identical output acetate bases to increase the image density.

The laser-printed document is not as good as the photographic original that the image-setter produces, especially for printing fine lines or printing small-size images. Usually the density of the image is not enough and the coverage is not good. In order to make up for this defect, manufacturers have specifically developed and produced a post-treatment spray and coating material. After the laser printer outputs the photographic original, the spray can be sprayed on the surface of the image, which helps to increase the hiding power of the image.

The photographic originals are used in the lithography. The originals are placed in the copy holder, and the photographic images are taken at the required magnification. According to the type of the film, positive or negative images are produced. Photosensitive films are automatically developed or placed on the developing disc for development. The film may be contaminated by the surrounding environment during development, but this method is still used by most people. One of its advantages is that it does not require that all manuscripts be produced by computers.

Film positioning

No matter how the positive film is made, positioning of the film is very important. It is necessary to ensure the orientation of the emulsion surface of the film during pad printing so that the emulsion surface is in close contact with the pad plate during exposure printing.
Because different printing processes have different film orientations, people often confuse the orientation of the film. The orientation diagram of the film describes how the image should be oriented during exposure exposure.

The orientation of the film is usually referred to as "emulsion facing down", which means that when we look at the film, the above is the image we need to print, and the emulsion should be on the back of the film.

When printing a positive image on a special paper with a laser printer, the printed image should be reversed because when the image of the printed paper is brought face down on the pad, the ink transfer agent will directly contact the surface of the plate.

When using a dry film imaging system, the image and the film are integrated, not attached to the surface, so there is no problem with the orientation of the film. This type of imaging sometimes causes "halation" of the image, and the sharpness of the edges is not good, especially when printing small-sized symbols or fine lines.

Be sure to ensure the cleanliness of the film as much as possible, especially when printing plastic plates. Be sure to remove all stains, ink, residual emulsions, etc. before printing. Pay particular attention to the area of ​​the image to be imprinted by the transfer head.

Image distortion

There are some distortions when the printing head prints on the surface of the printing material, but sometimes the original needs to be deformed, such as printing on the combined triangular surface, printing on the convex surface or printing at the edge, all require the original to be deformed. .

There are some tricks about manuscript deformation that can help the printer decide how to make the deformation and where to make the deformation, thus saving time.

1. Make some small grids on the printing plate. The size of the small grid depends on the size of the printed image. Squares with a side length of 5mm are commonly used. With multiple types of printing heads of different shapes and multiple printing, the print head with the smallest deformation will be found. As a reference standard, the original will be deformed as necessary, and a small square can be used as a deformation unit at a time. . When performing experiments on each type of print head, it is important to make a record of which kind of print head, where it is printed, and how large it is to compress. The procedure for making a deformed original using a small grid method is as follows:

● printing small grid;
● Measure the size of each small square deformation on the printed object;
• Make a deformed grid pattern in the computer by using the measured variance of the small squares;
Use deformed grid to make a distorted image for printing;
● Use an image with a distorted original to print on the object.

2. Print single letters using the "upside-down printing" method. The letter to be printed is circled and the print head is adjusted so that it can completely cover the printing area. The ink is gently coated on the raised letters, then the print head is pressed against the ink-coated letters to pick up the ink, and finally printed on the white paper. Such a distorted letter is printed on paper, the letter is entered into a computer using a scanner, and a deformed manuscript is drawn using a computer. The specific steps are as follows:

● Lightly spray on the letters with a small amount of ink or other transferable paint;
● Use the print head to pick up the ink on the letters;
● Print the letter pattern on the print head on the paper, use a scanner to input the printed deformation pattern into the computer, and create a distorted image on the computer to print on the printing plate. The raised letters are printed using the image-distorted original.

Original image size

When performing exposure printing, the original is placed on a steel plate (plastic plate) whose size does not exceed the area of ​​the steel plate (plastic plate) or the maximum area printed by the printer. The length and width of the original image in the open printing system should be controlled within 80% of the maximum length and width of the machine printable. If it exceeds this value, it will cause uneven ink coating, or the image is too close to the steel plate (plastic plate ) Edges, which cause the print heads to carry a lot of ink that should not be picked up. In a closed printing system, the image size should not exceed 90% of the ink tank diameter.

Also consider the working direction of the machine's ink plate when setting the original. If there is a straight line in the original, when placing the original, these straight lines should be perpendicular to the working direction of the inked plate, or have a small angle with the vertical direction. Correspondence between the eroded image in the printing plate and the way the inked plate works.

The best way is to check the positioning of the original image twice before you print the plate. Place laser prints or manuscripts together on a table to form a whole view, in accordance with the length of the working of the inked plate (in the multicolor printing press aligns the center axis of each plate), to see if all images are in one Straight line.

Source: Screen Printing