Screen printing 150 questions (34-42)

â–¼34. What are the basic conditions for choosing a light source when making screen printing plates?

When making screen printing plates, the basic conditions for selecting a light source are the following:

1 The intensity of the light source used must have a certain intensity. For any type of original, it should be able to meet the need for exposure in a short time.

2 Stability requirements The intensity of the light source used must be stable during exposure and must not be strong or weak. `

3 Light Source Illumination Uniformity The intensity of light used on the original must be the same. That is, the center of the original and the surrounding position must be under the same light intensity.

4 The light source used for high efficiency should be exposed in a short time and reach the exposure effect.

5 Operation, maintenance Simple selection of light source equipment is easy to operate and easy to maintain.

â–¼35. How to calculate the finest width of screen reproduction of fine lines?

In screen printing, the precision of reproduction of a replica is closely related to the screen. The number of screen meshes has an important influence on the reproduction of the fine lines of a manuscript. Therefore, when selecting a screen, careful selection must be made. The number of screen meshes must be selected. Corresponds to printed fine lines. The selected screen size has the ability to reproduce the fine lines of the original so as to ensure the printing effect.

Screen printing fine lines generally refers to line widths between 0.1-0.2 mm.

The finest width of the silkscreen reproduction fine line is the width value of the screen that can be used to reproduce the thinst line in screen printing. This is usually directly related to the width of the mesh and the mesh. When selecting a network, the following formula can be used as a basis for selecting a screen calculation. The formula for calculating the thinnest line width:

K=2S ten R where: K represents the width of the thinnest line, S represents the value of the screen wire width, and R represents the width of the mesh.

Calculation Example: If a 200 mesh/inch nylon screen is used, what is the finest width of the reproduced line?

Check the NM200 inch wire mesh, the wire diameter (S) is 0.05mm, and the mesh width (R) is 0.077mm. Substitute the above formula to obtain K=2×5077=177=0.177mm then the NM200 mesh/inch mesh reproduction The thinnest line width is 0.177 mm.

â–¼36 . How does the color of the screen affect plate making?

Wire mesh is white, yellow, amber, red and so on. The color of the screen has a great influence on the quality of the plate. Colored silk screens, such as yellow, red, and amber screens, can prevent light from diffusing when printed, while the use of white screen prints can cause noticeable diffusion. This is because the photosensitive material has a large absorption peak in the ultraviolet region, and since the white screen reflects white light when the light source is irradiated, it affects the photosensitive material. The irradiation light passes through the transparent part of the positive graphic film to reach the emulsion coating, and reflection, inflection-reflection, and diffuse reflection phenomenon occur when the irradiation light contacts the white screen, so that a floating phenomenon of the printing plate is vignetting. If colored screens are used, this phenomenon will be prevented because colored screens can absorb some of the irradiated light, thereby reducing the reflection intensity of the irradiated light, and colored screens reflect colored light. This colored light is inactive light, so it is sensitive to selection. Material does not affect, especially when the fine print is printed, the color of the screen is particularly important.

â–¼37. What is the relationship between the number of screen meshes and outlets?

The number of screen meshes and outlets only have a certain relationship when copying continuous originals. Use silk-screen printing technology to copy continuous color originals, if it is to directly use a continuous photographic film to print screen printing plates and printing is not the same as the manuscript of the same continuous tone reproduction. Because the screen printing plate presents only two states, namely, the mesh portion of the graphic and text is open, and the non-graphical portion of the mesh is closed and closed. And because a size of the screen mesh size and arrangement rules are uniform, the amount of ink passing through the screen mesh aperture during printing is basically the same, so that it cannot express the level change of the original image and text. In order to make the depth change of the original pattern fully reflected on the screen printing plate, a continuous tone original is made into a dot negative film. The changes in the depth of the manuscript will be expressed in terms of dots of different sizes and uniform density. The high density part of the screen printing plate has a large area of ​​dots, a small part of the dots with a low density, and the same part of the dots with the same density. The center distance between the dots and the dots is equal, and they are regularly arranged at a certain angle. Using this dot negative film to print a screen plate and print it, continuous reproductions can be obtained. Density of the network arrangement is usually expressed by the number of screen lines. The unit notation is line/inch or line/cm. Meaning means the number of dots arranged within the unit length.
â–¼38. What is the process of stretching the net?

Stretch net is the first process for making screen printing plates. The quality of the stretch net has a direct relationship with the quality of the plate. Therefore, we must carefully and carefully do a good job of preparatory work and stretch net work.
Firstly, according to the printing size, the stretch net is selected according to the printing frame size, and the net frame and the bonded surface of the wire mesh are cleaned. If the net frame is used for the first time, it needs to be lightly rubbed with fine sandpaper to make the surface of the frame rough. It is easy to improve the adhesion of the frame and the screen. If it is a used frame, it must be rubbed with sandpaper to remove the remaining glue and other substances. After cleaning the frame, before the stretching, first apply the adhesive on the surface that is in contact with the screen and dry it. When the net is stretched, the net is manually or mechanically stretched. After the net is stretched, the net is tightly attached to the net frame, and the net is brought into contact with the net frame. Part of the adhesive glue is applied again, and then dried. Note that the adhesive should not be coated too thick or too thin. When it is blown dry, it can be wiped with a rubber plate or a soft cloth, while applying a certain pressure. Make the screen and the frame more securely. After the adhesive is dry, loosen the external tension, cut off the screen around the outside of the frame, and then use a single adhesive tape on the screen and the frame to adhere to the site, so as to protect the wire The function of the net and the frame can also prevent the solvent or water from dissolving the adhesive when printing, so as to ensure the effective use of the screen plate. Finally, rinse the screen with clean water or detergent. After the screen is dry, it can be used for photosensitive coating (plate making).

â–¼39. There are several ways to stretch the net and what are their characteristics?

Stretching is an indispensable process for making screen printing plates. Stretch net is to firmly fix the screen to the frame under a certain tension. And make the stretched screen with a certain tension, to meet the sensitizer coating and other plate-making requirements. The size of the screen tension and the quality of the stretch net are directly related to the quality of the screen printing plate, and also have a direct impact on the printing quality. In the case of color printing, the stretching tension of each color version of the screen printing plate required to be colored must be kept basically the same so that the overprinting accuracy can be guaranteed. Only a good network can ensure the quality of plate making and printing. Stretch nets mainly include manual stretch nets and mechanical stretch nets. Manual stretching is a simple and traditional method. Usually applies to wooden frame. This method is to manually fix the screen to the wooden frame with nails, wood strips, adhesives, and other materials. The tension of the hand-stitched net can generally meet the requirements, but the tension is not uniform, the operation is troublesome and time-consuming, and the stretch netting quality is not easy to guarantee. This method is mostly used in occasions where there is a small amount of printing and printing accuracy is not required.

Mechanical stretch nets use mechanical forces to tighten the screen and bond it firmly to the frame with an adhesive. After the adhesive is dried, the frame is removed and sealed with a polyester film single-sided pressure-sensitive tape. To prevent the adhesive from dissolving and to make the screen printing plate look neat. Various materials can be used to stretch the net frame. The mechanical stretch net has the characteristics of uniform tension and short stretch time. Easy to grasp the required tension. Suitable for high-precision plate making.

â–¼40. How to choose the stretch angle?

The stretch angle refers to the angle between the warp and weft (wire) and the edge of the mesh frame.
There are two types of stretch nets, one is stretching nets and the other is diagonal stretching nets. The following describes the two methods of stretching.

The net is being stretched, the warp and weft of the screen are parallel and perpendicular to the four sides of the frame. That is, the latitude and longitude are 90 degrees to the edge of the mesh frame.

Obliquely stretched nets mean that the warp and weft of the screen have a certain angle with the four sides of the screen. Stretching the net in the form of stretched nets is relatively convenient for operation. Since the screens can be fully utilized, the leftovers of the screens can be saved and the screen waste can be reduced. However, in the case of color printing, the use of this type of stretch net plate making is prone to moiré, so the trapping printing generally does not use positive stretch net. The use of skewed stretched nets is beneficial to improve the printing quality, and it also has certain effect on increasing the amount of ink leakage. The disadvantage is that the screen is wasted. In the high printing precision requirements and color printing, the choice of stretch angle has a direct impact on the print quality, and the choice of stretch angle is not suitable, and moiré will appear. Therefore, the general copy of the printing, the commonly used stretch angle is 20 degrees to 35 degrees, in the printing of high-resolution circuit boards, due to the use of a higher mesh size, so the angle of 45 degrees to choose a comparison Suitable. In practice, in order to reduce the waste of screen mesh, generally copying prints are used to stretch the net.

â–¼41. How to determine the tension of the screen stretching net?

The screen printing accuracy is related to the precision of the screen printing plate, and the screen tension is one of the important factors affecting the quality of the screen printing plate. The tension of the screen is related to the material and strength of the frame, the quality of the screen, the temperature, the humidity, and the method of stretching the mesh. Usually in the case of hand-tensioning and no tension meter, the tension determination is mainly determined by experience. When stretching the net, the screen is stretched and the finger is pressed against the screen. Generally, the screen is pressed with a finger and the screen is felt. A certain degree of flexibility will do.

Tension meters are generally used to measure the screen tension when using stretchers and large frame stretch nets.
When using a stretcher to stretch the net, because the movement of the stretcher chuck (elastic adjustment) is controlled by the air pressure gauge, the screens of different textures have different pressure values ​​for the stretch net. Generally, the net pressure value of the net is 7-9 kg/cm2; the net pressure value of the nylon net is 8-10 kg/cm2; the net pressure value of the polyester net is 8-10 kg/cm2; The tension value of the stretched stainless steel mesh is 10--13 kg/cm2.
The above is the reference value of several kinds of wire mesh when stretching the net. The stretch net can refer to these data to stretch the net according to the actual situation to obtain an ideal stretch tension.

â–¼42. What kinds of splicers are commonly used in stretch nets?

The effect of the adhesive for the stretch net is to firmly bind the screen and the frame. There are many kinds of adhesives used for sticky nets. When they are selected, different adhesives should be used depending on the material of the frame and the type of screen. The main adhesives used for stretch nets are polyvinyl acetal (peave), vinyl chloride adhesive, rubber adhesive, phenolic-neoprene, nitrile rubber, and the like.