39. 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.

40. 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.

41. 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.

42. 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.

43. How does a vignette come about?

Vignette is a common phenomenon in screen printing plates. The main form of expression is that after the printing, the screen part on the screen printing plate is not fully formed into a through hole after development, and part of the mesh hole is still frozen by the photosensitive adhesive, and the graphic edge is not uneven. Qi, some meshes are translucent, and the edges of the images are like the wind rings around the moon. With this kind of screen printing printing, the edges of the imprints are not neat, and some of the text lines are missing. The size also changes. This reduces the print quality. There are two main reasons for producing vignetting. When using direct platemaking, a white screen is used. The white screen has reflection and scattering effects on the printing plate light source during the printing, so that the photosensitive material at the edge of the picture is sensitive to light. The degree of non-uniformity, resulting in vignetting; plate printing plate and the screen plate does not fit tightly, making the picture edge of the photosensitive material is not uniform, resulting in the emergence of vignetting.

44. How to prevent vignette?

In order to prevent the occurrence of vignetting phenomenon and improve the quality of plate making and printing, the following measures can be used to control it.
1 The use of colored silk screens in the plate-making process as much as possible without white screens, and use red, yellow, orange and other colored screens for plate making, use the color of the screen itself to absorb the light source of the printing plate, reduce the screen when the screen light Reflection, scattering, to prevent the appearance of vignetting.
2 Pre-painted varnish-preventing solution Pre-painted vignette-preventing solution is a solution of ethanol-cresol and ethanol-soluble red dyes that are blended at a given ratio. Before applying this solution, the screen must be cleaned and dried, then the varnish-preventing solution is applied and dried immediately. After drying, the photosensitive adhesive can be coated and the printing plate can be printed. This can prevent the appearance of vignetting. .
3 During the printing, pay attention to the plate and the screen plate to be affixed and tightly attached to ensure that the light-sensitive material is evenly distributed, so as to prevent the occurrence of vignetting.

45. Why are there pre-treatments for screens? What are the main treatments?

In the process of manufacture, transportation, storage, and use of the screen, dust, oil stains, etc. adhere to the surface of the screen. After the screen surface adheres with dust and oil, the combination of the screen and the photosensitive adhesive will be seriously affected, resulting in the quality of the screen printing plate. To reduce, in order to ensure that the screen and the photosensitive adhesive closely integrated, the screen should be pre-treated before coating the photosensitive adhesive. Screen processing is an important process for making screen printing plates. There are usually two methods for the pretreatment of the screen. Physical treatment and chemical treatment.

Physical treatment method: The purpose of this method is to polarize the surface of the screen to facilitate the combination of the screen and the photosensitive adhesive.

This method is pulverized using a pumice stone and a silicon carbide powder and treated by mechanically treating the surface of the mesh. Or the wire mesh is treated with silicone carbide powder, with the powder particles below 20 microns. After treatment, the screen is washed and dried with water and then used for plate making.

Chemical treatment: The phenol, cresol, and phosphoric acid corrosives are usually used to process the surface of the synthetic fiber mesh before the plate making process to roughen the surface of the mesh and achieve the purpose of tight bonding with the photosensitive resin. However, the use of phenol, cresol Phosphoric acid-based corrosives tend to damage the screen, so pay full attention to chemical treatment.

In addition to the polarization treatment on the screen surface, the screen can be degreased and cleaned with caustic alkali, cleaning agents, etc. The de-esterification cleaning method can effectively remove the oil, dust and other substances on the surface of the screen, making the screen and photosensitive adhesive Bonding is strong. After washing with the above solvent, rinse with water, dry it, and use it for plate making.

46. What is the principle of screen printing plate imaging?

From the foregoing, it can be seen that the screen is formed by weaving a certain material, and the latitude and longitude lines intersect to form a mesh. During the process of plate-making, a certain thickness of photosensitive resist is coated on the screen and dried, and a photosensitive film is formed on the screen, and then the positive-texture plate-making negative film and the coated screen are attached and exposed by a human exposure machine. In this way, the graphic portion of the screen is not cured due to light reception, and the non-graphic portion is cured by light. After developing and rinsing, part of the uncured photosensitive material is washed away, and the screen hole is a through hole. The non-graphic part of the photosensitive material solidifies the mesh. When printing, the silkscreen hole in the graphic part leaks ink, and the non-graphic part does not leak ink, thus obtaining the printing ink. This is the imaging principle of screen printing plates.

47. How is the screen printing plate developed and fixed?

Screen printing plate production process is relatively simple, with the continuous emergence of new photosensitive materials this feature is more prominent. Use modern photographic materials to make screen printing plates, use photographic plates and stencils coated with photosensitive materials to fit the printing plates, and put the frame in water, warm water or pharmaceutical water for about 10 minutes after printing, and the graphic shows Come out (different photosensitive material development time is also different). Then, the screen printing plate was gently scrubbed with water in a sponge, so that the mesh part of the picture and text basically became a through hole. Then the pressurized water is sprayed to remove the unexposed material left in the graphic part, so that all the screen holes in the graphic part are changed. Finally, the water mark is wiped and dried to make a screen printing plate.

48. How to determine the thickness of screen printing plate?

The thickness of the screen printing plate refers to the thickness after coating the photosensitive resin on the screen. That is, the thickness of the screen printing plate is equal to the sum of the thickness of the screen and the thickness of the photosensitive film.

The thickness of the screen printing plate is determined according to the printing requirements. Usually the height of the ink layer after printing is closely related to the thickness of the screen printing plate. Under normal circumstances, when the screen printing plate used is thicker, the ink layer after printing is used. On the other hand, if the screen plate used is thinner, the ink layer will be lower after printing. In special cases, it is required to use screen printing plates of thick-film photosensitive materials with a thickness of up to 1 mm. For example: Making Braille The thickness of the screen printing plate is generally between 0.1-0.250 mm.

With the direct platemaking method, the thickness of the screen printing plate is obtained by coating the photosensitive resist several times. The coating is applied once and dried once. The required thickness of the screen printing plate can be obtained by performing the cycle operations in sequence.
In the case of indirect plate-making or direct-to-plate printing, the thickness of the screen printing plate is obtained by selecting different thicknesses of the photosensitive film according to the printing requirements.

49. Platemaking accuracy and what factors?

When making screen printing plates, platemaking accuracy is mainly related to the following factors.
(1) Screens include the types of screens used, the height of the screen mesh, the thickness of the wire diameter, the size of the screen opening area, the size of the screen expansion and contraction, the method of weaving the screen, and the color of the screen.
(2) Frames, including the material of the frame, the size of the frame, and the strength of the frame.
(3) The size of the tension.
(4) The choice of stretch angle.
(5) The method of bonding the frame and the screen.
(6) Pre-treatment of the screen.
(7) Photosensitive materials, including sensitivity level, resolution level, adhesiveness of photosensitive materials, expansion and contraction rate of photosensitive film, photosensitive film thickness and photosensitive film hardness, water resistance, solvent resistance, abrasion resistance, and resistance to printing The rate of high and low.
(8) The sensitizing agent blending time, the time after blending.
(9) Coating method of a sensitizer.
(10) Drying methods and conditions, including drying temperature, humidity, and drying time.
(11) Printing, including the type of light source used, the length of the exposure time, and the distance between the substrate and the light source during exposure.
(12) The density of the photographic plate master.
(13) Development, including developing methods: water development, warm water development, drug development; development conditions: temperature, soaking time, type and use of the water sprayer, and the like.
(14) Drying temperature and time after development.