Screen printing 1000 questions (38-45)

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.

â–¼43. 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.

â–¼44. 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.

â–¼45. 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.

---Qi Lu Graphic Arts