Factors Affecting Screen Printing Quality

I. Introduction

Screen printing (hereinafter referred to as screen printing) is a classification other than offset printing, letterpress printing, and gravure printing, and the use of this type is particularly extensive. It is used in paper, plastics, leather, wood products, glass, ceramics, textiles and other industries. Currently, a large number of screen printing methods are used for the electronic telecommunications industry, such as the manufacture of printed circuits and the nameplate spleen. Although the use of silk screens is very wide, it is only an old and young industry that has been revived in 30 years. Therefore, people have only a preliminary understanding of silk screen printing, and they still do not understand much about the quality of screen printing. How to ensure the quality of silk screen printing has become our current discussion area. This article focuses on some of the quality issues in screen printing from the point of view of process requirements and how to control them so that the level of screen printing is further improved and the products are better decorated.

Second, screen printing template and its quality

Screen printing stencil manufacturing process of two methods

1. Direct method

Screen net frame stretch net preparation -> screen cleansing process -> repeatedly scratches the photosensitive emulsion -> after drying and the photographic plate covering the printing plate -> developing -> drying -> repairing -> sealing net.
Note during operation: (1) The screen must be completely removed before applying the photosensitive adhesive: (2) The number of times the adhesive is applied should be taken into account the accuracy of the printed pattern and the height of the screen mesh; (3) Make sure that the photofinisher and the photo-resist layer are tightly affixed. (4) The mesh should be used immediately after the cleaning process, and must not be contaminated.
2. Direct photosensitive film method

Wet the film or photo-resist -> dry -> strip the film base -> print -> develop -> repair etc.
Matters needing attention: (1) preservation of the film should be protected from light or under the yellow light; (2) using the corresponding film after the photosensitive adhesive.
Net material. From the quality requirements, the performance of common mesh materials is compared.

A. Nylon wire mesh:

(1) Has a certain degree of wear and tensile strength;
(2) Resilience and printed material permeability are all good;
(3) not resistant to acidic solution;
(4) High hygroscopicity, so poor dimensional stability;
(5) When using, pay attention to increase the tension on the screen, you can print fine products.

B. Polyester mesh:

(1) All the advantages of nylon mesh are:
(2) Strong acid resistance and chemical resistance;
(3) Since the hygroscopicity is lower than that of the nylon yarn, the dimensional stability is better.
(4) The decrease in strength of polyester filaments after exposure to sunlight is smaller than that of nylon filaments;
(5) Disadvantages: The bonding force with the currently used photosensitive adhesive (film) is slightly inferior, and the diameter of the monofilament is thicker than that of the nylon filament. Therefore, compared with the same through-hole rate of the nylon yarn, the ink of the polyester screen passes through the performance of the screen and is inferior to the nylon screen.
Mesh count - refers to the number of meshes per cm length. Inch refers to the number of meshes per inch;
Through hole rate - refers to the percentage of the mesh area of ​​the entire screen area.
Mesh width - refers to the side length of the square mesh;
Thickness - refers to the thickness of the screen;
Wire diameter - refers to the diameter of the monofilament of the woven wire mesh.

3. Stretch net requirements

In the screen printing process, the screen must be fixed on the frame so that the screen blade is in contact with the screen and the substrate after the screen is subjected to force. Once the force of the pressure disappears, the screen immediately Leave the substrate. Otherwise, the required clear pattern will not be printed. Therefore, the mesh itself must have a certain degree of elasticity, and it is not suitable if the mesh is too large or too small. In order for the screen to have a certain degree of elasticity, the screen must be stretched over the frame, so people think that to get an ideal template, the stretch net must be:
(1) The tension must be uniform throughout the screen.
(2) The screen hole area is almost square
(3) The stretched wire mesh ensures stable tension during the service life. Before the stretching net, the screen and the net frame must be subjected to force treatment so that their cold deformation can be stabilized.
(4) In the process of stretching the net, the force applied to the screen should be uniform and uniform, and the stage should be used as much as possible to increase the tension of the screen to promote the screen to have a stable “ageing” process. (The company sells all kinds of screens, sizes and sizes, from 80 to 420 meshes)
In order to achieve the above ideal conditions:
(1) It is best to use special stretch netting equipment for stretching nets, with a tension test table attached, and attention must be paid to the stretching of force 2-3 times for a period of 2 to 3 hours in a certain period of time. "Stating", after the deformation of the screen is stabilized, the tension is measured again, and the screen can be fixed on the frame after reaching the standard.
(b) There are screen printing requirements for overprinting. The lines, cutting direction and tension of the screen must be consistent.
(3) The latitude and longitude lines of the wire mesh when stretched shall be perpendicular to each other as far as possible under stress.
(d) Before the net is stretched, give the net frame a certain amount of stress. By doing the above, you will be able to draw a satisfactory screen.

4 pairs of black and white draft requirements

Drawing black manuscripts for the basic preparation process of photolithography, is the necessary process equipment before silk screen printing, the quality of the black and white manuscripts directly affect the accuracy and aesthetics of the finished product. Therefore, the size of the black and white manuscript is required to be accurate, the scribe line is smooth, the contrast between the manuscripts

Large, clear and beautiful characters, symbols, strokes and evenly arranged. In order to improve the accuracy of the black-and-white manuscripts, the drafts must be considered when considering the flexibility, manuscript storage, and storage of the used paper. For this purpose, the following requirements are proposed;

The requirements for painting black and white and its tools, materials, equipment paper: more than 250g non-reflective heavy forest paper or coated paper, the surface without impurities, stains, etc., absolutely flat. . Black and white drafting methods and requirements:

The ratio depends on the device. Normally, it is drawn as 2:1 and 4:1.

The front and back screen printing work pieces are required to have position requirements, and the seats are painted on the front and back sides of the same paper, such as double-sided printing plates.

For black-and-white drafts for multiple overprints, the lines of several sheets of paper should be considered during drawing, the positioning criteria should be the same, and the environment and weather conditions should be as consistent as possible. If necessary, they should be drawn under conditions of constant temperature and humidity. Take pictures and so on. Today, computer drawings and films are generally used.

5. The quality of silk screen printing (also known as screen printing)

In recent years, due to the extensive use of silk screens, the number of screen printing materials is also increasing. On the contrary, there are new requirements for screen printing quality. For this reason, the requirements for screen printing stencils should be dimensionally stable and the printing accuracy is high, and the screen printing rate is high. Therefore, the manufacture of silk screen dies plays an important role in the entire screen printing process. Here is a brief discussion of the main issues of silk screen printing.

(1) Line edges jagged:

Due to underexposure, the photosensitive material that absorbs light and crosslinks is not completely crosslinked, and development causes swelling and partial dissolution of the photosensitive paste. During the development process, the pattern is pulled by the external force, such as scratching. High-precision patterns use low-mesh screens to create burrs. If the above situation arises, the solution should be overcome one by one, but the key is to choose the correct exposure time.

(2) When the exposure time is suitable. If the part of the screen that is not supposed to appear is peeled off or if there is a residual glue remaining on the display, check the following questions. The contrast of the photographic plate is accurate and there is no contamination. The drying temperature of the photosensitive film before being exposed on the screen is too high, causing the thermal cross-linking of the film to fail, or the insufficient drying of the film, and the development of the film. The photographic plate and the photosensitive film do not cling closely to each other and have a refractive effect. The relationship between the distance between the light source and the workpiece and the exposure time is not adjusted properly, causing overexposure and underexposure. The position between the light source and the screen is not correct, causing an unequal exposure for the same amount of time. Sensitive adhesive (film) uneven, quality difference in different positions on the same screen plate.
(3) collapse network and collapse network model
1 After the screen is made, the drying temperature is too high, especially the deformation of the wooden frame is large (twisting, bending, stretching is irregular) causes the collapse of the mold.
2 Due to changes in the weather, the tension of the wire mesh changes to cause collapse and collapse of the net, especially after repeated repeated application cleaning.
(3) Improper handling of screens, especially in the case of incomplete cleaning before application of the photosensitive film, can easily cause film collapse during use.
(4) The photosensitive film (glue) is not firmly bonded to the screen.
1 Wire mesh decontamination to oil is not complete.
2 underexposed, check the lighting system.
3 The selected screen is incorrect, the mesh is too thick or the development rinse is not suitable.
4 After the film is coated or the photosensitive adhesive is dried too quickly, the film or adhesiveness quickly disappears.
5 poor performance of photosensitive materials.
(5) There are pinholes, pits, etc. in the template.
1 Check the photo studio.
2 Dust causes contamination of the screen and photosensitive film (glue) and is not clean on the plate glass.
3 underexposure, uncontrolled, light source, distance time.
4 development, rinse too much.
5 The film is too thin and the uniformity of melting of the photosensitive colloid and the sensitizer is poor.
In short, the quality problems of the network model are sometimes caused by a variety of factors. Therefore, it is necessary to analyze and inspect the various processes and solve them accordingly.
6. Measure the quality of a screen template should be assessed in the following areas:
First, the resilience of the screen during the screen printing process is relatively stable, and during the screen printing process, the screen approaches the original tension before screen printing.
Second, the printed material has good performance.
Thirdly, the edges of the pattern are smooth, without jaggies and pits;
Fourth, in the silk screen printing process, the original screen printing accuracy and resolution of the screen mold before screen printing are maintained.
Fifth, the long service life of the screen mold, namely, resistance to printing, abrasion resistance, and resistance to cleaning can be used repeatedly.
The first four points meet the fifth requirement according to the requirements of the manufacturing network model, so that the screen mold can achieve the greatest economic benefits. How to meet the requirements? This discussion is as follows:
(1) Thorough degreasing and decontamination must be performed on the stretched screen to maximize the bonding strength of the screen and glue.
1 Oil removal and decontamination methods Generally, organic solvents are used, and No. 120 aviation gasoline or acetone is used to wipe the front and back of the screen.
2 For single-piece production, the screen can be put into boiling water for half an hour before going to stretch the net, and still rubbing with organic solvent before applying the photo-resist film.
(2) Select the appropriate method of making the network model.
(3) Without affecting the requirements of the workpiece, the photosensitive film (glue) is selected to be as thick as possible so that (4) the correct exposure time is preferably selected so that the film (glue) of the light receiving portion is completely cross-linked.
(5) Reasonably select the photo-resist (film) layer that matches the properties of the printed material.
(6) The photosensitive film is treated with light and thermal curing after being developed to thoroughly crosslink it.

Third, the choice of printed materials

As the screen printing process with the development of the national economy. Need to develop, and the screen printing material is an integral part of the entire screen printing, screen printing must be based on the screen (template) and printed materials.

The current printing materials used in silk screen printing are indeed numerous, nothing more than three major categories;
1. Water-based printing materials
2. Solvent type printing material
(1) Physical drying type
(2) Oxidation drying type
(3) Chemical dry solid type
3. Due to the applicability of the UV-curable printing material, no printing material is universal, and the printing material with the highest bonding strength to the workpiece and the best screen printing performance should be selected according to the requirements of the workpiece material. The quality of screen printing. In order to achieve the purpose, the conditions that must be fulfilled for the printing materials according to the specific requirements of the workpiece are:
(1) In the silk screen printing process, the printing material can smoothly pass through the screen, and has good fidelity and leveling performance (fineness, printability, and covering performance are good) without drawing.
(2) The printed material cannot dissolve or swell the retina. The premise is that in the specific conditions of the workpiece, first select the printing material, and then select the screen.
(3) The color of the printed material and the workpiece should have obvious color difference, and the workpiece should have good adhesion.
(4) The printing material should have suitable viscosity and dryness suitable for printing.
1 It is not easy to dry when silk screen is printed, and it is quick-drying within the prescribed time after printing.
2 No dry (bubble) bubbles, cracking, etc. are allowed after the printed material is dry.
3 Thixotropic performance is better, (even if the imprint is thick, but not to open, aliasing, no pinholes, etc.).
(5) Printing materials should be used with low toxicity, low cost and wide resources.
(6) The printing materials should have high chemical stability, and the special-purpose printing materials should not be dissolved in the etching solution, and should not be peeled or foamed.
(7) Special printing materials should have high insulation resistance, high temperature resistance, etc.
(8) The silk screen printing plate after screen printing is convenient to clean, used as temporary printing material, it is convenient and simple to remove.

Fourth, substrate (workpiece) pre-treatment