Requirements and skills of screen stretching for t

Tension requirements and skills of silk printing

I. basis of tension:

in silk printing, the mesh number, diameter, spinning method and selected material of silk directly affect the tension of tension. During tension, the tension of silk is measured according to these parameters. When measuring tension, the tension point to be detected should be 10 cm away from the inner edge of the frame, otherwise the measured tension is inaccurate

the maximum Zhang Guo value that can be achieved by Sefen pet 1000 is shown in the table below. The maximum tension value indicated in the table is the display of specific wire strength, that is, the maximum tension value that can withstand and rebound for various mesh sizes and different wire diameters. If the tension value given in the table is exceeded, the wire will lose elasticity and become plastic deformation, which should be paid attention to in tension

unit of tension: Newton/cm (n/cm), which can be measured by Newton tensiometer. The tensiometer can measure the tension in the warp and weft directions. The so-called warp tension of silk is the tension in the winding direction of the whole roll of silk, that is, the tension in the direction of edge typing; The tension in the weft direction of silk is the tension in the width direction of silk

II. Tension requirements

1. Templates for different purposes require different levels of tension

for example, for color dot printing, in order to ensure the accuracy and reducibility of tone value, the tension n/cm is required. Fine print, dial, etc., tension n/cm. For general graphic printing, the tension is n/cm

for manual printing, rough printing products, or those who have no requirements for accuracy, size, etc., the tension is> 6n/cm. It is worth noting that in order to make the overprint position accurate, it is not only required that the plate tension should be greater than 10n/cm, but also that the tension of several overprinted templates must be consistent, which is more important for dot printing, otherwise it will accelerate the integration of production, learning and research of moire and hue; Accelerate the construction deviation of vertical innovation system of upstream and downstream phase structure

2. Why does dot printing require high tension templates? This is because

a, the template with higher tension can obtain a lower pitch. When the pitch is doubled, the deformation of printed graphics and texts will be increased by three times. Therefore, when the template tension is low, the inking is uneven and the dots expand and deform, affecting the hue

b, lower distance, less pressure can be used in silk screen printing, and the wear of the template is reduced, which prolongs the service life of the template

c, small scraping pressure is conducive to avoiding inking and deformation around the point, improving the sharpness of the point edge and ensuring the printing effect

III. factors affecting tension:

1. Tension equipment

a. pneumatic tension machine, which is a direct tension method. Because during tension, the cylinder support has been against the frame edge, and at the same time, it is subject to the reaction force of the wire being pulled, which makes the frame edge bend towards the center of the frame to obtain pressure. After the tension reaches a few, it is glued and removed after drying, so that the tension is more stable and it is easier to obtain tension. Disadvantages: A. in the whole process of stretching, the silk wind extends outward (around) close to the adhesive surface of the frame, so the frame sets resistance to the smooth extension of the silk, which is often impossible to achieve under high tension; B. The pneumatic tension is controlled by the air pressure. Therefore, if the anti air pressure is adjusted higher during the initial tension, it will be broken under the instantaneous and rapid large tension; C. The tension is controlled by the capacity of the cylinder, otherwise the high tension cannot be reached

b, mechanical or electromechanical pulling equipment can be flexible and changeable according to the needs of tension. Mechanical pulling machine belongs to indirect stretching method, so when the wire tension reaches the required tension, the frame rises and rests against the wire, so there is no influence of frame friction resistance on the wire during stretching, so the tension is easy to achieve uniformity

after stretching, the gap between the collets is allowed to be between 2mm-3mm, otherwise when the distance is far, the wire part that is not clamped by the collet will not get the tension directly, so that the tension of all the wires on the surface is uneven, the opening size is inconsistent with the wire diameter, and defects will appear in the printing, that is, the tension of the whole plate will be uneven with the silk printing, the ink volume will be inconsistent, the depth will be caused, the nesting will be inaccurate, etc. in serious cases, during the silk printing process, under the scraping and pressing of the scraper, Silk is easily broken, so we must pay attention to it

2. Frame

a. the material, section size, shape, flatness after processing, strength and aging treatment of the frame directly affect the tension of the version and the stability of tension in use. In most cases, the main reason is that the frame strength is not enough and leads to within ± 0.5% of the set value; The tension of the template decreases when it is used. Secondly, the degumming and stretching of the silk and the frame have exceeded the limit tension of the silk, which will also cause the decrease of the template tension. Therefore, the frame must be deburred on the bonding surface and Chuncai edge before use. After these treatments, the frame must be evenly coated with glue, and the silk will be firmly adhered to the glued frame when it is glued, so as to ensure that the cloth will not loose from the frame under large scraping pressure during the silk screen printing process. Note that the glue must penetrate into the bonding face of the silk to completely bond the silk to the frame. If necessary, when the glue is not completely dry, it is beneficial to help the silk completely stick to the frame after scraping and pressing the frame by hand

b, it is worth noting that the frame must have a section size (wall thickness) and elastic coefficient that can resist external large forces, otherwise it only pursues the plate tension and ignores the strength of the frame, which is an important reason for the instability and decrease of the plate tension

c, the frame is divided into aluminum alloy profiles and steel frames, and the elastic coefficient of steel is 2.9 times that of aluminum. Steel frames are expected to be used for versions requiring high tension. For example, the tension of the screen printing plate of the disc is about n/cm, and the cross-sectional area is required to be small, so the frame is made of steel plate of about 3mm, so that the set position is consistent, the overprinting is accurate, the hue is good, and it is not easy to produce moire, etc. in the process of four-color plate making and printing

d. for frames with position requirements and dot printing, the material, size, tension of several frames must be consistent with the cutting direction of the silk

e, improper ratio of frame size to printed graphic size will affect the printing accuracy and the stability of plate tension. Therefore, for large frames, it is generally required that the four sides of the printed figure should be at least 15cm away from the edge of the frame, and the medium and small size and frame should be 10cm. For those with higher accuracy, the length and width of the printed figure should be 1/3 of the length and width of the frame in the frame

3. Stretching methods and skills

a. standard stretching is used in all steel trade Ecology: the so-called standard stretching method is that after the wire is loaded into the collet, stretch to 70%-80% of the allowable tension of the wire within about 5 minutes, stay for about 10 minutes, pull to the allowable tension, stop for about 10 minutes, supplement to the allowable tension, stay for 10 minutes, supplement to the allowable tension, stick, and the whole stretching process requires 1 0 hours

b, 30 minute stretching method: this method means that after the drawing process is completed within 30 minutes, the wire will not be stretched repeatedly. The stretching process is:

(1) first make the tension in the warp and weft direction reach half of the requirements

(2) pause for about 60 seconds and then pull

(3) increase the tension by n/cm, stop for 60 seconds, and use this method to increase to the required tension

(4) stabilize the tension for about minutes, and then re tension to the required tension

(5) after repeating the step (4) again, it can be glued, and the whole stretching process is over

c, fast stretching method, as shown in the figure:

this method is to pull the tension of the wire to 15% of the required tension within 5 minutes and immediately stick

d. comparison of several stretching methods:

(1) fast stretching, the result of which indicates that it will have a loss of 2.5n/cm tension in the first hour. At the same time, due to the sudden change of the wire under the fast tension, the wire is deliberately easy to break, so this stretching method is not recommended

(2) the 30 minute stretching method provides higher plate tension than the fast stretching method. After 1 hour, the plate tension loses 1n/cm, and after 24 hours, the plate tension loses only 1.8n/cm

(3) the standard stretching is about 2 hours. The tension increased by increasing the stabilization time within 2 hours is not large. At the same time, the stretching time is also long, which does not save time. It is generally considered that the 30 minute stretching method is better, and it is recommended to use it

4. When stretching, loosen the four corners and leave adjustment allowance:

when stretching, if the four corners are not relaxed, because the distance between the corners is short, it belongs to the high tension area. When the wire elongation distance is very short, especially for the wire with high tension and low elongation, it is necessary to leave stretching allowance according to the size of the frame before stretching. Otherwise, during stretching, the tension has not reached the design tension parameter of the wire, That is, if it is not the real tension of the wire, it will be torn from the four corners of the frame in advance, which is a very important problem in stretching

5. Correct stretching process:

the correct stretching is: (1) tear off the irregular ends of the weft wire first, clamp them into the collet in equal quantities, then clamp the other end of the wire, and lay the wire flat

(2) flatten the wire and clamp the wires on both sides of the weft direction

(3) pull the warp to 1/2 of the required tension

(4) stretch the weft direction, and the tension is close to the full data of the required tension. At the same time, adjust the weft and warp direction to reach the required tension, and stop for 10 minutes

(5) adjust the tension in the warp and weft directions after 10 minutes to reach the required tension, about 10 minutes

(6) repeat (5) twice, stick

(7) Note: a. the collets on the four sides are close to each other, and the maximum allowable gap is within 3mm, so that the force on the wire is consistent. B. The opposite collets of the four sides correspond one by one to ensure that the hole is close to the square and the tension is uniform everywhere when the wire is stretched

6. After stretching, the version can be made after 24 hours

because of the high tension wire, its tension will decrease by about 10% - 20% after 24 hours. The tension loss due to the tension of the high-strength wire appeared in the first 4 hours, and then gradually stabilized. Therefore, the tension stability between stretches is the requirement of the internality of silk polymer materials, rather than the tension operation

version also change with the change of environment and temperature. This factor must be taken into account when making a fine version

7. Large changes in temperature and humidity and insufficient drying time after sticking and stretching make it easy to lose the plate tension. Under the condition of ensuring that all the wires and frames are closely pasted, the glue must be dried thoroughly, which not only stabilizes the tension, but also ensures that the solvent of the glue is helpful in cleaning the plate

8. Exceeding the maximum allowable tension value of the silk

means that although the silk does not break during the stretching process, the plate will also break naturally during the placing process under the impact of time, humidity and temperature, or during pretreatment. This is because the synthetic fiber has exceeded its extension length to reach plastic deformation and fatigue strength. When the environmental and external conditions change, it will break naturally, which is