First, stick back
The plastic film or transparent paper is printed with the illusion of dryness after printing, but the back side is sticky during the storage period after printing the reel or bag. In severe cases, the printed parts cannot be torn apart, and the graphic ink layer is stained, causing the product to be scrapped. This phenomenon is more likely to occur in high temperature seasons, but high temperature is not the only cause of back sticking, it is also closely related to a series of factors such as plastics, inks, solvents, air volume, light, speed and so on.
1. The solvent in the gravure ink fails to evaporate completely after printing.
(1) Plastic film printing is to dry the ink by solvent evaporation. Its volatilization speed is an important factor that determines the drying condition of the ink layer and affects the printing quality. The slower the solvent volatilizes, the better the reproducibility of the plate, the more beautiful the color of the printed product, but it is easy to stick; otherwise, it will produce the appearance of whitish imprint. Therefore, choosing a solvent with an appropriate volatilization rate is the key to plastic film gravure printing. In continuous register printing, the solvent must be completely evaporated during the running time of the film between the two printing stations, otherwise, the roller will stick when printing in the next station. (2) After the film is printed, the organic solvent in the ink is not completely volatilized in the drying system, and there is still heat after the film is rolled up, which brings the conditions of continued volatilization to the solvent remaining in the ink layer and forms adhesion. (3) High temperature and humid plum festival, the film is too tight or compressed after printing.
Countermeasure: The volatilization rate of the solvent in the ink depends not only on the boiling point of the solvent, vapor pressure, vapor latent heat, etc., but also on the temperature, humidity, air volume, solute and ink layer thickness of the operating environment. Therefore, the volatilization rate of the solvent in the ink should be adjusted at any time according to the change of conditions. At normal temperature, if the ink layer dries too fast, a solvent that evaporates slowly (such as Shanghai-made 74-92 plastic film slow-drying thinner) should be added; otherwise, a solvent that evaporates quickly (such as 74-91 plastic film should be added) Quick-drying thinner). It can also be diluted with a mixed solvent of xylene, ethanol and isopropanol (the three solvents are mixed in 1/3 ratio, and the use of a single solvent is not as good as the mixed solvent). If you feel the drying is too fast, you can use a little butanol to replace part of the ethanol (butanol has the effect of increasing the gloss of the ink). But be sure to add the appropriate amount to avoid affecting the drying of the printed parts.
The solvent for plastic film gravure printing is mainly alcohols and benzenes. Alcohols use ethanol and isopropanol, and benzenes use toluene and xylene. Due to the needs of nitrocellulose, esters such as ethyl acetate and butyl acetate should also be added.
In short, the organic solvent in the ink should be completely volatilized before winding. Slow down the vehicle speed properly and increase the air volume. After the film is printed, it is loosely stored in a wire braided basket to allow the ink layer to continue to be ventilated and dried. After curing, it is cut, bagged, and film printed and bagged. It is wrapped and placed vertically in the carton to reduce the film surface. Pressure.
2. The ink of plastic gravure ink is bad. The bonding material in some plastic gravure inks has a low melting point and is easy to stick when the operating environment temperature is high and the relative humidity is large.
Countermeasures: ①When conditions permit, install air conditioners in the operation workshop, control the suffocation temperature between 18 ℃ -20 ℃, and control the relative humidity below 65%; ②Replace the ink with good ink quality.
3. Plastic film thin printing and packaging is not suitable. Reasons: â‘ Plastic film processed from non-packaging resin; â‘¡ Insufficient opening agent in resin. Countermeasure: Replace the plastic film.
4. Effect of static electricity: Cause: Plastic film generates static electricity, which forms adhesion between the films. Countermeasure: Add antistatic stab to plastic.
2. Poor color accuracy
There are several reasons why plastic film color registration is not allowed:
1. The printing registration changes in the longitudinal direction (the direction of the transport side).
(1) The tension changes when the film is pulled. Reasons: Improper modulation of the roll brake of the unwinding device; too much roll material, or the diameter of the formed material is too large; improper setting of the speed of the three rollers; the surface friction on the surface of the three rollers decreased; the tension of the rewinding machine fluctuated. Countermeasure: Check and adjust each control device.
(2) The reason for the poor diameter difference of the printing plate cylinder: the increase in the diameter difference during plate making is inaccurate. Countermeasure: measure the diameter difference (method: see the plate making section).
(3) The rule line is abnormal. Cause: Insufficient density (dense), offset printing, etc. Countermeasures: Adjust the sensitivity of the photoelectric cell, and manually eliminate the disadvantages such as paste printing.
(4) The diameter of the printing plate cylinder is small. Reason: The accuracy of overprinting of smaller roller diameter is difficult to control. Countermeasure: If the length of the printing bag is short, the imposition method can be used, that is, more than two bags are printed on one printing roller.
2. The lateral registration is unstable. Reasons: (1) One side of the film is slack, and the film guide rollers are not parallel; (2) The printing pressure is uneven; (3) The plate cylinder and the impression cylinder form a taper; (4) The film tension changes, etc., causing the film to swing left and right. Countermeasure: Check and adjust each element one by one, such as repairing when the horizontal registration device is loose.
3. Poor printing fastness
The reasons for poor printing fastness of plastic film are as follows:
1. Poor corona treatment of polyolefin film. Countermeasure: Check whether the surface tension of PE and PP reaches above 40 dynes / cm.
2. The compatibility between the printing ink and the printed material is poor. The ratio of mixed solvent is poor. Countermeasure: Change the ink and adjust the solvent ratio.
3. The temperature and relative humidity of the printing workshop vary greatly. Countermeasure: Try to control the relative humidity of the workshop between 50% and 60%, but not too low. When it is lower than 40%, static electricity is easily generated. It is best to control the suffocation temperature at 18 ℃ ~ 23 ℃, and keep the environment clean and air circulation.
4. The plastic film itself is extremely poor in printability and is still unsatisfactory after corona treatment. Countermeasure: Replace printed materials.
4. Pattern size shift
1. When printing, the vertical pattern size shrinks. Reason: insufficient shrinkage margin. Due to the influence of the tension of the plastic film and the heating and drying during printing, the longitudinal dimension of the film is greater than the shrinkage of the printed graphics. Countermeasure: When making plate, the size of shrinkage should be kept properly; the film tension should be minimized.
2. When printing, the pattern size in the longitudinal direction is extended. Reason: The gravure size leaves too much for the shrinkage margin. Countermeasure: Increase film tension.
5. Poor reproducibility of printed materials
1. The highlight (bright part) is too deep. Cause: The printing plate is too deep and the ink content is too much. Countermeasures: (1) increase the mesh; (2) reduce the pigment in the ink; (3) increase the angle between the doctor blade and the surface of the printing plate.
2. The highlight is too shallow. Reason: The depth of the printing plate is not enough. Countermeasures: (1) Use dark-tone ink; (2) Reduce the angle between the doctor blade and the surface of the printing plate.
3. The dark tone (dark part) is too deep. Cause: The printing plate is too deep. Countermeasure: Due to insufficient ink drying, the ink layer may be agglomerated, and if necessary, the plate should be changed again.
4. Dark tone density is not enough. Reason: The printing plate depth is not enough. Countermeasure: Use dark ink.
5. The dark and light junction of the middle tone ink is obvious. Cause: Poor ink quality. Countermeasure: Replace the ink with good characteristics.
Sixth, the ink layer is dry and the conjunctiva is poor
Reasons: 1. Insufficient hot air temperature, wind speed, air volume and exhaust volume. Countermeasure: Adjust the drying device.
2. The ink layer has the illusion of drying conjunctiva after printing, and the ink layer still contains residual solvent. After the film is wound up, there is still residual heat of about 40 ° C, which is suitable for the solvent to continue to move, and there is a certain pressure during the winding up, thus causing the danger of stick back. Countermeasures: (1) The ink layer should be completely dried after printing; (2) The film should be loosely packed in the wire basket, and after the film is cooled, rewind, cut, and make the system when there is no sticking back. bag.
Seven, the gloss of the printed product is thin
Cause: Ink dilution is excessive.
Countermeasure: Control the amount of thinner.
8. The imprinted edges are not smooth, and the background color is spotty
Cause: The viscosity of the ink is too low and the solvent is used too much.
Countermeasure: Add new ink to increase viscosity.
9. The printed layer has orange peel marks
Cause: The ink dries too fast, the density is too light, the leveling is poor, the thixotropy is large, there is static electricity, and the printing speed is too slow.
Countermeasures: Find out the reasons and take corresponding measures.
10. The printed layer has scratches
Reason: The guide roller is scratched, the ink is mixed with sand, the scraper is not flat, and the angle between the scraper and the printing plate is inappropriate.
Countermeasure: Corresponding treatment.
11. Thin tone
Reasons: (1) Thin ink color: (2) Poor color light reflection.
Countermeasures: (1) Adjust the ink properties; (2) Use the white ink to lay the background color for printing, and then register the colors in sequence. Inside printing, the gradation version first overprints the three primary colors, and finally lays the background color.
12. Bite color
In overprinting, the second color ink sticks and pulls away the first color ink.
Cause: The ink layer fails to dry completely after the first color printing, the pressure of the second color is too high, or the machine speed is too fast, or the hot and cold air is poor, and the printing suitability of the substrate is poor, which can cause color seizure.
Countermeasures: Find out the reasons and take corresponding measures. â‘ Add an appropriate amount of quick-drying agent; â‘¡Slow down the speed of the vehicle; â‘¢Reduce the pressure of the impression cylinder; â‘£Increase the amount of cold and hot air.
Thirteen, the print is rough, there are many blank spots, the field has moire, and the tone is not full.
Reasons: ①Too much solvent; ② The ink is thick; ③ The ink is too thin; ④ The volatilization is too fast; ⑤ The ink has poor fluidity.
Countermeasures: ①reduce the amount of solvent; ② replace the ink or re-grind; ③ adjust the ink to be thick; ④ moderately slow the drying solvent; ⑤ replace the ink.
In addition to the blocking phenomenon caused by the chemical reaction caused by the ink itself (rarely rare), the remaining factors can be summarized into five points:
(1) During printing, with a large amount of solvent volatilization, the viscosity of the ink increases, and it is easy to dry inside the network cable. If it is not cleaned in time, it will cause serious blockage.
(2) In the printing process, when the accompanying blade is far away from the printing roller or the angle is unreasonable, the diameter of the plate roller is large, or the depth of the plate cylinder into the ink tank is not enough, the hot air from the dryer affects the surface of the plate cylinder Cause serious blockage.
(3) Impurities are mixed in the ink.
(4) The engraving depth (or gravure corrosion depth) of the plate cylinder wire is not suitable.
(5) When printing, the temperature is too high, the solvent in the ink volatilizes too fast, and it is also easy to cause blockage of the plate surface.
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