Causes and countermeasures of several common faults in gravure printing
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First, the color registration accuracy is poor
Plastic film covers are not allowed for the following reasons:
1. In the vertical direction (direction of the carrier), the printing registration changes.
(1) Tension variation during film pulling. Reason: The drum brake of the unwinding device is improperly modulated; the reel is too much, or the diameter of the bobbin is too large; the speed of the three rollers is improperly set; the surface friction of the three rollers is reduced; the tension of the rewinding machine is varied. Countermeasure: Check and adjust each control device.
(2) Reasons for the poor diameter difference of the plate cylinder: The increase in the diameter difference during plate making is not accurate. Countermeasure: Measure the diameter difference (method: see the plate making section).
(3) The rule line is abnormal. Cause: Insufficient density (density), printing offset, etc. Countermeasures: Adjust the sensitivity of the photocell, and manually eliminate the drawbacks such as the printing paste.
(4) The diameter of the plate cylinder is small. Reason: Smaller roll diameter overprinting accuracy is difficult to control. Countermeasure: If the length of the printing bag is short, the imposition method can be used, that is, two or more bags are printed on one printing roller.
2. Horizontal registration is unstable. the reason:
(1) The film is relaxed while the film roll is in parallel;
(2) The printing pressure is not uniform;
(3) The plate cylinder and the impression cylinder form a taper;
(4) The film tension is changed, and the film is formed to swing left and right. Countermeasure: Check and adjust each element one by one. If the horizontal registration device is loose, repair it.
Second, poor printing fastness
There are several reasons for poor printing fastness of plastic film:
1. Poor treatment of polyolefin film corona. Countermeasure: Check whether the surface tension of PE and PP reaches 40 dynes/cm or more.
2. The printing ink has poor affinity with the material to be printed. Poor mixture ratio. 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%, it is easy to generate static electricity. It is best to control the temperature at 18 ° C ~ 23 ° C, 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. Action: Replace the printed material.
Third, the pattern size shift
1. When printing, the longitudinal pattern shrinks in size. Cause: The shrinkage allowance is insufficient. Due to the influence of the tension of the plastic film and the heat drying during printing, the longitudinal dimension of the film is larger than that of the printed image. Countermeasure: The size of the shrinkage should be properly retained during the plate making; the film tension should be minimized.
2. When printing, the pattern size in the longitudinal direction is elongated. Cause: The gravure size is too much for the shrinkage allowance. Countermeasure: Increase film tension.
Fourth, the print rendition is poor
1. The highlight (bright part) is too deep. Cause: The plate is too deep and contains too much ink.
Countermeasures:
(1) increase the number of nets;
(2) reducing the colorant in the ink;
(3) Appropriately increase the angle between the doctor blade and the surface of the plate.
2. The highlight is too shallow. Cause: The depth of the plate is not enough.
Countermeasures:
(1) using dark-toned inks;
(2) Appropriately reduce the angle of the doctor blade to the surface of the printing plate.
3. The dark tone (dark part) is too deep.
Cause: The plate is too deep.
Countermeasure: Due to insufficient drying of the ink, the ink layer may be agglomerated, and it is necessary to re-type if necessary.
4. Dark density is not enough.
Cause: The plate is not deep enough.
Countermeasure: Use dark tone ink.
5. The depth of the intermediate adjustment ink is obvious.
Cause: Poor ink.
Countermeasure: Change the ink with good characteristics.
Fifth, After the printing, the ink layer is dry and conjunctiva is poor.
Reasons:
1. Hot air temperature, wind speed, air volume, and exhaust volume are insufficient.
Countermeasure: Adjust the drying device.
2. After printing, the ink layer has the illusion of dry conjunctiva, and the ink layer still contains residual solvent. After the film is wound, there is still about 40 °C of residual heat, suitable for the solvent to continue to move, and there is a certain pressure when winding, thus causing the damage of back viscosity.
Countermeasures:
(1) The ink layer should be thoroughly dried after printing;
(2) Printing firstly loosely collects the film in the wire basket. After the film is cooled, it is re-rolled, cut, and bagged when the film is not back-sticked.
Sixth, printed gloss is thin
Cause: The ink is over-diluted.
Countermeasure: Control the amount of thinner.
Seventh, the edge of the imprint is not smooth, the background is spotted
Cause: The ink viscosity is too low and the solvent is used excessively.
Countermeasure: Add new ink to increase viscosity.
Eighth, orange-striped markings on the printed layer
Cause: The ink dries too fast, the concentration 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 cause and take corresponding measures.
Ninth, the print layer has scratches
Cause: The guide roller is scratched, the sand is mixed with the ink, the blade is not flat, and the angle between the blade and the plate is not appropriate.
Countermeasure: Corresponding processing.
Tenth, thin color
Reasons: (1) The ink color is thin: (2) The color light reflection is poor.
Countermeasures: (1) adjust the ink; (2) the surface is first white ink to lay the ground color, and then the order color. In the print, the layer version first overprints the three primary colors, and finally lays the ground color.
Eleventh, bite color
In overprinting, the second color ink sticks off the first color ink.
Reason: After the first color printing, the ink layer is not completely dried, the second color pressure is large, or the machine speed is too fast, or the hot and cold air is bad, and the substrate printing is poor in suitability, which can cause biting.
Countermeasures: Find out the cause and take corresponding measures. 1 add an appropriate amount of quick-drying agent; 2 slow down the speed; 3 reduce the pressure of the impression cylinder; 4 increase the amount of cold and hot air.
Twelfth, the print is rough, the blank point is full, the field has moiré, and the tone is not full.
Reason: 1 too much solvent; 2 ink is thicker; 3 ink is too thin; 4 is too fast; 5 ink is poor in fluidity.
Countermeasures: 1 reduce the amount of solvent; 2 replace the ink or re-grind; 3 adjust the ink is thick; 4 properly slow dry solvent; 5 replace the ink.
In addition to the blocking phenomenon caused by chemical reactions caused by the ink itself (not uncommon), the remaining factors are summarized into five points:
(1) In printing, with the large amount of solvent volatilization, the viscosity of the ink increases, and it is easy to dry and solid inside the cable. If it is not cleaned in time, it will lead to serious blockage.
(2) In the printing process, if the blade is far from the printing roller or the angle is unreasonable, the diameter of the plate roller is large, or the depth of the plate roller entering the ink tank is insufficient, the hot air blown from the dryer affects the plate surface of the plate. Caused serious blockage.
(3) Impurities are mixed in the ink.
(4) The engraving depth of the plate roller (or the gravure depth of the gravure) is not suitable.
(5) When printing, if the temperature is too high, the solvent in the ink will evaporate too quickly, and the layout may be blocked.