After the printing of plastic film or transparent paper, there are dry artifacts after printing, but the phenomenon of back stickiness occurs again during the storage period after printing the roll or bag. In severe cases, the parts cannot be torn between the prints, and the ink layer of the image is shifted to cause the product to be scrapped. This phenomenon is more likely to occur in the hot season, but the high temperature is not the only cause of back tack. It is also closely related to a series of factors such as plastic, ink, solvent, air volume, light, and speed.
1. The solvent in the gravure ink is not completely evaporated after printing.
(1) Plastic film printing is drying the ink by solvent evaporation. The volatilization rate is an important factor that determines the drying of the ink layer and affects the printing quality. The slower the volatilization of the solvent, the better the reproducibility of the plate, the more beautiful the print color, but easy to stick; Therefore, the choice of a suitable solvent for evaporation speed is the key to the gravure printing of plastic films. In continuous color printing, the solvent must be completely evaporated after the film is in operation between the two printing stations. Otherwise, the rollers will stick when printing at the next station. (2) After the film is printed, the organic solvent in the ink is not completely evaporated in the drying system. After the film has been reeled, there is still residual heat, and the solvent remaining in the ink layer brings about the condition of continuing to volatilize and forms the blocking. (3) High-temperature and wet festivals. After the printing, the film is wound up tight or under pressure.
Countermeasures: The volatilization rate of the solvent in the ink depends not only on the boiling point of the solvent, vapor pressure, latent heat of the vapor, but also on the conditions of the operating environment such as temperature, humidity, air volume, solute, and ink layer thickness. Therefore, the volatilization rate of the solvent in the ink should be adjusted at any time according to the change of conditions. Under normal temperature, if the ink layer is dried too fast, slow-evaporating solvent (such as Shanghai-made 74-92 type plastic film slow-drying thinning agent) should be added; otherwise, fast-evaporating solvent (such as 74-91 plastic film) should be added. Fast drying thinner). It can also be diluted with a mixed solvent of xylene, ethanol, and isopropanol (the three solvents are each mixed in a ratio of 1/3, and it is better to use a single solvent than the mixed solvent). If you feel too dry, you can use a little bit of butanol to replace some of the ethanol (butanol has the effect of increasing the luster of the ink). However, it must be noted that the amount of addition should be appropriate so as not to affect the drying of the prints.
Solvents for the gravure printing of plastic films, mainly alcohols, together with benzenes. Ethanol and isopropanol are more commonly used for alcohols, and toluene and xylene are more commonly used for benzenes. Due to the requirement of nitrocellulose, esters such as ethyl acetate and butyl acetate are also added.
In short, the organic solvent in the ink is sought to completely evaporate before being wound up. Slow down the speed and increase the air volume. After the film is printed, it is loosely placed in a wire woven basket so that the ink layer can continue to be ventilated and dried. After solidification, the film is cut, bagged and film-printed and then bagged and placed in a carton to reduce the film surface. The pressure between.
2. Plastic gravure ink ink is bad. Some plastic gravure inks have a low melting point of the linking material and are prone to sticking when the operating temperature is high and the relative humidity is high.
Countermeasures: 1 When the conditions permit, the operation of the workshop to install air conditioners, the temperature control of any temperature between 18 °C ~ 20 °C, relative humidity controlled at 65% or less; 2 replace ink with good ink.
3. Plastic film thin print packaging is not suitable. Reasons: 1 non-packaging resin processed plastic film; 2 resin opening agent is insufficient. Countermeasure: Replace the plastic film.
4. Electrostatic effects: Causes: Static electricity is generated from the plastic film, and films are formed to adhere to each other. Solution: Add antistatic stings to the plastic.
Second, the color accuracy is poor
There are several reasons why plastic film registration is not allowed:
1. Vertical direction (ship direction) printing register changes.
(1) Tension variation during film pulling. Causes: The reel brake of the uncoiling device is not properly prepared; the reel material is too much, or the diameter of the reel material is too large; the three roller speeds are set incorrectly; the surface friction of the three rollers is reduced; the tension of the rewinding machine is changed. Countermeasures: Check and adjust each control device.
(2) The reason for the poor diameter of the plate cylinder: The increase in the diameter difference during plate making is inaccurate. Countermeasure: Measuring the diameter difference (method: See the plate making section).
(3) Abnormal rules. Cause: Insufficient density, printing offset, etc. Countermeasures: Adjust the sensitivity of photocells, and artificially eliminate defects such as plate stencils.
(4) The diameter of the plate cylinder is small. Reason: The smaller roller diameter overlay accuracy is difficult to control. Countermeasures: If the length of the bag is short, the imposition method may be used, ie, two or more bags may be printed on one roller.
2. Horizontal registration is unstable. Causes: (1) The film is loose on one side and the guide roller is not parallel; (2) The printing pressure is not uniform; (3) The plate cylinder and the impression cylinder are tapered; (4) The film tension changes, etc., and the film swings sideways. Countermeasures: Check and adjust each element one by one, such as when the lateral register device is loose.
Third, poor print fastness
The reasons for poor fastness of plastic film printing are as follows:
1. The corona treatment of the polyolefin film was poor. Countermeasures: Check whether the surface tension of PE and PP reaches 40 dyne/cm or more.
2. The printed ink and printed material have poor affinity. Poor mixture ratio. Countermeasures: Replace the ink and adjust the solvent ratio.
3. The printing shop temperature and relative humidity change greatly. Countermeasures: As far as possible, the relative humidity of the workshop should be controlled between 50% and 60%, but it should not be too low. Below 40%, static electricity is easily generated. 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. Countermeasure: Replace the printed material.
Fourth, the pattern size shift
1. When printing, the longitudinal pattern shrinks. Reason: Insufficient shrinkage margin. Due to the tension of the plastic film and heat drying during printing, the longitudinal dimension of the film is larger than the shrinkage of the printed image. Countermeasures: The size of shrinkage should be properly left when making plate; the tension of the film should be minimized.
2. The length of the pattern in the longitudinal direction is elongated during printing. Cause: The gravure size left too much for the shrinkage margin. Countermeasures: Increase the film tension.
Fifth, the print product level is poor
1, high light (bright) too deep. Cause: The printing plate is too dark and contains too much ink. Countermeasures: (1) increase the mesh; (2) reduce the pigment in the ink; (3) appropriately increase the angle between the doctor blade and the surface of the plate.
2. High light is too shallow. Cause: The depth of the plate is not enough. Countermeasures: (1) Use dark-toned ink; (2) Appropriately reduce the angle between the doctor blade and the plate surface.
3. Dark tone (dark part) is too deep. Cause: The plate is too deep. Countermeasures: Due to the lack of ink drying, the ink layer may be agglomeration, and if necessary, it must be reprinted.
4. Dark tone density is not enough. Cause: The plate is not deep enough. Countermeasure: Use dark color ink.
5. The depth of the intermediate tone ink junction is obvious. Cause: Poor ink. Countermeasure: Replace ink with good characteristics.
Sixth, the Indian ink dried conjunctiva is bad
Causes: 1. Insufficient hot air temperature, wind speed, air volume, and exhaust volume. Countermeasure: Adjust the drying device.
2. After printing, the ink layer has the illusion of dry conjunctiva. The ink layer still contains residual solvent. After the film has been rewinded, there is also residual heat at about 40° C., which is suitable for the solvent to continue its activity, and the pressure at the time of rewinding has a certain amount of pressure, thereby causing the danger of back tackiness. Countermeasures: (1) The ink layer should be thoroughly dried after printing; (2) The film should be first loosely packed in a wire basket after printing, and the film should be cooled, and the film should be rewinded, cut, and then re-rolled after the film is cooled. bag.
Seventh, the printed product gloss is thin
Cause: Ink dilution is excessive.
Countermeasures: Control the amount of diluent.
Eight, 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.
Countermeasures: Add new ink to increase viscosity.
Nine, the Indian layer from orange peel markings
Reasons: The ink is drying too fast, the concentration is too light, the leveling property is poor, the thixotropy is large, there is static electricity, the printing speed is too slow.
Countermeasures: Find out the reasons and take corresponding measures.
Ten, printed layer scratches
Causes: The roller was scratched, sand was mixed in the ink, the blade was uneven, and the angle between the blade and the plate was not proper.
Countermeasures: Corresponding processing.
Eleven, thin shades
Reasons: (1) Ink color is thin: (2) Poor color reflection.
Countermeasures: (1) adjust the ink; (2) surface printing with a white ink to lay the background color, and then sequentially color. In India, the first edition was printed with three primary colors, and the background was finally laid.
Twelve, bite color
In overprinting, the second color ink sticks off the first color ink.
Reason: After the first color ink layer is not completely dry, the second color pressure is too large, or the machine speed is too fast, or hot and cold air is bad, poor substrate printing, can cause bite.
Countermeasures: Find out the reasons 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.
Thirteen, rough print, blank dot full version, moire in the field, tune is not full
Reasons: 1 too much solvent; 2 ink is thick; 3 ink is too thin; 4 volatile too fast;
Countermeasures: 1 reduce the amount of solvent; 2 replace the ink or re-grinding; 3 adjust the ink thick; 4 appropriate slower dry solvent; 5 replace the ink.
In addition to the phenomenon of blockage caused by the chemical reaction caused by the ink itself (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 inside the cable. If it is not cleaned in time, it will cause serious blockage.
(2) In the printing process, when the squeegee is located far from the printing roller or the angle is unreasonable, the diameter of the printing roller is large, or the depth of the platen roller entering the ink tank is not enough, the hot air blown from the dryer affects the surface of the platen roller. Caused serious blockage.
(3) Ink is mixed with impurities.
(4) Edition roller network engraving depth (or photogravure corrosion depth) is not suitable.
(5) When printing, the temperature is too high to make the solvent in the ink volatilize too quickly, and it is easy to cause blockage on the layout.
Causes and countermeasures of common gravure failures
First, stick back