Printing Defects
Misregister errors are common defects in gravure printing. The phenomenon occurs during the multicolor printing process, where each color does not align perfectly, resulting in a certain degree of error.
First Aspect: Personnel Quality
Because misregister errors are objectively present, employees are required to have strong work abilities. Printing is a meticulous job that requires attention, precision, and a sense of responsibility. Employees should conduct self-checks, self-inspections, observe from multiple perspectives, and promptly identify problems.
Second Aspect: Equipment Factors
The overall accuracy and performance of the equipment, tension control during registration, heating and air supply systems, coordination of impression rubber rollers, plate rollers, plate shafts, guide rollers, etc., determine the temperature, pressure, parallelism, elongation, and winding during registration. The misregistration system itself sets limits on its precision.
Inappropriate incremental increase of the plate roller can lead to inaccurate longitudinal misregistration, and deformation and insufficient elasticity of the impression roller rubber can occur. Maintaining the impression roller in good condition is essential.
Third Aspect: Operational Process
Improper substrate tension settings, too small tension leading to substrate slack, causing both longitudinal and lateral misregistration, or too large tension leading to unstable lateral shrinkage of the substrate and misregistration. Tension should be adjusted accordingly.
Inappropriate temperatures, especially if the oven temperature is too high, and the cooling roller temperature is too high, can cause film deformation. For less smooth films, the preheating oven temperature should be appropriately increased. Imbalance in the air intake and exhaust in the oven can cause film shaking, and the air intake and exhaust should be measured and controlled using an anemometer.
Improper plate roller installation, such as eccentricity during plate roller installation, must be cleaned before installing the plate roller.
Doctor Blade Lines
Causes:
Ink-free lines: Mainly caused by the inclusion of hard impurities in the ink, such as sand. These hard particles embed in the doctor blade edge and cause the lines. Sometimes, these doctor blade lines may appear as dashed lines, posing a risk of damage to both printing and doctor blades.
Coarser continuous inked doctor blade lines: This is mainly due to larger, soft particles or impurities lifting the inked doctor blade or damage to the doctor blade edge. This is the simplest and most common type of doctor blade lines.
"Meteor" shaped doctor blade lines: These lines generally appear in areas with shallow or unengraved dots on the printing plate roller, appearing intermittently and causing significant harm to product quality. The causes of these doctor blade lines are complex, and various explanations have been given over the years from the perspectives of ink, plate making, printing environment, etc. However, there is still no complete and perfect explanation, with the best being the "active particle hypothesis."
Fine filamentous continuous doctor blade lines: These lines, resembling fine hairs, often appear during the printing process and are difficult to observe on the printing machine. By the time they are noticed, a considerable quantity of products may already be printed, posing a significant risk to quality. These lines can be divided into two types: one is soft and can be wiped away with a bamboo knife, mainly caused by impurities, while the other starts as hair-like and gradually becomes band-shaped, cannot be wiped away with a bamboo knife, and has complex causes involving ink, plate making, environmental temperature, humidity, doctor blade hardness, etc. Specific problems require specific analysis and treatment. In general, there are several aspects:
a. Regarding the plate roller, if the surface roughness is poor, the chrome-plated layer on the surface of the roller is not hard enough, or the plate roller is too smooth without lubrication, it is prone to these lines.
b. Regarding ink, insufficient fineness, too large pigment particles, and the inclusion of granular impurities during ink use can cause these lines. Improper ink dilution, leading to solvent impact, and poor anti-static properties of the ink can also cause these lines. The use of high-hardness pigments can also result in lines.
c. Regarding the environment, the temperature, humidity, and cleanliness of the air in the printing workshop can also affect these lines.
d. Regarding the doctor blade, harder doctor blades produce more doctor blade lines than softer ones. Additionally, if doctor blade lines appear and are not promptly addressed, a large bundle of doctor blade lines may be generated around the area as the doctor blade moves back and forth.
Countermeasures:
When installing the doctor blade, it must be flat, careful, and the doctor blade edge should be checked for damage before installation. After installation, it must be carefully ground. If doctor blade lines appear during the printing process, the machine must be stopped, and careful grinding is required. It is strictly forbidden for grinding oil to mix into the ink.
The environment must be controlled to reduce dust and eliminate faults. It is essential to keep the color printing environment sealed. Air filters can be installed, and dust content can be measured to produce high-quality products.
Doctor blade lines caused by the plate roller generally occur in the shallow part of the network, such as when the plate is rough, the transition is uneven, and the network depth is inappropriate. Generally, these can be treated by grinding the plate with metallographic sandpaper. When grinding the plate, force should be applied evenly, especially for severe cases, which should be returned to the plate-making factory for treatment.
Reduce ink viscosity. If the viscosity is too high, the ink on the plate dries too quickly, relatively increasing the friction between the plate roller and the doctor blade, making it difficult for the doctor blade to scrape off the ink lines. If the lines are caused by ink, it is generally due to impurities or uneven dissolution of the ink. Filtration of the ink or reselection of solvents with stronger solubility or ink replacement should be performed.