Common Problems and Solutions for Standard and Color Printing of PP Woven Bags

I. Poor Ink Adhesion on Substrate

1.1 Ink Adhesion Mechanism
Wetting: Adhesion strength is the result of the interaction of intermolecular forces between the printing substrate film and the ink binder molecules.

Compatibility: The polarity of the ink binder and the substrate is similar, resulting in good compatibility.

1.2 Causes and Countermeasures for Poor Adhesion Insufficient

corona treatment on the substrate surface, failing to achieve the required tension, or poor storage environment and prolonged storage time after treatment causing surface tension decay and failure. The surface tension of each batch of film should be checked before use. Substrates often have lubricants, antistatic agents, and moisture-proofing agents added during processing. Over time, these additives slowly penetrate from the film to the surface, forming a weak interface layer that affects ink adhesion. Therefore, the film should be used within its expiration date, and the surface tension should be tested before use. A dyne pen test can be used. Incorrect ink type used, mixing different types of ink. There are many types of gravure inks, including surface printing and reverse printing inks. Different materials require different inks, and various special inks must be used accordingly; otherwise, adhesion will be insufficient. The operating instructions should be checked before production. Ink whitening. Ink deterioration and whitening cause a drastic decrease in adhesion. Ink incomplete drying. Insufficient drying results in excessive residual solvent. Furthermore, a certain drying temperature is extremely beneficial for improving the adhesion of some resin binders.

II. Registration Error
Registration error is a common defect in printing. In multi-color printing, each color cannot completely overlap, resulting in a certain deviation.

2.1 Personnel Qualification
Because registration error is an objective reality, employees must possess extremely strong work abilities. Printing is a meticulous task, requiring dedication, attentiveness, and a strong sense of responsibility. During work, self-inspection and observation are essential to promptly identify problems.

2.2 Equipment Factors
The precision and performance of the entire equipment, the tension control during registration, the heating and air supply system, and the coordination of the impression roller, plate roller, plate shaft, and guide roller systems all determine the temperature, pressure, parallelism, elongation, and winding during registration. The registration system itself further limits its precision.

The color registration photoelectric device should be well-maintained and operated correctly. For example, if the halogen light source is weak, replace it with an optical fiber or halogen bulb. The distance between the reflector and the scanning head should be correctly adjusted, generally around 8mm. Improper increase in printing roller tension can easily lead to inaccurate longitudinal color registration, deformation of the printing roller rubber, and insufficient elasticity. The printing roller must be kept in good condition.

2.3 Operating Process

Inappropriate substrate tension setting: Too little tension results in a loose substrate, leading to inaccurate longitudinal and transverse color registration; too much tension causes transverse shrinkage of the substrate and unstable longitudinal color registration. Adjust the tension accordingly. Inappropriate temperature, especially excessively high oven or cooling roller temperatures, can deform the film. For uneven films, the preheating oven temperature should be appropriately increased. Uneven airflow in the oven causes film vibration. Use an anemometer to measure and control the airflow. Improper printing roller installation, such as eccentricity, requires thorough cleaning of the ink from the end caps before installation.

3.1 Causes of Doctor Blade Marks
Uninked marks are mainly caused by hard inorganic impurities, such as sand, mixed in with the ink. These hard particles become embedded in the doctor blade edge, causing marks. These types of doctor blade marks typically appear on the printing rollers in areas with very shallow or unengraved dots, appearing intermittently and posing a significant threat to product quality. The causes of these doctor blade marks are complex, and over the years, explanations have been offered from aspects such as ink, plate making, and the printing environment, but no single explanation is perfect yet. Regarding ink: Insufficient ink fineness, excessively large pigment particles, or particulate impurities introduced during ink use; improper ink dilution leading to solvent shock; poor antistatic properties; and poor lubrication. The use of high-hardness pigments can also produce lines. Regarding the environment: The temperature, humidity, and cleanliness of the air in the printing workshop also have an impact. Regarding the doctor blade: Harder doctor blades produce more marks than softer ones. Furthermore, if doctor blade marks are not addressed promptly, a large cluster of marks can form around the original mark due to the back-and-forth movement of the doctor blade.