Missing Dots
Patterns created by the superimposition of two or more light-colored screens in printing, exhibiting intermittent instances of dot-free transfer, resembling the night sky with scattered stars. Here and there, you may notice a few white dots, often appearing as small circular areas lacking ink. This phenomenon is distinct from ink shortage or plate blockage. Consider a bowl produced by overlaying blue and red light-colored screens. If you observe star-like white dots on the bowl's surface, careful observation will reveal whether these dots exhibit a blue or red cast. If it's a blue cast, it indicates a red dot, and vice versa.
Common Causes:
High viscosity of the current ink color
Small angle of the current ink color's doctor blade (blade too flat)
Elevated environmental temperature
High viscosity of the preceding related color
High viscosity of an unrelated but large-area color (less common)
Remedies:
Reduce the viscosity of the current ink color
Increase the angle of the current ink color's doctor blade
Lower the temperature in the printing workshop (using an electrostatic ink absorption device, if available, may enhance effectiveness)
Decrease the viscosity of the preceding color (e.g., if reducing the viscosity of red ink doesn't work in the case of red-blue overprinting, consider reducing the viscosity of blue ink)
Reduce the viscosity of the preceding unrelated but large-area color (e.g., in the case of Kangshifu's classic condensed milk-flavored milk tea with blue-red-yellow overlapping, if blue and red dots appear, and the base color (first color pearlescent white) is hollow in the pattern area, lowering the viscosity of the base color to ≤12.0 seconds immediately improves dot retention.)
Adhesion Issues
Poor adhesion between ink and the original film, where ink can be easily scraped off or detached. A common inspection method is to use 3M tape to adhere to the printed ink surface, then peel off the tape at a speed of 1m/second. Generally, if 90% of the ink does not come off, it is considered acceptable. Transparent tape is often used during printing for a simple adhesion test because it has a stronger adhesive force than 3M tape and is more cost-effective.
Common Causes:
Insufficient corona treatment intensity on the original film
The original film has expired
The corona surface of the original film is reversed
Incorrect use of ink or mixing of different ink types
Incorrect solvent use
Poor drying (ink not completely dry)
Poor adhesion of the ink itself
Remedies:
Check corona treatment intensity and use original films with acceptable corona treatment
Discontinue the use of expired materials and use materials within their expiration date
Change the printing surface
Segregate different types of inks, especially in gravure printing, and clearly label and store leftover inks to prevent mixing when reused
Use the correct solvent type and ratio
Ensure the drying machine provides adequate temperature