Reasons for deviation in printing accuracy
1. When making multi-color overprint (halftone printing) screens, the screens are dried at different temperatures, thereby causing errors in printing accuracy.
2. The screen has low tension or overprint (halftone) screen tension.
3. Drying with hair dryer, screen drying uneven.
4. The use of too old, unsturdy, and deformed grids to make plates, resulting in deviations in the printed image due to poor stability of the frame (these problems can be detected with negatives)
5. When multi-color printing, the net distance is not used.
6. Squeegee deformation.
7. Affected by some factors caused by changes in the size of the substrate
Causes of moiré
· Mismatches of half-tone printed cable and screen mesh (the number of screens should be 2.5, 3.75 or 6.25 times that of the cable).
• The halftone printing screen and the screen line have an incorrect angle.
The RZ value is too high (should be less than 8).
· The template is too thick or too thin.
• Compared to the wire diameter, too small dots will be lost (the dot range should be 5-95%).
Cause of color change
1. The consistency of the ink changes (different viscosities) due to evaporation of the solvent.
2. Change the squeegee angle during printing.
3. Change the scratch hardness in the printing process.
4. Change the speed during printing.
5. Scratched edge damage.
Causes of premature stencil damage
1. Insufficient rubber grease.
2. Insufficient drying before exposure.
3. Insufficient exposure time.
4. The net distance is too high.
5. The ink blade is too dry and the pressure is too great.
6. Use the wrong cleaning solvent for cleaning (including a small amount of water).
Tonometer
For screen printers, a tensiometer is an instrument that measures and monitors the screen level during the stretching or re-use. The unit of measurement is Newtons per centimeter, which is simplified to N/CM.
Note: 1 Newton is the force measured by accelerating the 1 kilogram of material on an area of ​​one square meter.
Thickness gauge
The instrument is mainly used in the following areas:
Printed circuit board (PCB);
Glass ornament
Ceramic ornament
Ceramic applique.
The print quality will depend on the control of the conditions of each factory during actual printing.
Thickness gauges are increasingly being used by all screen printing manufacturers certified by quality standards certification bodies.
Surface roughness measuring instrument
The instrument has been used in the engineering field for many years, and the screen printing industry was in the late 80's. The tester can show the flatness of the stencil surface. The sign of the surface roughness value is RZ, and an RZ value of 0 indicates that the template is very smooth.
Surface moisture meter
For best results, the reticle needs to be fully dried before exposure. If it is not completely dried out, the moisture contained in the photoresist will cause the photosensitive agent to be insufficiently sensitive, resulting in underexposure of the template, and even worse, there will be a phenomenon of release. The dryness of the screen is not easy to judge. The contact humidity meter is an ideal test instrument. It can eliminate people's concerns about the moisture content of the stencil to be exposed.
Ray Detector
In the stencil exposure process, two important parameters need to be controlled: the energy of the UV light and the distribution of light in the stencil area. Although most quality exposure machines are now equipped with an optical integrator as compensation for the UV output of the wave, it is not easy to measure the distribution of the incident light and the state of the UV light radiation density.
When the center of the exposure frame and the density of the light in the farthest corner are at the same level, the desired distance is achieved.
When using this measurement, wear UV-proof glasses and do not prolong unnecessary exposure.
Source: China Packaging News