In the high-gloss screen printing silicone process, the problem of printing bubbles will directly affect the surface flatness, glossiness and yield of the product, especially in scenarios that require high transparency or mirror effects (such as electronic equipment housings and automotive interior parts). Bubble defects will significantly reduce the value of the product. The following is a systematic solution to this problem, covering four dimensions: materials, processes, equipment and environmental control:
I. Material selection and pretreatment
1. Silicone system optimization
Choose low-viscosity, low-surface tension silicone
High-viscosity silicone is easy to encapsulate air and has poor fluidity. It is recommended to use addition-type or condensation-type silicone with a viscosity of ≤5000mPa·s (such as Dow Corning SE1700, Shin-Etsu KE-45T), and reduce the viscosity by adding 0.5%-2% silicone oil diluent to reduce the risk of bubble residue.
Add defoamer
Add 0.1%-0.3% polyether-modified siloxane defoamer (such as BYK-066N) to the silicone to accelerate bubble rupture by reducing surface tension. Attention should be paid to the compatibility of the defoamer with the silicone system. It is recommended to conduct a small test to confirm it first.
2. Screen and ink pretreatment
Screen cleaning and degreasing
Use isopropyl alcohol or special screen cleaning agent to thoroughly clean the screen, remove impurities such as grease, dust, etc., and avoid bubbles attached due to uneven surface tension.
Pre-degassing of ink
If two-component ink is used, vacuum degassing (pressure ≤-0.095MPa, time 5-10 minutes) is required after mixing to remove tiny bubbles introduced during the stirring process.
II. Process parameter optimization
1. Printing parameter control
Scraper pressure and angle
Pressure: 0.15-0.25MPa (too high will squeeze air into the silicone layer, too low will not fill enough).
Angle: 70°-80° (acute angle can reduce the friction between silicone and screen and avoid air entrapment).
Printing speed
Control the speed at 15-25mm/s to ensure that the silicone fills the mesh evenly and does not cause air entrapment due to too fast speed.
Ink return knife pressure
The ink return knife pressure should be slightly lower than the scraper (about 0.1MPa) to avoid pressing air into the screen when returning ink.
2. Printing environment control
Temperature and humidity
Temperature: 20-25℃ (too high temperature will accelerate the curing of silicone and shorten the time for bubbles to escape).
Humidity: 40%-60%RH (condensation silicone is sensitive to humidity, and high humidity may accelerate the curing reaction).
Air cleanliness
Operate in a Class 100 clean room to avoid dust particles becoming bubble attachment points.
III. Equipment improvement and auxiliary tools
1. Screen and scraper upgrade
Screen selection
High-tension screen: tension ≥24N/cm (such as Sefar 165T) to reduce air entrainment caused by screen deformation during printing.
Coating treatment: Use anti-static, low surface energy coated screen (such as Kirlin KC-300) to reduce the adhesion between silicone and screen and facilitate bubble escape.
Scraper material
Use polyurethane scraper (hardness 70-75 Shore A), its flexibility can reduce the shear force on silicone and avoid bubble generation.
2. Auxiliary degassing equipment
Vacuum printing table
During the printing process, vacuum is continuously drawn (pressure ≤-0.08MPa), and negative pressure is used to accelerate the escape of bubbles.
Pre-pressing treatment of release film
Before printing, use a silicone roller to apply 0.3-0.5MPa pressure to the substrate to pre-press the release film to remove the air between the substrate and the release film.
IV. Operation process specifications
1. Layered printing and curing
Multiple thin coatings
The thickness of a single print is controlled at 20-30μm, and the target thickness is achieved through multiple printings (such as 80μm requires 3 printings). After each printing, let it stand for 2-3 minutes to allow bubbles to escape naturally.
Step curing
Initial curing: After printing, first cure at 60℃ for 10 minutes to initially cross-link the silicone surface and fix the shape.
Final curing: Heat up to 120℃ for complete curing to avoid rapid curing at high temperature causing internal bubble expansion.
2. Defect detection and rework
Online detection
Use AOI automatic optical inspection instrument to monitor bubble defects in real time, with a positioning accuracy of ±0.05mm.
Rework process
For the local bubble area, use laser heating (power 5-10W) to soften the silicone locally, then use a silicone needle to inject the same batch of silicone to fill, and finally solidify the whole.