Bubbles are one of the most common quality issues in laminated glass production. By carefully controlling materials, environment, and process parameters, bubbles can be effectively reduced or avoided. Below are some key prevention methods explained in simple terms.
1. Raw Material Control
The base glass should be flat, defect-free, and of reliable quality. If the glass surface has small pits, scratches, or impurities, air can get trapped during processing and form bubbles. Some low-quality glass may already contain tiny bubbles that become more visible after lamination.
The PVB interlayer film is equally important. It should have uniform thickness and the correct moisture content-typically between 0.3% and 0.6% for architectural glass.
If the film contains too much moisture, it will release vapor during the high-temperature, high-pressure stage, creating bubbles. If it's too dry, it becomes brittle, reducing the strength of the laminated glass and increasing the chance of air being trapped during lamination.
2. Lamination Process Optimization
The lamination room should be kept at 20 ± 3°C and 23 ± 3% humidity. This stable environment helps keep the PVB film in good condition, preventing deformation or excess moisture absorption.
When laying the film:
- Workers should wear lint-free gloves and avoid direct contact with the glass or PVB film.
- The film should be laid flat, not stretched, and carefully aligned with the glass edges.
- Leave 0.5–2 mm of film extending beyond the glass edge to allow for slight shrinkage.
After lamination, seal the glass edges evenly with a vacuum rubber ring and perform a cold vacuum extraction for at least 5 minutes. This step removes as much trapped air as possible before heating, which greatly reduces the risk of bubbles later on.
3. Preheating and Pre-Pressing
During this stage, both temperature and pressure must be precisely controlled.
For example, when using Sekisui (S-LEC) PVB film:
- First heating stage: Furnace temperature 80–100°C; glass surface temperature 40–48°C at the edges and 38–45°C in the middle.
- Second heating stage: Furnace temperature 180–200°C; glass surface temperature 70–80°C at the edges and 65–75°C in the middle.
If the temperature is too high, the glass and film bond too quickly, trapping air inside.
If it's too low, the film won't stick properly, leaving open edges that allow air to enter.
- Pressure must also be controlled carefully:
- Too much pressure can damage the glass.
- Too little pressure leads to poor adhesion and edge sealing, making bubbles more likely.
4. Key Points for Autoclave Processing
Before placing laminated glass into the autoclave, separate each piece with spacers and secure them on the rack. Close the door tightly, then start pressurizing.
- When the internal pressure reaches around 0.75 MPa, stop adding air. This helps the glass layers release trapped air before full bonding occurs.
- When the temperature reaches about 130°C, hold it steady and start the second pressurization to around 1.2 MPa. Maintain this temperature and pressure for 1–1.5 hours.
The combination of high temperature and high pressure allows the PVB film to bond fully with the glass and absorb any remaining air or moisture, effectively removing bubbles.
Finally, reduce the autoclave temperature to 50°C while slowly releasing pressure and air. Once fully vented, open the autoclave and remove the finished glass.
5. Conclusion
By maintaining strict control over raw materials, environment, and production parameters, laminated glass manufacturers can effectively prevent bubbles and ensure superior product quality.
