Incomplete degassing of EVA film in laminated glass causes the a finished product to be full of bubbles. How to salvage it?

After completing a batch of laminated glass, it was cut open and reveal to be covered in bubbles of different sizes and densities, rendering the entire batch unusable. Don't rush to destroy equipment; the problem may not be the cladding machine, but the planting during degassing. Degassing is not a a simple "vacuuming" process, but a complex one. Each step leaves a bubble.
First, understand where the bubbles come from. Bubbles in laminated glass originate from three sources: first, air trapped in an EVA film; second, residual air between two glass panes; and third, gas released by EVA as it decomposes at high temperatures. The first two depend on procedure, while the third depends on formulas. Most people focus on the first two and ignore the third, which is why bubbles appear even after vacuuming and pressurizing.
EVA begins to soften and flow above 110°C. If the temperature rises too fast in this range, volatiles and dissolved gases in the film precipitate rapidly, forming microbubbles. Bubbles range in diameter from 0.1mm to 0.5mm and are almost invisible to the naked eye but appear dense when cut open. Worse, while larger bubbles can be removed by vacuum, smaller bubbles are completely trapped by the high-viscosity EVA melt.
Three main sins of Degassing Failure
culprit 1: Insufficient Vacuum or incorrect Vacuuming Timing
Many production lines are vacuumed at the same time, or even heated first and then vacuumed. This is not right. The correct sequence is: first, vacuum at room temperature to remove all air between the glasses; the vacuum level should be at least -0.095 MPa. Keep the pressure on for 3-5 minutes, then turn up the heat. If heated before vacuuming, the EVA softens, releasing the gas much faster than vacuuming, leaving no time for bubbles to escape.
Another detail that is often overlooked is that the volume of vacuum pumps must match the size of the glass. For small glass plates up to 6mm thick, a standard rotary vane pump is sufficient. However, if you produce large pieces of paper more than one metre long, the vacuum lines may be too long or poorly sealed, resulting in an actual vacuum of only -0.06 MPa. This means the air inside the slab cannot be completely cleared. Actual test data shows that for every 0.01 MPa difference vacuum, the bubble residue rate increased by approximately 15%.
The second reason is that the temperature is rising too fast for the gas to escape
If EVA is heated from room temperature to a softening point of 110°C inside 5 minutes, when the EVA boils, the dissolved gas and trace amounts of moisture in the film are immediately released, creating a numerous microbubbles. These bubbles are "frozen" in a highly viscous melt and cannot be destroyed by subsequent pressurization.
The correct heating rate is 2-3 degrees Celsius per minute, and it requires about 30-40 minutes from room temperature to 100 degrees Celsius. Slow heating allows sufficient time for the gas to settle gradually and be sucked away, rather than explode all at once. Many people compress the heating time to 15 minutes to increase production, saving half an hour but wasting an entire batch.
Sin 3: Not having enough time to hold the bubble under control
After the lamination temperature reaches 110°C, it needs to be maintained at 0.3 to 0.5 MPa for 15 to 20 minutes. The pressure is not intended to "flatten" the bubbles, but to allow the EVA melt to flow fully, sealing any remaining bubbles and squeezing microbubbles in the membrane to the edges to vent.
If left in place for less than 10 minutes, the EVA will not flatten completely and bubbles will be sealed in the middle of the membrane, leaving white spots when cut open. It also makes no sense to stick to it for more than 30 minutes, and can even lead to excessive EVA flow and insufficient edge bonding. 15-20 minutes is the best time.
What to Do If Bubbles Have Already Formed?
If a bubble has formed, do not panic and deal with it in a timely manner.
For bubbles larger than 2 mm in diameter and smaller than 5 mm in diameter, a syringe can be used to puncture holes in the edges of the glass and inject an adhesive for local repair. However, this is only a surface repair and structural strength has been compromised.
For sealed bubbles less than 1 mm in diameter, the entire batch must be downgraded or scrapped. No reprocessing process can completely eliminate dense microbubbles because they disperse within the membrane, preventing the injection of adhesives and the removal of heat.
Basic solution: Degassing as a stand-alone process, not as a patch aid.
First, a vacuum. Vacuum at room temperature for 5 minutes to more than -0.095 MPa, then raise temperature. Orders are crucial.
Second is slow heating. Heating rate to 2-3 degrees Celsius / min.
Third, enough pressure should be put on implementation. Maintain 0.3-0.5 MPa at 110°C for 15-20 minutes to bring the EVA to full level and release air.
Fourth is the control membrane. Use the EVA membrane on the day of opening and store in a place where humidity does not exceed 50%. Once the membrane absorbs moisture, evaporation during lamination becomes a source of bubbles, rendering suction ineffective.
In short: degassing is not a simple act, but a complete process. Vacuums, warmth and pressure are indispensable, the sequence cannot be reversed, and time cannot be omitted. In the process of defoaming, every minute you steal ends up as a bubble on the glass, shards falling one by one to look at you.