Windshield Lamination Process Secrets

The complex nature of lamination defects; individual processing conditions, and different materials create a situation where the only working solutions are individually adjusted processing parameters.

I was inspired writing this article after I concluded that majority of my windshield production related cases included findings causing lamination defects and encouraged further more after seeing lines performing under conditions that were not supposed to produce even one good piece of laminated glass. In my experience the producers sometimes over look the conditions that contribute to reduced lamination process performance. The lamination process is in key-role in complete windshield production unit performance and it’s results and quality are directly linked with bending and pre-processing activities. In this article I will review the lamination process and most common lamination defect. LAMINATION PROCESS Lamination defects are typically within the top 5 waste (scrap) reasons in production of laminated windshields. Dominating lamination process in production of windshields contains the following process cycle: PVB Assembly -> 2. Vacuum conveyor (cold and hot)-> 3. Autoclave Alternatives are existent such as vacuum boxes, but the processing principles remain. The entire process seems simple, however following the basic guide lines of clean environment work and common processing parameters alone is not enough to maintain and improve the production yield levels. Without perfectly adjusted processing parameters the process can result with high number of re-autoclaved and waste pieces due to lamination defects. Waste created at this stage of production chain is particularly expensive and re-autoclaved pieces create workflow to unnatural direction. Furthermore re-autoclaved pieces are especially harmful for productivity as the autoclave is typically the bottleneck in laminated glass production lines, due to the limited capacity and long process cycle taking up to 4 hours. TYPICAL LAMINATION DEFECT Defect known as “bubbles” is probably the dominating lamination defect caused by air left between the glass sheets. This lamination defect is controversial: the bubbles do not appear every time when a little air is left between the laminate or when gapping between the glass pairs is monitored. The defect seems to arise when few contributing factors are existent. The fact that there seems to be no single dominating factor that ultimately creates this defect makes the elimination difficult. The “Bubble”-defect appears in different sizes and locations. Typically the defect appears as “bubble”-areas in the very edges of the glass, while also larger individual bubbles appear: both in edges as well more towards to the middle section. Larger “bubbles” appearing in the very edge of glass can be commonly fixed with re-autoclave cycle adding clips that help sealing the glass edges. However larger bubbles in the inner parts of the glass, typically between the glass corner and mould hinge line can be impossible to fix. WHY BUBBLES APPEAR? There are several conditions in each processing stage that can affect the lamination performance. Pre-Processing Cutting size variations will cause severe difficulties in positioning of the glass pair in the lamination. Default in positioning of glass pairs in PVB assembly will cause unwanted gapping between the glass   sheets. Extensive amount of separating powder will cause minor surface variations in the glass pairs inner surfaces reducing adhesion of the PVB and glass. Poor washing water characteristics can affect the adhesion. Therefore the hardness of the water is monitored. Gapping between glass pairs created during bending process is also potentially increasing the risk of defects. Gapping between the glass sheets created due to temperature difference between the glass sheets or mould issues will contribute to appearing bubbles. Gapping affect is very product dependent, however gapping < 0,8 mm is usually not considered to cause “bubbles”. In any case gapping is a contributing factor and together with other contributing factors will create circumstances where the “bubble”-defect is more likely to appear. Lamination process conditions: Lamination clean environment conditions; temperature, and relative humidity should be in suitable levels. PVB storage should also maintain correct temperature and relative humidity. Cold-Suction of the laminate is also in important role. The cold-suction time should be sufficient enough to relive almost all the air from the glass before the edges are sealed. Usually most common reason for appearing bubbles is processing default in the very beginning of the heat treatment (hot-vacuum) enabling the glass edge to close too early. The hot de-airing phases heating should be conducted gradually and evenly so that the glass edges do not seal before all air is relieved from the sandwich Autoclaving process  The autoclaving process is the final real treatment process of a typical windshield that will define the lamination and end product quality. The process temperature and pressure curves can be controlled in great detail with modern autoclaves. Correct temperature and pressure set points are required to gain perfect results. With wrong set points the bubbles can appear days after the process. HOW TO PREVENT BUBLES DEFECT? Start with perfecting the pre-processing activities; cutting size, washing water quality control, bending process controls and quality. Follow to lamination activities controlling and measuring all the actions carefully: Releasing the PVB, careful positioning at assembly, sufficient cold-vacuum, controlled heating in hot-vacuum, correct actions with vacuum ring assembly and disassembly, and finally perform perfectly adjusted autoclave process with correct heating and pressure curves. When all production activities are well documented, staff performs according to production instructions and process parameters are repeated to perfection the identification of all variations is considerably easier. Safety Glass Experts provides services to study your current production activities performing a complete production Present State Evaluation. The evaluation will provide you complete, objective, external experts assessment and findings of your production current status. This information enables efficient approach solving any production defects, reducing waste, improving efficiency and line personnel know-how. CONCLUSIONS Processing conditions and used materials vary; therefore the followed process guidelines must be adapted to individual situations. Lamination process conditions must be well controlled to enable adjustments and identification of process variations when defects appear. Lamination process includes various minor details that potentially affect the results and results are linked with bending and pre-processing quality. Mastering these details is essential. Manufacturers suffering from any lamination difficulty will receive immediate solutions through our Remote Expert Service or on-site visits conducted by our experts. Safety Glass Experts International Oy Ltd Talviseisaus 2 D 8 FI-20400 Turku Finland Phone: +358 400 979 300 Fax: +358 2 6518 2539 Email: info@sge.fi Web: www.sge.fi All graphics, photographs, and text appearing in this article belong to Safety Glass Experts International Oy Ltd. Redistribution or commercial use is prohibited without express written permission.

Photos: http://www.sge.fi Last review: January, 2011