De-Airing PVB Flat Glass Laminates with Hot-Rollers
I realized few weeks back that I have not written public material in a while. Going through my notes made for potential topics for new articles didn’t help. Writers block. This week it finally it dawned on me that I should write about de-airing with nipper rollers/hot-rollers/. I’ve written extensively about windshield manufacturing, but only random pieces relating to the architectural/construction glass, but I work with architectural glass these days almost as much as with vehicle glass. So this time it is flat glass and I will try to keep the article short and simple, let’s see how that works.
De-airing flat glass laminates with hot-roller oven follows the same fundamentals as any PVB de-airing process. Thus, what we must accomplish in the process in chronological order is;
1. Air Removal
2. Edge Sealing
3. Preliminary adhesion/bond
Standard hot-roller oven consists two de-airing rollers, pre-heat oven, and primary oven. Some ovens allow oscillation and most ovens also consist a “air stirring” system which both are to promote heating of sometimes thick multilayer laminate.
If you have read my posts about lamination you’ve come to find that I always highlight the importance of air removal. Most often I meet situations where lamination defects are believed to be a result of some complex pattern of conditions, where in fact they are simply results of poor de-airing process.
The process starts with the laminate travelling on the conveyor to the pre-heat section. At this stage, the laminate should be heated only slightly to relax the PVB so that it will become softer and slightly tacky prior to the first hot-rollers. It is important not to over-heat the laminate, because this will cause premature edge seal, especially with thicker units, since the edges can seal simply due to the weight of the glass layers. I personally like to run the laminate through the first set of hot-rollers as cold as possible. The target in the first rollers is to literally to push the air out of the laminate and crush the surface pattern. Pressure of the rollers are typically around 5 bars. It is important to note here in-regards the pressures the system indicates that they are most often not precise values and I personally trust the set values of the machines only once I have verified data gathered with load cell for example.
After this the laminate transfers on the conveyor to the main oven. The main oven temperatures are higher to get the PVB soft and tacky. The required temperatures of the laminate can be achieved running moderate conveyor speeds and working with processing temperature or if available using the oscillation function. In the processing of flat laminates unique glass types and coated glass are very common. These parts regularly need higher temperatures and slower conveyor speeds, or alternatively oscillation. My preference is slow conveyor speeds-from 0.8 to 0.3 m/min- to allow the unit to heat as uniform as possible, soaking it in the main oven. Remember that IR-heating-even if you have little stirring fans- is only penetrating the very surface of the glass, thus majority of the glass thickness is heated by conduction. This is a challenge in short hot-roller ovens because you can easily over heat the first layer(s) of multilayer laminate.
Once the laminate exits the main oven it is subject to the second hot-rollers. Here the laminate needs to pressed with >5 bar pressure to create the edge seal and preliminary adhesion in order to prepare the glass for the Autoclave process.
Over the years, I have noticed that there is a common tendency to run the hot-rollers fast to put out more product. In these scenarios, most often operators do not “read the glass” With this I mean looking carefully what happens before and after both hot-rollers gathering understanding what temperatures, roller pressures, and conveyor speeds are optimal to achieve best results. Clear edges of the laminate and decent opacity/transparency of the glass should be expected after the process.
Luckily in the autoclave process we have the ability to facilitate more favorable conditions to the laminates that do not have a decent edge seal. However in most cases the poor edge seal could be corrected, corners or parts in the laminate indicating poor edge sealing clamps/clips can be applied to avoid unnecessary repair cycles.
I don’t want to subject this article to temperature targets, because ranges can be wide and thus misleading. Every hot-roller de-airing oven is a individual process which is subject to number of lamination process variables that can vary significantly between plants and even production lines.
All though during the autoclaving the residual air is dissolved in the PVB, but once the laminate is exposed to sun light, changing temperatures and other climate conditions over the limit residual air in the solution will react and can begin to form bubbles and delamination. Quantity of residual/trapped air is best evaluated reading the glass, hence commonly applies that the more transparent the laminate is the less air it has in it.
Article by Mika Eronen