An easy rule of thumb is that the hotter the glass, the worse the quality. One reason for overheating is the attempt to avoid the risk of breakages.
The colder the glass exits the furnace, the better the quality. However, the risk of breakage increases. In some cases, overheating may be needed to compensate for bad edge work or hole finishing.
There are differences in roller settings in tempering furnaces. When the distance between consecutive rollers is longer, the glass travels a longer distance unsupported. Longer roller pitch results in a larger peak-to-valley roller wave value, thus creating a visually more noticeable roller wave effect. Roller shape accuracy along the whole roller length also plays an important role in roller wave quality.
Faster oscillation movement in the end of the heating cycle reduces the time that the glass is without roller support, and thus improves the roller wave quality. A low oscillation speed has the opposite result. Improving roller wave There are specific reasons for roller wave to appear. Here are four steps to raise the quality of glass. The single most important factor in reducing the amount of roller wave is avoiding overheating. This is achieved by better heating control in the heating section.
Make sure you have the necessary measurement system to control the glass temperature after heating. The best is an infrared scanner. Additionally, a furnace with better heating control gives you a wider temperature window for working without breakages. If you have a problem with roller wave, the first thing to do is to decrease the heating time. Another rule of thumb to practically any quality problem is to:. Today there are automatic systems that measure the glass quality and give automatic feedback to the system to change parameters, for example, the heating time.
This is a tricky one since this is something that is not easily controllable. This tip is only valid if you are choosing your machinery. When choosing a new tempering line, pay attention to the roller pitch. Afterwards, it is extremely challenging to change it. What you can do with existing machinery is to check and measure the total indicated runout TIR of your rollers and make sure you use high-quality rollers in the process.
They have a direct impact on your quality. Different manufacturers have different specifications for rollers, even if they come from the same supplier. Make sure your roll supplier provides you with high-quality rollers. Get Started Here:. Get Free Estimates! Glass Dictionary. An A-Z glossary of all things glass-related. Lookup Glass-Related Terms. Glass Detective. Need customized help on glass?
Ask Glass Detective. March 30, With both heat-strengthening and tempering, distortion is a possibility. Optical image distortion occurs in glass for many reasons, including glazing pressure, wind load, temperature and barometric pressure changes, or even changes in altitude between where a glass is made and where it is installed. Because of its fluidity at higher temperatures, glass also is inherently susceptible to roller wave, bow and warp while it is being heat-treated.
In addition, the following tips are suggested in order to minimize the potential impact of glass distortion that is inherent with heat-treating processes:. While there is no industry standard to quantify permissible heat-treated glass roller wave, a tolerance of 0. Vitro Architectural Glass formerly PPG glass recommends using a millidiopter specification instead, if it is available.
Safety is a prime consideration when determining the type of heat-treating process necessary for a project. Deciding whether to use heat-strengthened or tempered glass depends on the specific application. For example, heat-strengthened glass can be selected for applications that do not specifically require a safety glass product; and tempered glass should be used wherever safety glass is a requirement.
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