Six mistakes to avoid when tempering glass

Materials World magazine
1 Dec 2017

Sami Kelin*, Product Manager at Glaston, Finland, explains some common mistakes made in the tempering of glass. 

Glass tempering is a bit like cooking – you can have the latest and greatest equipment in your kitchen, but the food can still turn out bad. To be a great cook takes much more than having the best equipment – it’s about understanding the processes. You have to know the ingredients and techniques as well as the equipment. 

Automation makes things a bit easier. This is what we try to do with our tempering furnaces – to automate some of the processes to achieve more consistent quality and to minimise operational mistakes. Nowadays, tempering furnaces come equipped with sensors, allowing the operator to pull data and measurements. 

In the long run, automation will make operating a furnace easier. But, glass processors need to do many things correctly to reach high-quality tempered glass. 

I have toured the world, seen hundreds of glass processing factories and helped processors take the next step in their production. 

Below, I recall common mistakes I have witnessed in the tempering process. 

Not learning the basics 

I’ve seen processors who have skipped the basics and ended up with quality problems before they even start to get the proper results. Not an ideal scenario. 

To me, the tempering line is not a magic black box. The behaviour of glass is relatively simple to understand – if you don’t heat or cool the glass uniformly, it will bend. If you heat it too much, it will have waves. 

These are the basics that should be clear to anyone operating a processing machine. Manufacturers often offer their clients the basic documentation and training packages that clearly state the fundamental facts. But, make sure you select a partner who can also train and instruct you and your employees. 

Take time to learn the process basics. This might take a while, but you’ll benefit from it later. For example, when you introduce new coatings, it is easy to look into the heating section and see how the glass behaves in the process. If you know the basics of heat transfer, you’ll find it easier to adjust the process yourself. 

Neglecting quality

The tempering line is usually among the biggest investments in the glass-processing factory, thus the primary attention is directed here. However, if we look at the end product’s quality, as well as the production yield, we can conclude that the pre-processing – including the cutting, grinding, washing and more – is just as important as the tempering. 

The pre-processing quality has a direct connection to the tempering line’s yield when it comes to breakage and production quality. Sometimes, I see processing plants that have acquired a top-rank tempering line but have left the pre-processing systems untouched and out of date. 

Poor cutting or edge work results in more breakage in the tempering line. Conversely, if the edge work is good, the glass can run at a lower temperature in the heating section, which leads to better optical quality and less breakage. For example, diamond-polished thick glass can be processed at a temperature about 20 degrees lower than normal seamed glass. This has a major impact on quality and, of course, glass breakage. 

Maximising loading efficiency 

Very rarely does the tempering line become the bottleneck of the factory. Still, many obsess over the highest possible loading efficiency by putting both small and large glass sheets into the same load – long side leading and short side leading in the same pattern but with a different orientation. 

Mixing up the loading patterns with different glass orientation – short versus long edge leading – will result in different optical quality. If these glasses are installed in the same building, you’ll see a different optical reflection and, most likely as a result, end up with reclamation. 

Maximising loading efficiency is a good thing, and it will increase your energy efficiency as well. The point is not to compromise the quality. 

Having a heating time that is too long 

The percentage of glass breakage has been a tempering line operator’s most important key performance indicator for years. For this reason, operators naturally do everything they can to decrease the waste. 

One way to prevent breakage is to increase the heating time. This will, unfortunately, have a negative impact on the glass quality. The higher the glass temperature is, the worse the quality. 

Note that quite often operators think that poor cutting or grinding quality come from having a temperature that is too low in the tempering process. In this case, the operator is correcting the wrong problem and, as a result, has poor quality glass with poor edge work. 

Running thick and thin glass in a similar way 

Avoid putting a thick piece of glass into a very hot furnace – you don’t want to hear the boom. That is the noise of a thick glass breaking in the furnace. In this case, the breakage is caused by heat shock. As a result, the furnace needs to be cooled, and the broken glass removed. This process takes about two shifts to complete. 

Also, when running a thick glass of around 10–19mm, the quenching pressure the operators apply is often too low. A common rule is that low pressure prevents the glass from breaking in the chiller. Most often, the case is almost the opposite – do not use pressure that is too low in the production of thick glass. 

Removing the ghost the wrong way

Many operators try to fix the white haze issue by operating the line to run the glass sad – a common way to describe the direction of the bow of the glass (corners bow up/happy or down/sad) in the furnace. In other words, the corners of the glass touch the rollers but the centre does not. 

This does eliminate the white haze, also known as ghosting. But, running thick glass sad will harm the rollers. The ghosting can chip off some glass from the sides or cause markings to the rollers. As a result, the following loads will have problems and the thin glasses will not stay stable. 

Here’s my rule of thumb – don’t exaggerate the sadness of the glass. 

It seems that as we become more comfortable with our work, the more mistakes we make, and the more we think we know, the more likely it is that our choices lead us down the wrong path. That’s why even experts should always keep the basics in mind. 

*Sami Kelin is a Product Manager at Glaston Machines. He joined the company in 2000 and has worked in different positions and travelled the world, gathering extensive hands-on experience of both automotive and architectural glass processing.

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