Miika Äppelqvist
This week, we’re dealing with questions about the dots on a glass surface that appeared after tempering and the equipment that can measure the glass flatness:
- We have a furnace made in China and have a lot of dots on the glass surface after tempering. The dots appear only on one side of the glass, the side that has contact with the rollers. Can you give any advice?
- What type of equipment is used to measure the flatness of the glass in addition to a standard straight-edge ruler?
For this week’s questions, see our full video response below!
As always, remember to learn, share and succeed!
As we can see in the picture, those defects are all around the glass. That tells us that there is something happening or some mistakes or errors happening all around the furnace. There are many sources for this kind of challenge.
From the furnace side, it can be that:
When it comes to the loading side, the following aspects need to be considered:
On the chiller side:
There are a lot of online measurement tools available on the market for the local bow. For example, iLooK online quality measurement system measures all the values online when the glass is processed and transferred to the unloading section of the tempering line. Unfortunately, for the overall bow of the glass, there is no reliable automatic online measurement device. A longer straight-edge liner can be used to check the overall bow of the glass.
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Glastory 
Good morning to you. I would like to raise this question to you about scratches mark and white haze appear on our low e NE 370 after tempering. The same picture as posted above but the spot or scratches mark and white haze is on the coated side. Can you please advise me because this is the problem when we tempered 6mm low e NE 370.
Very interesting since if the glass is tempered coated side up there should be no mechanical reason to cause scratch or haze on the coated side.
I don’t have a definitive answer but I will try to raise couple of questions that you can consider when trying to fnid the root cause:
– Haze could appear if the coating is too old or there is too long time between preprocessing and tempering. Then the coating may have been deteriorated, although invisibly.
– Is this experience based on only one order batch of this coating type, i.e. can the batch be defected somehow?
– Is it absolutely certain that glass is not tempered coating side against the rollers and that the marks are on the coating side?
– Could the defects been there before tempering? They may not have been visible but tempering process will bring them more clearly to sight.
This is tricky one! I hope the above consideration can help you forward?
The coating could be cracking due to too high of temperatures or overcooking. Hard to say without seeing the imperfections or knowing your recipes though.