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Roller Wave Distortion

• What is roller wave distortion?
• Minimising the effects of roller wave distortion

What is roller wave distortion?

• When glass is ToughenedGlass, it may be passed through the toughening oven either horizontally or vertically. For horizontal toughening the glass is supported on horizontal rollers. For vertical toughening the glass is suspended by tongs. Roller wave distortion is only an issue with horizontal toughening.
• BS EN 12510-1, para 9.1.2, and BS EN 14179-1, para 11.1.2, have identical wording:
  "While the hot glass is in contact with the rollers during the toughening process, a surface distortion is produced by a reduction in surface flatness, known as ‘roller wave’. Roller wave is generally noticed in reflection."
• Although both these paragraphs are in sections titled "Optical distortion", neither standard includes any quantitative criteria for assessing roller wave distortion. The American standard ASTM C1048 also addresses the issue of roller wave distortion but, like the BS EN standards, provides no guidance on the maximum allowable distortion.
• ASTM C1048, Paragraph 7.4.5, states further that regardless of the flatness achieved, the perceived reflective distortion is primarily due to the symmetry of the object being reflected. For instance, linear objects (telephone poles and building curtain walls) and moving objects (such as cars, etc.) may appear distorted, whereas irregular free-form objects (such as trees, clouds) may have little perceived distortion.
• An example of roller wave distortion is shown below:

Minimising the effects of roller wave distortion

• Roller wave distortion cannot be eliminated entirely from horizontally toughened glass.
• The primary way of minimising the visual effect of the distortion is to ensure that the roller waves are aligned horizontally. (You will note that they have been aligned vertically in the example above.)
• Although there appear to be no definitive national or international standards defining the maximum allowable distortion, NBS include (identical) values in two of their specification clauses - H11/741 and H13/617. These specify:
  - For 3 to 5 mm thick glass: 0.5 mm.
  - For 6 to 10 mm thick glass: 0.3 mm.
  - For 12 mm and thicker glass: 0.15 mm.
• Although not stated specifically, the assumption is that these are the maximum permitted values by which the glass may deviate from a straight line measured between the 'peaks' of the waves. You will note that there are no criteria for the wave 'length' although this will have an effect on the perceived distortion.
• It is believed that these values were provided by representatives of the European glass industry on the basis that they were economically achievable by 'reputable' glass manufacturers.
• You should be aware that, even if these values are included in your glazing specifications, you may encounter resistance from glass suppliers on the basis that there are no national standards covering this issue.
• You should also be aware that, in extreme combinations of high reflectivity (e.g. some coated glasses) and linear objects (e.g. the scaffold poles in the example above), even glass complying with the "NBS" values above may show visible roller wave distortion.

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