Mastering quality standards of glass production

1. Introduction

The quality requirements for glass used in modern applications have reached a level that requires close quality control and continuous analysis of the production process. A perfect example of current architecture with the highest quality glass is the cladding on the newest addition to the London skyline and Europe’s tallest building: the skyscraper „The Shard“. The clear blue glass used in this building meets the highest cosmetic standards to fulfill the architect’s vision of making the building appear like a glistening shard of glass on the horizon with the upper floors almost becoming invisible in certain weather conditions, while also controlling solar gain in the building to make it energy efficient.

2. New solutions to well-known problems

It is easy to imagine how cosmetic imperfections in the glass panes or the coating would have adversely affected the intended effect of “The Shard”. Although variations in the coating thickness would not necessarily reduce the solar gain control functionality of the glass pane, they would result in a visible change in contrast and color appearance of the glass. This color-shift effect would be apparent with every change in the angle of view.

The challenge for manufacturers of architectural glass or glass coating line owners therefore is to find process control solutions that help them make a perfect product that meets these quality demands. This requires modern, automated solutions that are able to catch even the smallest defects and monitor overall variations in glass properties, such as the aforementioned variations in the homogeneity of the coating. In order to ensure close quality control, automated optical inspection systems can be integrated in glass production lines to inspect the glass after crucial production steps. Ideally they are placed after every critical step to ensure that process deviations are caught as early as possible.

This serves three purposes:

Firstly, it helps to ensure the final product is of the quality level specified by the customer and guaranteed by the manufacturer which increases customer satisfaction and reduces the risk of product recalls.Secondly, it helps to save money in costly production steps such as coating, in two ways: Not only does it ensure that no more than the necessary amount of coating material is used which helps to reduce to expense on coating material – it helps to exclude glass that is not of the required quality from costly subsequent production steps.And thirdly real-time process feedback helps to optimize glass quality control without delays between detection of defects or coating variations and adaptation of process parameters.

Two innovative solutions to optimize the detectability of smallest defects and monitor variations in the glass coating are Sirius Light Technology (SLT) and Multiple Image Defect Analysis (MIDA) from Dr. Schenk: SLT uses ultra-bright LED illumination units to direct more light at the glass in different angles. This illuminates defects from all perspectives and by multiplexing between different wavelengths more defects are revealed more reliably and in greater detail than with conventional illumination techniques. SLT can be customized to use exactly the right amount and kind of light to detect the defects that matter to the manufacturer and the customer.

For optimum results SLT is combined with MIDA, which gathers multiple images of one defect in a single scan pass and combines the data for complete information regarding e.g. core and distortion size. MIDA helps to distinguish critical from non-critical defects and catch glass defects such as bubbles, inclusions, tin defects, stones, knot lines or reams as well as coating defects.

Modern automatic inspection systems must do more however, than detect local defects. For close process control they must be combined with monitoring of layer properties, such as coating homogeneity, covering the complete width of the glass. Dr. Schenk’s GlassMeasure (an option that is integrated in the GlassInspect solution) can use existing traversing measurement heads and combine them with a line camera to measure e.g. thickness variations.

3. Being prepared for changing applications

When a manufacturer decides to invest in a complete quality control system it should be more than a solution for the present: It should also be fit for the future, i.e. grow with and adapt to changing needs the manufacturer is faced with. At the heart of Dr. Schenk’s GlassInspect is an open concept with a defect classification tool that can adapt to multifaceted requirements. In float glass applications for example, the needs of architecture and automotive customers will differ regarding defect sizes, critical / non-critical classification and optical parameters, such as refraction index and distortion. GlassInspect easily and quickly adapts to those differences. At the same time additional parameters, including for example thickness measurement results from complementary devices, can be used for complete process analysis. All the data that GlassInspect has gathered can be exported to MS Excel for instance for further offline evaluation. In parallel the data is stored in a data SQL data base for comprehensive analysis with standard SQL-commands.

4. Summary

The continually rising demands for the quality of glass require new, better solutions that are up for the task of detecting not only smallest local defects but overall variations in e.g. low-e coating on the glass. Dr. Schenk’s EasyInspect does both using the innovative concepts SLT and MIDA and GlassMeasure for layer properties monitoring. The open system enables manufacturers to adapt to changing quality requirements and thus secures their investment into the future.

Dr. Schenk GmbH offers inspection and measurement solutions for automated quality assurance and production process control - a key success factor in the making/converting of plastics, glass, metal, PV modules, textiles & nonwovens, and the semiconductor industry.

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