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Selecting the Right Glass for Exposure Units

(July 2007) posted on Fri Jul 06, 2007

Replacing glass is a part of life for those who use exposure units. Coudray examines the ways in which various types of glass affect exposure efficiency.

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By Mark A. Coudray

Modern sheet glass is made using the float process, invented by Sir Alastair Pilkington in the late 1950s. His inven¬tion was to float a ribbon of molten glass on a bed of molten tin. Contaminants settle out as the molten glass flows, and the liquid-metal bed delivers an absolutely dead-flat sheet, free of any seed defects. The under surface, which touches the metal, is called the tin side. It can have a slight metallic sheen that’s often polished off. The upper surface of the glass is called the air side. Flame polishing ensures a perfectly uniform upper side. The sheet is finished with a pass through an annealing oven, called a lehr, where the glass’s internal stresses are reduced during controlled cooling. Pinkington licensed the process imme¬diately, and within a decade, it became the primary method by which window glass was produced.

Float glass is relatively strong with a tensile strength of 3500-4000 psi. The problem with this type of glass is that when it’s broken, it shatters into very dangerous shards that can cause serious injury or death. That’s not a risk I’m willing to take. A process call-ed tempering, also known as hardening or strengthening, is often carried out to increase the glass’s strength and change its internal stresses. Tempered glass is still breakable, but the tensile strength is increased to 10,000-24,000 psi. Edge compression increases from 4000 to 9500 psi. Furthermore, when the glass is broken, it breaks into small cubes called dice (Figure 1). Tempered glass is much safer than plate glass but about 25% more expensive.

Typical applications for tempered glass are windshields, shower doors, partitions, and more. Float glass is usu¬ally tempered or hardened with heat, but it can also be tempered chemically. The treatment does not alter any of the glass’s physical or chemical properties, such as color, clarity, chemical composi¬tion, and light transmission. The harden¬ing process affects the top and bottom layer of the glass and leaves the interior core unhardened. The glass will shatter when this surface layer is penetrated, but small scratches or nicks will not generally penetrate the surface layer.

Do not confuse tempered and safety glass. Safety glass consists of two or more layers of tempered glass lami¬nated with a layer of plastic (typically polyvinyl butyral or PVB) between. The PVB forms a barrier that keeps objects from penetrating the glass.


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