What type of glass to select for your touchscreen?

Article courtesy of Zytronic

To create the best touchscreen solution for challenging environments, it will come as no surprise that different types of glass are used. Standard soda-lime glass contains a small amount of iron, which gives it a greenish hue – this is noticeable when looking at the edge of a piece of glass or through a very thick sheet. Low iron glass (also sometimes called ‘water white’ glass) is made using carefully selected raw materials to minimise the amount of iron and reduce this tinting effect, making it ideal for applications where the true replication of the displayed image colours beneath the touchscreen is critical.

At Zytronic, full sheets of glass, in the required thickness, are cut to any shape or size, up to around 90 inches (2.28 m) on the diagonal. As the touch sensors are all produced in-house, using a uniquely flexible manufacturing process, custom designs are possible with no to low tooling fees, and in any quantity from a single prototype to many 1000’s of units.

Once the glass is cut to shape CNC machines can cut holes and slots into the screens so that peripheral devices such as credit card readers, ticket printers and NFC detectors can be mounted behind the finished touch sensor when installed in a kiosk. Other surface features, such as dimples and grooves or raised markings may now also be added to the glass, to further customise the screen and help guide the user’s fingertips when not looking at the touch sensor. The on-site CNC machines are also used to grind, profile or polish the glass edges for aesthetics or safety. For more sophisticated designs, Zytronic has dedicated ovens which bend the glass to specific radii, matching the latest curved displays to create an immersive, ‘wraparound’ experience for the touchscreen user.

What type of glass to select for your touchscreen

Once the glass has been machined, features such as graphics and logos may be added. Ceramic inks are highly scratch resistant and recommended for rugged applications, while polymer-based inks offer a broader range of colours. In applications where the most accurate colour replication is critical, it is usually recommended that the touch sensors are made from low iron glass.

Many types of special glass coatings can be used to enhance the touch sensor’s functionality in certain environments. For example, for ‘smart’ interactive mirror applications, glass coated in a highly reflective, but semi-transparent metal oxide layer is used to produce the touchscreen. This enables the interactive display to act as a mirror when not powered, but to be visible when switched on. Different levels of mirrored coatings are available depending on the amount of reflectivity required together with the viewability of the underlying display.

In strongly lit indoor and outdoor applications, reflections from the touchscreen surface can impair the readability of the underlying image, unless a high brightness display is used. Anti-reflective coated glass helps to reduce light reflected from the surface by around 75%, making it suitable for brightly lit locations while maximising image clarity and crispness. In direct sunlight, glare can impede users from interacting with a touchscreen. By manufacturing the touch sensor from chemically etched anti-glare glass, the sub-microscopically roughened surface scatters the reflected light and provides a durable, cost-effective method of enabling a user to view information on the display in outdoor applications. The slightly matt surface of anti-glare glass has the added benefit of improving how fingers ‘glide’ over the touchscreen – sometimes referred to low ‘stiction’, enhancing user experience on applications where lots of dynamic, moving touchscreen interaction is required.

In addition to its transparency, glass is a far stronger material than you may imagine. Zytronic takes full advantage of this property and can manufacture and laminate touch sensors in thicknesses up to around 30mm. As a result, touchscreens can be produced which can pass practically any impact test, including industry standards such as UL60950, IK10, UL22 and others. Zytronic regularly designs touch sensors for extreme applications such as the control panels used in mines, oil rigs and petrochemical facilities, which must conform to ATEX, IECEx and other standards governing the use of electrical equipment in explosive atmospheres.

More commonly, glass with a thickness of 3mm or more is strengthened by thermal toughening. This involves heating the glass to just above its strain point (between 500 – 600 degrees C in soda-lime glass) and then rapidly cooling it back to ambient within a few seconds using regulated compressed air. This in-house process creates alternate opposing forces in the glass, the surface in compression and the interior, under tension.  The resulting toughened glass, often referred to as being vandal resistant, is around five times stronger than annealed glass and has the added benefit that, if broken the tension within the glass causes it to break into relatively safe, small cube-like pieces. Zytronic thermally toughens and tests all its glass in accordance with BSI/ISO 12150.

For glass less than 3mm thick, or for applications requiring even greater levels of impact resistance, Zytronic can produce touch sensors using chemically strengthened glass. Here, the cut glass panels are placed in a bath containing molten salt (NaCl) for several hours. During this time, smaller potassium ions in the glass surface are replaced by larger sodium ions from the liquid salt. The longer the glass is left in the bath, the deeper into the glass the ion exchange occurs (also called “depth of layer” or DoL). As in thermal toughening, this process creates compressive forces in the surface of the glass, which make it up to around eight times stronger than annealed glass, however if shattered, chemically strengthened glass tends to break into large pieces like untreated glass, so system designers need to keep this failure mode in mind when designing touch systems for public use.

If you want to learn more about Zytronic solutions, please contact us at sales@eurocoincomponents.com.