Display bonding

An exciting challenge for dosing technology!

(PresseBox) ( Töging a. Inn, )
You're no doubt familiar with this problem: You're standing in bright sunlight and are trying to decipher something on your mobile or smartphone. This same thing happens with monitors, tablets, flat-screen TVs and all other devices with this sort of display. The reason for this is light refraction in the gap between the actual display and the sheet of glass covering it.

To minimise such unwanted reflections with the latest generation of devices, a production process has been developed that fills the gap between the display and glass with a suitable fluid material and makes it as small as possible. As mobile phones, like all other devices, are getting thinner and thinner and competitors market differences in the 1/10mm range, these layers are also in the order of 1/10-2/10mm.

What challenges do such demands make on dosing technology manufacturers? To start with, the bonding materials used here have an extremely low viscosity level (max. 1,000mPa-s) and would, depending on the manufacturer's technology, simply run off the glass as soon as they are applied. It is also essential for these bonding fluids to be absolutely free of trapped air during application. Otherwise it would be result in air bubbles on the display.

To precisely define the gap between the display and glass/touch surface, beads e.g. of silicone are applied around the edges to act as a dam. Here it is important to ensure that there is not too little or too much product at the transition between the beginning and end of the bead. Endless piston dosing technology from ViscoTec is ideally suited to this production step. The 3RD4 dispenser can be used to apply extremely fine beads ranging between approx. 0.08-0.25mm in the required area. This high-precision, pulsation-free micro-dosing system is combined with a stepless speed control function. Thanks to the linear dependence between the dispenser speed and the dosing speed, storing dosing parameters for clean transitions/bead finishes in the control system becomes cinch. This is achieved by starting application of the bead at an increasing dosing speed and then balancing out the wedge shape produced at the start of the process by reducing the speed at the end of dosing.

Once the dam has cured, the filler has to be added to the reservoir created. 2-component silicones or 1-component materials such as UV acrylates are used here depending on the production process. They however have all one thing in common: it is essential to avoid any air bubbles. Here ViscoTec makes use of different degassing systems which are adapted to the material and required dosing rate. One example is the ViscoTreat-Inline product preparation system. It is equipped with a separate control system for the preparation and product feed process. It offers automatic regulation for filling the degassing tank, the primary pressure to the downstream dispenser and additionally, the negative pressure required and permitted for each product in various stages, from degassing during filling through the negative pressure applied for degassing to the negative pressure or atmospheric pressure with lengthy idle periods.

Something of particular interest to production plants: This system can be incorporated inline in the production line. This means that production does not need to stop when refilling the degassing tank as the ViscoTec pump used to draw off the material from the container can also perform extraction with a counteracting vacuum. If a 2-component bonding material is used, both components must be degassed separately prior to mixing.

Dispensers from the RD series equipped with ViscoTec's own ViscoPro-C dosing drive are suitable for applying 1-component bonding materials. The advantage here is that no external controllers are required here as all parameters such as the freely selectable suckback can be directly programmed in the motor's built-in electronics. The required quantity, after all around 10ml for a 3.5" display with a 0.18mm gap, is applied in a matter of seconds, generally using portal systems.

And when it comes to dosing 2-component materials, ViscoTec can also offer reliable, high-precision products with its ViscoDuo-V systems. Special dispenser technology is used here for precise adjustment of the mixing ratio with the help of the viscosity-independent, valveless design. The associated 2-component dosing control system also offers all necessary features for process monitoring, e.g. monitoring of dosing pressure and pot-life time.

The highly innovative display market will no doubt continue to present process technology and in particular dosing technology with new challenges. Thanks to its team of experienced process technicians ViscoTec can eagerly look forward to facing up to such demands. An exciting time is in store!
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