By Valentin Bonnifet,

Field Application Engineer at Nextflow Software

 

Flat screens are everywhere in our daily life, from our smartphones to our television, but also spread in a wide range of industries. Everywhere one needs to handle data from a computer, a flat screen is used. Since the decline of cathodic displays and, more recently, plasma displays, two technologies have been sharing the market of visual human machine interface (HMI): Liquid Crystal Display (LCD), which uses liquid crystal polarization to  alter light transmission , and Organic Light-emitting diode (OLED) , composed by a matrix of organic P-N junctions. Both display technologies are delicate and could suffer irreversible damages even under basic mechanical stress. For this reason, a chemically strengthened glass is usually glued in front of the display, to provide resistance to bumps and scratches. This process – referred as optical bonding is extremely competitive – especially within the touch panel devices industry – as it directly affects the usability and the visual ease. 

The visual quality does not only depend on glass grade but also on the bonding itself, and more precisely, on the adhesive material and its repartition. Today, silicone glues and epoxy resins are the most widely used adhesive materials. Epoxy adhesive has the specificity of resulting from the mixture of a hardener agent and the above-mentioned epoxy resin. Once both components are blended, time is pressing before the glue hardening. Thus, when the bonding is not perfect, the entire display must be withdrawn from the assembly line. And here comes the fluid problem: how to homogeneously spread the resin over the glass surface in order to guarantee a crystal-clear device and reduce the material loss? Indeed, one could overload the glass with epoxy resin to ensure a homogenous adhesive distribution. But resin is expensive and getting rid of the overflow – without smear display corner – is tricky during an industrial process. 

Computational Fluid Dynamics (CFD), by enabling the numerical testing of a wide range of possibilities, can help display manufacturers to master the resin distribution.

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The epoxy bonding process is split in three major steps: the glue deposit, the glass closure and the resin polymerization. Since the gluing property is not activated until the last phases, resin flow is only driven by viscosity and surface tension. The glue viscous behavior strongly affects the deposit process. Its viscosity, which mostly comes from the resin rather that from the hardener agent, strongly depends on the temperature and on the glue composition. Surface tension will be more prominent in case of low viscosity glue.  

The Lagrangian and multiphysics features of Smooth Particle Hydrodynamics (SPH) makes this methodology particularly suitable for such flows. Resin deposit and glass closure computations, performed with the SPH-flow solver, are presented. Here, the considered fluid is an Epoxy with a relatively low viscosity of approximatively 1 Pa.s. This viscosity is about one order of magnitude higher than engine oil and one order of magnitude lower than honey.

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