LEDs | electrical and electronic


proportion of the potential shrinkage takes place before the fi nal moulding stage, so sink marks can normally be totally eliminated by over-moulding with the post- moulding. In addition, lower injection pressures result in lower internal stresses.


Engel, Arburg and Sumitomo (SHI) Demag are among a number of injection machinery makers that have also carried out work in multi-layer injection moulding of thick section lenses.


Silicone solutions The LED lighting market is also providing new


opportunities for silicones, already well known for their high temperature resistance across a range of markets. Dow Corning has recently introduced what it says is an innovative new thermal management silicone technol- ogy for LED lamps and luminaires. It says silicones can increase effi ciency, expand LED design options and provide greatly improved processability.


“These products are fast becoming the materials of


choice in critical lamp and luminaire applications,” says Hugo da Silva, global industry director, Next-Generation Lighting. “Our new innovations will raise the bar with cost-effective options for combating higher tempera- tures.” Four new silicone-based materials are designed to


improve performance and reliability of LED lighting through enhanced thermal management. Dow Corning says they will enable the integration of multiple functions into fewer parts, or to incorporate smaller or more complex features. “High-heat applications are where mouldable


silicones shine,” says da Silva. “They easily withstand temperatures of 150°C and higher without signifi cant loss of optical or mechanical performance. These qualities will become more attractive as LED sources increasingly deliver more intense white light from comparatively smaller package sizes, and as customers seek smaller lamps and luminaires with higher


BMS has developed multi-layer moulding technologies for production of thick collimator lenses at high speed and accuracy


Bayer has developed special optical grades of PC that can transmit more light at greater thickness


luminous fl ux, which will also drive up temperatures at the device level.”


As lumen densities increase, the package tempera-


tures within today’s high-brightness LEDs are already reaching 150°C. This poses challenges for epoxy encapsulants conventionally used to seal LED pack- ages, as well as for traditional secondary optics materials. Mouldable silicones retain their excellent optical stability and transparency even after prolonged exposure to temperatures upwards of 150°C, exhibiting comparatively little or no yellowing. Da Silva says that, like conventional silicones,


optical-grade silicones are well-suited to precision moulding applications. Before cure, the viscosity of silicones decreases as heat increases. This allows silicone resins to be injected into a mould at lower pressures than what is typical for other materials. For example, their low viscosity enables replication of micrometer-sized features on a lens surface that, in turn, offers advantages in enhancing, focusing or direct- ing light output. In addition, the comparatively low shrinkage of mouldable silicones helps minimize or prevent warping in components that integrate straight sections, such as the back of semispherical optics.


Click on the links for more information:  www.polyone.com  www.sabic.com  www.dsmep.com  www.ticona.com  www.rtpcompany.com  www.coolpolymers.com  www.materialscience.bayer.com  www.engelglobal.com  www.arburg.com  www.sumitomo-shi-demag.eu  www.dowcorning.com


www.injectionworld.com April 2013 | INJECTION WORLD 29


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