Optical moulding | processing
Clear benefi ts of multi-layer
The emerging LED luminaire market is expected to see demand for precision optical plastic parts grow strongly over the next fi ve years. With polycarbonate expected to capture a large share of these new applications, Bayer MaterialScience (BMS) has been exploring new multi-layer moulding techniques that promise shorter cycle times and higher optical quality. The higher refractive index of PC compared to PMMA – 1.59 against 1.49 – gives it an inherent advantage in production of thick section lenses, according to Bayer MaterialScience advanced processing specialist Christoph Klinkenberg, as lenses can be up to 20% thinner. This, together with the higher heat dissipation rate of PC, means that simply switching from PMMA to PC can deliver a moulding cycle time reduction of between 30 % and 50% in a standard optical lens production. However, achieving further cycle time reductions
without compromising optical performance requires a different approach to lens moulding. Klinkenberg says BMS engineers have carried out detailed work compar- ing the cycle time savings achievable using multi-layer moulding methods, including double and triple-layer systems, and have achieved typical cycle time savings of up to 45% compared to single-layer parts produced
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BMS has developed new multi-layer moulding modelling tools enabling it to slash cycle times for production of thick section polycarbonate optical parts. Chris Smith reports
using industry-standard injection compression moulding techniques. All moulding trials were carried out using the company’s Makrolon LED grades of PC, which have been developed for high quality LED lens production. Klinkenberg says optical performance is infl uenced by a number of factors, including surface properties such as shape, waviness and roughness as well as internal factors such as transmission, light extinction, dispersion, frozen-in stress and variation in refractive index. It can also be infl uenced by material contamina- tion and degradation. A successful moulding process has to accommodate all of these potential risk factors. “With the help of simulation methods we developed
Finished multi-layer
polycarbonate lens moulding (top) beside a fi rst shot
premoulding
October 2012 | INJECTION WORLD 25
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