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optical moulding | Technology

(heat-cool) mould temperature control system. This used a combination of conventional heating and CO2 cooling to cycle the temperature of the mould surface during the cycle – a high temperature during injection of the resin followed by an extended cooling period. According to Sumitomo Demag, the high mould

temperature during the filling phase reduces stress in the moulding and allows better surface properties to be achieved while the intensive cooling phase means the cycle time can be kept within economically viable limits – in this two-cavity example to 70s. Sumitomo Demag says melt quality is also a key

Above: A Kronacher rotary mould for overmould- ed LED lens production

overmoulding – is not critical to final part quality. In the Krallmann method, a thick first stage moulding is produced and demoulded while the core of the part is still at a temperature above the polymer Tg. This pre-moulding is then further cooled outside of the mould, which can be achieved without extending the processing cycle. Once the pre-moulding core has cooled to below the Tg of the resin it can be reintro- duced to the mould and overmoulded with two very thin surface layers. These thin surface layers allow a high quality final surface to be produced while enabling fast removal of thermal energy. “Using this intermediate cooling technology, we

think we can reduce the cycle time for this 15mm thick lens from 140s using a conventional three-layer process to between 60 and 80s,” says Steinbichler. He says that an automotive headlamp using the technology will go into production before the autumn.

Right: LED lenses

produced at Arburg’s

Technology Days

Overmoulding prospects Carclo’s Grassi says the company utilises overmoulding techniques in its LED optic production units today, but not for shortening cycle times. “Where we use over- moulding is more typically to add mechanical features onto an optical part or to make an array. When we do that we are treating the optic as one own moulding and then the array as another. In that way you have more room to process the optic portion,” he says. Grassi says multi-layer moulding technology may be

applicable to some simpler lens designs, where internal stresses in the part are not such a high concern. However, each project will have to be considered with care to ensure the cycle time savings are not out- weighed by the higher cost of the more complex moulds and machinery. At last year’s Fakuma show Sumitomo SHI Demag

demonstrated production of a single layer thick walled lens in PMMA using GWK’s Teco Vario variotherm

22 INJECTION WORLD | September 2013

factor in optical production (the company’s technology development manager Thomas Brettnich discussed this in an optical moulding article in the September 2012 edition of Injection World At the K show this year the company will launch its SL plasticising system, which features an upstream dosing screw and a plasticising screw with no melt compres- sion zone. The company claims this arrangement, in combination with a switch-controlled non-return valve, results in a highly homogenous melt with minimal variation in melt viscosity making it well suited to production of technically demanding parts and optical quality components. The SL plastification system will be available as an

option for fully-electric machines in the company’s SE-EV line, which extends from 50 to 180 tonnes. Materials manufacturers are also working to develop

grades capable of meeting the demanding requirements of producers of LED optics. “The LED market remains exceptionally dynamic, calling for highly adaptable mate- rial technologies that can enable rapid design changes, support higher-efficacy LED designs and help drive down persistent cost barriers to adoption,” says David Wildgoose, general manager engineering resins within Sabic’s US-based Innovative Plastics division. Sabic IP has developed its Lux line of polycarbonates

specifically for optical applications such as lenses and light guides, offering high light transmission (a claimed 91% at 1mm thickness for the Lux-C grade), good flow and long term colour stability. The most recent

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