Optical moulding | processing
Top tips for optical moulding
Transparent plastics are used today in a wide variety of applications requiring high optical quality, such as fl at, thin wall components for lamp covers, electronic device displays and headlamp lenses. Increasingly, however, plastics are now replacing glass in far more technically demanding industrial optical applications such as thick-walled optical fi bres, prisms or lenses. Industrial optical designers in fi elds such as sensor engineering, luminaires and automotive lighting are attracted to plastics for their light weight and the greater range of forming options, which make economi- cal production in high volumes a reality. However, achieving the level of optical precision required for these demanding applications is a challenging task. Optical function can be compromised by inaccuracy in the contour (the shape accuracy, waviness or roughness of the moulding), while internal stresses can affect transmission, extinction and dispersion of light. Injection moulding polymer optics requires a high
degree of precision in terms of mechanical engineering, tooling and process engineering, in combination with a high level of reproducibility and cleanliness of the production environment. The injection moulder must possess the highest levels of processing know-how in order to determine the appropriate process-relevant parameters and tolerance limits, and to establish permanent quality control procedures. For smaller to medium quantities, optical lenses are
usually manufactured using conventional injection moulding technology with a maximum of eight cavities connected via a cold runner system. The cross sections
www.injectionworld.com Production of optical parts is a
challenging task. Thomas Brettnich explains the common pitfalls and how to avoid them
of the manifold and gate are matched to the thickness of the lens so as to maintain an effective holding pressure for as long as possible. This results in cross section dimensions of up to 25mm. Holding pressure times of up to eight minutes and cycle times of up to 20 minutes may be required with thick-walled optical parts to achieve a high level of precision of the optical surfaces. From a mould fi lling perspective, to prevent jetting
effects, fl ow marks and silver streaks in the often voluminous cavities, it must be possible to implement injection profi les with injection speeds of a minimum of 1 mm/s and with very smooth transitions between the individual speed levels. Achieving the very high levels of quality required for many of the new polymer optic applications requires the use of more complex and non-standard moulding techniques. Machine controls should, therefore, provide the option to integrate processes such as compression moulding or multi-layer processing. It must also be possible to trigger external sequences, such as variotherm (heat-cool) mould temperature control, via
September 2012 | INJECTION WORLD 37
New genera- tion lighting
technologies such as LEDs are driving demand for
optical quality components
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