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Precision Injection Molding How to Make Polymer Optics for High Volume and High Precision Applications

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Precision Injection Molding How to Make Polymer Optics for High Volume and High Precision Applications PLASTIC OPTICS Precision Injection Molding How to Make Polymer Optics for High Volume and High Precision Applications Whether it is a cellphone camera • Low material cost THE AUTHOR or a head-up display – polymer optics is Optical polymers are within a range of at the heart of many high tech devices 5…30 €/kg. Compared to optical glass and the market demand for such pre- this makes a notable difference. RALF MAYER • Plain and fast mass production cision optical components is growing Dr.-Ing. Ralf Mayer Injection molded lenses are finished in year by year. The main application fields attended University of one step to optical quality without the are in the automotive industry, in the Kaiserslautern and holds need for additional finishing steps, such medical field (disposable optics), in sen- a degree in mechani- as polishing. Compared to glass, the cycle sor and information technology. Beside cal engineering. In 1997 he received a times are very low which makes injection lightweight and economic advantages, doctor´s degree in the fields of thin film molding suitable for mass production. polymer optics enables completely new coating and sputtering. After that he • High degrees of freedom in the design worked on the development of high solutions in optics. The key to all this is With injection molding almost every speed scales at Wipotec and than joined precision manufacturing. surface shape (e. g. diffractive, freeform, SiemensVDO where he was responsib- nano structure) becomes feasible without le for the optical design of automotive extra costs. Hence this process is well sui- Basic assessment of Precision head-up-displays. Today he in charge ted for the mass production of deman- Injection Molding of Viaoptic´s development department. ding optical elements. Because of this His main tasks are developing and de- Injection molding is well known from the advantage glass optics and polymer op- signing optical systems from scratch for field of plastics production. The conventi- tics are sometimes combined (e. g. as an e. g. industrial sensors, LED lighting or onal injection molding technology is not aspheric field lens) to improve imaging automotive driver assistance systems like accurate enough for optical parts produc- quality at reasonable costs. head-up-displays. tion. To achieve the necessary precision, it • Excellent automation possibilities is necessary to optimize the whole process Modern injection molding machines are chain. fully automated and computer control- After several years of continuous deve- led in every parameter. Together with an ●● lopment, precision injection molding (PIM) autonomous handling system and ad- Dr.-Ing. Ralf Mayer became a technology that helps to satisfy vanced process control, it is easy to set Viaoptic GmbH growing market demands in reasonable pri- up flexible manufacturing cells. These Ludwig Erk Str. 7 ced but highly functional precision optics. cells are capable of running whole pro- 35578 Wetzlar, Germany Table 1 shows the differences in the process cess chains like molding, testing, coating Tel.: +49 (0) 6441/9011-35 E-mail: [email protected] and the materials between conventional and packaging. Website: www.viaoptic.de molding and precision injection molding. • Integration of mechanical functionality Injection molding enables the designer to incorporate mechanical mounts, like lens Typical Features of Precision mounts, snappers and other fixture ele- Typical Part Specifications Injection Molding ments together with optical functionality Today, polymer optical components offer into one part, which reduces the number Due to the manufacturing technology, a number of unique features. Traditionally, of elements or may increase alignment polymer optical components have certain they are particularly well suited for large accuracy of optical components. limitations in their dimensions. Precision production lots and low costs. But mean- injection molding allows for a lens diame- while they also offer some features, where ter from 1 mm to 100 mm, lens thickness Applications of PIM they are clearly superior to glass optics: in- may be chosen between 1 and 30 mm. tegrated optical or mechanical functionality Precision injection molding is developed The diameter to thickness ratio should be is one example. The most important fea- to enable an economic mass production of in the range of 1:1 and 5:1. The optically tures will be discussed in the following. precise spherical, aspheric, diffractive and used area may then be between 1 mm² and • Low weight freeform plastic lenses and mirrors with 50.000 mm². Technical tolerances for poly- Optical polymers have approximate- high accuracy and good to excellent opti- mer optics are summarized in table 2. ly half the density of glass. Hence low cal surface finishing. Figure 1 briefly shows In general one should keep in mind that weight designs are possible. possible application fields. feasible tolerances are directly dependent 46 Optik & Photonik December 2007 No. 4 © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim PLASTIC OPTICS on material properties and parts terial specific feature shrinkage is geometry. The polymer material dependent on process parame- Automotive • Rain Sensor also defines the optical proper- ters and typically exhibits some • Head-Up Displays • Steering Angle Sensors ties. Today, the materials allow batch to batch variations. Hence Laser Technology Life-Science for refraction with n = 1.49…1.7 shrinkage prediction in injection • Laser Lenses/Optics • Diagnostics • Beam Shaping • Scale Magnifiers (1.9). The Abbe numbers can be molding is still a quite challenging • Syringes between 28 and 58. problem and practical experience with material-behavior is irrepla- Logistics Illumination • Scannersystems • Motion Detectors ceable. To improve the shrinka- • Photosensors • Collimation Optics Limitations of • • ge behavior inorganic fillers like Optical Sensors Reflectors Polymer Optics CE-/IT-Applications TiO nano particles are commonly • Handy-Lenses • Scanner Optics Due to material properties, po- used. Generalized: dimensional • Optical Storage lymer optics are more sensitive stability improves with a higher to changes in the environmental glass transition temperature and FIGUre 1: Applications for precision polymer optics. conditions. Typical effects are: larger difference between service shrinkage, warpage (during pro- temperature and glass transition Part cessing), thermal and mechanical temperature. Design stress, water absorption, heat de- Because of the above reasons flection. The parts service condi- VIAOPTIC prefer materials like Design Review tions are usually more important COCs or COPs due to their low not feasible when dealing with polymer optics water take up and their tempe- feasible than with glass optics. Depending rature stability. Polycarbonate is Tool Design on the expected performance, not well suited for high precision not feasible the temperature range can be applications because the relatively feasible between – 40 °C and 150 °C. Addi- high water take up causes swelling tionally, chemical impact (even in which will spoil high tolerances Tool Making residual concentrations) may have immediately. Additional water va- part design / tool revision a large impact on the lifetime of por from the part is not very wel- Non-Optical Mold-Parts Optical Mold-Parts polymer optics. come during thin film processes. For further reading on the me- HSC, Polishing, EDM, HSC Diamond Turning chanisms of dimensional instability Polymer Materials in polymers appropriate literature The material properties are re- such as [1] is recommended. Quality Check Quality Check sponsible for process capability n. ok ok Tool ok n. ok and manufacturability of the pro- Premises for Precision Assembly ducts and hence for feasible tole- Injection Molding rances. There are a wide variety of Molding: polymers available today. But if it Looking at injection molding pro- Identifying Stable Process comes to precision injection mol- cesses one can realize five different Parameters (cpk-Analysis) ding of optical components only a fields which are very important for tool correction by „pre“ shaping few are left, especially if the parts precision injection molding. Quality Check: Molded Part are to be coated with dielectric anti-reflex coatings. n. ok Expert Knowledge Important material properties ok are e. g. viscosity, melt tempera- To achieve the tightest part to- Controlled Series Production ture, glass transition temperature, lerances one has to accept that FIGUre 2: From Design to part: High Precision injection mol- water take up and gas absorption. precision injection molding alrea- ding starts with parts design and ends with the molded part. The latter are important parame- dy starts at the optical design of ters for thin film coating processes the parts. Further it is essential to on polymers, because those coa- consider the optical design, the n-dependency ting processes are commonly run- mechanical design, the mold-pro- ning with process temperatures cess development and the mold- R-dependency above 80 °C and require a water machine development as parts and residual gas free atmosphere. of an integrated design process Mold shrinkage is another im- with very strong interactions. You d-dependency portant measure of a given ma- cannot do one without the other! terial to accurately replicate fine Hence it is necessary to employ product features and meet tight highly skilled and experienced dimensional tolerances. Amor- design engineers who can under- phous
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