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

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 polymers typically exhi- stand and handle tasks like optical bit lower shrinkage (0.3…0.8 %) design, part design, tool design, than semi-crystalline polymers finite element analysis and mold Figure 3: Comparison of the impact of n-index tolerances to (1…3 %) [1]. Besides being a ma- flow analysis. dimensional tolerances.

Figure 4: MOORE Nanotech 250UPL diamond turning a freeform lens.

Even if most operations during injection molding processes are fully automated and controlled by computers today, the presence of skilled and well educated operators is still essential. High precision injection molding processes are operating on the edge of the capabilities of injection molding technology. Typical are tide process windows and therefore a demand for continuous monitoring and manual adjus- ting critical process parameters and that is where the human factor is very important. Figure 5: Worn out diamond tool tip. Design Experience To obtain maximum part performance with Furthermore, one has to keep in mind that the achievable product quality. Product to- minimum parts tolerances the part design is the refractive index of polymer lenses is less lerances in the micron range require tools very important (see figure 2). The part de- accurate than with glass lenses. From our which are dimensionally stable within the sub sign decides about the feasible tolerances. experience you cannot expect more than micron range. There is no common wisdom Some common design rules from our ex- the 2nd to 3rd decimal of stability of the re- in tool design and there are a lot of success- perience are: fractive index under standard environmen- ful tool concepts but there is one important • preferably constant wall thickness in parts tal conditions. Let’s look at the sensitivity of thing to mention: Dimensional stable tools • no wall-thickness leaps, smooth transi- some lens parameters in comparison: require a high rigid design and adequate tions The focal length of a spherical lens with material choices with an adequate heat tre- • keep a reasonable minimum wall thick- 2 radii R ½, center-thickness d and refrac- ating. The importance of the latter is often ness (material dependent) tive index n is defined by: neglected. Hardened steel for example tends • no holes etc. near optical active surfaces to change its dimensions in the sub micron because of resulting flow lines f = (n – 1) · (1 – 1 + (n – 1) · d and micron range even without load if the • avoid material accumulations, they are R1 R n · R1 · R2 microstructure change during austenite to prone to sinkmarks. martensite transition is not totally finished or One can easily derive the sensitivities ∆ to stopped, by e. g. cryogenic treatment [2]. radius changes thickness changes and re- The Company fractive index changes from: Tool Shop Capabilities ∆ = ∂f , ∆ = ∂f , ∆ = ∂f Viaoptic GmbH n ∂n R ∂R d ∂d Besides design considerations the tooling is Wetzlar, Germany a very important part of injection mold pro- Looking for example at a lens with n = 1.5, cesses. The molds have to be machined and

VIAOPTIC GmbH is a professional part- d = 3 mm, R1 = –R2 = 10 mm the nominal assembled to very tight tolerances because ner in the development and manufac- calculated focal length will be 10.52 mm. what you give away here in terms of tole- ture of optical components, mechanical Graphically one will get the dependencies rances is hardly retrieved later in the injec- parts and sub-assemblies made of plas- shown in figure 3 for a parameters change tion mold process and/or will for sure further tic material, with advanced competence from –1 %…to 1 %. narrow the process window. Important for in: Selection of materials, Design of in- From Figure 3 one may obtain a diffe- precision tool making is a temperature con- dividual parts and sub-assemblies, Pro- rent significance of mechanical tolerances trolled environment. From technology point totyping, Tool design, Tool fabrication, and it should be clear that it is worthwhile of view precise high speed multi axis milling Manufacturing with injection moulding, to think twice about tight geometrical to- machines are thought to replace EDM tech- Sub-assembly Fabrication and quality lerances in a design, which may become a nology more and more. But from our point assurance. problem during injection molding. of view you need both in very high qualities. www.viaoptic.de Tool design is of the same importance When it comes to the quality of the optical as part design. The tool directly defines mold inserts VIAOPTIC relies on a MOORE

Nanotech 250 uPL (see fi gure 4) for single (see fi gure 5) in the cutting edge will be point diamond turning for ready to use tool seen later on the molded part. Meet us at inserts (without the need for polishing). For high precision machining you need A big disadvantage of diamond turning high precision data formats. *.iges and Booth 6146 today is that there is no process to cut direct- *.step fi les proved not to be accurate en- ly into ferrous materials, such as steel, that ough (fi gure 6). This is why the authors use would wear out the diamond pretty fast. You direct surface descriptions, e. g. using the always need a nickel coating on the steel in- mathematical polynomial of if that is not serts which may be an additional risk in feasible NurBS. terms of durability. Currently VIAOPTIC joins a research project developing a heat treat- Injection Molding Machinery ment process for some tool steels to enable single point diamond turning in optical qua- The injection molding machine is of course Got the need lity. The initial results look very promising. another important element of the process The availability of high precision tooling chain. Within the machine the polymer for speed? machines is only half the truth. The other is melted and reproducibly injected into part is the cutting/milling tool itself. e. g. for the mold. This requires a precise control the new UsB2000+ single point diamond turning high precisi- of all process temperatures, the displace- on and accurate grinded monocrystalline ment volume, the injection speed, the enables 1,000 full tool-tips are needed. At VIAOPTIC we pay cavity-pressure etc. Product quality in the spectra per second much attention to the surface and edge fi - micron range requires micron precision nish of this tools. even the tiniest defects machinery!

TABLE 1: Differences in the process and the materials between conventional molding and precision injection molding. Process Features Precision Injection Molding (PIM) Conventional Molding critical phase post fi lling fi lling mold temperature high low polymer temperature high low cycle time long short 4000 sony 2048-element ILX511 detector toshina 3648-element tCd1304Ap detector packing pressure high medium 3000 injection velocity low high 2000 major diffi culties sinkmarks, warpage, shrinkage short shots, fl ash 1000 thin walled parts easy diffi cult intensity (counts) thick walled parts diffi cult easy 0 200 300 400 500 600 700 800 starting Wavelenght (nm) Material Features Precision Injection Molding (PIM) Conventional Molding detector sensitivity Comparison glass transition temperature high medium water uptake very low NA Features usB2000+ stiffness high NA rohs and Ce compliant melt viscosity low low Integration time 1 ms to 65 seconds compliance low NA resolution ~0.3-10.0 nm fWhM neW sony ILX511B silicon CCd array Various gratings (200-1100 nm range) TABLE 2: Typical tolerances of optical components made of polymers. 8 digital user-programmable GpIos Low Cost Quality Standard Quality State of the Art User interface software spectrasuite Focal Length +/– 3…5 % +/– 2…3 % +/– 0.5…1 % Curvature error +/– 3…5 % +/– 2…3 % +/– 0.5…1 % Irregularities 6…10 2…6 0.5…2 (@ 25 mm diameter) fringes fringes fringes Geometry error 20…50 µm 5…20 µm 0.5…5 µm (arbitrary surfaces) Surface Quality 80/50 60/40 40/20 (scratch/dig) for more information see us at roughness ra 10…15 nm 5…10 nm 2…5 nm Photonics West - Booth 6146 or Centering Accuracy +/– 3 min +/– 2 min +/– 1 min contact our sales team via sales@ oceanoptics.eu or call us at +31 Center Thickness +/– 0.1 mm +/– 0.05 mm +/– 0.01 mm 26 3190500 or +49 711 34 1696 0 Diameter +/– 0.1 mm +/– 0.05 mm +/– 0.01 mm reproducibility 1…2 % 0.5…1 % 0.3…0.5 %

Common to both processes is the injection of a ”hot“ polymer melt into a (compared CAD-Data transfer to machinery via: to the melt temperature) cold mold cavity. This will introduce additional internal stress • *.step x, y, z- NURBS • *.iges during the cool down and may spoil lenses point cloud Non-Uniform • … Rational B-Splines for applications in polarized light [3]. To overcome this potential show stopper one may temper lenses, which is practically not feasible feasible feasible geometry geometry geometry feasible because they will lose their shape, accuracy accuracy accuracy or to use compression-injection molding ~2…5 µm ~1 µm <1 µm processes which have some big advantages = not sufficient! = just ok! = ok! here. Figure 6: Data formats Molding processes are very complex with in brief comparison. a lot of different (often counteracting) parameters and influence factors (see figure 8).

material preparation: dehumidifying, dedusting … Summarizing injection molding process parameter ranges, some important are: • Mold temperature: 90 °C up to 170 °C cooling area • Compound temperature: 180 °C up to 330 °C • Cycle time: 30 sec to several minutes • Packing pressure: part/material specific (up to >1000 bar) • Injection velocity: mold/part/material Mold Machine Mold Machine specific

The mold process in PIM is a part-specific process and has to be developed for each part separately. Process developments are based on the experience of the process engi- packaging coating temp. and humidity-controlled, cleanroom cell metrology neers, on smart process evaluation strategies and statistic process evaluation methods. Common process strategies can be found in Figure 7: Manufacturing Cell (schematic). [3] and might serve well as a starting point. Some important parameters, such as shrinkage, are not only material specific but Mandatory for PIM machines is control- Further common requirements for a ”clean“ process and part specific too. In our expe- ling those parameters within a closed loop. manufacturing cell are: rience they cannot be determined accura- Furthermore, all process relevant mecha- • thermo controlled environment accor- tely enough for part tolerances in the low nical movements of the machine should ding to customer‘s specifications micron range without molding trials. Hence be of highest precision (e. g. parallelism • constant humidity according to customer‘s VIAOPTIC is relying on another strategy for of mold mounting plates) and all relevant parts specification (water absorption) high precision optical parts (see figure 2). machine parts of high stability. Due to their • to the highest standard cleaned and First we start designing and building a drive concept electrically driven injection dried process-air (no condensates) mold tool without taking shrinkage into ac- molding machines have clear advantages in • local granulate dryers and dedusters count. In a second step we develop an injec- terms of accuracy and reproducibility and • local granulate degassing processes and tion molding process with the smallest part should be preferred for PIM. prompt use to part deviations and the widest process Another important point are the envi- • automated part handling systems: for part window. Then we are checking the molded ronmental conditions in which the parts are handling during and after injection and parts and “pre” correct the shape of the molded. For parts with tolerances in the low for packing and measuring parts as well mold. This procedure enables high process µm range a climate controlled, de-dusted, • special devices to cut sprue without dust stability and high precision as well, but re- dehumidified environment is mandatory. or debris quires the right equipment in the metrology This is particularly true if the parts should • as less human interaction as possible and tool shop department and some mathe- be coated afterwards. The manufacturing matical skills in the design department. cell should than be set up according to the Injection Molding Process customer‘s specification for ”reference-con- Future Outlook ditions“. The injection molding processes can be Because there are different customer spe- classified into two types: conventional- in Precision injection molding paves the way cifications and because climacontrolling and jection molding and injection-compression for polymer optical components into high dedusting large areas is expensive, setting up molding. Injection-compression molding is demanding high volume precision applica- an appropriate manufacturing cell is a good preferred for molding parts with micro struc- tions. Back in the early seventies when VIA- alternative (see schematic in Figure 7). tures, e. g. lenses with diffractive structures. OPTIC started to produce the first polymer

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Figure 8: Parameters for Injection Molding.

optical part for a LEICA camera (a viewfin- Literature der screen) nobody could imagine what is [1] Greener, Jehuda; Precision Injection Molding, possible today. But there is still room for Hanser 2006 some improvements which I would like to Beam P2 P1 [2] Holden, Frank C.; A Review of Dimensional In- profile summarize here: stability in Metals, Memorandum 189, Defense Wavelength 635 – 980 nm (optional 405 or 1550 nm) Metals Information Center, 1964 Process and Parts Metrology: • Line width ≥ 0.008 mm • Laser lines with uniform [3] Bäumer, Stefan (ed.); Handbook of Plastic Op- intensity distribution and constant line width P1 + P2 • easy to use, reliable and accurate tics, Wiley-VCH 2005 (< 0.1µm) non-tactile metrology for aspheric and freeform surfaces (reflective Fiber Optics and transparent) • improvement of in-mold sensor systems polarization maintaining, for laser beam sources 370 - 1700 nm Mold Design:

• improvements in mold flow analysis for Laser Beam Coupler for Singlemode B high precision parts fiber A inclined • easy to use software for mold design/ fiber coupling axis C mold flow analysis • high precision diamond cutting of ferrous materials D

E Polymers: A Laser beam coupler • temperature stable optical polymers B Laser source D Fiber collimator C Singlemode fiber (PM) E Microfocus optic with a larger n and n range • more accurate and temperature stable refractive index Laser Sources • less prone to ”micro cracking“ with fiber optics from Schäfter+Kirchhoff

Mold Machine Design: 1 • high rigidity

• high thermal stability 2 • high mechanical accuracy • accuracy and repeatability of process parameters • sensor technology: melt-flow, viscosity, 1 HeNe laser pressure … 2 DPSS laser (two fiber system) • cleanliness, e. g. oil spillage, dust through 3 Diode laser wear … 405 - 1083 nm Not shown: • easy to clean EC laser, 3 Argon laser, … Made in Germany

Kieler Straße 212 • D-22525 Hamburg / Germany [email protected] www.SuKHamburg.de