Fabrication of Complex Optical Components

Total Quality Management (TQM) is a long term business strategy. It is based on the three principles Customer, Associate and Process-Orientation. All activities within TQM focus on the continuous improvement of the company’s performance. Such a holistic approach is especially auspicious for cross-sited production chains producing sophisticated components such as highly complex optical elements. Within the sub project M5 Quality Chain Management, the TQM approach was used to develop an internet based software tool. The so called Process Chain Manager (PCM) supports the coordination of the process chains. Especially the aspect of Process-Orientation represents a crucial element within the PCM. The PCM helps to manage the different process steps as well as to coordinate their interfaces within a single process chain. Additionally, it represents the aspects Associates, who are responsible for a smooth process flow as well as Customers, who are located at the end of each process chain and are demanding high quality standards.

A key condition for the successful manufacturing of optical plastics parts and a major challenge at the same time is to develop high-performance injection and injection-compression molds. Particularly in the field of optics, the stringent requirements placed on part dimensions and the homogeneity of the internal properties lead to a large number of requirements in terms of the mold concept.

In order to meet these requirements, a modularly-designed injection-compression mold has been developed. The mold features a cone-alignment to ensure a precise centering of the two mold inserts to each other. In injection-compression molding processes, it is necessary to seal the cavity before the polymer is injected. Therefore, a spring-supported sealing ring is implemented that seals the cavity after the two mold inserts are aligned. In this position, the mold is not closed completely and it is possible to mold polymer optics using both injection molding and injection-compression molding.

A second mold has been designed with two cavities to ensure symmetrical forces and hence to increase the reproduction accuracy. Injection-compression molding can be performed either via the closing movement of the injection molding machine or the movement of internal hydraulic pistons. Using this concept it is possible to compare the two compression techniques. During the compression process a gate closure prevents the melt from flowing back into the plasticizing unit. Furthermore, the mold features a parting line locker, which is needed when the compression cores are moved by the machine movement.

Over the past years, more and more polymers have been applied for manufacturing optical components and have won an increasing market share for spectacle glasses, cameras, or other lens, or light guide applications. The manufacturing of optical components in plastics competes functionally and economically to the material “glass”. For optical applications the technological advantages of plastics enable an increasing substitution of glass.

Injection molding and injection-compression molding allow a comparatively cheap one-step-manufacturing of high precision optical plastics components. Both processes are primary forming techniques which have a significant potential to produce optical plastics parts in mass production.

Using injection-compression molding, a combination of the injection molding and pressing process, a high molding accuracy combined with uniform inner properties of the molded parts can be obtained. Excellent process knowledge is a prerequisite to achieve and meet the required tolerances of high-precision plastics parts for imaging optics in the micron range.

The success of plastic optics relies on the availability of molds needed for mass production. This chapter deals with the machining of molds with continuous surfaces exhibiting aspheric and free-form shapes, while the machining of molds with discontinuous surfaces (prisms, facets, Fresnel structures, etc.) will be discussed in Chapter 6. Molds may be classified according to size, shape, and tolerance requirements. As a mold material for replication of plastic optics, a nickel-phosphorous plated steel alloy is frequently chosen, which meets temperature and wear resistance requirements and can be machined with monocrystalline diamond tools. Basically, there are three methods for machining asymmetric shapes: raster milling, ball-end milling, and tool- or slide-servo turning. The discussion of this chapter includes material response to diamond machining, selection of machining parameters, programming and data handling, machining strategy, and achievable surface roughness and figure accuracy. Since the machining of molds is still the most cost-effective factor of the production chain, a reduction of set-up and machining times remains the biggest challenge for future research.

This chapter will discuss the state of the art and advances in the development of dedicated processes for mold structuring by diamond machining with defined cutting edge geometry. The advantages and disadvantages of the individual processes, e.g. the achievable geometry spectrum or the required machining time, will be discussed. Particular attention will be given to the Diamond Micro Chiseling (DMC) process which was developed within the SFB/TR4 for structuring molds with discontinuous prismatic microstructures. Utilizing a novel tool kinematics and custom built V-shaped diamond tools, this process enables the machining and replication of such structures in optical quality.

The ultraprecision machining of steel alloys with optical surface finish could not be achieved in the past as catastrophic wear of the mono-crystal diamond tool occurs inevitably. A recently developed solution of this problem is a thermo-chemical treatment of the steel e.g. by custom nitriding or nitrocarburizing. This process leads to a compound layer at the steel surface, in which the iron atoms are bonded to nitride or carbon nitride, a layer which is diamond machinable without significant tool wear. This chapter covers all facts about the thermo-chemical treatment and the subsequent diamond cutting process: different nitriding and nitrocarburizing processes to produce a dense thick compound layer on steel are described. Results from the diamond machining of these layers show that an optical surface quality is achievable and the chemical tool wear can be suppressed. Furthermore, differences between the machining of non-ferrous metals and thermo-chemically treated steel alloys are discussed. Finally, examples for the replication of glass and plastic optics with moulding inserts made of thermo-chemically treated steel are given. Therefore, this chapter gives an overview over the whole process chain for the replication of complex optical parts with nitrided or nitrocarburized steel moulds.

In this chapter, two new process chains are introduced using novel processes for the machining of complex molds by diamond grinding and diamond profile grinding in combination with a machine-based abrasive profile polishing. Finest grained diamond grinding wheels with a metal bond provide superior grinding results. But the combination of a metal bond and very fine grains causes great difficulties for the trueing and dressing operation using conventional methods. For the machining of rotational symmetric mold inserts the novel process of electrochemical in-process dressing can be applied to achieve optical surface qualities without a subsequent polishing process.

For more complex shapes a grinding wheel with a micro-profile is needed. The pre-dressing and profiling of the difficult to machine grinding wheels is done by the unconventional Wire-EDM process. To achieve a surface roughness of the profiled mold in optical quality a subsequent polishing process is necessary. Therefore the novel process of diamond profile grinding in combination with a subsequent machine-based profile polishing is introduced.

The replication of ultra precise optical components requires molds with an extremely low surface roughness, a minimum of defectivity and high shape accuracy. To meet these demands polishing is essential. In comparison to other fields of application, polishing molds combines high demands and new geometries, e.g. aspherical cavities or structured surfaces for cylindrical lens arrays. Due to a lack of suitable polishing strategies, tools and innovative tool machines, the molds often have to be polished manually. In this chapter the authors discuss the major aspects for deterministic polishing starting from basic investigations on the material removal mechanisms to process strategies, tool development and design of innovative polishing tool machines. The materials dealt with reach from steel to advanced ceramics and tungsten carbide. The addressed geometries are smooth molds with a continuous surface as well as structured surfaces. The discussed issues look at the needs in replication of both plastic and glass optics.

Precision glass molding is becoming a promising technology for fast production of complex optical glass components in high volume. It is a replication process and becomes economical after a few batches. The glass molding process can be holistically described by a process chain which starts with the design of the optical component and modeling of the glass molding process using numerical simulation. An ultraprecision grinding process is applied to manufacturing the molds. Chemically insert coatings are necessary in order to prolong mold life because the mold has to withstand high mechanical and thermal loads. The parameters of the subsequent molding steps such as temperature or pressure depend on the type of glass and the size of the optical component. The last step of the process chain is the qualification of the optical components by measuring their optical properties using precision metrology.

Optical molds for precision glass pressing are frequently coated to improve their wear and oxidation resistance, and to reduce sticking of the hot glass to the mold surface during the glass pressing process. Currently PVD (Ti,Al)N, CrN, ZrN and PtIr coatings are used as protective coatings for these molds. Novel nano crystalline PVD Ti-Ni-N and Ti-Cu-N coatings and thin sol-gel ZrO₂ coatings were developed and tested as new protective coatings for glass pressing molds. Optical molds for injection molding of plastic lenses are frequently coated with thick diamond machinable electroless nickel coatings since steel cannot be machined with diamond tools. These nickel coatings have several drawbacks such as residual porosity, limited hardness and temperature resistance. Therefore, thick sol-gel silica based coatings were developed and tested as an alternative for the electroless nickel coatings.

Measurements of the form and the roughness during the machining process or the replication process with molds is still difficult and highly inaccurate. An adequate roughness measuring system is based on the analysis of the scattered speckle intensity distribution, emerging from the workpiece surface. This contribution covers the theoretical description of the scattered light measuring process, which is based on the scalar Kirchhoff theory and a ray tracing model of the light propagation in an optical fiber considering physical optics.

Highest accuracy in optical, non tactile measurement of form deviations of molds or the molded optics in the machine is achieved by interferometry. A phase shifting digital holographic measurement method is described in more detail. This comprises an automated alignment set-up including the features of auto focusing and subsequent refocusing, the use of computer generated holograms (CGHs) as well as different approaches for a systematic determination and evaluation of form deviations.

Aspects of the measurement and manufacture of mainly optical components from early times, the present and the future are described. Two instrumental types having the best pedigree and most potential are singled out namely the stylus method and optical methods with, in the latter case, scanning white light interferometry. Critical issues are highlighted and some examples of future requirements and possible developments are given.