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Integrated process chain view

Mastery of the process chain as essential prerequisite

Successful use of precision molding requires mastery of a process chain consisting of multiple individual steps. Depending on the optical design and the glass specifications, the glass blank form, the tool, and the tooling system are derived in the context of engineering and design. Next comes tool making, which consists primarily of precision machining and tool coating. After completion of tool making, the glass blank is heated to the thickly flowing state, then molded into the desired shape and measured. The measurement results are the basis for any later iteration cycles of the process chain steps described below.

Engineering and Design
Die Prozesskette zum Präzisionsblankpressen ist komplex und durch zwei Stränge gekennzeichnet: Das Glas und das Werkzeug. Zudem sind Iterationen, d.h. Rückschritte in der Prozessverkettung notwendig
Prozesskette Präzisionsblankpressen
Werkzeugsystem mit zwei asphärischen Formeinsätzen
Werkzeugsystem
FEM-Analyse

Preform design, FEM analysis, and tool system design

Based on the technical specifications of the glass optics, the first step in the process is to derive a suitable tooling system and glass blank design. To achieve the highest possible centering accuracy, optimum tolerance management has an important role. Form and dimensional accuracies generally fall in the micrometer range, since they significantly influence the centering accuracy of the later optic, for example. During selection of the material, the requirements of the glass deformation process at high temperatures are in tension with the capabilities of the fabrication technology. A combination of different high-performance materials is therefore unavoidable.

Due to high fabrication costs, the glass deformation process is simulated at Fraunhofer IPT using the Finite Element Method (FEM). First, a process model is created. The actual glass deformation process at high temperatures is modeled using a viscous-elastic simulation model. During the heating and cooling phases, it is now possible to examine the behavior of the hot glass in the different phases of the process. The simulation also allows a prediction of the resulting glass geometry if sufficient material data is available. This information permits the prevention of glass shrinkage in the tool, significantly reducing the costs of iteration. Different examples have shown that form errors can be predicted with a precision of about 1 µm.

Tool making
Quelle: Fraunhofer IPT
Werkzeugsatz Linsenarray
Diamandwerkzeuge
Diamandwerkzeuge

Precision machining and coating

The tool mold inserts that deform the glass and are therefore in continuous contact with it must satisfy a complex requirement profile. Due to the high reactivity of technical glasses in this temperature range and the often high surface pressures, materials must be corrosion-resistant, heat-resistant, hard, and wear-resistant. They must also be extremely leak-proof and capable of being machined at optical surface quality. These requirements are met by binder-poor hard metals based on tungsten carbide, as well as certain high-performance ceramics.

Next, the form inserts are fabricated by ultra-precision grinding with point contact kinematics and polished by hand. The key to success here lies in tool making due to the imaging character of molding technology. Molding tools are subject to the same requirements with regards to shape and surface as the glass optics themselves ‚€“ but with materials that are much more difficult to machine. Every micro-defect, the slightest shape deviation, is replicated onto the surface of the glass and will make unwanted rework on the glass optics necessary.

Since none of the materials listed is fully inert during molding, additional optical coating systems are applied to the tool surface that represent a diffusion barrier between the glass and the mold. These coating systems are applied using CVD or PVD sputter technologies and are based on noble metals or ceramics, based on the application. In addition to inertness, the essential requirement of the coating system is to maintain high surface quality and outstanding adhesion to the basic material. The coats must also be extremely thin (submicrometer range) and homogeneous in order to retain the high dimensional accuracy of the basic shape.
Links
Werkzeugbau Ultrapräzisionsschleifmaschine ULG-100 D

Precision molding
Pressprozess
GMP 207 HV

Molding and measurement

Using precision molding, the glass blank is next heated to a viscous state, then molded into the desired shape and finally measured. Due to the high-precision machining with very small tolerances, similarly precise measurement technology is a must. This permits the detection of the iteration steps needed, which then trigger the corresponding subsequent process steps.
Links
Präzisionsblankpressen Präzisionsblankpressmaschine Toshiba GMP 207 HV

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