Precision Glass Moulding - Mould Manufacturing

Mould Manufacturing

This article describes how mould inserts are manufactured for precision glass moulding.

In order to ensure high quality standards metrology steps are implemented between each process step.

1. Powder processing: This process step is responsible for achieving grain sizes suitable for pressing and machining. The powder is processed by milling the raw material.
2. Pressing: This step does the pre-forming of "green" raw bodies of the mould inserts.
3. Sintering: By sintering, the pre-formed green bodies are compressed and hardened. In order to do this the green body is heated to a temperature below the melting temperature. The sintering process consists of three phases: First, the volume and the porosity is reduced and secondly, the open porosity is reduced. In the third phase, sinter necks are formed which enhance the material's strength.
4. Pre-Machining: The step of Pre-Machining creates the main form of the optical insert. It typically contains four process steps. These steps are grinding the inner/outer diameter, grinding the parallel/end faces of the insert, grinding/lapping of the fitting of insert, and finally, the near-net-shape grinding of the cavity. Normally, the cavity is only pre-machined to a flat or a best-fit sphere.
5. Grinding: Grinding or finish-machining creates the final form and the surface finish of the cavity in the mould insert. Usually, the finish is carried out by grinding; a subsequent polishing step is optionally required. Finish grinding can require several changes of the grinding tool and several truing steps of the tool. Finish-machining of the mould is an iterative process: As long as the machined mould shows deviations from the nominal contour in the measurement step after grinding, it has to be reground. There is no well-defined border between pre-machining and fine grinding. Throughout the grinding process of the cavity, the grain size of the tool, the feed rate and the cutting depth are reduced whereas machining time increases. Convex surfaces are easier to manufacture. The necessary steps of workpiece preparation are the mould alignment and the mould referencing. Grinding tool alignment, grinding tool referencing and grinding tool truing also have to be done. After that polishing can be necessary to remove the anisotropic structure which remains after grinding. It can be performed manually or by a CNC-machine.
6. Coating: Coating is the process step in which a layer is applied on the cavity surface of the optical insert which protects the mould against wear, corrosion, friction, sticking of glass and chemical reactions with glass. For coating the surface of moulds by physical vapour deposition (PVD), metals are evaporated in combination with process-gas-based chemicals. On the tool surface, highly adherent thin coatings are synthesized. Materials for coatings on optical inserts are Platinum-based PVD (mostly iridium-alloyed, standard), diamond-like carbon (not yet commercially available), SiC (CVD) on SiC-ceramics (not yet commercially available, have to be post-machined) or TiAlN (not yet commercially available). To achieve a homogeneous layer thickness, the mould's position has to be changed during coating. To prepare the mould for the coating the surfaces have to be degreased, cleaned (under clean room or near-clean room conditions) and batched. Especially the cathode of the machine has to be cleaned. After this process the workpiece has to be debatched.
7. Assembly: In this process step the optical insert and the mould base are combined to the assembled mould. For one optical element two mould inserts are necessary which are assembled outside the machine. For the assembly height measurement and spacer adjustment are essential.
8. Moulding Tests: This step determines whether the mould creates the specified form and surface quality. If mould is not suitable, it has to be reground. It is part of an iterative loop. The assembly of the mould has to be put into the machine to start the try-out-moulding.

In order to save the quality and enable an early warning in case of any problems between every single step there has to be a step of measurement and referencing. Besides that the time for transport and handling has to be taken into account in the planning of the process.