In the mid-1990s, the Processes group started working on the development of micro-injection molding technology for the realisation of micro- and nanostructured components in plastics, ceramics and metals. In the field of micro injection molding and related special processes, the entire processing chain is considered, further developed and optimised. In detail, this concerns the process steps of compounding, shaping, debinding and sintering as well as the comprehensive characterisation of both the molding material and the sintered components. The experimental work is flanked by comprehensive design activities and simulations of mold filling.
In recent years R+D exceeded the area of micro technology and nowadays covers the whole size range from macro to micro applications.
The following main areas of work are currently being pursued:
Micro injection molding:
In micro injection molding, the molding compound is injected into a specially temperature-controlled mold in which microstructured mold inserts are integrated. Such mold inserts can be produced, for example, by processes of mechanical microtechnology (micromachining), by laser micromachining, X-ray or UV lithography according to the LIGA process or combinations of these and other processes. Since these manufacturing processes frequently result in blind hole-like cavities which can no longer be vented through backward holes, micro injection molding tools are often equipped with evacuating devices. The injection molding process is carried out isothermally or variothermally, i.e. with heating before injection and cooling before demolding, depending on the component to be molded. The last process variant is particularly recommended when molding high aspect ratios (> 5). After cooling and after the mold has been opened, the injection-molded parts are usually removed by a handling unit/robot. Almost all thermoplastics as well as thermoplastic elastomers can be used as material.
Subsequently to the injection molding of so-called feedstocks, i.e. organic binders highly filled with metal or ceramic powders (Powder Injection Molding, PIM), the components are debound after the injection molding process and finally sintered into metal (Metal Injection Molding, MIM) or ceramic parts (Ceramic Injection Molding, CIM). With powder injection molding, a process has become established in macroscopic molding technology that allows the high cost-effectiveness of injection molding series production to be used for the manufacture of metal or ceramic products. For this purpose, further process steps are being implemented in the process sequence. For example, the feedstocks are used for powder injection molding. These mixtures, which are prepared as homogenously as possible, contain 50-60 vol.% powder in an organic binder. The latter usually consists of a low-molecular wax component, a thermoplastic and additives mainly for homogeneous distribution of the powder particles in the organic matrix. Otherwise the binder is used exclusively for molding (=> green compact) and must be removed after injection molding (=> brown compact). This debinding step can be performed thermally, pyrolytically, catalytically, by dissolution or combinations thereof. The parts can then be sintered to approx. 95-99 % of the theoretical density. The sintering atmosphere is either high vacuum, hydrogen or nitrogen or a mixture of both gases. When the carbon content in a steel has to be adjusted, hydrogen/methane mixtures will be used. The high porosity of the brown stock leads - depending on the powder content in the feedstock - to a linear sinter shrinkage of the parts in the range of 15-23 %. This must be compensated by an appropriate allowance of the injection mold. The tolerances currently achievable with powder injection molding are ± 0.5 % of the nominal dimension. Through thorough process optimization, values of ± 0.3 % to ± 0.1 % (in special cases) can also be achieved.