(for generative fabrication processes)

In a stricter sense, rapid prototyping is understood to be a process that allows for the generation of 3-dimensional models from various materials on the basis of computer data exclusively. This process is of generative character and, hence, differs from the erosive character of conventional fabrication methods [1].

Stereolithography and selective laser melting are generative fabrication processes for rapid prototyping used by the IMVT.

The development of novel microstructured devices usually is very time-consuming. First, given or assumed boundary conditions have to be transferred to a CAD model. Prototype fabrication then results in a first real or functional model for the test rig. The results obtained in the test rig serve as a basis for optimizing the design or for model variations. This development cycle usually is repeated several times, until an optimized prototype is available that meets the given boundary conditions. With the help of a rapid prototyping process, the time needed for prototype fabrication can be reduced to a few days. The development cycle of complex geometries in particular takes far less time, due to the time gained in prototype fabrication. This allows for a new development strategy. For example, uncertain design assumptions can be verified using a first, possibly simplified, prototype in the test rig. The use of generative fabrication processes additionally results in a new freedom of construction. It is now possible to design constructions that were not permissible in the past for technical reasons or required too high an expenditure. Consistent use of this new constructive and technical freedom allows for an optimum problem solution.

At the IMVT, stereolithography is applied as a generative process for the fabrication of plastic microstructures or microstructured components. These components are built up in a layerwise manner. Liquid plastic is polymerized locally in thin layers by laser light (photopolymerization).

Fig. 1 shows the principle underlying the process. A platform that can be moved in z direction carries the model to be built. This building platform is lowered from layer to layer by a defined layer thickness. Then, the lowered layer height is filled up with liquid plastic. A thin layer of liquid plastic is applied onto the previous solid layer. After this, this layer is exposed and, hence, polymerized.

((F-Theta Objektiv = F-theta lens; Modell = model, Bauplattform = building platform; Vorratsbehälter = storage tank; Flüssiger Kunststoff = liquid plastic))

The IMVT stereolithography system (Fig. 2) has been constantly further developed for the manufacture of microstructures. For example, the laser spot diameter was reduced from the initial value of 250 µm to 50 µm. With a laser spot diameter of 50 µm, it is now possible to build 100 µm wide channels and minimum wall thicknesses of 100 µm. This corresponds to a structural resolution of 100 µm.

The examples (Fig. 3) illustrate that also microchannel structures integrated in the component can be produced easily by means of stereolithography.

The cross-flow heat exchanger component is provided with channels of 200 µm x 800 µm in cross section, channel length is 5 mm. The unit for measuring residence times also is equipped with channels of 200 µm x 800 µm in cross section. Construction time of a cross-flow heat exchanger is about 2 hours. The measurement unit was manufactured within about 10 hours.

Selective laser melting is a generative fabrication process for the manufacture of metal components. These components are set up in layers. Layerwise design may be compared in principle with that of stereolithography. On a building platform, a thin layer of metal powder is applied. Then, the powder is molten locally by laser beam energy. The building platform is lowered by a defined layer height, coated again, and molten again. This process is repeated until the component is set up completely.

Fig. 4 shows the SLM system of the Institute for Micro Process Engineering.

The major prerequisite for the direct manufacture of microstructures by laser melting is a structural resolution smaller than 100 µm. This means that wall thicknesses smaller than 100 µm are built up or molten reproducibly and in a gas-tight manner from metal powder. Structural resolution and component density are influenced mainly by energy supply and exposure strategy. Research at IMVT is dedicated to studying the influence of these process parameters when fabricating of microstructured components and to gathering sufficient experience.

It is a current objective of the experiments to reach a homogeneous structure quality of the welding beads molten from metal power and to optimize the surface quality of the components. The material used is stainless steel powder of the type 1.4404 (X2 CrNiMo 17 12 2) with a grain size ranging from 10 µm to 30 µm.

Fig. 5 shows a building platform with test structures produced by layerwise melting of stainless steel powder. The height of the powder layer is 50 µm. The surface and welding bead quality reached so far is illustrated below.

Fig. 6 presents a general view. The four views are marked red in the CAD model. Current results reveal defect-free welding beads with a width of 80 µm and a height of 50 µm. Single powder grains that have been molten partly adhere to the surface. These powder grains cover less than 10% of the surface.

Selective laser melting is suited in principle for the fabrication of microstructures with a structural resolution smaller than 100 µm.

In addition to the classical fabrication methods, IMVT applies stereolithography and selective laser melting for rapid prototyping. When used in microfabrication, these methods have the potential of further refining the structural resolution.

[1] Gebharth, A.: Rapid Prototyping, Hansa Verlag, München 1996.