Lithography-based Additive Manufacturing

Lithography-based additive manufacturing processes include all 3D printing processes in which liquid photo resins are polymerized and, thus, cured using UV or visible light. Starting with stereolithography (SL) developed in the 1980s, a large number of different process variants have been developed to date and established in many applications for lithography-based manufacturing.

Irradiation can be carried out, for example, with laser radiation via a scanner system or with LED radiation, whereby the light is modulated in two-dimensions with the aid of digital light processing (DLP) or via LCD masks. With such processes, structural resolutions of between 20 µm and 100 µm can be achieved at maximum process speeds in the range of a few cubic centimeters per hour. Both stereo-lithography and DLP processes can also achieve structure resolutions of down to 1 µm by using high magnification optics, but then at a correspondingly reduced process speed. Even higher resolutions of < 1 µm can be achieved with multiphoton polymerization (MPP, also known as direct laser writing), although the build-up rate is lower here and is typically in the order of 1 mm³/h. In most processes, two-dimensional layers are generated, thus fabricating the component successively. For components with curved surfaces, this layer-by-layer construction forms surfaces with steps in the range of the lateral resolution or layer thickness. Quasi-continuous process control or volumetric methods eliminate the artifacts of layer-by-layer buildup, which has a positive effect on surface quality. 

In addition, significant time savings can in principle be achieved compared with processes in which work is carried out layer by layer. When considering if lithography-based additive manufacturing process is cost-effective, research must weigh up the required product properties, the photo resins that can be used and the advantages and disadvantages of the individual processes against each other.

The inherent versatility of lithography-based processes not only applies to the processes but also to the materials used. The underlying photo resin consists of a reactive monomer or oligomer base, an absorbing component that limits the penetration depth of the radiation, and so-called photo initiators that decompose through light absorption and thereby start polymerization. In most cases, this combination leads to the formation of thermosetting or elastomeric plastics. By varying the functional groups and the backbone of the reactive components used and by adding fillers, research can achieve a wide variety of product properties. Moreover, a wide range of materials is available, from hard-brittle materials with moduli of elasticity in the gigapascal range and elongations at break of ~1 percent, to soft-elastic materials with Shore A hardness > 50 and elongations at break > 150 percent, to abrasion-resistant or flame-retardant materials. Biocompatible materials approved for the manufacture of Class IIa medical devices can also be used with lithography processes. Optical properties of the components range from highly transparent to colored-transparent to opaque-matte-colored when using filled photo resins. 

When the photo resins are filled with solid particles, components can be indirectly created from materials that are not photo reactive. Ceramic or glass-filled photo resins can, for example, be printed and subsequently sintered in such a way that the polymer matrix can be liquified or burned out to produce ceramic or glass components. Burn-out molds can also be produced using wax-containing material, which can be embedded in a thermostable matrix material. After printed parts have been burned out in the furnace, metal components can then be cast.

Different process types can be advantageously combined to use complementary process specifications. For example, Fraunhofer ILT has designed a process version with an MPP processing head and a DLP processing head, allowing component areas with small structural elements to be cured at high resolution (MPP) and other areas to be cured more quickly (DLP) to ensure economical production. Alternatively, a low-resolution DLP processing head can be combined with a high-resolution one for optional large-area or high-resolution exposure.

The patented TwoCure® process of Fraunhofer ILT has decisive advantages over other process variants. By using a material with two curing mechanisms – thermally reversible and photochemically irreversible – there is no need for the use of support structures anymore, s because layers to be cured are mechanically held in place by underlying thermally solidified layers. Support structures are component elements to be cured, which are necessary in many processes to support overhanging structures. These have to be digitally created, printed and then usually removed manually. This requires time and material effort and will impair the surface quality of the printed components.

The TwoCure® process also eliminates the need to position components in contact with the build platform during printing. This provides the basis for a process design that allows for continuous production. The thermally solidifi ed block is mechanically moved and heated outside the process area, releasing the manufactured components from the photo resin matrix. This continuous printing approach can be integrated into existing process chains, also because manual post-processing steps such as the removal of support structures are no longer required. 

For some applications, it makes sense to use smaller wavelengths and thus larger photon energies. This may be the case when highly transparent systems are desired. The absorption cross-section of absorbing molecules is usually much higher for light in the UV-C range than for longer wavelengths. This way, absorbers no longer need to be used and the photo initiator concentration can be reduced. In the fi eld of thiol-ene click chemistry, the additivation of initiators can be dispensed with completely since the thiol monomers contained can be initiated directly. When such initiators are no longer needed, the production of biocompatible components can be expanded since initiators often cause cytotoxic behavior. In addition, the step-growth mechanism of thiol-ene photoreactions offers further advantages for material properties. However, UV-C-based lithography systems are not commercially available. For this reason, a UV-C-SL system was developed at Fraunhofer ILT. The beam source is a frequency quadrupled Nd:YAG laser with a wavelength of 266 nm and a power of 70 mW at a resolution of 50 µm. It has an installation space of 50*50*50 cm³. 

A limiting factor in material selection is viscosity. Highly viscous materials cannot be used because enormous forces are generated here during layer generation, which can lead to component defects or cracks in the bottom of the vat. Monomers and oligomers for the production of mechanically and thermally highly stable polymers are often highly viscous. In order to process these materials, reactive diluents are usually added, which, however, can adversely affect the mechanical properties. One way to reduce the viscosity is to heat the photo resins in the process. At Fraunhofer ILT, equipment has been converted so that work can be carried out at elevated temperatures of up to 80°C, which has made it possible for the institute to develop highly viscous materials that can subsequently be used in hot or volumetric processes.

A next step in the further development of lithography-based manufacturing processes is volumetric curing. This means that curing is no longer performed only at the surface of the reaction vessel, but at arbitrary coordinates in the build space. As a result, a layer-by-layer build would no longer be absolutely necessary and the build platform would no longer have to be moved through the usually highly viscous resin. This movement is not only time-consuming, but it also often results in an enormous force impact on the generated component due to fl owing photo resin. Volumetric 3D printing is not readily available (with the exception of MPP) because the combination of irradiation wavelength and material results in Lambert-Beer absorption at the surface of the resin. This means that curing occurs primarily at the surface – regardless of where the radiation is focused.

Another option for volumetric 3D printing was demonstrated by xolo GmbH. An initiator system is used here, which is activated in two excitation stages via two different wavelengths. If light with two different wavelengths is irradiated in the reaction volume from different directions, polymerization takes place only in the overlap region. In the ZIM cooperation project »Polymeric Optic Xolography POX« founded by the German Federal Ministry of Education and Research BMBF, Fraunhofer ILT, together with xolo GmbH and the Chair of Optical Systems Technology TOS of RWTH Aachen University, is further developing corresponding printers, initiator systems, monomer and oligomer bases to enable higher resolutions and more diverse material properties. 

At Fraunhofer ILT, light is understood as a non-contact high-precision tool that can be used to process and analyze materials on a microscopic and macroscopic level. In addition to having extensive knowledge of beam sources, light properties, beam shaping and measurement methods, the institute has 3D printing laboratories with diverse process models such as DLP and LCD-based printers, MPP systems, UV-C-SL systems or also with the various special systems described above. The equipment technology is complemented by a chemical materials laboratory, which provides the appropriate analytics for rheological analyses, dynamic mechanical analyses, tensile elongation tests, refractive index measurements and absorption measurements of undiluted photo resins. Finished processed components can be measured and characterized using geometricaloptical and infrared spectroscopic methods.