Vat photopolymerization - shaping of plastic-ceramic composites and technical ceramic components
Additive manufacturing of filigree high-performance ceramics, multiphase composites or molds for die casting
The chemical and physical interactions between ceramic particles and organic resin components are complex – the photosensitive organic-inorganic system is far from being fully developed and state of the art for all types of ceramics. Consequently, the focus of our research is on the development of suitable resin suspensions.
Knowledge in organic polymer chemistry, solid-state and surface chemistry as well as ceramic processes are combined in our company to be able to move this development forward quickly and individually.
Surely we always focus on our customers goals: undistorted, crack-free and sharply contoured ceramics with predictable density and strength.
In addition to customized material and process development, we offer feasibility studies or A-Z parts production. Whatever our customers need – filigree structural ceramics, customized functional ceramics or moulds for thermoset process – we are pleased to receive demanding geometries.
Additive Manufacturing Process
What is Vat photopolymerization?
The process of Vat Photopolymerization originally stands for the layer-by-layer additive manufactoring of plastic solid bodies. However, according to the current state of the art, it can also be used as a vehicle for shaping plastic-ceramic composites and as a shaping step for technical ceramic components.
“Vat” stands for the tub into which a mass of liquid organic molecules is filled. In this mixture, also known as resin, the molecules have a metastable “predetermined linking point” that allows them to combine selectively to form a solid polymer at the desired moment – through an energy input in the form of electromagnetic radiation.
If the first polymerization processes begin adhering to a building platform that can be moved into the tub for wetting with resin mass, three-dimensional components can gradually grow on this building platform in a layered structure. If ceramic particles have previously been dispersed in the resin mass, they are also part of the thermoset solid bodies.
How does Vat Photopolymerization (VPP) work in detail?
First, a CAD software is used to generate a digital image of the part, for example in .stl format. Afterwards it will be divided into several layers which determine the number of printing cycles.
A UV-reactive system consists of a reaction resin (oligomer/monomer mixture) and a photoinitiator. When exposed to a photon of a certain wavelength, the initiator molecules decompose into radicals that initiate the chain reaction of polymerization.
If a resin suspension is filled with ceramic powder, dispersants and rheological additives are usually also involved. Functionality of rheology and exposure sensitivity of the suspension must be guaranteed to get sharply contoured green bodies.
The component is manufactured on a building platform which allows z-axis moving inside the liquid resin towards the exposure window. The exposure takes place from below through this transparent base of the vat. A defined layer thickness (between 25µm and 200µm) is built up every exposure cycle. The UV light required for curing is focused locally on the resin surface (UV-LED combined with micro mirror DLP technology is increasingly used). In principle, the projection optics used enable precise curing of filigree structures if resin parameters and device parameters interact well.
Meanwhile the curing exposure cycles, the growing component is lifted and detached from the transparent bottom of the vat – so that resin can reflow. The torso is then moved back to the bottom of the vat leading a resin-filled gap in value of layer hight and the next curing exposure is started.
The polymerization kinetics work in such a way that the gel point is first reached at 20% monomer conversion. As of now, due to kinetic hindrance the rate of polymerization decelerates progressively. The maximum conversion is reached at about 80 %. From this degree of polymerization on, the kinetics are so decelerated that no further conversion can be observed. If very solid green bodies are required (e.g. dental composites), the polymerization must be completed in a downstream process in a post-curing unit equipped with a UV source and mirrors.
If the goal is an organic-free technical ceramic, the pronounced organic matrix of the green body is removed by debinding and then the porous ceramic brown body is sintered to a dense microstructure.
What is Vat photopolymerization particularly suitable for?
Even pure thermoset parts manufacturing offers many options, e.g. for the fast, uncomplicated and inexpensive construction of casting moulds, whose shape can meet the highest demands for filigree if required.
This method is particularly suitable for plastic-ceramic composites, since the thermosetting polymer matrix is very strong in itself (bending strength: 120 – 160 MPa) and gains even greater strength, hardness and abrasion resistance due to the ceramic particle filling. Two examples of composites that are accessible via resin suspensions are finest structured, precisely fitting dental applications or plastic components filled with ceramic fibres and thus reinforcing structure.
Vat Photopolymerization is also particularly suitable for the production of small components made of technical ceramics. The open-pored microstructure of the fired ceramic, which is inherent in all additive production methods and also occurs here, can be optimized with surface chemical and ceramic-technical understanding (adjusting by filling degree of the resin suspension, adjustment of the grain size distribution, selection of a sinter-active material, selection of precisely fitting sintering curves and post-processing: infiltration/glazing steps).With this know-how it is possible to produce ceramic components with VPP, which are comparable to conventionally produced ceramics in their properties.
The use of Vat Photopolymerization plus development costs is worthwhile for prototypes and small series. The advantages of the process matter the most when the goal is to produce geometries, which cannot be produced with conventional manufacturing processes.