Vat photopolymerization - shaping of plastic-ceramic composites and technical ceramic components
Undistorted, crack-free and sharply contoured ceramics with predictable density and strength
The chemical and physical interactions between ceramic particles and organic resin components are complex – the system is far from being fully developed. 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 advance this development quickly and individually.
In doing so, we naturally have your goal in mind: undistorted, crack-free and sharply contoured ceramics with predictable density and strength.
In addition to individual material and process development, we offer feasibility studies and A-Z component production. We are pleased to receive demanding geometries and/or physical functional specifications, whether in the field of thermoset mould making, plastic-ceramic composites or filigree structural or functional ceramics.
What is Vat photopolymerization?
The process of Vat photopolymerization originally stands for the layer-by-layer additive structure of plastic 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” here 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 breaking 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 molded parts.
How does Vat Photopolymerization (VPP) work in detail?
First, CAD/CAM is used to generate a file of the solid, for example in .stl format. This is divided into layered work packages and the number of pressure cycles as well as building platform and blasting procedures are determined.
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 filled with ceramic powder is to be used, dispersants and rheological additives are usually also involved. Functional rheology and exposure sensitivity of the suspension must be guaranteed so that sharply contoured green bodies can be built up by additives.
The component is manufactured on a building platform which is lifted out of the vat containing the liquid resin. Exposure takes place from below through a transparent base in the tub. A defined layer thickness (between 30µm and 200µm) is built up. 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 the finest structures if suspension parameters and device parameters interact well.
In between, the growing component is lifted and detached from the transparent bottom of the tub together with the last cured layer – so that resin can flow. The torso is then moved back to the bottom of the tub at layer thickness intervals and the next curing exposure is started.
The polymerization kinetics function in such a way that the gel point is first reached at 20% monomer conversion. From this point on, the degree of polymerization increases at an increasingly slower rate. The maximum conversion is reached at about 80 %. From this degree of polymerization onwards, the kinetics are so slow that no further conversion can be observed in the foreseeable future. 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 target 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.
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 molds, whose shape can meet the highest demands for filigree if required.
Finely structured, precisely fitting dentures or plastic components filled with ceramic fibers and thus reinforced are two examples of components that are accessible via resin suspensions. 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.
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 screws: 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 pays off for unique specimens (individual medical devices) and small series (e.g. samples for empirical optimization of components before completion of the time-consuming and cost-intensive injection molding tool). The advantages of the process are particularly evident when geometries are realized that cannot be produced conventionally.