2023 has been an eventful year at WZR ceramic solutions – mainly because of the new 3d printer. With the CeraFab S65 (VPP-printer) from Lithoz we got a new AM method in our house, whose potential is used to the fullest for our public funded project Redox3D.
The ceramic material, that is used to achieve the goal of the project is the technical ceramic cerium oxide. Only a few might have heard about this fancy oxide, that’s why it’s not surprising that there are no commercial VPP-suspensions for this material.
The development for this project therefore starts from scratch.
How do we do this?
In this article we would like to give you a short behind the scenes of our engineering-team that develops the suspensions. But before that, we would like to give a short summary of the VPP process.
VPP is an abbreviation for Vat photopolymerization. It includes a term for a container and the chemical process of linking molecules (polymerisation) by light. In practice, this means that a polymer mixture (with particles) is placed in a glass tank that can be irradiated with light from below or above. If a building platform is then placed in this suspension, polymerisation is started by targeted exposure at the desired location (e.g. outline of the component). In this way, the component is built up layer by layer.
But up to the point of building a component, it’s a long road with a lot of tests.
Firstly, the choice of powder is important for the suspension production. As with other additive manufacturing processes, it is also important to pay attention to the particle size in VPP, as this has a major influence on the strength, strength and edge sharpness of the component. The refractive index of the particles also plays an important role in VPP, as it influences the curing depth of the suspension.
We are currently working with a cerium oxide powder that we grind to a particle size of approx. 1-2 µm. As soon as the cerium oxide powder has the right grain size, it can be weighed in together with the photopolymer, the photoinitiator and the co-initiator. As the photoinitiators become reactive on contact with UV light, the mixture must no longer come into contact with light. For this reason, we have also covered the windows in our VPP laboratory with orange-coloured foil. As the proportion of solids in the suspension should be as high as possible, we also use liquefiers to obtain a flowable suspension.
With the right mixing ratio of powder to organic material, the suspension has roughly the consistency of melted chocolate. This is significantly thinner than the pastes that we use for 3D screen printing or material extrusion, but is necessary so that the suspension can be distributed well and evenly in the tank. The device is then filled and the printing process can start.
Firstly, we “print” an adhesive layer over the entire surface of the building platform. The components should adhere to this and can then be easily and non-destructively removed from the platform. Only then is the first layer of the component printed on.
With VPP, the time required to print a component depends on the exposure time and the layer thickness. As cerium oxide, for example, has a high refractive index, the curing depth is low. We therefore work with layer thicknesses of 10 µm. In combination with a long exposure time, printing a component with a height of approx. 20 mm takes over 17 hours, so it is always a good idea to position several components on the platform at the same time. If there are overhangs, care must also be taken to ensure that support structures are introduced. Even with heavy/large components, enough support on the construction platform must be ensured, because nothing is more annoying than seeing the next morning that the long-awaited component is half-finished in the tub because it was too heavy and has fallen off. Our engineering team also had to experience this, as every additive mixture and every component behaves differently.
But it is not only the printing process that requires development. Once the green body has been removed from the build platform and cleaned, the organic material, which accounts for up to 50 % of the total volume, must be burnt out. In order not to damage the component, the debinding curve must be well matched to the corresponding additives. Finding the right combination of as fast as possible and as gentle as necessary is the crucial point here. A TGA/DSC or dilatometer measurement can provide good indications of the temperature ranges in which dwell times need to be incorporated. With our cerium oxide components, for example, debinding currently takes about a week.
The sintering process can only begin once the components have survived the debinding process without cracking. As with other additive manufacturing processes, the sintering temperature depends on the material and the grain size – in the case of our cerium oxide, for example, this is between 1600°C and 1650°C.
Anyone who visited our booth at Formnext 2023 will have already seen the terracotta-coloured components made of CeO2. The first SEM images of our material have now also been added.
