Just a few years ago, 3Dprinting was stillaniche product. Today, it has long been relevant to the market. The marketvolume for 2024 is estimated at 27.52 billion US dollars, with an annual growth rate of 23.6% forecast until 2032. However, despite the rapid increase, there are limits, for example in terms of production speed or printing accuracy. The electrophotographic 3D printing process at TH Köln could overcome these limitations.
In an interview with K-Mag, Prof. Dr. Danka Katrakova-Krüger explains the advantages of this process and which industries could benefit from it.
From idea to implementation: the path to an innovative printing process
Image: Prof. Dr. Danka Katrakova-Krüger; Copyright: TH Köln
Prof. Dr. Danka Katrakova-Krüger, Laboratory for Materials at the Institute for General Mechanical Engineering, Cologne University of Applied Sciences. Copyright: TH Köln
How did the idea for the research project come about and what were the most important goals of the project?
Prof. Dr. Danka Katrakova-Krüger: Our company partner mz Toner already had experience in “flat” printing in the field of ceramic decoration. However, in order to obtain three-dimensional objects from this, an additional step was required in which the previously printed (multiple) layers are stacked and bonded together. Toners made of suitable polymer materials had to be developed for this purpose. The process has great potential for application, for example for spare parts in space where there is no or low gravity, as no loose powder is used, but the particle sizes are such that good mechanical properties can be achieved.
Can you explain the printing process used in your method in more detail?
Katrakova-Krüger: The new electrophotographic 3D printing process works as follows: First, an electrical charge image is selectively generated on a charged photo drum, in a similar way to laser printing. The toner mixture accumulates at the points of the charge image, creating a layer of the image. The polymer particles of this layer are then transferred from the photo drum to the transfer medium and pre-fixed. In a separate machine, the individual layers of the transfer medium are superimposed and fixed to create the finished three-dimensional component.
Image: Schematic representation of the electrophotographic 3D printing process, in which plastic particles are selectively applied by a charged photo drum; Copyright: mz Toner Technologies
Figure 1: Electrical charge image is generated. Copyright: mz Toner Technologies
Image: Diagram of the two-step process in electrophotographic 3D printing, showing the transfer and fixation of the plastic layers; Copyright: mz Toner Technologies
Figure 2: Layers are transferred and fixed by the transfer medium. Copyright: mz Toner Technologies
Challenges and solutions in the development process
hat particular challenges had to be overcome in the development of this new 3D printing process?
Katrakova-Krüger: There were essentially three hurdles that had to be overcome: toner, transfer medium and stamping machine. During development, particular focus was placed on the toners, which are a complex mixture of material to be printed, in our case polymer, carrier material and many other additives in the right quantity and morphology. Setting the triboelectric charge is also a demanding task. The next challenge was to find a suitable transfer medium. On the one hand, this must ensure good adhesion of the printed layers and, at the same time, enable good release during the stamping process. There were various aspects to consider during stamping, in particular positioning accuracy, pressure distribution and temperature in the installation space. The optimum process windows for the different polymers then had to be determined.
Comparable material properties and competitive advantages
Image: 3D-printed plastic tensile specimens, prepared for mechanical tests to determine material properties; Copyright: mz Toner Technologies
Printed tensile samples on transfer medium. Copyright: mz Toner Technologies
What material properties and mechanical characteristics do the printed components have, and how do they compare with components from other manufacturing processes such as the fused layer process or injection molding?
Katrakova-Krüger: An electrophotographically printed tension rod made of ABS has a tensile strength of around 1900 MPa, a modulus of elasticity of around 40 MPa and an elongation at break of just over 2 percent. This means that our tensile strength is 25 percent higher than that of FDM (1627 MPa) and 20 percent lower than that of injection molding (2300 MPa). The modulus of elasticity of our sample is 43% higher than that of FDM (22 MPa) and only 12% lower than that of injection molding (44 MPa) for the same material. The elongation at break is similar to FDM at around 3%, but significantly lower than the values achieved in injection molding (12%). However, we are continuously working on further improving the mechanical properties.
What advantages does this process offer, for example for industrial applications, compared to other 3D printing processes?
Katrakova-Krüger: Thanks to the high printing speed of electrophotography and the two-step process, high production rates can be achieved. The use of small polymer particles enables a high print resolution. It is also possible to process different materials in one component and produce several parts at the same time. As the process is not dependent on gravity, an application in space would also be possible.
What is the current throughput of the developed 3D printing prototype and what factors influence the number of workpieces that can be produced?
Katrakova-Krüger: The maximum build-up rate depends on the 2D printer size and the number of stamping machines. In the 2D area, printing speeds of up to 5 m² per minute are currently possible. It is possible to print several layers of a component or to print different components at the same time. Up to 8 different toner materials per layer can be printed simultaneously. The overall production speed increases the more transfer units work in parallel. In principle, the process is able to compete with injection molding in terms of quantities.
Industrial applications and potential for the future
Image: Close-up of a 3D-printed plastic tension rod, used to measure tensile strength and other material properties; Copyright: TH Köln
Finished tension rod (here ABS). Copyright: TH Köln
What role could this technology play in the future of additive manufacturing and which industries could benefit in particular?
Katrakova-Krüger: Electrophotographic 3D printing is a serious competitor to injection molding. In contrast to classic injection molding, this process does not require any specially manufactured, expensive tools. Therefore, different components can be produced with the same machine setup without any retooling. This leads to savings in storage, manufacturing and retooling costs and enables full automation of the process in line with Industry 4.0. In addition, the process is currently being extended to other materials, meaning that electrophotographic 3D printing should also be available for metallic and ceramic materials in the long term. Many areas of application are conceivable: in aerospace, which triggered the development, but also in electronics, for solar cells, batteries, and why not also for consumer goods such as household electrical appliances – there would be no limits to material and color combinations.
