Casting the turbine wheel
A joint aid project involving voxeljet looked to 3D printing technology to fix a defective wind turbine wheel at a small hospital in Walga, Ethiopia.
Collaborators from Germany and Switzerland put their heads together to resolve the issue led by Swiss-based Turbal - a medium-sized family-run firm with half a century of experience in constructing turbines.
Augsburg-based 3D printing specialist voxeljet played a pivotal role in the project and was supported by the expertise of solar and heat technology specialist H Lenz from Niederuzwil, in addition to the steel foundry Wolfensberger from Bauma.
The task was to find a way to reconstruct the turbine wheel by using innovative techniques. The conventional production of wheels is a very cumbersome, labour-intensive and expensive process because it requires the manual production of several sand core segments and complicated undercuts.
However, voxeljet's advanced 3D printing technology provided an elegant, rapid and cost-effective solution.
Chief Executive Officer at voxeljet Dr Ingo Ederer stated: "Once we heard about the problem, we immediately decided to help. Our 3D print technology is virtually predestined for cases such as these."
He added: "The state-of-the-art printing equipment at our service centre allows for the economical and tool-less production of high-quality sand moulds and cores for metal casting based on CAD data, even for a batch size of one. Producing a sand core for the wheel is a routine undertaking for us."
Once the required CAD data for the wheel was available, voxeljet began the process of producing the flow-carrying interior for the wheel. In this instance, the monoblock sand core - which is created on a fully-automated basis using 3D printing - replaces many manually-produced core segments that are strung together. In contrast to the conventional manufacture of moulds - in which the production of model plates or core boxes alone can take several weeks - 3D printing makes it possible to print smaller sand moulds in as little as a few hours.
The 250 mm mould for the wheel was built in layers over a period of almost five hours. Precision is considered a factor in this process - an accuracy of 0.2 mm in the X and Y direction is setting new trends. In principle, voxeljet's large-format printers can produce moulds of particle material in dimensions of 4 x 2 x 1 m.
The moulds are created without cumbersome and expensive mould setups, and are produced in a fully-automated process purely based on CAD data using the layer building method, which consists of the repeated application of 300 μm-thick quartz sand layers that are selectively glued together with a binder using the print head of the system.
This method offers enormous production-related advantages that affect quality, production targets and profitability equally. In this case, 3D printing resulted in higher component accuracy, a significant reduction in subsequent machining as well as an excellent surface quality and contour precision.
After the printing process is complete, the mould only has to be unpacked and cleaned of excess sand - and it is finished. The simple and easy handling of the moulds means that they can be transported to the foundry without complications. Due to their low binder content of 1.2 per cent and a pressure of 220N/cm², the sand cores are comparable to those derived from conventional series production. The moulds can be used immediately for further processing and are ready for casting.
Dr Ederer explained: "In this case, we decided on a combination of a 3D-printed sand core for the complicated turbine geometry and a conventionally produced exterior mould. This means that we use the advantage of 3D printing where it pays off the most - for the production of the complicated interior.
"Instead of many individual core segments which are strung together, the mono sand core impresses with higher component accuracy, smaller tolerances and fewer cleaning requirements, and does away with the need for many core separation devices."
In many cases, it pays to produce both the interior as well as the exterior mould using the 3D printing method. In the final analysis, it is a question of unit numbers and individual circumstances that will determine how the exterior mould should be produced.
The Wolfensberger steel foundry, which is growing increasingly reliant on voxeljet sand moulds for the production of complex cast part prototypes, was again very impressed by the quality of the 3D prints for this project. The cast also impressed Turbal AG.
The 'help for self-help' project by the participating companies successfully restored the production of electricity at the clinic in Ethiopia.
"In our opinion, non-bureaucratic direct-help projects such as these are very effective and also ensure that help goes exactly where it is needed. voxeljet was pleased to be able to make a meaningful contribution to a meaningful project", concluded Dr Ederer.
