At Mecklenburger Metallguss (MMG) in Waren, ship propellers for large cargo and cruise ships with a diameter of up to twelve meters are manufactured using a complex casting process. These propellers are usually designed and optimized directly for the respective ship. For this reason, the batch sizes are very small, sometimes down to one-off production. Today, the entire production process, both in the foundry and in the associated mechanical processing, is characterized by a particularly high proportion of manual work.

The potential to stand out from the competition through efficient production and superior product quality is very limited with manual production. At the same time, competition in the shipbuilding industry is increasing due to the fact that demand for ships has fallen dramatically in recent years, particularly in 2016. In order to survive in this tight market, suppliers must operate globally, respond flexibly to customer requirements and offer first-class product quality.

At present, a negative mold made of sand concrete is produced by repeatedly molding a conventional positive mold made of wood. The positive molds milled from wood have a very long production and delivery time and no longer allow for adjustments once the order for production has been placed. One approach to shortening the long production times and meeting special customer requirements is to make the production of casting molds more flexible and faster by using generative processes.

On the one hand, this is due to the production of the actual negative mold as well as the positive mold. A generative process could address precisely these disadvantages and give the foundry a decisive advantage over the competition. Existing methods for the additive manufacturing of sand casting molds are not suitable due to the special requirements of the casting process. Furthermore, 3D printers available on the market neither offer the necessary workspaces nor are they optimized for high material output.

This gave rise to the development of a large-volume 3D printer together with MMG, which covers a working area of two by two by two meters and thus makes it possible to generatively produce positive molds for the casting of ship propellers with a diameter of up to four meters, which are to be used in the yacht sector in particular.

Using the casting manufacturing process, accuracies of between 100 and 1,000 micrometers are achieved in the production of ship propellers. In comparison, the achievable accuracies for milling are ten to 100 micrometers and for sintering even two to ten micrometers.

The accuracy requirements for the mold can therefore be considered comparatively low, meaning that cost-intensive precision linear technology can be largely dispensed with. The focus is on the longevity of the drive system as well as the rigidity and dimensional accuracy of the test setup.

The design of the frame structure of the large-format 3D printer corresponds to a cube with a non-moving print bed and is equipped with highly rigid aluminum profiles and additional stiffening elements to withstand the loads, especially jerky changes in direction of the extruder during the printing process. Due to the very rigid basic structure and the desired parallelism of the axes, it is possible to operate the X and Y axes with only one motor each in combination with guide rails. The deviations between the target and actual contour of the component to be printed in the X and Y directions are only a few 1/10 millimeters. The Z-axis, on the other hand, has two motors, each with running rails attached to the base frame, which moves the extruder together with the X- and Y-axes. Due to the motor arrangement on two of the four sides of the cube, the absolute accuracy in the Z direction is only one millimeter, depending on the extruder position.

The extruder used processes thermoplastic in the form of filament and has a one-millimeter nozzle, which makes it possible to melt six times the volume of material compared to a standard 0.4-millimeter nozzle. This ratio is also reflected in the production time of a propeller to be manufactured. This means that the production time is reduced as the nozzle diameter increases.

By selecting a defined layer height of 0.5 millimetres and a process speed of around 60 millimetres per second, depending on the material, it is possible to produce a propeller blade for the manufacture of a positive mould in around 200 hours. The current production process for a positive mold made of wood takes around one month. Further advantages of the molds made of thermoplastic material are their recyclability and the savings in storage costs, as the molds can be melted down again and processed into new filament once a ship's propeller has been completed. This saves resources and protects the environment.

Klötzer Fh-IGP, Lauer Fh-IGP, Dryba Fh-IGP, Flügge LS Fertigungstechnik UR/as

Fraunhofer Research Institution for Large Structures in Production Engineering IGP

Briefly explained:
The Fh-IGP in cooperation with the LS Manufacturing and Joining Technology at the University of Rostock
The Fraunhofer Research Institution for Large Structures in Production Engineering IGP, based in Rostock, is one of the 72 institutes of the Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V., Munich. Research at the Fh-IGP focuses on the production and manufacture of large structures. On the basis of applied research, concepts for product and process innovations for future industries such as shipbuilding and steel construction, energy and environmental technology, rail and commercial vehicle construction as well as mechanical and plant engineering are developed and realized within the framework of various projects with cooperation partners. As part of a cooperation agreement, the Fh-IGP works together with the chairs of Production Engineering and Joining Technology at the Faculty of Mechanical Engineering and Marine Technology at the University of Rostock and is a member of the Fraunhofer Transport Alliance and various research associations and networks.

Scientific Society for Assembly, Handling and Industrial Robotics (MHI e.V.) © MHI

Briefly explained:
The MHI e.V.
The Wissenschaftliche Gesellschaft für Montage, Handhabung und Industrierobotik e.V. (MHI e.V.) is a network of renowned university professors - institute directors and chair holders - from German-speaking countries. The members conduct both fundamental and application-oriented research on a wide range of current topics in the fields of assembly, handling and industrial robotics. Further information on the society, its members and activities: www.wgmhi.de.