A multi-hardness concept for multipurpose printed parts

Sirris can 3D print TPU by SLS, creating different hardness zones, either by concentrating on their internal structure or on sintering parameters.

SLS (Selective Laser Sintering) is a rapidly developing additive manufacturing process involving a powder bed, increasingly applied for the production of finished parts. Although it essentially involves polyamides, including PA12, over the past 3 to 4 years, other thermoplastics have come to enhance the range of available materials.
Thanks to an SLS machine with adaptable control parameters, Sirris can produce via additive manufacturing, not only using polyamide but also other materials such as polyurethane, polypropylene, etc.

Thermoplastic polyurethane (TPU) offers interesting prospects thanks to its flexibility at room temperature. It also offers new possibilities in terms of design (shoe soles, flexible connectors, shock-protection elements). TPU’s Shore hardness values range from 70 to 90 ShA. Implementation is relatively simple and the majority of service providers propose these materials to their clients.

However, TPU part functionalities can also be improved by applying the 'multi-hardness' concept. One single part can comprise different zones of varying hardness to satisfy a range of purposes. There are two possible approaches: working on energy density during powder sintering or integrating lattice structures within the parts.

The first approach requires command of the process’s laser parameters: laser energy, scanning speed and hashing interval. This approach enables varying hardnesses to be acquired, but on a relatively small scale, from 70 to 90 ShA. Furthermore, an excessively low energy density may deteriorate the part’s mechanical properties (delamination, etc.).
The second approach is not associated with the process but with the part’s basic geometry. By integrating lattice structures within the part, apparent hardness can be reduced to 40-50 ShA.

By combining machine parameter control and the integration of lattice structures in parts, Sirris can innovate in terms of part design by, in particular, drawing advantage from the introduction of hardness gradients.

The example below illustrates a multipurpose 'multi-hardness' part produced by Sirris. It is a push scooter wheel, produced as a single part comprising different zones:

  • The wheel rim has a hardness value of 90 ShA.
  • The tyre, comprised of:
    • an internal lattice structure area
    • an external skin with a hardness of 80 ShA

This work was conducted within the framework of the IAWATHA ERDF project.