When components are not specifically designed for SLM (like 90% of current products), supports are critical to ensure fabrication success as well as high and efficient production yield. Optimising build orientation is the 1st step to minimise the amount of supports by finding. Choosing the most appropriate type of support is the 2nd step. These supports need to be strong yet easy to remove while using minimal material.
Supports
A support structure is composed of two functional areas: the main support itself and the teeth. The teeth are the connections between the main support and the part. Their role is to minimise the contact area (and facilitate removal), act as efficient localised mini heat-sinks (for improved surface roughness) and fix the 1st overhangs layers in place (secure against warping).
Various types of structure geometries [4].
The main peg must be strong enough to withstand upper vertical weights, horizontal recoating and thermal stresses [1]. Teeth design can be fine tuned so as to define top length, based interval between teeth and teeth height: to close, and the teeth degrade the surface roughness; too far and they become too weak to be efficient.
There are various geometries available for the main support. A point support is used for very small features, a web support for circular areas, a line support for narrow down facing areas. The most widely (and successfully) used is the block structure. Block supports are applied for large overhangs. Their design is based on a grid of lines fabricated along the x and y directions (in the substrate plane). The distance between lines, ie hatching, can be varied.
Typical support teeth [4].
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To facilitate supports removal, each block can in turn be fragmented and the pegs (main support) can be perforated with diamond or rectangular shapes [2,3].
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Sacrificial geometries
A few commercially available software automatise the generation of supports (Magics,…). Their proprietary algorithms identify surfaces inclined over a user-defined angle threshold to build supports reaching towards the platform. However, it is sometimes more sensible to actually add sacrificial geometries: they will limit the amount of supports structure required or the time involved to remove them. A typical sacrificial geometry is, for instance, a platform oriented at 45deg incidence with solid supports at each extremity. Something that would look similar to an extruded ‘V’ letter and that can generally be wire-cut quickly.
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This is especially viable when some very tall block supports are required. They are naturally prone to distortion (warping) and tend to increase the risk of unrecoverable fabrication failure.
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Teeth structure [4].
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To sum up
Considerations such as build success probability, operator time and skill levels, all need to be taken into account when dealing with supporting structure. The quantity of supports impacts the long term commercial viability of the process. Teeth influence surface roughness. It is critical to build the component at an optimised angle to minimise the amount supports (hence material) or the post processing time. Using the right type of structure, most frequently fragmented blocks with hollows trunks and strong teeth, is also very important. Where components are not suitably (re)designed for selective laser melting, supports structure play a significant role in fabrication success and commercial viability.
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References
[1] Venuvinod PK, Ma W. Rapid prototyping – laser-based and other technologies. Kluwer Academic Publishers 2003. ISBN: 1-4020-7577-4.
[2] Pauwels J, Swaelens B, Vancraen W. Method for supporting an object made by means of stereolithography or another rapid prototype production method. Patent US5595703 A; 1997.
[3] Venuvinod PK, Ma W. Rapid prototyping: laser-based and other technologies. US: Springer; 2004.
[4] F. Calignano, “Design optimization of supports for overhanging structures in aluminum and titanium alloys by selective laser melting,” Mater. Des., vol. 64, pp. 203–213, Dec. 2014.
[1] Venuvinod PK, Ma W. Rapid prototyping – laser-based and other technologies. Kluwer Academic Publishers 2003. ISBN: 1-4020-7577-4.
[2] Pauwels J, Swaelens B, Vancraen W. Method for supporting an object made by means of stereolithography or another rapid prototype production method. Patent US5595703 A; 1997.
[3] Venuvinod PK, Ma W. Rapid prototyping: laser-based and other technologies. US: Springer; 2004.
[4] F. Calignano, “Design optimization of supports for overhanging structures in aluminum and titanium alloys by selective laser melting,” Mater. Des., vol. 64, pp. 203–213, Dec. 2014.