A structural and 3D printing analysis of a design entry in the Grabcad NASA challenge.

This part is designed to be 3D printed in space. It is to be used as a complementary Handrail Clamp Assembly (HCA) which is currently utilized by astronauts to provide rigid mounting locations in the International Space Station.

The original, full design can be found on the Grabcad website here. The design was made by Alberto Beretta, you can find his company website here.

We analysed the most important section of the design, the snapping fixation clip, to be able to provide some feedback to the design for future design improvements. We performed a non-linear finite element structural analysis, using simulation software, with a focus on finding the points in the snap-clip that may suffer from high stress. If the stresses exceed the material capabilities, then we can expect damage in these stress regions.

 

 

The material that is specified for this 3D printing operation is ABS. Typical, unfilled ABS used in 3D printers has an ultimate tensile strength (UTS) of about 41MPa. This is the tensile stress that the material can withstand before it ruptures (breaks). The von Mises stress is calculated within the volume of the part to give an indication of where yielding may occur. The simulations indicated a maximum von Mises stress of around 85MPa, more than double the UTS, so we have a pretty reliable indication that the material will break at some point. By creating a threshold, as can be seen in the film above, we can see the zone of the part that is being subjected to stresses above the UTS of 41MPa. These are the zones that will most likely break during the insertion of the clip into its holder. What is also noticeable is the displacement or deformation of the part. We see that the ridge of the receiving end (on the right) is bending upwards to receive the clip (on the left), even though an attempt has been made to design the clip so that it should bend down to be able to clip into place.

This design could be improved, by, for example, reducing the width of the clip or designing the clip with thinner walls or less support, so that the clip bends and not the receiver.

Seeing as this part is designed to be 3D printed on the international space station using an additive printer, we also performed a review of the design. Using our extensive knowledge in this type of 3D printing, we can assess some of the issues that may occur by using this kind of manufacturing technique. The part has been designed with many overhangs, engraved lettering and features and small edge rounds or chamfers.

Overhangs are difficult to print without using a printer that is capable of generating support material. Support material increases the overall print time and requires finishing after the print is complete, to remove the excess material.

Engraved lettering and features also often require support material, but also results in longer printing paths and increased printing time.

Edge rounds and chamfers are usually fine features that cannot be printed with enough resolution using layer heights of 0.25 or 0.3mm (typical for small FDM printers –¬†Fused Deposition Modelling).

This design could be improved for 3D printing by removing the above mentioned features, if possible and also ensuring that the printing direction is carefully considered. The part should always be orientated so that the layer-by-layer direction reduces overhangs. A typical example is a cup or mug. The cup should be printed with its base as the first layer of the print. If the cup is printed with its rim as the first layer, then the base will be printed last and will be “floating” without support.

This part needs to be readdressed in both structural and manufacturing aspects. The part of the clip that is intended for deformation, does not deform as required, generating too much stress on the receiving end of the part, probably ending in catastrophic failure. The part could also be optimised for 3D printing by following some typical guidelines.
  • Project Type: Strucural Simulation and 3D Printing Analysis
  • Tools Used: Simscale
  • Project Date: January 2015