Damage around screw holes seems a common issue. It could have occurred due to a combination of factors. Screw holes create points of stress concentration in the chassis. When a screw is tightened, the material around the hole experiences higher stress than other areas of the chassis. In 3D-printed parts, this is exacerbated by the fact that the material is often not as strong as traditionally manufactured materials. In fact, sharp edges and corners around screw holes can act as stress risers, where cracks can initiate and propagate under load or vibration.
Because 3D printing techniques such as FDM builds parts layer by layer, it can create weak points between layers (layer adhesion). The bonding between layers is often weaker than the material itself, making these areas more prone to cracking under stress, especially around high-stress areas like screw holes. 3D-printed parts are mostly anisotropic, meaning they have different strength properties in different directions. The layer orientation relative to the screw holes can lead to weaker bonds and increased susceptibility to damage.
When screws are tightened, especially metal screws in plastic parts, heat can be generated due to friction. This heat can cause local expansion of the material, followed by contraction when it cools down, potentially leading to cracking. And when the screw material has a different thermal expansion coefficient than the 3D-printed material (this is almost always the case since the screws are metal while the 3 printed parts are non-metal), thermal cycling in the form of heating and cooling can cause stress around the screw holes.
The use of captive nuts may be able to reduce the stress on the 3D-printed material itself, but additional space will be required to accomodate the nuts.
