FDM is a filament-based extrusion 3D printing method that is the most widely used. In the FDM process, an object is built by selectively depositing melted material in a pre-determined path layer-by-layer. In this method, each new layer must be supported by the layer beneath it. In FDM, the primary load on the printed part is the gravitational force of its own. If your model has an overhang which is not supported by anything below, you may need to add additional 3D printing support structures to ensure a successful print. However, depending on the material properties, re-orienting the model can significantly reduce the number of supports (Figure 5.5b).

The printable threshold overhang angle (PTOA) is defined to the smallest angle between the ground and the overhang that can be fabricated without printing support structures beneath the overhang. As PTOA decreases, the overhang is more susceptible to the load due to gravity, and more adhesion inside the material is needed. Therefore, PTOA is related to the density of the material, and the stiffness of the material. [1] If a smaller PTOA is wanted, the material should have a lower density and a higher stiffness. (If you want more information on the modeling of PTOA, please refer to [1]).

Question

You are to print an iPad holder shown in Figure 5.5c. Here, the available categories of FDM printing material in are ABS, PLA and Polycarbonate. Different from the beam-shaped support in the SLA process, the support added in the FDM process usually occupies the entire space under the overhang, which uses a significant amount of printing material and are often difficult to remove. A structure that can be printed without any support is preferred in an acutal design. Therefore, identifying the PTOA for an overhang structure is important for 3D printing planning. Here, please determine which material of the three given above can tolerate the smallest PTOA ???? without needing supports. Why? Provide your reasoning and work using the mechanics of material knowledge we learned in class.