# Geometrical Degrees of Freedom for Cellular Structures Generation: A New Classification Paradigm
## 3 Highlights of Manufacturability Challenges of Cellular Structures  
One of the main challenges in AM is that the manufacturability of components is at the intersection of the design space, material properties and process physics, where the interactions are not fully understood. Furthermore, the effect of the process parameters on the manufacturability of lattices depends on the AM technique used. This section presents the set of design limitations relevant to each technology. The parameters that affect manufacturability can be divided into two categories: the lattice design parameters and parameters related to the AM process. For the lattice design, this includes the size of the RVEs (unit cells) and the primitive properties (the strut angles and diameters), which is typically be dictated based on the resolution of the AM technology (design guidelines) and experience with each material and machine (design rules). For AM parameters, each technology has a specific set of inputs; such brief examples are the scanning speed, in the case of powder bed processes, or the extrusion speed in the case of material extrusion processes. In the context of this work, manufacturability refers to the capacity to achieve three things: dimensional fidelity, low surface roughness, and low porosity defects. It is important to have a firm understanding of those elements to be able to determine how the mechanical properties, among others, will be affected. Table 2 summarizes the various design constraints with example works across the more common AM fabrication techniques [114]. Mechanical properties will not be explored in this context since such properties are dependent on the material used. One design limitation is the need to remove unused material from inside the lattice structure. For the powder bed AM process such as Laser Powder Bed Fusion (LPBF) or Selective Laser Melting (SLM), Selective Laser Sintering (SLS), Binder Jetting (BJ), and Electron Beam (EBM) powder bed fusion, the lattice inner voids need to be large enough to allow powder to flow freely and to exit the volume during de-powdering. [PERSON] and [PERSON] [115] mention a minimum size requirement for SLM and SLS, although such requirement is likely dependent on the powder size distribution and material rheological properties. For stereolithography (SLA), the unused resin flows freely out and only requires an exit opening if the lattice is fully enclosed. For Fused Deposition Modeling (FDM) and Material Jetting (MJ), internal material removal requirements are largely driven by the need to remove support structures via mechanical or chemical processes, if such support structures are present. If the lattice structures contain overhangs as defined by a technology class-specific threshold angle, the overhanging features need to be supported through the generation of expendable support structures. This can be mitigated either by changing the lattice design, or by modifying the build orientation of the product. For multi-material capable systems like FDM or MJ, support structures can be printed out of dissolvable materials. For FDM, all overhang needs to be supported due to gravity. [PERSON] and [PERSON] [115] and [PERSON] et al. [116] illustrate a few design rules for FDM. Support structures are also necessary for LPBF and EBM. For LPBF, support structures act as thermal dissipation structures, to prevent warping and material vaporization due to the accumulation of residual stress and thermal insulating properties of powders [117], respectively. [PERSON] and [PERSON] [115] and [PERSON] et al. [118] show a few design guidelines regarding overhangs and slopes for LPBF. For EBM, support structures are technically only necessary for anchoring components to the build plate, as the powder around the part is sintered by the process into a powder cake. The powder cake provides enough mechanical support and heat dissipation properties for the object [117].