# Function-Based Design Principles for Additive Manufacturing
## 5 Discussion
Neither heuristics nor crowdsourced DPs provide a means for systematic application of AMK, but rather designers must go through each heuristic or DP and evaluate if they can be applied in their context. On the other hand, the presented DPs provide a systematic search and application of AMK through the FFDP matrix. When a designer is looking at how to solve a particular function, the FFDP matrix narrows down the entire list of DPs and provides only DPs applicable for the case. Finally, the three approaches involve different scopes of AMK. The crowdsourced DPs provide the broadest AMK, which, besides design solutions, provide guidance for conducting and improving the design process and guidance for detail design to ensure the printability of created designs. On the other hand, AM design heuristics only contain knowledge about conceiving and improving design solutions, and in that view, the derived DPs are very similar to heuristics. However, they contain only design knowledge that can be related to functions, and do not refer to design requirements (e.g., recyclability, weight, or aesthetics) in the manner of heuristics. The comparison of DPs with similar AMK sources provides an initial validation of the derived DPs. However, this research has two main limitations that must be discussed: the scope of the gathered AMK and the verification and validation. DPs were derived through an inductive approach and the analysis of existing AM products. The analysis was carried out until the number of derived DPs converged to a finite number. In our case, this occurred after analysing 40 products. According to [PERSON] et al. [27], inductive approaches based on the analysis of products and asymptotic convergence have a sample size that goes from as low as 3 products up to 190 products. The conducted analysis falls into that scope and is similar to [PERSON] et al. [11], who reported 80 observations needed for deriving 90% of their crowdsourced AM DPs. These similarities and asymptotic convergence provide confidence that the derived set of function-based AM DPs covers a broad scope of AMK. However, it must be emphasised this is not a closed set of DPs, as the analysis included a few specific AM domains. If new products and AM domains are included in the analysis, new DPs could emerge. Furthermore, as DPs cover the current state of AM, with further development of AM technologies, but also with the new and creative application of AM to solve product functions, it is probable that new DPs will emerge. Another limitation of the study is the verification and validation of the DPs. If the four-step procedure for validation of early-phase DfAM design supports is followed [40] the presented approach is only validated through two steps. Firstly, a case study needed for verifying the feasibility of the DPs was conducted. It showed the DPs were a suitable tool for searching for AM-based solutions through the function structure of a product, and enabled the creation of concepts that utilised the unique design possibilities of AM. Secondly, the DPs were compared with other similar validated DfAM approaches. The observed similarities provide an initial validation of the DPs. However, for complete validation, controlled validation and workshop validation are needed. They are planned for future research, but at the moment, they are outside of the scope of this paper.