## 5 Conclusions  
According to the results of this research, the following conclusions were obtained:  
1. With the actual experimental facilities and electronic devices, 64.6% of PP scraps, 61.7% of ABS scraps, and 63.9% of SMA scraps in mass respectively were successfully characterized, with diameters from 14-23 mm. The feeding velocity of the tested scraps was 1-2 pieces/s.
2. The larger the variation of the material coefficient \(k_{\mathrm{C}}\), the higher the efficiency of characterization/separation, and _vice versa_. The value of \(k_{\mathrm{C}}\) depends only on material mechanical properties.
3. Fitting curves demonstrated that scraps with lower diameters were expected to be more easily recognized and separated than scraps with larger diameters. However, the impact eigen-frequency response of smaller scrap was located beyond the hearing threshold. In this case, a professional AE signal acquisition device was necessary.
4. Characterization and separation by using impact AE completely avoided the influence of black, dark or color dyes, as well as the supporting additives in vehicle plastics, which caused great difficulty in other sorting methods.
5. Different automobile enterprises have their own formulas for dying, modification, and reinforcement on vehicle plastics. By way of commercial confidentiality, the compositions of plastic materials are difficult to obtain from manufacturers, even with the aim of recycling. The method of using impact AE recognition could avoid this problem, since it concentrates only on the mechanical properties of vehicle plastics, which are normally open in design.  
6. With the actual experiment facilities and devices, not all impact AE signals were available for analysis. More convenient facilities and sensitive signal acquisition devices are necessary. The crushing process and facilities also need to be modified, in order to increase the scrap proportions with the appropriate particle size.
7. This research concentrated on sorting of plastic scraps with flake or quasi-flake structures. Although this shape of plastic scraps makes up the vast majority, the development of an impact frequency response model for other scrap shapes is also necessary in subsequent research.  
Generally, this research verified the feasibility of sorting ASR plastic materials by using impact acoustic emission processing and characterization. This method can be an appropriate solution for optimization of existing ELV plastics' recycling technologies. Further in-depth research in signal processing and the elastic collision model are required. Additionally, this feasibility study should be further extended to other kinds of vehicle plastics.  
This research was financially supported by projects \"China Fundamental Research Funds for the Central Universities 2015 QNA28\" and \"The Funds for Creative Research Groups of China 51421003\", The contribution of [PERSON] and colleagues in department of processing and recycling, RWTH Aachen University in Germany were highly appreciated by the authors.  
The author [PERSON] contributed research and experimental work in this study. And the authors [PERSON] and [PERSON] contributed data analysis work and revision of this paper.  
The authors declare no conflict of interest.