## 6 Discussion
### Limits and Future Developments  
The main content of this paper is to put forward the concept of 3 DIM, study the construction method of 3 DIM and develop the corresponding 3D interpreted model construction tool. Although the method and tool achieve the goals of evolving the product definition model and improving the quality and capability of product model, there are still some limits:The proposed 3 DIM integrates and fuses four types of design knowledge including design intent, design justification, design decision and design option, and supports design knowledge in different carrier forms such as pictorial, symbolic, linguistic, virtual and algorithmic. However, this division of design knowledge has certain universality. For different enterprises and products, the specific contents of design knowledge are different. For example, for consumer electronic products, the design intent mainly considers the needs of users, while for industrial equipment, the design intent needs to consider the functional performance indicators of products. Therefore, for different enterprises and products, the design knowledge division of 3 DIM needs to be sorted out more finely to support the implementation of 3 DIM;
2. The construction process of 3 DIM includes design knowledge annotation and information extraction and knowledge fusion. The design knowledge annotation process mainly depends on the designer to select which knowledge to annotate. On the one hand, this process has a certain subjectivity, which affects the effect of knowledge annotation. On the other hand, the annotation process requires a certain amount of human and material resources and increases the workload of designer. To solve this problem, it is necessary to apply the idea of knowledge management to the process of design knowledge annotation, so as to provide some automation means such as design knowledge query, retrieval. Thus the time and cost caused by the process of design knowledge annotation can be reduced. In the aspect of information extraction and fusion algorithm, its main limitation is that the algorithm is only suitable for dealing with STEP as the neutral file format, but cannot deal with IGES, STL and other neutral file formats. To solve this problem, we need to analyze and compare the common neutral formats, research a more general information extraction and knowledge fusion algorithm, and adopt different strategies to adapt different types of neutral files in the specific implementation process, so as to make the application scope of the algorithm wider;
3. The 3D interpreted model construction tool developed in this paper is implemented based on the CATIA system, so it cannot be directly transplanted to UG NX, CREO and other CAD systems. In fact, the heterogeneous problem of CAD system is a very common and difficult. The best solution is to separate the design knowledge annotation module from the CAD system to form an independent tool. However, the independent tools need a lot of software development work to realize the basic functions such as CAD modeling and MBD annotation. For scientific research and engineering application, an acceptable method should be to develop corresponding 3D interpreted model construction tools for different CAD systems, which is also the main measure we plan to solve this problem.