STUDY OF THE IMPACT OF THE RAPID PROTOTYPING METHOD ON THE PERFORMANCES OF A DESIGN PROCESS

Transcription

1 STUDY OF THE IMPACT OF THE RAPID PROTOTYPING METHOD ON THE PERFORMANCES OF A DESIGN PROCESS Daniel-Constantin Anghel, Nadia Belu University of Pitesti, Romania KEYWORDS Rapid prototyping, DSM, design experiment, impact study ABSTRACT - The level of product quality is established by means of constructive technological solutions adopted by designer during the design stage. In order to realize the product which responds to diverse customer requirements, the designers use a lot of methods and means. One of them is the prototyping method. The designers use the prototype to propose, to testing and to develop their theories and solutions. This paper presents a study about the impact of using the rapid prototyping method on the performance of a design process, for an automotive product. The study was conducted to University of Pitesti in the Automotive Engineering Research Center. Two design experiments were conducted for the same product. The first experiment used conventional methods of prototyping and the second used the rapid prototyping. We followed the evolution of parameters: cost, duration, number of iterations needed to quantify the impact of the two used methods. To represent and to analyze the performance of studied experiments there were used instruments as: DSM (Design Structure Matrix), Gantt, a graphical representation tool based on the model proposed by Pahl and Beitz. INTRODUCTION Product design is a process dynamic and complex. It can be defined as the idea creation, concept development, prototyping, testing and manufacturing of an artifact. The designers conceptualize and evaluate ideas, making them tangible through products in a more systematic approach. A prescriptive model of design process was proposed by Pahl and Beitz (2). It is almost impossible to have a once through execution of tasks in a design process. One or more repetition of the tasks is necessary to obtain the expected results. These repetitions are called iterations. Iteration is an activity that occurs in all design projects. Iterations are fundamental for the design process and several authors underlined their importance; however this subject is still not well tackled in the literature. Iteration can be defined as the repetition of design tasks in order to improve the design solution being performed. 155

2 Planning and clarifying the task Conceptual design Embodiment design Detail design ARTEFACT Figure 1. Pahl and Beitz s model of design Figure 2 shows an iteration process between three design tasks. In this example, task B needs the result of task A, task C needs the one of task B, and task A the one provided by task "C". This type of design problems could only be resolved through an iteration process, which starts with a rough or a preliminary information of one of the results x, y or z. Figure 2. Design iteration Pahl and Beitz, in their work (2) define iterations as a process by which a solution is approximated step by step. Smith et al. (3) state that design iteration is the repetition of design tasks due to the arrival or discovery of new information. Yassine et al. (5) affirm that iteration is a typical characteristic of any complex engineering development project due to the coupling and interdependency between the development tasks. Osborne (1) has observed that iteration is a significant component of the product development cycle time and represent about one third to two thirds of project effort. It is then important to consider the iteration aspects of design tasks when developing a design process model. We present below some models that consider the iterative aspect of design. One of the most important model families is based on the design structure matrix (DSM). Design Structure Matrix (DSM) based models (3), (5), (4) have been extensively used to capture and display the iterative structure of engineering design. 156

3 A B C D E F G H I J K A X X B X X X C X X D X E X X X F X X G X X X X H X X X X I X X X X J X K X Figure 3. A DSM representation DSM uses a matrix representation of the design process (figure 3). The DSM matrix is square with one task by column and by row. Information flows between tasks are indicated in the off-diagonal elements of the matrix. Two types of information flows are distinguished: feed forward (lower diagonal elements) and feedback (upper diagonal elements). With this representation, cyclic information flows are easily captured and the need for iterations is identified. Matrix elements are manipulated in an attempt to eliminate or minimize the number of upper diagonal elements. This process is known as partitioning. Figure 4 represent the reordered elements of the DSM shown in figure 3. A C F D J B E G H I K A X X C X X F X X D X J X B X X X E X X X G X X X X H X X X X I X X X X K X X Figure 4. Rearranged DSM matrix By this method we analyze the dependencies between tasks of the two experiments. THE RAPID PROTOTYPING Rapid Prototyping can be defined as a group of techniques used to quickly fabricate a scale model of a part or assembly using three-dimensional computer aided design (CAD) data. 157

4 Figure 5. The 3D part in Catia V5 The prototyping soft takes virtual designs from computer aided design, transforms them into thin horizontal cross-sections and then creates each cross-section in physical space, one after the next until the model is finished. The Zprinter machine reads the data from a CAD drawing and lays down successive layers of liquid and powder and in this way builds up the model from a series of cross sections. These layers, which correspond to the virtual cross section from the CAD model, are joined together to create the final shape. Figure 6. The Zprinter machine ZCorp 310 plus The standard data interface between CAD software and the machines is the STL file format. An STL file approximates the shape of a part or assembly using triangular facets. Smaller facets produce a higher quality surface. 158

5 To make the part presented in figure 5, we have used a composite powder (ZP131) and an epoxy binder. The soft used by Zprinter has the possibility to simulate the printing process, step by step, and to make an estimation of binder usage, number of layers necessary to make the prototype, of total time etc, very efficiently for the designer in the process of decision making. Figure 7. Estimation report of printing by Zprinter soft CONVENTIONAL PROTOTYPING The material of the conventional prototype is a polyamide. The properties of this material are the same with the material for the final part, but different to the prototype realized by the rapid method. We can make the mechanical or thermal tests and a lot of other tests: geometrical tests, functional tests, technological tests, the quality of surfaces etc. In order to realize the conventional prototype, a CAD/CAM soft is necessary. For our work, the soft Catia V5 was chosen. All the operations necessary was calculated and simulated by Catia V5. In the NC Manufacturing, the milling operations were conducted under prismatic machining module. Three manufacturing programs were made, in order to realize the process: Roughing, Contour driven and Pocketing. All of the three programs was optimized and simulated by the appropriate tools and the NC code was generated. In the figure 8 we show a screenshot of the manufacturing module. 159

6 Figure 8. The prototype manufacturing program in Catia V5 PRESENTATION AND ANALYSIS OF EXPERIMENTS The experiments analyzed in this paper were made at the University of Piteşti, in the Automotive Engineering Research Center, by a local team. The goal of the design experiments was to design a part in CATIA V5 and to create two types of prototypes: by the conventional method and by rapid prototyping method. Our intention in this study is to observe the design activity performed by a team of designers. Video-based observational techniques were used in this experiment to provide useful and reach record of the design process that is then used by different researchers in different ways to study different issues. During the design process the designers cooperate in order to perform together the design of the part, to set up the manufacturing parameters and the rapid prototyping parameters. In order to realize the conventional prototyping, the designers have made the tasks below: 1. Modeling the part in Catia V5 2. Realize the manufacturing program in Catia V5 a. Roughing b. Contour driven c. Pocketing 3. Simulation and optimization the manufacturing programs 4. Realize the NC code 5. Preparing the NC for the manufacturing process 6. Preparing and install the blank on the NC 7. Part execution 8. Part painting 160

7 For the rapid prototyping, the tasks are: 1. Modeling the part in Catia V5 2. Realize the prototyping program by the Zprint soft 3. Preparing printer 3D (Zprint) 4. Printing the part 5. Solidification the part 6. Removing the part on the printer and cleaning the part 7. Part painting To illustrate the relationship between the tasks a DSM matrix was performed for each case x 2 x 3 4 x x 5 x 6 x 7 x Figure 9. DSM representation of tasks for the rapid prototyping process 1 2a 2b 2c x 2a x x x 2b x x 2c x x x 3 x x x 4 x 5 6 x x x x 7 x x x 8 x Figure 10. DSM representation of tasks for the conventional prototyping process We observed in case of the conventional prototyping process, a larger number of elements on the right side of the main diagonal. This indicates a large number of iterations. In our analysis, iterations are considered as the repetition of design tasks. So, when a task is executed for the first time by the designers, each new switch to this task will be considered as a new iteration. In the iteration process, the rework may concern the entire task or just a part of it. The figure below shows the Gantt diagram of the design process as it was observed. Figure 11. Gantt representation of two processes 161

8 After conducting two tests for the two experiments, we find following: For the case of conventional prototyping, the number of tasks required is higher, between tasks 1, 2a, 2b, 2c, 3, 4, there is a block independent of the other tasks. We have observed a large number of iterations needed to optimize programs manufacturing. These iterations are necessary to obtain a product correct, but have the effect of increasing the duration process. The number of tasks is less for the case of rapid prototyping, the links between them are in series, a single task could be made in parallel with others, is the task 5. However, the time for this process is inferior compared with each other. It produces an 18% reduction in time. Half the time difference is needed for iterations to optimize the manufacturing program. Concerning the costs, the conventional prototyping is much more expensive compared to rapid prototyping. For our study we have a report 4-1 between conventional and rapid prototyping. CONCLUSIONS The prototypes are an important role in the design process. It is used to aid the designers to view the characteristics of the product, to simulate the functions of the product or to construct the injection mold. The advantage of rapid prototyping is the reduction of manufacturing times. The piece is more complex and the difference with a traditional production increases. In addition, rapid prototyping provides new opportunities for manufacturing. The benefits to have and use a prototype are: detect design problems ; testing alternatives ; validate the industrial feasibility ; forms and optimize the cost of tooling ; minimize the risk of modification ; refine the operational characteristics ; have a media object ; carry out mechanical and thermal tests, etc ; have a media object and avoid potential conflicts. REFERENCES (1) Osborne, S. M.: Product development cycle time characterization through modeling of process iteration. PhD Thesis, MIT, (2) Pahl, G., Beitz, W.: Engineering Design: A systematic approach. Springer-Verlag, (3) Smith, R. P., Eppinger, S. D.: Deciding between sequential and concurrent tasks in engineering design. Concurrent Engineering: Research and Applications, Vol. 6, N 1, 1998, (4) Steward, D.V.: The design structure system: A method for managing the design of complex systems. IEEE Transactions on Engineering Management, Vol. EM-28, N 3, 1981, pp (5) Yassine, A., Chelst, K., D. Falkenburg.: A Decision Analytic Framework for Evaluating Concurrent Engineering. IEEE Transactions on Engineering Management, Vol. 46, No. 2, 1999, pp