Design for rapid prototyping: implementation of design rules regarding the form and dimensional accuracy of RP prototypes.

Popescu, Diana

Abstract: Theoretically, due to the additive manner of manufacturing, RP processes can build geometrical shapes, impossible to obtain using traditional methods, thus providing a high degree of freedom for the designer. However the practical experience shows that RP processes have limitations which impose restrictions in the prototype design. Similar with the traditional manufacturing, the Design for Manufacturing concept can be also applied to Rapid Prototyping (RP) processes, especially when it comes to building functional prototypes. The design of RP prototypes has to be based on the characteristics of the RP process: materials, accuracy, support structures, build-up orientation, mechanical properties, cost/time, quality, process parameters,minimal wall thickness, etc.

Key words: Design for Rapid Prototyping, benchmarking, accuracy, process parameters

1. INTRODUCTION

The necessity of studying the application of the "design for manufacturing" concept to Rapid Prototyping processes ("design for RP") results from the critical analysis of the aspects characterizing both the RP processes themselves and the prototypes built with these processes. Thus, despite de continuous improvement of the fabrication accuracy and surface quality, and despite the enlargement of the range of construction materials, the layered fabrication implies a number of issues that must be considered when designing RP prototypes (especially for functional prototypes). Some of the most important are the following (Popescu, D., 2001):

* Surface quality. The staircase effect, innate to layered fabrication, is mainly responsible for the surfaces quality of the prototypes. The designer, when prescribing the roughness values, must know that, for instance, the surfaces oriented parallel or perpendicular to the building direction have the best quality. Also it is necessary to prescribe a roughness which can be achieved using RP.

* Accuracy. Each RP process/machine builds the prototype with accuracy dependent on a number of factors, among which the process parameters settings and the build-up orientation can be mentioned. Also, geometrical features of different dimensions/orientations are built with different accuracies. In these conditions, for each process, the fabrication accuracy must be known so that the designer to be sure that all the designed geometrical features will be built according with his/her specifications. Therefore it is important that this information to be available during the design process, in order to take the best design decisions.

* Ability to build prototypes with very small/large dimensions. The working space of the RP machine determines the maximal dimensions of the prototype. For large prototypes, it must be known from the design phase if they can be built from one or many pieces, because this decision can influence the conception of the geometrical form of the prototype. For prototype with very small dimensions or with very fine details, the ability of RP machine to construct them comes into attention.

* Mechanical characteristics. They are determined by the type of RP process and the construction materials, but also by the layer filling strategy. All these are influencing mechanical properties such as: tensile/compressive strength, impact strength, environmental resistance (humidity, temperature), etc.

* Building orientation. As mentioned before, the building orientation influences many of the prototype characteristics, and sometimes finding the best orientation can be a complicated task (especially for prototypes with complex geometry). According to the chosen build-up orientation, the designer will know if certain geometrical features can be built or not, if they require support structures and how the building cost/time will be affected. Hence, it will be highly important for the designer to know and analyze all these aspects.

As can be seen, there are plenty aspects a designer must consider, and in order to keep all this information under control, it is necessary to systematize and present it in a form easy to use during the prototype modeling stage. That implies to create an application added-on the modeling software, offering knowledge about the RP processes.

In this context, the current paper is focusing on developing a methodology for the "design for RP" approach, by setting the framework for elaborating a set of design rules, in particular regarding the dimensional and form accuracy aspects.

2. LITERATURE REVIEW

Applying the concept of "design for manufacturing" to RP by elaborating sets of design rules to assist the designer in the conception phase of the prototype, represent a subject not approached in the literature, to the best of our knowledge.

However, there are some complementary researches like those from University of Loughborough, Rapid Manufacturing Research Group, "Design for Rapid Manufacture" project (Hague, R.J., 2006), which assume that RP processes are able to produce the prototypes at the exact specifications of the designer, which is not always true, as shown above. Therefore, the researches from University of Loughborough are focused on testing materials and redesigning existent parts for RP fabrication, and not on the task of elaborating sets of design rules for RP prototypes and implementing them in a software application for supporting the designer's work, as is the case for the methodology presented in this paper.

The analysis of the literature in the field shows a number of comparative studies, which are intended to help evaluating RP systems and processes according to certain criteria. Due to the lack of standardization in the RP field, in the design and manufacturing of test parts, but also regarding the measuring and testing methods, researchers have created their own test parts. Thus, using various benchmark parts, the performances of different RP processes in terms of accuracy, surface quality, mechanical properties, geometrical complexity of prototypes build time/cost, are assesed (Gibson, I., 2002), (Dimitrov, D. et al., 2006), (Makesh, M., et al, 2005).

[FIGURE 1 OMITTED]

The benchmarks from literature can be classified in two main categories: geometrical and process. The evaluation parameters for these benchmarks depend on the intended application of the prototype (visualization, fit and form, medicine, etc.), but most of the time these parameters are: cost, manufacturing time, surface quality and accuracy.

1. Geometrical. Geometric benchmarks are built for evaluating: form and dimensional accuracy; RP processes ability to build different geometrical features; surface quality; repeatability in building certain features; ability to build prototypes with very large/small dimensions.

2. Process. In this category are those benchmarks that test the mechanical properties of parts and those which refer to the process characteristics and allow the evaluation of the performances of process/machine.

3. DESIGN FOR RAPID PROTOTYPING APPROACH

3.1. Analysis of the geometrical RP benchmarks

In figure 1 are presented steps required for developing the approach design for RP, in particular from the point of view of dimensional and form accuracy. Due to the existence of a relative large variety of test parts for evaluating RP processes, the literature is offering information about prototypes' different features and their manufacturing resolution. In this context, an analysis of the literature must be the first step of the methodology. When it comes to geometric benchmarks, each of the test parts presented in the literature contains a certain number of features (holes, slots, threads, chamfers, slope walls, overlap features, very thin walls, etc.) of different sizes and at different locations, which can be used by a designer in defining the geometrical form of a product. But there are also geometric features that have not been studied yet and which will be identified and evaluated according to the established criteria.

In order to complete the data set gather from literature, the second step of the methodology will be to model a test part containing new features and to manufacture them using different RP processes, extracting design which will be implemented in a software application in order to assist de designer. Depending on the RP process used for fabrication, the application will offer information referring to the possibility of that RP process to build the prototype with the prescribed values of accuracy.

It is important to mention that:

* New rules can be added to the set, as other aspects concerning the RP prototypes/processes will be studied and/or presented in the literature (for instance, regarding surface quality or build-up orientation);

* The design guidelines can be implemented using any advanced modeling software;

* This approach can be also applied to other RP processes, for obtaining sets of design rules for prototypes.

3.2. Extraction of the set of design rules

The set of guide rules can be obtained by using two sources:

* The literature in the field, especially the conclusions of the benchmark studies which are using test parts with different geometrical features. Based on the studied benchmarks, a list of geometrical features in different positions and dimensions should be elaborated, along with information about the manufacturing accuracy of each feature;

* Building, measuring and evaluating a new test part, which geometry and dimensions include geometrical entities not studied before, this way filling the gaps in the data set. Thus, all the information about the form and dimensional accuracy can be gathering together, in order to cover the largest possible range of situations the designer can meet during the designing phase. Based on this information, sets of design guidelines will be elaborated, one for each RP process considered.

4. CONCLUSIONS

Software implementation of this methodology can offer important advantages:

* Decreased design time and cost. The situations when it is necessary to redesign the product because it cannot be manufactured according to the designer's specifications;

* The designer must not have extensive information regarding a certain RP process, benefiting of the RP specialists knowledge gathered in the form of guidelines;

* The set of design rules can be modified, enlarged or improved, according to the evolution in the RP field.

5. REFERENCES

Dimitrov, D., s.a. (2006), Investigating the achievable accuracy of three dimensional printing, Rapid Prototyping Journal, vol.12,n.1, pp.42-52, ISSN 1355-2546

Gibson, I. (2002), Software Solutions for Rapid Prototyping, J.Wiley Press, ISBN 1-86058-360-1

Grimm, T. (2005), 3D Printer Dimensional Accuracy Benchmark, Time Compression Magazine, 13-18/5 Hague, R.J. et al. (2002), Design for Rapid Manufacture, Proceedings of the Rapid Prototyping and Manufacturing Conference, Cincinnati, USA, 10 pp (CD ROM)

Mahesh, M., s.a. (2004), Benchmarking for comparative evaluation of RP systems and processes, Rapid Prototyping Journal, vol.10, n.2, pp123-135, ISSN 1355-2546

Popescu, D. (2003), Fabricatia rapida a prototipurilor, rocedee de fabricare rapida a prototipurilor, Editura Aius, ISBN9739490-83-2