EDITABLE PHYSICAL MODELS FOR CONCEPTUAL DESIGN. A.F. Lennings. J.J. Broek. I. Horváth. W. Sleijffers. A. de Smit. Delft University of Technology.
EDITABLE PHYSICAL MODELS FOR CONCEPTUAL DESIGN
A.F. Lennings J.J. Broek I. Horváth W. Sleijffers A. de Smit Delft University of Technology Sub-Faculty of Industrial Design Engineering ICA Research group The Netherlands www.io.tudelft.nl/reseacht/ica
ABSTRACT
KEYWORDS
The application of Rapid Prototyping during Conceptual Design is generally called Concept Modelling, without any further reference to what exactly is meant. In this paper the design process will be examined more closely, applying design methodology, and it will be shown that two distinctly different types of activities can be recognized. All (sub)processes start with divergent activities: gathering information and generating design concepts, followed by convergent activities like evaluating and choosing the best concept. It will be shown as well that these two activities are so different in nature that two specific types of Concept Modelling are needed to truly support each. The convergent activities can very well be supported by the current generation of RP systems, using Proof-OfConcept models. To truly support the divergent activities a different types of models is needed, to be called Editable Models. These are models that support the synthesis phase of the design process by giving real-time feedback to the designer. RP systems capable of producing editable models do not yet exist: the paper will only describe the requirements and advantages, also using a real-life application example.
Concept Modelling, Physical Concept Modelling, Rapid Prototyping, Conceptual Design, Design Methodology.
1.
INTRODUCTION
Rapid Prototyping has by now grown to a generally accepted and mature technology. The special type of RP called Concept Modelling is not yet generally accepted: as we will show this new technology has not even yet been clearly defined. Obviously in this context Physical Concept Modelling is meant (PCM), in contrast to the meaning of Concept Modelling as a special type of CAD application. The first written reference to Physical Concept Modelling that we have found was made in 1995, in a paper called "The Divergence of Rapid Prototyping Systems" [Throup, 1995]. In this article an observation is made that further developments in RP technology will divert into two different directions: at the one side so-called "High-end" modelling, leading to small series production and ultimately Rapid Manufacturing on demand; at the other side Concept Modelling, leading to 3D printers/plotters. Several authors wrote about this new type of RP, and also some new RP systems were marketed as 'Concept Modelers': the Actua from 3D Systems, the Genisys from Stratasys, the Z402 from Z
Corp, and several systems based on light CNC milling machines. These new systems were presented having advantages (over 'traditional' RP systems) such as: simple operation, plug and play, desktop, office friendly, environmentally safe, and low-cost. Current RP research worldwide is primarily focused on Rapid Manufacturing and Rapid Tooling [Hunt, 1998]. In fact, in this survey apart from Delft University of Technology no other academic research institute reports an interest in 'Concept Modelling', and only a few do mention the keywords 'design' and 'product development'.
Wohlers defines a 3D Printer, not a Concept Modeler, and Lennings gives a futurist definition. So in fact no clear definitions have been found. More important is that both authors do define the hardware. We have found that this is an incorrect approach, as it is not the machine that is a Concept Modeler, it is the application of the machine that makes it a Concept Modeler. We can explain this using a few examples. - Service provider Materialise in Belgium promotes its "Next Day Service" as specifically aimed at Concept Modelling. Still they use stereolithography, a "high-end" RP technology.
In this paper we will present some results of our research in the field of PCM. We will give a definition, based on the design process (section 2). We will look into the design process (3), and conclude that two distinctly different applications of PCM can be found, needing different types of models (4). After discussing both applications (4,5) we will present application areas (6) and a real-life example (7).
- [Gibson, 1998] reports about an Actua (a "Concept Modeler") in Denmark that is primarily used to produce patterns for metal casting applications.
2. WHAT IS PHYSICAL CONCEPT MODELLING
Any definition of Concept Modelling thus has to be application oriented. We will follow the observation of Throup that was mentioned before, and distinguish two types of RP. We will give two definitions, each of them based on the purpose of the application.
Although we could not find any clear definitions of Concept Modelling, we did find two publications covering the type of RP system to be used for Concept Modelling.
- Light CNC milling machines can be used to quickly produce Concept Models in foam, as well as to produce prototype moulds in tooling board or aluminum. - Nearly 36 % of all RP models are being used for visualization purposes [Wohlers, 1998], most models produced on "high-end" RP systems.
The first is [Wohlers, 1998], where well-known RP consultant Terry Wohlers states: "A 3D Printer is a less-costly and less-capable variation of RP Technology" (to be used for Concept Modelling). So, while avoiding the phrase "Concept Modeler", he emphasizes price and accuracy of the system. The second is our own [Lennings, 1998], where Lennings gives a definition of a Concept Modeler based on the most important requirements. He then defines a true Concept Modeler as "any RP system that is: 1. Priced below USD 10,000; 2. Office friendly; 3. Build time max 15 min; 4. Push-button operation." Obviously such a machine is not yet available, however we are sure that in a few years time it will be. Both definitions are in fact of limited value, as they are very time-dependant: prices and build times are continuously decreasing, and capacities are continuously increasing.
Figure 1: the cost incurred (spent) during the design process.
The first type of RP is Pre-Production Prototyping: make a prototype and test it as a last check, just before ordering the manufacturing tooling. This makes sense, as from that moment on the costs will steeply rise: manufacturing tooling, mass production, marketing effort, distribution. See figure 1: the prototype will be used for Verification, between the activities ‘Engineering’ and ‘Manufacturing’. At that time possible design errors still can be fixed at reasonable cost: any later it will be much more expensive. Main requirement for a pre-production prototype is that it resembles the final product as closely, in as many aspects, as possible: it will thus have to be a completely functioning prototype. Most research in RP is aimed at creating more and more realistic prototypes: ultimately this type of RP will lead to Manufacturing on demand. The second type of RP is Physical Concept Modelling, for now to be roughly defined as any application of RP during the design process. So not only during concept design, but during all stages of the design process, until the design has been finished and the pre-production prototypes can be made. For this type of RP it is not needed to take all aspects into account (as it is for pre-production prototyping): resembling the final product in one or a few aspects is sufficient. This definition is not yet very clear, we will elaborate later when distinguishing two different types of Concept Modelling. The goal of Physical Concept Modelling is to assist the designer in making correct decisions, like technical choices concerning the functioning of the new product and styling choices concerning its appearance. Making first time right decisions will save both time and money. In contrast to PreProduction Prototyping, Concept Modelling must be used in an interactive way. Speed and ease of use are important issues to have designers accept and take advantage of this new design tool. As already said in the introduction, this application of RP technology is not (yet) generally accepted within the design community. Though the potential advantages are recognized, the actual use still is low. We state [Gibson, 1998]: "While significant progress has been made in RP Technology and its integration into manufacturing, the technology still has many challenges to overcome in crossing the barrier, which aligns the technology as a tool in product development." (Scott Loose, Queensland Manufacturing Institute). A next observation
comes from [Gribnau, 1999], who states that "the physical models are snapshots in time and not useful during the actual modeling of the design when the design is manipulated." Having outlined the differences between PreProduction Prototyping and Physical Concept Modelling, we have to distinguish from Virtual Concept Modelling as well. The difference of course is clear: the model being or not being physical. Two major functional difference do result: using a physical model a complete simulation (all aspects) is possible, using a virtual model not, as then only certain aspects are modeled (totality); and a physical model can be touched, handled, ‘fondled’ (palpability). For now we will leave the question open in which cases physical is needed, and in which cases virtual will be sufficient. No further attention will be given to PreProduction Prototyping and Virtual Prototyping. We will now only look into Physical Concept Modelling, and in order to do so we will start with the design process.
3. DESIGN METHODOLOGY. As our definition of PCM is based on the design process, we will first summarize the main characteristics of (conceptual) design, including some effects on prototyping. - Concept design is an intuitive process. Creation of a new product starts with some vague and fuzzy notions in the designers mind, to be made more and more exact using tools like hand drawn sketches, physical models and computer software. In fact this characteristic is a major obstacle for the use of 3D CAD tools, which do not (yet) support vague notions. The obstacle is even bigger for the use of RP, for which perfect solids are needed. - Design is an iterative process: a concept will be evaluated, changed, re-evaluated, changed again, etcetera. Changes may be minor and concern some details only, which should be supported by the designers tools. - Design is synthesis oriented: starting with a list of requirements, a solution must be found that fulfills all wishes. - Designers work aspect oriented: they solve problems one by one, concentrating on one specific aspect of the design at the same time. For instance
its styling, its functioning, ergonomics, etc. Obviously all these aspects are strongly interdependent and cannot be optimized separately. Still, as controlling/optimizing all aspects at once is not possible, the designer will concentrate on various aspects one by one. So concept models typically focus on simulating one specific aspect. All other aspects of the design do not need to be simulated at that moment. A shape model for instance does not need to function, or have the correct weight (this is all in contrast to the Pre-production prototypes mentioned before, that need to resemble the final product as closely, in as many aspects, as possible). This is valid for all types of concept models. - Design is a mental process, in which the designers train of thought produces new concepts. Any interruption can be harmful here, so all tools used must be able to provide real-time support - or at least be very fast. Waiting one or more days, as is custom for current RP processes and acceptable for Pre-Production Prototypes, prohibits the use of the tool during the design process. This apart from the time-pressure on all design activities. So no next day models, but next hour or even same hour models instead [Wohlers, 1999]. Accuracy is less important here than for pre-production use, and may be traded in for speed. - Apart from these methodological issues, the cost is important as well. Designers tools need to be priced sufficiently low to make the cost of using them unimportant. Several models of the design process have been published [Roozenburg, 1995], clearly showing its iterative character and the sub-processes involved, using graphs to illustrate various characteristics. A well known graph that applies to many design (sub)processes is shown in figure 2. It shows the information contents of a (detail) design against the time axis. Starting with very little information (for instance an idea only), the information contents grows (divergence) as data is gathered, requirements are defined, and (most important) alternative proposals are generated. This idea generation is called the synthesis phase, and is in fact the kernel of the design process. During the second part of the design process the information contents again becomes smaller (convergence) by making more and more decisions, always choosing one solution out of a number of alternatives. At the end just one solution (the final) is left.
Figure 2. The design process (or any subprocess) consists of a diverging part, followed by a second part that again converges to the final solution.
Obviously the model shows a very simplified view of the design process, ignoring many subprocesses and cyclic activities. Still it will prove to be very useful to further explore the use of Physical Concept Modelling by designers.
4. TYPES OF CONCEPT MODELS Based on the model of the design process just described, our observation is that the convergent part of the design process is supported by the currently available Concept Modelling systems. A concept model from such system can be used to prove whether or not a particular solution does fulfill a certain requirement, so can be used to choose one solution out of a number of alternatives: convergence. We would like to name this type of Concept Models as Proof-of-Concept models, after [Horton, 1995]. This name exactly shows the use of this type of Concept Modelling: the designer has generated one or more possible solutions for a certain design problem and wants to test which solutions are valid, or which is the best. As the solutions have been designed already (assuming this implies modelling a geometry in 3D CAD), the current generation of commercially available Concept Modelers and 3D Printers is perfectly capable of producing these proof-of-concept models. The earlier divergent part of the design process is not supported by any current RP system. This part of the design process is in fact to be called Con-
cept Modelling: the time when the (still vague) design proposals are generated and elaborated, when series of alternative design concepts are produced. In this part of the design process the characteristics intuitive, iterative and syntheses oriented are most important. To really support the designer at this synthesis stage a completely different type of prototyping is needed, in fact one that is closely linked to geometry definition. A tool that can help the designer to 'think in space', just like drawing a sketch helps to materialize some vague notion into a form. A model that can easily be changed, just as flexible as the sketch on paper. No ‘snapshots in time’, but direct (real-time) feedback showing the current virtual geometry status in physical form. We would like to refer to this type of Concept Models as editable models.
5. EDITABLE MODELS As said before, an important characteristic of the design process is its iterative nature. Typically for any aspect to be designed, a number of optimization cycles are needed in order to reach the final solution. For every next cycle a few details concerning the currently reviewed aspect have been changed, while the rest of the design still is the same. The current generation of RP systems can only support the designer by producing a completely new model for every next iteration step: a time consuming and expensive process. We state that in order to really support product synthesis, which is the core activity in the conceptualization phase, physical concept models need to be editable. A new generation of Concept Modelling systems should be capable of just changing one specific detail of an existing model. RP systems capable of creating such editable models do not yet exist. Two basic possible ways to create editable models are: Replacing: substitution of improved parts, and Reshaping of improved parts. The authors are developing a first implementation of a new RP technology capable of creating models that are editable by replacement: Free Form Thick Layered Object Manufacturing (FF-TLOM). This technology supports incremental fabrication of concept models in low-density rigid PS foam. In order to achieve editability, the model is composed of separate parts (segments) that can easily be removed, interchanged, and reassembled after a de-
sign change. The FF-TLOM technology is specially targeted at the creation of large-sized prototypes with freeformed outer surfaces. It will be presented during this conference in more detail by [Broek, 2000]. As far as we know, RP technologies that are capable of creating models that are editable by reshaping do not yet exist. The idea however is not original: RP artist Michael Rees promotes the idea of real-time interactive editable models, calling it "Digital clay": any change in the (physical) clay will result in a changed CAD model, any change in the CAD model will result in a deformation of the clay. The methodological impact of using such editable concept models is great: instead of evaluating the design concept in an iteration, using a series of subsequent concept models, now the editable model can be used as an interactive tool, giving the designer real-time feedback on any change applied during the product synthesis. This will strongly enhance the quality of the resulting design, and shorten the time needed to create several acceptable alternative solutions. Currently a designer will generate a series of virtual geometries, have them made physical using ‘traditional RP technology that costs at least a day, will evaluate and remodel the geometry, again make prototypes, etc: ‘Snapshots in time’. Each RP interruption causes the designer to shift to a different project and loose his/her train of thought. With the proposed ‘editable models’ the designer will have a real-time physical feedback. He then can improve and proceed immediately. The long-term objective even reaches further, offering an integration between input and output device: any change to the computer model instantly changes the physical model, and any change to the physical model instantly changes the computer model. Integration of such tools in the design process will indeed influence design methodology. Many technological issues will have to be treated before such tools will become available, including the input technology (reverse engineering) needed and the high build speed. Do note that we do not propose and describe a new technology here, we just mention the designers needs and outline the characteristics of a potential solution. The integration of input and output functionality obviously introduces many new computational is-
sues as well. Even more important are the methodological questions: - are (indirect) computer tools at all suited to assist in tasks like generating new geometry design concepts, or are the traditional (direct) hand tools like sketching, claying and sculpting foam per definition better suited for this aim ? - is clay in fact an intuitive and useful input tool for most designers ? - are physical models at all needed, or will they be completely replaced by virtual models ? What is the extra value offered by using physical models ?
6. APPLICATION AREAS OF CONCEPT MODELLING When discussing the application of Physical Concept Models, it is important to recognize that several different applications of Concept Models do exist, which might result in different types of models needed. This categorization applies to all types of Concept Models: to proof-of-concept models as well as to editable models. Some types will however be applied only later in the design process. We do distinguish between the following applications: - Shape models The best known application of Rapid Prototyping during Concept Design is the creation of styling models or shape models. These models represent the outer appearance of the design, and are meant for visual, tactile and ergonomic evaluation. Important are the advantages of touching, feeling, easily looking from all sides, in one word the "palpability" of the physical model. The evaluation needed can be a visualization of the total design, or a more detailed freeform surface quality evaluation. The model can be partial or whole.
pletely finished: color, gloss, texture, decoration, lettering, etc. Other uses for these models include Customer panel testing and Photography for marketing purposes. - Physical behavior testing models Models that are used to simulate certain behavior of (a part of) the design, like strength or stiffness. All these applications do in fact involve a type of simulation of the final product or of one of its aspects, used when generating new design concepts or for verification of the current design. In addition to using the prototype for evaluation of a design concept, a different purpose can be to stimulate design group discussions. The computer screens in current design studios do not give colleagues a clear view of what is being designed (like the former drawing boards did). So colleagues are no longer induced to make spontaneous remarks and comments on what they see, which before in many cases lead to discussion. A concept model surely will attract all colleagues and start a vivid discussion, in which the designer can greatly benefit from the accumulated design experience.
7. APPLICATION EXAMPLE Since the technology of editable models has not yet been implemented, we cannot present any application example of its use. We will show an example design project of a packaging, in which other types of RP have been used, and clearly indicate where and why the use of editable models would have meant an improvement.
- Functional models Models needed to test the functioning of some part of the design. This can be a mechanical part, electronic (breadboard model), ergonomic, etc. - Presentation models Models needed to present the design or design variation to an outsider. This can be the manager, the marketing department, the client, etc. In fact these are shape models as well, but now com-
Figure 3. One of the design concepts, that have been created in 3D CAD and presented using Virtual Models (computer renderings).
Figure 4. A number of Physical Concept Models that have been used to select the final design.
MONA is Holland’s largest manufacturer of desserts, producing many different types of yogurts, puddings, mousses, etc [Mona]. As these products must severely compete on the shop’s shelves, the packaging design (both geometry and graphics) is of great importance. This project concerns one of Mona’s luxury puddings, the 'Bavarois', for which an attractive new bowl had to be designed. The pudding was already available in a family size bowl, and in addition now the marketing department requested a bowl fit for 2 persons. In what perhaps seems a simple design project, a surprisingly large number of requirements were present. These concerned the volume (contents), the family resemblance with the existing bowl, the dimensions (exact diameter prescribed to fit the existing filling machines, maximum height prescribed to fit between the shops shelves), produceability of the packaging (no steep walls and sharp
corners where the material would become too thin), the label position (clearly visible from
the front when standing upright, cylindrical surface to enable application of a label), stacking and destacking without any problem an the automated production line, stability on the vibrating production line, etc. In this project a number of design concepts have been created, using sketches on paper, handmade models, and finally 3D CAD. ‘Rough’ CAD concepts only: intersecting surfaces, no solid geometry, not yet checked for the requested volume. Just meant to choose which shape to be used. This creation of alternative solutions, which is in fact the real synthesis phase of this design process, proved to be time-consuming. Completely freeformed geometry like this bowl is very difficult to sketch on paper, and in fact to imagine. Even the use of advanced CAD tools, enabling good quality renderings (virtual prototypes) as shown in figure 3, is insufficient for the designer to enable a good
mental view of the design. And that is exactly what is needed during synthesis.
Figure 5: DeskProto screen, showing the 3D geometry as imported from STL file, and part of the NC toolpaths (the first layer) for the concept model.
To solve this problem, in this project hand-made models in foam and plaster have been used to assist in correctly defining the geometry in the 3D CAD system. Here a real-time physical feedback from the CAD system, a capability to produce editable models, would have saved a lot of time. An editable model would physically represent the current CAD geometry, and thus assist the designer real-time, interactive, during the synthesis of a new concept geometry. The concepts were presented to the marketing department using renderings as shown in figure 3 and also more detailed virtual models including product graphics and a (virtual) environment. It was soon clear that the use of only virtual models was insufficient in this case as well: for an accurate evaluation by the marketing department (personnel without a technical background) physical concept models were required. So a number of concept models have been produced, which proved to be very effective during the discussions needed to choose a direction for further detail design: see figure 4. This is an example of an application of proof-ofconcept models: these physical models were created from finished design concepts, to assist in choosing (convergence) the correct concept for the engineering design phase. These same models have been used by the marketing department for a few customer panel evaluation sessions. Here again,
physical models were needed to present the designs to the panel, and to compare them with existing products .
Figure 6. A concept model being milled in foam on a light CNC milling machine.
The concept models were machined in light PUR foam (ca 80 kg/m3), using the DeskProto software [DeskProto] and a desktop CNC milling machine: see figures 5 and 6. Communication between 3D CAD software and DeskProto was done using STL files. Note that in fact these were invalid STL files, as in the concept modeler CAD system used, no true solids were available, and as several orphan surfaces were present. DeskProto can easily process this type of geometry, which is a big advantage in this early phase of the design process. For these styling block models machining from one side was sufficient. Using this setup the models could be produced very quickly: no finishing or painting has been applied. This resulted in very low cost as well: no financial barriers for creating a large series of concept models. The application area in this case was the shape model: only the outer appearance was evaluated. As said, the use of renderings only was insufficient: the possibility to touch and ‘fondle’ the models was needed to really evaluate the quality of the shape. After selecting a ‘winning’ design, this design has been detailed and refined, and as many virtual checks as possible have been performed to verify the products features. For instance it is quite easy to calculate the bowls volume from a computer
model. Any other dimensional requisites are easy to check as well.
a prototype tooling material. See the result in figure 7.
Figure 8. The resulting product: a pudding packaging
Figure 7: Part of the pilot series, used as Pre Production Prototypes, for a final check of the new design before committing to the large expense of tooling and marketing.
Still not all aspects of the new design could be evaluated in a virtual environment. For some aspects this is due to the fact that the reality to be checked just never has been modeled (or would be too complex to model). For instance checking the easy and complete removal of all pudding from the bowl when turning the bowl upside down to put the pudding on a plate. More important is the fact that only known problem areas (aspects) can be virtually checked. Any virtual simulation only checks the aspects that are expected to be ‘dangerous’ and have thus been modeled into the virtual environment. To detect any unexpected problem area a real-life test is needed. This is valid for any design project: as a final pre-production evaluation at least one physical prototype is needed. In this case, before final approval of the design, a large pilot series of functioning prototypes (real bowls) have been manufactured, to be filled on MONA's pudding production line for a final test. For a packaging like this the test-series involves several thousands of products. This series of PreProduction Prototypes have been manufactured using a 'soft-tool': CNC machined by DeskProto in
Indeed some new problems were found, leading to a few final adjustments of the design. For instance it became clear that the stacked bowls did not all have the same orientation (rotation), and that in some cases the crests of the inner bowl jammed with the lowest parts of the outer bowl. Due to this jamming the bowls sometimes did not de-stack. As a result of this test, the profile depth of the waves has been reduced in the final design. As this fine design (see figure 8) is 'packaging only' it is sold at a very low price. All illustrations are courtesy of Delft Spline Systems [Deskproto] and Mona [Mona], both from the Netherlands.
8. CONCLUSIONS After having given definitions for both PreProduction Prototyping and Physical Concept Modelling, we have shown that two distinctly different types of concept modelling do exist. The first to be applied during the divergent parts of the design process (idea- and solution-generation). The second type, with different requirements, to be used during the convergent parts (evaluation and decision). We have called concept models to be used during the convergent activities Proof-ofConcept Models; these models can very well be created using currently available RP systems. To support the designer with physical models during divergent activities, Editable Models are needed.
RP systems capable of creating editable models do not yet exist.
We have shown the need for editable models, and described requirements and applications. Obviously much more research will be needed to realize this aim. First methodology oriented research, to find out when exactly concept models are needed, when physical and when virtual. We plan to start this line of research by questioning product designers about the tools that they need for specific tasks. Next to that, technology oriented research is needed as well, finding new technologies that enable models that can be (automatically) reshaped as the CAD model is changed. This futurist notion of reshapeability, of a “virtual clay”, might well prove to have serious impact on the design process.
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