Phase model

From WikID

Figure 1: Phase model of the Product Design Process.[1]

According to Roozenburg and Eekels, design processes are structured according to the basic design cycle[1]. That establishment leads almost imperatively to the statement: effective design processes should be structured in this manner. The cycle, which is found descriptively, changes into a norm for effective designing. We can therefore also consider the basic design cycle as a prescriptive model for designing. But because the basic design cycle is so general and abstract, it offers in this form insufficient scope for the purposeful structuring of design projects in practice. The basic design cycle and the idea of spiral-like development can, however, be worked out into a phase model of the design process . To that end the design process is divided into groups of related activities, which lead to a certain stage of development of a design, such as a functional design or function structure, a principal solution, a concept, and the like. These designs in the making correspond with the provisional designs. They are not alternative design proposals for the same problem, but further elaborations, at an increas­ingly concrete level. The end of each phase can be taken as a decision point. Herein lies the importance of phase models. At the decision points one looks back on the work performed, and the results obtained are weighed against the goals of the project. Phase models therefore urge a regular evaluation of the project: reject, do a step back, or continue to the following phase.

Phase models are based on the idea that a design in the making can exist in three different ways[1]:

  1. As a function structure; a representation of the intended behaviour (the functions) of a product and its parts
  2. As a solution principle; defines the working principle, or mode of action, of a product or a part thereof. It specifies (in generic terms) the function carriers or ‘organs’ of which a product should be built up, to fulfil its internal and external functions.
  3. As an embodied design. a design in the more usual meaning of the word. It is a description, usually as a drawing, of the geometrical and physico-chemical form of a product and its parts.

The function structure

Figure 2: Example of a function structure (from student report).

A product is a system. There is hardly any product that does not consist of various parts. Products are built up of components, which in turn contain parts, and even a part might be composed of different materials. Products are systems and can themselves be part of more comprehensive man-machine systems. The set of invariant relations is called the structure of a system. Systems are continuously changing, but their structure constrains the number of possible transformations. Because of its structure, a system remains as it is. Each system has an environment, and contains sub-systems. In a function structure, the sub-systems and the system as a whole are represented by their functions. It is an abstract representation that does not refer to concrete shape and material of the physical parts of the system. The function structure is an important methodological tool; it provides an aid for thinking about the mode of action of a product, without enforcing premature decisions on its embodiment.

The solution principle

Figure 3: Example of a solution principle (from student report).

A function structure is a model of the intended behaviour of a material system; it shows what internal functions must be realized by (not yet concretely defined) elements, so that the system as a whole can fulfil its external overall function. Designers try to realize this behaviour by thinking up concrete parts and components for the internal functions. For each part its place in the whole is established, as well as its precise geometry and materials. A solution principle is an idealized (schematic) repre­sentation of the structure of a system or a subsystem. The characteristics of the elements and the relations are qualitatively deter­mined. But a solution principle already establishes essential characteristics of the form of the product. Just as the overall function of a system is the resultant of a number of subfunctions, a solution principle for a product as a whole arises from the combination of solution principles for its parts. The overall solution principle, which is chosen for further development, is called the principal solution. The core of designing - reasoning from function to form - is especially evident in the creation of a principal solution, for the principal solution marks the transition of the abstract functional structure to the concrete material structure of the product to be developed. Reasoning from function to form does not lead to a unique answer. Any function can therefore be realized with different physical effects, and these can be worked out into different solution principles and an overall principal solution.

The embodied design

Figure 4: Example of an embodied design (from student report).

A principal solution is already a first design proposal because it embodies decisions on the geometry and material of the new product. It is, however, not more than an outline design proposal, which deals with physical feasibility only. It is a technical possibility that has to be worked out to some extent, before it can be evaluated against non-technical criteria as well. The development of a principal solution to a detailed design can be seen as a process of establishing increasingly accurate, and more nu­merous characteristics of the new product, such as:

  1. The structure of the entire product (the arrangement of the parts)
  2. The shape
  3. The dimensions
  4. The material
  5. The surface quality and texture
  6. The tolerances
  7. The manufacturing method

A product design is ready for production if all design properties have been specified definitively and in all required detail. Because usually many properties have to be considered, and the relations among them are complex, the development of a principal solu­tion into a detailed definitive design usually requires some stages in between. Typical intermediate stages are the design concept and the preliminary design (or sketch design). In a design concept a solution principle has been worked out to the extent that important properties of the product - such as, appearance, operation and use, manufacturability and costs – can be assessed, beside the tech­nical-physical functioning. One should also have a broad idea of the shape and the kinds of materials of the product and its parts. A preliminary design is the following and last stage before the definitive design. Characteristic for this stage is that, for at least the key parts and components of the product, the layout and shape and main dimensions have been established, and the materials and manufacturing techniques have been determined.

Figure 5: Phase model of the product design process by Pahl and Beitz[1].
Figure 6: Phase model of the product design process by VDI[1].

Models of the design process have been developed since the early 1960s. In engineering design, two well-known phase models of the design process are the Pahl and Beitz model and the VDI Guideline[1].


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Roozenburg N. and Eekels J. (1998) Product Design: Fundamentals and Methods, Wiley, Chichester, 2nd ed.
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