The Iterative Design Process

A Cyclic Approach to Design

The iterative design process is the cyclic, step-by-step framework through which most engineering design projects progress. Unlike linear design, the process loops back: after evaluating a prototype, the designer returns to the design stage to improve it, then makes and evaluates again. The iterative process is structured into two main stages — Design and Make and Evaluate — each containing several distinct activities that you must be able to describe.

Stage 1 — Design: Analysing the Brief

Every iterative cycle begins with a thorough analysis of the design brief. The designer reads the brief carefully to identify what the product must do, who will use it, any constraints (cost, size, materials), and what success looks like. This analysis informs every later decision — if the brief is misunderstood, the entire cycle produces a product that fails to meet the client's needs.

Stage 1 — Design: Researching Product Requirements

Once the brief is understood, the designer researches what the product must achieve. Research falls into two types. Primary research gathers first-hand information from direct sources — interviews with potential users, focus groups, user observations, and the use of tables of anthropometric data to understand body dimensions. The advantage of primary research is that it is specific to the product; the disadvantage is that it is time-consuming and expensive to gather.

Secondary research uses existing published information — books, design guides, technical standards, and market research reports. Analysing existing products is a key secondary research activity: the ACCESS FM framework (Aesthetics, Cost, Customer, Environment, Size, Safety, Function, Materials and Manufacturing) provides a structured checklist for evaluating a competitor or similar product. Product disassembly — physically taking apart an existing product — is a secondary research technique that reveals how it is constructed and what materials are used. Secondary research is faster and cheaper than primary research but may be out of date or not specific to the new product.

Stage 1 — Design: The Engineering Design Specification

After research, the designer writes an engineering design specification. The specification converts the client's needs into precise, testable criteria — for example, specifying that a product must weigh no more than 500 g, withstand a force of 200 N, or be produced for under £15. The specification is used throughout the iterative cycle to check whether the design is progressing in the right direction.

Stage 1 — Design: Generating Design Ideas

With a specification in place, the designer generates ideas. At the start of the first cycle, generation means creating new concepts through sketching (freehand drawings showing proportions, features and materials) and modelling (physical mock-ups or virtual 3D models). In later iterations, generation refers to the modification or improvement of an existing design in response to evaluation feedback. The ability to move fluidly between sketching and modelling — and between first ideas and refined revisions — is central to the iterative approach.

Stage 2 — Make and Evaluate: Modelling

The make-and-evaluate stage begins with modelling. Models are made before a final prototype to test specific aspects of the design without the full cost of manufacture. The reasons for modelling are to test proportions (does the product look and feel the right size?), to test scale (does it fit the intended space or the user's hand?), and to test function (does it move, connect or operate as intended?).

Modelling takes two forms. Virtual modelling uses CAD software to create a digital representation; this allows dimensions to be checked, components to be assembled on screen, and simulations of stress or movement to be run — all before any physical material is used. Physical modelling creates a tangible object from card, foam, clay or other cheap materials; this allows the designer to assess ergonomics, user interaction and aesthetics in a way that a screen image cannot replicate.

Stage 2 — Make and Evaluate: Manufacture and Comparison

After modelling, the designer manufactures or modifies a prototype — a working version of the product built to check whether it meets the brief and specification. The prototype is then evaluated: its dimensions, function and quality are compared against the requirements of the design brief and specification. Where the prototype falls short, the cycle restarts — the designer returns to the design stage, modifies the idea, and makes a revised prototype. This loop continues until the product meets all specification criteria or until the project constraints (time or budget) require a final decision.