Additive Manufacturing and Rapid Prototyping
What Is Additive Manufacturing?
Additive manufacturing is the opposite of cutting or drilling: instead of removing material, you build a part up, one thin layer at a time. A digital 3D model (usually saved as an STL file) is sliced into hundreds of horizontal layers by software, and the machine deposits or fuses material along each slice until the full shape is complete. Because no specialist tooling — moulds, dies, jigs — is required, a new design can go from computer screen to physical object in hours rather than weeks.
Additive manufacturing builds parts layer by layer from a digital model (STL file). No moulds or dies are needed, so tooling cost is effectively zero for a one-off part.
Fused Deposition Modelling (FDM)
Fused deposition modelling (FDM) is the most common form of 3D printing found in schools and small workshops. A spool of thermoplastic filament — typically PLA or ABS — is fed into a heated nozzle. The nozzle melts the plastic and extrudes it in precise paths across the build platform. Each layer bonds to the one below as it cools, and the platform drops slightly before the next layer is deposited. A support structure is printed under overhanging sections and removed afterwards.
FDM is ideal for concept models, ergonomic prototypes and functional parts that do not need very smooth surfaces. Typical layer thickness is 0.1–0.3 mm. The main limitations are visible layer lines on the surface and relatively low mechanical strength compared with injection-moulded parts. Consumer FDM printers such as the Bambu Lab range have made this technology genuinely accessible at under £300.
Stereolithography (SLA) cures liquid photopolymer resin with an ultraviolet laser. The build platform starts just below the surface of the resin vat; the laser traces each layer, hardening only the areas it touches. The platform then drops by one layer thickness and the next cross-section is cured. SLA produces parts with an extremely smooth surface finish and fine detail, making it popular for jewellery, dental models and appearance prototypes in product design studios. Resin is more expensive than FDM filament and uncured resin requires careful handling as a chemical irritant.
Rapid prototyping using SLA is particularly valued when a design team needs a high-fidelity model to present to a client before committing to expensive injection-mould tooling. Dyson engineers, for example, use SLA models to evaluate cyclone geometry before the tooling investment is made.
Metal Sintering
Metal sintering (also called selective laser sintering or SLS when applied to metals, or DMLS — Direct Metal Laser Sintering) spreads a thin layer of metal powder across the build platform and a laser selectively fuses the particles. Unfused powder acts as its own support structure and is recycled after the build. The result is a fully dense metal component with mechanical properties close to those of a traditionally machined part.
Metal sintering is used across aerospace, automotive and medical engineering for parts that are geometrically too complex to machine conventionally — internal cooling channels inside a turbine blade, for instance, or a lightweight titanium bracket for an aircraft seat with internal lattice structures. Rolls-Royce uses metal sintering for low-volume turbine components. The process is slow and the machines cost hundreds of thousands of pounds, so it is reserved for high-value, low-volume parts.
FDM (thermoplastic filament), SLA (UV-cured resin) and metal sintering (laser-fused powder) are three distinct additive routes. FDM is cheapest; SLA gives the best surface finish; metal sintering produces functional metal parts with complex geometry.
Additive Manufacturing vs Mass Production
A common error is to describe additive manufacturing as a mass-production method. It is not. The table below captures the key differences.
Tooling cost: Additive manufacturing requires no moulds, dies or fixtures — the digital model is sent straight to the machine. Injection moulding, by contrast, requires a steel mould that can cost £20,000–£200,000 before a single part is made. Additive manufacturing therefore has zero tooling cost, making one-off and low-volume production economically viable.
Speed of iteration: A designer can update a CAD model and have a revised prototype in hand within hours. With traditional tooling, modifying a mould takes days or weeks and costs thousands of pounds. This is why product development teams use additive manufacturing for rapid prototyping during the design iteration phase.
Geometric freedom: Additive processes can produce internal cavities, lattice structures and undercuts that are impossible or prohibitively expensive to machine. Conventional subtractive machining is limited by tool access; additive is limited only by the digital model.
Production volume: Additive manufacturing is slow per part. A single FDM print of a mobile phone case takes 2–4 hours. Injection moulding cycles in 30–60 seconds. For volumes above a few thousand units, injection moulding almost always has a lower unit cost once the tooling cost is amortised.
Material waste: Additive manufacturing adds material only where needed — waste is minimal. Subtractive processes (milling, turning) can waste 90% of the starting billet for complex aerospace parts. This makes additive manufacturing more sustainable for expensive materials such as titanium.
Rapid Prototyping in the Engineering Design Process
Rapid prototyping sits between the design phase and the production phase. An engineer creates a CAD model, exports it as an STL file, and feeds it to an additive manufacturing machine. The resulting prototype is used to test fit, form and function before committing to expensive tooling. This compresses the design cycle significantly: Tesla’s Gigafactory uses additive manufacturing to iterate on manufacturing jigs and tooling fixtures at speed, avoiding the lead times of conventional fabrication. Once the design is finalised, mass production typically uses a different process — injection moulding, die casting or CNC machining — better suited to high volumes.