Purpose
Why prototype at all
Design prototyping has been standard engineering practice for a long time. The purpose is verification: designers and manufacturing engineers want confidence that the product as conceived on paper or on screen is what they actually have. Verification eliminates hidden errors and surprises that would otherwise emerge during manufacture, where they are costly.
Even when a design contains no errors, holding a prototype prompts changes — dimensional adjustments, aspect-ratio corrections, features relocated for easier access by users or maintenance personnel. That is a legitimate and common outcome, not a sign that the design was wrong.
Visualisation
Turning a physical object in the hand is the most effective way to understand it — better than a model on screen. Blind holes, complex interior detail and free-form surfaces are frequently misread from images no matter how experienced the reader.
Verification
Fast build speed gives designers time to modify and re-verify a design at reasonable cost and within the production schedule.
Function and fit
Assemblies can be built with clearances and loose parts intact, producing a functional article in a single operation.
Pattern making
Prototypes serve as patterns for investment casting and sand casting, and as masters for soft tooling.
Communication
A physical article settles arguments in review meetings far faster than a rendered image does.
Low-volume production
Beyond prototyping, the same processes now produce end-use parts where volume does not justify tooling.
By industry sector, the automotive industry has been by far the largest user, because of the geometric complexity typical of its models — engine blocks, intake manifolds, exhaust systems.
Process chain
The five steps, in order
Every RP process uses the same chain regardless of the underlying technology.
Create the CAD model
Part or assembly. RP requires no special modelling treatment — it is another application of the model, like mass properties or FEA.
Pre-process
Triangulate the model and export it. Then, in the machine software, orient the part, generate supports and slice.
Build
The machine deposits, cures or fuses material one layer at a time from the bottom up.
Post-process
Remove supports, clean, and where the process requires it, cure, infiltrate or heat-treat.
Finish
Sand, seal, paint or plate as the intended use demands.
Four universal concepts
What every RP process shares
Different vendors use different techniques and build different machines, but all of them work with the same four concepts.
A — Triangulation (tessellation)
The first step. The CAD software accesses the boundary representation of the model and converts all its faces to triangles, preserving the model topology. Resolution is under the designer's control, and it directly affects the quality and accuracy of the resulting physical part: higher triangulation resolution gives a better prototype.
Two points follow. First, resolution only affects non-planar faces — a flat face is already exactly representable by two triangles, and refining it achieves nothing but file size. Second, the appropriate resolution depends on purpose: use high resolution where the prototype is a final product or will be tested experimentally, and low or medium resolution where it is only for visualisation or verification.
B — Build orientation
Orientation is decided before slicing and it matters for several reasons.
| Consideration | Effect | Design response |
|---|---|---|
| Anisotropy | Properties vary by direction. Parts are typically weaker and less accurate in the Z direction than within the XY plane. | Orient so that the principal load runs within the layer plane, not across layers. |
| Build time | Orientation partly determines build time, because it sets the number of layers. | Placing the shortest dimension in Z reduces layer count and shortens the build. |
| Surface quality | Curved and inclined faces show stair-stepping that depends on their angle to the build plane. | Orient critical cosmetic or sealing faces to minimise stepping. |
| Support requirement | Overhangs below the process threshold need support, and supported faces finish worse. | Orient to reduce overhang area, and keep supports off critical faces. |
C — Layer thickness
Slicing software works within a layer thickness band that in practice runs from around 0.01 mm at the fine end to about 0.7 mm at the coarse end. The consequence is arithmetic: a 25 mm tall part is roughly 36 layers at the coarse setting and about 2,500 at the fine one. Thin layers give better surface finish and higher accuracy; thick layers build far faster. The choice is a direct trade of time against quality and should be made against the purpose of the prototype.
D — Support structure
Material cannot be deposited into empty space. Overhanging geometry, undercuts and bridges require a support structure built alongside the part and removed afterwards. Supports consume material and build time, mark the surfaces they touch, and take labour to remove — which is why orientation and support strategy are usually decided together rather than in sequence.
Consider a bottle with a curved base. Two orientations present themselves: horizontal or vertical. Horizontal minimises height and therefore layer count, but places the entire cylindrical wall as an overhang and puts stair-stepping on the visible circumference. Vertical increases layer count considerably, keeps the wall in the build plane where it prints cleanly, and confines the support problem to the curved base. For a visual prototype, vertical is the better answer — and the reasoning, not the answer, is what transfers to the next part.
Interface
The STL file as the interface
Generating a triangulated file from a CAD model is a single-step operation: save the model in STL format. The file contains the coordinates of the vertices of each triangular facet together with the outward normal vector of that facet — which is why the normal vector discussed in Part 11 turns up again here, now telling the machine which side of each facet is material.
The STL file is best understood as the interface between the CAD/CAM system and the RP machine. It is transferred by any ordinary means, opened in the machine software, and processed there to generate the build file. Assemblies can be saved as one file, in which case the assembly is built as a single prototype; saved separately, the components are built individually and must be assembled by hand, which for some assemblies is not possible at all.
An STL file is a triangulated approximation with no units declared, no material, no colour in its base form, no tolerances and no assembly structure. A curved surface is never exactly represented, only approximated to the chosen resolution. For prototyping this is entirely adequate; for exchanging design data it is not, and STEP is the correct choice. Confusing the two is a recurring and avoidable error.
Technologies
The principal build techniques
Stereolithography is generally considered the first RP technique. Several others followed, and each pairs a build material with a corresponding method of forming or curing it.
| Technique | Build material | Forming or curing method |
|---|---|---|
| Stereolithography (SLA) | Photopolymer resin | A laser solidifies each resin layer. |
| Laminated object manufacturing (LOM) | Paper or sheet stock | Layers are glued together and cut to shape with a knife or laser. |
| Selective laser sintering (SLS) | Thermoplastic or metal powder | A laser fuses powder particles, layer by layer. |
| Fused deposition modelling (FDM) | Thermoplastic; eutectic metals; wax | Molten material is deposited as beads following the layer cross-section boundary. |
| Solid ground curing (SGC) | Photopolymer resin | A light source hardens whole layers through a mask. |
| 3D printing (3DP) | Powdered ceramics and plastics, ABS, engineering thermoplastics | A binder or thermal process joins powder selectively. |
Work from the purpose backwards. A form-and-fit check wants the fastest and cheapest process available. A functional test wants a process whose material properties resemble the production material. A casting pattern wants a material that burns out or melts cleanly. A presentation model wants surface finish above everything. Selecting by machine availability rather than by purpose is how prototypes end up answering the wrong question.
Practice
Working the process well
Set resolution against purpose
Fine triangulation for tested or final parts; medium for verification. An unnecessarily fine STL of a large assembly is slow to transfer, slow to slice and no better in the hand.
Decide orientation and supports together
They trade against one another. Optimising layer count first and discovering the support burden second wastes a build.
Account for the finished dimension
Support removal, sanding and coating all change dimensions. Where a prototype must fit something, decide in advance which faces are datum faces and protect them.
Prototype to answer a question
Write down what the prototype is meant to establish before it is built. A prototype without a question is an expensive paperweight.
Key takeaways
- RP builds a physical part in layers from the bottom up, from a sliced CAD model, using the same five-step chain regardless of technology.
- Triangulation resolution affects only non-planar faces; set it against purpose, not by default.
- Orientation governs anisotropy, build time, surface quality and support burden simultaneously.
- Layer thickness in practice spans roughly 0.01 to 0.7 mm — a direct trade of build time against finish and accuracy.
- Supports are unavoidable for overhangs and should be planned with orientation rather than after it.
- The STL file carries facet vertices and outward normals; it is right for prototyping and wrong for design data exchange.
- Choose the build technique from the prototype's purpose, not from what happens to be free.
Series
Continue the pathway
The SOLIDWORKS Design Approach series works through computer aided design as an engineering discipline, from first principles to manufacture.