The short answer: your file decides more than the machine does

If you already have a CAD file, turning it into a physical prototype is a three-step job. Confirm the model is actually manufacturable, choose a process that matches what you need to learn from the part, and send a quote package a shop can price without asking you five questions. Most disappointments at this stage are not caused by the printer or the mill. They are caused by a model that renders beautifully on screen and was never checked against how a real part gets built.

The good news is that every one of those checks is cheap. Fixing geometry in CAD costs an afternoon. Fixing it after you have paid for a part, or worse, after a mold has been cut, costs real money.

Which file should you actually send?

Two families of file matter. Mesh files (STL, 3MF, OBJ) describe only the outer skin of your shape as a cloud of triangles. They are fine for 3D printing and useful for almost nothing else. Solid and boundary-representation files (STEP, IGES, Parasolid) preserve the true mathematical geometry, which is what a machine shop, a mold designer, or any engineer who picks up the project after you needs in order to machine the part, edit a dimension, or keep developing it.

Send both, and include the native file from whatever software you modeled in. The native file is the only one that keeps your sketches and parameters intact, so a design change becomes an edit instead of a redraw. If you are not sure which format does what, our comparison of STL and STEP file formats lays out the practical differences.

Audit the file before you order anything

Almost every model that arrives from outside a professional engineering workflow has at least one of these problems. Work through the list before you request quotes.

  • Open or duplicated geometry. Surfaces that never close into a watertight solid, zero-thickness walls, overlapping faces, inverted normals. All of it looks correct in the viewport and fails in build preparation, which is where you find out days later.
  • Walls too thin to survive. What you can draw is not always what a process can build. Each technology has a minimum wall thickness below which the part warps, tears, or simply does not form. The design rules for 3D printed parts are the fastest place to calibrate this.
  • Zero clearance between mating parts. In CAD two parts can touch perfectly. In reality you need a gap so they can be assembled. Parts modeled exactly nominal-to-nominal will not go together.
  • No critical dimensions or tolerances called out. Which measurements must be held, and where is drift acceptable? Without that, the shop guesses. Real achievable accuracy varies enormously by process, as our guide to 3D printing tolerances explains.
  • Units and scale. Rare, and brutal when it happens. A file saved in the wrong unit system arrives ten times too large or too small, and the mistake is only obvious when the box is opened.
  • No assembly logic. Screw bosses with no access for a driver, no place for a wire to run, a lid that cannot be lifted off without breaking something. This is the failure a rendering never shows you.

Choose the process from what you need to learn

The same file can become five very different prototypes. Before you order, answer three questions honestly: is this part meant to be looked at, used, or loaded; how many units do you need; and what is the real date it has to be in your hands. Those three answers narrow the options faster than any technical debate.

A form and grip study can be printed fast and cheap. A part that must carry load, resist heat, or survive hundreds of cycles needs a different material and often a different process entirely. Optically clear, flexible, or tightly dimensioned parts each push you toward a specific technology. Our overview of rapid prototyping techniques maps the trade-offs side by side.

It is also normal to split one model across processes: the enclosure printed, a load-bearing shaft machined from metal, a gasket cast in silicone. Nothing requires the whole assembly to come from one machine.

What a complete quote package contains

Shops quote faster and more accurately when they are not guessing. Send a STEP file plus the native file, a simple PDF drawing marking the handful of dimensions that matter and their tolerances, the material and finish you want (or a description of the environment if you do not know), the quantity, the date you need it, and a one-paragraph note on what the part has to do. That last paragraph often changes the recommendation more than the geometry does.

Get a manufacturability review while changes are still free

Before the first part is made, it is worth having an engineer look at the model the way the person who eventually has to produce it in volume would: draft angles, parting line placement, consistent wall thickness, a sane assembly sequence, fasteners that can be reached. Doing that review on the first prototype costs almost nothing, while the same change after tooling is committed is expensive and slow. The full logic is in our guide to design for manufacturing.

One pattern worth naming: a file built by an industrial designer is usually visually precise and manufacturing-naive — lovely surfaces, uneven wall thickness, no thought about how two halves close on each other. That is not sloppy work. It is a stage that simply has not happened yet.

If the file does not exist yet, or the budget is tight

Not everyone arrives here with a model. If you are starting from a hand sketch, the route is described in our walkthrough from sketch to 3D CAD model. And if the quotes coming back are higher than you expected, there is usually room to restructure the build rather than abandon it — see our practical list of ways to cut prototype costs without cutting what you are testing.

A clean CAD file is an asset

A properly built model shortens every future round, serves as the baseline for manufacturing quotes, and lets you approach several suppliers without redrawing anything. It is often worth more than the first prototype it produces. More material on building physical models at every stage is collected on our prototyping hub.

Have a CAD file and want to know whether it is genuinely ready to be built? Send us the details through our contact form and the Projects House engineering team will review the model, flag what needs fixing, and recommend the process that answers your specific question about the design.