3D printing vs injection moulding for low-volume production

How the two processes differ

Injection moulding forces molten plastic into a machined mould under pressure. Once the mould exists, a part can be made in seconds and every part comes out almost identical. The cost and the time sit at the start: the mould has to be designed, machined, tested and adjusted before production begins.

FDM 3D printing builds the part directly from your STL or 3MF file, one layer of melted filament at a time. There is nothing to make first, so the first part costs much the same as the hundredth. The cost sits in each part instead: material, and the hours the printer spends on it.

Why there is no universal crossover quantity

You may see a single number quoted as the point where moulding becomes cheaper. In practice it moves a long way from part to part, because both sides of the comparison change:

• The mould. A small, simple part with no undercuts needs a much cheaper mould than a large part with sliding cores or several cavities.
• The print. A small part that prints in minutes stays cheap at quantity; a large, solid part that takes hours per copy does not.
• The lifetime volume. What matters is how many you will make over the life of the design, not just the first order.
• How settled the design is. If the part is likely to change, each change costs little with printing and potentially a lot with a mould.
• What the part has to do. A cosmetic surface, a particular plastic or tight tolerances may rule one process in or out before cost comes into it.

A fair comparison is simple to set up: for moulding, add the mould cost to the per-part price multiplied by your lifetime quantity; for printing, multiply the per-part price at that quantity. Do it for two or three realistic quantities, using real quotes for your own part.

When low-volume FDM production makes sense

• The quantity is too small to justify a mould, now or over the product's life.
• The design is still changing, or you sell several variants in small numbers each.
• You need parts now, and a mould would take weeks you don't have — often called bridge production.
• Demand is uncertain, so making parts to order is safer than paying up front for tooling and stock.
• The part's shape suits printing and a layered finish is acceptable.

When injection moulding is likely to be more economical

• The design is final and volumes are high and steady.
• The part needs a moulded cosmetic surface, or a plastic that isn't available as a printing filament.
• The part is large or solid, so printer time makes each printed copy expensive.
• Tight tolerances or uniform strength in every direction are essential.

Design, tolerances and materials

A part designed for moulding usually prints well, but the reverse isn't always true: printed parts don't need draft angles and can have shapes a mould can't release. If you print now and may mould later, design with moulding in mind (even wall thickness, draft where it costs nothing) so the switch doesn't mean a redesign.

FDM tolerances are looser than moulding, and holes tend to print slightly small, so mating features need designed-in clearance (see why printed holes come out small). Layer & Loop Store prints files at the size they are supplied and doesn't guarantee dimensional tolerances, so test the fit on a sample before a run.

On materials, choose the printing filament for what the part has to do rather than trying to copy a moulding plastic exactly (see choosing a material). Layer & Loop Manufacturing confirms the material for each job during production review.

Getting a production estimate

If printing looks like the right route for your quantity, the quickest way to compare is a real price for your own part.

1. Have a production-ready STL or 3MF. Layer & Loop Store doesn't provide design or engineering changes, so the file should be final.
2. Upload it to the production estimate from Layer & Loop Manufacturing, the manufacturing service from Layer & Loop Store. It shows indicative prices across several quantities, and your file stays on your device until you submit the project.
3. Compare those figures with a moulding quote for the same part at the same quantities.