3D Printing or Injection Moulding? Costs, Quantities and Strength

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For single pieces, prototypes and small runs, 3D printing is almost always cheaper, because no mould has to be built. Injection moulding only pays off at larger quantities: the mould is expensive, but every further part is very cheap. The manufacturing service Xometry gives 250 to 2,000 parts as the typical point at which injection moulding becomes cheaper. Depending on the part, though, it can be well below or above that. On strength, injection moulding has an advantage: the parts have no layers. Printed FDM parts are weaker between the layers than along the lines.

How the processes differ

Injection moulding: plastic granules are melted and injected under pressure into a metal mould. After a short cooling time the finished part falls out. Most of the cost is in the mould.

3D printing (FDM): the printer lays down molten filament line by line and layer by layer. There is no mould, and each part costs roughly the same in material and print time. How exactly this works is explained in What is 3D printing?.

What an injection mould costs

For injection moulds, Formlabs gives a range from around 100 US dollars for a 3D-printed mould for small quantities to over 100,000 US dollars for complex steel moulds with several cavities.

The granules themselves are cheap, with Formlabs giving about 1 to 5 US dollars per kilogram. For comparison: filament costs a multiple of that, and current prices are in the filament overview.

Worked example from Formlabs

In one example, Formlabs calculates with three production runs. The figures are example values, not quotes:

This shows the principle: the larger the run, the less the mould cost weighs per part.

When does injection moulding pay off?

Xometry uses examples to show how much the break-even point varies:

• Small control knob: printed, it stays competitive up to about 2,000 pieces. Above that it costs about 0.80 US dollars per piece in injection moulding, not counting the mould.
• Large connector housing: printing alone already costs over 200 US dollars per part. The mould costs around 10,000 US dollars here, but injection moulding is cheaper from about 250 pieces.

Important: Xometry’s examples refer to industrial printing processes such as SLS and Multi Jet Fusion. For a part from your own FDM printer the calculation is different, because you mainly pay for filament, electricity and your time. How to work this out is explained in Calculating print costs.

Rule of thumb: the larger the part and the more pieces you need, the sooner injection moulding pays off. The smaller the part, the fewer pieces, and the more often the design still changes, the more likely 3D printing stays ahead. Xometry points out that design changes are expensive in injection moulding, because the mould has to be modified.

Strength: the problem with layers

FDM parts consist of lines and layers that only partly fuse with each other. Formlabs describes how tiny voids remain between the round lines, which is why printed parts are less dense than comparable injection-moulded parts. An FDM part is strongest along the printed lines and weaker across them and between the layers.

Experts call this anisotropy: the strength depends on the direction. Formlabs quotes a test in which the yield strength of an FDM part in the build direction upwards (Z axis) reached only about 55 % of the value in the X direction. This is a single test, and depending on material and settings the difference turns out differently. The direction remains important, though.

Injection-moulded parts, by contrast, are formed in one piece from the melt and have no layer boundaries.

What this means for you:

• Orient the part so that the main load acts along the layers and does not pull the layers apart.
• More walls often bring more strength than more infill, see Walls and top layers.
• Choose a suitable material: some filaments bond better between the layers than others. An overview is in Which filament for what?.

What is harder with injection moulding

Injection moulding places demands on the shape of the part that hardly matter in printing. Protolabs lists, among others:

• Draft angles: vertical surfaces need a slight taper so the part slides out of the mould. As a rule of thumb, Protolabs gives 1 degree per inch of depth.
• Uniform wall thicknesses: areas that are too thick lead to sink marks, warping and air pockets. Instead of solid walls, ribs are used.
• Undercuts: they make ejecting the part harder and need extra components in the mould such as slides. This makes the mould and production more complex. According to Protolabs, simple moulds without extra components save money.

A printed part, on the other hand, can have internal cavities, lattices or shapes that could not be removed from any mould. Protolabs notes that with prototypes from the 3D printer, suitability for injection moulding is often barely considered. If you want to go into series production later, you should plan for these rules early.

In short

If you want to sell printed parts, Calculating print costs provides the basis for the calculation.

Sources

• Mould costs, granule price, worked example with three runs: Formlabs: How to Estimate Injection Molding Cost?
• Break-even 250 to 2,000 parts, examples, design changes: Xometry: Injection Molding vs. 3D Printing
• Anisotropy of FDM parts, test with about 55 %: Formlabs: Validating Isotropy in SLA 3D Printing (Formlabs sells resin and SLS printers)
• Draft angles, wall thicknesses, ribs, undercuts: Protolabs: Injection Molding Basics and 9 Mistakes to Avoid When Designing Injection Molded Parts