3D printing or plastic injection moulding: how to choose?

Two opposing economic logics

3D printing and plastic injection moulding don't differ on quality but on the structure of their costs.

• Injection moulding relies on a metal mould machined specifically for the part. This mould represents a significant fixed cost, paid once before the first part. After that, each part comes out in a few seconds to a few tens of seconds, for a low unit cost.
• 3D printing has practically no fixed cost: no tooling, just a file to prepare. On the other hand, each part occupies a machine for minutes or hours, and its unit cost stays roughly constant regardless of quantity.

In other words, injection moulding becomes increasingly advantageous as quantity increases, while 3D printing is unbeatable for small quantities. Between the two lies a break-even quantity.

Comparing the two processes

Finding the break-even quantity

The reasoning is simple, even though the figures depend on each part. You need to compare the total cost of both options for a given quantity:

• Total injection cost = mould cost + (injected unit cost × quantity)
• Total 3D printing cost = printed unit cost × quantity

The break-even quantity is the one at which these two costs are equal. It's obtained by dividing the mould cost by the gap between the printed unit cost and the injected unit cost:

Break-even quantity ≈ mould cost ÷ (printed unit cost − injected unit cost)

Below this quantity, 3D printing costs less overall; above it, injection moulding wins out. To give an order of magnitude: an injection mould generally costs from a few thousand to several tens of thousands of euros, depending on the part's size, its complexity (side-actions, cores, number of cavities) and the mould material. With these gaps, the break-even quantity often falls between a few hundred and a few thousand parts. It drops for large parts that take a long time to print, and rises for small, simple parts that print quickly and in large numbers on the same plate.

What the calculation doesn't tell you

The break-even quantity is a starting point, not a decision. Several factors shift it:

• Volume uncertainty: if you're not sure you'll sell the planned quantity, a mould remains a gamble. 3D printing reduces this risk.
• Design stability: a product still likely to change will require a mould rework with every change.
• Time to market: selling several weeks earlier can be worth more than a lower unit cost.
• Stock and cash flow: injection moulding is cost-effective in large batches, which have to be stored and financed. Printing lets you produce as demand arises.
• Required properties: if the part must be certified in a specific material or reach properties that printing can't guarantee, injection moulding is required regardless of quantity.

Additional costs to factor in

An honest comparison doesn't stop at the mould and the unit price. On the injection side, add mould fine-tuning (trials, samples, corrections), any minimum order quantities per batch, mould storage and maintenance, and the cost of a modification if the design changes. On the 3D printing side, factor in each part's post-processing (support removal, finishing, inserts) and inspection, which repeat with every unit instead of being absorbed by the tooling. These items can shift the break-even quantity significantly, in either direction.

To estimate the printed unit cost of your part, the factors to consider are detailed in our guide to the price of 3D printing.

Bridge production: using both processes

The choice isn't always exclusive. A common strategy is to chain the processes together:

1. Printed prototypes to validate shape, assembly and function, with fast iterations. Our guide to rapid prototyping and small batches details this phase.
2. Printed pre-production run to deliver to the first customers, test the market and gather field feedback while the design settles.
3. Bridge production while the mould is being made: 3D printing supplies parts during the weeks needed to machine and fine-tune it.
4. Injection moulding once the design is locked and volumes are confirmed.
5. Back to printing at end of life: when volumes drop off, spare parts can be printed on demand instead of maintaining a stock or a mould.

This approach reduces the risk of a mould being commissioned too early and allows selling before the tooling is ready. It does, however, require designing a part compatible with both processes, or accepting a slight adaptation when switching to injection moulding.

Designing with what comes next in mind

If injection moulding is being considered down the line, a few precautions during the printed phase avoid a complete redesign:

• Uniform wall thicknesses: essential in injection moulding to avoid sink marks and warping, and they don't hinder printing.
• Draft angles: plan from the start for a slight angle on faces parallel to the mould opening direction.
• Limited undercuts: every undercut adds a side-action to the mould and raises tooling costs. What's free in 3D printing no longer is in injection moulding.
• Ribs rather than solid masses, with thicknesses less than that of the wall they support.
• Parametric model: much simpler to adapt than a mesh. See parametric model or mesh.

Conversely, if the part will remain printed, take full advantage of the design freedom: part consolidation, internal channels, lightweight structures. The rules specific to the process are explained in our guide to 3D printing technologies, and the choice of material in our guide to 3D printing materials.

In summary: how to decide

• A few units to a few hundred, design still in flux, urgent need or customisation: 3D printing.
• Several thousand identical parts, locked design, guaranteed volumes: injection moulding.
• In between: calculate the break-even quantity with real figures, then weigh it against commercial risk, lead time and cash flow. Bridge production often lets you avoid deciding too early.

To launch a pre-production run or bridge production, the Timelapse-3D workshop prints continuously across more than twenty machines, with round-the-clock production and delivery across France. You can also generate and price a simple part directly from the AI 3D Studio.