A misconception persists stubbornly in industry: take an existing machined or injection-molded part, send the file to a 3D printer – and save money. In practice, that rarely works. Not because 3D printing is worse, but because every manufacturing process has its own design logic. Understanding that means choosing the right tool for the right job.
The Fundamental Difference
CNC machining removes material. It starts with a blank and mills away everything that isn’t part of the component. The result: highly precise parts with tight tolerances, metallic materials, smooth surfaces – but always with the drawback of tooling, fixturing and setup time. Every added complexity costs.
Injection molding fills a mold with liquid plastic. Once the tool is made, thousands of identical parts can be produced in seconds. Per-unit costs drop dramatically – but the tool costs between €5,000 and €100,000, and changes to it are expensive or impossible.
FDM 3D printing builds up additively. No tool, no mold, no setup effort. The geometry lives in the file. That means maximum flexibility, low entry costs – and clear limits on quantity and material properties.
The 1:1 Fallacy
The most common problem with 3D printing orders: designers deliver a machined-part design optimized for multi-axis machining. Flat faces, drilled holes, symmetric geometry – everything a milling machine likes.
The 3D printer can print that. But it can’t print it well. Thin, unsupported elements vibrate. Horizontal bores come out as ellipses. And strength in the Z direction is a fraction of the XY strength.
Printing a machined-part design is like driving a sports car on winter tires: it works, but it’s not what the system was built for.
The solution: additive geometries. Undercuts, internal channel structures, lattice infill, organic shapes – everything that overwhelms a milling machine is free in 3D printing. Rethinking a part for printing often results in a lighter, cheaper and functionally superior part.
Economic Thresholds: The Three Zones
Zone 1: Prototypes and Single Parts (1–10 units)
Here, 3D printing is almost always the right choice – for plastic parts without extreme tolerance requirements.
- No tooling, no minimum order value
- Lead time: 2–5 business days instead of 3–8 weeks
- Changes cost nothing but a new file
- Per-unit cost is higher than with series processes – but that doesn’t matter for 1–10 parts
Zone 2: Small Batches (10–500 units)
This is the critical decision zone. The question is: how stable is the design?
If the part is still being iterated → 3D printing. No tooling that would need adjusting with every change.
If the design is frozen → evaluate injection molding or machining, depending on geometry and material.
For high-performance polymers like ASA or PETG-HF, 3D printing can remain economically competitive up to 300–500 units – because injection-molding tools for technical plastics are often disproportionately expensive.
Zone 3: Series from 500 Units
From here on, injection molding or automated CNC generally wins. Tooling costs amortize, and per-unit costs drop below what FDM printing can achieve.
Exception: parts with high geometric complexity that are difficult to demold in injection molding. And parts that exist in multiple variants – here 3D printing can cover the variant diversity without needing a separate tool for each variant.
Cost Comparison: A Worked Example
| Process | Tooling cost | Per-unit cost (estimate) | Break-even |
|---|---|---|---|
| FDM 3D printing | €0 | €15–80 | – |
| CNC machining | €0–500 setup | €40–200 | from ~50 units |
| Injection molding | €8,000–30,000 | €1–5 | from ~500–2,000 units |
The figures vary considerably by geometry, material and supplier – but the ratios stay stable.
What 3D Printing Can’t Do
Honesty matters here too. FDM 3D printing is not a universal tool:
- Metal parts: not possible with FFF (except metal-composite filaments that are sintered afterward – a different process)
- Tolerances under ±0.1 mm: achievable only with post-processing
- Surface finishes Ra < 1.6: require sanding or painting
- Temperatures above 150 °C continuous: only with specialty materials (PC, PEI/Ultem) – outside our standard portfolio
Asking the Right Question
Before requesting a part, a simple checklist pays off:
- How many units do I need over the next 12 months?
- Is the design still in development or frozen?
- What temperature and chemical exposure must the part withstand?
- Which tolerances are functionally critical?
The answers determine the process – not the material datasheet. If unsure: send an inquiry. The feasibility assessment is free.