Design for FDM printing: a practical guide
By the Print a Part engineering team
FDM (fused deposition modelling) printers build parts layer by layer by extruding thermoplastic. Because of this layered process, design decisions directly affect part strength, surface finish, cost, and overall print success. This guide explains how to design parts that print reliably and perform as intended.
The figures in this guide are general FDM design guidance, not a guarantee of the accuracy of any particular part. If a dimension is critical, tell us in the order notes.
1. Understanding how FDM works
FDM prints parts one layer at a time, bonding each new layer to the one below.
As a result:
- Parts are strongest along the X and Y axes
- Parts are weaker along the Z axis (between layers)
- Good geometry helps ensure parts are strong, cost-effective, and printable.
Design tip: Always consider how forces will travel through the layers to avoid weak points.
2. Part size and build volume
Our standard build volume takes parts up to 350 × 350 × 340 mm in one piece.
Parts larger than this must be split and assembled after printing. While this is common, it can increase production time and may affect strength depending on joint design.
Designing within the build volume generally results in faster, more reliable prints.
3. Wall thickness
Recommended minimum: 1.2 mm, three extrusion widths from a standard 0.4 mm nozzle.
Absolute minimum: 0.8 mm, two extrusion widths. Walls this thin may print inconsistently or lack durability, so keep them to features that carry no load.
Recommended for strength: 2.0 mm or greater.
Consistency is important. Wall thickness should align with extrusion widths to avoid inefficient toolpaths and weak areas.
Design tip: Use consistent wall thickness throughout the part for improved strength and print reliability.
4. Strength through geometry
In FDM printing, geometry has a greater impact on strength than infill percentage.
To improve strength:
- Increase wall thickness rather than only increasing infill
- Add ribs instead of solid blocks
- Use fillets at internal corners to reduce stress concentration
- Align load paths with the X/Y direction wherever possible
Sharp internal corners are common failure points and should be avoided.
5. Fillets and chamfers
Fillets (rounded corners):
- Distribute stress more evenly
- Improve layer bonding
- Reduce the likelihood of cracking
Chamfers (angled edges):
- Reduce support requirements
- Improve edge durability
- Create cleaner surface transitions
Design tip: Replace flat horizontal overhangs with 45° chamfers to reduce or eliminate the need for support material.
6. Overhangs and support
FDM printers cannot print unsupported material in mid-air.
Overhangs up to about 45° from vertical usually print without support. Steeper overhangs need support material, which:
- Increases print time and cost
- Requires post-processing
- Can affect surface finish
Best practice: Design self-supporting features and orient parts to minimise the need for support.
7. Model integrity
All models must be a single, watertight solid.
Avoid:
- Gaps or open surfaces
- Overlapping geometry
- Disconnected bodies
Clean, closed geometry ensures predictable slicing and reliable printing.
8. Fine details
FDM printers are limited by nozzle diameter and layer height.
Very small details may:
- Lose definition
- Appear softened
- Fail to print accurately
Design tip: Make features comfortably larger than the minimum printable size to ensure clarity and durability.
9. Bed adhesion
The first layer is critical to print success.
Parts with:
- Flat contact surfaces
- Balanced geometry
- Even weight distribution
are less likely to warp or detach during printing.
Tall, narrow, or top-heavy designs may require additional consideration during setup.
10. Print resolution
Typical FDM layer heights range from 0.1–0.3 mm:
- Low: Fastest and most economical
- Standard: Balanced speed and surface finish
- High: Smoother surface detail
For functional parts, strength is influenced far more by geometry and orientation than by very fine layer heights.
11. Designing for efficient production
Parts designed with manufacturing in mind:
- Print faster
- Use less support material
- Cost less to produce
- Scale more reliably
Simple, well-considered geometry almost always outperforms overly complex designs.
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