Design Guide

Fused Deposition Modeling

Our design guidelines for Fused Deposition Modeling (FDM) include important information to improve part quality, minimize costs, and reduce overall manufacturing time. By following the guidelines, you can produce high-quality parts, reduce expenses, and improve productivity.

If you have any questions regarding our design guidelines, you can contact our technical team by telephone, email or chat with us online.

Download Guidelines as PDF


Fused Deposition Modeling (FDM) melts thermoplastic filament and extrudes it through a heated nozzle onto a build platform. Known for its versatility, ease of use, and ability to produce functional prototypes and end-use parts alike in a wide range of materials.

Maximum Build Volume
914mm x 609mm x 914mm (36" x 24" x 36")

FDM produces parts up to 36 inches in a single build. Parts larger than the build volume can be printed in sections and assembled.

Fortus 450 (16" x 14" x 16") : ±0.25 mm (±0.010"), or ±0.2%, whichever is greater.
Fortus 900 (36" x 24" x 36") : ±0.762 mm (±0.030"), or ±0.5%, whichever is greater.

Tolerances as tight as +/- 0.127 mm (+/-0.005") can be achieved on request. If tighter tolerances are required, please provide an engineering drawing when requesting a quote.

Details in the Z axis will be rounded up to the nearest slice.

Layer Height
125 - 330 microns

Standard layer height is 254 microns (0.010″) unless otherwise stated.

Industrial FDM machines can print parts with custom layer thickness, in the range of 125 - 330 microns thick, depending on the settings specified. Thicker layers typically result in better adhesion and better mechanical properties.

Surface Finish
Visible Layer Lines

Parts made with FDM will always show visible layer lines, especially on curved surfaces. However, the visibility of layer lines can be reduced with smaller layer heights at the expense of print speed.

Wall Thickness

Thicker geometries and walls with variable thickness are at risk of deformation due to shrinkage and stress. In general, follow wall design rules for injection molding to achieve a more uniform and consistent part.

Minimum Wall Thickness
1mm (0.039”)
Supported walls are connected to two or more sides and are thick enough to support the model.
Minimum Unsupported Wall Thickness
1mm (0.118”)
Unsupported walls are connected on only one side or edge.
Minimum Pin Diameter
3mm (0.197”)

The minimum pin size depends on a combination of parameters such as; part orientation, nozzle diameter, and length of the pin.

Minimum Detail Size

The quality of engraved and embossed features depends heavily on part orientation and the resulting thermal effects of printing. Forge Labs recommends using a 3-5 degree draft angle when extruding text as the extra taper from the draft can help preserve intricate details.

Debossed Details
Minimum Width: 1mm (0.039”)
Minimum Depth: 0.3mm (0.011”)

Debossed details are recessed features on your model. Making these too small can result in closed gaps and loss of detail.

Embossed Details
Minimum Width: 1mm (0.039”)
Minimum Depth: 0.5mm (0.019”)

Embossed details are raised features on your model. These usually resolve better than engraved details.

Extruded text should have a wall thickness of at least 1.5mm (0.059") when printed flat, and 2mm (0.079") when
printed vertically.

Minimum point size: 16pt

Minimum suggested text size on the top or bottom build plane of a FDM model is 16 point boldface. Minimum suggested text size on vertical walls is 10 point bold. 

In most cases the supports generated to support text on a vertical wall can be eliminated to save time and material.

Part Clearances

The mechanical performance of printed assemblies is sensitive to variables such as print orientation, layer height and machine tolerances. It is best to test and refine any mated parts before committing to a full production run.

Minimum Clearance
0.5mm (0.019”)

Clearance is the distance between two moving parts on hinges, joints, mating parts, etc. We require a minimum of 0.5mm on all sides.

Minimum Press Fit
0.1mm (0.004")

For a tight press fit, add a small offset, and chamfer the leading edge to ease entry. Always print an initial test part to dial in dimensions.

Minimum: 0.6 mm (0.023")

This defines the minimum allowed gap between a bush and a bolt, to avoid bonding of the hinge parts. 

Holes & Gaps

Hole accuracy can vary depending on print orientation, thickness of surrounding material and support structures. For tight tolerances, consider printing center marks and post-machining them instead.

Minimum Hole Size
0.5mm (0.019”)

Depending on orientation, circular holes can resolve as ovals – if higher accuracy is required, printing the hole smaller and then drilling it out is recommended.

Minimum Gap Size
0.5mm (0.019”)
Gaps or slots can fuse if made too narrow, and support material may be difficult to remove. If possible, orient gaps along the xy-plane for highest accuracy.
Minimum Drain Hole Size
4 mm (0.15”)

Hollow features may be filled with soluble support material, and must be designed with drain holes so that the support can be dissolved and removed during cleaning. Larger parts or parts with complex internal features may require larger holes to allow support material to fully dissolve.

Part Geometry

Part geometry can be optimized for 3D printing to reduce material usage, increase strength and rigidity, and take advantage of machine capabilities.


Fillets can be used to reduce stress concentrations and increase the strength of your part. 

When designing fillet features across inside and outside corners, it is best to have them share a common center point in order to maintain a consistent wall thickness.

Bosses and Ribs

Bosses and ribs are an effective way to add strength and stiffness to a part while keeping material consumption to a minimum, this can also reduce build time and use less support material. 

Bosses and ribs can generally be matched to part thickness

Support Structures

A separate support material is used to hold the part up during building. All materials aside from ULTEM use soluble support material that is dissolved, leaving no imperfections on the surface of the part. This soluble support is excellent for parts with more complex internal volumes. Support for ULTEM has to be manually removed and can leave some minor scratches on parts.


Infills are used in FDM printing to save material and print time while maintaining the overall mechanical properties of the part. Parts will be printed solid unless otherwise specified. 

‘Sparse fill’ prints large volumes with a honeycomb structure instead of solid plastic. This can help to dramatically reduce manufacturing costs.

Hollow walls and enclosures will be filled with support material, and must be designed with drain holes so that the support can be dissolved and removed during cleaning.

Threads & Inserts

FDM is not well suited for printing threads, so inserts are recommended. As a general rule, avoid printing hardware.

3D Printed Threads

Very large threads can be printed, but most threads do not perform well in FDM. The use of inserts is recommended.

Tapping & Threading

Design parts with holes sized for tapping, or center marks for drilling a pre-tapped hole. You may need to reference a tap and drill chart to figure out which hole size corresponds to a given thread. 

Tapping removes material, puts pressure on the surrounding geometry, and fastener locations are often points of stress, so make sure there is enough material surrounding the tapped hole.

Tapping FDM parts often produces weaker threads that are not suitable for frequent re-use, especially when the threads run along the layer lines of the part.

Heat Set Inserts

Threaded inserts can be glued in, or heated and pressed in.

Heat set inserts require a soldering iron with an installation tip, which is used to heat the metal insert. This softens the plastic walls, allowing the insert to be pushed into the hole or boss before the plastic hardens around the insert.

The boss should be designed around the heat set insert intended to be used. The diameter should be set to the maximum diameter of the insert, keeping in mind the offset for the potential thermal shrinkage of the outside walls. We recommend adding a chamfer or fillet around the edge of the boss to allow a small amount of plastic to flow over the top edge of the insert.

Post Processing

While printed parts are highly functional and accurate, there are a variety of ways to modify them after printing to enhance durability or aesthetics.

Sectioning, Joining, and Bonding

Parts too big to fit on the print tray can be split in CAD and printed in multiple parts, then glued or press fit.  Use joinery features, such as tongue and groove, to ensure the sections are aligned and bond tightly. 

Depending on orientation, material and other properties, an offset may be required to make sure the parts fit together properly. We recommend printing a small test part to dial in the offset before printing the full set of parts.

Parts can be machined to tighter tolerances using traditional or CNC subtractive manufacturing equipment.

Parts can be sanded, primed, and painted for an excellent surface finish.

Parts can also be coated with a lacquer, varnish or clear coat for various custom finishes. These finishes can improve wear resistance, UV resistance, surface hardness, water tightness and limit marks and smudges on the surface of the part.

Vapor Smoothing

Vapor smoothing uses acetone vapor to increase plasticity of the material allowing it to flow, smoothing layer lines. With enough exposure, the resulting finish can be uniform and glossy, although this comes at the expense of dimensional accuracy.

This is only applicable to ABS and ASA materials.