FDM Design Guideline

Jack Lie CNC machining expert

Specialize in CNC Milling, CNC Turning, 3D Printing, Urethane Casting, and Sheet Metal Fabrication Services.


FDM (Fused Deposition Modeling) 3D printing technology is the most affordable method for rapid and low-cost prototyping. Its products is suitable for various application, including functional parts like enclosure. However, in order to guarantee FDM parts quality, we need to consider its technology limitations and constraints in designing process, try to find the best solution to eliminate impact in printing process.


Bridging in FDM always occur between two support points or anchor points. In some case, if bridging don’t have support of initial layer structure, it cannot be avoid that printing material will sag. Bridges are normally used in horizon axis of holes in object walls and hollow in top or roof layer. In order to reduce impact of bridging, we recommend decrease bridge distance and add necessary support in design. Support provide a temporary platform for layer of bridge printing, and then remove support once printing process finish. In this step, marks and micro damage on parts surface should be noticed.

Design notice:

It is difficult to avoid sagging or mark of support in FDM bridges printing process, unless bridge length is less than 5mm.

Separate bridge into different parts, and form final parts in post-processing for smooth surface.

Vertical Axis Holes

FDM often prints vertical axis hole of micro size. Normally hole diameter will reduce as printing process. Once nozzle prints vertical axis holes, it will compress new printed layer, this compressing force will deform round layer into a wider and flatter shape. Then contact are of previous layer will increase to improve adhesion. This give rise to diameter decrease in hole printing process. This issue is particular for small diameter holes with great ratio of hole diameter and nozzle diameter.

Design notice:

With high accuracy requirement of hole diameter, we recommend further drilling to correct diameter.


Overhangs issues in FDM is this most common quality problems. It always occur once new printing layer material is only supported partially by below layers. Inadequate support will result to poor layer adhesion, bulging or curling.

Overhang angle less than 45 degrees will not affect parts quality. At 45 degrees, new printed layer has 50% support of previous layer, which will provide sufficient support and adhesion for further building. Above 45 degrees, additional support is need to guarantee new printed layer won’t bulge down. New printed layer become too thin at overhang edge will occur curling issue, finally result to deformation upward in various cooling cause.

Design notice:

Provide support for overhang angle over 45 degrees to eliminate limitations. Post-process to remove support marks on final surface.


In reason of FDM print nozzle is circular, so all corners and edges will have radio equal to nozzle size. This result that all features cannot be perfect square as design.

In sharp edges and corners, the first layer of material printing is significant, as the case in vertical holes, once nozzle prints each layer, it will compress printing material down and increase adhesion stress. So in the initial print layer, this will create a flare as “elephant foot”. This will impact further FDM parts assembly, because the flare will outside the desired dimension.

Warping is another issues related to first print layer in FDM. Compare to PLA material, ABS is more vulnerable to warp in reason of high printing temperature. Base layer is printed firstly, and it cools as other hot layers are still printing. This different cooling process will give rise to base layer curling and shrinking away form build plate.

Adding design of chamfer or radius on parts edges in build plate contact area will reduce all these problems. This also assist the component removal form build plate when printing process is finished.

Design notice:

For functional FDM parts with assemble or overall dimension requirement, add 45 degree chamfer or radius on all edges in build plate.

High precision form, we recommend other technologies like SLA or Polyjet.

Vertical Pins

Vertical pins are usually produced by FDM for parts assemble and alignment. This functional features are important of dimension accuracy in FDM process.

Large pins more than 5mm in diameter has strong connection with rest parts by printing of perimeter and infill. Less than 5mm only by perimeter without infill will result to weak connection and easy broken. In worst case, it cannot be created because of no enough layer material to adhere.

Design notice:

Add small fillets on pin base once vertical pin diameter less than 5mm.

Advanced design

There are several key factors need to be considered in FDM printing, reduce support requirement, part orientation and part building direction on build platform.

Model parts splitting

Splitting up a model will reduce its complexity, save cost and time. Overhangs need too much support can be removed by splitting complex shape into two simple sections, which can be printed individually. Then glue there two separate sections together if required.

Hole orientation

Avoid support for holes by changing printing orientation. It is difficult to remove horizontal support in holes, but rotate build direction to 90 degrees will eliminate support requirement. For multiple hole in different directions, consider blind holes firstly, then holes from smallest to largest diameter, finally critical size holes.

Build direction

Because of FDM nature anisotropic, it’s important to understand component application and success way of building form design. The lack of continuous material paths and concentration press will result to inherent weaker in one direction of FDM components. All layers are printed as round rectangle, joints area between each layer are small valleys, this will create stress concentration, which form potential cracks.