[3D Printing News] How to Design 3D Printed Parts for FDM Printers
Learn how to optimize common design features in FDM 3D printing, such as bridging, overhang, pins, and vertical shaft holes.
Table of contents
introduce
bridging
Vertical shaft hole
Overhang
corner
positioning nails
Advanced Design
Rules of thumb
introduce
As the most cost-effective 3D printing technology on the market, FDM is ideal for rapid and low-cost prototyping and can be widely used in a variety of applications. It can also be a suitable solution for functional parts such as housings.
FDM extrudes molten filaments along a predetermined path onto a pre-built surface.
When the material is compressed, it cools and forms a solid surface, providing a foundation for the construction of the next layer of material.
It repeats layer by layer until the printed object is complete.
Like all manufacturing methods, FDM has some limitations on what can be printed.
This article will introduce these limitations and discuss methods that can be implemented during the design phase to reduce their impact on print quality.
bridging
When FDM bridging occurs, it is usually because the printer needs to print between two supports or anchor points.
Since there is no support for the initial layer to be printed (there is nothing to build on), and gaps need to be "filled," the material will tend to sag.
Bridging typically occurs in horizontal shaft holes in the walls of an object or in the top layer (or roof) of a hollow section.
Print bridge spacing is 25mm, 35mm and 45mm
One solution to reduce the impact of bridging is to reduce the distance between bridges, but this depends on the design constraints of the parts. Another solution to avoid sagging is to use supports.
The support provides a temporary construction platform for building the bridging layer.
Once printing is complete, the support material is removed. This may leave marks or damage on the surface where the support is attached to the final part. 
The surface roughness of the FDM-printed mosaic image after removing the support.
Key design considerations: Due to the nature of FDM, unless the bridge thickness is less than 5mm, sagging or marks on the supporting material will exist.
If a level and smooth surface is required, a more advanced solution is to divide the design into different sections, or consider alternative post-processing techniques.
Vertical shaft hole
FDM often prints vertical shaft holes that are too small. The reason for this diameter reduction during the printing process is generally:
1. When the nozzle prints around the vertical shaft hole, it compresses the newly printed layer onto the existing build-up layer to help improve adhesion.
2. The compression force of the nozzle changes the shape of the extruded layer from a circle to a wider and flatter shape (see figure below).
3. This increases the contact area with the previously printed layer (improving adhesion), but also increases the width of the extruded portion.
4. Finally, the diameter of the printing holes is reduced.
Printing small-diameter holes can cause particular problems due to the ratio of hole diameter to nozzle diameter. 
The variation in the slicing program and the actual diameter of the vertical hole are due to the compression of the extrusion profile.
The size will be too small, depending on the printer, slicing software, hole size, and material.
Reducing the diameter of the vertical axis hole is typically considered within the slicing software, but accuracy may vary and multiple test prints may be required to achieve the desired precision.
If high precision is required, drilling may be necessary after printing.
Key design considerations : If the diameter of the vertical axis hole is critical, it is recommended to reduce the print size and then drill the hole to the correct diameter.
Overhang
Sagging is one of the most common print quality issues associated with FDM. It occurs when the printed layer material is only partially supported by the underlying layer, resulting in a bulge.
Similar to bridging, insufficient support provided by the surface beneath the building layer can lead to poor layer adhesion, protrusion, or curling. 
The impact of increasing the hang angle (in increments of 5 degrees) on print quality. The maximum display angle is 70 degrees.
Depending on the material, it is usually possible to print a raised section without losing quality up to 45 degrees.
At 45 degrees, the newly printed layer will have 50% support in its first layer. This provides sufficient support and adhesion. Above 45 degrees, support is needed to ensure that the newly printed layer does not bulge downwards and away from the nozzle.
Another problem that occurs during printing is curling. Newly printed layers become thinner at the convex edges, causing cooling differentials that result in upward warping (see above image).
Key design considerations: Restrictions on overhang can be eliminated by using supports for wall angles greater than 45 degrees. For larger overhangs, additional support is required.
Unless post-processing is performed, markings will appear on the final surface.
corner
Because the print nozzles in FDM are circular, the radii of the corners and edges will be equal to the nozzle size. This means that these functions will never be perfectly square.
For sharp edges and corners, the first layer of printing is particularly important. As discussed above regarding the vertical axis hole, the nozzle compresses the printing material to increase adhesion as it prints each subsequent layer. This initial layer of printing creates an outward-expanding shape commonly referred to as the "elephant's foot."
This will affect the ability to assemble FDM parts because the outward protrusion exceeds the specified dimensions. 
Another common problem associated with the initial print layer of FDM printing is warping, which can be seen in the side view of the large image foot feature on the initial print layer.
Compared to PLA, ABS is more prone to warping due to its higher printing temperature.
The base layer is the first layer to be printed and cooled, because the other heated layers are printed on top.
This can lead to differential cooling and may cause the underlying layers to leave the printing platform during shrinkage.
Adding a chamfer or radius along the edge of the part that contacts the printing platform will reduce the impact of these problems.
Once printing is complete, this will also help remove objects from the printing platform.
Key design considerations: If the assembly or overall dimensions are critical to the function of the FDM part, then all edges in contact with the printing platform, including...
Pay attention to 45-degree chamfers and radii. For high-precision shape and fit testing, other technologies such as SLA or Polyjet are recommended.
positioning nails
Positioning pins are typically printed in FDM when parts need to be assembled or aligned.
Given that these functions are usually functional, it is important to know that FDM can accurately print the dimensions of the positioning pins.
Larger nails (diameter greater than 5mm) are printed with their circumference and filler, and are strongly integrated with the rest of the print. Smaller nails (diameter less than 5mm)
It can only print the perimeter without the fill.
This can create discontinuities between the printed parts and other components, resulting in weak, easily broken connections. In the worst case, the small nails may fail to print because there isn't enough printing material to bond the new layer.
The printing of positioning pins with decreasing diameters (from 25 to 5 mm) indicates that the upper diameter of the printout has become too small to print accurately.
Regularly adjusting the correct printer calibrations (optimal layer thickness, print speed, nozzle temperature, etc.) can reduce the likelihood of staples falling off. Adding a radius to the base of the staple eliminates concentrated pressure at that point and increases strength.
For critical nails with a diameter less than 5mm, inserting a ready-made nail into the print hole may be the best solution.
Key design considerations: If your design includes nails smaller than 5mm in diameter, you can add a small rounded corner to the bottom of the nail. If functionality is critical, consider designing a space for a hole and nail, drilling the hole to the correct size and inserting a ready-made nail.
Advanced Design
Several key considerations when using FDM printing include minimizing the number of supports required, part orientation, and the orientation of parts on the printing platform.
Decompose your model
Typically, disassembling a model reduces its complexity, saving cost and time. Protruding sections requiring extensive support can be removed by simply dividing the complex shape into separately printed parts. If needed, these parts can be glued back together once the printing is complete. 
Disassemble the model to eliminate the need for support.
Hole positioning
Changing the printing orientation is the best way to avoid supports in the holes. Removing supports in horizontal axis holes is often difficult.
However, by rotating the construction direction by 90 degrees, the need for support is eliminated.
For parts with multiple holes in different directions, prioritize blind holes first, then proceed from the smallest to the largest hole diameter, and finally the critical hole size. 
Repositioning the horizontal shaft hole eliminates the need for a support.
Establish direction
Due to the anisotropic nature of FDM printing, understanding the application of parts and how to build them is crucial for the success of a design.
Due to the orientation of the layers, FDM parts are more fragile in one direction. 
The lack of a continuous material path and the concentrated pressure generated at each joint contribute to this defect.
Because the layers are printed as rounded squares, the seams between each layer actually resemble small depressions. This creates concentrated pressure where cracks form.
Rules of thumb
• If the bridge thickness exceeds 5mm, sagging or marks on the support material may occur. This problem can be eliminated through component design or post-processing.
• For important vertical hole diameters, if high precision is required, it is recommended to drill after printing.
• For corners greater than 45 degrees, additional supports should be added to allow the FDM printer to print.
• On all edges of the FDM part, including 45-degree chamfers or radii that contact the printing platform.
• For applications using small vertical nails, add a small rounded corner at the bottom or consider inserting ready-made nails into the printed holes.
• Disassembling the model into parts, repositioning holes, and specifying the building direction are all factors that can reduce costs, speed up the printing process, and improve design strength and print quality.
Original source: https://www.3dhubs.com/knowledge-base/how-design-parts-fdm-3d-printing#advanced-design