Learn how to optimize common design features (such as bridging, overhangs, pins, and vertical axis holes) for FDM 3D printing.
Table of Contents
Introduction
Bridging
Vertical Axis Holes
Overhangs
Corners
Locating Pins
Advanced Design
Rules of Thumb
Introduction
As the most affordable 3D printing technology on the market, FDM is ideal for rapid and low-cost prototyping and can be widely used in various applications.It can also be a suitable solution for functional parts (such as enclosures).
FDM extrudes molten filament onto a pre-built surface along a predetermined path.
As the material is extruded, it cools, forming a solid surface that provides a base for the construction of the next layer of material.
This process is repeated 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 discuss these limitations and methods that can be implemented in the design phase to reduce their impact on print quality.
Bridging
When bridging occurs in FDM, it's typically because the printer needs to print between two supports or anchor points.
Since there is no support for the initial layer of the print (nothing to build on) and a gap needs to be "filled," the material will tend to sag.
Bridging usually occurs in horizontal axis holes in the walls of an object or in the top layer (or roof) of hollow sections.
Printing bridge distances of 25mm, 35mm, and 45mm
One solution to reduce the effects of bridging is to reduce the distance of the bridge, but this depends on the design constraints of the part. Another solution to avoid sagging is supports.
Supports provide a temporary construction platform for building bridge layers.
Once printing is complete, the support material is removed. This can leave marks or damage on the surface where the support connected to the final part.
FDM printed jigsaw pieces showing surface roughness after support removal
Key design considerations: Due to the nature of FDM, unless the bridge thickness is less than 5mm, sagging or support marks will be present.
If a horizontal and smooth surface is required, advanced solutions involve splitting the design into different parts or considering other forms of post-processing.
Vertical Axis Holes
FDM often prints vertical axis holes that are undersized. The reasons for the diameter reduction during the printing of holes are:
1. When the nozzle prints the perimeter of a vertical axis hole, it compresses the newly printed layer onto the existing build layer to improve adhesion.
2. The compression force of the nozzle changes the shape of the extruded layer from circular to a wider and flatter shape (see image below).
3. This increases the contact area with the previously printed layer (improving adhesion), but also increases the width of the extruded section.
4. The result is a reduction in the diameter of the hole being printed.
This can be a particular problem when printing small diameter holes due to the ratio of the hole diameter to the nozzle diameter.
Varying slicer programs and actual vertical hole diameter changes due to compressed extrusion profile
Undersizing will depend on the printer, slicing software, hole size, and material.
Vertical axis hole diameter reduction is often accounted for within the slicing software, but accuracy can vary, and multiple test prints may be required to achieve the desired accuracy.
If high precision is required, drilling the hole after printing may be necessary.
Key design consideration: If the diameter of vertical axis holes is very critical, it is recommended to print them undersized and then drill them to the correct diameter.
Overhangs
Overhang issues are among the most common print quality problems associated with FDM. Overhangs occur when printed layer material is only partially supported by the layer below.
Similar to bridging, insufficient support provided by the surface below the build layer can lead to poor layer adhesion, protrusions, or curling.
The effect of increasing overhang angle (increasing by 5 degrees upwards) on print quality. The maximum angle shown is 70 degrees.
Depending on the material, an overhang can generally be printed without loss of quality up to 45 degrees.
At 45 degrees, the newly printed layer will have 50% support from the first layer. This provides sufficient support and adhesion. Above 45 degrees, supports are needed to ensure that
the newly printed layer does not sag downwards and away from the nozzle.
Another problem that appears during printing is curling. The newly printed layers become thinner at the overhanging edges, leading to differentiated cooling and upward warping (see image above).
Key design considerations: Limitations on overhangs can be overcome by using supports for wall angles greater than 45 degrees. For larger overhangs that require supports,
marks will appear on the final surface unless post-processed.
Corners
Since the print nozzle in FDM is circular, the radius of corners and edges will be equal to the size of the nozzle. This means that these features will never be perfectly square.
For sharp edges and corners, the first printed layer is particularly important. As discussed above for vertical axis holes, when the nozzle prints each layer, it compresses
the printed material to improve adhesion. When printing the initial layer, an outward expansion commonly known as "elephant's foot" occurs.
This affects the ability to assemble FDM parts, as this outward protrusion exceeds the specified dimensions.
Side view of elephant's foot feature visible on the initial layer of an FDM print
Another common problem associated with the initial printed layer of FDM prints is warping.
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, as other heated layers are printed on top.
This can lead to differentiated cooling and may cause the bottom layer to lift off the print bed as it shrinks.
Adding a chamfer or radius along the edges of the part that contact the print bed will reduce the impact of these issues.
This will also help remove the object from the print bed once printing is complete.
Key design considerations: If assembly or overall dimensions are critical to the function of the FDM part, then all edges in contact with the print bed, including a 45-degree chamfer or radius,
should be considered. For high-precision shapes and fit testing, other technologies like SLA or Polyjet are recommended.
Locating Pins
Locating pins are often FDM printed when parts need to be assembled or aligned.
Given that these features are typically functional, it is crucial to understand the dimensions of locating pins that FDM can accurately print.
Large pins (diameters greater than 5mm) are printed with perimeters and infill, having a strong connection to the rest of the print. Smaller diameter pins (diameters less than 5mm)
can only be printed with perimeters and no infill.
This creates a discontinuity between the print and other parts, with only a weak connection, making them prone to breakage. In the worst case, small pins may fail to print due to
insufficient printing material to adhere the new layer.
Printing locating pins with gradually decreasing diameters (from 25 to 5mm), illustrating how the upper diameter becomes too small to print accurately.
Properly adjusted printer calibration (optimal layer thickness, print speed, nozzle temperature, etc.) can reduce the likelihood of pins falling off. Adding a
radius to the base of the pin will eliminate concentrated stress at that point and increase strength.
For critical pins with diameters less than 5mm, inserting an off-the-shelf pin into a printed hole may be the best solution.
Key design considerations: If your design includes pins with diameters less than 5mm, you can add a small fillet at the base of the pin. If functionality is very important,
consider designing to include a hole and a pin location, drill the hole to the correct size, and insert an off-the-shelf pin.
Advanced Design
Several key considerations when printing with FDM are how to reduce the amount of support needed, part orientation, and the build direction of the part on the print bed.
Disassemble Your Model
Often, disassembling a model can reduce its complexity, saving cost and time. Overhangs requiring extensive support can be removed by simply splitting complex shapes into separately printed parts.
If needed, these parts can then be glued together once printing is complete.
Disassembling the model to eliminate the need for supports
Hole Orientation
Changing the print orientation is the best way to avoid supports in holes. Removing supports in horizontal axis holes is often difficult,
but by rotating the build direction by 90 degrees, the need for supports can be eliminated.
For parts with multiple holes in different orientations, prioritize blind holes, then holes from smallest to largest diameter, and then critical hole sizes.
Repositioning horizontal axis holes can eliminate the need for supports
Build Direction
Due to the anisotropic nature of FDM printing, understanding the application of the part and how it is built is crucial for the success of the design.
FDM parts are weaker in one direction due to the layer orientation.
The lack of continuous material paths and concentrated stress generated by each layer joint can cause this defect.
Since layers are printed as rounded squares, the seam between each layer effectively acts as small valleys. This creates concentrated stress where cracks can form.
Rules of Thumb
• If bridges exceed 5mm, sagging or support marks may occur. Splitting the design or post-processing can eliminate this problem.
• For critical vertical hole diameters, if high precision is required, drilling after printing is recommended.
• Wall angles greater than 45 degrees require added support for FDM printers to print.
• Include a 45-degree chamfer or radius on all edges of FDM parts that contact the print bed.
• For applications using small vertical pins, add a small fillet at the base or consider inserting off-the-shelf pins into printed holes.
• Model disassembly, hole repositioning, and specifying build 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