Learn how to optimize common design features for FDM 3D printing (such as bridging, overhangs, pins, and vertical holes).
Table of Contents
Introduction
Bridging
Vertical 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 along a predetermined path onto a pre-built surface.
As the material is extruded, it cools to form a solid surface, providing a foundation for the construction of the next layer of material.
This process 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 address these limitations and discuss methods that can be implemented during 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's no support for the initial layers being printed (nothing to build upon), and a gap needs to be "filled," the material tends to sag.
Bridging usually occurs in horizontal holes within the walls of an object or in the top layers (or roofs) of hollow sections.
Printing bridges of 25mm, 35mm, and 45mm
One solution to reduce the impact of bridging is to shorten the bridge distance, but this depends on the design constraints of the part. Another solution to avoid sagging is support structures.
Supports provide a temporary 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 supports connect to the final part.
FDM printed jigsaw piece showing surface roughness after support removal
Key design considerations: Due to the nature of FDM, sagging or marks from support material will be present unless the bridge thickness is less than 5mm.
If a horizontal and smooth surface is required, advanced solutions involve splitting the design into different sections or considering other forms of post-processing.
Vertical Holes
FDM often prints vertical holes that are undersized. The reasons for the diameter reduction during the printing process are generally:
1. When the nozzle prints the perimeter of a vertical hole, it compresses the newly printed layer onto the existing build layers to improve adhesion.
2. The compressive force of the nozzle changes the shape of the extruded layer from a circle 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 hole diameter to nozzle diameter.
The actual diameter change of a vertical hole with varying slicing programs is due to the compression of the extrusion profile.
The amount of undersizing will depend on the printer, slicer software, hole size, and material.
The reduction in vertical hole diameter is usually accounted for within the slicer software, but accuracy can vary, and multiple test prints may be required to achieve the desired precision.
If high precision is required, post-print drilling may be necessary.
Key design considerations: If the diameter of a vertical hole is critical, it is recommended to print it undersized and then drill it to the correct diameter.
Overhangs
Overhang issues are one of the most common print quality problems associated with FDM. Overhangs occur when a printed layer of material is only partially supported by the layer below it.
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, it is usually possible to print an overhang without significant loss of quality up to 45 degrees.
At 45 degrees, the newly printed layer will have 50% support from the first layer. This allows for 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 arises during printing is curling. The newly printed layers at the overhanging edges become thinner, leading to differential cooling and upward warping (see image above).
Key design considerations: The limitations of overhangs can be eliminated by using supports for wall angles greater than 45 degrees. For larger overhangs requiring support,
marks will appear on the final surface unless post-processed.
Corners
Because the print nozzle in FDM is round, the radius of corners and edges will be equal to the size of the nozzle. This means these features will never be perfectly square.
For sharp edges and corners, the first printed layer is particularly important. As discussed above regarding vertical holes, when the nozzle prints each layer, it compresses the printed material to improve adhesion.
This creates an outward-flared shape in the initial print layers, commonly known as "elephant's foot."
This can affect the ability to assemble FDM parts, as this outward protrusion exceeds the specified dimensions.
Side view of elephant's foot feature visible in the initial layers of FDM prints.
Another common issue related to the initial print layers of FDM 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 leads to differential cooling and can cause the bottom layer to lift off the print bed as it shrinks.
Adding chamfers or fillets 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 FDM parts, all edges in contact with the print bed should include a 45-degree chamfer or fillet.
For high-precision shapes and fit testing, other technologies such as 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 often functional, it's crucial to understand the dimensions of locating pins that FDM can accurately print.
Large pins (diameter greater than 5mm) are printed with a perimeter and infill, creating a strong connection with the rest of the print. Smaller diameter pins (diameter less than 5mm)
can only be printed with a perimeter and no infill.
This creates discontinuities between the print and other parts, resulting in weak connections that are prone to breakage. In the worst case, small pins may fail to print due to insufficient material to adhere the new layer.
Printing locating pins with gradually decreasing diameters (from 25 to 5mm), illustrating how the upper diameters become too small for accurate printing.
Regularly adjusting correct printer calibration (optimal layer thickness, print speed, nozzle temperature, etc.) can reduce the likelihood of pins failing. Adding a radius to the base of the pin
will eliminate that point as a stress concentration and increase strength.
For critical pins with a diameter smaller 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 smaller than 5mm in diameter, you can add a small fillet to the base of the pin. If functionality is critical,
consider designing a hole and pin location, drilling the hole to the correct size, and inserting 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 orientation of the part on the print bed.
Decompose your model
Often, disassembling a model can reduce its complexity, saving cost and time. Overhanging parts that require extensive support can be removed by simply splitting complex shapes into individually printable sections.
If needed, these sections can be glued together once printing is complete.
Disassemble the model to eliminate the need for supports.
Hole Positioning
Changing the print orientation is the best way to avoid supports in holes. Removing supports in horizontal holes is often difficult,
but rotating the build orientation by 90 degrees eliminates the need for supports.
For parts with multiple holes in different orientations, prioritize blind holes first, then from smallest to largest hole diameter, followed by critical hole sizes.
Repositioning horizontal holes can eliminate the need for supports.
Build Orientation
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 will be weaker in one direction due to layer orientation.
The lack of a continuous material path and stress concentrations at each layer joint cause this defect.
Since layers are printed as rounded squares, the seams between each layer act like small valleys. This creates stress concentrations 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, it is recommended to drill after printing.
•Wall angles greater than 45 degrees require added support for FDM printers to print.
•Include a 45-degree chamfer or fillet 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 an off-the-shelf pin into a printed hole.
•Model decomposition, hole repositioning, and specified 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