Fused deposition modeling (FDM) is an affordable 3D printing technology that’s favored for rapid and low-cost prototyping. Nevertheless, like other additive manufacturing methods, FDM has its limitations and constraints when it comes to designing a 3D printable part.
On the one hand, designing is done in a digital space, where one can explore the most complex geometries. On the other hand, 3D printing any arbitrary part can be a nightmare, especially when the model doesn’t conform to the basic mechanics of additive manufacturing.
In this guide, we dive into the most crucial considerations that one must stick to when designing for FDM. This may not be the ultimate guide, but we’re confident that any tinkerer can use it as a base for understanding the fundamentals of FDM design.
The main design concerns in FDM originate from the shape and size of the hot deposited plastic, which may vary from one printer to another. Therefore, we’ll be using figures from industry experts (including reputable printer manufacturers and 3D printing service providers).
As a designer, expect slight deviations from these figures, which will ultimately be at your discretion.
A bridge is a strand of plastic suspended between two anchor points. Depending on their design, some printed parts will simply require bridges.
The problem is, as the bridge gets longer, the bottom layers droop, which means they won’t stick well to the upper layers.
With active cooling, an Ultimaker printer can produce a bridge as long as 25 mm, but this figure may change depending on the printer.
Solution #1: Reduce the length of the bridge.
When you want to print between two anchor points or supports, but you have nothing to build upon, the extruded material is going to sag. Your best bet is to reduce the distance of the bridge, though this may be limited by the part’s design constraints.
When the bridge is shorter, it’ll be easier to print between two supports (or anchor points) without the need for support.
Solution #2: Include supports.
Supports can act as temporary build platforms for bridged layers. Once the part is printed, the support can be removed, though this can leave marks on the surface of the printed piece.
In FDM, there will always be a variation between what you have in the slicer and the actual diameter of a 3D printed hole. Horizontal holes, for example, are notorious for not being perfectly round, while vertical holes often come out smaller than intended.
A horizontal hole faces the problem that its face is composed of multiple layers with varying degrees of overhang. As such, some sections of the curve might not come out perfectly round.
The reason a vertical hole is often smaller than expected is that the compressing force of the nozzle squishes the perimeter lines next to the hole. If these lines turn out wider than intended, the hole will be narrower than intended.
The narrower the hole is designed, the more likely these phenomena are to cause problems.
Solution #1: Change the shape of the hole.
Instead of designing a circular hole, go for a teardrop shape, which your FDM printer can easily handle.
With horizontal holes, this will give you a margin of error on droopage. Tear-dropping limits overhangs to 45 degrees and makes large holes printable. With vertical holes, you’re allowed some slack with fitted parts.
This solution works especially well with functional parts, where the appearance of the hole is not as important. If, however, you’re against it, or the design simply cannot accommodate it, create a thin support membrane to hold up the top of the hole.
Solution #2: Design small and drill after.
The amount of variation will often depend on the printer in question, the size of the hole, the slicing software, and the material. Getting an accurate diameter may require you to perform several test prints.
However, if you want a much higher accuracy, or if your hole has a tight tolerance, print the hole undersized and drill it to the correct diameter.
Solution #3: Design a wider hole.
MachineDesign recommends that you design your hole with a diameter that is greater than 1 mm (0.04”) if you want it to retain a circular shape.
With vertical holes, if you know the amount by which the diameters are reduced, you can simply design the hole to be that much wider. Of course, this may require some testing…
(Note that MachineDesign is using what Xometry offers for FDM parts.)
FDM works by depositing layer after layer, which means that each layer is normally supported by the layer underneath. If, however, your model happens to have an overhang (which is more than 45 degrees), you’ll need support structures to achieve a successful print.
Unfortunately, supports can sometimes be impossible to remove and will often leave hard-to-remove marks on your prints.
For more information, check out our in-depth guide on supports.
Solution #1: Reduce support material by adjusting the design or re-orienting the model.
While designing your model, try to make it as independent of supports as possible.
Additionally, re-orienting a model can help with the amount of support required. Some platforms, like Meshmixer, will do this for you automatically.
Solution #2: Adjust support variables.
If you must use supports, consider adjusting variables like the support pattern, density, or overhang threshold to get appropriately sturdy supports that are easy to remove.
Solution #3: Design custom supports.
A good number of CAD tools have the capability of automatically generating supports for your print. However, depending on the design, they may not always succeed. Furthermore, such supports can be hard to remove and will usually leave ugly marks on your print.
Use a program like Meshmixer to create custom supports for your 3D print. If done properly, you’ll produce supports that are successful regardless of the design. Additionally, you can save material and printing time using particular types of supports.
Sometimes, overhangs will simply be unavoidable. Usually, an overhang can be printed without loss of quality up to 45 degrees (depending on the material). However, anything beyond that may require support.
The length of an overhang can also determine whether it will be printable or not. Ultimaker, for example, recommends a maximum length of between 1–2 mm for an unsupported 90° overhang. Going beyond 2 mm will leave you with an unprintable shape.
Solution #1: Limit or reduce overhangs.
Always try to limit or reduce overhangs in your model. This is achievable by keeping an eye on overhanging geometry and manipulating the design to avoid protruding structures. It can be a bit of work, but if your design allows it, you’ll save yourself a lot of trouble.
Solution #2: Consider bridging instead of a steep overhang.
If you must use overhangs, and if the design allows, go for a bridge instead of a steep overhang. Using support material would be an alternative, but supports will mostly scar the surface of your print, besides being a waste of plastic and time.
The YHT Rule
The “YHT rule” is a great way to remember which shapes are printable. It goes like this:
Threads, like those of a screw, are generally a hassle to print, especially if they are small and detailed.
Solution #1: Round the crests and roots.
Avoid sharp edges when designing built-in threads. Instead, go for rounded crests and roots. In plastic parts, sharp edges are stress concentrators.
Solution #2: Add “dog point” heads.
A dog point screw features a flat, unthreaded tip that stretches from one end of the screw. This tip is helpful for locating a groove on a shaft.
Starting a thread can be a challenge if you lack such a dog point head. Incorporating one into your threaded design should make it easier to print. Note that it should be at least 0.8 mm in length.
Solution #3: Use taps or dies for smaller threads.
Smaller threads are difficult to print and are not recommended for FDM printers, particularly if the threaded part needs to go into a hole with a diameter of 1.6 mm or less. An easy alternative is to use a tap (or a die) to make smaller threads.
Taps and dies are metalworking tools used for creating threads in almost any metal. A tap cuts a thread on the inside surface of a hole while a die forms an external thread on a rod or cylinder.
In the context of 3D models, fillets are rounded corners, whether internal or external. Being an additive process, FDM 3D printing doesn’t benefit from filleting as much as other manufacturing techniques. Nevertheless, fillets can still be extremely helpful in certain cases.
The primary use case for fillets is to strengthen fragile protruding features. Without additional support around their bases, these structures are liable to snap off, even during printing.
Fillets are useful for reducing stress concentrations and increasing the overall strength of the 3D printed part. When you introduce fillets, maintaining consistent thickness on the print is critical.
Solution: Add a fillet where necessary.
In general, use the largest fillet possible to increase strength.
An embossed two-dimensional detail is one that is slightly raised from a surface, while a debossed one is sunken into the surface.
When embossed or debossed text or other small details are below certain tolerances, like filament thickness, printing them in FDM can be troublesome or even impossible.
Solution #1: Use reliable measurements.
According to Machine Design, the width and depth of protruding or cut-out text shouldn’t go below a certain limit. In both cases, the minimum is around 1 mm (0.04″), but making them even larger is recommended in order to avoid potential issues.
Solution #2: Opt for larger fonts.
Naturally, larger and simpler fonts will perform better than smaller and complicated ones.
When designing your model, you should always take into account the printing material you’re going to use. Plastic like ABS, for example, is prone to warping due to its high print temperature. PLA and PETG, on the other hand, are much easier to use.
Solution #1: With warp-prone materials, go for a large base.
When using a material like ABS, try to increase the size of your print’s base to ensure the extruded plastic remains attached to the build plate.
Solution #2: With flexibles, omit small details and pay attention to manufacturer specifications.
Flexibles don’t do well with small details. Omitting or minimizing small features in your design can therefore go a long way to preventing printing failure.
When considering printing material, also pay attention to manufacturer specifications. An identical filament acquired from one manufacturer may have unique characteristics when purchased from a different manufacturer.
Since the nozzle compresses the printing material as it prints each layer, you’re likely to have “elephant’s foot” on the initial print layer, which will cause your print to protrude outside the specified dimensions.
Solution #1: Print with a raft.
Printing with a raft will essentially ensure that, if elephant’s foot does occur, it won’t affect your print. That’s because the raft, which is in direct contact with the print surface, takes the hit instead.
Solution #2: Add chamfers or fillets to the bottom edges.
To avoid this classic problem, consider adding a chamfer or a fillet to the bottom edges of your design. This will minimize the effect and prevent material from extending beyond the model’s boundaries.
A large design may not fit within the build space of your 3D printer, while a complex design is likely to require difficult or impossible supports.
Solution: Split up your model.
If you really can’t avoid having a model that’s bulky or complex, your best option might be to split it into multiple pieces. In the case of a large model, this will ensure that your printer can actually produce it.
Meanwhile, splitting a complex model should only be done if you can take advantage of its shape with respect to the printing process. For example, splitting the shell in the above photo ensured two very clean prints instead of one potentially problematic print.
In either case, you’ll need to glue the pieces together.
You have to get the clearances right, especially when you’re dealing with mechanical parts that need to be fitted together. 3D printers aren’t always very precise, and this leaves you with the task of making sure parts have the right gap between each other if they are to mate or assemble as expected.
You don’t want parts sticking together when they’re not meant to, and this means accommodating for sagging or bulging. How big the clearance should be is something you must investigate. More work is needed when working with more intricate shapes; the compensation has to be just right.
Solution #1: Increase slot size.
When it comes to the “accepting” piece, you may need to design the slot to be slightly larger than what it should be in theory. The easiest way of figuring out the right clearances is by printing out a test piece and taking measurements.
Solution #2: Design with shrinkage in mind.
You should also keep in mind that plastic shrinks. This depends on several factors, including the type of plastic, the shape of the part, the print bed (whether it’s heated or not), and the surrounding structures.
While setting clearances, you may also need to compensate for how plastic shrinks. Get the right figures by performing print tests. After numerous tests, you will instinctively know how much you need to compensate. It may take a little time, but you’ll end up saving even more later on.
When designing an FDM model with intricate details, it’s crucial to have an understanding of the minimum feature size that each 3D printing process can produce. Normally, the minimum level of detail that can be created has a connection with the mechanics and capabilities of the 3D printer.
Solution #1: Know the smallest printable features.
Your printer’s resolution dictates how much detail your prints can have. Have a precise understanding of the XY resolution and the layer height before you add intricate details to your design.
If the minimum layer height is 0.1 mm, you won’t have any luck trying to accomplish anything less than 0.1 mm in the Z-direction.
Test prints are a great way to determine what your printer is capable of.
Your 3D printer is only capable of producing a thin wall up to a certain point. Beyond this given point, it becomes impossible to print the wall because it is too thin.
Solution #1: Add thickness.
Always make sure to incorporate the thickness of walls into your design. If you forget and have to do it later, it could mean trouble for your model.
Wall thickness should typically be two or three times the nozzle’s width, so at least 0.8 mm for a standard 0.4-mm nozzle.
Solution #2: Add ribs.
Ribs can be added if a wall is too thin and you want to avoid deformation (warping). Ribs distribute the force applied to your printed structure and generally make the print stronger.
Of course, ribs should also follow wall thickness tolerances and shouldn’t be too thin. MachineDesign recommends 1.5 mm (0.06”) as a safe minimum rib thickness.
Essentially, a model is manifold when it is “closed”, meaning it can’t have any “open sides”. For this reason, non-manifold geometry can be represented digitally, but cannot exist in the real world.
Therefore, since the mesh of a 3D model is represented by edges, vertices, and faces, a design has to be manifold for it to be printable. Otherwise, you’ll have errors that cannot define the geometry of your 3D model.
Non-manifold designs are your worst 3D printing nightmare, but they occur fairly often because 3D design virtually has endless possibilities. For example, you may end up with walls that have no thickness, or shapes that overlap. These features imply your model isn’t realizable or “watertight”.
Solution: Ensure that your design is manifold.
The best way to make a model manifold, apart from redesigning it, is to process it using an STL repair tool. Many such tools exist, and some free examples include Netfabb, Meshmixer, and Meshlab.
In FDM, orienting parts can have an enormous impact on strength and appearance. Concentric features, for example, will be best when they are printed in parallel to the XY-plane.
Objects that are produced in layers are naturally strong in the direction normal to their layers, but weak in the parallel direction. For example, if you want to print a rod that will be subjected to bending, make sure to print it flat against the build surface. In this way, you can ensure that the layers travel the length of the piece.
Solution: Optimize orientation for strength and appearance.
When designing, think about what kind of forces your object will be subjected to and in which directions. As much as possible, try to orient the model so that the layers will “absorb” these strains.
Here are some specific examples to consider:
Feature image source: TheMagicmodel / YouTube
License: The text of "How to Design Parts for FDM 3D Printing" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.
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