Which shapes and fits are the most difficult to print (with FDM)?

[stijn]

For the knowledge base we’re writing an article about printing complex shapes. What I’m looking for is a comprehensive list of complex shapes and fits. These shapes and fits will then be covered per technology (e.g. how FDM deals with large overhangs).

What I have now:

• Large overhangs
• Intricate shapes
• Thin walled structures
• Integrated assemblies
• Living hinges
• Snap- & push fits (with tight tolerances)

EDIT: Suggestions in comments:

• Tall / narrow object that
• Threads

I’ve left small details and features out of the categorization on purpose as we’ll dedicate a separate article to that. Wondering if there’s something you’d change or add here?

[cobnut]

Threads? It’s difficult to get a decent quality thread unless it’s printed along the axis and that can sometimes mean positioning the model in a way that affects other aspects (for example, two threads at right angles to each other in the same model).

Tall, narrow objects? If they must be printed vertically, it can be difficult to get sufficient adhesion to the bed to avoid toppling as the print gets higher (and the torque on the base increases).

Watertight objects? What looks like a perfect print will probably still leak like a sieve unless you work hard on the settings and choose the right material.

[Evan_3]

I would also add (relative) small contact area with the print bed, especially with tall objects. Edit: i see someone was ahead of me, I agree with him.

[stijn]

Thanks, added threads and tall / narrow objects to the list. Watertightness I think is less of a shape characteristic.

[Perry_1]

This: A-form RNA Helix Stand by codygeary - Thingiverse

Very tough print.

Supports everywhere, including inside the model, in every nook and cranny, wow.

Supports created an issue where they were very hard to remove, hard to support each round part of the dna, and difficult to remove the supports. I printed it, spent the time to make it look excellent, and the customer was thrilled. But it was a tough one, that if the customer had the $ to go with a non FDM print, it would have been better.

[stijn]

Thanks Perry. Indeed looks like a tough print for a desktop FDM printer. This model could be a good example of an intricate shape.

[SOC3D]

Spheres.

[Perry_1]

Can I push back on some of this?

I print very good prints with:

threads

spheres

objects that have very little build plate area

tall narrow objects

— All the time!

Watertight is a different story. If it is a bowl that can be printed in vase mode, I can make it watertight. If no vase mode: not watertight.

You can see examples on my hub’s pictures. The small object is threaded. The bowl is watertight.

I don’t want the knowledge base to be misleading. Most of these items can be printed if you have skills with your printer and it is tuned correctly.

I can post pictures or gcode of the following:

Tall thin,

Small build plate contact,

Thread, lot’s of threads, of any size!

Spheres.

Again, don’t want to be argumentative, but I do want to be accurate.

[Evan_3]

Maybe we should specify a very small build area. I had an problem with an pillar that was 1*1 mm on the build platform, but 100mm high. It was supposed to be support, but it didn’t make it to the end.

[shintashi]

for me, faces, holes, and really long pieces, where diagonals more than 4cm start to contract causing curling. Most recently i had a 28mm statue print perfectly except for the head and hat, which merged together forming a balloon animal. I haven’t been able to print anything with a feature smaller than 1/8" to 1/16" and objects larger than 4 inches lengthwise actually print better vertically, but they lose all strength and function along the grain.

[shintashi]

agreed. I made some spherical knobs for a machine and the bottom portion of the spheres caved inward rather than following the round shape of the upper half. I think it had to do with my infill.

[stijn]

I understand your concerns @Perry_1 and I’ll definitely make sure the article communicates the capabilities and limitations of FDM printers fairly.

The article won’t insinuate that the shapes you mention cannot be printed with FDM. Instead, the article will explain why printing such models with FDM is more challenging and requires experience from the hub.

[toybuilder]

Bottom of spheres are challenging, especially if modeled with a 'point contact" at the bottom, where successive layers are at an extreme overhang angle and have more nozzle path versus the previous layer. Most support structures, by default, will not actually come in contact with the sphere surface so the sphere has a tendency to detach from the bed.

Another challenge is a high-aspect-ratio part, especially if it tapers down to a really small section - the nozzle will usually remain in contact with the small area and will keep it hot - causing the prints to just sag away.

Parts with tight tolerances are not a problem if your printer does not have backlash – slowing the printer is key to avoid ringing which tends to cause the part to be out of tolerance.

Parts with interior supports that are hard to remove are difficult, too.

[MindFuLL]

Thank you!
-Jesse

[MindFuLL]

I agree. My entire business is based exactly on these kinds of difficult prints. One of my biggest clients is an international bottle manufacturer and I print all their prototype bottles/caps. I print single wall, watertight, dimensionally accurate, bottles with functional threads every week.
If you post some guide saying that FDM cannot do these things(or cannot do them well) it could be DETRIMENTAL to what I do. Explaining why they are more challenging is great and can help the customers understand why they might cost more to produce but I just want to double down on what @Perry_1 said.
-Jesse