FDM Considerations

Introduction

In this section, I just wanted to go over some of the key mechanical considerations you should take into account when conceiving of a part you’d like to 3D Print.

Tolerance & Manufacturability Considerations

Your printer is limited in the size of depositions it can make, and the spacing it can create between depositions. In many *macro* cases, for big parts, you don’t need to pay this a lot of mind, but when you get into small sub-millimeter features, this size comes into consideration. 2 Big things to consider for this:

  • If a hole is small enough, it may not be possible to create
  • If a solid feature (IE a small finger) is too narrow, it will either not be printed, or be printed thicker than specified.

Of course your deposition size is important to consider for hole tolerances (how accurate the hole diameter is) as well. This will very from machine to machine (and especially how well it is calibrated) so for this I always recommend printing test pieces to characterize the tolerance for yourself (and in some case you can just up-scale your hole to account for the tolerance)

There’s also strength requirements to consider, and two big factors that can help meet those requirements are infill density and material type. However I’m not going to go into those topics yet.

Shear & Print Orientation

Not only do the magnitudes of forces need to be considered, but also the direction of the forces. You can always make changes to your design to reinforce areas to account for these forces, you should also consider how your print orientation is going to affect the strength of the part:

There’s not always an ideal orientation, but if you’re using supports and strengths are really critical, you can also shift your design to be printed at a 45 degree angle:

Once we get into our material science section, we’ll talk a bit more about why these interfaces are the weakest.

Curl

Okay. I can’t completely postpone the materials talk, especially for this section. Curl is a tendency for a part to pull away from the build plate and “curl” up. Let’s talk about this intuitively – Let’s start with our simplest 3D print; a single line:

When that line is deposited (let’s say it’s a common plastic like PLA) it’s sitting roughly at 200C. Then the line cools, and what happens when things cool? They shrink:

It’s not exactly linear, but the extrusion shrinks in each dimension proportionally to that dimension, the width shrinks a little bit, and the length shrinks a lot more (in absolute units, but once again in percentages, they are roughly the same).

But that’s not a very exciting print. Now let’s suppose we print a line, and then deposit another line on top of that:

The blue line at the bottom was deposited first, and it’s had a little bit of time to cool down, so it’s already started to shrink. But now it’s somewhat being heated up, and bonded with the new layer that’s on top of it. The blue layer’s trying to shrink but at all point’s it’s being “pulled up” but the slightly longer, warmer extrusion above it. It can shrink in thickness dimension, but this will cause the extrusion to curl along it’s length:

A real life example of this looks like:

Obviously this can screw up your dimensional accuracy, but even worse, it can cause your print to pop off of the print bed. This can sometimes be solved by printing with a skirt:  a single layer of extra extrusions added around your print to “help hold it down”. It’s critical that it’s only a single layer because if it is several layers, it would encounter the same curl issue.

Back in the day I was working with a very large 3D Printer, attempting to print furniture. Really long, and thick extrusions resulted in really big curling problems. And to be honest this is very much a contemporary issue that needs solving.

Overhangs

Remember that 3D Printing is a layer-by-layer process. You build on top of previous layers. However, say you want to make a nail-shape. If you were building from the tip to the head, the head would would have it’s outer diameter being built over nothing. This problem can sometimes be solved a few ways:

  • Supports: The slicer can generate material to support the overhang, that will need to be removed later.
  • Printer Tuning / Slicing Tricks: It seems like every day there are new features that you can enable in your slicer to better create these overhangs without the need for supports
  • Engineering Design: If possible, try and avoid designing overhangs. If you instead add small angles so the overhang has something in the design to build off of, you’re going to have an easier time. On top of that, the overhanging part will be better supported structurally once the part is in use:
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