Lesson 11.2: Designing 3D Printed Parts That Survive FTC Loads
Technical Context
3D printing lets a team make a part that fits their exact geometry in an afternoon. It also lets them make a part that looks correct and fails at a fraction of the load an equivalent aluminum bracket would carry.
The difference is almost entirely in how the part is designed for the process.
Printed Parts Are Anisotropic
An FDM part is built from stacked layers, and the bond between layers is weaker than the material within a layer. A load that tries to pull layers apart, or to peel them, fails at a small fraction of the part's apparent strength.
Treat layer orientation as a primary design constraint for printed parts.
Orient the part so the load runs across layers, not along them. A bracket that carries a bending load should be printed so the layers run parallel to the bending stress rather than perpendicular to it.
The practical implication is that print orientation is a design decision that belongs in the drawing or the notebook, not a choice the person operating the printer makes for convenience.
Print orientation is a design decision, not a printer setting. Record it on the drawing or in the notebook, because the person running the print will otherwise choose it for convenience.
Print two copies of a bracket in different orientations and load them by hand until they fail. The difference is usually obvious and it teaches the principle faster than any explanation.
Design Rules That Matter
Fillet every internal corner. Sharp internal corners concentrate stress and are where printed parts crack. Even a small radius helps substantially, and it costs nothing.
Avoid thin unsupported walls. A wall a couple of perimeters thick has very little strength. If a feature carries load, give it enough thickness to contain several perimeters of solid material.
Increase perimeters before increasing infill. Strength in a printed part comes mostly from the perimeter shells, not from the infill. Four to six perimeters with moderate infill is stronger, for the same print time, than two perimeters with high infill.
Design holes oversize or drill them after. Printed holes come out undersize, per Lesson 11.1.
Do not print threads for structural use. Printed threads strip. Use a heat set insert, a captured nut, or a through hole with a nut and washer.
Never rely on a press fit into printed plastic. The material creeps under sustained load, so the interference relaxes over weeks. This applies especially to bearings, per Lesson 4.3.
Material Selection
| Material | Character | Use for |
|---|---|---|
| PLA | Stiff, easy to print, brittle under impact | Jigs, fixtures, prototypes, non-load parts |
| PETG | Tougher than PLA, slightly flexible | Light brackets, guides, guards |
| ABS or ASA | Tough, heat resistant, harder to print | Parts near motors, outdoor exposure |
| Nylon | Tough, wear resistant, absorbs moisture | Gears, wear surfaces, high load parts |
| CF filled nylon | Stiff and strong, abrasive to nozzles | Structural printed parts |
A common and reasonable team pattern is PLA for everything during prototyping and PETG or nylon for the parts that stay on the competition robot.
PLA loses stiffness at temperatures a robot can reach, particularly for a part mounted against a motor that has been working, and a robot left in a vehicle on a warm day can exceed it easily. If a printed part sits near a heat source, use PETG, ABS, ASA, or nylon.
When a Printed Part Is the Wrong Answer
Printing is the right choice for complex geometry, for parts you need quickly, and for things that would be difficult to machine. It is the wrong choice when:
- The part is a simple flat bracket that aluminum would do better and faster
- The part carries a high load in a direction that stresses layer bonds
- The part locates a bearing or another precision feature by interference
- The part will be near a motor and made from a low temperature material
- The part is safety critical, since printed parts fail with less warning
Iterating Printed Parts
Printing's real advantage is iteration speed. Use it deliberately:
- Number every version in the model and print the version number into the part itself, in a face that stays visible. Identifying which of four similar brackets is on the robot is otherwise guesswork.
- Keep the failed versions and photograph them for the notebook, since a row of four brackets showing progressive reinforcement is exactly the iteration evidence judges look for.
- Print a spare of every printed part that is on the competition robot, and put it in the pit box.
Fill-in-the-Blank Practice
- Printed parts are weakest when the load tries to separate the
__________, so orientation must be chosen deliberately. - Strength in a printed part comes mostly from the
__________rather than from infill. - Structural threads should not be printed; use a heat set
__________, a captured nut, or a through bolt.
Show answers
- layers
- perimeters (shells)
- insert
Exercise
Take one printed part currently on your robot. Determine its print orientation and the direction of the main load. If the load stresses the layer bonds, reprint it in a better orientation and load both by hand to compare. Record the result.
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