Parametric, FDM-optimized angle bracket

Introduction

I found myself needing angle brackets in a bunch of my designs, and also saw some interesting videos on making strong parts (and in one case, strong angle brackets) with FDM printing in mind. Since I also wanted to practice my parametric CAD skills, I made this ultra-parametric, FDM-optimized, design. 

Parameterization options

You can use this project to generate a bracket with any of the following modified for your needs:
 

• Arm length (how large a span you want the bracket to support)
• Width (with guarantees that it won't be too thin based on screw choice)
• Screw diameter
• Washer (or cap) outer diameter (you can choose to use a washer or not)
• How many screw holes you want per side
• How thick you want the ‘brace’ for the screw head or washer
• Nozzle diameter

The provided STL file is based on the values shown in the screenshot of a spreadsheet.

How to parameterize

To make a bracket suitable to your needs, follow these steps:

1. Open the project file (the .FCstd) in the realthunder FreeCAD version (or FreeCAD 1.0 or later, when it releases).
2. In the ‘Combo View’, select the ‘Parameters’ spreadsheet and open it.
3. Set the values to your liking. Make sure to press ‘Enter’ after editing each value so the model recomputes.
4. In the ‘Combo View’, select the ‘bracket’ part.
5. Go to File → Export, and choose the format and filename you want. FreeCAD can export, among others, STL, STEP and 3MF.
6. If you like, in the ‘Combo View’, select the ‘Print settings’ spreadsheet and check the settings.

Optimization ideas

The design makes a few choices, designed to make it strong and easy to print. More precisely, the ideas are as follows:
 

• The bracket is designed as one solid piece, oriented with the hypotenuse side on the bed. This ensures that compression forces on the bracket try and ‘crush’ the layers, rather than try and rip them apart.
• The cut-off corners mean that there aren't weak spindly bits near the edges, or a small pointy bit at the top. This both improves printability and means that parts won't snap off the ends.
• To resist compression even more, a ribbed pattern is applied across the bracket. This stiffens it considerably, similarly to a corrugated iron sheet or something similar.
• The 45-degree rule is adhered to throughout: there are no overhangs or bridges (apart from tiny internal ones).
• The holes for the screws are two-part: a wider access hole, and a narrower screw hole. This creates a flat ‘brace’ for the screw head (or a washer) to rest against, better distributing any compression force from tightening. See the 3D model image for a clearer view of this, and the pictures of how screws and washers insert.
• Print settings designed to give strength based on nozzle diameter as a parameter.

My sources for these techniques, and other recommendations in this project:

• CNC Kitchen, Texturing 3D Prints for Strength.
• Slant3D, Corner Brackets | Design for Mass Production 3D Printing.
• CNC Kitchen, Extrusion Width - The magic parameter for strong 3D prints?
• My Tech Fun, Ultimate Nylon filament test - Polymaker CoPA vs PA6-GF vs PA6-CF vs PA12-CF.
• CNC Kitchen, INFILL pattern and SHELLS - How to get the maximum STRENGTH out of your 3D prints?
• My Tech Fun, Ultimate comparison: Polymaker PolyLite filaments (PLA, PETG, ABS, ASA, PC)
• Prusa Knowledge Base, Modeling with 3D printing in mind.
• Voron Design, Materials Selection.
• My Tech Fun, Carbon Fiber ASA vs regular ASA by 3DO.
• Prusa Knowledge Base, Infill Patterns.
• My Tech Fun, How strong is Carbon Fiber PETG? Extrudr PETG vs PCTG vs XPETG CF.

Print settings

The project will generate print settings based on your parameterization, which will be in the ‘Print settings’ spreadsheet. To save some time on common settings, the print settings for 0.4mm and 0.6mm nozzles are summarized below. These use the PrusaSlicer names: other slicers should have similar settings, but they might be differently named.

Print materials

If the bracket isn't going to bear much weight, or failure isn't catastrophic, use whatever you like. Otherwise, read on.

For this purpose, you want a material that has good tensile strength, stiffness and low material creep. Below, we summarize common materials, and explain their suitability:
 

• PLA: Strong and stiff, but is creep-prone and brittle. Additionally, it has poor temperature resistance (PLA prints are known to deform in hot cars). Thus, it is not suitable.
• PETG: Not as strong or as stiff, but not creep-prone, as long as it's cold (below about 50C). PETG is also ductile, which means it won't fail suddenly. Thus, it is suitable at low loads when cold.
• ABS (or ASA): Both materials (which are mechanically near-identical) aren't as strong or as stiff, but have excellent creep resistance (both cold and warm) and aren't brittle. Thus, they are recommended. At the same time, they do require an enclosure to print.
• PC: One of the strongest and stiffest materials on this list, but somewhat brittle. It's also difficult to print due to hygroscopicity and warping tendencies. Thus, it is suitable.
• Nylons (all kinds): Despite being one of the strongest materials listed here, nylons are not stiff at all, and have the worst creep of any material listed here. Thus, they are not suitable.

Additionally, we consider carbon-fibre (or glass-fibre) filled variants of the above. Both fills increase tensile strength and stiffness significantly, at the cost of brittleness. They also improve creep, both warm and cold, as well as reducing warping, making it easier to print. At the same time, they both require a hardened nozzle. Based on this, we consider:
 

• Filled PLA to be unsuitable: despite the added creep resistance, it is even more brittle, for added strength and stiffness it doesn't really need. Additionally, its temperature resistance does not meaningfully improve.
• Filled PETG is suitable: filling improves its downsides, and while it becomes less ductile, it's still a strong choice. However, its warm creep is still worth bearing in mind, despite being improved.
• Filled ABS (or ASA) is suitable: while the added warp prevention can make the material easier to print, the benefits are negligible given the suitability of the base material.
• Filled PC is recommended: while it is somewhat brittle, the combination of strength, stiffness, temperature resistance and improved printability make it a great choice.
• Filled nylons (of all kinds) are unsuitable: while in theory, the combination is effective, since the fill covers the material weaknesses of nylons, in practice, their creep is still too severe for continuous loads without deformation.

To summarize: ABS (or ASA), PC-CF or -GF are your best choices; ABS (or ASA)-CF and -GF are fine too; PETG (with or without fill) is OK as long as temperatures stay low; PLA and nylons (with or without fill) are not a good idea.

Disclaimer

I am not a mechanical engineer. None of these statements have been tested extensively. A metal part, all other things being equal, will be stronger than a plastic one. If you are someone who knows something about this, and see something wrong or misleading, please let me know so I can correct it, ideally with sources.