When you start to design your own models, there are some obvious constraints that you must take into account. Laws of gravity do apply to 3D printing, and you cannot print a piece in midair. Models must also be able to lie flat on the build plate or be supported, and your model must fit within your printer’s build volume. However, there are less obvious features that you should consider when designing and modeling:
- The minimum supported wall thickness
This constraint describes the smallest thickness that you can assign to your walls if you want them to print. Slicers like Cura have an assignable setting for Line Width, and will overlook any geometry features smaller than that value. Some designers just double the size of the nozzle diameter to estimate the minimum supported wall thickness and work from there, but you can also calculate the minimum wall thickness by doubling the Line Width. The Line Width is set in your slicer and is determined by the width of a typical line extruded by your printer. For example, if you’re using Cura with PLA and a 0.4mm nozzle, then the recommended Line Width setting is 0.35mm for Layer Heights set between 0.1mm and 0.15mm. Double the Line Width of 0.35mm to get the minimum wall thickness of 0.7mm. You are doubling the number because you generally print with two shells: one for inside and one for outside. With a Layer Height of 0.2mm, and a Line Width setting of 0.4mm, your minimum wall thickness will be 0.8mm (double the Line Width value). Single pass printing creates features that are lightweight, but can buckle. For more strength and reliability design for double pass printing. Double the minimum wall thickness in your design, use the minimum wall thickness value when setting Wall Thickness and adjust your Wall Line Count to 2.
- Maximum overhang length
If you design a part that includes an unsupported 90° overhang, then the maximum overhang length you will be able to print without using support will be between 1mm and 2mm.
- Maximum unsupported overhang angle
How much of your model can you print at an angle without using support? This constraint is material dependent. With PLA and cooling fans, you should be able to print parts with angles up to 55°. However, as a rule, you will generally be able to print unsupported parts successfully for any angle up to 45°.
- Maximum horizontal span/bridge
This property is mostly affected by the quality of the material you're printing with and how close the bridge is to the heated bed. Generally you will want to slow the print speed down and reduce the temperature to achieve bridging. You should be able to achieve a bridge of about 25mm across if you enable active cooling. For more information on this topic see Bridging.
- Minimum embossed detail
0.5mm is universally readable from a distance, while 0.2mm is visible on close examination. As a rule of thumb the embossed details should be no less than your Wall Thickness setting.
- Minimum engraved detail
Engraved details are similar to embossed details, except that they are more likely to produce fluctuations in the model wall about 5mm-10mm away from the feature in the direction that the extruder is traveling in. The imperfections are usually attributed to loose belts, or too high a print speed.
- Minimum allowance
When connecting parts together you have two basic options:
- Press Fit, Force-Fit or Interference-Fit occurs when parts are held together by friction.
- Sliding Fit or Free-Fit occurs when the connection between parts allows for movement.
Every company’s 3D printer is slightly different and you will have to test your printer and settings to determine your tolerances. Generally if you are designing a 3D printed press-fit part, allow for a 0.2mm offset from the interior feature. You may want to go as small as 0.1mm on each side, but once you’ve connected a part with an allowance of less than 0.2mm you will not be able to remove that part without breaking the model. The minimum allowance for a moving or rotating part is 0.4mm on each side. Basically you are separating your parts by a wall thickness. For gears and other complex intermeshing parts, use a 0.5mm offset to make up for any errors in X-Y accuracy.
- Minimum Hole Diameter
While this is somewhat dependent on material and settings, a safe assumption is that the diameters of your holes need to be at least twice the value of your Line Width setting.
See also: 3 Design tips for FDM 3D printing