CAD Design Rules for 3D Printing
Think about the production of a part. All kinds of objects currently around you may have been manufactured by a process called Additive or Subtractive Manufacturing. In Subtractive Manufacturing the process starts with a block of wood or metal where there’s cut away of material. Additive Manufacturing deposits material layer upon layer.
Many years working with all sorts of CAD programs I have learned you can’t just design a model just because it looks cool. Technically you can, but there needs to be a balance as to how much time and waste of material is worth it. To help you understand all the facets that go into designing a model based on the tools available I have simplified the many factors into five different categories primarily focused on Fused Deposition Modeling (FDM) 3D Printers. 
#1 – Tessellation & No Gaps
a) Watertight Tessellation
b) Level of Tessellation
#2 – Amount of Material
a) Wall Thickness (Hollow Parts)
b) Infill Strength
c) Engraved or Extruded Wording
d) Fillets or Chamfers
e) Hole Size
f) Minimize Material
#3 – Best for Orientation
a) Print Direction (Tension vs Load)
b) High Width to Height Ratio
c) Runoff (45 Degress or less, 5mm or less)
#4 – Best for Support
b) Bridging (5mm max without support)
c) Large Horizontal Holes
#5 – Best for Assemblies
a) Design Size
b) Allow for Clearance
#1 – Tessellation & No Gaps
a.) Watertight Tessellation
If the 3D model has any type of tessellation issue, or if there is a hole in the model due to bad tessellation the 3D print will look bad or fail.
A badly tessellated CAD model can be due to inverted triangles, unintended holes, overlapping or intersecting triangles.
Inverted triangles, which have a back side and “normal” side, will confuse the 3D printer as it will not know which is which or where to begin or end the print layer. Any holes that are not intentional within the design will also cause the 3D printer to not know where to begin or end. Overlapping triangles or surfaces will cause unnecessary clumps in the printed model. And, intersecting triangles will cause the print to fail as the 3D printer will not be sure where to go.
Inspect your model tessellation with your analysis tools within your CAD or Slicer program. For more details on fixing tessellated CAD models check out our CAD Design Errors to Fix Before 3D Printing article.
b.) Level of Tessellation
A CAD model is considered to have low or high tessellation, which is the amount of triangles that make up the 3D object. So, what is the difference between a low or high tessellated model? Low tessellation CAD models have less triangles that are larger in size. Although the model has more of a pixelated look, a low tessellated model helps to reduce the file size. High tessellated models print higher quality, detailed and much smoother prints.
To ensure high quality prints check your CAD application settings to ensure the model is at the desired amount of tessellation.
#2 – Amount of Material
a.) Wall Thickness (Hollow Parts)
Wall thickness is the outer boundary of your 3D printed object. The wall thickness is formed by the number of times the extruder will lay filament around the perimeter of the model. If the wall thickness is too thin the print may fail or the infil may show through a bit. The final print may be too frail and easily get destroyed with any tension or load applied to the model.
Your CAD or Slicer program should have a wall thickness analysis tool to check the thickness of a model. The recommended minimum of a wall thickness is 1.2mm while taller prints should be 2mm thick for sufficient strength. But, use the right balance. Consider how much stress the print should be able to handle along with the amount of material you are okay with using.
b.) Infill Strength
Infills are not really part of the CAD model design, however it greatly impacts the strength and durability of the printed object. Different imfill patterns and percent of infill will also impact the weight and amount of material used. Infills are managed in the Slicer program prior to printing.
Infills less than 20% may be a good option for minimizing the amount of material used and can support about 50% of strength. But this is not a good option for heavy load or tension.
Consider using different infill patterns with higher percent of material to improve strength and durability. To maximize the fatigue life of the 3d printed object consider 75% or greater density infill. Use a cross stitching pattern to ensure both tension and load strength for all sides of the model.
c.) Engraved or Extruded Wording
Logos, pictures, and Letters can either be engraved or extruded onto the surface of the 3D model. However, if the embossed letters are too thin then melted material may seep into the lettering or the letters may easily wear off. If the engraved or extruded letters are too small then it may not show in the print.
The recommended size for engraved logos and letters is a minimum of .5mm wide and .2mm deep. For extruded logos and letters we recommend a minimum of .9mm wide and .2mm high. Keep in mind any embossed lettering or logos will not turn out on the floor layer.
d.) Fillets or Chamfers
Fillets and chamfers are a great way to remove sharp corners, save on material, and prevent warping. Without fillets or chamfers excess material can form what is called “Elephant” effect where some of the melted material seeps on to the print bed.
To prevent the Elephant effect add small chamfers of .3mm on the downward facing edges where the print meets the print bed. To relieve stress concentration areas vulnerable to warping use 4mm fillets. Fillets and chamfers are commonly used to remove sharp edges and unnecessary overhang especially if connecting to other parts.
e.) Hole Size
Whether holes are part of design look or functional use for add screws holes turn out much better if printed flat along the XY plane, or spline along the Z axis. Holes less than 2mm may disappear in the print as the print material may seep and enclose the hole anyway.
If the hole needs to be less than 2mm consider adding the hole after the print is done. If it is necessary to have accurate size holes consider undersizing the hole and later drill it out to the proper tolerance. Slightly smaller holes can help with proper fitting of threaded screws.
f.) Minimize Material
Due to the nature of Additive Manufacturing, the amount of material deposited onto the object impacts the cost of 3D printing as well as the duration of the printing time. With this in mind reduce print areas where possible. Design your model in a way that saves on material use yet retains the strength and durability of the print. Identify the important areas. For example, on a jigsaw plate only the area around the drill holes need to have thick enough walls where the rest of the plate does not have to be as thick.
#3 – Best for Orientation
a.) Print Direction (Tension vs Load)
If you are just learning about 3D CAD modeling, one of the first things you’ll need to know is the XYZ orientation. Prints are made based on an XYZ Global Coordinate system. Since 3D printers print along a flat surface, which is the XY plane, prints are stronger in this direction. This is because the print along the XY plane is a single layer of material. 3D printed objects are weaker along the Z axis because it takes multiple layers of material to form the 3D print along this axis.
b.) High Width to Height Ratio
There are a lot of factors that go into producing quality prints with 3D printers, one of which is deciding what orientation will be most efficient. This can be frustrating because there is a lot to think about as far as size, material waste and strength of the print. What is the best orientation for the design? How thick is each layer? How much runoff or angles are there? How much support is required if in a certain orientation?
Maybe you have been running into problems with your printed object because the 3D model could be printing more efficiently at a different orientation, or something is wrong with your 3D printer. Check out our 3D printer troubleshooting guides to see what else could be going wrong with your 3D print.
c.) Runoff (45 Degrees or less, 5mm or less)
When 3D printers dispense material onto the print bed or layered on top of another layer, there needs to be some kind of support. Otherwise, with nothing to print onto, prints will start drooping immediately or produce a stringy spaghetti mess.
It’s true some angles and runoffs can print without support. 3D printers can handle dispensing material up to a 45 degree angle or 5mm runoff lip. A runoff lip is an area where a horizontal run off exists or there is material overflow from the previous layer.
We recommend running a test print with a sample model to see how well your 3D printer can handle angles and bridges.
If tension or load strength ultimately does not matter then choose the orientation with the least required support.
#4 – Best for Support
When printing objects with sharp overhangs gravity is not your friend. Prints with angles sharper than, or less than, 45 degrees will start to warp as the weight of the printed material gets heavier. Small runoffs may be okay since material is sticky enough to stick to its side, however runoffs larger than 5mm will start to droop if there is no support structure. A runoff is an area with absolutely no layer beneath it.
When modeling your CAD design keep in mind you need to include support beams in order to prevent your 3D print from failing. Any area with sharp angles (less than 45 degrees) or overhangs larger than 5mm need to have clearance for support beams and to then be cleared away during the post-processing.
Keep in mind overhang quality is also dependent on the printing material properties. Check out Prusa’s Chart on 3D filaments to learn about the different material properties.
b.) Bridging (5mm max without support)
Bridging occurs when a horizontal overhang is dispensed with no layer beneath and is supported by support structures on either side. Without support prints will begin to droop, or sag. For prints with bridging larger than 5mm in length we suggest using support structures to maintain quality.
c.) Large Horizontal Holes
Due to the way 3D printers print downward onto a print bed, holes in the vertical direction (spline along the Z-axis) are perfectly printed. When holes are printed horizontally (spline along the X or Y axis) the material starts to show more like a staircase.
Large holes actually form a little bit of an overhang while in the middle of printing. Therefore, we suggest using support structures especially if the overhang is larger than 5mm throughout the print time. This means holes that are 10mm in diameter will have this overhang during print time. You may consider using diamond or teardrop holes to prevent this overhang drooping effect.
#5 – Best for Assemblies
a.) Design Size
There are many things to consider when you need to modify your CAD model to be printable in your 3D printer. When the object you want to print is too large for your 3D printer the next best thing to do is to split the model into multiple smaller components. Or, you simply want to print in multiple colors or create a functional product with flexible joints. You will need to consider what kind of connector joints you want to model into the design.
Keep in mind larger models have a higher chance of warping so it’s good to use chamfers and fillets where possible. It is also good to use chamfers and fillets so your parts can fit together more easily.
b.) Allow for Clearance
When creating multiple parts to assemble after print is complete you will need to design the parts in a way to allow clearance for connecting joints. You will need to ensure holes and pins are in alignment and the female end is slightly larger than the male end.
Adding chamfers or fillets to connecting edges will prevent overhang and allow for clearance. Otherwise you may end up spending more time sanding the edges down for clearance. Or, you may risk destroying the print if you just try to jam the parts together.
For sleeve joints make sure there is enough clearance between the male and female ends. And, make sure there are no areas of obstruction where the parts meet.
Threaded holes are necessary for adding screws to join parts or assemblies together. These holes can be formed in a few different ways. The old fashion way is to drill or puncture a hole with a nail. With Fused Deposition Modeling (FDM) 3D prints you can also get away with printing slightly smaller holes then tapping the holes with a tapping tool. Depending on the density of the material, even regular screws can be used for tapping.
CAUTION: Modeling threads into a horizontal hole (spline along X or Y axis) will NOT work due to the fine detail overhangs it would try to create.
If your goal is to add thread directly onto the CAD model design, which is common for larger thread sizes, we suggest using a CAD program that has an automated thread feature.
If threads are smaller than M5 (size #10) then tapping should be done with threading tools after the print is done.
When designing your 3D CAD model think ahead how well the model will be able to print on the 3D printer. In this article we had highlighted many different modeling design guidelines that factor into producing the best quality print efficiently. To be able to print with the least amount of material and efficient amount of time. How to obtain a strong durable print with the least amount of material waste. And, many other factors that go into the detail of adding text or aligning holes.
When modeling in complex curves and surfaces be careful as to not lose any quality of the print due to a tessellation issue. Consider the CAD modeling errors you need to fix before going to print.