STL Tessellation Issues – CAD Modeling Errors You Need to Fix Before 3D Printing

Fix your 3D CAD modeling errors before going to print

CAD Modeling Errors

The data that goes into the 3D printer is essentially a 3D vectorized CAD model calculated into a text file to read into a 3D printer. This data could have some missing data, or if viewed in a CAD or Slicer program may have missing or corrupt surfaces. The most common format used for 3D printing is an STL file. STL, Stereolithography, calculates solid geometry into a 3D printed part by way of three dimension (XYZ) space. If you want to know more about STL check out our article on What is an STL file?

In some instances a bad print job could be due to the wrong temperature on the extruder or print bed. The print bed may not be flat. The nozzle might be jammed. Check out our Troubleshooting guide to 3D Printers for more information.

As an expert in CAD modeling I have seen how data can get lost in translation even if it is simply due to a bug in the translator. [1] These bad 3D CAD models can also be causing a bad print. In this article I have gathered the top ten most common issues found in CAD data that should be inspected before going to print.

1.) Trim or Unify Intersecting and Overlapping Triangles

When designing or modifying a CAD model for 3D Printing, especially with complex models, sometimes surfaces will run into or through each other. This usually happens when using the auto surface generating tools in CAD.

The issue with these intersecting surfaces is that the edges do not mesh with other triangle edges. This will cause the 3D printer to get confused as it will not be able to discern which side is the correct outer side, or not know where to start or stop the print.

The print must have watertight surfaces in order to print properly. But, when trimming or unifying surface data watch out for extra surfaces that may still linger inside the CAD model. These extra surfaces, also known as non-manifold edges, may still appear because an edge may still be sharing two faces. 

When this happens you will need to cut off the extra edge or delete the extra surface. Trim the triangle(s) so the edges are unified together. If surface data still appears in the model after trimming or splitting surfaces just delete the unnecessary surface.

2.) Stitch Bad Edges

Sometimes you may come across bad boundary edges, or tiny gaps, in your CAD model. When designing complex models gaps are more prone to occur as surfaces are typically auto-generated at once. This could also happen if two surface edges are not perfectly in alignment, or meshed together.

When gaps occur, your model is no longer considered watertight and your print will not turn out. However, you should be able to detect these errors with your CAD program. At this point you should be able to zoom into the model to look for these bad boundary edges and stitch these gaps. You might be better off deleting the surface area around the gap and recreating it.

3.) Fill Holes or Gaps

Unplanned holes or gaps can appear in the model while creating or modifying your CAD model. Either this can happen by accidently deleting part of a surface or data can be lost during a file translation.

As mentioned in the other points this will cause confusion for the 3D printer. The 3D printer does not have the proper data needed to print in the missing area. The 3D printer does not know where to start or end while printing a layer. The 3D printer may then try to print the outline of the triangle or continue printing when it should stop.

Again, at this point you should be able to zoom into the model to fill these holes. But, you might be better off deleting a larger area of surrounding hole and recreating the full surface.

4.) Avoid Duplicate or Overlapping Triangles

While designing the CAD model you may be more efficient creating multiple shapes and joining them together as opposed to creating an outline and extruding the surface from the enclosed outline. With multiple shapes you can join or subtract intersecting areas. While this may trim out the unnecessary surfaces you may inadvertently keep surfaces you thought were deleted. These unnecessary surfaces may confuse the 3D printer, or tell the printer to dispense material in those areas wasting time and material.

Zoom into the model to look for overlapping or duplicate surfaces. You can temporarily hide the outer layer and click on surfaces to see where they highlight. You should also be able to select these surfaces from the CAD’s assembly tree.

5.) Remove Wart-Like or Bad Surfaces

A wart-like, or noise, shell is essentially an extra area of triangles that had formed during the design or format conversion of the CAD file. This may have occurred when auto-generating a curved surface made up of multiple triangles, or some triangles were not deleted when removing other surfaces during construction.

These bad areas, or shells, can appear like wrinkles on a shirt or warts on skin which in turn reduces the print quality. You will want to zoom in to the model to remove and recreate these surfaces.

6.) Flip Inverted Normals

A 3D print is actually an object made up of a ton of triangles meshed together. These triangles have two sides,which the CAD application and Slicer program recognizes as the inside and outside of the object. The outer surface is called the “Normal” side. Sometimes a triangle could end up inverted, or flipped.

This inverted triangle confuses the 3D printer causing the material to dispense on the wrong side of the design and thus creating bad edges. The 3D printer may not know where to stop printing and may print in areas where the object should be hollow. Using your CAD tools you can check for inverted normals by seeing which direction the tools arrows point.

7.) Minimize file size

A low tessellated, pixelated-like, model will give you a small file, but will not give you a smooth print. A high tessellated, smooth and detailed, model will give you a larger file size because there are a lot more smaller triangles the computer’s memory has to store.

These high tessellated CAD models with fine detail, smoother surfaces, will also impact how much the Slicer program has to process and slide. You can use the Polygon Reduce Tool provided by the Slicer program to optimize the 3D model for proper 3D printing.

8.) Rescale your 3D model

Depending on the size of your 3D printer you may need to adjust the size of your print in order to fit the boundary size of your printer. Or, maybe your CAD model is just not at the desired scale. Resizing a CAD model does risk the chance of distorting some of the surface triangles.

Increasing the CAD model size of a print can improve the accuracy of the polygons, but also use up more material. Wall thickness should be thicker for taller prints to ensure strength and stability.

Keep in mind every model is made of triangles, or polygons. Thus, when resizing your CAD model smaller prints will have finer tiny triangles whereas larger prints will stretch out those polygons. 

9.) Check Wall Thickness

Wall thickness is one of the most important elements of a good quality and durable print. When modeling a part in CAD, even if the model is “water-sealed” an STL file is just surface data as opposed to a solid block. The 3D printer needs information about how thick to print the outer shell.

All too often a print has to be cancelled mid-way through because the print is failing due to incorrect thickness. A wall too thin can cause the infill pattern to show through the outer layer and can also break mid-print because it is not too strong to hold up. Also the printed object may not last long if heavy load or tension is applied to it. So, find the right optimal balance for a sturdy print as well as saving on material.

10.) Hollow parts

With 3D printing comes the cost of time and material. To ensure a quality and durable print you will want to print at a slow pace and have decent infill density. Printing a 100% solid object is not ideal as it will take hours and use more material than necessary. A completely hollow model will break if the print must handle heavy load.

The ideal infill density to improve model strength should be at least 75% infill. Be more efficient with hollowing out the model. Manage the outer wall thickness and adjust the infill density. 20% infill is about 50% in strength.

Bottom Line

To ensure a smooth print you should always check your CAD model in the CAD application or Slicer program. Look for missing or deleted surfaces. Zoom into the model to look closely at the edges on complex areas to check for wart-like surfaces or bad edges. Make sure the 3D CAD model representation is watertight so the 3D printer dispenses the material properly. Once your data looks good also check your Slicer program for wall thickness, layer thickness, infill density, and temperature controls. Follow our 3D Printer Troubleshooting Guide to help resolve any issues you might be having with your 3D Printer.

Programs to Manage Your .STL File

If you already have a 3D CAD model in .STL file format you can use any of the following FREE 3D CAD programs to make modifications. However, as these a free programs they will have their limitations on full CAD modeling functionality.


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