Design for 3D printing

Written by Jasper Bouwmeester

26th August 2020

Design guidelines for 3D printing

Just like any production process, 3D printing imposes some requirements on workflow and 3D models. In this post we will give a quick overview of the various aspects that lead to high quality products 3D printed on our machines. Several good guidelines are available from Sculpteo and 3D hubs. Most of the design rules stated for the SLA process are applicable to our technology as well. That said, we will add some further information, notes and/or rules in the sections below.

In this post we will address the following topics:

  • Our technology
  • Software and File formats
  • 3D scanning
  • Mesh Quality
  • Application
  • Wall thickness, Shell & internal structure
  • Corners & fillets
  • Support areas
  • Drain holes
  • Orientation

We hope this post will serve as a best practice guide to ensure optimal product quality from our 3D printing process. If there are any unanswered questions, complexities or remarks, do not hesitate to contact us!

Our technology

Our 3D printing technology is developed and build in-house and allows us to print very large products with high accuracy and detail. The process has similarities with the stereolithography (SLA) 3D printing process but tailored towards large cost-efficient products. The printers operated by Fiberneering have a build volume of 510 x 410 x 1000 mm. The resolution is approximately 150 microns in X, Y and Z directions.

We have a blog post to explain the differences and similarities of the various liquid resin technologies (SLA, DLP, LCD and CLIP).

Software and File formats

In order to print your products, we need a digital file describing the geometry. Many different 3D file formats exist and fortunately, we can accept almost all. The most standard file format used in 3D printing is the .STL file, this is the format that all files we receive are converted to.

3D modelling programs (e.g. Solidworks, Inventor, Spaceclaim, Rhino) typically use parametric files to accurately describe a model. Because relationships between parts of the model are defined, it is relatively easy to make changes to the part. For example, changing the radius of a fillet from 10 to 15mm is a very easy operation in most programs. Most programs have their own file formats, for Solidworks this is .SLDPRT for parts and .SLDASM for assemblies. There are open formats too, .STEP an .IGES being the most used. However, it is not possible to use .STEP or .IGES files directly in our 3D printing workflow, therefore we will convert it to .STL format when the product design is finished.

.STL (wikipedia) and .3MF (wikipedia, 3mf consortium) are both triangle based surface file formats. The triangles are connected in such a way that a surface – also called a mesh – is formed that approximates the shape of the product. Mesh models are always approximations due to the straight edges of the triangles. However, if the mesh is fine enough, the geometry obtained through printing can be very accurate. One important thing to consider is that a triangle based model is much harder to modify. The relationships between different aspects of the part no longer exist, so they cannot be adjusted. In the previous example of the fillet, the comput no longer knows it’s a fillet, so it is almost impossible to change the diameter of such a feature. Therefore, it’s important to do a conversion to an STL model only at the end of the design process.

We can accept most popular 3D files, e.g.: .STL .3MF .3DS .STEP .IGES .SLDPRT .SLDASM.

Our engineering team uses Solidworks for technical modelling. Products can be designed or improved by us and – if necessary – we can also perform finite element analysis with Ansys. The .STEP and .IGES files mentioned above can be opened in Solidworks. The preparation of our prints is done with Magics. In this program, the printer build volume is represented by a digital volume in which 3D models can be placed. With Magics, the products are fixed, scaled, shelled, given an internal structure, oriented and given support. The resulting platform is then sliced with an interval equal to the layer thickness into 2D slice files that can be read by the printer.

Next to the commercial programs mentioned before, there are some open source packages that are very helpful when modelling for 3D printing. Blender is very good for quick modelling and sculpting of meshes and Meshmixer has some useful tools specifically for mesh operations.

The way STL is set-up, it doesn’t really contain much data on scale and units of measure. Therefore, it’s important to ensure the scale of the STL file is as intended. Being a Dutch company, we use mm as base unit for our models – so a US / inch based model would be likely printed 25 times too small if we don’t adjust. Also some software packages, especially blender, can easily blow up or shrink your model significantly upon exporting, depending on the scale they are working in (fix for blender). If you’re not sure, just send us the intended dimensions together with the model and we can scale it accordingly.

3D scanning

It is very much possible and interesting to 3D print a digital scan of an actual product or model. Due to high detail present in most 3D scans this leads to highly detailed and impressive printed parts.

3D scans have a high tendency to contain holes and mistakes. When dealing with 3D scanned files extra care should be taken to ensure a watertight model. Current 3D scanning software is typically very capable of fixing any holes due to incomplete scanning data.

Recently we received a scan of an art piece to be printed. However, the scan had areas that were filled in very coarse by the software. On screen this did not really show, but once printed it looked far from perfect. Also, some features appeared in the scan that should have been removed. In this case they weren’t and then it gets printed. Key learning: make sure you are happy with your scan file before sending it for printing.

The quality of the print will be determined by the quality of the scan.

Mesh Quality

The triangle mesh quality determines the quality of the 3D printed product, any mistakes or coarseness will show in the final product. Unfortunately, the .STL format is quite prone to errors. These errors can frustrate the workflow, causing lost time but can also result in product defects. Because of this, always try to supply high quality and error-free .STL files.

Successfully exporting an STL through Soliworks

When you are happy with your part go to File > Save As and choose “STL (*.stl)”. Click the Options button. You will now see a screen like this:

Make sure that the deviation and angle settings are fine enough. We typically use a deviation less than 0.15mm and angle of 0.5 degrees. Click OK and then Save. The resulting file can become relatively large, depending on overall product size, anywhere between 0 and 1GB. Most other programs have similar ways to “Save As” or “Export” a model to an STL file.

Watertight Mesh

It is important for the mesh to be manifold / watertight. Non-manifold areas will cause errors in the slicing process. You can use Meshmixer (Analysis > Inspector) to check for non-manifold areas and repair them. We can also repair most of these errors.

Overlaps and orientation

All triangles in the STL file should be properly defined; they should not be overlapping each other. Especially sculpted models can have lots of overlapping triangles. It’s possible to fix this, however, there is a risk to slightly modify the model in the process. So always check back.

Triangle orientation, i.e. in which direction the normal points, is important to create solid areas. Unless this has gone wrong very badly it is easy to fix.

When working with .STL files, please make sure:

  • the triangle size is small enough to represent the product geometry
  • the mesh is watertight (there are no gaps)
  • triangles do not overlap and are properly oriented

Fortunately, many of these errors can be fixed by Fiberneering so even if the quality of your files is not optimal, do not hesitate to send them to us for review. It is always preferable to have a clean mesh to work with as any fix can cause unwanted geometry changes.

** Afbeelding met verschil Solidworks/Mesh **

Application

When designing a part for 3D printing, it is good practice to consider how the product will be used and which features are important. For instance, if the model is used as a casting or lamination mold one may want to add a release angle. If mechanical loads are exerted on the product you may want to consider a larger wall thickness and internal structure. Consider the following:

  • Is there a limit for the part mass?
  • What are the required tolerances?
  • What is the required surface finish?
  • Is a coating or post treatment required?

Wall thickness, Shell and Internal Structure

Lower wall thickness tends to give higher accuracy, but the resulting product may be less sturdy. On the other hand, higher wall thickness will typically be stronger, but there may be a higher tendency to warpage and unwanted deformations of the part. Therefore, there is no one wall thickness that works for every application.

We can print solid sections, hollowed products, hollowed products with an infill or products with a sandwich wall. Most products we print are a hollowed shell with an internal structure. The wall thickness of this shell is typically 0.75 to 3 mm, depending on the application. The combination of shell with internal structure produces a relatively lightweight product with very good mechanical properties.

For smaller details, a wall thickness of less than 0.75 mm can be used down to 0.3 mm.

Corners & fillets

Although 90 degree inside corners can be printed, we recommend using fillets where possible. Sharp inside corners can result in stress concentration and early failure.

Fillets help reduce stress concentration and are often easier to print than sharp corners.

Support areas

Added support structures form a connection between the build platform and the product, proper support is required for successful prints. When the part has finished printing, the support material is removed from the print. Locally this leads to very small break off points on the product which are cleaned up afterwards. This is usually done by means of local sanding.

Drain holes

Drain holes are added to each hollow print to allow excess resin to drain from the product when the print is finished. These drain holes can either be placed at locations where they are not a problem or can be closed later in the process.

Orientation

Products are often printed under various angles, to minimise the cross-sectional area that is printed in one step. This improves the overall tolerance level of the product.

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