Our daily business is rapid prototyping and small series production. Hundreds to thousands of prototypes and end-use parts are sent daily from our production facilities to engineers and product developers all over the world. When discussing a project, the most frequently discussed issue is that of the appropriate procedure or material. There are many factors that influence this choice, but most often the following:

• Costs: the cost per piece is calculated based on quantity and geometry of the part
• Production speed: with technologies like Stereolithography parts can be produced overnight, thermal technologies (e.g. Multi Jet Fusion / High-Speed Sintering HSS) take about 3 – 4 days
• Material properties: mechanical, chemical & heat resistance
• Desired visual effects: color, surface quality, etc.
• Accuracy / tolerances: typical deviations of the printed parts vs. the base dimensions

While the exclusion principle can often be applied to the first four factors, the question of accuracy / tolerance is often more complex. In the following article we will give you a brief overview of what you need to bear in mind when thinking about the accuracy and tolerances of 3D printing.

General information about tolerances in 3D printing

In most additive technologies, the dimensional tolerance is at least 0.1 mm. This means that the deviations in 3D printing are greater than in other technologies, such as injection molding or CNC machining. While tolerances of 0.1 mm are not uncommon when injection moulding plastics, it is quite possible to work in the hundredth scale when machining metal.

A common misunderstanding is the swapping of resolution and tolerance. If a Polyjet printer has e.g. resolutions (in Z direction) of only 15 µm, this does not mean that the tolerance is correspondingly low.

Error #1: STP -> STL conversion

In order to be used for 3D printing, an STL file must be available. This is exported from a CAD software or converted from a CAD file. Typical file format is the STP format. When converting STP to STL, inaccuracies in the hundredth scale can occur.

There is a particular problem with the surface. As this converts an algorithm (STP) into a ‘mesh’, in simple terms into a network of triangles, the interface therefore needs to be simplified. The lower the resolution is selected, the greater the deviation from the desired state. On the other hand, if the resolution is too high, the models get such enormous sizes that they can no longer be processed.

If you are not sure which resolution to choose, simply send us the STP file, we will choose the technically maximum possible resolution.

Export Resolution: 0,8
File size: 0,01 MB

Export Resolution: 0,1
File size: 0,03 MB

Export Resolution: 0,01
File size: 0,5 MB

Export Resolution: 0,001
File size: 3,7 MB

Error #2:
3D printing tolerances according to process

• SLA (Stereolithography):  ± 0,2 % (with a lower limit of ± 0.2 mm)
  SLA is the most accurate method with Polyjet. The diameter of the laser is usually around 100 -150 µm and is therefore significantly smaller than, for example, in the FDM process. Tolerances occur mostly due to shrinking. Attention with thin large surfaces, these can (as with most procedures) partly significantly warp.
• DLP: ± 0,1 – 0,2 % (with a lower limit of ± 0.1 – 0.2 mm)
  DLP works basically like stereolithography, only that here projectors are used instead of lasers. The tolerance depends on the resolution. Devices with very small installation spaces (= 10 cm side length). At this size, the systems generally have tolerances of around 0.1 mm.
• SLS (Selective Laser Sintering): ± 0,3 % (with a lower limit of ± 0.3 mm)
  SLS is one of the most popular rapid prototyping technologies. This is particularly due to the design flexibility (no support structures) and the robust material. This process works with heat, which unfortunately can lead to shrinkage or warpage.
• HP Multi Jet Fusion: ± 0,2 % (with a lower limit of ± 0.2 mm)
  HP Multi Jet Fusion is a novel process (also called High Speed Sintering) which, like laser sintering, works predominantly with polyamide 12. The factors that influence the tolerance in this technology are comparable to the ones in laser sintering.
• FDM/FFF: ± 0,5 % (with a lower limit of ± 0.5 mm)
  FDM printers usually print from ABS and PLA plastics. During cooling, the ABS shrinks to about 8%, while the PLA shrinks only about 2%. These are only guide values and may vary depending on the PLA or ABS used.
  A distinction must be made between systems. While desktop or semi-professional systems work with tolerances in the above-mentioned range, high-end FDM printers (e.g. Stratasys systems) deliver lower tolerances of up to 0.1 mm. However, the printing costs of these systems are much higher.
• SLM (Direct Metal 3D Printing): ± 0,5 % (with a lower limit of ± 0.5 mm)
  In general comparable to laser sintering, however, the heat conductive properties of metal is significantly different from plastics. Furthermore, the temperatures are much higher. This technology causes similar problems as laser sintering. The temperatures cause warpage. All in all, the parts are almost pore-free, with surface roughnesses of up to 20 µm. Often 1 – 2 mm of additional material is applied to important surfaces in the construction and these surfaces are milled accordingly after printing. As a result, tolerances can be significantly improved and the surface can be brought to the desired value.
• Polyjet: ± 0,1 – 0,2 % (with a lower limit of ± 0.1 – 0.2 mm)
  The parts printed with Polyjet are very accurate because UV radiation is used instead of heat. Even with thin-walled models, shrinkage or stresses are usually hardly measurable.

Error #3:
Design & Orientation for 3D Printing

A compact component is usually built much more precisely than a very filigree one. Furthermore, it is often the case that one and the same model, which is oriented differently in the printer, leads to different results. It is therefore helpful to indicate which surfaces are functional surfaces so that we can take this into account when printing.

Error #4:
Post-processing

Every 3D printing requires post-processing. In Stereolithography/DLP, FDM & Polyjet require support structures or mass (polyjet) must be removed. All of these processes are hand-grinded and blasted with compressed air (with different blasting agents depending on the material). In laser sintering, the surface is roughened and is therefore vibratory finished.

If you have high tolerance requirements, we usually reduce this work to a minimum. This reduces the surface quality, but the tolerance is lower. In this case, too, it is best if you discuss your requirements with us so that we can select the best possible process chain for you.

External links

• A very nice article about tolerances and reproducibility in FDM printing can be found here.
• Information on what to consider when designing in Solidworks can be found here.

About 3Faktur: 3Faktur is a 3D printing service bureau. We are specialized in the production of plastic parts in low (rapid prototyping) or larger (rapid manufacturing) quantities. Get in touch.