Surface Roughness In 3D Printing

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Surface Roughness In 3D Printing

This article explains what roughness is in 3D printing, provides surface roughness measurements for SLS, MJF, FDM, DMLS, Carbon DLS, and Polyjet 3D prints and highlights the benefits of surface treatments to achieve a smooth surface for 3D printed parts.

The surface quality of 3D printed parts is not similar for every process and every raw material. It varies greatly depending on the 3D printing technology employed, the printer quality and the build material. The design of the part to be printed and the position and orientation of the part during build also influence the surface quality.

Before choosing an additive manufacturing process, one needs to consider the roughness (measured in most cases in Ra) of the 3D printed parts. For the technologies that use support structures, like fused deposition modeling (FDM), special care needs to be taken since there is a contact point between the support structures and the actual part which has a different surface quality altogether.

Evaluating surface roughness of 3D printing processes

The surface roughness of a material is basically measured in terms of Ra and Rz values. Ra, in its simplest terms, refers to the average roughness of a surface, whereas Rz is the difference between the highest “peak” and the lowest “valley” on the surface. The R values are usually measured in µm.

In simple terms, Ra can be said as the deviation from the ideal surface plane usually measured in microinches or micrometres (µm). The higher the Ra value, the rougher the surface. The main regulation governing surface roughness is ISO 25178 which measures the surface roughness in terms of Ra and Rz values.

The roughness of a surface depends on a variety of factors, including the design of the part, the manufacturing process used to create it and the manufacturer. Even if you select a 3D printing technology with a supposedly great surface finish, the parts might still present some stepping marks or have a slightly curved surface, causing a decreased surface roughness quality.

Graphic shows the reduction of surface roughness with Levels of Smoothing compared to the raw part using an EOS PA 2200 sample part (Source: Develop3D)

How to measure surface roughness in 3D printing?

To measure the surface roughness of 3D print, the easiest and most accurate way is to use a roughness tester, or “roughness meter”. This instrument measures the roughness of a surface. It is commonly used in manufacturing and engineering applications where the texture and surface finish of a material is critical to the functionality of the end product.

The roughness tester typically consists of a motorized drive unit that moves the stylus across the surface at a constant speed, a transducer that converts the mechanical movement of the stylus into an electrical signal, and a display unit that shows the roughness measurement.

Footage showing how to measure surface roughness of a 3D print with the roughness tester 

Let us take a look at the surface roughness variations and differences of 3D printed parts according to the technological categories. All the measurements were done in house and are average. One may find different results with parts printed with a different material or by a different provider.

Surface roughness for Powder bed fusion (PBF) processes

PBF includes technologies like multi jet fusion (MJF), selective laser sintering (SLS) and Direct metal laser sintering (DMLS) where the build materials are usually polymers and metal powders. This powdered build material is fused together to form the part as per the CAD design with the help of infrared in MJF and laser in SLS and DMLS.

The surface texture is matte and is not as smooth as SLA but it doesn’t require support structures. Holes and pores can occur on the surface, depending on the size of the powder grain. Reused powder will cause more nonuniformities on the surface.

A completely smooth, glass-like surface cannot be obtained on the printed surfaces, even after heavy treatment like blasting or sanding. However, the MJF process produces a less porous surface compared to SLS and has a smoother texture comparatively.

Close-up of an MJF PA 12 part, without any finishing option 

For PBF technologies, the cheap and ideal way of smoothing the surface is using sand paper. The other processes include grinding, sand or bead blasting and tumbling. Grinding and blasting will have little impact on tolerances and dimensions. Nevertheless, tumbling is an efficient process but is much slower compared to other surface finish methods.

In SLS, a near perfect poreless surface can only be achieved with vacuum infusions or a multi-stage process of surface filling and multiple grinding.

Close-up of an SLS PA 12 part, without any finishing option 

Surface roughness of fused deposition modeling parts

The table below details Ra and Rz values on both sides for an FDM 3D print:

MaterialSurface finishRaRz
ASAAs printed22.5 µm*114.9 µm

*This result was measured on the roughest surface, perpendicular to the layering. When we measured the part’s surface in the other direction, the spin of the tool measured along with the layers and the surface roughness was 0.9 µm.

Surface roughness for photopolymer droplets technology

The most common 3D printing process in this category is Polyjet. It is a rigid photopolymer additive manufacturing technology that works by jetting UV curable resin onto a build tray in a process that is somewhat similar to inkjet printing. It is well known for its speed and incredible precision. This technology also produces support structures which are removed later in the post processing. 

The surface quality is one of the best among all 3D printing technologies. One downside is the use of support structures and it impacts the related region when removed.

3D printed tiles printed with polyjet, as printed 

Surface finish options photopolymer process

If soluble support structures are used, no treatment is usually needed. If not, they need to be manually removed like in other technologies. Since it uses a deposited droplets system and the print head moves in X-Y direction, lines are visible along the movement of the print head. These can be removed in post processing for very smooth surfaces. The post-processing methods used to get improved surface quality are blasting and grinding.

Measured surface roughness of Polyjet 3D printed parts

The table below details Ra and Rz values on both sides for a Polyjet 3D print:

MaterialSurface finishRaRz
Photopolymer rigid As printed2.1 µm17.2 µm

Additive manufacturing processes’ surface roughness compared

TechnologyMaterialSurface finishRoughness (Ra)Roughness (Rz)
SLSAlumide® / PA 12 filled with aluminiumAs printed8 µm50.1 µm
SLSPA 12Bead blasted4.5 µm31.6 µm
SLSPA 11Dyed black7.5 µm47.6 µm
SLSPA 12Media tumbled7.1 µm45.5 µm
SLSPA 12Vapour polished + dyed black2.5 µm19.7 µm
MJFPA 12As printed10-12 µm59.9-69.4 µm
MJFPA 12Chemical vapour polished4.4 µm31.1 µm
MJFPA 12Dyed black5.8 µm38.7 µm
MJFPA 12Spray painted black5.5 µm37.1 µm
DMLSAluminium Al-Si10MgBead blasted10-12 µm59.9-69.4 µm
DMLSStainless Steel 316LBead blasted6 µm39.8 µm
DMLSTitan Grade 5Bead blasted5 µm34.4 µm
SLAIndustrial White, ABS-likeAs printed1.5 µm10.1 µm
Carbon DLSEPX 82As printed1.22 µm11.1 µm
FDMASAAs printed22.5 µm114.9 µm
PolyjetPhotopolymer rigidAs printed2.1 µm17.2 µm

Order parts for your 3D printing projects

The 3D printing surface roughness varies greatly with post processing. It not only improves the aesthetic value but also usability of the part for end use applications. Colouring methods like spray painting and dying can also be used to enhance the surface but it’s impact on surface roughness is highly debatable.

Xometry Europe offers 3D printing services online, for on-demand additive manufacturing projects, for both prototypes and large batches. With a network of more than 2,000 partners all over Europe, Xometry is able to deliver 3D printed parts in up to 7 days. Upload your CAD files to Xometry Instant Quoting Engine to get an instant quote with various manufacturing options available for polyjet 3D printing.

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