From Rough to Smooth: Surface Finishes in 3D Printing

The surface of a part is often the first thing you notice – long before dimensional accuracy or strength come into play. Whether a surface appears technically matte, feels smooth, or remains visibly rough has a major influence on how a part is perceived and used. Especially in additive manufacturing, surfaces differ significantly from familiar machined or cast components.

At the same time, surfaces are difficult to describe. Values such as Ra are technically correct, but without experience they are hard to interpret intuitively. Photos provide an impression, but they do not show how a surface actually feels or how it is perceived in everyday use. That is why it is helpful to look at surfaces not only from a metrological perspective, but also to compare them with well-known materials and objects.

This article is intended to support exactly that. It classifies typical surfaces in additive manufacturing, explains the differences between cast parts, simple FDM prints, and professional processes such as SLS and SLA – and helps to develop realistic expectations of the surface quality of components.

What does surface roughness (Ra) mean?

To be able to compare surfaces, engineering often uses the so-called Ra value. Ra stands for the arithmetic mean roughness and describes how much a surface deviates on average from an ideally smooth reference line. This value is given in micrometers, i.e. thousandths of a millimeter. The smaller the Ra value, the smoother the surface appears.

It is important to note: Ra is an average value. It does not describe the structure of the surface, only the average height of its irregularities. Two surfaces with the same Ra value can therefore feel different or appear very different visually. Nevertheless, Ra is a helpful characteristic value for generally classifying surfaces.

Why do surfaces differ depending on the manufacturing process?

The surface quality of a component does not occur by chance, but depends directly on the manufacturing process. In additive manufacturing, components are built up layer by layer. Material type, layer thickness, process parameters, and part orientation all have a decisive influence on the surface. As a result, each technology produces a characteristic, typical surface structure.

Conventional processes such as casting or machining also leave their own recognizable surface finishes. Classifying these differences meaningfully helps in selecting the right process for a given application.

SLS parts without post-processing

SLS parts are produced by fusing plastic powder using a laser. The result is an even, matte surface without visible layer lines. In the untreated state, it appears slightly grainy and porous, yet very homogeneous. Many users compare this surface to a fine sugar cube.

This typical SLS surface is deliberately technical and is very well suited for function-oriented applications where robustness, design freedom, and reproducibility are more important than a decorative appearance.

SLS parts with chemical smoothing

Chemical smoothing can be used to selectively refine the surface of SLS parts. Surface peaks are reduced and pores are closed without significantly changing the component geometry. The result is a much calmer, more homogeneous surface with a noticeably smoother feel.

Many customers compare chemically smoothed SLS parts with high-quality series-produced plastic components, such as controller housings or device enclosures. This combination of high functionality and improved surface appearance makes chemically smoothed SLS particularly attractive for visible or handheld components.

SLA parts

SLA parts are considered the smoothest among common polymer-based 3D printing processes. In stereolithography, liquid resin is cured layer by layer at very high resolution. As a result, layer steps are hardly noticeable, especially with suitable part orientation.

SLA surfaces can appear very smooth to almost glossy and are particularly suitable for design models, visual components, or applications with high aesthetic requirements. Functionally and mechanically, however, they differ clearly from SLS parts, which is why the intended application is always decisive.

FDM Parts

FDM parts are produced by extruding molten plastic filament layer by layer. This process results in clearly visible layer lines that strongly influence the overall appearance of the part. Depending on layer height, nozzle diameter, and part orientation, these lines may be more or less pronounced—but they never disappear completely.

FDM is by far the most widely used 3D printing technology in the hobby and maker community. Relatively simple machine technology, low equipment costs, and a broad range of available materials make the process easily accessible. This widespread use also strongly shapes the general perception of what “typical” 3D‑printed surfaces look like.

Untreated FDM surfaces often appear more structured to clearly stepped and are frequently compared to everyday objects such as ribbed plastics, consumer-grade 3D‑printed products, or roughly layered prototypes. In terms of haptics, they are usually rougher than SLS surfaces and far removed from the very smooth appearance characteristic of SLA parts.

Classifying surfaces instead of just measuring them

No surface is fundamentally better or worse than another. What matters is whether it fits the application. While rougher, technical surfaces are ideal for many functional parts, visual or design components often require smoother surfaces or additional post-processing. Ra values provide important guidance here, but only become truly tangible when compared with familiar materials.

Our graphical overview is designed to help with exactly that: it places typical additive manufacturing surfaces along familiar everyday references and supports realistic evaluation of processes and post-processing.

Note: The surfaces and Ra values shown are typical reference values intended for orientation. The actually achievable surface quality depends on material, part geometry, process parameters, and post-processing.

Surfaces in additive manufacturing cannot be meaningfully rated as “good” or “bad,” but only in the context of their application. Each process produces a characteristic surface that results from the process itself and can be deliberately influenced or post-processed. Anyone who understands the differences between FDM, SLS, and SLA and realistically classifies surfaces makes better decisions when it comes to technology selection, design, and post-processing. Comparisons with familiar materials and haptic references are often more helpful than pure numbers – and lead to results that are convincing both functionally and visually.