Additive manufacturing today delivers very high surface finishes that can rival those from injection molding. In the following article, you will read everything you need to know in order to obtain components with the desired, smooth surface:
In technology, the surface finish describes the roughness of a surface. The higher the surface quality, the smoother and higher the quality of the surface of a workpiece. In additive manufacturing, the surface quality depends on several factors. In addition to the printing process used, the object to be printed and the positioning in the working area of the 3D printer also influence the surface finish of the workpiece.
Today, modern 3D printing delivers very high surface qualities that are comparable to those of the injection molding process. In this article, you will learn which surface qualities are possible, which technologies are used, and how a nearly perfect surface can be achieved through chemical smoothing.
If you look at the surface of a material with a microscope, you can always see unevenness even on surfaces that are supposedly smooth as glass. These unevennesses consisting of "mountains and valleys" are called roughness in the technical field. Various parameters can be used to calculate the surface quality of a workpiece:
In additive manufacturing, the center roughness value (Ra) in the unit micrometer (µm), one millionth of a meter, is often used to compare surface qualities. The smaller the center roughness value, the finer and thus smoother the surface.
In 3D printing, the quality of a surface is directly dependent on the printing process used. While center-line roughness values in the low, single-digit micrometer range are possible with some processes even without reworking, other printing processes require manual or chemical reworking.
Stereolithography (SLA) and the technically closely related Digital Light Processing (DLP) are two of the oldest, but also most precise, processes in additive manufacturing. SLA printing is best suited for the production of filigree, small prototypes or illustrative models, whereas DLP printing can fully exploit its speed advantages in the 3D printing of series parts. The processes are based on selective curing of liquid polymers, and support structures are always required for printing.
The roughness of the surfaces of workpieces produced using the SLA or DLP processes is directly dependent on the alignment in the printer. The top side of the workpieces is already very smooth without post-processing and offers center roughness values in the low, single-digit µm range. The underside, however, is very uneven and rough due to the support structures. Due to the pore-free starting material, excellent surface values can easily be achieved by manual grinding or - for perfectly homogeneous surfaces - by corundum blasting. Maximum achievable center roughness values of approx. 0.4 µm can be obtained by corundum blasting followed by grinding.
Multi Jet Fusion (MJF) and Selective Laser Sintering (SLS) are powder-based processes in which a powdered starting material is selectively bonded by chemical or thermal processes. Support structures are not required in these processes (except in the exceptional case of metal printing). Immediately after printing, the surfaces of the workpieces are already of good quality and offer largely homogeneous surfaces. However, due to the nature of the material, tiny pores always appear on the surface of the workpiece, which cannot be completely eliminated even by manual grinding afterwards. Maximum achievable center roughness values for laser sintering are around 1.3 µm and 4.05 µm for HP Multijet Fusion, in each case with reworking by grinding or blasting. Almost completely pore-free surfaces can only be achieved with both processes by means of complex procedures such as vacuum infusion or surface filling with subsequent multiple grinding.
Fused Deposition Modeling (FDM) is one of the processes in which thermoplastics are first liquefied by means of an extruder and then hardened during cooling. Depending on the object to be printed, FDM requires support structures. Surface finish can be described as low to mid-range. The 3D print is virtually pore-free, although stronger grooves are visible in the Z direction due to the process. The underside of the printed components is generally quite smooth, and center roughness values of around 8.8 µm can be achieved on the upper side without reworking. When printing with extruded plastics, surface impairments at the points where support material is applied cannot be avoided. However, it is quite easy to improve the quality of the surface by using sandpaper, glass bead blasting or barrel finishing. Center-line roughness values of 4.5 µm by glass bead blasting down to 2.3 µm by grinding can be realistically achieved with FDM prints.
In the polyjet or multi-jet modeling printing process, ultra-fine photopolymer droplets are applied to a working platform by a print head, where they melt to form very thin layers. The polymers are immediately cured by UV light. The process always requires the use of support structures. On the other hand, the quality of the surfaces is very good, even without reworking. Due to the low layer thicknesses of max. 30 µm, very homogeneous, pore-free surfaces without perceptible grooves and ridges are produced during printing. At the contact points between the surface and the support structure, quality losses can only be prevented by using thermally soluble or water-soluble support materials. Without post-processing, workpieces printed with Polyjet or Multi Jet Modeling offer center roughness values of approx. 6 µm. Extremely smooth surfaces with center-line roughness values of 0.2 µm can be achieved by grinding, and a convincing 3.7 µm can still be achieved by blasting with corundum.
For printing using the DLP process, we recommend the plastics listed below if a very smooth surface is required. Further details on the materials can be found on the corresponding detail pages.
We recommend the following materials for printing using the SLA process:
For SLS and MJF printing, we offer the very interesting option of ordering a standard PA12 material with the post-processing option of chemical smoothing. This enables components that have a quality similar to injection molding.
What is chemical smoothing?
For chemical smoothing, a printed object is suspended in a chamber and flowed around with a chemically specified mist. The mist slightly dissolves the surface, causing pores to collapse and grooves to be smoothed. Material removal does not occur, so size allowances are not necessary in the design. At the same time, the surface is closed and thus sealed. This makes the workpieces washable and disinfectable. Chemically smoothed components have a very smooth, injection-molded, high-quality surface.
Picture right: A component in SLS PA12 standard (background) and treated with post-processing chemical smoothing (foreground).
Investment casting technology combines 3D printing with "classic" casting processes. For investment casting, 3D data is first printed with a wax printer. The models are freed from support structures and bonded to prefabricated abutments. A robot then dips the models into a liquid silicate ceramic, which is then sanded several times. Once the required layer thickness has been achieved, the wax model is released and the mold shell is fired in a sintering furnace at over 1000° Celsius. The desired alloy can then be melted down and poured into the preheated ceramic mold.
After cooling, the shell is removed mechanically and the finished castings can be reworked. Heat treatment for normalizing or tempering is applied to most alloys in investment casting. The surface quality in investment casting is identical to that in injection molding. Therefore, this hybrid technology offers a good alternative if a part has geometries too complex for other production processes.
Read more now: Investment Casting Blog
Additive manufacturing has made enormous progress in the area of surface quality in recent years. Through a clever combination of starting material and printing technology, high-quality surfaces can be produced that resemble those of injection-molded parts. Manual or chemical post-treatments can be used to smooth even quite rough 3D printed surfaces. The new, hybrid technology of investment casting additionally opens up new possibilities to realize surfaces of printed workpieces in maximum perfection.
When nesting, i.e. placing the components in the installation space, the Jellypipe Print Partners ensure that they are optimally aligned. If a component specifically requires a particularly high surface quality on one side, please create an "individual request". Describe the corresponding requirement so that the component can be placed accordingly during 3D printing, or we can recommend reworking if necessary.
Do you have further questions about surfaces and qualities? Our Solution Partners will be happy to help you: Contact Solution Partner
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