Additive manufacturing has experienced tremendous growth in recent years, evolving from a niche technology to an increasingly industrial approach that enables everything from prototyping to the production of end-use parts. However, despite the many advantages of 3D printing, especially in terms of design freedom and custom production, the technology alone cannot produce professional-quality finished parts. Usually, various post-processing processes are required to obtain a high-quality final model. This involves many different steps. This type of post-processing is often necessary to change the appearance of the blank, smooth the surface, adjust its size and shape, or increase its service life, so what surface treatments are available in 3D printing? What are the advantages and disadvantages of each?
Sanding is a popular method of finishing 3D printed parts and can be done manually, but automated tools are also available
Sanding is a popular method of finishing 3D-printed parts and can be done manually, but automated tools are also available. In 3D printed parts, especially those manufactured by extrusion methods, the line layer is very obvious. To remove these surface imperfections, one of the most common methods is sanding, using a rough material such as sandpaper of varying grits to remove layer lines and provide a blemish-free, uniform surface. This feature is useful if you want to apply additional coatings to the surface.
Sanding is usually started with coarse-grit sandpaper and progressively finer-grit sandpaper is used to obtain an increasingly smooth surface. However, be careful when sanding so as not to damage the geometry of the model. Also, a disadvantage of this method is that some points on the part are difficult to reach, especially small holes and undercuts. Sanding can be done manually, although it can be time-consuming. Therefore, there are also automatic grinding tools and machinery on the market.
Shot Peening and Sandblasting
Shot peening and sandblasting are other common techniques used for surface preparation in 3D printing, and they are often used on a variety of metal parts, including aluminum alloys, steel, titanium, copper, and other metals. These processes increase the strength and durability of parts, but there are some key differences between them. Shot peening uses small metal or ceramic balls that are blasted onto the surface of a part by high-velocity compressed air. This impact produces controlled plastic deformation in the surface layers of the part, increasing fatigue strength and reducing the likelihood of cracks and fractures. Shot peening also improves the corrosion resistance and adhesion of subsequent coatings.
Sandblasting, on the other hand, uses small beads of glass, metal, plastic, or other abrasive materials to clean, polish, or texture the surface of a part. Unlike shot peening, which deforms the outer shape of the part, grit blasting removes only the top layer of material. It is used to improve the aesthetics of parts, remove dirt and corrosion, and prepare surfaces for subsequent coatings.
The main difference between shot peening and grit blasting is quite simple. The first method creates plastic deformation on the surface of the part, increasing its ultimate strength and durability. The second method smoothes only the surface layer of the material, improving aesthetics and preparing the surface for subsequent coats. These post-processing techniques are particularly useful for parts subject to mechanical stress or strain, such as gears, springs, turbine components, and structural parts of aircraft and vehicles.
Polishing method: vibration polishing and barrel polishing
Different from the aforementioned method of processing parts one by one, the polishing system is used to process multiple 3D-printed objects at the same time, divided into vibration polishing and barrel polishing. Both techniques require the 3D-printed part to be placed in a drum or tumbler next to the abrasive, which when combined with tumbling or vibrating motion will create the friction needed for the part to achieve the optimum finish.
Can handle metal and plastic parts
While both methods can achieve a high-quality finish, vibratory polishing is generally better suited to achieve a smoother, more uniform surface by creating a more even distribution of material on the part. Vibration is therefore ideal for large parts or parts with rounded edges but a low level of detail. Tumbling, on the other hand, is based on a centrifugal cylinder system that applies a smoother motion, making it better for smaller, more delicate, and finer parts. Speed is another differentiating aspect of the two methods. While vibratory finishing is a faster technique, tumbling generally requires more time to achieve a high-quality surface. Depending on the desired finish and the materials used, tumbling can take anywhere from a few hours to a few days. In summary, vibratory finishing and barrel grinding are effective methods for post-processing metal and plastic parts, but they vary in motion, speed, surface finish, and applicability to different part geometries. Also, care must be taken when mixing different types of grinding media, as certain combinations can cause imbalances, resulting in uneven part finishes or damaged parts.
Vapor smoothing is another method of achieving a smooth surface for 3D printed parts, the main difference being that the finish will be glossy instead of matte. To do this, a gaseous solvent is used to melt the surface of the part until it is homogeneous. Once the part is exposed to the solvent in the vapor chamber, it is directed into the cooling chamber to prevent liquefaction. Liquefaction is the direct transformation of a solid or gaseous substance into a liquid state due to a fundamental change in its physical condition, this cooling ensures that only the surface melts and maintains the desired shape of the object.
The vapor smoothing process also fills pores on the exterior of objects and seals surfaces, making parts useful for containing liquids or gases. Although the technology is compatible with a wide range of thermoplastics, it should be noted that it cannot be used with certain materials because it may cause harmful chemical reactions. Incompatible plastics include polycarbonate (depending on the after-treatment machine), polyphenylsulfone (PPSF), ULTEM 1010, and ULTEM 9085.
Comparison of the original part with the steam-smoothed part
An alternative to steam smoothing is solvent impregnation. As the name suggests, it immerses the 3D-printed part in a solvent rather than exposing it to vaporizing chemicals. Although the results are very similar to those of vapor smoothing, maintaining dimensional accuracy is more difficult due to the faster and more violent action of the solvent. This method is often useful if the part is larger than the size of the vapor chamber.
Epoxy Resins for Surface Preparation: Coating and Penetrating
Epoxy resins are a class of reactive polymers containing epoxy groups (three-membered cyclic ethers containing two carbon atoms and one oxygen atom). When processing 3D printed parts, these resins enable a hermetic surface finish, making the part hermetic and increasing its resistance to high temperatures and certain chemicals. This method is ideal for parts that must face harsh operating conditions. When applying this type of resin, we have two options: coating and infiltration.
Epoxy coatings are usually applied by hand. This reduces costs by avoiding the purchase of expensive equipment, but at the same time increases the time and labor required for application. In addition, the technology is better suited for low-volume production, small-sized parts, or items that only need to seal part of their surfaces. However, some areas cannot be accessed using this technique, such as internal channels and undercuts. Also, it may not be ideal for parts that require precise dimensions, as the epoxy coating will add slightly to the thickness of the part.
Epoxy Resin Manual Application Example
Epoxy infiltration systems, on the other hand, overcome many of the limitations encountered with manual application. In the infiltration method, the part is dipped in epoxy resin and a vacuum chamber is used to introduce the resin into the object, filling the pores. The process takes about three hours to complete and is less labor-intensive, meaning it is more practical and faster when applied to large components. However, the main disadvantage is the higher cost compared to manual coating. In addition to the cost of the epoxy itself, it requires a vacuum chamber, and a heating furnace to preheat and cure the resin.
It is well known that additive and subtractive technologies can be used complementary to combine the advantages of both approaches. CNC machining, while not strictly a post-processing method, can be used as a method for high-quality surface finishing in 3D printing. This is especially interesting for technologies such as direct energy deposition (DED). In these products, since the metal is melted directly during the extrusion process, the resulting part has a very rough surface. A CNC machining step is always required to achieve a smooth and well-defined surface. There are many hybrid manufacturing solutions on the market, and integrating the two processes can speed up production.
△On the right is the CNC machined part