There are many different 3D-printing technologies available for metal components – and countless others are under development. Generally, most of the technologies build parts by some form of layering method. When choosing the process for a specific job, it is important to understand various 3D printer attributes. This included build surfaces and final finishes as they relate to several mass finishing techniques.

Metal Printing Technologies

Every printed part is unique. When post-processing a 3D-printed part, its often difficult to know where to begin to achieve a specific surface finish. As additive manufacturing (AM) technologies progress, they are becoming an increasingly more viable form of production and prototyping, AM can manufacture multiple parts simultaneously and create complex geometries.

Currently, two major types of laser AM involve either melting powder layer by layer within a powder bed – direct metal laser sintering (DMLS) and selective laser sintering (SLM) – or directly using electron-beam melting (EBM/EBAM). EBM is similar to SLS as it also prints from a powder for the 3D printers powder-bed; however, EBM uses electrons while SLS uses photons.

Bullets with varying degrees of surface finish
Bullets with varying degrees of surface finish

Image: Pick your post-processing technology. The surface finish can vary significantly for a #d-printed part depending on what post-processing method is used.

In EBM, a high-energy electron beam melts layers of powdered metal to create the desired form within a vacuum. Bon EBM and SLS utilize metal mixtures to create functional and aesthetic part characteristics. They can create parts made of various metals, including stainless steel, aluminum, nickel and copper.

There are multiple settings of the build process to consider.

Various process parameters can be adjusted, such as time, energy and build orientation, that will influence the finishing process and ultimately help achieve the surface finish goal.

Manufacturers often overlook three basic issues when post-processing 3D-printed parts directly after the build:

  • The potential for parts to have surfaces 3x to 20x the surface roughness of a machined or casted part
  • The final surface roughness and geometry
  • The cosmetic appeal of the part These issues should be considered in each part of the design process.

Typical surface roughness callouts on a part print are not applicable in the traditional sense. This is because the diffe-ence between a machined (or casted) surface and a printed surface can vary due to the build parameters.

The surface must be clearly indicated to allow for the variations and limitations of printed surfaces. For instance, without a secondary machining operation, the inset detail cannot be mass finished, so a compensation must be made to the part print. To address the shortcomings of AM surfaces, a post-process is often the best solution – rather than a secondary machining process – but even this must still be considered in the design process.

Surface geometry is hardly ever addressed at the design level of a part. However, a post-machining operation may call for a standard Ra 36 uin, with an added description, such as “removing machining lines”

Figure 1: The part on the left was additively manufactured “as printed”, while the one on the right was post-processed to a requested finish of Ra 36 uin or better. Notice the inclusions of the surface that remain even after a lengthy post-processing. (provided by Bel Air)

The before micrograph shows “as machined” and the after shows removal of machine li by mass finishing. In the AM world, because of the layered method of building, there always remains micro crevices, inclusions, due to the process.

Figure 1 shows an AM part that was “as printed” and one that was post-processed to a requested surface finish of Ra uin or better. Notice the inclusions of the surface, that remain even after a lengthy post-processing. The part manufacturer will need to decide whether the inclusions are acceptable and how to designate that during the design process.

Post-Processing Technologies

When choosing a post-processing finishing technology that will match your “manufacturing culture” and parts requirements, the first step is to scrutinize how the part function and aesthetic requirements will fit current manufacturing systems. Plugging a post-processing system into a production stream is no easy task. The process must scale with the number of parts produced, part size and geometry, as well as material composition. Keep in mind that current printing technologies produce a range of surfaces throughout the geometry of the part, so you will see significant variations among available printer technologies and build parameters.

Take time to understand the surfaces required on the whole part geometry, including interior surfaces. A common mistake is to replace an existing part with a 3D-printed one with the belief that replacing multiple parts and assemblies with one 3D build is an advantage.

The examples in Figure 2 show 3D-printed parts that were designed without taking into account the surface limitations and the needed functionality of the part. After all these criteria are considered, designers will need to match these requirements with the specific printer technology and available build parameters.

A number of post-processing technologies are used in AM. Mechanical mass finishing is the most common, as well as the most efficient and manufacturing friendly. While known as a tumbling procedure, mechanical mass finishing is a more sophisticated process that includes the newest high-energy equipment and abrasives to accomplish a more challenging task than those of traditional finishing.

It’s important to understand the benefits of a logistically simple process and the drawbacks of non-directional finishing-and that it’s an overall surface process. Consider this as a wet grinding procedure with a honing stone (process media), which can finish all surfaces that the energy from the machine can generate force.

Barrel and disc finishers both utilize this method to more easily process batches of parts in a cycle. A popular option in this category is the disc finisher, which comes in a wide variety of sizes to help scale with current production streams. Vibratory bowl finishers, meanwhile, are relatively inexpensive and also available in an array of shapes and sizes.

Accelerated chemical finishing uses a combination of mechanical and chemical techniques to achieve a mirror-like finish on metal parts. A chemical solution is used to produce a softened metal layer on the surface of a part, which is removed with a relatively gentle vibratory or disc finishing cycle. This results in parts that are extremely smooth. In addition to requiring significantly less time, the process is capable of uniformly finishing more complex geometries, and can finish smaller through-holes and more complex internal features than other processes.

Perhaps the most significant benefit of accelerated chemical finishing is that the process is better able to maintain par geometry compared with traditional finishing approaches. Th is particularly true for features such as gear teeth.

Electropolishing (also known as electrochemical polish anodic polishing or electrolytic polishing) uses an electro-chemical process to remove a micro-surface metal layer from a workpiece, reducing the surface roughness by leveling micro-peaks and valleys, improving the surface finish polish and making the surface reflective and shiny. However, the process usually isn’t very effective on 3D-printed parts unless a preliminary operation has been done to remove the gross layer imperfections from the layering process.

Figure 2: These parts were designed without taking into account surface limitations or the required part functionality.

Other Techniques

Producing internal holes is one of the biggest advantage of 3D printing complex parts. However, it’s difficult to s finish these printed surfaces without a secondary machining process. Abrasive flow with a putty-type media or pressurized waterjet slurry of the abrasive material are the two most common methods for finishing these internal surfaces. If the part requires an internal surface with a machined-like finish, design configurations during the initial phase are extremely important and callouts on the part print should indicate this accordingly. Although either method can accomplish a certain degree of internal finishing, the result may not be as good as a machines or fabricated surface.

A post-process can also include coating and plating operations. Both metal and plastic parts can benefit functionally and cosmetically from these processes. This can involve adding coatings of metal copper, nickel, silver, gold or palladium; and using a surface treatment can exponentially increase durability.

Coating options are also available and come in a wide range of colors. These coating and surface technologies also provide friction resistance to products designed for cutting and drilling, and they increase the overall lifespan of parts.

Leveraging AM Post-Processing

These technologies are all important to understand when considering a post-process for AM parts, as they play a crucial role in product design, function and manufacturing.

Knowing a part’s functional requirement for surface roughness and geometry will help manufacturers choose the best printer technology to fit their overall budget and production needs. The part model and print should indicate the surface finish requirements, and you’ll need a clear idea of the potential finishing procedures for production costing and manufacturing implementation. The next step is to work with a qualified supplier of the printer and post-processing system.

There is no better way to establish your best print than to choose a model, print a group of parts, and go through the exercise of finishing with different technologies. You have now completed the first step of 3D printing your component.