Mating post-processing with 3DP technology

Metal printing technologies are a form of additive manufacturing (AM) using lasers or electron beams to melt metal powders and create complex shapes. There are two main types of metal printing: powder bed fusion (PBF) and direct energy deposition (DED). PBF involves melting layers of metal powder in a bed using a laser [direct metal laser sintering (DMLS) or selective laser sintering (SLS)] or an electron beam [electron beam melting (EBM) or electron beam additive manufacturing (EBAM)]. DED involves melting metal powder or wire as it’s fed into a nozzle and deposited onto a substrate. Both methods can produce parts from various metals, such as stainless steel, aluminum, nickel, copper, etc. The quality and appearance of the printed parts depend on the process parameters, such as time, energy, and build orientation. These parameters can be adjusted to achieve different surface finishes, depending on the desired function and aesthetics of the parts.

To achieve the desired final finish for printed parts, consider three main factors often ignored by manufacturers: how to post-process the parts after the build, how to deal with the high surface roughness of the printed parts compared to machined or casted parts, and how to specify the surface requirements on the part print.

Post-processing is an essential step to improve the appearance and performance of printed parts. It can include removing excess material, smoothing rough surfaces, and enhancing mechanical properties of the parts. However, post-processing methods vary depending on the type and geometry of the parts, so you need to plan ahead and choose the most suitable method for the design.

Surface roughness is another challenge for printed parts. The surfaces of printed parts can be 3x to 20x rougher than machined or cast parts, depending on the printer technology and the build parameters. This can affect the functionality and aesthetics of the parts, especially for applications requiring high precision or smoothness. Therefore, adjust expectations and design accordingly. For example, figure 1 shows a machined part with standard surface and corner callouts. However, if the same part is printed, as shown in figure 2, the surface roughness will be much higher (150µ" to 300µ") and the inset detail won’t be reachable by mass finishing. To account for these differences, indicate the surface requirements on the part print in a manner reflecting the limitations of printed surfaces. For instance, you can use a special notation for inclusions (grade B) and allow for some variations in surface roughness.

The design level of a part usually ignores the surface geometry, but a post-machining operation may require a standard Ra 36µ", with an instruction such as remove machining lines, as shown in figure 3a. The micrograph in figure 3a shows as machined and figure 3b shows the elimination of machine lines by mass finishing. In the AM world, the layered building method may leave micro crevices, or inclusions, due to the process. Figures 4a and 4b show an AM part that was “as printed” and one post-processed to a desired surface finish of Ra 36µ" or better. The surface still has inclusions, even after a long post-processing.

The author doesn’t know any standard criteria to specify this at the time of this printing. The manufacturer of the part must decide if it’s acceptable or not and how to indicate it during the design process.

Choosing a post- processing method

Post processing is an important step in 3D printing, as it affects the function and appearance of the printed parts. Different post-processing methods suit different types of parts, depending on their shape, size, material, and surface requirements. Therefore, before choosing a post-processing method, one should consider the following factors:

● The function and aesthetic needs of the part

● The compatibility of the post- processing method with the existing manufacturing systems

● The scalability of the post-processing method with the production volume and part diversity

● The surface characteristics of the part – exterior and interior; how they vary with printer technology, build parameters

Some common mistakes to avoid when designing 3D-printed parts are:

● Replacing an existing part with a 3D-printed part without considering surface limitations, functionality

● Designing 3D-printed parts with complex, inaccessible surfaces that are hard to post-process

● Using a printer technology that doesn’t match surface requirements

The examples in (5a-5c) show 3D-printed parts post-processing. Figure 5a is a stainless-steel bone plate requiring a lower roughness finish; figure 5b shows a titanium medical implant requiring a machined finish; and figure 5c is a medical tool requiring a pristine surface finish and polish.

Mechanical mass finishing is a widely used and effective method for surface treatment. It involves using machines and abrasives to create a uniform and smooth finish on various parts. It’s also known as tumbling, but it’s more than just rotating or vibrating the parts in a container. It uses advanced equipment and media to achieve different levels of finishing, from deburring to polishing. However, this method also has some limitations, such as being non-directional and affecting the whole surface of the parts. It’s similar to wet grinding with a honing stone, but can only reach the areas where the media can apply pressure.

There are different types of machines using this method, such as barrel and disc finishers and vibratory bowl finishers. Each one has its own advantages and disadvantages, depending on the size, shape, and quantity of the parts. One of the most popular choices is the disc finisher (figure 6), which can handle large batches of parts in a short time. Another common option is a vibratory bowl finisher, which is relatively inexpensive and versatile, but may have limited capacity and control.

Accelerated chemical finishing is a method combining mechanical and chemical methods to create a shiny finish on metal parts. The method first uses a chemical solution to make a soft metal layer on the part’s surface. Then, it uses a mild vibratory or disc finishing cycle to remove that layer, leaving parts very smooth with almost no roughness. The method can finish complex shapes faster and more evenly than other methods and can also finish smaller holes and more complicated internal features.

One of the main advantages of the method is it preserves the shape of the parts better than traditional finishing methods. The main disadvantages are it’s not environment, health, and safety (EH&S) friendly and it’s not as efficient on rougher surfaces.

Electropolishing is a technique that removes a thin layer of metal from the surface of a workpiece, using an electro-chemical reaction. This reduces the surface roughness and makes the surface smoother, shinier, and more reflective. Electropolishing is not very effective on 3D-printed parts unless they’ve been pre-treated to smooth out layering defects.

One of the main benefits of 3D printing is creating complex parts with internal holes. However, these holes may need to be finished with a secondary process if a high-quality surface is required. Two common methods for finishing internal surfaces are abrasive flow and pressurized water jet slurry. These methods use a paste or a liquid with abrasive particles to polish the inside of the holes. However, these methods may not produce a surface as good as a machined or fabricated one. Therefore, it’s important to consider the design and specifications of the part before printing it.

Another type of post-process is coating and plating, which can be applied to metal and plastic parts. These processes can add a layer of metal or color to the surface of the part to improve its functionality or appearance. For example, metal coatings such as copper, nickel, silver, gold, or palladium can increase the durability and resistance of the part. Color coatings can also enhance the aesthetics of the part. These processes can be useful for products that need to cut, drill, or withstand friction.

About the author: Steven R. Alviti is president of Bel Air Finishing Supply Corp.

Bel Air Finishing Supply Corp.
https://belairfinishing.com