Expertise in post-processing is essential to meet customer expectations of 3D printed parts
Apart from 3D printing, what makes additive manufacturing unique are all the processes performed. AM first designs parts dedicated to additives, then programs them, optimizes the process, prints the parts, and finally post-processes them.
The promise of 3D printers to spit out finished parts in mass production is unrealistic. For example, simple plastic parts may require post-processing steps, such as removal of supports, and may require grinding, polishing, steam lubrication, machining, filling, painting, sealing, dyeing, and assembly.
In the AM market, the workshop is called the service bureau. This is because they must provide a series of services in order to turn the customer’s concept into reality. Similarly, when companies buy 3D printers, they usually seek more than just printed parts.
Printing parts that meet the requirements will require company employees to apply service bureau-style expertise, including post-processing.
Each addition process has its own challenges. For example, large-scale polymer parts usually require extensive finishing. They are made by extruding and stacking a layer of thermally melted plastic layer by layer.
When the plastic cools, it shrinks by about 0.5%. This may seem trivial, but for a 20-foot-long part, it is about 1 inch, which is a big error.
Tools are one of the main applications of large AM. Parts near the printer have a texture similar to corduroy. After printing and cooling, it must be processed. Machining can take longer than printing.
The prototype car or model has an A-level finish. The Oak Ridge National Laboratory reproduced the Shelby Cobra with the famous 3D printing method to celebrate the 50th anniversary of the iconic racing car. Before the post-processing, the vehicle looked good, but its surface was rough. With the help of the finishing experts of Tru-Design LLC, a process was developed to finish this large additive part.
Since the surface of the carbon fiber reinforced material is similar to concrete, it is very difficult to sand. The initial sanding and primer attempts are feasible, but they are not a long-term solution. The best results come from the use of a proprietary spray filler, which can be sanded and painted to achieve the car’s finish.
3D printing metal parts is completely different from printing plastic. Fused filament manufacturing (FFF) is a popular process for printing plastic parts that involves extruding molten plastic, which can also be used to produce metal parts. Metal FFF parts require a lot of post-processing.
In metal FFF, the raw material includes filaments with heavy metal powder-loaded plastic or wax binder. The adhesive must be removed after printing. The degreasing process can be carried out by heating or acid bath.
After debinding, the parts are sintered. Significant shrinkage occurs at this stage. The good news is that the shrinkage rate is predictable, so that you can print fairly accurate parts.
Another method of metal printing is adhesive jetting. It has post-processing requirements similar to FFF. Binder spraying involves spreading a layer of metal powder, and the printer head selectively sprays the binder in a pattern according to the design of the part. The adhesive is then cured and another layer of metal powder is spread.
After the part is processed, the powder around it must be removed-this process is called powder removal (or scaling). Sintering is then carried out, which also eliminates the binder. Finally, the parts are machined and polished.
The LMD (Laser Metal Deposition) machine is equipped with a laser that can melt sprayed metal powder or wire to build parts layer by layer. These parts usually need to be machined after printing to achieve an acceptable surface finish.
Mixers with deposition heads and full processing capabilities can be used to minimize part handling and achieve the goal of removing finished parts from the machine.
LPBF (Laser Powder Bed Fusion) is the most common method for manufacturing metal parts. The powder layers are stacked and fused with the focused laser beam. Although the LPBF process can print solid and accurate parts, it still requires post-processing.
Once the excess powder is removed from the 3D printed part, it is usually necessary to remove the part from its bottom plate. Some people use band saws to get the job done, but most people use wire cutters.
Depending on the application, the parts can be heat treated or stamped through a hot isostatic press to improve the microstructure of the material. Some high tolerance surfaces may be machined. Many polishing techniques, including sandblasting, can improve surface quality.
The key to mass production
Obviously, additive manufacturing is more than just 3D printed parts. When making customers truly understand the vision of the part, post-processing steps are as important as additional processes.
Expertise in post-processing is essential to ensure this. It is also crucial to complete the transition from 3D printing to serial AM.
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