Post Processing
It is important to understand how post-processing MJF parts plays into the larger production workflow for the technology, given the fact that HP Multi Jet Fusion technology (MJF) has been billed as a platform built for mass manufacturing.
What is AM Post Processing and Why is it Needed?
As is the case with most manufacturing processes, AM parts aren’t complete until they’ve undergone the necessary finishing steps. In 3D printing, this can mean anything from ultraviolet curing to machining off excess metal left by support structures. Whatever the technique, post-processing is required to make the part smoother, stronger, aestetically attractive or to match the specific tolerances of a job.
Post Processing with AM Processes and Machines
For some 3D printing technologies, the first step in post-processing includes the removal of the support structures needed to hold up elements of the model to prevent sagging or warping as it is printed. It’s important to note that technologies that use photopolymers will also require rinsing before supports can be removed. The model below took 22 hours to print and a further 2 hours for removing supports and cleaning. This is not true of MJF or selective laser sintering (SLS) technologies.
Once these supports are removed, parts are usually sanded and polished, a process that varies depending on the material and the technology. This may include bead blasting, sanding, tumbling or vapor smoothing.
Components made by some technologies require specific post-processing elements in order to strengthen the part. For SLA and DLP, this means ultraviolet curing. For some metal parts, heat treatment may be required.
Post Processing for HP Multi Jet Fusion Technology
MJF is a new technology that delivers certain advantages over legacy print processes, but there are still post-processing steps that are required before items can be considered finished. Worth noting, however, is that the HP Jet Fusion 3D Processing Station has the option of “Fast Cooling,” which allows prints to be cooled down more quickly so that they may be removed for more immediate processing.
Within the processing station for the Jet Fusion 4200, there is a vacuum used to remove powder. Once removed from the processing station, bead blasting or airblasting is performed to clear any remaining powder, not unlike SLS.
Bead Blasting: This process consists of shooting an abrasive media, usually a bead (size and type results in different surface finishes), at high pressure at a printed part with compressed air, knocking loose unfused powder while also smoothing the finish of the part. This can be done manually or automatically, with manual bead blasting relying on a foot pedal-driven system for propelling the beads as opposed to an automated tumbler, turntable or conveyer. Manual may be preferred for fragile parts.
Airblasting: Air blasting is required after bead blasting. After bead blasting, air blasting must be used to remove the remaining powder from the surface of the printed part using a closed cabin air pressure machine with a minimum air pressure of three bar.
Secondary Post Processing
After the necessary post-processing steps described above, parts may need further finishing to bring the part up to technical requirements. This includes methods for reducing surface roughness, as well as methods for changing the color or finish of the part, like dying, electroplating and painting.
Sanding: Post-processing techniques can range from manual to almost entirely automated. For example, a company may want to smooth their Multi Jet Fusion parts; this could be done with manual sanding, though it would take a long time and be cost-prohibitive. However, it may work for one-off objects or visual prototypes.
Vibratory Tumbling: “Vibratory tumbling is another method that can be used to smooth Multi Jet Fusion parts that is hands-off and largely automated,” she added. “Though it can take several hours, because the process does not require supervision and can process many parts at once it is very cost-effective. You can buy vibratory tumblers of different sizes, according to your particular specifications such as quantity and part size.”
Vibratory finishing can be performed as a wet or dry process. In wet vibratory tumbling, ceramic and plastic media are used and create a more polished finish, with less wear on the part, but produces waste from the liquid-abrasive media. The dry process is cleaner and wastes less, but may be more aggressive.
Chemical polishing: This process uses a chemical to smooth the surface of printed parts without impacting its mechanical properties, resulting in a controllable level of glossiness from matt to gloss to shiny.
Dying: In addition, not unlike other processes, MJF parts can be subject to any number of finishes. When coloring parts that have been printed on MJF 4200, dying can be performed, either manually in pots of hot water or using automated dying equipment.
Dyeing is the most common secondary post-processing technique of MJF users and may be best for parts that are visible or subject to wear, as the color penetrates the surface of the part. Dying white parts, rather than grey, offers a greater range of dying options. Manual dying, which usually involves leaving the part in an 80-100°C dye bath for about eight minutes, is comparatively inexpensive. Automated dying machines, however, may be more efficient, as they use specific programs for mixing the dye bath, as well as conditioning, dyeing, part rinsing, dye disposal, and cleaning.
Painting and Electroplating: Painting and plating are other options for coating Multi Jet Fusion parts. Performing surface smoothing first will help achieve the best results with the least additional effort. Since every industry has its own paint specifications, the best bet is to have samples done with existing paint suppliers. Hydrographs are another method of coating. An image or pattern is floated on water, and the part is dipped in it to transfer the pattern over. Given that a layer of material is applied in the process, hydrographs also result in a smoother surface.
Electroplating consists of dissolving a metal in a solution and attaching the metal particles to the surface of the printed part using an electric current. Before this process can be performed on a polyamide part, the part must be made electrically conductive through the use of electroless plating, gas activation, or a conductive coating.
Graphite Blasting: Graphite blasting uses the same process as bead blasting but aims for giving parts a uniform, metallic appearance, with glass beads and graphite projected at the part. This can also reduce friction between moving parts, though it is not recommended for final parts that are handled frequently.
