Compared to traditional manufacturing processes, 3D printing seems like a miracle: design almost anything, press a button, and create a real part. However, many potential users of the technology often have questions about the machining and finishing of printed parts.
This is especially true for new additive manufacturing (AM) technologies like the HP Jet Fusion 3D Printer. Given that HP Multi Jet Fusion (MJF) technology is known as a platform built for high-volume manufacturing, it is important to understand the role that post-processing of MJF parts plays in the larger manufacturing process of this technology.
Here we study the post-processing techniques required for a number of AM methods and compare them with MJF.
Like most manufacturing processes, AM parts aren’t complete until they undergo the necessary finishing steps. In 3D printing, this can mean anything from UV curing photopolymer components to machining excess metal left behind by support structures. Regardless of the method, post-processing is necessary to make the part smoother, stronger, or within certain tolerances.
For many 3D printing technologies, the first step in post-processing involves removing the support structures needed to support the model elements to prevent them from sagging or warping during printing. It is important to note that photopolymer methods also require rinsing before support removal. This is not the case with MJF or selective laser sintering (SLS) technology.
In particular, for fused filament fabrication (FFF), stereolithography (SLA), digital light processing (DLP), and powder bed fusion (PBF) methods such as selective laser melting, the support structure must be removed. For plastics, this may be as simple as using pliers. For metals, it may be as complex as CNC machining or wire cutting.
Once these supports are removed, the parts are usually sanded and polished, a process that varies depending on the material and technique. It may involve sandblasting, grinding, tumbling, or steam polishing.
Parts made with certain technologies require special post-processing elements to strengthen the part. For SLA and DLP, this means UV curing. Some metal parts may require heat treatment.
This brings us to SLS. This powder process requires the use of a powder removal station to separate the part from the green powder, where compressed air blows excess powder off the part. Sandblasting can then be used to remove any remaining powder. A roller can then be used to polish the rough surface of the part. Painting can add color to SLS parts, and spray paint, varnish, or other coatings can be used to provide a variety of finishes.
MJF is a new technology that offers some advantages over traditional printing processes, but still requires post-processing steps before the product is finished. But it’s worth noting that the HP Jet Fusion 3D Processing Station has a “Fast Cooling” option that allows prints to cool faster so they can be removed for more direct processing. Additionally, the latest HP Jet Fusion 5200 3D Printing Solution includes a natural cooling unit designed for cost-effective, continuous printing.
The Jet Fusion 4200 and 5200 systems have a vacuum powder removal unit in the processing station. After the powder is removed from the processing station, it is sandblasted, air blasted or water blasted to remove any remaining powder, similar to SLS.
Sandblasting: This process involves using compressed air to spray an abrasive (usually beads, the size and type of which can result in a variety of surface finishes) at high pressure onto the printed part, dislodging unmelted powder and smoothing the surface of the part at the same time. This can be done manually or automatically, with manual sandblasting using a foot pedal system that pushes the beads rather than automated rollers, turntables, or conveyor belts. Manual sandblasting is preferred for delicate parts.
Water jetting: This process involves spraying water and air onto the part to remove powder and may use abrasive blasting media to pre-treat the surface. This process is generally more expensive than sand blasting and is ideal for automated machining of complex geometries and cavities, as well as for reducing surface roughness without the need for additional post-processing (such as vibratory systems). And it produces no dust.
Sandblasting: After sandblasting, sandblasting is required, but water jet is not required. Some sandblasting machines have a sandblasting function. After sandblasting, the powder residue on the surface of the printed part must be removed by sandblasting, using a closed chamber air compressor with a minimum air pressure of 3 bar.
After the necessary post-processing steps described above have been completed, the part may require further finishing to meet specifications. This includes methods to reduce surface roughness, as well as methods to change the color or surface treatment of the parts, such as painting, galvanizing, and painting.
Sanding: Post-processing methods range from manual to almost fully automated. For example, a company may want to sand their multi-jet deposited parts; this can be done by hand sanding, although this can be time-consuming and expensive. 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 fused parts without human intervention, and it is largely automated,” she adds. “While it can take several hours, since the process is unsupervised and allows for multiple parts to be processed at once, it is very cost-effective. You can purchase vibratory tumblers in different sizes depending on specific specifications such as part quantity and size.”
Vibratory polishing can be a wet or dry process. Wet vibratory polishing uses ceramic and plastic materials to achieve a smoother surface and reduce component wear, but it does create waste due to the liquid abrasive media. Dry processes are cleaner and produce less waste, but can be more aggressive.
Chemical polishing: This process uses chemicals to smooth the surface of the printed part without affecting its mechanical properties, resulting in a controlled gloss from matte to glossy and shiny.
Dyeing. In addition, like other processes, MJF parts can be subjected to various surface treatments. Although there is a dedicated MJF line for full-color 3D printing (HP Jet Fusion 580/380 series), these systems are currently designed for small batches. When painting parts that are not printed on these machines, painting can be done either manually in a hot water pot or using automated painting equipment. The DY130 system from Girbau is the recommended solution for automated painting of MFJ parts.
Dyeing is the most common secondary post-processing method used by MJF users and is probably best suited for visible or wear-prone parts as the colour penetrates the surface of the part. Dyeing white parts (rather than grey) provides a wider range of colour options. Hand dyeing typically requires placing parts in a dye bath at 80–100°C for approximately eight minutes and is relatively inexpensive. However, automatic dyeing machines can be more efficient as they employ specific procedures for mixing the dye bath as well as conditioning, dyeing, rinsing the parts, treating the dyes and cleaning.
Painting and coating. Painting and coating are other options for coating parts that have been welded using the multi-jet process. Smoothing the surface beforehand will help achieve the best results with minimal additional effort. Since each industry has its own paint specifications, it is best to ask your existing paint supplier for samples. Watermarking is another method of coating. Place an image or pattern on water, then submerge the part in the water to transfer the pattern to it. Watermarking also creates a smoother surface due to the layer of material deposited during the process.
Electroplating involves dissolving a metal in a solution and then using an electric current to attach metal particles to the surface of a printed part. Before this process can be performed on a polyamide part, it must be made electrically conductive through the use of chemical coating, gas-activated coatings, or conductive coatings.
Graphite Sandblasting: Graphite sandblasting uses the same process as shot blasting, but the goal is to give the part a uniform metallic appearance by projecting glass beads and graphite onto the part. It can also reduce friction between moving parts, but is not recommended for final parts that are handled frequently.
The time and cost required for post-processing can vary depending on the finishing needs. While painting parts is quick and cheap, other finishing processes, such as electroplating, can be more expensive and time-consuming. However, there are ways to reduce the amount of post-processing required, such as adding textures to the 3D model to mask layer lines.
It’s also worth noting that design and post-processing methods are not always compatible right away. For example, an airbrush may not be able to access the internal lattice structure, and surface treatments may not cover the part evenly.
HP is exploring ways to improve the efficiency of MJF part post-processing. This includes fully automated post-processing, providing a complete solution for manufacturers of manufactured parts.
Beyond post-processing improvements, MJF technology continues to evolve in other ways. HP seems to be regularly releasing MJF and Metal Jet technology, which the company promises will change the manufacturing and 3D printing industry. With that in mind, there may be more exciting 3D printing news coming from HP.
Post time: Jan-15-2025