Guide to SLS Post-Processing Techniques: Media Blasting, Smoothing, Coating, Coloring, and More
Selective laser sintering (SLS) 3D printing is a proven manufacturing technique with high-performance materials—and those materials can go even further in performance and appearance when paired with advanced post-processing methods.
Read our guide to learn about both the basics of post-processing SLS 3D printed parts and advanced methods to smooth, coat, and color finished prints.
Basics of SLS Post-Processing
Like other powder-based additive manufacturing technologies, SLS parts must be cleaned of excess powder after printing concludes. Once your part has finished printing and is removed from the 3D printer, there are three core steps you need to take: part extraction, powder recovery, and media blasting.
Step 1: Part Extraction
Before post-processing can begin, SLS prints need to cool down, which can take up to 14 hours on a fully packed Fuse 1 build. Fuse Series printers have a touchscreen feature that indicates the recommended cooling time.
This cooldown process consists of two stages.The first step is to wait for the build chamber to cool to ≤ 100°C, when it can be removed from the printer and transported to the Fuse Sift station. This industry-leading rapid cooldown time allows you to start a new print within 1-2 hours of your previous print job.
The second stage of the cooldown process takes place within the Fuse Sift. Once completed, the part extraction can begin. The Fuse Sift will slowly raise the powder cake out of the build chamber, allowing you to separate sintered parts from unsintered powder, while a fan pulls any dispersed powder into a filter. Unsintered powder should fall away from the prints with the touch of your hand, exposing the completed prints.
Since SLS 3D prints do not require support structures, there is no need to remove excess printed material. Fuse Series 3D printers use a proprietary semi-sintered shell called Surface Armor to protect the surface of parts as they print for consistent part quality and mechanical properties. This shell is removed by gently scrubbing the surface of the part with a brush.
Step 2: Powder Recovery
Any powder left unsintered after a Fuse Series print can be recycled and used again, reducing waste and material cost. The Fuse Sift uses a vacuum to recover loose powder, and a tumbler to mix the recovered powder with unused powder for use in future prints. The percentage of new powder added is called the refresh rate.
Different 3D printers and powders have different refresh rates. Fuse 1+ 30W's inert gas environment enables printing at lower refresh rates than its predecessor, the Fuse 1.
Step 3: Media Blasting
After basic cleaning, media blasting (also called sand blasting) is recommended to fully depowder an SLS part. This is particularly useful for clearing the semi-sintered Surface Armor out of negative features, which can be hard to reach with a brush.
Media blasting, also called sand blasting, is recommended to fully depowder an SLS part. The part on the left has been media blasted and the part on the right has not. Note the semi-sintered Surface Armor remains in the negative features even after initial depowdering.
There are multiple types of media blasting systems. Formlabs recommends siphon-feed systems for SLS 3D parts. Siphon-feed systems use the Venturi effect to create suction at the end of a tube that intersects with the compressed air line. Siphon-feed systems typically operate at a blasting pressure that is strong enough to remove excess powder without any negative effect to the printed part. Higher-pressure systems may risk pitting or otherwise damaging the surface of the print.
Media blasting systems need significant air flow to work, with most cabinets requiring a compressor that can achieve 16 CFM at 80 psi to effectively propel abrasive particles and clean SLS parts. Having shop air provided by an industrial compressor will make the blasting process more seamless as it will enable continuous blasting without pressure loss.
The basic siphon-feed options on the market are:
Benchtop blasters: Desktop media blasters tend to range from $150 to $1,000 in price, and their compact form factor may fit in smaller workspaces. Note that the lowest-end units may be of poor quality, which can lead to issues like equipment downtime or media escaping the cabinet.
Floor blasters: Standing blasters are moderately more expensive and take up more space, but are still relatively affordable, starting around $1,000. Many come with air media separators (vacuums that connect to the side of the blast cabinet, keeping the air free of airborne media and powder) that keep the surrounding workstation clean.
Tumble blasters: Tumble blasters contain a rotating drum with a stream of air and media pointed at the parts inside the drum. This is an automated blasting process, where parts are placed in the machine and left alone until the blasting cycle is completed, which typically takes about 20 minutes. Tumble blasters are very well suited for cleaning large volumes of small, robust parts. They are not suitable for delicate parts or parts with fine features, and are likely not worth the expense for a smaller volume of parts, as these start above $20,000.
A few manufacturers, including AMT and DyeMansion, offer media blasters that are designed for 3D printed parts. Both are expensive, starting around $35,000, but these are the gold standard for user experience and efficiency in high-volume use.
Overview of Advanced SLS Post-Processing Techniques
Each of these processes has their own benefits and drawbacks, and each can impact different material properties to different degrees. When considering these workflows, including whether to bring the equipment and skills in-house or to outsource it, consider the capital cost and skill required.
The summary below shows the barrier to entry and potential outcomes of different advanced post-processing techniques on SLS 3D printed parts.
|Method||Barrier to Entry (Cost & Skill)||Smoothness||Chemical Resistance||Surface Hardness||Color||Conductivity|
|Liquid Polymer Coating||Low||✔||✔||✔||✔✔|
These steps can be done in sequence to achieve multiple attributes. In this article, we will discuss each option separately. If you are interested in stacking post-processing techniques to achieve a specific outcome, contact our team to speak with a specialist who can consult on your use case.
Options for Smoothing SLS Parts
After cleaning and media blasting, SLS 3D printed parts have a grainy, sandy surface texture. Here, we explore two techniques to smooth the surface: vibratory tumbling, which gently removes material, and vapor smoothing, which adds a glossy finish to parts.
Looking for smooth parts right out of the printer? Consider the Form 3+ or Form 3L stereolithography (SLA) 3D printers, which use liquid resin and produce parts with a naturally smooth surface finish.
Vibratory tumbling, also known as vibro-polishing or media tumbling, is a finishing process that smooths the surface of SLS 3D printed parts by placing them in a tumbler with other media. This produces a matte to semi-gloss finish that is satiny smooth to the touch. In our experiments, we observed an average reduction in surface roughness of 80%.
Vibro-polishing machines have a bowl, barrel, or tumbler that agitates the parts alongside pellets of media, such as ceramic chips, plastic pellets, or organic material like walnut shells, that gently abrade the surface. Cycle times typically vary from two to eight hours, and machines can run without a technician’s supervision.
Vibratory tumblers start at $100 for small benchtop models, making them an economical choice for any manufacturing setting. Larger, free-standing models are better suited for high-throughput production, and start around $3,000.
The part pictured below was tumbled with ceramic media in the Mr. Deburr 300DB for six hours.
Vapor smoothing bathes SLS 3D printed parts in vaporized chemical solvents that create a controlled chemical melt of the 3D printed material, filling minute cavities and forming a smooth, glossy shell around the part. Notably, the smoothing effect is not limited to line-of-sight geometries, making it suitable for complex parts with internal channels or other negative features.
Providers of vapor smoothing equipment and services claim that finished parts experience reduced bacterial growth, and may even be safe for food contact. Formlabs has not verified these claims, and we recommend you check with the equipment manufacturers for further safety information.
Vapor smoothing equipment is expensive, starting at $60,000, so businesses producing parts in smaller volumes may wish to send parts to be vapor smoothed at a service provider.
Options for Coating SLS Parts
Coating the exterior surfaces of a 3D printed part with a performance material confers additional properties that the 3D printed material can’t achieve on its own. Such properties include increased strength, conductivity, or chemical resistance. Many options exist for coating SLS 3D printed parts, and different coatings can be layered to achieve different properties. This section has a brief explanation of each and when they are beneficial.
Coating 3D printed parts in metal may be done through electroplating, also known as electrodeposition. In this process, the 3D printed part is first coated in a conductive spray, then submerged in a chemical bath, where an electrical current transfers metal from a source material (the anode) to the surface of the 3D printed part (the cathode).
Parts can be plated in many metals, including chromium, nickel, zinc, and titanium. Manufacturers will sometimes layer multiple types of metals sequentially to achieve the right balance of mechanical, electrical, and cosmetic properties.
Although it is possible to electroplate SLS printed parts with a home or DIY setup, inexperienced users risk exposure to hazardous chemicals and may struggle to get a quality result. For any structural parts, we recommend sending parts to a specialized contract manufacturer. Download our white paper for a list of electroplating services by region and job size.
The part pictured below was plated with nickel by RePliForm, Inc.
Strength from Metal: Strategies and Use Cases for Electroplating SLA Parts
Read on to learn how engineers are adding metal to resin 3D prints, and why hybrid metal parts can open doors to a surprising range of applications, including (but not limited to) end-use strength and durability.
Ceramic coatings, like those made by Cerakote, are applied in a thin film by spraying a polymer with ceramic additives directly onto the part. No primer is required, although parts need to be thoroughly media blasted with specialized media, and cleaned with a wax and grease remover to make sure no contaminants remain on the surface. Cerakote offers both air-cure and oven-cure formulations.
Ceramic coatings are among the highest-performance options for advanced post-processing techniques. Parts are chemically resistant and mechanically strong, and there are a number of color options available. Cerakote also offers specialized products for corrosion resistance, low friction, and more properties.
Cerakote has certified applicators listed on their website, and they also offer certifications for shops looking to bring ceramic coating capabilities in-house.
The part pictured below was coated with Cerakote H-140 Bright White by a trained applicator.
Powder coating is a dry application of polymer in powder form to a substrate material. It is applied electrostatically and cured with heat, light, or both. Many powder coating materials require very high temperatures, which rules out thermoplastics, like those often used with SLS 3D printers, as a substrate. Some powder coating materials, that can be applied at lower operating temperatures, are suitable for use with SLS 3D printed parts.
UV curable powder coatings were developed specifically with heat sensitive substrates in mind. These coatings are also applied electrostatically and flowed with low heat. Once the powder has melted, the coating is exposed to high intensity ultraviolet light, which cures it. UV powders require significantly less heat and time to process compared to thermoset powder coatings, while providing comparable performance properties, without damaging or warping the printed part.
Similar to other types of coating, powder coating can increase chemical resistance, surface hardness, and mechanical strength. Powder coating is particularly well-suited to thick coatings and geometries that would be difficult to reach or evenly coat with a liquid application process, like painting or dipping, where the material can run and drip.
There are a number of liquid polymer coatings on the market that provide different degrees of smoothing and chemical resistance. They may be clear, tinted, or opaque; may be applied by dipping, brushing, or spraying; and may be based on different chemistries, such as epoxy, lacquer, and polyurethane.
When it comes to materials and application methods, there is a lot of variation in both performance and difficulty to apply. Some of these can be purchased at a hardware store and applied with a minimal setup; some require industrial setups to apply.
The part pictured below was coated with an inexpensive clear gloss spray paint.
Options for Coloring SLS Parts
Whether for prototypes or final parts, you may wish to modify the color of all or some of a part. Dyeing results in a consistent, semi-permanent color, even in hinges and interior cavities, but it isn’t possible to use dyeing to lighten the color of a part. Painting offers more control and customizability, but is typically a more manual process and may chip with abrasion or heavy use.
Because SLS 3D printed parts are porous, they can be dyed with consumer-grade dyes. Many SLS 3D printing materials are nylon or polyamide, which are also used in textiles, so fabric dye designed for use with synthetics should suit.
Dyeing 3D printed parts can be done with household materials, in industrial vats, or with any level of sophistication in between. Whatever your setup, parts 3D printed in a nylon-based material, like those available on the Fuse Series, shouldn’t require any special steps—the directions recommended by the dye manufacturer should suffice.
Because the barrier to entry is low on both cost and skill level required, dyeing is a great way to experiment with advanced post-processing of SLS 3D prints. And, because it scales easily, it’s also suitable for production applications.
The parts pictured below were dyed with RIT fabric dye.
Painting may be preferable to dyeing when parts need to be brightly colored or color-matched to a reference. Spray painting provides the most even coverage and easier scalability, while painting by hand allows for greater detail. Both acrylic and enamel paints work well with powder printed parts. Other paints may work well, but Formlabs has not extensively tested these.
For best adhesion, use a primer that is designed for use with plastics. Spraying primer results in more consistent coverage. Additionally, a heat gun or hair dryer can be used to accelerate the drying process.
For an in-depth tutorial on spray painting 3D prints, check out our how to priming and painting guide.
The part pictured below was painted with an inexpensive spray paint.
Explore SLS 3D Printing Solutions
Ready to explore bringing SLS 3D printing in-house for any of the smoothing, coating, or coloring techniques outlined here? Formlabs is a trusted provider of accessible 3D printing solutions for organizations of all sizes, with its flagship SLS 3D printer, the Fuse 1+ 30W.
Curious to get hands-on with an SLS material? Request a free sample part, shipped right to you.