SLS Post-Processing Guide: Media Blasting, Smoothing, and Dyeing Parts
Selective laser sintering (SLS) 3D printing has long been the domain of expensive, high-end industrial printers, oftentimes putting it out of reach for most business and product designers, or forcing them to outsource high-performance parts. But as prices have declined, the demand for in-house nylon printing has increased. This is because nylon 3D printed parts are proven, high-quality thermoplastics with mechanical properties comparable to those created with conventional manufacturing methods like injection molding.
Due to its use of nylon powder to create parts, SLS 3D printing has a distinct post-processing workflow. Plus, excess powder can be saved and reused, unlike the excess resin that must be washed off of a stereolithography (SLA) part after printing.
In this guide, we cover everything you need to know about post-processing SLS parts, from removing the powder cake from the printer, media blasting, and dyeing.
SLS Post-Processing: The Basics
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
- Media Blasting
In this section, we will be covering the post-processing steps for SLS parts printed on the Formlabs Fuse 1 3D printer and cleaned using the Fuse Sift powder recovery station. Users may have their own, custom cleaning station that works in a similar manner to the Fuse Sift. This section also includes how to remove and cool down the build chamber, which is the container into which the Fuse 1 deposits powder.
Similar to stereolithography (SLA) 3D printing, many factors will influence how successful and easy post-processing will be. For example, the volume of parts packed into the build chamber, part geometry, part orientation, part size, and packing distance will all impact print quality and ease of powder removal.
Once parts have been extracted from your SLS 3D printer, it is important not to lose any loose powder. Powder recovery is a critical part of SLS post-processing that does not exist when using FDM or SLA technologies. Because nylon powders are reusable in the SLS printing process, every ounce that can be saved is important. SLS 3D printers such as the Fuse 1 can operate with up to 70% reclaimed powder, meaning there is significant cost savings associated with collecting powder during post-processing.
The first post-processing step for SLS 3D printers is the cooling process. For print jobs that take over 24 hours to complete - due to the part being large or due to a tightly packed build chamber - you are going to need to wait 10 hours for the build chamber to cool off before you continue to post-process the parts. In some cases, you can reduce cooling times by opening the door of the Fuse 1 to save multiple hours.
Rapidly cooling down an SLS build chamber could warp or distort certain geometries and parts. When trying to cool down your build chamber faster, take care to note if it impacts part quality. One safe method is to follow the touch screen on the Fuse 1, which will provide guidance on how many hours are left until the machine has finished its cooldown. For other SLS printers, set a timer and come back later.
Since SLS 3D prints do not require support structures, there is no need for snipping and sanding support nubs, or using a hammer and spatula tool to remove a print from a build platform. Large chunks of excess powder should fall away from the prints with the touch of your hand, exposing the completed prints. At this point, a small brush can be used to remove remaining powder from the surface of the part. For small spaces, a toothbrush sized item can be used.
To fully complete the part extraction process, many users will need a media blaster to remove the final residue of excess powder. When creating SLS 3D prints on the Fuse 1, parts will have a semi-sintered shell called Surface Armor, which may also have to be removed.
We will discuss media blasting and Surface Armor in more detail below.
During part extraction, it is important to collect all excess powder for reuse. The optimal way to achieve this is to use a vacuum-based recovery unit to collect excess powder.
How much recycled powder can be used will differ between each SLS 3D printer. The Fuse 1 operates with a 30% material refresh rate using Nylon 12, meaning you can print with up to 70% recycled powder, indefinitely without degrading the mechanical properties of the printed parts. Fuse Sift can dispense and mix used and new powder automatically so you can reduce or even completely eliminate waste and control your powder supply.
Storing & Mixing Powder
Some SLS printers may have multiple hoppers for new and used nylon powder. Others, like the Fuse 1, may have a single hopper that combines the two. The Fuse Sift automatically mixes new powder with recycled powder, which then can be inserted directly into the Fuse 1 for optimal printing. For those using other devices, reclaimed power would have to be manually mixed.
The Fuse 1 consistently produces high quality prints using a refresh rate of 30 – 50% for Nylon 12 Powder. This means you can print with up to 70% reclaimed powder. No matter your SLS 3D printing setup, collecting and mixing reclaimed powder should be seen as a vital part of your post-processing workflow.
At this point, you have completed part extraction and powder recovery. The recovered powder has been sorted and mixed with new powder for the next print cycle. You can now perform media blasting to remove any final residual powder from your parts.
Media Blasting SLS Parts To Perfection
In order to fully remove all traces of excess powder, SLS parts need to be treated with a media blaster (also commonly referred to as a sand blaster). Media blasting uses compressed air and particulate media to remove unwanted powder from parts. Typically used for paint removal and cleaning surfaces before they are coated or painted, media blasters can be used to depowder the surface of an SLS part.
The Formlabs Fuse 1 uses a proprietary semi-sintered shell called Surface Armor to protect the surface of parts as they print for consistent part quality and mechanical properties. During post-processing, this shell is removed by scrubbing the surface of the part. The Surface Armor should fall away easily during regular post-processing.
While the majority of the unsintered powder should easily fall off your parts once removed from the Fuse 1, the powder that is close to your parts (1-2 mm surrounding your parts) will be more difficult to remove and often cannot be simply brushed away. This is why media blasting is a vital post-processing step for many users.
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 lower blasting pressure, and are therefore more gentle on parts. Using a system that’s too highly powered can damage parts, changing their dimensional accuracy. Siphon feed sandblasting systems are typically more gentle on parts but strong enough to remove excess powder.
Media blasting systems need significant air flow to work. Most cabinets require 16CFM at 80 psi to effectively propel abrasive particles and clean SLS parts. Most compression systems are not going to be able to supply this type of air flow for more than a few minutes at a time. For smaller SLS parts and projects, this will be fine.
Having shop air provided by a large industrial compressor will make the blasting process much more seamless as it will enable continuous blasting without pressure loss. However, using a cheap household compressor such as this silent electric compressor made by Husky (20 Gal. Vertical Electric-Powered Silent Air Compressor Model #3332013) should be sufficient for blasting both smaller jobs and larger jobs with breaks in between to allow the tank to repressurize.
Media Blaster Options for SLS 3D Printing
An extensive list of sandblasters that are on the market can be found below. In general, these machines can be classified into four categories:
- Benchtop media blasters
- Floortop media blasters
- Tumbling blast cabinets
- SLS-specific depowdering blast cabinets
Benchtop blasters ($1,000 and under)
Benchtop blasters are recommended for users on a budget, typically with small- to medium-sized print volumes per week. Desktop media blasters tend to range from $150 to $1,000 in price, and their compact form factor often makes sense for SLS parts. Cheaper desktop units often come with a slew of issues, such as poor cabinet sealing, which can lead to media escaping the cabinet. We’ve also seen cheap units suffer from poor sealing of pneumatic fittings, which wastes compressed air and creates loud noises that disrupt work spaces. Before purchasing a benchtop media blaster, make sure it comes with (or you already own) a dust collection system. When done right, desktop media blasters offer a small footprint and attractive price tag. The Abrasive Blasting Cabinet by Econoline is an example of an affordable desktop media blaster.
Floor blasters (Over $1,000)
Floortop blasters can be more expensive, with most machines costing over $1,000, but offer a larger working space and are considered industrial quality. Many come with well functioning air media separators (vacuums that connect to the side of the blast cabinet, keeping the air free of airborne media and powder) helping keep your workstation clean and parts free of excess powder. At Formlabs, we primarily use a Trinco 24 x 18 x 23" Working Dimensions Free-Standing Pressure Sandblaster.
Tumble blasters contain a rotating drum with a blasting gun pointed at the SLS 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. Tumble blasters are best suited for cleaning large volumes of smaller parts. However, this process is not recommended for delicate parts. Tumble blasting systems were not designed with SLS parts in mind, and their metal buckets can ding or break off fragile pieces of your 3D print. With a cycle time of approximately 20 minutes, tumble blasters can greatly improve your workflow efficiency. However, due to the need to use a benchtop or floortop blaster for delicate parts, Tumble blasters can be inefficient.
SLS-specific media blasters exist at the same high-end spectrum as the tumble blasters. Offering completely automated solutions for removing powder from delicate parts, these blasters are expensive but are market leaders for heavy SLS use cases. AMT’s PostPro DP and Dyemansion’s Powershot C are two example SLS specific media blasters. Both machines cost about $35,000, not including service plans and consumables. The cycle times for both machines are about 10 minutes to fully depowder parts.
Additional Post-Processing Steps
Vibratory polishing, 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 vibratory tumbler. This extra step produces a satin-like surface finish, and allows parts to achieve smoothness that are typically impossible with media blasting alone. The result of vibro polishing is a smoothness that gets rid of the rough exterior of the sintered nylon parts. Parts that are vibro polished are more scratch and dirt resistant than those that are not.
The process of vibro-polishing begins with putting batches of SLS printed parts into a vibratory bowl containing pellets of small media, such as ceramic chips. During operation, the tumbler containing both the media and SLS parts vibrates to create friction between the media and the workpieces. The media would then lap the surface of the SLS parts, rubbing the parts to slowly smooth the surfaces.
Typical processing time for vibratory polishing is around four to eight hours. Due to the abrasive nature of the media, some sharp edges will be softened and more delicate features would be prone to breaking. You should always follow the vibro-polishing guidelines to ensure that your parts meet the minimum viable thickness so that more fragile features would not break under the tumbling forces.
Before dyeing a SLS part, some users may want to deploy vapor smoothing. Vapor smoothing is a finishing option for SLS printed parts that uses vaporized chemical solvents to create shiny, smooth surfaces similar to an injection molded part. Vapor smoothing can be used in various 3D printing technologies such as powder bed fusion, including SLS and MJF, as well as Fused Deposition Modelling (FDM). The result of vapor smoothing is a smoothed printed part that also retains its original mechanical properties.
Unlike media tumbling, vapor smoothing does not remove material from the workpiece. The vaporized chemical solvent gets dispersed onto the parts surface inside of the chamber during operation. The result of it is a layer of chemical melt on the part surface that liquifies the part material in order to achieve an even surface across all surface exposure.
The same finishing agent can be used to vapor smooth many different materials, including ABS, polycarbonate and PA12. You can see examples of this process on the AMT website, which sells vapor smoothing systems.
Dyeing and Painting
Once parts are cleaned, users may want to change their color. Two popular methods are spray painting and dyeing.
Steps for spray painting SLS parts are similar to that of other 3D printed parts. First, SLS parts should be mounted and covered in multiple thin layers of primer. Then apply the spray paint to the surface of the part.
Due to the often complex nature of SLS parts, dyeing is frequently preferable to painting. Oftentimes, SLS parts come with moving parts, hinges, and internal crevices that can be hard to paint. You may choose to add paint for prints with large, flat surfaces and parts that do not contain complex lattices or internal sections that can be easily viewed.
Dyeing SLS parts is easy, and we recommend using basic RIT Proline Dye in the color of your choice. RIT Proline offers safety guidelines, available on their website. Before you begin, make sure you have gloves, an apron to protect your clothes, safety glasses, and a location with excellent ventilation. RIT Proline Dye contains instructions on the back of the package, which should be sufficient for a successful dyeing process. We will reiterate the dyeing steps below.
Typically that process will start with setup. The required items are:
- RIT Proline Dye (SDS)
- Hot plate
- Dye pot, stainless steel
- Stopwatch timer
- Liquid measuring cup
- Dye scoop, tongs, other utensils
- Paper towels, drying trays, drop cloths, etc.
- Acetone for cleaning
Dyeing SLS Parts
The steps on the back of the package should be as follows:
- Heat 1 L of water to boiling.
- Add dye to the water. Stir until dissolved.
- Drop parts in and start the timer.
- Remove parts at target time.
Standard stove top dyeing instructions detailed on the back of the RIT Proline package are suitable for dyeing parts printed in Formlabs Nylon 12. No salt, vinegar, or dish detergent was added to the dye bath. Parts do not require rinsing or washing after air drying.
The coloring process can be controlled by changing the amount of time the parts remain in dyed water. After just 30 seconds, some coloring will begin to soak into the SLS part. We found that the richest color comes after being soaked in water for 30 minutes.
Once the part is taken out of the hot water, rinse it for one minute under cold water to remove any excess dye left on the surface of the part. Rinsing should not notably affect color compared to unrinsed parts. Both rinsed and unrinsed parts left to air dry overnight did not transfer color to skin when handled.
The use of light-colored dyes present one downside to working with SLS parts. Since powders for the Fuse 1 are darker, such as Nylon 12, colors such as yellow will look tainted. Yellow dyes on Nylon 12 will look dark, more akin to a yellow-green color. For brighter colors, a user may have to add paint to the dyed parts.
Formlabs SLS 3D printing users are already dyeing SLS parts created on the Fuse 1. Partial Hand Solutions is dedicated to advancing technology for amputees of all ages. Since the company's inception in 2007, they’ve provided functional solutions for many active soldiers and individuals with partial hand and finger amputations, along with children with more extensive prosthetic requirements.
Learn More About SLS 3D Printing
Post-processing SLS parts is not a daunting task. After a few prints, most users are washing, post-curing, painting, sanding, and more with ease. Bringing your creation to life has never been easier with affordable, desktop 3D printing and the knowledge of how to post-process your parts.
Want to hold a high-quality SLS part in your hands? Order a free sample part below.
Want to learn more about SLS 3D printing? Download our extensive guide, where we cover the selective laser sintering process, the different systems and materials available on the market, the workflow for using SLS printers, the various applications, and when to consider using SLS 3D printing over other additive and traditional manufacturing methods.