Post-Processing for SLS Parts: Streamlining Production
Selective Laser Sintering printers are, commonly, bulky industrial machines that can cost hundreds of thousands of pounds. But, since the release of the Formlabs Fuse 1, the first benchtop SLS machine in the market, SLS technology and processes have become …
Selective Laser Sintering printers are, commonly, bulky industrial machines that can cost hundreds of thousands of pounds. But, since the release of the Formlabs Fuse 1, the first benchtop SLS machine in the market, SLS technology and processes have become much more accessible for everyone to incorporate into smaller productions. For more information on this unique system, read this article.
Those that are entering into this technology must be aware that workflows are significantly different from those of FDM and SLA desktops. Even when the Fuse 1 is more affordable than other SLSs, it is still a substantial investment where manufacturers must know how to boost their productivity, lead times and ROI to its maximum.
This article focuses on post-processing stages for SLS machines, often overlooked in the production process as a whole. We’ll examine common practice processes and the surprising versatility that SLS parts can achieve. But first, why focusing so much on post-processing?
SLS systems are remarkable for their outstanding efficiency for fast production of dimensionally accurate and high-performance nylon parts in large batches. But, without a doubt, their top advantage is that, unlike FDM and SLA, parts are not dependant on support structures. Since they get buried under the unsintered powder, you can print all the parts that can fit inside the build chamber’s volume in one cycle. This advantage increases design freedom, reduces lead times, and guarantees fewer surface defects.
So, again, why should we care about post-processing? Well, like any other 3D printing technology, SLS has its limitations. The resulting parts are porous, their surfaces grainy, colour options are few (Only white, black and natural colours. In the Fuse 1 case, only black), and that nylon tends to absorb moisture.
Adding to that, removing the parts from the powder and cleaning them is a process that requires extra labour and adds significant downtimes. Maybe if you print a single part with easy-to-work geometries is not that big of a deal. However, if we talk about large batches, this issue could represent an unexpectedly massive bottleneck.
Now that SLS part production is entering the end-use consumer market as a great opportunity, post-processing can become a heavy barrier in lead times and mass consumption quality. As a response to this, many companies like Dyemansion focus entirely on developing automated solutions for SLS post-processing. Formlabs has solutions for post-processing too, the Fuse Sift. This machine’s design focuses on streamlining post-processing seamlessly from the Fuse 1.
Let’s focus now on the processes themselves.
Basically, removing the dust and cleaning. This is the only process that we necessarily require to do regardless of the application. Removing the dust can be done manually with the help of brushes, but we recommend using a compressed air gun and bead blasting if possible to improve efficiency. Moreover, it’s a common practice to add powder release holes in the design of parts with internal geometries.
After removing the powder, it wouldn’t be bad to clean the part with soap and water. Although SLS surface finishes are very stable when comparing to imperfections that often come with FDM parts, applying some filler can help fix any defect that may arise.
Maybe the best advantage of leaving the parts without any additional post-processing is that SLS parts have high dimensional accuracies. Since there aren’t further modifications like smoothing or coatings, you can easily remain within the tolerance range. On the other hand, these surface finishes are quite rough and therefore not the best for all intended uses. Furthermore, these finishes are not watertight and won’t deliver the best mechanical performance and aesthetics.
But what about automation? Another advantage of SLS systems is that the powder is highly reusable, but recollecting it and remixing it into the cartridge can be a very daunting task. Thankfully, Formlabs is aware of this and developed a whole station just for this issue, the Fuse Sift.
This machine can collect, store and remix the resulting powder directly from the build chamber and into the cartridge. To further improve lead times, we recommend investing in one extra cartridge and one extra build chamber. This solution enables you to start a new printing cycle directly without waiting for the Fuse Sift to finish.
To reduce the notable SLS grainy finishing and the layered look that all printed parts have, smoothing is the obvious way to go. Achieving this for SLS parts is highly feasible since nylons are very resilient materials for abrasion, being very tough and thermal resistant (PA-12 has 171°C heat deflection). By taking this into account, SLS parts perform relatively well over high friction and high-pressure processes that would require more care if it was with less resilient materials.
When we think about smoothing, the first idea that comes to mind would probably be sanding. And, yes, Nylon 12 parts are sandable; however, it can be very labour intensive and lacks consistency. On the contrary, investing in SLS machines means investing in repeatable and fast production. Consequently, media polishing has become the standard process for SLS smoothing.
Parts are engulfed with tiny ceramic beads inside a tumbler, where a vibration motion generates compression forces to smoothen their surfaces. The result is what we can call a satin finish, kind of a middle ground between matte and glossy.
As a consistent and labour-relieved process, multiple parts can go in one cycle, making it more efficient than other processes like sanding or bead blasting. However, the first downside is that it extends lead times significantly. Every cycle can take hours (Approximately overnight), and the dimensions of the container limit part sizes and quantities.
The other downsides relate more to geometrical and dimensional limitations. First and most obvious, surface thicknesses slightly reduce, manufacturers must make sure not to fall bellow tolerances. The following are:
- Rounded edges, it is impossible to preserve sharp edges with media polishing
- Fine details and engravements might either become blurry or even disappear completely
- Delicate features or thin walls might break during the process, especially with the presence of stress concentrators and elongated features
- Regarding holes or cavities, the beads won’t reach internal surfaces as easily as external surfaces. Moreover, if holes are too small, beads will likely get stuck or won’t fit at all.
All in all, manufacturers must take some design considerations for post-processing, like establishing minimum thicknesses and hole diameters.
Another way of smoothening surfaces is by chemical solvents. Although polyamides have exceptional chemical resistance, some compounds like cresols, dimethyl formamide (DMF) and phenols can do the work. Unlike media polishing, surface finishes can become completely glossy like injection moulded parts.
The application approach is to gradually expose the part to the chemical steam in a closed chamber to affect the least possible original dimensions. But more than dimensional accuracy, you must be very careful with safety measures. Since these solvents release toxic and corrosive gases, it is imperative to have all the required protective gear and ventilation systems.
Additionally, solvents can also work as bonding agents to weld part surfaces. A safer alternative to this is the use of epoxy adhesive, which works fine with nylon parts.
In contrast to the subtractive nature of smoothing processes, coating processes are additive. In fact, the porous surface of SLS parts enables easier adhesion. But, just as with smoothing processes, adding coatings affect dimensional accuracy by increasing thickness, so this is a key design and manufacturing factor to consider.
Generally speaking, coatings enhance not only aesthetics but also performance. For instance, they can provide extra UV protection. Many post-processing purposes like colouring, sealing and even metal plating are highly feasible for SLS parts.
The porous and hydrophilic surfaces of SLS parts make them ideal for dyeing bath processes. However, in the case of Fuse 1 parts, this is not possible since its consumables are only black. The surprising reason for this is because this is one of the reasons why this machine is so affordable. Since its cost-effective laser is less potent than its industrial counterparts, the material must be black for optimal energy absorption.
So, can we still give a colouring to a Fuse 1 part? The answer is yes. Spray paint coatings (Ideally acrylic paint) are highly versatile, providing a wide range of possible results, from very matte to very glossy. Before applying the paint, primer layers must be added first. Once the part is completely covered in primer, spraying thin paint layers, rather than thick, is vital to avoid dripping, pooling or any irregularity. As you can see, spray painting requires highly skilled labour to get decent results, but it is also automatable.
The porous and hygroscopic nature of SLS parts can make them unusable for many applications. For instance, parts can expand after absorbing moisture, which negatively affects tolerances. However, this same porosity helps consolidate sealant infiltration into the part. By adding lacquer coatings, SLS parts can achieve watertight and enhanced mechanical performance, not to mention a unique glossy appearance.
Common lacquer choices in the market are silicones and vinyl-acrylates, while polyurethanes are not recommended for nylon parts. You can apply it either by hand or by dipping the part. Just make sure the lacquer properly reaches internal geometries like cavities and holes. Lastly, widening holes just enough for the sealant to flow comfortably during this coating process is a desirable design practice.
Now, we cannot leave without talking about metal coatings. Though electroplating submersion is not as easy to do with plastics as it with metals, high-quality results are entirely possible. By adding a thin layer of stainless steel, copper, nickel, chrome, gold, silver, titanium, among others, parts can improve in terms of mechanical performance en electrical conductivity. With a resulting metallic appeal, parts become visually indistinguishable from real metal parts. The main drawback is that this process is, generally speaking, the most expensive on this list.