Post-processing in 3D printing – how do SLS, SLA and FDM compare?
Among many differences between current techniques of 3D printing there’s one that concerns all – the post-processing, that needs to be applied, after you take your part out of any printer – because there’s always something that needs to be done. Let’s take a closer look at what actions need to be taken in case of each technology, and how time-consuming these processes really are.
There are several things that need to be done after printing something with an FDM printer. First of all, you absolutely have to remove the support structures. There are several ways to do that. The most common one is manual removal – it can be done using popular tools you can find in any workshop, it can be relatively fast (depends on how complicated and detailed the object is), but also inaccurate: does not remove layer lines, striations, blemishes and can leave marks, diminishing the overall accuracy and appearance of the print – important details can be removed by mistake (it happens quite often, especially with long and thin details). You can also opt to use dissolvable supports from PVA (soluble in water), HIPS (soluble in d-limonene) or other materials. These, however, come with a different set of pitfalls, as improper dissolving can irrevocably damage your print or leave holes that will later need to be filled to make it whole again. It should also be noted, that it takes a lot of time, at least in case of desktop printers – process of dissolution PVA in water, or HIPS in d-limonene are quite slow. You also need specialized equipment, like glass containers or even ultrasonic bath. Finally, solutions resulting from the dissolution of supports are subject to special disposal regulations, usually they can not be simply poured into the sink. On the other hand, some filaments (like basic ABS) simply could not stick to PVA, which means sometimes you just can’t use it.
After you remove the support structures, the steps that need to be taken differ. If there are gaps in the print, they’ll need to be filled (with epoxy resin, autobody filler or an ABS and acetone compound). Usually, the surfaces also need to be polished. It can be done manually, with rotary power tools or with the use of additional techniques such as sanding (produces really smooth surfaces, but isn’t recommended for prints with 2 or less perimeter shells, or ones with small, intricate details – if done too aggressively, can also impact accuracy of the part). You need to be really careful with power tools, however, since too much friction force produces heat that can literally melt your print. Other techniques you can consider are vapor smoothing (can smooth out blemishes and diminish layer lines, but negatively affect tolerances and strength, due to “dissolving” of the outer shell and alterations in the material’s properties) or dipping, for example in acetone – it works very quickly, but the smoothing is very strong, so tolerances are heavily affected. Also, the chemical reaction during this process is really powerful and often breaks ABS prints – a day or two later you’ll notice multiple small cracks on their surface. In many cases the print also needs to be glued or welded together from separate parts, which is usually quite noticeable and affects the model’s visual quality.
After that, the only thing left is priming and painting, or coating the print in different kinds of materials (epoxy, metal) – and voila, your work is done.
Just like in FDM printer’s case, parts made in an SLA printers have to have their supports removed, before any other actions can be taken. In addition, after the printing process the models have to be cleaned of residual resin in aqueous solutions with various types of chemical additives (depending on the material used) also it is necessary to harden them by curing process (which can be made with chemical additives, heat or solar/ultraviolet radiation). These processes are time-consuming and require laboratory conditions – solutions resulting from dissolution of resins usually have to be disposed of by storing them in special containers and handing them to the appropriate services. Post-processing can be done manually or by power tools, but other methods can be used in addition (or instead) to produce better results. For example, you can use sanding or polishing to get a smoother surface (if you add some extra material to the print – around 0,1 mm – you can maintain good accuracy) or better yet – wet sanding – to achieve pretty much perfect smoothness (albeit at the cost of lower accuracy on the supported side and also possibly some lighter spots here and there, due to the use of water). You can also finish your print using mineral oil, which is kinda similar to wet sand, except it adds another, oily layer, which helps hide the typical discoloration. It also lubricates the surface and reduces friction, but may make it hard to paint.
And since we’ve already mentioned painting – it’s often the next, logical step. Spray-painting the printed part not only helps conceal the obvious layer lines (especially if it wasn’t sanded or otherwise polished to absolute smoothness), but also protects your work from too severe UV exposure, preventing yellowing and post curing.
The main difference of the SLS technique in comparison to the previous mentioned ones, is that it takes a little more time to set up the print, but you save yourself a lot of trouble later on. One of the reasons for that is that to achieve maximum cost and time efficiency, you need to print many parts at once – it’s best to fill the powder chamber to maximum capacity, and that has to be planned ahead of time. The placement must be right, so that dimensional accuracy isn’t negatively affected. The machine has to be heated up (and cooled afterwards). The good thing is that – once it’s started printing – an SLS printer is completely autonomous and doesn’t require any kind of supervision. Another boon is that you can print complex structures without the need to assemble them from different pieces.
The first thing you need to do during the actual post-processing of an SLS-printed item is cleaning up the unused/unsintered powder, that has served as support during printing. It’s usually done through the use of compressed air, but you can also do it manually, using rice or copper fiber brushes. Coming straight out of the printer, the parts have a powdery, grainy finish, so depending on their intended use, some work may have to be done on the surface to achieve optimal results. You can use sanding. You don’t need a lot of specialized equipment – in most cases sandpaper, precision knife and some pliers are quite enough, but if you like, you can also use an electric sander. If your print isn’t too delicate and doesn’t need sharp edges, you can opt for vibro polish, which provides excellent smoothness and can be done on many items at once. Another great way to finish work on a print is sand-blasting – it creates a homogeneous, evenly abraded and matte surface that scatters light evenly.
After that you can dye your parts and/or apply a different kind of coating to make them more durable or watertight and that’s it – the print is done.
Which technology is better?
There’s no question: SLS holds the advantage. Readying the printer for work and dusting off the spare powder after it’s done requires nowhere near as much time and effort as the careful removal of support structures, filling of possible gaps, gluing/welding stuff together, and all other things that are often needed just to make a part usable. The SLS technique also produces the most accurate and complete print currently possible, right off the bat, and supports printing of multiple parts at once. Whether you need many simple items, or a smaller number of very complex ones – SLS remains a better choice.
The lack of support structures and the fact that there’s no need to put parts together after getting them out of the printer – at least in case of small enough prints, since, obviously, models that won’t fit into the printer will still need to be assembled from smaller parts – are the deciding factors here. All the other post-processing actions – things like polishing, painting, (also through special methods mentioned in this article, like sand-blasting, vapor smoothing, dipping or vibro polish), coating them with different substances – all that can be done regardless of the printing method used and can therefore be described as finishing touches. They may, or may not be needed, depending on the situation, as not every printed part needs to be smooth, painted or coated with something, to be usable. Each and every single one, however, needs to be whole and to accurately represent the intended design – which, in case of FDM and SLA printing methods, requires additional post-processing work.